0001564590-21-014190 | 10-K | XBRL Viewer

Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2020	Mar. 09, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
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 Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		41,622,060
Entity Public Float			$ 533,953,963
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Small Business	true
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Shell Company	false
Entity File Number	001-38537
Entity Tax Identification Number	81-0710585
Entity Address, Address Line One	One Kendall Square
Entity Address, Address Line Two	Building 300
Entity Address, Address Line Three	Suite 201
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	914-8420
Document Annual Report	true
Document Transition Report	false
Entity Incorporation, State or Country Code	DE
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Security Exchange Name	NASDAQ
ICFR Auditor Attestation Flag	false
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE Part III of this Annual Report on Form 10-K incorporates by reference certain information from the registrant’s definitive Proxy Statement for its 2021 annual meeting of shareholders, which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant’s fiscal year end of December 31, 2020. Except with respect to information specifically incorporated by reference in this Form 10-K, the Proxy Statement is not deemed to be filed as part of this Form 10-K.

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 259,682	$ 187,043
Prepaid expenses and other current assets	7,560	8,658
Total current assets	267,242	195,701
Property and equipment, net	3,064	3,696
Restricted cash	492	492
Other assets	436	625
Total assets	271,234	200,514
Current liabilities:
Accounts payable	2,682	3,949
Accrued expenses and other current liabilities	13,693	9,854
Deferred rent	239	214
Total current liabilities	16,614	14,017
Deferred rent, net of current portion	276	484
Total liabilities	16,890	14,501
Commitments and contingencies (Note 6)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 10,000 shares authorized and no shares issued or outstanding as of December 31, 2020 and 2019
Common stock, $0.0001 par value; 150,000 shares authorized as of December 31, 2020 and 2019; 41,569 and 31,673 shares issued as of December 31, 2020 and 2019, respectively; 41,569 and 31,643 shares outstanding as of December 31, 2020 and 2019, respectively	4	3
Additional paid-in capital	518,756	330,714
Accumulated deficit	(264,416)	(144,704)
Total stockholders’ equity	254,344	186,013
Total liabilities and stockholders' equity	$ 271,234	$ 200,514

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
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	41,569,000	31,673,000
Common stock, outstanding	41,569,000	31,643,000

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating expenses:
Research and development	$ 87,236	$ 54,974
General and administrative	32,992	20,835
Total operating expenses	120,228	75,809
Loss from operations	(120,228)	(75,809)
Other income (expense):
Interest income	719	2,934
Other expense, net	(203)	(90)
Total other income, net	516	2,844
Net loss	(119,712)	(72,965)
Comprehensive loss	$ (119,712)	$ (72,965)
Net loss per share —basic and diluted	$ (3.31)	$ (2.66)
Weighted-average number of common shares outstanding—basic and diluted	36,206	27,432

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Public Offering [Member]	ATM Facility [Member]	Common Stock [Member]	Common Stock [Member] Public Offering [Member]	Common Stock [Member] ATM Facility [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Public Offering [Member]	Additional Paid-in Capital [Member] ATM Facility [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2018	$ 122,184			$ 2			$ 193,921			$ (71,739)
Beginning balance, shares at Dec. 31, 2018				23,807,000
Vesting of restricted stock awards and units, shares				123,000
Exercise of stock options	502						502
Exercise of stock options, shares				236,000
Issuance of common stock, net of offering costs		$ 129,465			$ 1			$ 129,464
Issuance of common stock upon public offering, net of offering costs, shares					7,475,000
Issuance of common stock under 2018 employee stock purchase plan	27						27
Issuance of common stock under 2018 employee stock purchase plan, shares				2,000
Stock-based compensation expense	6,800						6,800
Net loss	(72,965)									(72,965)
Ending balance at Dec. 31, 2019	186,013			$ 3			330,714			(144,704)
Ending balance, shares at Dec. 31, 2019				31,643,000
Vesting of restricted stock awards and units, shares				31,000
Exercise of stock options	$ 270						270
Exercise of stock options, shares	147,335			148,000
Issuance of common stock, net of offering costs		$ 163,849	$ 8,130		$ 1			$ 163,848	$ 8,130
Issuance of common stock upon public offering, net of offering costs, shares					9,350,000	384,000
Issuance of common stock under 2018 employee stock purchase plan	$ 166						166
Issuance of common stock under 2018 employee stock purchase plan, shares				13,000
Stock-based compensation expense	15,628						15,628
Net loss	(119,712)									(119,712)
Ending balance at Dec. 31, 2020	$ 254,344			$ 4			$ 518,756			$ (264,416)
Ending balance, shares at Dec. 31, 2020				41,569,000

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Public Offering [Member]
Issuance of common stock upon public offering, offering costs	$ 697	$ 525
ATM Facility [Member]
Issuance of common stock upon public offering, offering costs	$ 67

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (119,712)	$ (72,965)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	15,628	6,800
Depreciation and amortization expense	1,207	850
Deferred rent expense	(183)	(172)
Other	20
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,098	(4,888)
Other assets	84	(241)
Accounts payable	(881)	1,028
Accrued expenses and other current liabilities	3,936	1,920
Net cash used in operating activities	(98,803)	(67,668)
Cash flows from investing activities:
Purchases of property and equipment	(1,177)	(1,585)
Net cash used in investing activities	(1,177)	(1,585)
Cash flows from financing activities:
Exercise of stock options	270	502
Net cash provided by financing activities	172,619	129,994
Net increase in cash, cash equivalents and restricted cash	72,639	60,741
Cash, cash equivalents and restricted cash at beginning of period	187,535	126,794
Cash, cash equivalents and restricted cash at end of period	260,174	187,535
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses		582
Common stock offering costs incurred but unpaid at period end	205	104
Reconciliation of cash, cash equivalents and restricted cash reported within the consolidated balance sheets:
Cash and cash equivalents	259,682	187,043
Restricted cash	492	492
Cash, cash equivalents and restricted cash at end of period	260,174	187,535
2018 Employee Stock Purchase Plan [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock, net of offering costs	166	27
Public Offering [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock, net of offering costs	164,053	$ 129,465
ATM Facility [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock, net of offering costs	$ 8,130

Nature of the Business

12 Months Ended

Dec. 31, 2020

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 treatment regimen.

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.

The Company has devoted substantially all of its efforts to research and development, business planning, acquiring operating assets, seeking protection for its technology and product candidates, and raising capital. Since inception, the Company has funded its operations through sales of preferred stock and common stock. As of December 31, 2020, the Company had an accumulated deficit of $264,416. 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 and cash equivalents on hand as of December 31, 2020 of $259,682 will be sufficient to fund current planned operations and capital expenditure requirements for at least the next twelve months from the filing date of this Annual Report on Form 10-K with the SEC. 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

12 Months Ended

Dec. 31, 2020

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.

