0001193125-19-145337 | 10-Q | XBRL Viewer

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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2019	May 09, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	AVRO
Entity Registrant Name	AVROBIO, INC.
Entity Central Index Key	0001681087
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		24,147,025
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Small Business	true

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 108,476	$ 126,302
Prepaid expenses and other current assets	5,203	3,718
Total current assets	113,679	130,020
Property and equipment, net	2,864	2,634
Other assets	825	825
Total assets	117,368	133,479
Current liabilities:
Accounts payable	3,458	2,784
Accrued expenses and other current liabilities	6,477	7,822
Total current liabilities	9,935	10,606
Deferred rent, net of current portion	645	689
Total liabilities	10,580	11,295
Commitments and contingencies (Note 11)
Stockholders' equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized and no shares issued or outstanding as of March 31, 2019 and December 31, 2018
Common stock, $0.0001 par value; 150,000,000 shares authorized as of March 31, 2019 and December 31, 2018; 24,076,762 and 23,959,903 shares issued as of March 31, 2019 and December 31, 2018, respectively; 23,954,241 and 23,806,628 shares outstanding as of March 31, 2019 and December 31, 2018, respectively	2	2
Additional paid-in capital	195,628	193,921
Accumulated deficit	(88,842)	(71,739)
Total stockholders' equity	106,788	122,184
Total liabilities and stockholders' equity	$ 117,368	$ 133,479

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, authorized	10,000,000	10,000,000
Preferred stock, issued	0	0
Preferred stock, outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, authorized	150,000,000	150,000,000
Common stock, issued	24,076,762	23,959,903
Common stock, outstanding	23,954,241	23,806,628

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 12,446	$ 5,647
General and administrative	5,254	2,141
Total operating expenses	17,700	7,788
Loss from operations	(17,700)	(7,788)
Other income (expense):
Interest income	657	158
Change in fair value of preferred stock warrant liability		(12)
Change in fair value of derivative liability		(587)
Other expense	(60)	(13)
Total other income (expense), net	597	(454)
Net loss	(17,103)	(8,242)
Comprehensive loss	(17,103)	(8,242)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(17,103)	(8,242)
Accretion of issuance costs on redeemable convertible preferred stock		(2,243)
Net loss attributable to common stockholders-basic and diluted	$ (17,103)	$ (10,485)
Net loss per share attributable to common stockholders-basic and diluted (Note 10)	$ (0.72)	$ (4.51)
Weighted-average number of common shares used in computing net loss per share attributable to common stockholders-basic and diluted	23,893,696	2,324,790

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) - 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]
Beginning balance at Dec. 31, 2017	$ (23,135)	$ 1,500	$ 25,000			$ 339	$ (23,474)
Beginning 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
Issuance of series B redeemable convertible preferred stock to settled accrued liability of license cost				$ 500
Issuance of series B redeemable convertible preferred stock to settle accrued liability of license cost, shares				233,765
Accretion of issuance costs related to redeemable convertible preferred stock	(2,243)			$ 2,243		(339)	(1,904)
Stock-based compensation expense	109					109
Vesting of restricted stock awards	0	$ 0	$ 0	0	$ 0	0	0
Vesting of restricted stock awards, shares					30,753
Net loss	(8,242)						(8,242)
Ending balance at Mar. 31, 2018	(33,511)	$ 1,500	$ 25,000	$ 61,000		109	(33,620)
Ending balance, shares at Mar. 31, 2018		3,333,333	31,450,499	28,519,322	2,335,926
Beginning balance at Dec. 31, 2018	122,184				$ 2	193,921	(71,739)
Beginning balance, shares at Dec. 31, 2018					23,806,628
Stock-based compensation expense	1,455					1,455
Exercise of stock options	$ 252					252
Exercise of stock options, shares	116,859				116,859
Vesting of restricted stock awards	$ 0	$ 0	$ 0	$ 0	$ 0	0	0
Vesting of restricted stock awards, shares					30,753
Net loss	(17,103)						(17,103)
Ending balance at Mar. 31, 2019	$ 106,788				$ 2	$ 195,628	$ (88,842)
Ending balance, shares at Mar. 31, 2019					23,954,241

