0001193125-19-085321 | 10-K | XBRL Viewer

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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Mar. 20, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	AVRO
Entity Registrant Name	AVROBIO, INC.
Entity Central Index Key	0001681087
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Small Business	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		24,073,876
Entity Public Float			$ 360,591,820

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 126,302	$ 5,963
Prepaid expenses and other current assets	3,718	345
Total current assets	130,020	6,308
Property and equipment, net	2,634	349
Other assets	825	365
Total assets	133,479	7,022
Current liabilities:
Accounts payable	2,784	527
Accrued expenses and other current liabilities	7,822	2,098
Total current liabilities	10,606	2,625
Warrant to purchase redeemable convertible preferred stock		35
Derivative liability		371
Deferred rent, net of current portion	689	126
Other long-term liability		500
Total liabilities	11,295	3,657
Commitments and contingencies (Note 14)
Redeemable convertible preferred stock (Note 8)		26,500
Stockholders' equity (deficit):
Preferred stock, $0.0001 par value; 10,000,000 shares authorized and no shares issued or outstanding as of December 31, 2018; No shares authorized, issued or outstanding at December 31, 2017
Common stock, $0.0001 par value; 150,000,000 and 51,000,000 shares authorized as of December 31, 2018 and 2017, respectively; 23,959,903 and 2,581,474 shares issued as of December 31, 2018 and 2017, respectively; 23,806,628 and 2,305,173 shares outstanding as of December 31, 2018 and 2017, respectively	2
Additional paid-in capital	193,921	339
Accumulated deficit	(71,739)	(23,474)
Total stockholders' equity (deficit)	122,184	(23,135)
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	$ 133,479	$ 7,022

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, authorized	10,000,000	0
Preferred stock, issued	0	0
Preferred stock, outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, authorized	150,000,000	51,000,000
Common stock, issued	23,959,903	2,581,474
Common stock, outstanding	23,806,628	2,305,173

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development	$ 35,095	$ 15,191
General and administrative	11,148	3,195
Total operating expenses	46,243	18,386
Loss from operations	(46,243)	(18,386)
Other income (expense):
Interest income	1,726	57
Change in fair value of preferred stock warrant liability	(162)	(17)
Change in fair value of derivative liability	(1,629)	(283)
Other expense	(53)	(19)
Total other expense, net	(118)	(262)
Net loss	(46,361)	(18,648)
Comprehensive loss	(46,361)	(18,648)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(46,361)	(18,648)
Accretion of issuance costs on redeemable convertible preferred stock	(2,243)	(85)
Net loss attributable to common stockholders-basic and diluted	$ (48,604)	$ (18,733)
Net loss per share attributable to common stockholders-basic and diluted (Note 13)	$ (3.62)	$ (8.38)
Weighted-average number of common shares used in computing net loss per share attributable to common stockholders-basic and diluted	13,435,478	2,235,865

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) Equity - USD ($) $ in Thousands	Total	Series Seed Redeemable Convertible Preferred Stock [Member]	Series A Redeemable Convertible Preferred Stock [Member]	Series B Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Begining balance at Dec. 31, 2016	$ (4,579)	$ 1,500	$ 7,500			$ 247	$ (4,826)
Begining balance, shares at Dec. 31, 2016		3,333,333	5,714,286		2,172,068
Issuance of series A redeemable convertible preferred stock dividend	0	$ 0	$ 0	$ 0	$ 0	0	0
Issuance of series A redeemable convertible preferred stock dividend, shares			3,720,864
Issuance of redeemable convertible preferred stock,			$ 17,415
Issuance of redeemable convertible preferred stock, shares			22,015,349
Vesting of restricted stock awards	0	0	$ 0	0	$ 0	0	0
Vesting of restricted stock awards, shares					133,105
Stock-based compensation expense	109					177
Accretion of issuance costs related to redeemable convertible preferred stock	(85)		85			(85)
Net loss	(18,648)						(18,648)
Ending balance at Dec. 31, 2017	(23,135)	$ 1,500	$ 25,000			339	(23,474)
Ending balance, shares at Dec. 31, 2017		3,333,333	31,450,499		2,305,173
Issuance of redeemable convertible preferred stock,				$ 58,257
Issuance of redeemable convertible preferred stock, shares				28,285,557
Vesting of restricted stock awards	0	$ 0	$ 0	$ 0	$ 0	0	0
Issuance of series B redeemable convertible preferred stock to settle accrued liability of license cost				$ 500
Vesting of restricted stock awards, shares					123,026
Issuance of series B redeemable convertible preferred stock to settle accrued liability of license cost, shares				233,765
Stock-based compensation expense	2,196					2,196
Accretion of issuance costs related to redeemable convertible preferred stock	(2,243)			$ 2,243		(339)	(1,904)
Issuance of common stock upon IPO	104,013				$ 1	104,012
Issuance of common stock upon IPO, shares					6,035,151
Reclassification of warrants to purchase redeemable convertible stock into warrants to purchase common stock	197					197
Conversion of redeemable convertible preferred stock into common stock	87,500	$ (1,500)	$ (25,000)	$ (61,000)	$ 1	87,499
Conversion of redeemable convertible preferred stock into common stock, shares		(3,333,333)	(31,450,499)	(28,519,322)	15,320,213
Exercise of warrants to purchase common stock	0	$ 0	$ 0	$ 0	$ 0	0	0
Exercise of warrants to purchase common stock, shares					6,091
Exercise of stock options	$ 17					17
Exercise of stock options, shares	16,974				16,974
Net loss	$ (46,361)						(46,361)
Ending balance at Dec. 31, 2018	$ 122,184				$ 2	$ 193,921	$ (71,739)
Ending balance, shares at Dec. 31, 2018					23,806,628

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement of Stockholders' Equity [Abstract]
Issuance of redeemable convertible preferred stock, net of issuance costs	$ 2,243	$ 85
Issuance of common stock upon IPO, net of issuance costs	$ 2,628

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (46,361)	$ (18,648)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,196	177
Depreciation and amortization expense	380	45
Amortization of deferred offering costs	37	24
Impairment loss of property and equipment	235
Deferred rent expense	(105)	134
Change in fair value of preferred stock warrant liability	162	17
Change in fair value of derivative liability	1,629	283
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,037)	(326)
Other assets	(115)	(218)
Accounts payable	2,018	176
Accrued expenses and other current liabilities	5,313	1,454
Other long-term liability		500
Net cash used in operating activities	(37,648)	(16,382)
Cash flows from investing activities:
Purchases of property and equipment	(1,832)	(383)
Net cash used in investing activities	(1,832)	(383)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	58,257	17,415
Proceeds from issuance of common shares upon completion of initial public offering, net of offering costs	104,013
Payment of dilution liability	(2,000)
Exercise of stock option	17
Payments of issuance costs on debt facility		(44)
Net cash provided by financing activities	160,287	17,371
Net increase in cash and cash equivalents	120,807	606
Cash, cash equivalents and restricted cash at beginning of period	5,987	5,381
Cash, cash equivalents and restricted cash at end of period	126,794	5,987
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses	256	11
Property and equipment held for sale	19
Issuance of warrants associated with debt facility		18
Deferred offering costs included in accrued expenses		85
Purchase of property and equipment paid for by landlord	842
Accretion of issuance costs related to redeemable convertible preferred stock	$ 2,243	$ 85

Nature of the Business	12 Months Ended
Nature of the Business	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business AVROBIO, Inc. (the “Company” or “AVROBIO”) is a clinical stage gene therapy company focused on developing potentially curative ex vivo lentiviral gene therapies to treat rare diseases following a single dose. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including but not limited to, risks associated with completing preclinical studies and clinical trials, receiving regulatory approvals for product candidates, development by competitors of new biopharmaceutical products, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize revenue from product sales. Through December 31, 2018, the Company has funded its operations primarily with proceeds from the sale of series Seed redeemable convertible preferred stock (the “Series Seed Preferred Stock”), series A redeemable convertible preferred stock (the “Series A Preferred Stock”), series B redeemable convertible preferred stock (the “Series B Preferred Stock”) (the Series Seed Preferred Stock, the Series A Preferred Stock and the Series B Preferred Stock are collectively referred to as the “Preferred Stock”) and common stock through the Company’s initial public offering (“IPO”). The Company has incurred recurring losses since its inception, including net losses of $46,361 and $18,648 for the years ended December 31, 2018 and 2017, respectively. In addition, as of December 31, 2018, the Company had an accumulated deficit of $71,739. The Company expects to continue to generate operating losses for the near future. The future viability of the Company is dependent on its ability to raise additional capital to finance its operations. The Company’s inability to raise capital as and when needed could have a negative impact on its financial condition and ability to pursue its business strategies. There can be no assurance that the current operating plan will be achieved or that additional funding will be available on terms acceptable to the Company, or at all. The Company believes the cash and cash equivalents on hand as of December 31, 2018 of $126,302 will be sufficient to fund its operations and capital expenditure requirements through at least the next twelve months from the filing date of this Annual Report on Form 10-K with the SEC.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).

Principles of Consolidation

The accompanying consolidated financial statements include the accounts of AVROBIO, Inc. and its wholly owned subsidiaries. All intercompany transactions and balances have been eliminated in consolidation.

Use of Estimates

The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, stock-based compensation expense, the valuation of equity and derivative instruments and the recoverability of the Company’s net deferred tax assets and related valuation allowance. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ materially from those estimates.

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Cash and cash equivalents include cash held in banks and amounts held in interest-bearing money market accounts. Cash equivalents are carried at cost, which approximates their fair market value.

Restricted Cash

As of December 31, 2018 and 2017, restricted cash consisted of $492 and $24, respectively, used to secure the letters of credit for the benefit of the landlord in connection with the Company’s lease agreements (Note 14). These amounts are classified as other assets in the Company’s consolidated balance sheets.

Concentrations of Credit Risk

The Company has no significant off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and restricted cash. Periodically, the Company maintains deposits in accredited financial institutions in excess of federally insured limits. The Company deposits its cash and cash equivalents in financial institutions that it believes have high credit quality and has not experienced any losses on such accounts and does not believe it is exposed to any unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

Deferred Issuance Costs

The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred issuance costs until such financings are consummated. After consummation of the equity financing, these costs are recorded as a reduction of the proceeds generated as a result of the offering. Should the planned equity financing be abandoned, the deferred issuance costs will be expensed immediately as a charge to operating expenses in the consolidated statements of operations. As of December 31, 2017, the Company recorded deferred issuance costs of $85 within other assets on the consolidated balance sheet related to the Series B Preferred Stock offering which was consummated in January 2018 (Note 8).

Property and Equipment

Property and equipment are recorded at cost. Depreciation and amortization is calculated using the straight-line method over the following estimated useful lives of the assets:


		Estimated Useful Life
Laboratory and office equipment		5 years
Computer equipment and software		2 years
Leasehold improvements		Lesser of lease term or 10 years

Upon disposal, retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Expenditures for repairs and maintenance that do not improve or extend the lives of the respective assets are charged to expense as incurred.

Impairment of Long-Lived Assets

Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. During the year ended December 31, 2018, the Company recorded an impairment loss of $235 on long-lived assets related to leasehold improvements.

Fair Value Measurements

Certain assets and liabilities of the Company are carried at fair value under GAAP (see Note 3). Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

•

Level 3—Unobservable inputs that are supported by little or no market activity that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

Segment Information

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer (“CEO”). The Company and the CEO view the Company’s operations and manage its business as one operating segment. All material long-lived assets of the Company reside in the United States.

Research and Development Costs

Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, facilities costs, depreciation, third-party license fees, and external costs of outside vendors engaged to conduct preclinical development activities and clinical trials as well as to manufacture research and development materials. Non-refundable prepayments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized. Such amounts are recognized as an expense as the goods are delivered or the related services are performed or until it is no longer expected that the goods will be delivered or the services rendered.

The Company has entered into various research and development related contracts with parties both inside and outside of the United States. The payments to these agreements are recorded as research and development expenses as incurred. The Company records accrued liabilities for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.

Stock-Based Compensation

For stock-based awards issued to employees and members of the Company’s board of directors (the “Board”) for their services on the Board, the Company measures the estimated fair value of the stock-based award on the date of grant and recognizes compensation expense for those awards over the requisite service period, which is generally the vesting period of the respective award. The Company issues stock-based awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. The Company has not issued any stock-based awards with performance- or market-based vesting conditions. The Company accounts for forfeitures as they occur.