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.

Reclassifications

Restricted cash totaling $492 as of December 31, 2019 has been reclassified from “Other assets” to “Restricted cash” in order to conform to the current period presentation. Additionally, the Company has reclassified $214 which was previously classified as “Accrued expenses and other current liabilities” as of December 31, 20219 to “Deferred rent” in order to

conform to the current period presentation. These reclassifications had no effect on the consolidated results of operation or cash flows for the year ended December 31, 2019.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires that the Company make estimates and judgments that may affect the reported amounts of assets, liabilities and expenses and the related disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. On an on‑going basis, the Company evaluates its estimates, judgments and methodologies. The Company bases its estimates on historical experience and on various other assumptions that are believed to be reasonable, the results of which form the basis for making judgments about the carrying values of assets and liabilities. Actual results may differ from these estimates. Changes in estimates are reflected in reported results in the period in which they become known.

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of three months or less at acquisition to be cash equivalents. As of December 31, 2020 and 2019, cash and cash equivalents were primarily held in interest-bearing money market funds.

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, 2019, the Company recorded deferred issuance costs of $133 within other assets on the consolidated balance sheet related to a follow-on offering of common stock completed in February 2020. There were no amounts deferred as of December 31, 2020.

Fair Value Measurements

Certain assets and liabilities of the Company are carried at fair value under GAAP. 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— Fair values are determined utilizing prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access.

•

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.

The carrying amounts of the Company’s financial instruments, which include cash equivalents, accounts payable, and accrued expenses, approximated their fair values as of December 31, 2020 and 2019 due to the short‑term nature of these instruments.

The Company has evaluated the estimated fair value of financial instruments using available market information. The use of different market assumptions, estimation methodologies, or both, could have a significant effect on the estimated fair value amounts. See Note 4 “Fair Value of Financial Assets and Liabilities” for further discussion.

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		2 years
Leasehold improvements		Lesser of lease term or 10 years

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. The Company did not record any impairment loss during the years ended December 31, 2020 and 2019.

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 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.

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’ equity (deficit) which includes certain changes in equity that are excluded from net income (loss). Comprehensive loss has been disclosed in the accompanying consolidated 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, 2020 and 2019, the Company recorded foreign exchange losses of $203 and $90, respectively, in other expense.

Research and Development Expenses

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 related 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 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. Since the Company’s adoption of ASU 2018-07 in 2018, 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 its initial public offering (“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, 2020, 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.

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.

Net Loss per Share

Basic net loss per share is computed by dividing net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration of potential dilutive securities. Diluted net loss per share is computed by adjusting the weighted-average shares outstanding for the potential dilutive effects of common stock equivalents outstanding during the period calculated in accordance with the treasury stock method. For purposes of the diluted net loss per share calculation, stock options and restricted stock units 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, basic and diluted net loss per share were the same for all periods presented.

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. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure.

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 November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808)—Clarifying the Interaction between Topic 808 and Topic 606, or 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 adopted ASU 2018-18 during the quarter ended March 31, 2020. The adoption did not have a material impact on the consolidated financial statements.

In August 2018, the FASB issued ASU No. 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40), or 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 adopted ASU 2018-15 during the quarter ended March 31, 2020. The adoption did not have a material impact on the consolidated financial statements.

In August 2018, 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 adopted ASU 2018-13 during the quarter ended March 31, 2020. The adoption did not have a material impact on the consolidated financial statements.

Recently Issued Accounting Pronouncements

In February 2016, the FASB issued ASU No. 2016-02, (Topic 842) Leases, or ASU 2016-02. ASU 2016-02 requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases. The ASU will also require new qualitative and quantitative disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. For public entities, ASU 2016-02 is effective for fiscal years beginning after December 15, 2018. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, ASU 2016-02 is effective for the Company for the year ended December 31, 2022, and all interim periods thereafter. 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. Based on this evaluation the Company currently expects that its operating lease commitments will be subject to the new standard and recognized as right-of-use assets and operating lease liabilities upon adoption of this standard, which will increase the Company’s total assets and total liabilities.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, or ASU 2016-13. ASU 2016-13 requires that credit losses be reported as an allowance using an expected losses model, representing the entity's current estimate of credit losses expected to be incurred. The accounting guidance currently in effect is based on an incurred loss model. For available-for-sale debt securities with unrealized losses, this standard now requires allowances to be recorded instead of reducing the amortized cost of the investment. ASU 2016-13 is effective for non-emerging growth companies (“EGCs”) for fiscal years beginning December 15, 2019 and interim periods within those fiscal years, and will be effective for the Company for fiscal years beginning after December 15, 2022 and interim periods within those fiscal years, assuming the Company remains an EGC. Early adoption is permitted. The Company is currently evaluating the effects the adoption of ASU 2016-13 may have on its financial statements.

In November 2019, the FASB issued ASU 2019-11, “Codification Improvements to Topic 326, Financial Instruments – Credit Losses”, or ASU 2019-11. ASU 2019-11 is an accounting pronouncement that amends ASU 2016-13, “Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” The amendments update guidance on reporting credit losses for financial assets. These amendments affect loans, debt securities, trade receivables, net investments in leases, off balance sheet credit exposures, reinsurance receivables, and any other financial assets not excluded from the scope that have the contractual right to receive cash. The amendments in both ASU 2016-13 and ASU 2019-11 are

effective for annual reporting periods beginning after December 15, 2019, including interim periods within those fiscal years. As a result of the Company having elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act, 2016-13 and ASU 2019-11 are effective for the Company for fiscal years beginning after December 15, 2022, and interim periods within those fiscal years. The Company is currently evaluating ASU 2016-13 and ASU 2019-11 and their impact on its consolidated financial statements and financial statement disclosures.

In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes, or ASU 2019-12. ASU 2019-12 eliminates certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. It also clarifies and simplifies other aspects of the accounting for income taxes. ASU 2019-12 is effective for non-EGCs for fiscal years beginning after December 15, 2020 and interim periods within those fiscal years and will be effective for the Company for fiscal years beginning after December 15, 2021 and interim periods beginning after December 15, 2022, assuming the Company remains an EGC. Early adoption is permitted. The Company is currently evaluating the effects the adoption of ASU 2019-12 may have on its financial statements.

License Agreements

12 Months Ended

Dec. 31, 2020

License Agreements [Abstract]

License Agreements

3. License Agreements

Agreement with The University of Manchester

On September 30, 2020, the Company entered into an agreement (“MPSII License Agreement”) with The University of Manchester, England (“UoM”), whereby UoM granted to the Company an exclusive worldwide license under certain patent and other intellectual property rights, subject to certain retained rights, to develop, commercialize and sell an ex vivo lentiviral gene therapy for use in the treatment of Hunter syndrome, or mucopolysaccharidosis type II (“MPSII”). As consideration for the MPSII License Agreement, the Company agreed to pay UoM an upfront, one-time fee of $8,000, which was recognized as research and development expense during the year ended December 31, 2020.