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Statement of Stockholders' Equity [Abstract]
Issuance of redeemable convertible preferred stock, net of issuance costs	$ 2,243

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (17,103)	$ (8,242)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	190	21
Stock-based compensation expense	1,455	109
Amortization of deferred offering costs		11
Impairment loss of property and equipment		235
Deferred rent expense	(40)	(8)
Change in fair value of preferred stock warrant liability		12
Change in fair value of derivative liability		587
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,485)	(189)
Other assets		(306)
Accounts payable	671	1,438
Accrued expenses and other current liabilities	(1,426)	352
Net cash used in operating activities	(17,738)	(5,980)
Cash flows from investing activities:
Purchases of property and equipment	(340)	(263)
Net cash used in investing activities	(340)	(263)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		58,263
Proceeds from exercise of stock options	252
Payments of initial public offering costs		(55)
Net cash provided by financing activities	252	58,208
Net (decrease) increase in cash, cash equivalents and restricted cash	(17,826)	51,965
Cash, cash equivalents and restricted cash at beginning of period	126,794	5,987
Cash, cash equivalents and restricted cash at end of period	108,968	57,952
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses	336	88
Property and equipment held for sale		19
Deferred offering costs included in accrued expenses		507
Purchase of property and equipment paid for by landlord		181
Accretion of issuance costs related to redeemable convertible preferred stock		2,240
Reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheets:
Cash and cash equivalents	108,476	57,928
Long-term restricted cash (included in other assets)	492	24
Cash, cash equivalents and restricted cash at end of period	$ 108,968	$ 57,952

Nature of the Business	3 Months Ended
Nature of the Business	Mar. 31, 2019
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. On June 20, 2018, the Company’s registration statement on Form S-1 relating to its initial public offering (“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, all series Seed redeemable convertible preferred stock (the “Series Seed Preferred Stock”), series A redeemable convertible preferred stock (the “Series A Preferred Stock”) and 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”) then outstanding converted into an aggregate of 15,320,213 shares of common stock. Through March 31, 2019, the Company has funded its operations primarily with proceeds from the sale of the Preferred Stock and common stock through the Company’s IPO. The Company has incurred recurring losses since its inception, including net losses of $17,103 and $8,242 for the three months ended March 31, 2019 and 2018, respectively. In addition, as of March 31, 2019, the Company had an accumulated deficit of $88,842. Although the Company has incurred recurring losses and expects to continue to incur losses for the foreseeable future, the Company expects that its existing cash, cash equivalents and marketable securities will be sufficient to fund current planned operations and capital expenditure requirements for at least the next twelve months from the date of issuance of the financial statements contained in this Form 10-Q. However, 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.

Summary of Significant Accounting Policies	3 Months Ended
Summary of Significant Accounting Policies	Mar. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies *Basis of Presentation* The accompanying condensed 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”). The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the audited annual consolidated financial statements as of and for the year ended December 31, 2018, and, in the opinion of management, reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of March 31, 2019, and the results of its operations, changes in stockholders’ equity and its cash flows for the three months ended March 31, 2019 and 2018. *Basis of Presentation (continued)* The results for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, filed with the Securities and Exchange Commission (“SEC”) on March 25, 2019. The accompanying Unaudited Condensed Consolidated Financial Statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the Unaudited Condensed Consolidated Financial Statements. As of March 31, 2019, there have been no changes to the Company’s significant accounting policies as described in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018. *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. *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”. *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 13. *Recently Issued Accounting Pronouncements* 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 2018-18clarify 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. *Recently Issued Accounting Pronouncements (continued)* 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 August 2019, The FASB issued ASU 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 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 adopted ASU 2016-15 during the quarter ended March 31, 2019. The adoption did not have a material impact on the condensed 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 2016-02supersedes 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

3 Months Ended

Mar. 31, 2019

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 March 31, 2019 and December 31, 2018:


	Fair Value Measurements as of March 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	108,233	$	—	$	—	$	108,233
Restricted cash		492		—		—		492

	$	108,725	$	—	$	—	$	108,725


	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

During the three months ended March 31, 2019 and 2018, there were no transfers between Level 1, Level 2 and Level 3.