Prior to the adoption of Accounting Standards Update (“ASU”) No. 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, the measurement date fornon-employee awards was generally the date the services are completed, resulting in financial reporting period adjustments to stock-based compensation during the vesting terms for changes in the fair value of the awards. After adoption of ASU 2018-07, the measurement date for non-employee awards is the later of the adoption date of ASU 2018-07, or the date of grant, without change in the fair value of the award. For stock-based awards granted to nonemployees subject to graded vesting that only contain service conditions, the Company has elected to recognize stock-based compensation expense using the straight-line recognition method.

The Company classifies stock-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s cash compensation costs are classified.

Given the absence of an active market for the Company’s common stock prior to the IPO, the Company and the Board, the members of which the Company believes have extensive business, finance, and venture capital experience, were required to estimate the fair value of the Company’s common stock at the time of each grant of a stock-based award. The Company and the Board determined the estimated fair value of the Company’s equity instruments based on a number of factors, including external market conditions affecting the biotechnology industry sector. The Company and the Board utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants’ Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the fair value of its common stock. Each valuation methodology includes estimates and assumptions that require the Company’s judgment. These estimates and assumptions include a number of objective and subjective factors in determining the value of the Company’s common stock at each grant date, including: (1) prices paid for the Company’s redeemable convertible preferred stock, which the Company had sold to outside investors in arm’s-length transactions, and the rights, preferences, and privileges of the Company’s redeemable convertible preferred stock and common stock; (2) valuations performed by an independent valuation specialist; (3) the Company’s stage of development; (4) the fact that the grants of stock-based awards involved illiquid securities in a private company; and (5) the likelihood of achieving a liquidity event for the common stock underlying the stock-based awards, such as an IPO or sale of the Company, given prevailing market conditions.

The fair value of each stock option award is estimated on the date of grant using the Black-Scholes option pricing model. As there was no public market for its common stock prior to June 21, 2018, which was the first day of trading, and as the trading history of the Company’s common stock was limited through December 31, 2018, the Company determined the volatility for awards granted based on an analysis of reported data for a group of guideline companies that issued options with substantially similar terms. The expected volatility has been determined using a weighted-average of the historical volatility measures of this group of guideline companies. The Company expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. The Company has not paid, and does not anticipate paying, cash dividends on its common stock; therefore, the expected dividend yield is assumed to be zero.

See Note 10 for the assumptions used by the Company in determining the grant date fair value of stock-based awards granted, as well as a summary of the stock-based award activity under the Company’s stock-based compensation plan for the year ended December 31, 2018.

Warrant to Purchase Preferred Stock

Prior to the IPO, the Company classified the warrant for the purchase of shares of its redeemable convertible preferred stock (see Note 7) as a liability on its consolidated balance sheets as the warrant was a free-standing financial instrument that may require the Company to transfer assets upon exercise. The preferred stock warrant liability was initially recorded at fair value upon the date of issuance and was subsequently remeasured to fair value at each reporting date. Changes in the fair value of the warrant to purchase preferred stock were recognized as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss.

The Company utilized the Black-Scholes option-pricing model, which incorporates assumptions and estimates, to value the warrant. The Company assessed these assumptions and estimates on a quarterly basis as additional information impacting the assumptions was obtained. Estimates and assumptions impacting the fair value measurement included the fair value per share of the underlying redeemable convertible preferred stock issuable upon exercise of the warrant, the remaining contractual term of the warrant, the risk-free interest rate, the expected dividend yield and the expected volatility of the price of the underlying redeemable convertible preferred stock.

Upon the IPO, the warrant to purchase preferred stock was converted to a warrant to purchase common stock. The carrying amount of the warrant to purchase preferred stock as of the date of IPO was transferred to the account of additional paid in capital. No further revaluation was needed for the warrant to purchase common stock.

Comprehensive Income (Loss)

Comprehensive income (loss) is defined as the change in stockholders’ equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. Comprehensive income (loss) includes net income (loss) as well as other changes in stockholders’ (deficit) equity which includes certain changes in equity that are excluded from net income (loss). Comprehensive loss has been disclosed in the accompanying statements of operations and comprehensive loss and equals the Company’s net loss for all periods presented.

Foreign Currency Translation

The functional currency of the Company’s international operations in Canada and Australia is the U.S. dollar. Accordingly, all operating assets and liabilities of these international subsidiaries are remeasured into U.S. dollars using the exchange rates in effect at the balance sheet date or historical rates, as appropriate, while expenses are remeasured into U.S. dollars at the average rates in effect during the period. Any differences resulting from the remeasurement of assets, liabilities, and operations of the Canadian and Australian subsidiaries are recorded within other income (expense), net in the consolidated statements of operations and comprehensive loss. During the years ended December 31, 2018 and 2017, the Company recorded foreign exchange losses of $51 and $19, respectively, in other expense.

Income Taxes

Deferred tax assets and liabilities are determined on the basis of the differences between the consolidated financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established.

The Company accounts for uncertain tax positions recognized in the consolidated financial statements by prescribing a more-likely-than-not threshold for financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Leases

The Company categorizes leases at their inception as either operating or capital leases. On certain lease arrangements, the Company may receive rent holidays or other incentives. The Company recognizes lease costs on a straight-line basis once control of the space is achieved, without regard to deferred payment terms, such as rent holidays, that defer the commencement date of required payments or escalating payment amounts. The difference between required lease payments and rent expense has been recorded as deferred rent and other accrued expenses and other current liabilities in the accompanying consolidated balance sheets. Additionally, incentives received are treated as a reduction of costs over the term of the agreement, as they are considered an inseparable part of the lease agreement.

Net Income (Loss) per Share Attributable to Common Stockholders

Net income (loss) per share attributable to common stockholders is determined using the two-class method, which includes the weighted-average number of shares of common stock outstanding during the period and other securities that participate in dividends (a participating security). In periods of income, the redeemable convertible preferred stock would be considered participating securities because the shares include rights to participate in dividends with the common stock; however, the redeemable convertible preferred stock is not considered a participating security in periods of loss as they do not have an obligation to share in the Company’s net losses.

Under the two-class method, basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period. Diluted net income (loss) per share attributable to common stockholders is computed using the more dilutive of (1) the two-class method or (2) the if-converted method. The Company allocates net income first to the holders of Preferred Stock based on dividend rights under the Company’s certificate of incorporation and then to preferred and common stockholders based on ownership interests.

Subsequent Event Considerations

The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the consolidated financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. See Note 17.

Emerging Growth Company Status

The Company is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act, or JOBS Act, and may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. The Company may take advantage of these exemptions until the Company is no longer an “emerging growth company.” Section 107 of the JOBS Act provides that an “emerging growth company” can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected to use the extended transition period for complying with new or revised accounting standards and as a result of this election, its consolidated financial statements may not be comparable to companies that comply with public company effective dates. The Company may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of an offering or such earlier time that it is no longer an “emerging growth company”.

Recently Adopted Accounting Pronouncements

In June 2018, the FASB issued ASU No. 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). ASU 2018-07 amends the FASB Accounting Standards Codification (“ASC”) to expand the scope of FASB ASC Topic 718, Compensation-Stock Compensation, to include accounting for share-based payment transactions for acquiring goods and services fromnon-employees. The amendments in ASU 2018-07 are effective for all entities for annual periods, and interim periods within those annual periods, beginning after December 15, 2018. Early adoption is permitted. The Company early adopted this guidance at the beginning of the fourth quarter of 2018. There was no material impact on the consolidated financial statements as a result of this adoption.

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes existing revenue recognition guidance under GAAP. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The standard defines a five-step process to achieve this principle and will require companies to use more judgment and make more estimates than under the current guidance. The Company expects that these judgments and estimates will include identifying performance obligations in the customer contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts. The Company early adopted ASU 2014-09 at the beginning of the fourth quarter of 2018. The adoption of ASU 2014-09 did not have an impact on the Company’s consolidated financial statements as the Company does not currently have any revenue-generating arrangements.

In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”), which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The standard is effective for annual periods beginning after December 15, 2017. Early adoption is permitted, including adoption in any interim period, for (1) public business entities for reporting periods for which financial statements have not yet been issued and (2) all other entities for reporting periods for which financial statements have not yet been made available for issuance. The Company adopted this guidance at the beginning of the fourth quarter of 2018. There was no material impact on the consolidated financial statements as a result of this adoption.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230) Restricted Cash (“ASU 2016-18”), which requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents and restricted cash. Therefore, amounts described as restricted cash should be included with cash and cash equivalents when reconciling the beginning of period and end of period amounts shown on the statement of cash flows. The Company early adopted this guidance at the beginning of the fourth quarter of 2018. ASU 2016-18 is effective on a retrospective basis. The cash, cash equivalents and restricted cash balances as of December 31, 2018 and 2017, which are presented in the Company’s consolidated statements of cash flows subsequent to the adoption of ASU 2016-18, consisted of the following:


	December 31,
	2018		2017
Cash and cash equivalents	$	126,302	$	5,963
Other assets		492		24

Cash, cash equivalents and restricted cash	$	126,794	$	5,987

Recently Issued Accounting Pronouncements

In August 2018, the FASB issued Accounting Standards Update No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, or ASU 2018-13. The amendments in ASU 2018-13 eliminate, add, and modify certain disclosure requirements for fair value measurements. The amendments are effective for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted for either the entire ASU or only the provisions that eliminate or modify requirements. The amendments with respect to changes in unrealized gains and losses, the range and weighted-average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty are to be applied prospectively. All other amendments are to be applied retrospectively to all periods presented. The Company is currently evaluating the effects the adoption of ASU 2018-13 will have on its disclosures.

In August 2018, the FASB issued ASU No. 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40) (“ASU 2018-15”). ASU 2018-15 updates guidance regarding accounting for implementation costs associated with a cloud computing arrangement that is a service contract. The amendments under ASU 2018-15 are effective for interim and annual fiscal periods beginning after December 15, 2019, with early adoption permitted. The Company does not expect the adoption of ASU 2018-15 to have a material impact on its consolidated financial statements.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808)—Clarifying the Interaction between Topic 808 and Topic 606 (“ASU 2018-18”). The amendments in ASU 2018-18 clarify that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606 when the collaborative arrangement participant is a customer in the context of a unit of account. The amendments under ASU 2018-18 are effective for interim and annual fiscal periods beginning after December 15, 2019, with early adoption permitted. The amendments in ASU 2018-18 should be applied retrospectively to the date of initial application of ASC 606. The Company does not expect the adoption of ASU 2018-18 to have a material impact on its consolidated financial statements.

In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception. Part I applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within ASC Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. For public entities, the amendments in Part I of this update are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. For all other entities, the amendments in Part I of this update are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted for all entities, including adoption in an interim period. The Company is currently evaluating the impact that the adoption of ASU 2017-11 will have on its consolidated financial statements.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”), to address diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. For public entities, the standard is effective for annual periods beginning after December 15, 2017, including interim periods within those fiscal years. For all other entities, the standard is effective for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. The Company is currently evaluating the impact that the adoption of ASU 2016-15 will have on its consolidated financial statements.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today. ASU 2016-02 supersedes the previous leases standard, ASC 840, Leases. For public entities, not-for-profit entities and an employee benefit plan that files financial statements with the U.S. Securities and Exchange Commission (SEC), the standard is effective for public entities for annual periods beginning after December 15, 2018 including interim periods within those fiscal years. For all other entities, the standard is effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements.

Fair Value of Financial Assets and Liabilities

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

3. Fair Value of Financial Assets and Liabilities

The following table presents information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicates the level of the fair value hierarchy utilized to determine such fair values as of December 31, 2018 and 2017:


	Fair Value Measurements as of December 31, 2018 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	126,047	$	—	$	—	$	126,047
Restricted cash		492		—		—		492

	$	126,539	$	—	$	—	$	126,539


	Fair Value Measurements as of December 31, 2017 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	5,684	$	—	$	—	$	5,684
Restricted cash		24		—		—		24

	$	5,708	$	—	$	—	$	5,708

Liabilities:
Derivative liability	$	—	$	—	$	371	$	371
Warrant to purchase redeemable convertible preferred stock		—		—		35		35

	$	—	$	—	$	406	$	406

During the years ended December 31, 2018 and 2017, there were no transfers between Level 1, Level 2 and Level 3.