As part of the agreement, the Company is obligated to make milestone payments of up to an aggregate of $80,000 upon the achievement of specified development and regulatory milestones, to pay royalties, on a product-by-product and country-by-country basis, of a mid-single digit percentage based on net sales of products licensed under the agreement and to pay a low double digit percentage of any sublicense fees received by the Company. The next anticipated payment milestones under the MPSII License Agreement include $2,000, which would become due following the date of regulatory approval of the clinical trial application for the investigator-sponsored Phase 1/2 clinical trial sponsored by UoM, and $4,000, upon the dosing of the first patient in the investigator-sponsored Phase 1/2 clinical trial sponsored by UoM.

Unless terminated earlier, the agreement expires upon the later of 15 years from the effective date or the expiration of the last valid claim of the licensed patents, subject to certain surviving rights and obligations. UoM and the Company can each terminate the agreement in the event of the bankruptcy or insolvency of the other party, or a material breach by the other party and failure to cure such breach within a certain period of time. UoM has the right to terminate the agreement in the event of certain actions relating to challenge or opposition to the licensed intellectual property brought by the Company or its affiliates or sublicensees.

Concurrently with the MPSII License Agreement, the Company entered into a collaborative research funding agreement with UoM (“CRFA”). Under the CRFA, the Company has agreed to fund the budgeted costs of an investigator-sponsored Phase 1/2 clinical trial to be sponsored by UoM in connection with the development activities under the MPSII License Agreement, which are currently estimated to equal approximately £9,100 in the aggregate.

For the year ended December 31, 2020, the Company recognized $565 of costs related to the CRFA.

Agreements with University Health Network (“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, 2020 and 2019, the Company recorded research and development expense related to this agreement with UHN of $344 and $783, respectively, which consists of reimbursable funded study trial costs and license maintenance fees. No milestone fees were incurred related to the Fabry license agreement in the years ended December 31, 2020 and 2019.

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 and agreed to pay UHN up to $2,000 upon the closing of an IPO if certain criteria are met. The fair value of the shares issued to UHN of $480 and the upfront fee was expensed upon the execution of the agreement. Upon the closing of the IPO in 2018, as the criteria were met, the Company paid UHN $2,000. 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, 2020 and 2019, the Company recorded research and development expense related to this agreement with UHN of $37 and $38, respectively, which consists of license maintenance fees. No milestone fees were incurred related to the Interleukin 12 license agreement in the years ended December 31, 2020 and 2019.

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. The license agreement was amended in February 2018 and again in January 2020 to, among things, provide that BioMarin would supply the Company with certain technology materials. 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 the Company’s Series B Preferred Stock financing in January 2018. 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 expenses related to the license were recorded for the years ended December 31, 2020 and 2019.

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.

For the year ended December 31, 2020 the Company recorded no research and development expense related to milestone payments. For the year ended December 31, 2019 the Company recorded $2,000 related to milestone payments.

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, 2020 and 2019.

Fair Value of Financial Assets and Liabilities

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

4. 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, 2020 and 2019:

	Fair Value Measurements as of December 31, 2020
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	44,416	$	—	$	—	$	44,416
	$	44,416	$	—	$	—	$	44,416

	Fair Value Measurements as of December 31, 2019
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	186,797	$	—	$	—	$	186,797
	$	186,797	$	—	$	—	$	186,797

During the years ended December 31, 2020 and 2019, there were no transfers between levels.

- 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

Supplemental Balance Sheet Information

12 Months Ended

Dec. 31, 2020

Balance Sheet Related Disclosures [Abstract]

Supplemental Balance Sheet Information

5. Supplemental Balance Sheet Information

Prepaid expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:

	December 31,
	2020		2019
Tax incentive refund	$	2,558	$	1,916
Prepaid research and development expenses		3,145		4,915
Prepaid insurance		973		897
Prepaid compensation benefits		609		362
Interest income receivable		4		224
Other current assets		271		344
	$	7,560	$	8,658

Property and equipment, net

Property and equipment, net consisted of the following:

	December 31,
	2020			2019
Laboratory and office equipment	$	3,965		$	3,456
Leasehold improvements		1,377			1,340
Computer equipment		143			134
		5,485			4,930
Less: Accumulated depreciation and amortization		(2,421	)		(1,234	)
Property and equipment, net	$	3,064		$	3,696

Depreciation and amortization expense for the years ended December 31, 2020 and 2019 was $1,207 and $850, respectively.

Restricted Cash

As of December 31, 2020 and 2019, the Company had restricted cash as presented in the table below, which consists of cash used to secure letters of credit for the benefit of the landlord in connection with the Company’s lease agreements. The cash will be restricted until the termination or modification of the lease arrangement.

	December 31,
	2020		2019
Restricted cash	$	492	$	492

Accrued Expenses

Accrued expenses consisted of the following:

	December 31,
	2020		2019
Compensation and benefit costs	$	6,402	$	3,028
Research and development expenses		5,847		5,794
Consulting and professional fees		1,096		917
Other liabilities		348		115
	$	13,693	$	9,854

Commitments and Contingencies

12 Months Ended

Dec. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6. Commitments and Contingencies

Lease Agreements

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. 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 restricted cash as of December 31, 2020 and 2019.

On August 31, 2018, the Company entered into a sub-lease agreement for lab space located in Cambridge Massachusetts, United States, which was set to expire in October 2020. On June 9, 2020, the Company amended the terms of the sublease, which is now set to expire in April 2022. The annual lease payments are subject to a 5% increase each year. In accordance with the lease agreement, the Company is required to maintain a security deposit of $283, which was recorded in restricted cash as of December 31, 2020 and 2019.

On June 1, 2020, the Company entered into a lease agreement for office space located in Toronto, Ontario, Canada, which is set to expire in June 2025. The annual lease payments are fixed for years 1 and 2, and then subject to a 6.67% increase for years 3 through 5. In accordance with the lease agreement, the Company is required to maintain a security deposit of CAD $27, which was recorded in other long-term assets as of December 31, 2020.

The Company recorded rent expense of $2,352 and $2,257 during the years ended December 31, 2020 and 2019, respectively.