Valuation of the Warrant to Purchase Preferred Stock

At March 31, 2018, the Company had outstanding a warrant to purchase shares of Series A Preferred Stock that was issued to a lender in connection with a loan and security agreement entered into in 2017. 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.

Prior to the IPO, the Company classified the warrant as a liability on its consolidated balance sheets as the warrant is 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 warrant to purchase preferred stock as of the date of IPO was transferred to additional paid in capital.

The Company recognized a loss of $12 in the condensed consolidated statements of operations and comprehensive loss for the three months ended March 31, 2018 related to the change in fair value of the warrant.

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 was 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 was recorded as research and development expense in January 2016.

On March 31, 2018, the Company remeasured the fair value of the derivative, using current assumptions, resulting in an increase in fair value of $587, which was recorded in other expense in the accompanying condensed consolidated statements of operations and comprehensive loss for the three months ended March 31, 2018. 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 condensed consolidated statements of operations and comprehensive loss for the three months ended September 30, 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

3 Months Ended

Mar. 31, 2019

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:


	March 31, 2019		December 31, 2018
Tax incentive refund	$	1,268	$	1,325
Prepaid research and development costs		2,950		981
Prepaid insurance		227		316
Interest income receivable		200		220
Prepaid rent		83		81
Other current assets		475		795

	$	5,203	$	3,718

Property and Equipment, Net

3 Months Ended

Mar. 31, 2019

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

5. Property and Equipment, Net

Property and equipment, net consisted of the following:


	March 31, 2019			December 31, 2018
Laboratory and office equipment	$	2,044		$	1,624
Leasehold improvements		1,260			1,260
Computer equipment and software		135			134

		3,439			3,018
Less: Accumulated depreciation and amortization		(575	)		(384	)

	$	2,864		$	2,634

Depreciation and amortization expense for the three months ended March 31, 2019 and 2018 was $190 and $21, respectively.

Accrued Expenses and Other Current Liabilities

3 Months Ended

Mar. 31, 2019

Payables and Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

6. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:


	March 31, 2019		December 31, 2018
Compensation and benefit costs	$	1,437	$	2,616
Research and development costs		4,086		3,969
Consulting and professional fees		484		320
Other liabilities		470		917

	$	6,477	$	7,822

Redeemable Convertible Preferred Stock and Common Stock

3 Months Ended

Mar. 31, 2019

Text Block [Abstract]

Redeemable Convertible Preferred Stock and Common Stock

7. Redeemable Convertible Preferred Stock and Common Stock

Redeemable Convertible Preferred Stock

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.

In January 2018, the Company issued and sold 28,519,322 shares of Series B Preferred Stock, at a price of $2.1389 per share, for total proceeds of $58,757, net of issuance costs of $2,243.

As of March 31, 2018, the Preferred Stock consisted of the following:


	March 31, 2018
	Preferred Shares Authorized		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
Series B preferred stock		28,519,322		28,519,322		61,000,000		61,000,000		6,902,064

		63,491,857		63,303,154	$	87,500,000	$	87,500,000		15,320,213

Upon closing of the IPO, the Preferred Stock as of March 31, 2018, were converted into 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

As of March 31, 2019 and December 31, 2018, the authorized capital stock of the Company included 150,000,000 shares of common stock, $0.0001 par value and 10,000,000 shares of undesignated preferred stock. As of March 31, 2019 and 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 March 31, 2019, no cash dividends have been declared or paid.

Common Stock Reserved for Future Issuance

At March 31, 2019 and December 31, 2018, the Company has reserved the following shares of common stock for future issuance:


	March 31, 2019		December 31, 2018
Shares reserved for vesting of restricted stock awards		124,821		153,276
Shares reserved for exercise of outstanding stock options		2,986,048		2,164,101
Shares reserved for issuance under the 2018 Stock Option and Grant Plan		528,134		385,561
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		461,266		223,200

Total shares of authorized common stock reserved for future issuance		4,100,269		2,926,138

Stock-Based Compensation

3 Months Ended

Mar. 31, 2019

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

Stock-Based Compensation

8. Stock-Based Compensation

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:


	Three Months Ended March 31,
	2019			2018
Expected option life (years)		6.00			6.07
Risk-free interest rate		2.53	%		2.72	%
Expected volatility		80.57	%		84.00	%
Expected dividend yield		—	%		—	%

The following table summarizes the Company’s stock option activity for the three months ended March 31, 2019:


	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2018		2,164,101		$	7.75		8.77	$	23,083
Granted		1,002,579		$	16.30
Exercised		(116,859	)	$	2.16
Cancelled or forfeited		(63,773	)	$	3.23

Outstanding as of March 31, 2019		2,986,048		$	10.94		8.72	$	34,524

Exercisable as of March 31, 2019		673,946		$	2.02		6.73	$	13,502
Vested and expected to vest as of March 31, 2019		2,986,048		$	10.94		8.72	$	34,524

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 three months ended March 31, 2019 and 2018 was $11.41 and $6.99, respectively.

Restricted Common Stock

The following table summarizes the Company’s restricted common stock activity for the three months ended March 31, 2019:


	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2018		153,275		$	0.42
Granted		2,300		$	15.65
Vested		(30,754	)	$	0.42
Forfeited, canceled or expired		—		$	—

Issued and unvested as of March 31, 2019		124,821		$	0.70

The total fair value of restricted common stock vested during the three months ended March 31, 2019 and 2018 was $13 and $13, respectively.

Stock-Based Compensation

Stock-based compensation expense was allocated as follows:


	Three Months Ended March 31,
	2019		2018
Research and development	$	860	$	36
General and administrative		595		73

Total stock-based compensation expense	$	1,455	$	109

As of March 31, 2019, total unrecognized compensation cost related to the unvested stock-based awards was $22,083, which is expected to be recognized over a weighted-average period of 3.61 years.

License Agreements	3 Months Ended
License Agreements	Mar. 31, 2019
Text Block [Abstract]
License Agreements	9. 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. *Agreements with UHN (continued)* 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 three months ended March 31, 2019 and 2018, the Company recorded research and development expense related to this agreement with UHN of $194 and zero, respectively, which consists of reimbursable funded study trial costs. No milestone or maintenance fees were incurred related to the Fabry license agreement in the three months ended March 31, 2019 and 2018. 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 three months ended March 31, 2019 and 2018, the Company recorded research and development expense related to this agreement with UHN of $38 and $41, respectively, which consists of license maintenance fees. No milestone fees were incurred related to the IL-12 license agreement in the three months ended March 3 I, 2019 and 2018. *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. No milestone fees related to the license were recorded for the three months ended March 31, 2019 and 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 next anticipated payment under this Agreement is $2,000, and would become due following the treatment of the first patient in the planned Phase 1/2 clinical trial of AVR-RD-04 in cystinosis. 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. No milestone fees related to the license were recorded for the three months ended March 31, 2019 and 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 milestone fees related to the license were recorded for the three months ended March 31, 2019 and 2018.

Net Loss per Share Attributable to Common Stockholders

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Net Loss per Share Attributable to Common Stockholders

10. 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:


	Three Months Ended March 31,
	2019		2018
Options to purchase common stock		2,986,048		1,788,750
Restricted common stock		124,821		245,548
Redeemable convertible preferred stock (as converted to common stock)		—		15,320,213
Warrants to purchase redeemable convertible preferred stock (as converted to common stock)		—		6,850

Commitments and Contingencies	3 Months Ended
Commitments and Contingencies	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies *Lease Agreement* 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. 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 September 30, 2018. *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 three months ended March 31, 2019 and 2018 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 March 31, 2019 and December 31, 2018, or royalties on future sales. No milestone or royalty payments under these agreements are expected to be payable in the immediate future, except as disclosed in Note 9. See Note 9 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. Further, the Company indemnifies its directors and officers who are, or were, serving at the Company’s request in such capacities. The Company’s maximum exposure under these arrangements is unknown at March 31, 2019. The Company does not anticipate recognizing any significant losses relating to these arrangements. 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	3 Months Ended
Related Party Transactions	Mar. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions *UHN* In connection with the Company’s entry into a license agreement with UHN on January 27, 2016, the Company issued UHN 1,161,665 shares of its common stock. Upon the closing of the IPO, as UHN’s fully-diluted percentage ownership of the Company was reduced within a range of specified percentages, the Company was obligated to pay UHN an amount up to $2,000, which was paid in July 2018. See Note 3 for further discussion on the accounting treatment for this provision. For the three months ended March 31, 2019 and 2018, the Company recognized $232 and $41, respectively, of research and development expense related to the license agreements with UHN (Note 9). *Others* For the three months ended March 31, 2019, the Company recorded expenses of $356, related to a sublease to rent lab space, provided by an entity affiliated with a member of the Board. No such expense was recorded for the three months ended March 31, 2018. For the three months ended March 31, 2019 and 2018, the Company recognized zero and $38, respectively, related to consulting services provided by an entity affiliated with an officer of the Company and a member of the Board.