Valuation of the Warrant to Purchase Preferred Stock

The liability for the warrant to purchase redeemable convertible preferred stock in the table above is composed of the fair value of a warrant to purchase shares of Series A Preferred Stock that was issued to a lender in connection with a loan and security agreement in 2017 (the “Loan Agreement”) (Note 7). The fair value of the warrant to purchase preferred stock was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy.

In order to determine the fair value of the warrant to purchase preferred stock, the Company utilized available facts and circumstances to estimate the number of shares of Series A Preferred Stock for which the warrant would ultimately be exercisable. The Company then used the Black-Scholes option-pricing model, which incorporates assumptions and estimates, to value the warrant to purchase preferred stock. Estimates and assumptions impacting the fair value measurement included the fair value of the underlying shares of Series A Preferred Stock, the remaining contractual term of the warrant, risk-free interest rate, expected dividend yield and expected volatility of the price of the underlying preferred stock. The Company determined the fair value of the underlying preferred stock based on various valuation methodologies. The Company lacks company-specific historical and implied volatility information of its stock. Therefore, it estimated its expected stock volatility based on the historical volatility of publicly traded guideline companies for a term equal to the remaining contractual term of the warrant. The risk-free interest rate was determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining contractual term of the warrant. The Company estimated no expected dividend yield based on the fact that the Company has never paid or declared dividends and does not intend to do so in the foreseeable future. Upon the IPO, the warrant to purchase preferred stock was converted to a warrant to purchase common stock. The carrying amount of warrant to purchase preferred stock as of the date of IPO was transferred to the account of additional paid in capital.

The following table sets forth a summary of changes in the fair value of the Company’s preferred stock warrant liability for which fair value was determined by Level 3 inputs:


	Warrant Liability
Balance as of December 31, 2016	$	—
Initial fair value of warrant to purchase redeemable convertible preferred stock		18
Change in fair value		17

Balance as of December 31, 2017	$	35
Change in fair value		162
Conversion of preferred stock warrant to common stock warrant		(197	)

Balance as of December 31, 2018	$	—

Valuation of Derivative

In January 2016, in connection with a license agreement entered into with University Health Network (“UHN”), and as part of the initial consideration for the license, the Company issued 1,161,665 shares of common stock to UHN pursuant to a stock purchase agreement (the “Stock Purchase Agreement”). The Stock Purchase Agreement contained a provision requiring the Company to make a cash payment to UHN of up to $2,000 if UHN’s fully diluted ownership is reduced within specified percentages as part of an IPO by the Company. The Company concluded the anti-dilution feature represented a derivative instrument and should be measured at fair value, with changes in fair value recognized as a gain or loss to other income (expense), net in the consolidated statements of operations and comprehensive loss. The initial fair value of the derivative of $49 was recorded as research and development expense in January 2016.

On December 31, 2016 and 2017, the Company remeasured the fair value of the derivative, using current assumptions, resulting in an increase in fair value of $39 and $283, respectively, which was recorded in other expense in the consolidated statements of operations and comprehensive loss for the years ended December 31, 2016 and 2017, respectively.

On June 21, 2018, in connection with the Company’s IPO, the Company remeasured the fair value of the derivative to $2,000 as the Company was required to pay the dilution payment as mentioned above, which the Company paid in July 2018. An increase in fair value of $1,629 was recorded in other expense in the accompanying consolidated statement of operations and comprehensive loss for the year ended December 31, 2018.

The following table sets forth a summary of changes in the fair value of the Company’s derivative liability for which fair value is determined by Level 3 inputs:


	Derivative Liability
Balance as of December 31, 2016	$	88
Change in fair value		283

Balance as of December 31, 2017		371
Change in fair value		1,629
Payment		(2,000	)

Balance as of December 31, 2018	$	—

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Prepaid Expenses and Other Current Assets

12 Months Ended

Dec. 31, 2018

Text Block [Abstract]

Prepaid Expenses and Other Current Assets

4. Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:


	December 31,
	2018		2017
Tax incentive refund	$	1,325	$	—
Prepaid research and development costs		981		163
Prepaid insurance		316		—
Interest income receivable		220		—
Prepaid rent		81		122
Other current assets		795		60

	$	3,718	$	345

Property and Equipment, Net

12 Months Ended

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

5. Property and Equipment, Net

Property and equipment, net consisted of the following:


	December 31,
	2018			2017
Laboratory and office equipment	$	1,624		$	126
Leasehold improvements		1,260			240
Computer equipment and software		134			28

		3,018			394
Less: Accumulated depreciation and amortization		(384	)		(45	)

	$	2,634		$	349

Depreciation and amortization expense for the years ended December 31, 2018 and 2017 was $380 and $45, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2018

Payables and Accruals [Abstract]

Accrued Expenses

6. Accrued Expenses

Accrued expenses consisted of the following:


	December 31,
	2018		2017
Compensation and benefit costs	$	2,616	$	794
Research and development costs		3,969		831
Consulting and professional fees		320		267
Preferred stock issuance cost		—		85
Other liabilities		917		121

	$	7,822	$	2,098

Loan Agreement and Warrant to Purchase Preferred Stock	12 Months Ended
Loan Agreement and Warrant to Purchase Preferred Stock	Dec. 31, 2018
Statement of Stockholders' Equity [Abstract]
Loan Agreement and Warrant to Purchase Preferred Stock	7. Loan Agreement and Warrant to Purchase Preferred Stock On June 23, 2017, the Company entered into the Loan Agreement with a lender, which provided for the issuance of term loans of up to $10,000, subject to the achievement of various development and corporate milestones. Any outstanding principal amounts under the Loan Agreement accrued interest at a floating per annum rate equal to the greater of 1% and the “prime rate,” as defined in the Loan Agreement, minus 3%. The Loan Agreement contained customary indemnification obligations and customary events of default. In the event of default by the Company under the Loan Agreement, the lender would be entitled to exercise their remedies thereunder, including the right to accelerate the debt, upon which the Company may be required to repay all amounts then outstanding under the Loan Agreement or the lender may take possession of the collateral securing the Loan Agreement. The Loan Agreement also includes certain restrictions on, among other things, the Company’s ability to incur additional indebtedness, change the name or location of its business, merge with or acquire other entities, pay dividends or make other distributions to holders of the Company’s capital stock, make certain investments, engage in transactions with affiliates, create liens, open new deposit accounts, sell assets or pay subordinated debt. As of October 31, 2018, the Company had not drawn down from the facility and the Loan Agreement expired on October 31, 2018. In connection with the Loan Agreement, the Company agreed to issue a warrant to the lender for the purchase of up to 188,702 shares of the Company’s Series A Preferred Stock with an exercise price of $0.7949 per share. The warrant expires on June 22, 2027. The warrant was initially exercisable for the purchase of up to 28,305 shares of Series A Preferred Stock and could become exercisable for up to an additional 160,397 shares of Series A Preferred Stock based on the amounts drawn under the Loan Agreement. On the issuance date of the warrant, the Company recorded a deferred financing cost and a liability for the warrant to purchase preferred stock in the Company’s consolidated balance sheet equal to the issuance-date fair value of the warrant. Upon closing of the IPO, the carrying amount of the warrant to purchase preferred stock as of the date of IPO was transferred to the account of additional paid in capital (Note 3). On the date of issuance, the fair value of the warrant was determined to be $18. The Company recognized a loss of $162 and $17 in the consolidated statements of operations and comprehensive loss for the years ended December 31, 2018 and 2017, respectively, related to the change in fair value of the warrant. On July 13, 2018, the lender of the Loan Agreement exercised its common stock warrants to purchase 6,850 shares of common stock on a net basis, resulting in the issuance of 6,091 shares of common stock.

Redeemable Convertible Preferred Stock

12 Months Ended

Dec. 31, 2018

Text Block [Abstract]

Redeemable Convertible Preferred Stock

8. Redeemable Convertible Preferred Stock

As of December 31, 2016, the authorized capital stock of the Company included 22,380,952 shares of $0.0001 par value preferred stock, of which 3,333,333 shares were designated as Series Seed Preferred Stock and 19,047,619 shares were designated as Series A Preferred Stock. As of December 31, 2017, the authorized capital stock of the Company included 34,972,535 shares of $0.0001 par value preferred stock, of which 3,333,333 shares were designated as Series Seed Preferred Stock and 31,639,202 shares were designated as Series A Preferred Stock.

In January 2016, the Company issued and sold 3,333,333 shares of Series Seed Preferred Stock at a price of $0.45 per share, for total proceeds of $1,398, net of issuance costs of $102.

In July 2016, the Company issued and sold 5,714,286 shares of Series A Preferred Stock, at a price of $1.3125 per share, for total proceeds of $7,297, net of issuance costs of $203. The terms of the Series A Preferred Stock Purchase Agreement included the obligation of the investors to purchase, and the Company to sell, up to 13,333,333 additional shares of Series A Preferred Stock at $1.3125 per share contingent upon the achievement of certain specified milestones or at the election of a required vote by the investors and the Board. The Company concluded that the right to participate in the future issuance of Series A Preferred Stock did not meet the definition of a freestanding financial instrument, as the rights were not legally detachable from the Series A Preferred Stock.

In March 2017, the Company amended certain provisions of the Series A Preferred Stock. The changes included issuing additional instruments through a preferred stock dividend in the amount of 3,720,864 shares of Series A Preferred Stock, which effectively repriced the outstanding shares of Series A Preferred Stock, changing the purchase price for future shares, and decreasing the redemption price per share such that total redemption value was not affected. Concurrent with the amendment, the Company issued 4,403,070 additional shares of Series A Preferred Stock at a purchase price of $0.7949 per share, for total proceeds of $3,452, net of issuance costs of $48. Additionally, the number of shares of Series A Preferred Stock which could be issued in the future was changed to 17,612,279 shares. The Company assessed the changes to the terms and concluded that it represented a material change to substantive terms of the instrument, and therefore represented, for accounting purposes, an extinguishment and re-issuance of the Series A Preferred Stock outstanding at the time. As the carrying value of the outstanding shares of Series A Preferred Stock prior to the extinguishment was deemed to equal the aggregate fair value of the reissued shares and the shares issued as a dividend, no gain or loss was recognized on extinguishment of the Series A Preferred Stock.

In October 2017, the Company issued and sold an additional 17,612,279 shares of Series A Preferred Stock, at a price of $0.7949 per share, for total proceeds of $13,963, net of issuance costs of $37.

Prior to the IPO, the authorized capital stock of the Company included 63,491,857 shares of $0.0001 par value preferred stock, of which 3,333,333 shares have been designated as Series Seed Preferred Stock, 31,639,202 shares have been designated as Series A Preferred Stock and 28,519,322 shares have been designated as Series B Preferred Stock.

On January 19, 2018, the Company entered into a stock purchase agreement for the sale of 28,285,557 shares of Series B Preferred Stock for $2.1389 per share. The total gross proceeds received were $60,500 and issuance costs were $2,243. In addition, the Company issued 233,765 shares of Series B Preferred Stock to BioMarin as required by the Company’s license agreement with BioMarin (Note 11).

As of December 31, 2017, the Preferred Stock consisted of the following:


	December 31, 2017
	Preferred Shares Authorize		Preferred Shares Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series Seed preferred stock		3,333,333		3,333,333	$	1,500,000	$	1,500,000		806,711
Series A preferred stock		31,639,202		31,450,499		25,000,000		25,000,000		7,611,438

		34,972,535		34,783,832	$	26,500,000	$	26,500,000		8,418,149

Upon closing of the IPO, the Preferred Stock as of December 31, 2017, together with Series B Preferred Stock issued in January 2018, were converted to 15,320,213 shares of common stock. The holders of the Company’s Preferred Stock had certain voting, dividend, and redemption rights, as well as liquidation preferences and conversion privileges. All rights, preferences, and privileges associated with the preferred stock were terminated at the time of the Company’s IPO in conjunction with the conversion of all outstanding shares of Preferred Stock into shares of common stock.