The following table summarizes the future minimum lease payments due under operating leases as of December 31, 2020:

Year Ending December 31,
2021	$	2,078
2022		1,297
2023		207
2024		146
Thereafter		73
	$	3,801

Other Funding Commitments

As of December 31, 2020, the Company had several ongoing clinical and non-clinical studies for its various pipeline programs. The Company enters into contracts in the normal course of business with contract research organizations and clinical sites for the conduct of clinical trials, professional consultants for expert advice and other vendors for clinical supply manufacturing or other services. These contracts are generally cancellable, with notice, at the Company’s option and do not have significant cancellation penalties.

Guarantees

The Company enters into certain agreements with other parties in the ordinary course of business that contain indemnification provisions. These typically include agreements with directors and officers, business partners, contractors, landlords and clinical sites. Under these provisions, the Company generally indemnifies and holds harmless the indemnified party for losses suffered or incurred by the indemnified party as a result of the Company’s activities. These indemnification provisions generally survive termination of the underlying agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification provisions is unlimited. However, to date the Company has not incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. As a result, the estimated fair value of these obligations is minimal.

Litigation

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, 2020 and 2019, 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 as of December 31, 2020 and 2019, 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 3 “Licenses Agreements” 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.

Common Stock

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Common Stock

7. Common Stock

As of December 31, 2020 and 2019, 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 December 31, 2020 and 2019, no undesignated shares of preferred stock were 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, 2020, no cash dividends have been declared or paid.

Public Offerings

In July 2019, the Company closed an underwritten public offering of 7,475,000 shares of its common stock at a public offering price of $18.50 per share (the “July 2019 Follow-on Offering”), which included 975,000 shares of the Company’s common stock resulting from the full exercise of the underwriters’ option to purchase additional shares at the public offering price, less underwriting discounts and commissions. The net proceeds to the Company from the July 2019 Follow-on Offering, after deducting underwriting discounts and commissions and other offering expenses payable by the Company, were $129,464.

In February 2020, the Company closed an underwritten public offering of 4,350,000 shares of its common stock at a public offering price of $23.00 per share (the “February 2020 Follow-on Offering”). The net proceeds to the Company from the February 2020 Follow-on Offering, after deducting underwriting discounts and commissions and other offering expenses payable by the Company, were $93,627.

In June 2020, the Company sold an aggregate of 384,140 shares of common stock under its “at-the-market” facility (the “ATM Facility”) for net proceeds, after deducting commissions and other offering expenses payable by the Company, of $8,130. As of December 31, 2020, approximately $41,549 of common stock remained available for future issuance under the ATM Facility.

In November 2020, the Company closed an underwritten public offering of 5,000,000 shares of its common stock at a public offering price of $15.00 per share (the “November 2020 Follow-on Offering”). The net proceeds to the Company from the November 2020 Follow-on Offering, after deducting underwriting discounts and commissions and other offering expenses payable by the Company, were $70,221.

Common Stock Reserved for Future Issuance

As of December 31, 2020 and 2019, the Company has reserved the following shares of common stock for future issuance:

	December 31,
	2020		2019
Shares reserved for vesting of restricted stock awards		—		30,252
Shares reserved for exercise of outstanding stock options		5,559,545		3,414,445
Shares reserved for vesting of restricted stock units		1,198		2,300
Shares reserved for issuance under the 2018 Stock Option and Incentive Plan		3,877,478		332,513
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		762,900		459,595
Shares reserved for issuance under the 2019 Inducement Plan		365,050		1,800,000
Shares reserved for issuance under the 2020 Inducement Plan		1,700,000		—
Total shares of authorized common stock reserved for future issuance		12,266,171		6,039,105

Stock-Based Compensation

12 Months Ended

Dec. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

8. 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 have been made under 2015 plan.

Shares that expire, are terminated, surrendered or cancelled 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 Board, compensation committee or other designated 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 Board or compensation committee (the “Plan Evergreen”). This number is subject to adjustment in the event of a stock split, stock dividend or other change in its capitalization.

On April 16, 2020, the Board adopted an amendment to the 2018 Plan (the “Amendment”), to (i) increase the number of shares of common stock currently reserved for issuance under the 2018 Plan by 3,300,000 shares and (ii) automatically terminate the 2018 Plan’s annual increase (or “evergreen”) provision after January 2022. The Amendment was approved by the Board on June 4, 2020 and the Company’s stockholders on June 4, 2020.

The number of shares of common stock available for future grant under the 2018 Plan was 3,877,478 as of December 31, 2020, which does not include the shares added to the 2018 Plan reserve on January 1, 2021 as a result of the Plan Evergreen for the year ended December 31, 2020.

During the years ended December 31, 2020 and 2019, the Company granted options to purchase 3,019,663 and, 1,677,967 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 sto

ck 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 “ESPP Evergreen”). 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.

During the years ended December 31, 2020 and 2019, the Company issued 13,425 and 1,671 shares, respectively of common stock. The total number of shares of common stock that may be issued under the ESPP was 762,900 shares as of December 31, 2020, of which 762,900 shares remained available for future grant, and which does not include the shares added to the ESPP reserve on January 1, 2021 as a result of the ESPP Evergreen for the year ended December 31, 2020.

2019 Inducement Plan

The Company’s 2019 Inducement Plan (the “2019 Plan”) was adopted by the Board on December 11, 2019. The purpose of the 2019 Plan is to allow the Company to grant equity awards to new employees as inducements material to such new employee’s acceptance of employment with the Company. The Company intends that the shares underlying the 2019 Plan be reserved for persons to whom the Company may issue securities without stockholder approval as an inducement pursuant to Rule 5635(c)(4) of the Nasdaq marketplace rules.

The Company initially reserved 1,800,000 shares of its common stock for the issuance of awards under the 2019 Plan.

The number of shares of common stock available for future grant under the 2019 Plan was 365,050 as of December 31, 2020.

2020 Inducement Plan

The Company’s 2020 Inducement Plan (the “2020 Plan”) was adopted by the Board on December 9, 2020. The purpose of the 2020 Plan is to allow the Company to grant equity awards to new employees as inducements material to such new employee’s acceptance of employment with the Company. The Company intends that the shares underlying the 2020 Plan be reserved for persons to whom the Company may issue securities without stockholder approval as an inducement pursuant to Rule 5635(c)(4) of the Nasdaq marketplace rules.

The Company initially reserved 1,700,000 shares of its common stock for the issuance of awards under the 2020 Plan.

The number of shares of common stock available for future grant under the 2020 Plan was 1,700,000 as of December 31, 2020.