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	*Basis of Presentation* The accompanying condensed 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”). The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the audited annual consolidated financial statements as of and for the year ended December 31, 2018, and, in the opinion of management, reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of March 31, 2019, and the results of its operations, changes in stockholders’ equity and its cash flows for the three months ended March 31, 2019 and 2018. *Basis of Presentation (continued)* The results for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, filed with the Securities and Exchange Commission (“SEC”) on March 25, 2019. The accompanying Unaudited Condensed Consolidated Financial Statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the Unaudited Condensed Consolidated Financial Statements. As of March 31, 2019, there have been no changes to the Company’s significant accounting policies as described in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018.
Use of Estimates	*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.
Emerging Growth Company Status	*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”.
Subsequent Event Considerations	*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 13.
Recently Issued Accounting Pronouncements	*Recently Issued Accounting Pronouncements* 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 2018-18clarify 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. *Recently Issued Accounting Pronouncements (continued)* 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 August 2019, The FASB issued ASU 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 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 adopted ASU 2016-15 during the quarter ended March 31, 2019. The adoption did not have a material impact on the condensed 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 2016-02supersedes 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 (Tables)

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

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


	Fair Value Measurements as of March 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	108,233	$	—	$	—	$	108,233
Restricted cash		492		—		—		492

	$	108,725	$	—	$	—	$	108,725


	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

- 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)

3 Months Ended

Mar. 31, 2019

Text Block [Abstract]

Summary of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:


	March 31, 2019		December 31, 2018
Tax incentive refund	$	1,268	$	1,325
Prepaid research and development costs		2,950		981
Prepaid insurance		227		316
Interest income receivable		200		220
Prepaid rent		83		81
Other current assets		475		795

	$	5,203	$	3,718

Property and Equipment, Net (Tables)

3 Months Ended

Mar. 31, 2019

Property, Plant and Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following:


	March 31, 2019			December 31, 2018
Laboratory and office equipment	$	2,044		$	1,624
Leasehold improvements		1,260			1,260
Computer equipment and software		135			134

		3,439			3,018
Less: Accumulated depreciation and amortization		(575	)		(384	)

	$	2,864		$	2,634

Accrued Expenses and Other Current Liabilities (Tables)

3 Months Ended

Mar. 31, 2019

Payables and Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:


	March 31, 2019		December 31, 2018
Compensation and benefit costs	$	1,437	$	2,616
Research and development costs		4,086		3,969
Consulting and professional fees		484		320
Other liabilities		470		917

	$	6,477	$	7,822

Redeemable Convertible Preferred Stock and Common Stock (Tables)

3 Months Ended

Mar. 31, 2019

Text Block [Abstract]

Summary of Preferred Stock

As of March 31, 2018, the Preferred Stock consisted of the following:


	March 31, 2018
	Preferred Shares Authorized		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
Series B preferred stock		28,519,322		28,519,322		61,000,000		61,000,000		6,902,064

		63,491,857		63,303,154	$	87,500,000	$	87,500,000		15,320,213

Summary of Common Stock Reserved for Future Issuance

At March 31, 2019 and December 31, 2018, the Company has reserved the following shares of common stock for future issuance:


	March 31, 2019		December 31, 2018
Shares reserved for vesting of restricted stock awards		124,821		153,276
Shares reserved for exercise of outstanding stock options		2,986,048		2,164,101
Shares reserved for issuance under the 2018 Stock Option and Grant Plan		528,134		385,561
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		461,266		223,200