Common Stock

12 Months Ended

Dec. 31, 2018

Federal Home Loan Banks [Abstract]

Common Stock

9. Common Stock

As of December 31, 2017, the authorized capital stock of the Company included 51,000,000 shares of common stock, $0.0001 par value.

On June 1, 2018, the Board approved for filing immediately following the effectiveness of the Company’s registration statement in connection with its IPO, the Fourth Amended and Restated Certificate of Incorporation, which, among other matters: (i) authorized 150,000,000 shares of common stock, $0.0001 par value and (ii) created 10,000,000 shares of undesignated preferred stock. As of December 31, 2018, no undesignated preferred stock was outstanding.

In accordance to the Fourth Amended and Restated Certificate of Incorporation, the holders of the common stock shall have the exclusive right to vote for the election of directors of the Company and on all other matters requiring stockholder action, each outstanding share entitling the holder thereof to one vote on each matter properly submitted to the stockholders of the Company for their vote; provided, however, that, except as otherwise required by law, holders of common stock, as such, shall not be entitled to vote on any amendment to any amendment to a certificate of designations of any series of undesignated preferred stock that alters or changes the powers, preferences, rights or other terms of one or more outstanding series of undesignated preferred stock if the holders of such affected series of undesignated preferred stock are entitled to vote, either separately or together with the holders of one or more other such series, on such amendment pursuant to a certificate of designations of any series of undesignated preferred stock.

Through December 31, 2018, no cash dividends have been declared or paid.

Reverse Stock Split

On June 1, 2018, the Board approved a 1-for-4.312 reverse stock split of the Company’s common stock. The reverse stock split was approved by the stockholders on June 7, 2018 and became effective on June 7, 2018. Stockholders entitled to fractional shares as a result of the reverse stock split received a cash payment in lieu of receiving fractional shares. All share and per share data shown in the accompanying consolidated financial statements and related notes have been retroactively revised to reflect the reverse stock split. Shares of common stock underlying outstanding stock options and other equity instruments were proportionately reduced and the respective exercise prices, if applicable, were proportionately increased in accordance with the terms of the agreements governing such securities. Shares of common stock reserved for issuance upon the conversion of the Company’s Preferred Stock were proportionately reduced and the respective conversion prices were proportionately increased.

Initial Public Offering

On June 20, 2018, the Company’s registration statement on Form S-1 relating to its IPO was declared effective by the SEC. The IPO closed on June 25, 2018 and the Company issued and sold 5,247,958 common shares at a public offering price of $19.00 per share for net proceeds of $90,103 after deducting underwriting discounts and commissions of $6,980 and other offering expenses of approximately $2,628. Simultaneously, on June 25, 2018, the Company issued and sold 787,193 additional common shares, pursuant to the full exercise of the underwriters’ option to purchase additional shares, for net proceeds of $13,910 after deducting underwriting discounts and commissions of $1,047. Thus, the aggregate net proceeds to the Company from the IPO, after deducting underwriting discounts and commissions and other offering costs, were $104,013. Upon the closing of the IPO, the Preferred Stock then outstanding converted into an aggregate of 15,320,213 shares of common stock.

Common Stock Reserved for Future Issuance

At December 31, 2018 and 2017, the Company has reserved the following shares of common stock for future issuance:


	December 31,
	2018		2017
Shares reserved for Series Seed Preferred Stock outstanding		—		806,711
Shares reserved for Series A Preferred Stock outstanding		—		7,611,438
Shares reserved for vesting of restricted stock awards		153,276		276,301
Shares reserved for exercise of outstanding stock options		2,164,101		1,034,961
Shares reserved for issuance under the 2015 Stock Option and Grant Plan		—		143,717
Shares reserved for issuance under the 2018 Stock Option and Grant Plan		385,561		—
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		223,200		—

Total shares of authorized common stock reserved for future issuance		2,926,138		9,873,128

Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation

10. Stock-Based Compensation

Amended and Restated 2015 Stock Option and Grant Plan

The Company’s Amended and Restated 2015 Stock Option and Grant Plan, (the “2015 Plan”) provides for the Company to issue restricted stock awards and restricted stock units, or to grant incentive stock options or non-statutory stock options. Incentive stock options may be granted only to the Company’s employees including officers and members of the Board who are also employees. Restricted stock awards and restricted stock units and non- statutory stock options may be granted to employees, members of the Board, outside advisors, and consultants of the Company.

The total number of common shares that may be issued under the 2015 Plan was 2,008,564 shares. Following the IPO, no further grants will be made under 2015 plan.

Shares that expire, are terminated, surrendered or canceled under the 2015 Plan without having been fully exercised will be available for future awards under the 2018 Plan (as defined below). In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for future awards.

The 2015 Plan is administered by the Board. Equity awards granted to employees and members of the Board typically vest over four years.

2018 Stock Option and Incentive Plan

The Company’s 2018 Stock Option and Incentive Plan (the “2018 Plan”) was adopted by the Board on June 1, 2018 and approved by stockholders on June 7, 2018 and became effective upon the effectiveness of the Company’s Registration Statement on Form S-1. The 2018 Plan replaced the 2015 Plan as the Board determined not to make additional awards under the 2015 Plan following the pricing of the Company’s IPO. The 2018 Plan allows the compensation committee to make equity-based and cash-based incentive awards to its officers, employees, directors and other key persons (including consultants).

The Company initially reserved 616,300 shares of its common stock for the issuance of awards under the 2018 Plan. The 2018 Plan provides that the number of shares reserved and available for issuance under the plan will automatically increase each January 1, beginning on January 1, 2019, by 4% of the outstanding number of shares of our common stock on the immediately preceding December 31, or such lesser number of shares as determined by its compensation committee. This number is subject to adjustment in the event of a stock split, stock dividend or other change in its capitalization.

The number of options available for future grant under the 2018 Plan was 385,561 as of December 31, 2018.

During the years ended December 31, 2018 and 2017, the Company granted options to purchase 1,277,528 and, 433,102 shares, respectively, of common stock to employees, nonemployees and members of the Board.

2018 Employee Stock Purchase Plan

The Company’s 2018 Employee Stock Purchase Plan (the “ESPP”) was adopted by the Board on June 1, 2018 and approved by stockholders on June 7, 2018 and became effective upon the effectiveness of the Company’s Registration Statement on Form S-1. The ESPP is intended to qualify as an “employee stock purchase plan” within the meaning of Section 423(b) of the Code. The ESPP initially reserves and authorizes the issuance of up to a total of 223,200 shares of common stock to participating employees. The ESPP provides that the number of shares reserved and available for issuance will automatically increase each January 1, beginning on January 1, 2019 and each January 1 thereafter through January 1, 2028, by the least of (i) 1% of the outstanding number of shares of our common stock on the immediately preceding December 31; (ii) 1,115,700 shares or (iii) such number of shares as determined by the ESPP administrator. The number of shares reserved under the ESPP is subject to adjustment in the event of a stock split, stock dividend or other change in our capitalization.

The total number of common shares that may be issued under the ESPP was 223,200 shares as of December 31, 2018, of which 223,200 shares remained available for future grant.

As of December 31, 2018, the initial purchase period under the ESPP has not yet commenced.

Stock Option Valuation

The assumptions that the Company used to determine the grant-date fair value of stock options granted to employees and members of the Board were as follows, presented on a weighted-average basis:


	Year Ended December 31,
	2018			2017
Expected option life (years)		6.06			6.08
Risk-free interest rate		2.74	%		1.93	%
Expected volatility		81.60	%		84.54	%
Expected dividend yield		—	%		—	%

The following table summarizes the Company’s stock option activity for the year ended December 31, 2018:


	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2017		1,034,961		$	0.78		8.94	$	3,427
Granted		1,277,528		$	12.78
Exercised		(16,974	)	$	0.98
Cancelled or forfeited		(131,414	)	$	2.54

Outstanding as of December 31, 2018		2,164,101		$	7.75		8.77	$	23,083

Exercisable as of December 31, 2018		538,214		$	1.22		7.97	$	8,311
Unvested as of December 31, 2018		1,625,886		$	9.92		9.04	$	14,771

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the underlying stock options and the estimated fair value of the Company’s common stock for those stock options that had exercise prices lower than the estimated fair value of the Company’s common stock.

The weighted-average grant-date fair value of the Company’s stock options granted during the years ended December 31, 2018 and 2017 was $10.91 and $1.50, respectively.

Restricted Common Stock

The Company has granted restricted common stock with time-based vesting conditions to certain employees of the Company. The purchase price of the restricted stock awards are determined by the Board. Unvested shares of restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. The Company has the option to repurchase the restricted stock awards at the original purchase price if the grantee terminates its working relationship with the Company prior to the vesting date.

The following table summarizes the Company’s restricted common stock activity for the year ended December 31, 2018:


	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2017		276,301		$	0.42
Vested		(123,026	)	$	0.42
Forfeited, canceled or expired		—		$	—

Issued and unvested as of December 31, 2018		153,275		$	0.42

The total fair value of restricted common stock vested during the years ended December 31, 2018 and 2017 was $52 and $55, respectively.

Stock-Based Compensation

Stock-based compensation expense was allocated as follows:


	Year Ended December 31,
	2018		2017
Research and development	$	920	$	80
General and administrative		1,276		97

Total stock-based compensation expense	$	2,196	$	177

As of December 31, 2018, total unrecognized compensation cost related to the unvested stock-based awards was $12,097, which is expected to be recognized over a weighted-average period of 3.48 years.