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,
	2020			2019
Expected option life (years)		6.00			5.62
Risk-free interest rate		0.76	%		2.09	%
Expected volatility		76.02	%		76.83	%
Expected dividend yield		—	%		—	%

The following table summarizes the Company’s stock option activity for the year ended December 31, 2020:

	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2019		3,414,445		$	12.08		8.43	$	29,353
Granted		3,019,663		$	18.74
Exercised		(147,335	)	$	1.83
Cancelled or forfeited		(727,228	)	$	19.70
Outstanding as of December 31, 2020		5,559,545		$	14.97		7.65	$	13,511
Exercisable as of December 31, 2020		1,934,685		$	10.37		5.40	$	11,188

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 aggregate intrinsic value of options exercised during the years ended December 31, 2020 and 2019 was $2,077 and $3,514, respectively.

The weighted-average grant-date fair value of the Company’s stock options granted during the years ended December 31, 2020 and 2019 was $12.28 and $11.61, respectively.

Restricted Common Stock

The Company has granted restricted common stock (or restricted stock awards) 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 stock awards 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. There were no unvested restricted stock awards as of December 31, 2020.

Restricted stock units

Restricted stock units represent an unsecured promise to grant at no cost a set number of shares of common stock upon vesting. With respect to restricted stock units, recipients are not entitled to cash dividends and have no voting rights during the vesting period.

The following table summarizes the Company’s restricted stock award and restricted stock unit activity for the year ended December 31, 2020:

	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2019		32,552		$	1.50
Granted		—		$	—
Vested		(31,354	)	$	0.96
Forfeited, cancelled or expired		—		$	—
Issued and unvested as of December 31, 2020		1,198		$	15.65

The total fair value of restricted stock awards and restricted stock units vested during the years ended December 31, 2020 and 2019 was $29 and $50, respectively.

Stock-Based Compensation

Stock-based compensation expense was allocated as follows:

	Year Ended December 31,
	2020		2019
Research and development	$	8,207	$	3,646
General and administrative		7,421		3,154
Total stock-based compensation expense	$	15,628	$	6,800

As of December 31, 2020, total unrecognized compensation cost related to unvested stock-based awards was $36,045, which is expected to be recognized over a weighted-average period of 3.01 years.

As of December 31, 2020, the Company modified certain awards in conjunction with employee terminations. The modification provided for the extension of the post-employment exercise period. The modifications resulted in approximately $417 of additional research and development expenses for the year ended December 31, 2020.

401(k) Savings Plan

12 Months Ended

Dec. 31, 2020

Compensation And Retirement Disclosure [Abstract]

401(k) Savings Plan

9. 401(k) Savings Plan

The Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. Eligible employees may make pretax contributions to the 401(k) Plan up to statutory limits. At the election of the Board, the Company may elect to match employee contributions. Currently, the Company makes matching contributions at a rate of 50% of the first 6% of employee contributions. The Company recorded $468 and $224 of expenses related to its 401(k) match for the years ended December 31, 2020 and 2019, respectively.

- Definition

The entire disclosure for retirement benefits.

+ References

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+ Details

Income Taxes

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Income Taxes

10. Income Taxes

For the years ended December 31, 2020 and 2019, 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, 2020 and 2019.

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

	Year Ended December 31,
	2020			2019
Federal income tax expense at statutory rate		21.0	%		21.0	%
State income taxes, net of federal benefit		5.6	%		5.2	%
Permanent differences		-0.6	%		-0.9	%
Foreign rate differential		0.1	%		0.2	%
Research and development tax credits		2.3	%		2.1	%
Change in valuation allowance		-28.3	%		-27.6	%
Provision to Return		-0.1	%		0.0	%
Effective income tax rate		0.0	%		0.0	%

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,
	2020			2019
Deferred tax assets:
U.S., foreign and state net operating loss carryforwards	$	57,705		$	33,594
Research and development credits		4,383			1,716
Capitalized start up and organizational costs		29			32
Equity based compensation		2,944			543
Licensing agreements		4,004			1,756
Accruals and other		1,587			993
Total deferred tax assets		70,652			38,634
Valuation allowance		(70,492	)		(38,425	)
Net deferred tax assets	$	160		$	209
Deferred tax liabilities:
Property and equipment	$	(160	)	$	(209	)
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, 2020 and 2019, 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, 2020 and 2019. The valuation allowance increased $32,066 and $19,968 during the years ended December 31, 2020 and 2019, respectively, due primarily to net operating losses generated.

As of December 31, 2020 and 2019, the Company had U.S. federal net operating loss carryforwards of $213,991 and $121,836, respectively, that may be available to offset future income tax liabilities. The U.S. federal tax operating loss carryforwards of approximately $17,743 will expire at various dates through 2037. Approximately $196,248 of the U.S. federal tax operating losses can be carried forward indefinitely. As of December 31, 2020 and 2019, the Company also had U.S. state net operating loss carryforwards of $201,642 and $115,129, respectively, which may be available to offset future taxable income. These losses expire at various dates through 2040.

As of December 31, 2020 and 2019, the Company has federal research and development tax credit carryforwards of $3,677 and $1,547, respectively. Included in the $3,677 of federal tax credit carryforwards are $1,351 of orphan drug credits. 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, 2020 and 2019, the Company had state research and development tax credit carryforwards of approximately $894 and $214, respectively, available to reduce future tax liabilities which expire at various dates through 2034. For all years through December 31, 2020, 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 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, 2020, the Company recorded $589 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 percentage points, 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 the United States, Australia and Canada, 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, 2016 through December 31, 2019. 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.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Net Loss per Share

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Net Loss per Share

11. Net Loss per Share

For purposes of the diluted net loss per share calculation, stock options, unvested restricted stock awards and unvested restricted stock units 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,
	2020		2019
Options to purchase common stock		5,559,545		3,414,445
Restricted stock awards and units		1,198		32,552

Related Party Transactions

12 Months Ended

Dec. 31, 2020

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 in 2018, 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 of $2,000, which was paid in July 2018. For the years ended December 31, 2020 and 2019, the Company recognized $381 and $821, respectively, of research and development expense related to the license agreements with UHN. Refer to Note 3 “License Agreements” for additional information regarding the UHN license agreements.

Others

For the years ended December 31, 2020 and 2019, the Company recorded expenses of $1,488 and $1,451, respectively, related to a sublease to rent lab space, provided by an entity affiliated with a member of the board.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Basis of presentation

Basis of Presentation

Principles of Consolidation

Segment Information

Reclassifications

conform to the current period presentation. These reclassifications had no effect on the consolidated results of operation or cash flows for the year ended December 31, 2019.

Use of Estimates

Cash and Cash Equivalents

Concentrations of Credit Risk

Deferred Issuance Costs

Fair Value Measurements

•

Level 1— Fair values are determined utilizing prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access.