Total shares of authorized common stock reserved for future issuance		4,100,269		2,926,138

- 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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65888546&loc=d3e21300-112643

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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+ Details

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

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:


	Three Months Ended March 31,
	2019			2018
Expected option life (years)		6.00			6.07
Risk-free interest rate		2.53	%		2.72	%
Expected volatility		80.57	%		84.00	%
Expected dividend yield		—	%		—	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the three months ended March 31, 2019:


	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2018		2,164,101		$	7.75		8.77	$	23,083
Granted		1,002,579		$	16.30
Exercised		(116,859	)	$	2.16
Cancelled or forfeited		(63,773	)	$	3.23

Outstanding as of March 31, 2019		2,986,048		$	10.94		8.72	$	34,524

Exercisable as of March 31, 2019		673,946		$	2.02		6.73	$	13,502
Vested and expected to vest as of March 31, 2019		2,986,048		$	10.94		8.72	$	34,524

Summary of Restricted Stock Unit Activity

The following table summarizes the Company’s restricted common stock activity for the three months ended March 31, 2019:


	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2018		153,275		$	0.42
Granted		2,300		$	15.65
Vested		(30,754	)	$	0.42
Forfeited, canceled or expired		—		$	—

Issued and unvested as of March 31, 2019		124,821		$	0.70

Schedule of Stock-based Compensation Expense

Stock-based compensation expense was allocated as follows:


	Three Months Ended March 31,
	2019		2018
Research and development	$	860	$	36
General and administrative		595		73

Total stock-based compensation expense	$	1,455	$	109

Net Loss per Share Attributable to Common Stockholders (Tables)

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

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


	Three Months Ended March 31,
	2019		2018
Options to purchase common stock		2,986,048		1,788,750
Restricted common stock		124,821		245,548
Redeemable convertible preferred stock (as converted to common stock)		—		15,320,213
Warrants to purchase redeemable convertible preferred stock (as converted to common stock)		—		6,850

Nature of the Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Jun. 25, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Nature Of Organization And Operations [Line Items]
Net loss		$ (17,103)	$ (8,242)
Accumulated deficit		$ (88,842)		$ (71,739)
Net proceeds from issuance of common stock	$ 104,013
Convertible preferred stock converted into common stock			15,320,213
IPO [Member]
Nature Of Organization And Operations [Line Items]
Common stock, shares issued and sold	5,247,958
Issuance price per shares	$ 19.00
Net proceeds from issuance of common stock	$ 90,103
Underwriting discounts and commissions and offering costs	6,980
Other offering expenses	$ 2,628
Over-Allotment Option [Member]
Nature Of Organization And Operations [Line Items]
Common stock, shares issued and sold	787,193
Net proceeds from issuance of common stock	$ 13,910
Underwriting discounts and commissions and offering costs	$ 1,047

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 ($)	Mar. 31, 2019	Dec. 31, 2018
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 108,725	$ 126,539
Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	108,233	126,047
Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	492	492
Level 1 [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	108,725	126,539
Level 1 [Member] \| Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	108,233	126,047
Level 1 [Member] \| Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 492	$ 492

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)		3 Months Ended
	Jan. 27, 2016	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018	Jul. 31, 2018
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Fair value input transfer between levels		$ 0		$ 0
Change in fair value of preferred stock warrant liability				12,000
Common stock issued for initial consideration for license		2,300
Increase in fair value of derivative			$ (1,629,000)	$ (587,000)
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] \| 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

Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Prepaid Expense and Other Assets, Current [Abstract]
Tax incentive refund	$ 1,268	$ 1,325
Prepaid research and development costs	2,950	981
Prepaid insurance	227	316
Interest income receivable	200	220
Prepaid rent	83	81
Other current assets	475	795
Total	$ 5,203	$ 3,718

Property and Equipment, Net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 3,439	$ 3,018
Less: Accumulated depreciation and amortization	(575)	(384)
Property plant and equipment, net	2,864	2,634
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	2,044	1,624
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	1,260	1,260
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 135	$ 134