License Agreements	12 Months Ended
License Agreements	Dec. 31, 2018
Text Block [Abstract]
License Agreements	11. License Agreements *Agreements with UHN* Fabry License Agreement— On January 27, 2016, the Company entered into an agreement with UHN, pursuant to which UHN granted the Company an option to enter into an exclusive license under the UHN intellectual property related to Fabry disease in accordance with the pre-negotiated licensing terms. On November 4, 2016, the Company exercised its option and entered into a license agreement with UHN, pursuant to which UHN granted the Company an exclusive worldwide license under certain intellectual property rights and a non-exclusive worldwide license under certain know-how, in each case subject to certain retained rights, to develop, commercialize and sell products for use in the treatment of Fabry disease. In addition, for three years following the execution of the agreement, UHN granted the Company an exclusive option to obtain a license under certain improvements to the licensed intellectual property rights as well as an option to negotiate a license under certain other improvements. Under this agreement, the Company paid an option fee of CAD $20, an upfront license fee of CAD $75, plus the annual license maintenance fee for the first year. Thereafter, the Company is also required to pay UHN future annual license maintenance fees until the first sale of a licensed product in certain markets. The Company is also obligated to make future milestone payments in an aggregate amount of up to CAD $2,450 upon the achievement of specified milestones as well as royalties on a country-by-country basis of a low to mid-single-digit percentage of annual net sales of licensed products and a lower single-digit royalty percentage in certain circumstances. Additionally, the Company has agreed to pay a low double-digit royalty percentage of all sublicensing revenue. The agreement requires the Company to meet certain performance milestones within specified timeframes. UHN may terminate the agreement if the Company fails to meet these performance milestones despite using commercially reasonable efforts and the Company is unable to reach agreement with UHN on revised timeframes. The Company’s royalty obligation expires on a licensed product-by-licensed product and country-by-country basis upon the latest to occur of the expiration or termination of the last valid claim under the licensed intellectual property rights in such country, the tenth anniversary of the first commercial sale of such licensed product in such country and the expiration of any applicable regulatory exclusivity in such country. Unless terminated earlier, the agreement expires upon the expiration of the Company’s royalty obligation for all licensed products. UHN can terminate the agreement if the Company fails to make any payments within a specified period after receiving written notice of such failure, or in the event that the Company fails to obtain or maintain insurance. Either the Company or UHN may terminate the license agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. The Company can voluntarily terminate the agreement with prior notice to UHN. For the years ended December 31, 2018 and 2017, the Company recorded research and development expense of $15 and $16, respectively, which consists of the license option fee, the upfront fee and maintenance fees. Interleukin 12 License Agreement— On January 27, 2016, the Company entered into an exclusive license agreement with UHN, pursuant to which UHN granted the Company a license to certain patent rights for the commercial development, manufacture, distribution and use of any products or processes resulting from development of those patent rights related to Interleukin 12. Upon execution of this agreement, the Company paid an upfront license fee of CAD $264. In addition, as part of the initial consideration for the license, the Company issued to UHN 1,161,665 shares of the Company’s common stock. The fair value of the shares issued to UHN of $480 and the upfront fee was expensed upon the execution of the agreement. In addition, the Company agreed to pay UHN up to $2,000 upon the closing of an IPO if certain criteria are met. This obligation was considered a derivative instrument and was initially recorded at fair value of $49 (Note 3). The Company is also required to pay UHN future annual license maintenance fees of CAD $50 on each anniversary of the effective date of the license agreement prior to expiration or termination and potential future milestone payments of up to CAD $19,275 upon the achievement of specified clinical and regulatory milestones. The Company also agreed to pay UHN royalties of a low single-digit percentage of net sales of licensed products sold by the Company. If the Company grants any sublicense rights under the license agreement, the Company has agreed to pay UHN a low double-digit royalty percentage of any sublicense income received by the Company. The agreement requires the Company to meet certain diligence requirements based upon specified milestones. The agreement expires on the later of the date the last patent rights expire in the last country or ten years from the date of first sale. UHN can terminate the agreement if the Company fails to make any payments within a specified period after receiving written notice of such failure, or in the event that the Company fails to obtain or maintain insurance. The Company can voluntarily terminate the agreement with prior notice to UHN. Either the Company or UHN may terminate the license agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. For the years ended December 31, 2018 and 2017, the Company recorded research and development expense related to this agreement with UHN of $41 and $151, respectively, which consists of upfront fees, the fair value of the shares and derivative instrument issued to UHN, and license maintenance fees and development milestone payments. *Agreement with BioMarin Pharmaceutical Inc. (“BioMarin”)* On August 31, 2017, the Company entered into a license agreement with BioMarin, pursuant to which BioMarin granted the Company an exclusive worldwide license under certain intellectual property rights owned or controlled by BioMarin to develop, commercialize and sell products for use in the treatment of Pompe disease. As consideration for this agreement, the Company paid an upfront license fee of $500 in cash and issued 233,765 shares of Series B Preferred Stock to BioMarin at the time of our Series B Preferred Stock financing in January 2018. Both the upfront cash payment of $500 and the value of the shares Series B Preferred Stock issued of $500 were recorded as research and development expense during the year ended December 31, 2017. The Company is also obligated to make future milestone payments of up to $13,000 upon the achievement of certain specified milestones and agreed to pay BioMarin royalties of a low single-digit percentage of net sales of licensed products sold by the Company or its affiliates covered by patent rights in a relevant country. The Company recorded research and development expense related to this agreement with BioMarin of $1,000 related to the license for the year ended December 31, 2017. No expenses related to the license were recorded for the year ended December 31, 2018. Unless terminated earlier, the agreement expires upon the expiration of the Company’s royalty obligation for all licensed products throughout the world. BioMarin and the Company can terminate the agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. The Company may terminate the agreement at will upon written notice to BioMarin. BioMarin has the right to terminate the agreement upon the Company’s bankruptcy or insolvency, or in the event of any challenge or opposition to the licensed patent rights or related actions brought by the Company or its affiliates or sublicensees, or if the Company, its affiliates or sublicensees knowingly assist a third-party in challenging or otherwise opposing the licensed patent rights, except as required under a court order or subpoena. *Agreement with GenStem Therapeutics, Inc. (“GenStem”)* On October 2, 2017, the Company entered into a license agreement with GenStem, pursuant to which GenStem granted the Company an exclusive worldwide license, subject to certain retained rights, under certain intellectual property rights owned or controlled by GenStem to develop, commercialize and sell products for use in the treatment of cystinosis. Under this agreement, the Company paid an upfront license fee of $1,000 and is required to make payments upon completion of certain milestones up to an aggregate of $16,000. The Company also agreed to pay GenStem a tiered mid to high single-digit royalty percentage on annual net sales of licensed products as well as a low double-digit percentage of sublicense income received from certain third-party licensees. The Company’s royalty obligation expires on a licensed product-by-licensed product and country-by-country basis on the eleventh anniversary of the first commercial sale of such licensed product in such country or the expiration of the last valid claim under the licensed patent rights covering such licensed product in such country, whichever is later. Unless terminated earlier, the agreement expires upon the expiration of the Company’s royalty obligation for all licensed products throughout the world. GenStem and the Company can terminate the agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. The Company may terminate the agreement at will upon the specified prior written notice to GenStem. The Company recorded research and development expense of $1,000 for the year ended December 31, 2017, which consisted of upfront fees related to the license. No expenses related to the license were recorded for the year ended December 31, 2018. *Agreement with Lund University Rights Holders* On November 17, 2016, the Company entered into a license agreement with affiliates of Lund University, along with certain other relevant rights holders that may be added from time to time, pursuant to which such rights holders granted to the Company an exclusive worldwide license, subject to certain retained rights, under certain intellectual property rights to develop, commercialize and sell products in any and all uses relevant to Gaucher disease. As consideration for the license, the Company is required to make payments in connection with the achievement of certain milestones up to an aggregate of $550. The agreement expires on the latest of (i) the twentieth anniversary of the end of a certain research project the Company is funding pursuant to an agreement with Lund University, (ii) the expiration of the term of any patent filed on the licensed rights that covers a licensed product, (iii) the expiration of any applicable marketing exclusivity right and (iv) such time that neither the Company nor any sublicensees, partners or contractors are commercializing a licensed product. Either the Company or the rights holders acting together may terminate the license agreement if the other such party commits a material breach and fails to cure such breach within a certain period of time, or if the other party enters into liquidation, becomes insolvent, or enters into composition or statutory reorganization proceedings. No expenses related to the license were recorded for the years ended December 31, 2018 and 2017.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

12. Income Taxes

For the years ended December 31, 2018 and 2017, the Company did not record a current or deferred income tax expense or (benefit) due to current and historical losses incurred by the Company. The Company’s operations are predominantly based in the United States and the Company’s foreign subsidiaries generated de minimis losses for the years ended December 31, 2018 and 2017.

A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate to income taxes as reflected in the consolidated financial statements is as follows:


	Year Ended December 31,
	2018			2017
Federal income tax expense at statutory rate		21.0	%		34.0	%
State income taxes, net of federal benefit		5.2			4.9
Permanent differences		(0.8	)		(0.3	)
U.S.—TCJA		—			(14.8	)
Foreign rate differential		0.2			(0.2	)
Research and development tax credits		1.1			1.3
Change in valuation allowance		(26.5	)		(24.9	)
Provision to Return		(0.2	)		—

Effective income tax rate		—	%		—	%

Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. The significant components of the Company’s deferred tax assets and liabilities are comprised of the following:


	December 31,
	2018			2017
Deferred tax assets:
U.S., foreign, and state net operating loss carryforwards	$	15,815		$	5,183
Research and development credits		454			95
Capitalized start up and organizational costs		35			39
Equity based compensation		185			29
Derivative liability		—			101
Licensing agreements		1,258			800
Accruals and other		948			239

Total deferred tax assets		18,695			6,486
Valuation allowance		(18,457	)		(6,466	)

Net deferred tax assets	$	238		$	20

Deferred tax liabilities:
Property and equipment	$	(238	)	$	(20	)

Total deferred tax liabilities	$	—		$	—

The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. As of December 31, 2018 and 2017, based on the Company’s history of operating losses, the Company has concluded that it is not more likely than not that the benefit of its deferred tax assets will be realized. Accordingly, the Company has provided a full valuation allowance for deferred tax assets as of December 31, 2018 and 2017. The valuation allowance increased $11,991 and $4,568 during the years ended December 31, 2018 and 2017, respectively, due primarily to net operating losses generated.

As of December 31, 2018 and 2017, the Company had U.S. federal net operating loss carryforwards of $57,279 and $19,007, respectively, that may be available to offset future income tax liabilities. U.S. federal tax operating loss carryforwards of approximately $17,504 will expire at various dates through 2037. Approximately $39,774 of the U.S. federal tax operating losses can be carried forward indefinitely. As of December 31, 2018 and 2017, the Company also had U.S. state net operating loss carryforwards of $54,212 and $18,866, respectively, which may be available to offset future taxable income. These losses expire at various dates through 2038. As of December 31, 2018 and 2017, the Company had foreign net operating loss carryforwards of $1,201 and $296, respectively. The foreign net operating losses can be carried forward indefinitely.

As of December 31, 2018 and 2017, the Company has federal research and development tax credit carryforwards of $339 and $34 respectively. The Company qualifies for, and has elected to, apply part of its federal research credits against its payroll tax liability in accordance with certain provisions of the Internal Revenue Code. The amount applied towards the Company’s payroll tax liability is capped at $250 per year. The federal research credits generated in excess of the $250 cap are able to be carried forward for 20 years. As of December 31, 2018 and 2017, the Company had state research and development tax credit carryforwards of approximately $146 and $119, respectively, available to reduce future tax liabilities which expire at various dates through 2033. For all years through December 31, 2018, the Company generated research credits but has not conducted a study to document the qualified activities. This study may result in an adjustment to the Company’s research and development credit carryforwards.

As the Company carries out extensive research and development activities, it seeks to benefit from the Australian research and development tax credit cash rebate regime. Under this regime, the Company’s Australian subsidiary, AVROBIO Australia, may be eligible to surrender the trading losses that arise from its research and development activities for a payable tax credit of up to approximately 43.5% of eligible research and development expenditures. Qualifying expenditures largely comprise employment costs for research staff, consumables, certain internal overhead costs and subcontracted expenditures as part of research projects for which the Company does not receive income. For the year ended December 31, 2018, the Company recorded $1.3 million in research and development tax credits as an offset to research and development expenses.

Under the provisions of the Internal Revenue Code, the net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50 percent, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed numerous financings since its inception, which may have resulted in a change in control as defined by Sections 382 and 383 of the Internal Revenue Code, or could result in a change in control in the future.

The Company files income tax returns in Australia, Canada, the United States, and in several states. The foreign, federal and state income tax returns are generally subject to tax examinations for the tax years ended December 31, 2015 through December 31, 2017. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by foreign tax authorities, the Internal Revenue Service, or state tax authorities to the extent utilized in a future period.

On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (the “TCJA”). This legislation reduced the U.S. corporate tax rate from the existing rate of 34% to 21% for tax years beginning after December 31, 2017. As a result of the enacted law, the Company was required to revalue deferred tax assets and liabilities existing at December 31, 2017 from the 34% federal rate in effect through the end of 2017, to the new 21% rate. This revaluation resulted in a reduction to the Company’s net deferred tax asset of $2,779. This amount was offset by a corresponding reduction to the Company’s deferred tax asset valuation allowance. The other provisions of the TCJA did not have a material impact on the December 31, 2017 financial statements.

In December 2017, the SEC staff issued SAB 118 to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of H.R.1. The Company did not record any adjustments in the year ended December 31, 2018 to provisional amounts that were material to its financial statements. As of December 31, 2018, the Company’s accounting treatment is complete.

Net Loss per Share Attributable to Common Stockholders

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss per Share Attributable to Common Stockholders

13. Net Loss per Share Attributable to Common Stockholders

For purposes of the diluted net loss per share calculation, stock options, unvested restricted stock, Preferred Stock and the warrant to purchase shares of Series A Preferred Stock are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, the weighted-average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same.

The following potentially dilutive common stock equivalents, presented based on amounts outstanding at each period end, were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods indicated:


	Year Ended December 31,
	2018		2017
Options to purchase common stock		2,164,101		1,034,961
Restricted common stock		153,276		276,301
Redeemable convertible preferred stock (as converted to common stock)		—		8,418,149
Warrants to purchase redeemable convertible preferred stock (as converted to common stock)		—		6,850

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

14. Commitments and Contingencies

Lease Agreements

In August 2017, the Company entered into a sub-lease agreement for laboratory space located in Cambridge Massachusetts, United States, which expires in August 2020. The annual lease payments are subject to a 3% increase each year. The Company recognizes rent expense on a straight-line basis over the lease period and has recorded deferred rent for rent expense incurred but not yet paid. On December 4, 2018, the Company terminated the sub-lease without any penalty. The leasehold improvement cost was written off to rental expenses during the year ended December 31, 2018.