•

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		2 years
Leasehold improvements		Lesser of lease term or 10 years

Impairment of Long-Lived Assets

Leases

Comprehensive Income (Loss)

Foreign Currency Translation

Research and Development Expenses

Stock-Based Compensation

Income Taxes

Net Loss per Share

Subsequent Event Considerations

Emerging Growth Company Status

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2020

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		2 years
Leasehold improvements		Lesser of lease term or 10 years

Fair Value of Financial Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

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

	Fair Value Measurements as of December 31, 2020
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	44,416	$	—	$	—	$	44,416
	$	44,416	$	—	$	—	$	44,416

	Fair Value Measurements as of December 31, 2019
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents—money market funds	$	186,797	$	—	$	—	$	186,797
	$	186,797	$	—	$	—	$	186,797

- 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

Supplemental Balance Sheet Information (Tables)

12 Months Ended

Dec. 31, 2020

Balance Sheet Related Disclosures [Abstract]

Summary of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:

	December 31,
	2020		2019
Tax incentive refund	$	2,558	$	1,916
Prepaid research and development expenses		3,145		4,915
Prepaid insurance		973		897
Prepaid compensation benefits		609		362
Interest income receivable		4		224
Other current assets		271		344
	$	7,560	$	8,658

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following:

	December 31,
	2020			2019
Laboratory and office equipment	$	3,965		$	3,456
Leasehold improvements		1,377			1,340
Computer equipment		143			134
		5,485			4,930
Less: Accumulated depreciation and amortization		(2,421	)		(1,234	)
Property and equipment, net	$	3,064		$	3,696

Summary of Restricted Cash

	December 31,
	2020		2019
Restricted cash	$	492	$	492

Summary of Accrued Expenses

Accrued expenses consisted of the following:

	December 31,
	2020		2019
Compensation and benefit costs	$	6,402	$	3,028
Research and development expenses		5,847		5,794
Consulting and professional fees		1,096		917
Other liabilities		348		115
	$	13,693	$	9,854

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Summary of Future Minimum Lease Payments Due Under Operating Leases

The following table summarizes the future minimum lease payments due under operating leases as of December 31, 2020:

Year Ending December 31,
2021	$	2,078
2022		1,297
2023		207
2024		146
Thereafter		73
	$	3,801

Common Stock (Tables)

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Summary of Common Stock Reserved for Future Issuance

As of December 31, 2020 and 2019, the Company has reserved the following shares of common stock for future issuance:

	December 31,
	2020		2019
Shares reserved for vesting of restricted stock awards		—		30,252
Shares reserved for exercise of outstanding stock options		5,559,545		3,414,445
Shares reserved for vesting of restricted stock units		1,198		2,300
Shares reserved for issuance under the 2018 Stock Option and Incentive Plan		3,877,478		332,513
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		762,900		459,595
Shares reserved for issuance under the 2019 Inducement Plan		365,050		1,800,000
Shares reserved for issuance under the 2020 Inducement Plan		1,700,000		—
Total shares of authorized common stock reserved for future issuance		12,266,171		6,039,105

- 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.

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+ Details

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Disclosure Of Compensation Related Costs Sharebased 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,
	2020			2019
Expected option life (years)		6.00			5.62
Risk-free interest rate		0.76	%		2.09	%
Expected volatility		76.02	%		76.83	%
Expected dividend yield		—	%		—	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the year ended December 31, 2020:

	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2019		3,414,445		$	12.08		8.43	$	29,353
Granted		3,019,663		$	18.74
Exercised		(147,335	)	$	1.83
Cancelled or forfeited		(727,228	)	$	19.70
Outstanding as of December 31, 2020		5,559,545		$	14.97		7.65	$	13,511
Exercisable as of December 31, 2020		1,934,685		$	10.37		5.40	$	11,188

Summary of Restricted Stock Award and Restricted Stock Unit Activity

The following table summarizes the Company’s restricted stock award and restricted stock unit activity for the year ended December 31, 2020:

	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2019		32,552		$	1.50
Granted		—		$	—
Vested		(31,354	)	$	0.96
Forfeited, cancelled or expired		—		$	—
Issued and unvested as of December 31, 2020		1,198		$	15.65

Schedule of Stock-based Compensation Expense

Stock-based compensation expense was allocated as follows:

	Year Ended December 31,
	2020		2019
Research and development	$	8,207	$	3,646
General and administrative		7,421		3,154
Total stock-based compensation expense	$	15,628	$	6,800

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Schedule of Reconciliation of Income Tax Expense (Benefit)

	Year Ended December 31,
	2020			2019
Federal income tax expense at statutory rate		21.0	%		21.0	%
State income taxes, net of federal benefit		5.6	%		5.2	%
Permanent differences		-0.6	%		-0.9	%
Foreign rate differential		0.1	%		0.2	%
Research and development tax credits		2.3	%		2.1	%
Change in valuation allowance		-28.3	%		-27.6	%
Provision to Return		-0.1	%		0.0	%
Effective income tax rate		0.0	%		0.0	%

Schedule of Deferred Tax Assets and Liabilities

	December 31,
	2020			2019
Deferred tax assets:
U.S., foreign and state net operating loss carryforwards	$	57,705		$	33,594
Research and development credits		4,383			1,716
Capitalized start up and organizational costs		29			32
Equity based compensation		2,944			543
Licensing agreements		4,004			1,756
Accruals and other		1,587			993
Total deferred tax assets		70,652			38,634
Valuation allowance		(70,492	)		(38,425	)
Net deferred tax assets	$	160		$	209
Deferred tax liabilities:
Property and equipment	$	(160	)	$	(209	)
Total deferred tax liabilities	$	—		$	—

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

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

	Year Ended December 31,
	2020		2019
Options to purchase common stock		5,559,545		3,414,445
Restricted stock awards and units		1,198		32,552

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ (264,416)	$ (144,704)
Cash and cash equivalents	$ 259,682	$ 187,043

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) Segment	Dec. 31, 2019 USD ($)
Significant Of Accounting Policies [Line Items]
Number of operating segment \| Segment	1
Restricted cash	$ 492,000	$ 492,000
Deferred rent		214,000
Deferred issuance costs	0	133,000
Impairment loss of long-lived assets	0	0
Foreign exchange losses	$ 203,000	$ 90,000
ASU No. 2018-18 [Member]
Significant Of Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
ASU No. 2018-15 [Member]
Significant Of Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
ASU No. 2018-13 [Member]
Significant Of Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
Expected Dividend Yield [Member]
Significant Of Accounting Policies [Line Items]
Expected cash dividend yield	0

Summary of Significant Accounting Policies - Schedule of Property and Equipment Estimated Useful Life (Detail)	12 Months Ended
	Dec. 31, 2020
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful life	5 years
Computer Equipment [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	10 years
Property and equipment estimated useful life	Lesser of lease term

