0001564590-21-041725 | 10-Q | XBRL Viewer

Print Document View Excel Document

Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2021	Jul. 30, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
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		43,623,066
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Small Business	true
Entity Current Reporting Status	Yes
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 Quarterly 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

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 226,438	$ 259,682
Prepaid expenses and other current assets	7,081	7,560
Total current assets	233,519	267,242
Property and equipment, net	3,990	3,064
Restricted cash	492	492
Other assets	94	436
Total assets	238,095	271,234
Current liabilities:
Accounts payable	2,029	2,682
Accrued expenses and other current liabilities	14,665	13,693
Deferred rent	259	239
Total current liabilities	16,953	16,614
Deferred rent, net of current portion	150	276
Total liabilities	17,103	16,890
Commitments and contingencies (Note 6)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 10,000 shares authorized; no shares issued or outstanding
Common stock, $0.0001 par value; 150,000 shares authorized; 43,623 and 41,569 shares issued and outstanding as of June 30, 2021 and December 31, 2020, respectively	4	4
Additional paid-in capital	543,690	518,756
Accumulated deficit	(322,702)	(264,416)
Total stockholders’ equity	220,992	254,344
Total liabilities and stockholders’ equity	$ 238,095	$ 271,234

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2021	Dec. 31, 2020
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	43,623,000	41,569,000
Common stock, outstanding	43,623,000	41,569,000

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Operating expenses:
Research and development	$ 22,544	$ 20,866	$ 41,024	$ 39,140
General and administrative	8,831	7,991	17,235	16,306
Total operating expenses	31,375	28,857	58,259	55,446
Loss from operations	(31,375)	(28,857)	(58,259)	(55,446)
Other (expense) income:
Interest income	9	62	19	694
Other expense	(21)	(33)	(46)	(49)
Total other (expense) income, net	(12)	29	(27)	645
Net loss	(31,387)	(28,828)	(58,286)	(54,801)
Comprehensive loss	$ (31,387)	$ (28,828)	$ (58,286)	$ (54,801)
Net loss per share — basic and diluted	$ (0.74)	$ (0.80)	$ (1.39)	$ (1.57)
Weighted-average number of common shares outstanding — basic and diluted	42,510	36,105	42,067	34,886

Condensed 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]	Retained Earnings [Member]
Beginning balance at Dec. 31, 2019	$ 186,013			$ 3			$ 330,714			$ (144,704)
Beginning balance, shares at Dec. 31, 2019				31,643,000
Stock-based compensation expense	6,721						6,721
Exercise of stock options	96						96
Exercise of stock options, shares				32,000
Vesting of restricted stock awards and units, shares				21,000
Issuance of common stock, net of offering costs		$ 93,628	$ 8,130		$ 1			$ 93,627	$ 8,130
Issuance of common stock upon public offering, net of offering costs, shares					4,350,000	384,000
Issuance of common stock under the 2018 employee stock purchase plan, shares				2,000
Net loss	(54,801)									(54,801)
Ending balance at Jun. 30, 2020	239,787			$ 4			439,288			(199,505)
Ending balance, shares at Jun. 30, 2020				36,432,000
Beginning balance at Mar. 31, 2020	256,525			$ 4			427,198			(170,677)
Beginning balance, shares at Mar. 31, 2020				36,011,000
Stock-based compensation expense	3,899						3,899
Exercise of stock options	61						61
Exercise of stock options, shares				25,000
Vesting of restricted stock awards and units, shares				10,000
Issuance of common stock, net of offering costs			8,130						8,130
Issuance of common stock upon public offering, net of offering costs, shares						384,000
Issuance of common stock under the 2018 employee stock purchase plan, shares				2,000
Net loss	(28,828)									(28,828)
Ending balance at Jun. 30, 2020	239,787			$ 4			439,288			(199,505)
Ending balance, shares at Jun. 30, 2020				36,432,000
Beginning balance at Dec. 31, 2020	254,344			$ 4			518,756			(264,416)
Beginning balance, shares at Dec. 31, 2020				41,569,000
Stock-based compensation expense	9,326						9,326
Exercise of stock options	$ 850						850
Exercise of stock options, shares	196,872			197,000
Issuance of common stock, net of offering costs			14,550						14,550
Issuance of common stock upon public offering, net of offering costs, shares						1,829,000
Issuance of common stock under the 2018 employee stock purchase plan	$ 208						208
Issuance of common stock under the 2018 employee stock purchase plan, shares				28,000
Net loss	(58,286)									(58,286)
Ending balance at Jun. 30, 2021	220,992			$ 4			543,690			(322,702)
Ending balance, shares at Jun. 30, 2021				43,623,000
Beginning balance at Mar. 31, 2021	232,930			$ 4			524,241			(291,315)
Beginning balance, shares at Mar. 31, 2021				41,766,000
Stock-based compensation expense	4,691						4,691
Issuance of common stock, net of offering costs			$ 14,550						$ 14,550
Issuance of common stock upon public offering, net of offering costs, shares						1,829,000
Issuance of common stock under the 2018 employee stock purchase plan	208						208
Issuance of common stock under the 2018 employee stock purchase plan, shares				28,000
Net loss	(31,387)									(31,387)
Ending balance at Jun. 30, 2021	$ 220,992			$ 4			$ 543,690			$ (322,702)
Ending balance, shares at Jun. 30, 2021				43,623,000

Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Public Offering [Member]
Issuance of common stock upon public offering, offering costs				$ 419
ATM Facility [Member]
Issuance of common stock upon public offering, offering costs	$ 67	$ 67	$ 67	$ 67

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities:
Net loss			$ (58,286)	$ (54,801)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	$ 299	$ 301	589	584
Stock-based compensation expense			9,326	6,721
Deferred rent expense			(106)	(108)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets			478	1,987
Other assets			343	76
Accounts payable			(653)	(930)
Accrued expenses and other current liabilities			1,005	3,019
Net cash used in operating activities			(47,304)	(43,452)
Cash flows from investing activities:
Purchases of property and equipment			(1,344)	(1,065)
Net cash used in investing activities			(1,344)	(1,065)
Cash flows from financing activities:
Proceeds from exercise of stock options			850	96
Payment of offering costs			(204)
Net cash provided by financing activities			15,404	101,854
Net (decrease) increase in cash, cash equivalents and restricted cash			(33,244)	57,337
Cash, cash equivalents and restricted cash at beginning of period			260,174	187,535
Cash, cash equivalents and restricted cash at end of period	226,930	244,872	226,930	244,872
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses			171
Reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheets:
Cash and cash equivalents, end of period	226,438	244,380	226,438	244,380
Restricted cash	492	492	492	492
Cash, cash equivalents and restricted cash at end of period	$ 226,930	$ 244,872	226,930	244,872
Public Offering [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock				93,628
ATM Facility [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock			14,550	$ 8,130
ESPP [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock			$ 208

Nature of the Business

6 Months Ended

Jun. 30, 2021

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 clinical-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 incurred recurring losses since its inception, including net losses of $58,286 and $54,801 for the six months ended June 30, 2021 and 2020, respectively. In addition, as of June 30, 2021, the Company had an accumulated deficit of $322,702. The Company has primarily funded these losses through the proceeds from sales of common and preferred stock. 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 June 30, 2021 of $226,438 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 Quarterly Report on Form 10-Q with the Securities and Exchange Commission (“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

6 Months Ended

Jun. 30, 2021

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

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

The unaudited condensed consolidated financial statements have been prepared on the same basis as the audited annual consolidated financial statements as of and for the year ended December 31, 2020, and, in the opinion of management, reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of June 30, 2021, and the results of its operations for the three and six months ended June 30, 2021 and 2020, its statements of stockholders’ equity for the three and six months ended June 30, 2021 and 2020 and its statement of cash flows for the six months ended June 30, 2021 and 2020.