On January 12, 2018, the Company entered into a lease agreement for office space located in Cambridge, Massachusetts. The lease agreement expires in January 2023, with a landlord who is an affiliate of the landlord of the Company’s prior lease facility. The annual lease payments are subject to a 3% increase each year. The Company recognizes rent expense on a straight-line basis over the lease period and has recorded deferred rent for rent expense incurred but not yet paid. The Company received a tenant incentive allowance of $842 in 2018. Such incentive allowance is being amortized as a reduction of rent expense on a straight-line basis over the lease period. In accordance with the lease agreement, the Company is required to maintain a security deposit of $209, which was recorded in other assets as of December 31, 2018. In contemplation of this agreement, the Company terminated its prior lease agreement.

On August 31, 2018, the Company entered into a sub-lease agreement for lab space located in Cambridge Massachusetts, United States, which expires in October 2020. The annual lease payments are subject to a 3% increase each year. In accordance with the lease agreement, the Company is required to maintain a security deposit of $283, which was recorded in other assets as of December 31, 2018.

The Company recorded rent expense of $1,295 and $335 during the years ended December 31, 2018 and 2017, respectively.

The following table summarizes the future minimum lease payments due under operating leases as of December 31, 2018:


Year Ending December 31,
2019	$	1,806
2020		1,563
2021		705
2022		726
2023		61

	$	4,861

Legal Proceedings

The Company, from time to time, may be party to litigation arising in the ordinary course of business. The Company was not subject to any material legal proceedings during the years ended December 31, 2018 and 2017 and to the best of its knowledge, no material legal proceedings are currently pending or threatened.

Other

The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at December 31, 2018 and 2017, or royalties on future sales of specified products. No milestone or royalty payments under these agreements are expected to be payable in the immediate future. See Note 11 for discussion of these arrangements.

The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company agrees to indemnify, hold harmless, and to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners, in connection with any U.S. patent or any copyright or other intellectual property infringement claim by any third-party with respect to the Company’s products. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

Related Party Transactions	12 Months Ended
Related Party Transactions	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	15. Related Party Transactions *UHN* In connection with the Company’s entry into a license agreement with UHN (Note 11) on January 27, 2016, the Company issued UHN 4,800,000 shares of its common stock. As a result of the issuance of common stock, UHN owned 4.45% and 9.65% of the Company’s fully diluted equity as of December 31, 2018 and 2017, respectively. Upon the closing of the sale of shares of common stock in an IPO, if UHN’s fully-diluted percentage ownership of the Company is reduced within a range of specified percentages, then the Company is obligated to pay UHN an amount up to $2,000. See Note 3 for further discussion on the accounting treatment for this provision. For the years ended December 31, 2018 and 2017, the Company recognized $56 and $167, respectively, of research and development expense related to the license agreements with UHN. Refer to Note 11 for additional information regarding the UHN license agreements. The Company recorded research and development expenses of $0 and $3 related to participation on the scientific advisory board and consulting services performed by a member of the board who is affiliated with UHN during the years ended December 31, 2018 and 2017, respectively. As of December 31, 2018 and 2017, there was $0 and $10, respectively, included in accrued expenses on the Company’s consolidated balance sheet related to these services. *Others* For the years ended December 31, 2018 and 2017, the Company recorded expenses of $348 and $0, respectively, related to a sublease to rent lab space, provided by an entity affiliated with a member of the board. For the years ended December 31, 2018 and 2017, the Company recorded expenses of $40 and $86, respectively, related to consulting services provided by an entity affiliated with an officer of the Company and a member of the Board. The entity is also a shareholder of the Company and owned 0.46% and 1.00% of the Company’s fully diluted equity as of December 31, 2018 and 2017, respectively. For the year ended December 31, 2017, the Company recorded expenses of $15, related to services provided by an entity affiliated with a member of the Board and the use of office space. The lease was terminated in February 2017 and the services were no longer being provided as of December 31, 2017. In August 2017, the Company entered into a sub-lease agreement with an entity affiliated with the Company’s CEO. The executive resigned from the affiliated entity in September 2017. The Company recorded rent expense of $31 under this sub-lease agreement prior to such resignation for the two month period in 2017. See Note 14 for additional information on the terms of these sublease agreements.

Benefit Plans	12 Months Ended
Benefit Plans	Dec. 31, 2018
Postemployment Benefits [Abstract]
Benefit Plans	16. Benefit Plans The Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. In August 2018, the Company’s Board approved a company matching contribution of up to 3%, effective October 1, 2018. The Company made matching contributions of $35 to the plan during the year ended December 31, 2018 and made no contributions to the plan during the year ended December 31, 2017.

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	17. Subsequent Events The Company has completed an evaluation of all subsequent events after the audited balance sheet date of December 31, 2018 through the filing date of this Annual Report on Form 10-K with the SEC, to ensure that these consolidated financial statements include appropriate disclosure of events both recognized in the consolidated financial statements as of December 31, 2018, and events which occurred subsequently but were not recognized in the consolidated financial statements. The Company has concluded that no subsequent events have occurred that require disclosure, except as disclosed within these consolidated financial statements.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of presentation

Basis of Presentation

Principles of Consolidation

Principles of Consolidation

Use of Estimates

Use of Estimates

Cash and Cash Equivalents

Cash and Cash Equivalents

Restricted Cash

Restricted Cash

Concentrations of Credit Risk

Concentrations of Credit Risk

Deferred Issuance Costs

Deferred Issuance Costs

Property and Equipment

Property and Equipment

Property and equipment are recorded at cost. Depreciation and amortization is calculated using the straight-line method over the following estimated useful lives of the assets:


		Estimated Useful Life
Laboratory and office equipment		5 years
Computer equipment and software		2 years
Leasehold improvements		Lesser of lease term or 10 years

Impairment of Long-Lived Assets

Impairment of Long-Lived Assets

Fair Value Measurements

Fair Value Measurements

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Segment Information

Segment Information

Research and Development Costs

Research and Development Costs

Stock-Based Compensation

Stock-Based Compensation

Prior to the adoption of Accounting Standards Update (“ASU”) No. 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, the measurement date for non-employee awards was generally the date the services are completed, resulting in financial reporting period adjustments to stock-based compensation during the vesting terms for changes in the fair value of the awards. After adoption of ASU 2018-07, the measurement date for non-employee awards is the later of the adoption date of ASU 2018-07, or the date of grant, without change in the fair value of the award. For stock-based awards granted to nonemployees subject to graded vesting that only contain service conditions, the Company has elected to recognize stock-based compensation expense using the straight-line recognition method.

Warrant to Purchase Preferred Stock

Warrant to Purchase Preferred Stock

Comprehensive Income (Loss)

Comprehensive Income (Loss)

Foreign Currency Translation

Foreign Currency Translation

Income Taxes

Income Taxes

Leases

Leases

Net Income (Loss) per Share Attributable to Common Stockholders

Net Income (Loss) per Share Attributable to Common Stockholders

Subsequent Event Considerations

Subsequent Event Considerations

Emerging Growth Company Status

Emerging Growth Company Status

Recently Adopted Accounting Pronouncements

Recently Adopted Accounting Pronouncements


	December 31,
	2018		2017
Cash and cash equivalents	$	126,302	$	5,963
Other assets		492		24

Cash, cash equivalents and restricted cash	$	126,794	$	5,987

Recently Issued Accounting Pronouncements

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Schedule Of Property and Equipment Estimated Useful Life

Property and equipment are recorded at cost. Depreciation and amortization is calculated using the straight-line method over the following estimated useful lives of the assets:


		Estimated Useful Life
Laboratory and office equipment		5 years
Computer equipment and software		2 years
Leasehold improvements		Lesser of lease term or 10 years

Schedule Of Cash And Cash Equivalents And Restricted Cash

The cash, cash equivalents and restricted cash balances as of December 31, 2018 and 2017, which are presented in the Company’s consolidated statements of cash flows subsequent to the adoption of ASU 2016-18, consisted of the following:


	December 31,
	2018		2017
Cash and cash equivalents	$	126,302	$	5,963
Other assets		492		24

Cash, cash equivalents and restricted cash	$	126,794	$	5,987

Fair Value of Financial Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2018

Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis


	Fair Value Measurements as of December 31, 2018 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	126,047	$	—	$	—	$	126,047
Restricted cash		492		—		—		492

	$	126,539	$	—	$	—	$	126,539


	Fair Value Measurements as of December 31, 2017 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	5,684	$	—	$	—	$	5,684
Restricted cash		24		—		—		24

	$	5,708	$	—	$	—	$	5,708

Liabilities:
Derivative liability	$	—	$	—	$	371	$	371
Warrant to purchase redeemable convertible preferred stock		—		—		35		35

	$	—	$	—	$	406	$	406

Warrants to Purchase Redeemable Convertible Preferred Stock [Member]

Summary of Changes in Fair Value Determined by Level 3 Inputs

The following table sets forth a summary of changes in the fair value of the Company’s preferred stock warrant liability for which fair value was determined by Level 3 inputs:


	Warrant Liability
Balance as of December 31, 2016	$	—
Initial fair value of warrant to purchase redeemable convertible preferred stock		18
Change in fair value		17

Balance as of December 31, 2017	$	35
Change in fair value		162
Conversion of preferred stock warrant to common stock warrant		(197	)

Balance as of December 31, 2018	$	—

Derivative Liabilities [Member]

Summary of Changes in Fair Value Determined by Level 3 Inputs

The following table sets forth a summary of changes in the fair value of the Company’s derivative liability for which fair value is determined by Level 3 inputs:


	Derivative Liability
Balance as of December 31, 2016	$	88
Change in fair value		283

Balance as of December 31, 2017		371
Change in fair value		1,629
Payment		(2,000	)

Balance as of December 31, 2018	$	—

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Prepaid Expenses and Other Current Assets (Tables)

12 Months Ended

Dec. 31, 2018

Text Block [Abstract]

Summary of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:


	December 31,
	2018		2017
Tax incentive refund	$	1,325	$	—
Prepaid research and development costs		981		163
Prepaid insurance		316		—
Interest income receivable		220		—
Prepaid rent		81		122
Other current assets		795		60

	$	3,718	$	345

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following:


	December 31,
	2018			2017
Laboratory and office equipment	$	1,624		$	126
Leasehold improvements		1,260			240
Computer equipment and software		134			28

		3,018			394
Less: Accumulated depreciation and amortization		(384	)		(45	)

	$	2,634		$	349

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2018

Payables and Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

Accrued expenses consisted of the following:


	December 31,
	2018		2017
Compensation and benefit costs	$	2,616	$	794
Research and development costs		3,969		831
Consulting and professional fees		320		267
Preferred stock issuance cost		—		85
Other liabilities		917		121

	$	7,822	$	2,098

Redeemable Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2018

Text Block [Abstract]

Summary of Preferred Stock

As of December 31, 2017, the Preferred Stock consisted of the following:


	December 31, 2017
	Preferred Shares Authorize		Preferred Shares Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series Seed preferred stock		3,333,333		3,333,333	$	1,500,000	$	1,500,000		806,711
Series A preferred stock		31,639,202		31,450,499		25,000,000		25,000,000		7,611,438

		34,972,535		34,783,832	$	26,500,000	$	26,500,000		8,418,149

Common Stock (Tables)

12 Months Ended

Dec. 31, 2018

Federal Home Loan Banks [Abstract]

Summary of Common Stock Reserved for Future Issuance

At December 31, 2018 and 2017, the Company has reserved the following shares of common stock for future issuance:


	December 31,
	2018		2017
Shares reserved for Series Seed Preferred Stock outstanding		—		806,711
Shares reserved for Series A Preferred Stock outstanding		—		7,611,438
Shares reserved for vesting of restricted stock awards		153,276		276,301
Shares reserved for exercise of outstanding stock options		2,164,101		1,034,961
Shares reserved for issuance under the 2015 Stock Option and Grant Plan		—		143,717
Shares reserved for issuance under the 2018 Stock Option and Grant Plan		385,561		—
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		223,200		—

Total shares of authorized common stock reserved for future issuance		2,926,138		9,873,128