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 190	$ 21

Accrued Expenses - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Accrued Liabilities, Current [Abstract]
Compensation and benefit costs	$ 1,437	$ 2,616
Research and development costs	4,086	3,969
Consulting and professional fees	484	320
Other liabilities	470	917
Accrued expenses	$ 6,477	$ 7,822

Redeemable Convertible Preferred Stock and Common Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Jun. 25, 2018
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		63,491,857	63,491,857
Redeemable convertible preferred stock, par value		$ 0.0001
Redeemable convertible preferred stock, shares issued and sold			63,303,154
Proceeds from issuance of redeemable convertible preferred stock			$ 58,263
Common stock, shares authorized		150,000,000		150,000,000
Common stock, par value		$ 0.0001		$ 0.0001
Undesignated preferred stock		10,000,000		10,000,000
Undesignated preferred stock outstanding		0		0
Cash dividends		$ 0
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
Redeemable convertible preferred stock, shares issued and sold			3,333,333
Series A Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		31,639,202	31,639,202
Redeemable convertible preferred stock, shares issued and sold			31,450,499
Series B Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		28,519,322	28,519,322
Redeemable convertible preferred stock, shares issued and sold	28,519,322		28,519,322
Redeemable convertible preferred stock, sales price per share	$ 2.1389
Proceeds from issuance of redeemable convertible preferred stock	$ 58,757
Redeemable convertible preferred stock, net of issuance costs	$ 2,243

Redeemable Convertible Preferred Stock - Summary of Preferred Stock (Detail) - USD ($)	Mar. 31, 2019	Mar. 31, 2018	Jan. 31, 2018
Temporary Equity [Line Items]
Preferred Shares Authorized	63,491,857	63,491,857
Preferred Shares Issued		63,303,154
Preferred Shares Outstanding		63,303,154
Carrying Value		$ 87,500,000
Liquidation Preference		$ 87,500,000
Common Stock Issuable Upon Conversion		15,320,213
Series Seed Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	3,333,333	3,333,333
Preferred Shares Issued		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
Preferred Shares Issued		31,450,499
Preferred Shares Outstanding		31,450,499
Carrying Value		$ 25,000,000
Liquidation Preference		$ 25,000,000
Common Stock Issuable Upon Conversion		7,611,438
Series B Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	28,519,322	28,519,322
Preferred Shares Issued		28,519,322	28,519,322
Preferred Shares Outstanding		28,519,322
Carrying Value		$ 61,000,000
Liquidation Preference		$ 61,000,000
Common Stock Issuable Upon Conversion		6,902,064

Redeemable Convertible Preferred Stock - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Mar. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	4,100,269	2,926,138
2018 Stock Option and Incentive Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	528,134	385,561
2018 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	461,266	223,200
Restricted Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	124,821	153,276
Employee Stock Option [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	2,986,048	2,164,101

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)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected option life (years)	6 years	6 years 25 days
Risk-free interest rate	2.53%	2.72%
Expected volatility	80.57%	84.00%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of options, Outstanding beginning balance	2,164,101
Number of options, Granted	1,002,579
Number of options, Exercised	(116,859)
Number of options, Cancelled or forfeited	(63,773)
Number of options, Outstanding ending balance	2,986,048	2,164,101
Number of options, Exercisable	673,946
Number of options, Vested and expected to vest	2,986,048
Weighted average exercise price, Outstanding beginning balance	$ 7.75
Weighted average exercise price, Granted	16.30
Weighted average exercise price, Exercised	2.16
Weighted average exercise price, Cancelled or forfeited	3.23
Weighted average exercise price, Outstanding ending balance	10.94	$ 7.75
Weighted average exercise price, Exercisable	2.02
Weighted average exercise price, Vested and expected to vest	$ 10.94
Weighted average remaining contractual term, Outstanding balance	8 years 8 months 19 days	8 years 9 months 7 days
Weighted average remaining contractual term, Exercisable	6 years 8 months 23 days
Weighted average remaining contractual term, vested and expected to vest	8 years 8 months 19 days
Aggregate intrinsic value, Outstanding balance	$ 34,524	$ 23,083
Aggregate intrinsic value, Exercisable	13,502
Aggregate intrinsic value, vested and expected to vest	$ 34,524