License Agreements - Additional Information (Detail) £ in Thousands, $ in Thousands							1 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2020 GBP (£)	Oct. 02, 2017 USD ($)	Nov. 17, 2016 USD ($)	Jan. 27, 2016 USD ($)	Jan. 27, 2016 CAD ($) shares	Aug. 31, 2017 USD ($) shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)
License Agreement [Line Items]
Accrued upfront license								$ 565,000
Common stock, issued \| shares								41,569,000	31,673,000
Research and development								$ 87,236,000	$ 54,974,000
Preferred stock issued \| shares								0	0
University of Manchester Agreement [Member] \| MPSII License Agreement [Member]
License Agreement [Line Items]
Upfront license fee								$ 8,000,000
Milestone payments	$ 80,000,000
Milestone payment paid	2,000,000
Milestone payment due	$ 4,000,000
Agreement Expiration Period								15 years
Agreement Expiration Description								upon the later of 15 years from the effective date or the expiration of the last valid claim of the licensed patents
Research and development expense \| £		£ 9,100
UHN Agreement [Member]
License Agreement [Line Items]
Research and development expense								$ 344,000	$ 783,000
Milestone fees								0	0
UHN Agreement [Member] \| Fabry License Agreement [Member]
License Agreement [Line Items]
Upfront license fee						$ 75
Milestone payments						2,450
Option fee						20
UHN Agreement [Member] \| Interleukin 12 License Agreement [Member]
License Agreement [Line Items]
Upfront license fee						264
Milestone payments						19,275
Milestone fees								0	0
Annual maintenance fees						$ 50
Common stock, issued \| shares						1,161,665
Fair value of shares issued					$ 480,000
Payments upon closing of an initial public offering					$ 2,000,000					$ 2,000,000
Research and development								37,000	38,000
BioMarin Pharmaceutical Inc [Member]
License Agreement [Line Items]
Upfront license fee							$ 500,000
Preferred stock issued \| shares							233,765
Milestone payments								13,000,000
Expenses related to license								0	0
GenStem Therapeutics Inc [Member]
License Agreement [Line Items]
Upfront license fee			$ 1,000,000
Milestone payments			$ 16,000,000						2,000,000
Research and development								0
Lund University Rights Holders Agreement [Member]
License Agreement [Line Items]
Milestone payments				$ 550,000
Expenses related to license								$ 0	$ 0

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 ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 44,416	$ 186,797
Money Market Funds [Member] \| Cash Equivalents [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	44,416	186,797
Level 1 [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	44,416	186,797
Level 1 [Member] \| Money Market Funds [Member] \| Cash Equivalents [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 44,416	$ 186,797

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair value input transfer between levels	$ 0	$ 0

Supplemental Balance Sheet Information - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Supplemental Balance Sheet Information Disclosures [Abstract]
Tax incentive refund	$ 2,558	$ 1,916
Prepaid research and development expenses	3,145	4,915
Prepaid insurance	973	897
Prepaid compensation benefits	609	362
Interest income receivable	4	224
Other current assets	271	344
Total	$ 7,560	$ 8,658

Supplemental Balance Sheet Information - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 5,485	$ 4,930
Less: Accumulated depreciation and amortization	(2,421)	(1,234)
Property and equipment, net	3,064	3,696
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	3,965	3,456
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	1,377	1,340
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 143	$ 134

Supplemental Balance Sheet Information - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Supplemental Balance Sheet Information Disclosures [Abstract]
Depreciation and amortization expense	$ 1,207	$ 850

Supplemental Balance Sheet Information - Summary of Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Restricted Cash And Investments [Abstract]
Restricted cash	$ 492	$ 492

Supplemental Balance Sheet Information - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Compensation and benefit costs	$ 6,402	$ 3,028
Research and development expenses	5,847	5,794
Consulting and professional fees	1,096	917
Other liabilities	348	115
Accrued expenses	$ 13,693	$ 9,854

Commitments and Contingencies - Additional Information (Detail) $ in Thousands, $ in Thousands					12 Months Ended
	Jun. 09, 2020	Jun. 01, 2020	Aug. 31, 2018	Jan. 12, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 CAD ($)
Commitments And Contingencies Disclosure [Line Items]
Lease agreement expiration month and year	2022-04	2025-06	2020-10	2023-01
Percentage of annual increase in rent	5.00%	6.67%		3.00%
Tenant incentive allowance				$ 842
Annual lease payments, description					The annual lease payments are fixed for years 1 and 2, and then subject to a 6.67% increase for years 3 through 5
Rent expense					$ 2,352	$ 2,257
Restricted Cash [Member]
Commitments And Contingencies Disclosure [Line Items]
Security deposit in connection with lease					283	283
Other Long-term Assets [Member]
Commitments And Contingencies Disclosure [Line Items]
Security deposit in connection with lease							$ 27
Cambridge, Massachusetts \| Restricted Cash [Member]
Commitments And Contingencies Disclosure [Line Items]
Security deposit in connection with lease					$ 209	$ 209

Commitments and Contingencies - Summary of Future Minimum Lease Payments Due Under Operating Leases (Detail) $ in Thousands	Dec. 31, 2020 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2021	$ 2,078
2022	1,297
2023	207
2024	146
Thereafter	73
Total	$ 3,801

Common Stock - Additional Information (Detail) - USD ($)	1 Months Ended				12 Months Ended
	Nov. 30, 2020	Jun. 30, 2020	Feb. 29, 2020	Jul. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Class of Stock [Line Items]
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 shares of preferred stock outstanding					0	0
Cash dividends					$ 0
July 2019 Follow-on Offering [Member]
Class of Stock [Line Items]
Proceeds from issuance of common stock, net of offering costs				$ 129,464,000
July 2019 Follow-on Offering [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued and sold				7,475,000
Issuance price per shares				$ 18.50
Over-Allotment Option [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued and sold				975,000
February 2020 Follow-on Offering [Member]
Class of Stock [Line Items]
Proceeds from issuance of common stock, net of offering costs			$ 93,627,000
February 2020 Follow-on Offering [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued and sold			4,350,000
Issuance price per shares			$ 23.00
ATM Facility [Member]
Class of Stock [Line Items]
Common stock, shares issued and sold		384,140
Proceeds from issuance of common stock, net of offering costs		$ 8,130,000			8,130,000
Common stock remained available for future issuance					$ 41,549,000
November 2020 Follow-on Offering [Member]
Class of Stock [Line Items]
Proceeds from issuance of common stock, net of offering costs	$ 70,221,000
November 2020 Follow-on Offering [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued and sold	5,000,000
Issuance price per shares	$ 15.00

Common Stock - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2020	Apr. 16, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	12,266,171		6,039,105
Restricted Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance			30,252
Employee Stock Option [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	5,559,545		3,414,445
Restricted Stock Units [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	1,198		2,300
2018 Stock Option and Incentive Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	3,877,478	3,300,000	332,513
2018 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	762,900		459,595
2019 Inducement Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	365,050		1,800,000
2020 Inducement Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	1,700,000