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

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+ Details

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Assumptions that Used to Determine Grant-Date Fair Value of Stock Options Granted to Employees and Members of Board

The assumptions that the Company used to determine the grant-date fair value of stock options granted to employees and members of the Board were as follows, presented on a weighted-average basis:


	Year Ended December 31,
	2018			2017
Expected option life (years)		6.06			6.08
Risk-free interest rate		2.74	%		1.93	%
Expected volatility		81.60	%		84.54	%
Expected dividend yield		—	%		—	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the year ended December 31, 2018:


	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2017		1,034,961		$	0.78		8.94	$	3,427
Granted		1,277,528		$	12.78
Exercised		(16,974	)	$	0.98
Cancelled or forfeited		(131,414	)	$	2.54

Outstanding as of December 31, 2018		2,164,101		$	7.75		8.77	$	23,083

Exercisable as of December 31, 2018		538,214		$	1.22		7.97	$	8,311
Unvested as of December 31, 2018		1,625,886		$	9.92		9.04	$	14,771

Summary of Restricted Stock Unit Activity

The following table summarizes the Company’s restricted common stock activity for the year ended December 31, 2018:


	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2017		276,301		$	0.42
Vested		(123,026	)	$	0.42
Forfeited, canceled or expired		—		$	—

Issued and unvested as of December 31, 2018		153,275		$	0.42

Schedule of Stock-based Compensation Expense

Stock-based compensation expense was allocated as follows:


	Year Ended December 31,
	2018		2017
Research and development	$	920	$	80
General and administrative		1,276		97

Total stock-based compensation expense	$	2,196	$	177

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Reconciliation of Income Tax Expense (Benefit)

A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate to income taxes as reflected in the consolidated financial statements is as follows:


	Year Ended December 31,
	2018			2017
Federal income tax expense at statutory rate		21.0	%		34.0	%
State income taxes, net of federal benefit		5.2			4.9
Permanent differences		(0.8	)		(0.3	)
U.S.—TCJA		—			(14.8	)
Foreign rate differential		0.2			(0.2	)
Research and development tax credits		1.1			1.3
Change in valuation allowance		(26.5	)		(24.9	)
Provision to Return		(0.2	)		—

Effective income tax rate		—	%		—	%

Schedule of Deferred Tax Assets and Liabilities


	December 31,
	2018			2017
Deferred tax assets:
U.S., foreign, and state net operating loss carryforwards	$	15,815		$	5,183
Research and development credits		454			95
Capitalized start up and organizational costs		35			39
Equity based compensation		185			29
Derivative liability		—			101
Licensing agreements		1,258			800
Accruals and other		948			239

Total deferred tax assets		18,695			6,486
Valuation allowance		(18,457	)		(6,466	)

Net deferred tax assets	$	238		$	20

Deferred tax liabilities:
Property and equipment	$	(238	)	$	(20	)

Total deferred tax liabilities	$	—		$	—

Net Loss per Share Attributable to Common Stockholders (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Schedule of Dilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share


	Year Ended December 31,
	2018		2017
Options to purchase common stock		2,164,101		1,034,961
Restricted common stock		153,276		276,301
Redeemable convertible preferred stock (as converted to common stock)		—		8,418,149
Warrants to purchase redeemable convertible preferred stock (as converted to common stock)		—		6,850

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Schedule Of Future Minimum Lease Payments Under Operating Lease

The following table summarizes the future minimum lease payments due under operating leases as of December 31, 2018:


Year Ending December 31,
2019	$	1,806
2020		1,563
2021		705
2022		726
2023		61

	$	4,861

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Net loss	$ (46,361)	$ (18,648)
Accumulated deficit	(71,739)	(23,474)
Cash and cash equivalents	$ 126,302	$ 5,963

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant Of Accounting Policies [Line Items]
Restricted cash	$ 492	$ 24
Impairment loss of property and equipment	235
Foreign exchange losses	$ 51	19
Series B Preferred Stock [Member]
Significant Of Accounting Policies [Line Items]
Deferred issuance costs		$ 85
Expected Dividend Yield [Member]
Significant Of Accounting Policies [Line Items]
Expected cash dividend yield	0

Summary of Significant Accounting Policies - Estimated Useful Lives of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2018
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful life	5 years
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful life	2 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful life	Lesser of lease term
Property and equipment estimated useful life	10 years

Summary of Significant Accounting Policies - Statements of cash flows subsequent to the adoption of ASU (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 126,302	$ 5,963
Other assets	492	24
Cash, cash equivalents and restricted cash	$ 126,794	$ 5,987	$ 5,381

Fair Value of Financial Assets and Liabilities - Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 126,539	$ 5,708
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		406
Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	126,047	5,684
Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	492	24
Derivative Liabilities [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		371
Warrants to Purchase Redeemable Convertible Preferred Stock [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		35
Level 1 [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	126,539	5,708
Level 1 [Member] \| Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	126,047	5,684
Level 1 [Member] \| Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 492	24
Level 3 [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		406
Level 3 [Member] \| Derivative Liabilities [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		371
Level 3 [Member] \| Warrants to Purchase Redeemable Convertible Preferred Stock [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		$ 35

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)		12 Months Ended
	Jan. 27, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 31, 2018
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Fair value input transfer between levels		$ 0	$ 0
Increase in fair value of derivative		(1,629,000)	$ (283,000)	$ (39,000)
IPO [Member]
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Dilution payment					$ 2,000,000
License Agreement [Member]
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Provision for maximum cash payment	$ 2,000,000	$ 2,000,000
License Agreement [Member] \| Research and Development Expense [Member]
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Initial fair value of the derivative	$ 49,000
License Agreement [Member] \| Stock Purchase Agreement [Member]
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Common stock issued for initial consideration for license	1,161,665

Fair Value of Financial Assets and Liabilities - Summary of Changes in Fair Value of Preferred Stock Warrant Liability for Fair Value Determined by Level 3 Inputs (Detail) - Level 3 [Member] - Warrants to Purchase Redeemable Convertible Preferred Stock [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 35
Initial fair value of warrant to purchase redeemable convertible preferred stock		$ 18
Change in fair value	162	17
Conversion of preferred stock warrant to common stock warrant	$ (197)
Ending balance		$ 35

Fair Value of Financial Assets and Liabilities - Summary of Changes in Fair Value of Derivative Liability for Fair Value Determined by Level 3 Inputs (Detail) - Level 3 [Member] - Derivative Liabilities [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 371	$ 88
Change in fair value	1,629	283
Payment	$ (2,000)
Ending balance		$ 371

Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Prepaid Expense and Other Assets, Current [Abstract]
Tax incentive refund	$ 1,325
Prepaid research and development costs	981	$ 163
Prepaid insurance	316
Interest income receivable	220
Prepaid rent	81	122
Other current assets	795	60
Total	$ 3,718	$ 345

Property and Equipment, Net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 3,018	$ 394
Less: Accumulated depreciation and amortization	(384)	(45)
Property plant and equipment, net	2,634	349
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	1,624	126
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	1,260	240
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 134	$ 28

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 380	$ 45

Accrued Expenses - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities, Current [Abstract]
Compensation and benefit costs	$ 2,616	$ 794
Research and development costs	3,969	831
Consulting and professional fees	320	267
Preferred stock issuance cost		85
Other liabilities	917	121
Accrued expenses	$ 7,822	$ 2,098

Loan Agreement and Warrant to Purchase Preferred Stock - Additional Information (Detail) - USD ($)		12 Months Ended
	Jun. 23, 2017	Dec. 31, 2018	Dec. 31, 2017
Class of Warrant or Right [Line Items]
Maximum issuance of term loans	$ 10,000,000
Loan expiration date		Oct. 31, 2018
Fair value of warrants at issuance		$ 18,000
Loss related to change in fair value of warrant		$ 162,000	$ 17,000
Purchase of common stock		6,850
Issuance of common stock		$ 6,091
Prime Rate [Member]
Class of Warrant or Right [Line Items]
Floating interest rate per annum	3.00%
Series A Preferred Stock [Member]
Class of Warrant or Right [Line Items]
Warrant to purchase shares	188,702
Warrant exercise price	$ 0.7949
Warrant expiration date		Jun. 22, 2027
Shares purchase upon exercise of warrant	28,305
Additional shares purchase upon exercise of warrant	160,397
Minimum [Member]
Class of Warrant or Right [Line Items]
Floating interest rate per annum	1.00%

Redeemable Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands					1 Months Ended	12 Months Ended
	Jan. 19, 2018	Oct. 31, 2017	Jul. 31, 2016	Jan. 31, 2016	Mar. 31, 2017	Dec. 31, 2018	Dec. 21, 2018	Dec. 31, 2017	Jun. 25, 2018	Dec. 31, 2016
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized						63,491,857		34,972,535		22,380,952
Redeemable convertible preferred stock, par value						$ 0.0001		$ 0.0001		$ 0.0001
Redeemable convertible preferred stock, shares issued and sold								34,783,832
Proceeds from issuance of redeemable convertible preferred stock						$ 58,257		$ 17,415
IPO [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, sales price per share									$ 19.00
Preferred stock converted into common stock							15,320,213
Series Seed Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized						3,333,333		3,333,333		3,333,333
Redeemable convertible preferred stock, shares issued and sold				3,333,333				3,333,333
Redeemable convertible preferred stock, sales price per share				$ 0.45
Proceeds from issuance of redeemable convertible preferred stock				$ 1,398
Redeemable convertible preferred stock, net of issuance costs				$ 102
Series A Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized						31,639,202		31,639,202		19,047,619
Redeemable convertible preferred stock, shares issued and sold		17,612,279	5,714,286		4,403,070			31,450,499
Redeemable convertible preferred stock, sales price per share		$ 0.7949	$ 1.3125		$ 0.7949
Proceeds from issuance of redeemable convertible preferred stock		$ 13,963	$ 7,297		$ 3,452
Redeemable convertible preferred stock, net of issuance costs		$ 37	$ 203		$ 48
Issuance of redeemable convertible preferred stock dividend					3,720,864
Redeemable convertible preferred stock, to be issued					17,612,279
Series A Redeemable Convertible Preferred Stock [Member] \| Sales Milestone [Member] \| Maximum [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares issued and sold			13,333,333
Redeemable convertible preferred stock, sales price per share			$ 1.3125
Series B Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized						28,519,322
Redeemable convertible preferred stock, shares issued and sold	28,285,557
Redeemable convertible preferred stock, sales price per share	$ 2.1389
Proceeds from issuance of redeemable convertible preferred stock	$ 60,500
Redeemable convertible preferred stock, net of issuance costs	$ 2,243
Series B Redeemable Convertible Preferred Stock [Member] \| BioMarin Pharmaceutical Inc [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares issued and sold	233,765

Redeemable Convertible Preferred Stock - Summary of Preferred Stock (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Oct. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Jul. 31, 2016	Jan. 31, 2016
Temporary Equity [Line Items]
Preferred Shares Authorized	63,491,857	34,972,535			22,380,952
Preferred Shares Issued		34,783,832
Preferred Shares Outstanding		34,783,832
Carrying Value		$ 26,500,000
Liquidation Preference		$ 26,500,000
Common Stock Issuable Upon Conversion		8,418,149
Series Seed Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	3,333,333	3,333,333			3,333,333
Preferred Shares Issued		3,333,333					3,333,333
Preferred Shares Outstanding		3,333,333
Carrying Value		$ 1,500,000
Liquidation Preference		$ 1,500,000
Common Stock Issuable Upon Conversion		806,711
Series A Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	31,639,202	31,639,202			19,047,619
Preferred Shares Issued		31,450,499	17,612,279	4,403,070		5,714,286
Preferred Shares Outstanding		31,450,499
Carrying Value		$ 25,000,000
Liquidation Preference		$ 25,000,000
Common Stock Issuable Upon Conversion		7,611,438