The results for the three and six months ended June 30, 2021 are not necessarily indicative of the results to be expected for the year ending December 31, 2021, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2020, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020, filed with the SEC on March 18, 2021.

The unaudited condensed consolidated financial statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the unaudited condensed consolidated financial statements. As of June 30, 2021, there have been no changes to the Company’s significant accounting policies as described in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2020.

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.

Use of Estimates

The preparation of the unaudited condensed consolidated 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 ongoing 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.

Significant estimates relied upon in preparing the unaudited condensed consolidated financial statements include the determination of the fair value of share-based awards issued and the estimation of accrued research and development expenses.

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. After adoption of ASU 2018-07, the measurement date for non-employee awards is the later of the adoption date of ASU 2018-07, or the date of grant, without change in the fair value of the award. For stock-based awards granted to nonemployees subject to graded vesting that only contain service conditions, the Company has elected to recognize stock-based compensation expense using the straight-line recognition method.

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

Given the absence of an active market for the Company’s common stock prior to 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 up through December 31, 2020 based on an analysis of reported data for a group of guideline companies that issued options with substantially similar terms. Beginning with options granted in 2021, the Company estimates its expected stock volatility using a weighted-average calculation based on the historical volatility of the Company and publicly traded peer companies and 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.

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 its IPO or such earlier time that it is no longer an “emerging growth company”.

Subsequent Event Considerations

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

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-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

6 Months Ended

Jun. 30, 2021

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 expected to equal approximately £9,275 in the aggregate.

For the three months ended June 30, 2021 and 2020, the Company incurred $825 and $0, respectively, related to the CRFA. For the six months ended June 30, 2021 and 2020, the Company incurred $825 and $0, respectively, 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 three months ended June 30, 2021 and 2020, the Company recorded research and development expense related to this agreement with UHN of $24 and $147, respectively, which consists of reimbursable funded study trial costs. For the six months ended June 30, 2021 and 2020, the Company recorded research and development expense related to this agreement with UHN of $68 and $147, respectively, which consists of reimbursable funded study trial costs. No milestone or maintenance fees were incurred related to this agreement in the three or six months ended June 30, 2021 and 2020.

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 three months ended June 30, 2021 and 2020, the the Company did not record research and development expense related to this agreement with UHN. For the six months ended June 30, 2021 and 2020, the Company recorded research and development expense related to this agreement with UHN of $39 and $38, respectively, which consists of license maintenance fees. No milestone fees were incurred related to this agreement in the three and six months ended June 30, 2021 and 2020.

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 has 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 Company is also obligated to make future milestone payments of up to $13,000 upon the achievement of certain specified milestones and agreed to pay BioMarin royalties of a low single-digit percentage of net sales of licensed products sold by the Company or its affiliates covered by patent rights in a relevant country.

The Company has recognized no expenses related to the license for the three and six months ended June 30, 2021 and 2020.

Unless terminated earlier, the agreement expires upon the expiration of the Company’s royalty obligation for all licensed products throughout the world. BioMarin and the Company can terminate the agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. The Company may terminate the agreement at will upon written notice to BioMarin. BioMarin has the right to terminate the agreement upon the Company’s bankruptcy or insolvency, or in the event of any challenge or opposition to the licensed patent rights or related actions brought by the Company or its affiliates or sublicensees, or if the Company, its affiliates or sublicensees knowingly assist a third-party in challenging or otherwise opposing the licensed patent rights, except as required under a court order or subpoena.