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Apr. 16, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance	12,266,171	6,039,105
Number of options granted	3,019,663
Aggregate intrinsic value of options exercised	$ 2,077	$ 3,514
Options Granted, Weighted-average grant-date fair value	$ 12.28	$ 11.61
Unvested restricted stock awards	1,198	32,552
Total fair value of restricted stock awards and restricted stock units vested	$ 29		$ 50
Unrecognized stock-based compensation expenses	36,045
Unrecognized stock-based compensation expense, period for recognition		3 years 3 days
Additional research and development expenses	$ 417
Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance		30,252
Unvested restricted stock awards	0
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	3,877,478
Number of shares of common stock outstanding, increase, percentage	4.00%
Common stock reserved for issuance	3,877,478	332,513		3,300,000
Number of options granted	3,019,663	1,677,967
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
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	762,900
Common shares available for future grant	762,900
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 (the “ESPP Evergreen”).
Number of common shares issued	13,425	1,671
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
2019 Inducement Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares that may be issued under the plan	1,800,000
Common shares available for future grant	365,050
2020 Inducement Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares that may be issued under the plan	1,700,000
Common shares available for future grant	1,700,000

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, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Abstract]
Expected option life (years)	6 years	5 years 7 months 13 days
Risk-free interest rate	0.76%	2.09%
Expected volatility	76.02%	76.83%

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of options, Outstanding beginning balance	3,414,445
Number of options, Granted	3,019,663
Number of options, Exercised	(147,335)
Number of options, Cancelled or forfeited	(727,228)
Number of options, Outstanding ending balance	5,559,545	3,414,445
Number of options, Exercisable	1,934,685
Weighted average exercise price, Outstanding beginning balance	$ 12.08
Weighted average exercise price, Granted	18.74
Weighted average exercise price, Exercised	1.83
Weighted average exercise price, Cancelled or forfeited	19.70
Weighted average exercise price, Outstanding ending balance	14.97	$ 12.08
Weighted average exercise price, Exercisable	$ 10.37
Weighted average remaining contractual term, Outstanding balance	7 years 7 months 24 days	8 years 5 months 4 days
Weighted average remaining contractual term, Exercisable	5 years 4 months 24 days
Aggregate intrinsic value, Outstanding balance	$ 13,511	$ 29,353
Aggregate intrinsic value, Exercisable	$ 11,188

Stock-Based Compensation - Summary of Restricted Stock Award and Restricted Stock Unit Activity (Detail)	12 Months Ended
	Dec. 31, 2020 $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of shares, Issued and unvested beginning balance \| shares	32,552
Number of shares, Vested \| shares	(31,354)
Number of shares, Issued and unvested ending balance \| shares	1,198
Weighted average grant date fair value, Issued and unvested, beginning balance \| $ / shares	$ 1.50
Weighted average grant date fair value, Vested \| $ / shares	0.96
Weighted average grant date fair value, Issued and unvested, ending balance \| $ / shares	$ 15.65

Stock-Based Compensation - Schedule of Stock-based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 15,628	$ 6,800
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	8,207	3,646
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 7,421	$ 3,154

401(k) Savings Plan - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Compensation And Retirement Disclosure [Abstract]
Defined contribution plan, description	Currently, the Company makes matching contributions at a rate of 50% of the first 6% of employee contributions.
Employer matching contribution, percent of match	50.00%
Percent of employee contributions	6.00%
Defined contribution plan, expense	$ 468	$ 224

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Taxes [Line Items]
Deferred income tax expense (benefit)	$ 0	$ 0
Deferred tax assets change in valuation allowance	32,066,000	19,968,000
Deferred tax assets, operating loss carryforwards, domestic	213,991,000	$ 121,836,000
Deferred tax assets, operating loss carryforwards, not subject to expiration	196,248,000
Payroll tax liability capped per year	250,000
Income tax reconciliation tax credit research	$ 250,000
Income tax credit carryforward expiration period	20 years
Research and development tax credits	2.30%	2.10%
Percentage of cumulative changes in ownership interest subject to annual limitation	50.00%
Period of cumulative changes in ownership interest	3 years
U.S. federal [Member]
Income Taxes [Line Items]
Research and development tax credits carryforwards	$ 3,677,000	$ 1,547,000
Australia [Member]
Income Taxes [Line Items]
Income tax reconciliation tax credit research	$ 589,000
Australia [Member] \| Maximum [Member]
Income Taxes [Line Items]
Research and development tax credits	43.50%
Orphan Drug Credit [Member] \| U.S. federal [Member]
Income Taxes [Line Items]
Research and development tax credits carryforwards	$ 1,351,000
Expiration Through 2037 [Member]
Income Taxes [Line Items]
Deferred tax assets, operating loss carryforwards, subject to expiration	17,743,000
Expiration Through 2039 [Member]
Income Taxes [Line Items]
Deferred tax assets, operating loss carryforwards, state	201,642,000	115,129,000
Expiration Through 2033 [Member] \| State [Member]
Income Taxes [Line Items]
Research and development tax credits carryforwards	$ 894,000	$ 214,000

Income Taxes - Schedule of Reconciliation of Income Tax Expense (Benefit) (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rate	21.00%	21.00%
State income taxes, net of federal benefit	5.60%	5.20%
Permanent differences	(0.60%)	(0.90%)
Foreign rate differential	0.10%	0.20%
Research and development tax credits	2.30%	2.10%
Change in valuation allowance	(28.30%)	(27.60%)
Provision to Return	(0.10%)	0.00%
Effective income tax rate	0.00%	0.00%

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
U.S., foreign and state net operating loss carryforwards	$ 57,705	$ 33,594
Research and development credits	4,383	1,716
Capitalized start up and organizational costs	29	32
Equity based compensation	2,944	543
Licensing agreements	4,004	1,756
Accruals and other	1,587	993
Total deferred tax assets	70,652	38,634
Valuation allowance	(70,492)	(38,425)
Net deferred tax assets	160	209
Deferred tax liabilities:
Property and equipment	$ (160)	(209)
Total deferred tax liabilities		$ 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, 2020	Dec. 31, 2019
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	5,559,545	3,414,445
Restricted Stock Awards and Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	1,198	32,552

Related Party Transactions - Additional Information (Detail) - USD ($)		1 Months Ended	12 Months Ended
	Jan. 27, 2016	Jul. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Research and development expense			$ 87,236,000	$ 54,974,000
License Agreement [Member]
Related Party Transaction [Line Items]
Provision for maximum cash payment		$ 2,000,000
Research and development expense			381,000	821,000
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			$ 1,488,000	$ 1,451,000