Common Stock - Additional Information (Detail) $ / shares in Units, $ in Thousands			12 Months Ended
	Jun. 25, 2018 USD ($) $ / shares shares	Jun. 01, 2018 $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 $ / shares shares
Class of Stock [Line Items]
Common stock, shares authorized \| shares		150,000,000	150,000,000	51,000,000
Common stock, par value \| $ / shares		$ 0.0001	$ 0.0001	$ 0.0001
Undesignated preferred stock \| shares		10,000,000
Undesignated preferred stock outstanding \| shares		0
Cash dividends \| $			$ 0
Reverse stock split		1-for-4.312
Stock Split, conversion ratio		4.312
Convertible preferred stock converted into common stock \| shares				8,418,149
IPO [Member]
Class of Stock [Line Items]
Common stock, shares issued and sold \| shares	5,247,958
Issuance price per shares \| $ / shares	$ 19.00
Net proceeds from issuance of common stock \| $	$ 90,103
Net proceeds after deducting underwriting discounts and commissions and offering costs \| $	6,980
Underwriting discounts and commissions and offering costs \| $	$ 2,628
Over-Allotment Option [Member]
Class of Stock [Line Items]
Common stock, shares issued and sold \| shares	787,193
Net proceeds from issuance of common stock \| $	$ 13,910
Net proceeds after deducting underwriting discounts and commissions and offering costs \| $	1,047
Underwriting discounts and commissions and offering costs \| $	$ 104,013
Convertible preferred stock converted into common stock \| shares	15,320,213

Common Stock - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	2,926,138	9,873,128
Series Seed Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		806,711
Series A Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		7,611,438
2018 Stock Option and Incentive Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	385,561
2018 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	223,200
Restricted Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	153,276	276,301
Employee Stock Option [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	2,164,101	1,034,961
2015 Stock Option and Grant Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		143,717

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options granted	1,277,528
Options Granted, Weighted-average grant-date fair value	$ 10.91	$ 1.50
Total fair value of restricted common stock vested	$ 52	$ 55
Unrecognized stock-based compensation expenses	$ 12,097
Unrecognized stock-based compensation expense, period for recognition	3 years 5 months 23 days
2018 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares that may be issued under the plan	616,300
Common shares available for future grant	385,561
Number of shares of common stock outstanding, increase, percentage	4.00%
2015 Stock Option and Grant Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares that may be issued under the plan	2,008,564
Common shares available for future grant	0
Option vesting period	4 years
Number of options granted	1,277,528	433,102
2018 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares that may be issued under the plan	223,200
Common shares available for future grant	223,200
Employee Stock Purchase Plan, description	The ESPP provides that the number of shares reserved and available for issuance will automatically increase each January 1, beginning on January 1, 2019 and each January 1 thereafter through January 1, 2028, by the least of (i) 1% of the outstanding number of shares of our common stock on the immediately preceding December 31; (ii) 1,115,700 shares or (iii) such number of shares as determined by the ESPP administrator.
2018 Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares that may be issued under the plan	223,200
Number of shares of common stock outstanding, increase, percentage	1.00%
Number of common shares that may increase under the plan	1,115,700

Stock-Based Compensation - Summary of Assumptions that Used to Determine Grant-Date Fair Value of Stock Options Granted to Employees and Members of Board (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected option life (years)	6 years 21 days	6 years 29 days
Risk-free interest rate	2.74%	1.93%
Expected volatility	81.60%	84.54%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of options, Outstanding beginning balance	1,034,961
Number of options, Granted	1,277,528
Number of options, Exercised	(16,974)
Number of options, Cancelled or forfeited	(131,414)
Number of options, Outstanding ending balance	2,164,101	1,034,961
Number of options, Exercisable	538,214
Number of options, Unvested	1,625,886
Weighted average exercise price, Outstanding beginning balance	$ 0.78
Weighted average exercise price, Granted	12.78
Weighted average exercise price, Exercised	0.98
Weighted average exercise price, Cancelled or forfeited	2.54
Weighted average exercise price, Outstanding ending balance	7.75	$ 0.78
Weighted average exercise price, Exercisable	1.22
Weighted average exercise price, Unvested	$ 9.92
Weighted average remaining contractual term, Outstanding balance	8 years 9 months 7 days	8 years 11 months 8 days
Weighted average remaining contractual term, Exercisable	7 years 11 months 19 days
Weighted average remaining contractual term, Unvested	9 years 14 days
Aggregate intrinsic value, Outstanding balance	$ 23,083	$ 3,427
Aggregate intrinsic value, Exercisable	8,311
Aggregate intrinsic value, Unvested	$ 14,771

Stock-Based Compensation - Summary of Restricted Stock Unit Activity (Detail)	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of shares, Issued and unvested beginning balance	276,301
Number of shares, Vested	(123,026)
Number of shares, Forfeited, cancelled or expired	0
Number of shares, Issued and unvested ending balance	153,275
Weighted average grant date fair value, Issued and unvested, beginning balance \| $ / shares	$ 0.42
Weighted average grant date fair value, Vested \| $ / shares	0.42
Weighted average grant date fair value, Issued and unvested, ending balance \| $ / shares	$ 0.42

Stock-Based Compensation - Schedule of Stock-based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock based compensation expense	$ 2,196	$ 177
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock based compensation expense	920	80
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock based compensation expense	$ 1,276	$ 97

License Agreements - Additional Information (Detail) $ in Thousands					1 Months Ended	12 Months Ended
	Oct. 02, 2017 USD ($)	Nov. 17, 2016 USD ($)	Jan. 27, 2016 USD ($) shares	Jan. 27, 2016 CAD ($)	Aug. 31, 2017 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Jan. 27, 2016 CAD ($) shares
License Agreement [Line Items]
Research and development						$ 35,095,000	$ 15,191,000
Common stock, shares issued \| shares						23,959,903	2,581,474
Fair value of shares issued						$ 104,013,000
Preferred stock issued \| shares						0	0
UHN Agreement [Member] \| Fabry License Agreement [Member]
License Agreement [Line Items]
Option fee								$ 20
Upfront license fee				$ 75
Milestone payments				2,450
Research and development						$ 15,000	$ 16,000
UHN Agreement [Member] \| Interleukin 12 License Agreement [Member]
License Agreement [Line Items]
Upfront license fee				264
Milestone payments				19,275
Research and development						41,000	151,000
Common stock, shares issued \| shares			1,161,665					1,161,665
Fair value of shares issued			$ 480
Payments upon closing of an initial public offering			2,000,000
Fair value of derivative liability			$ 49,000
Annual maintenance fees				$ 19,275
BioMarin Pharmaceutical Inc [Member]
License Agreement [Line Items]
Upfront license fee					$ 500,000
Milestone payments						13,000,000
Research and development							1,000,000
Upfront cash payment							500,000
Expenses related to license						0
BioMarin Pharmaceutical Inc [Member] \| Series B Preferred Stock [Member]
License Agreement [Line Items]
Research and development							500,000
Preferred stock issued \| shares					233,765
GenStem Therapeutics Inc [Member]
License Agreement [Line Items]
Upfront license fee	$ 1,000,000
Research and development							1,000,000
Expenses related to license						0
Milestone payments	$ 16,000,000
Lund University Rights Holders Agreement [Member]
License Agreement [Line Items]
Expenses related to license						$ 0	$ 0
Milestone payments		$ 550

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred income tax expense (benefit)	$ 0	$ 0
Deferred tax assets change in valuation allowance	11,991,000	4,568,000
Deferred tax assets, operating loss carryforwards, domestic	57,279,000	19,007,000
Deferred tax assets, operating loss carryforwards, not subject to expiration	39,774,000
Deferred tax assets, operating loss carryforwards, foreign	1,201,000	$ 296,000
Payroll tax liability capped per year	250,000
Income tax reconciliation tax credit research	$ 250,000
Income tax credit carryforward expiration period	20 years
Percentage of cumulative changes in ownership interest subject to annual limitation	50.00%
Period of cumulative changes in ownership interest	3 years
Research and development payable tax credit, percentage	(1.10%)	(1.30%)
Federal corporate tax rate	21.00%	34.00%
Reduction in deferred tax liabilities	$ (2,779,000)
U.S. federal [Member]
Research and development tax credits carryforwards	339,000	$ 34,000
Australia [Member]
Income tax reconciliation tax credit research	$ 1,300,000
Australia [Member] \| Maximum [Member]
Research and development payable tax credit, percentage	43.50%
Expiration Through 2033 [Member] \| State [Member]
Research and development tax credits carryforwards	$ 146,000	119,000
Expiration Through 2037 [Member]
Deferred tax assets, operating loss carryforwards, subject to expiration	17,504,000
Expiration Through 2038 [Member]
Deferred tax assets, operating loss carryforwards, state	$ 54,212,000	$ 18,866,000

Income Taxes - Schedule of Reconciliation of Income Tax Expense (Benefit) (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rate	21.00%	34.00%
State income taxes, net of federal benefit	5.20%	4.90%
Permanent differences	(0.80%)	(0.30%)
U.S.-TCJA		(14.80%)
Foreign rate differential	0.20%	(0.20%)
Research and development tax credits	1.10%	1.30%
Change in valuation allowance	(26.50%)	(24.90%)
Provision to Return	(0.20%)
Effective income tax rate	0.00%	0.00%

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
U.S., foreign, and state net operating loss carryforwards	$ 15,815	$ 5,183
Research and development credits	454	95
Capitalized start up and organizational costs	35	39
Equity based compensation	185	29
Derivative liability		101
Licensing agreements	1,258	800
Accruals and other	948	239
Total deferred tax assets	18,695	6,486
Valuation allowance	(18,457)	(6,466)
Net deferred tax assets	238	20
Deferred tax liabilities:
Property and equipment	(238)	(20)
Total deferred tax liabilities	$ 0	$ 0

Net Loss per Share - Schedule of Dilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	2,164,101	1,034,961
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	153,276	276,301
Warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount		6,850
Redeemable Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount		8,418,149

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands				1 Months Ended	12 Months Ended
	Aug. 31, 2018	Jan. 12, 2018	Aug. 31, 2017	Aug. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Commitments And Contingencies Disclosure [Line Items]
Lease agreement expiration date		Jan. 31, 2023			Feb. 28, 2017
Percentage of annual increase in rent		3.00%
Tenant incentive allowance		$ 842
Security deposit in connection with lease					$ 209
Rent expense					1,295	$ 335
Sub-lease Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease agreement expiration date	Oct. 31, 2020		Aug. 31, 2020
Percentage of annual increase in rent	3.00%		3.00%	3.00%
Security deposit in connection with lease					$ 283
Rent expense				$ 31

Commitments and Contingencies - Schedule Of Future Minimum Rental Payment Under Operating Lease (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2019	$ 1,806
2020	1,563
2021	705
2022	726
2023	61
Total	$ 4,861

Related Party Transactions - Additional information (Detail) - USD ($)					1 Months Ended	12 Months Ended
	Aug. 31, 2018	Jan. 12, 2018	Aug. 31, 2017	Jan. 27, 2016	Aug. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Research and development expense						$ 35,095,000	$ 15,191,000
Accrued expenses						$ 7,822,000	2,098,000
Lease termination date		Jan. 31, 2023				Feb. 28, 2017
Rent expense						$ 1,295,000	$ 335,000
Officers and Board Members [Member] \| Consulting Services Agreement [Member]
Related Party Transaction [Line Items]
Issuance of common stock ownership percentage						0.46%	1.00%
Expense related to related party						$ 40,000	$ 86,000
Board Member [Member]
Related Party Transaction [Line Items]
Expense related to related party							15,000
License Agreement [Member]
Related Party Transaction [Line Items]
Provision for maximum cash payment				$ 2,000,000		2,000,000
Research and development expense						$ 56,000	$ 167,000
License Agreement [Member] \| Stock Purchase Agreement [Member]
Related Party Transaction [Line Items]
Common stock issued for initial consideration for license				1,161,665
Issuance of common stock ownership percentage						4.45%	9.65%
License Agreement [Member] \| Officers and Board Members [Member] \| Consulting Services Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense						$ 0	$ 3,000
Accrued expenses						0	10,000
Sub-lease Agreement [Member]
Related Party Transaction [Line Items]
Lease termination date	Oct. 31, 2020		Aug. 31, 2020
Rent expense					$ 31,000
Sub-lease Agreement [Member] \| Officers and Board Members [Member]
Related Party Transaction [Line Items]
Expense related to related party						$ 348,000	$ 0

Benefit Plans - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Aug. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans [Abstract]
Employer matching contribution, percent of match	3.00%
Employer contributions to defined contribution savings plans		$ 35	$ 0