Agreement with GenStem Therapeutics, Inc. (“GenStem”)

On October 2, 2017, the Company entered into a license agreement with GenStem, pursuant to which GenStem granted the Company an exclusive worldwide license, subject to certain retained rights, under certain intellectual property rights owned or controlled by GenStem to develop, commercialize and sell products for use in the treatment of cystinosis. Under this agreement, the Company paid an upfront license fee of $1,000 and is required to make payments upon completion of certain milestones up to an aggregate of $16,000. The Company also agreed to pay GenStem a tiered mid to high single-digit royalty percentage on annual net sales of licensed products as well as a low double-digit percentage of sublicense income received from certain third-party licensees. The Company’s royalty obligation expires on a licensed product-by-licensed product and country-by-country basis on the eleventh anniversary of the first commercial sale of such licensed product in such country or the expiration of the last valid claim under the licensed patent rights covering such licensed product in such country, whichever is later. Unless terminated earlier, the agreement expires upon the expiration of the Company’s royalty obligation for all licensed products throughout the world. GenStem and the Company can terminate the agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. The Company may terminate the agreement at will upon the specified prior written notice to GenStem.

The Company has recognized no expenses related to this agreement for the three and six months ended June 30, 2021 and 2020.

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.

The Company has recognized no expenses related to this agreement for the three and six months ended June 30, 2021 and 2020.

Fair Value Measurement

6 Months Ended

Jun. 30, 2021

Fair Value Disclosures [Abstract]

Fair Value Measurement

4. Fair Value Measurement

The following table presents information about the Company’s financial assets 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 June 30, 2021 and December 31, 2020:

	Fair Value Measurements as of June 30, 2021
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents — money market funds	$	225,191	$	—	$	—	$	225,191
	$	225,191	$	—	$	—	$	225,191

	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

The fair value of cash equivalents was determined through quoted prices by third-party pricing services.

During the six months ended June 30, 2021, 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

6 Months Ended

Jun. 30, 2021

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:

	June 30, 2021		December 31, 2020
Tax incentive refund	$	2,332	$	2,558
Prepaid research and development expenses		3,492		3,145
Prepaid insurance		39		973
Prepaid compensation benefits		702		609
Other current assets		516		275
	$	7,081	$	7,560

Property and equipment, net

Property and equipment, net consisted of the following:

	June 30, 2021			December 31, 2020
Laboratory and office equipment	$	5,266		$	3,965
Leasehold improvements		1,586			1,377
Computer equipment		149			143
		7,001			5,485
Less: Accumulated depreciation and amortization		(3,011	)		(2,421	)
Property and equipment, net	$	3,990		$	3,064

Depreciation and amortization expense was $299 and $301, respectively, for the three months ended June 30, 2021 and 2020. Depreciation and amortization expense was $589 and $584, respectively, for the six months ended June 30, 2021 and 2020.

Restricted Cash

As of June 30, 2021 and December 31, 2020, the Company had restricted cash of $492, 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.

Accrued Expenses

Accrued expenses and other current liabilities consisted of the following:

	June 30, 2021		December 31, 2020
Compensation and benefit costs	$	5,084	$	6,402
Research and development expenses		8,491		5,847
Consulting and professional fees		897		1,096
Other liabilities		193		348
	$	14,665	$	13,693

Commitments and Contingencies

6 Months Ended

Jun. 30, 2021

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6. Commitments and Contingencies

Legal Proceedings

The Company, from time to time, may be party to litigation arising in the ordinary course of business. The Company was not subject to any material legal proceedings during the six months ended June 30, 2021 and 2020 and to the best of the Company’s knowledge, no material legal proceedings are currently pending or threatened.

Other

The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at June 30, 2021 and December 31, 2020, or royalties on future sales. No milestone or royalty payments under these agreements are expected to be payable in the immediate future, except as disclosed in Note 3 “License Agreements.”