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options Granted, Weighted-average grant-date fair value	$ 11.41	$ 6.99
Total fair value of restricted common stock vested	$ 13	$ 13
Unrecognized stock-based compensation expenses	$ 22,083
Unrecognized stock-based compensation expense, period for recognition	3 years 7 months 9 days

Stock-Based Compensation - Summary of Restricted Stock Unit Activity (Detail)	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of shares, Issued and unvested beginning balance	153,275
Number of shares, Granted	2,300
Number of shares, Vested	(30,754)
Number of shares, Forfeited, cancelled or expired	0
Number of shares, Issued and unvested ending balance	124,821
Weighted average grant date fair value, Issued and unvested, beginning balance \| $ / shares	$ 0.42
Weighted average grant date fair value, Granted \| $ / shares	15.65
Weighted average grant date fair value, Vested \| $ / shares	0.42
Weighted average grant date fair value, Issued and unvested, ending balance \| $ / shares	$ 0.70

Stock-Based Compensation - Schedule of Stock-based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock based compensation expense	$ 1,455	$ 109
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock based compensation expense	860	36
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock based compensation expense	$ 595	$ 73

License Agreements - Additional Information (Detail) $ in Thousands					1 Months Ended	3 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 ($) shares	Mar. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jan. 27, 2016 CAD ($) shares
License Agreement [Line Items]
Common stock, shares issued \| shares						24,076,762	23,959,903
Research and development						$ 12,446,000		$ 5,647,000
Preferred stock issued \| shares						0	0
UHN Agreement [Member]
License Agreement [Line Items]
Milestone fees						$ 0		0
Annual maintenance fees						0		0
Research and development expense						194,000		0
UHN Agreement [Member] \| Fabry License Agreement [Member]
License Agreement [Line Items]
Option fee											$ 20
Upfront license fee				$ 75
Milestone payments				2,450
UHN Agreement [Member] \| Interleukin 12 License Agreement [Member]
License Agreement [Line Items]
Upfront license fee				264
Milestone payments				19,275
Annual maintenance fees				$ 50
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
Research and development						38,000		41,000
BioMarin Pharmaceutical Inc [Member]
License Agreement [Line Items]
Upfront license fee					$ 500,000
Milestone payments						13,000,000
Upfront cash payment									$ 500,000	$ 500,000
Expenses related to license						0		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
Expenses related to license						$ 0		$ 0
Milestone payments	16,000,000
Milestone payment due	$ 2,000,000
Lund University Rights Holders Agreement [Member]
License Agreement [Line Items]
Expenses related to license							$ 0		$ 0
Milestone payments		$ 550

Net Loss per Share - Schedule of Dilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Detail) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
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	2,986,048	1,788,750
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	124,821	245,548
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		15,320,213

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	Aug. 31, 2018	Jan. 12, 2018	Mar. 31, 2019	Sep. 30, 2018
Commitments And Contingencies Disclosure [Line Items]
Lease agreement expiration date		Jan. 31, 2023
Percentage of annual increase in rent		3.00%
Tenant incentive allowance		$ 842
Security deposit in connection with lease			$ 209
Sub-lease Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease agreement expiration date	Oct. 31, 2020
Percentage of annual increase in rent	3.00%
Security deposit in connection with lease				$ 283

Related Party Transactions - Additional information (Detail) - USD ($)		3 Months Ended
	Jan. 27, 2016	Mar. 31, 2019	Mar. 31, 2018
Related Party Transaction [Line Items]
Common stock issued for initial consideration for license		2,300
Research and development expense related to license agreements		$ 12,446,000	$ 5,647,000
Officers and Board Members [Member] \| Consulting Services Agreement [Member]
Related Party Transaction [Line Items]
Expense related to related party		0	38,000
License Agreement [Member]
Related Party Transaction [Line Items]
Provision for maximum cash payment	$ 2,000,000	2,000,000
Research and development expense related to license agreements		232,000	$ 41,000
License Agreement [Member] \| Stock Purchase Agreement [Member]
Related Party Transaction [Line Items]
Common stock issued for initial consideration for license	1,161,665
Sub-lease Agreement [Member] \| Officers and Board Members [Member]
Related Party Transaction [Line Items]
Expense related to related party		$ 356,000