The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company agrees to indemnify, hold harmless, and to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners, in connection with any U.S. patent or any copyright or other intellectual property infringement claim by any third-party with respect to the Company’s products. Further, the Company indemnifies its directors and officers who are, or were, serving at the Company’s request in such capacities. The Company’s maximum exposure under these arrangements is unknown as of June 30, 2021. The Company does not anticipate recognizing any significant losses relating to these arrangements. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

Stockholders' Equity

6 Months Ended

Jun. 30, 2021

Equity [Abstract]

Stockholders' Equity

7. Stockholders’ Equity

Common Stock

As of June 30, 2021 and December 31, 2020, 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 June 30, 2021 and December 31, 2020, no undesignated preferred stock was outstanding.

As of June 30, 2021, no cash dividends have been declared or paid.

Common Stock Reserved for Future Issuance

As of June 30, 2021 and December 31, 2020, the Company has reserved the following shares of common stock for future issuance:

	June 30, 2021		December 31, 2020
Shares reserved for exercise of outstanding stock options		8,412,822		5,559,545
Shares reserved for vesting of restricted stock units		772,511		1,198
Shares reserved for issuance under the 2018 Stock Option and Grant Plan		1,863,992		3,877,478
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		1,151,010		762,900
Shares reserved for issuance under the 2019 Inducement Plan		352,549		365,050
Shares reserved for issuance under the 2020 Inducement Plan		1,567,000		1,700,000
Total shares of authorized common stock reserved for future issuance		14,119,884		12,266,171

Stock-Based Compensation

6 Months Ended

Jun. 30, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

8. Stock-based Compensation

Stock Option Valuation

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

	Six Months Ended June 30,
	2021			2020
Expected option life (years)		6.07			6.00
Risk-free interest rate		0.76	%		0.88	%
Expected volatility		81.25	%		75.72	%
Expected dividend yield		—	%		—	%

The following table summarizes the Company’s stock option activity for the six months ended June 30, 2021:

	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2020		5,559,545		$	14.97		7.65	$	13,511
Granted		3,743,232		$	13.52
Exercised		(196,872	)	$	4.32
Cancelled or forfeited		(693,083	)	$	17.28
Outstanding as of June 30, 2021		8,412,822		$	14.40		8.25	$	5,974
Exercisable as of June 30, 2021		2,622,442		$	13.36		6.36	$	5,575

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 six months ended June 30, 2021 and 2020 was $1,729 and $546, respectively.

The weighted-average grant-date fair value of the Company’s stock options granted during the six months ended June 30, 2021 and 2020 was $9.31 and $13.16, respectively.

Restricted Stock Units

The following table summarizes the Company’s restricted common stock units for the six months ended June 30, 2021:

	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2020		1,198		$	15.65
Granted		771,600		$	9.63
Vested		(287	)	$	15.65
Forfeited, cancelled or expired		—
Issued and unvested as of June 30, 2021		772,511		$	9.64

The total fair value of restricted stock units vested during the six months ended June 30, 2021 and 2020 was $4 thousand and $21 thousand, respectively.

Stock-Based Compensation

Stock-based compensation expense was allocated as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Research and development	$	1,909	$	1,966	$	4,024	$	3,409
General and administrative		2,782		1,933		5,302		3,312
Total stock-based compensation expense	$	4,691	$	3,899	$	9,326	$	6,721

As of June 30, 2021, total unrecognized compensation cost related to the unvested stock-based awards was $62,167, which is expected to be recognized over a weighted-average period of 3.13 years.

Net Loss Per Share

6 Months Ended

Jun. 30, 2021

Earnings Per Share [Abstract]

Net Loss Per Share

9. Net Loss Per Share

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

	Six Months Ended June 30,
	2021		2020
Options to purchase common stock		8,412,822		5,586,794
Restricted stock units		772,511		11,570

Related Party Transactions

6 Months Ended

Jun. 30, 2021

Related Party Transactions

10. Related Party Transactions

UHN

For the three months ended June 30, 2021 and 2020, the Company recognized $24 and $147, respectively, of research and development expense related to the license agreements with UHN. For the six months ended June 30, 2021 and 2020, the Company recognized $107 and $185, 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 three months ended June 30, 2021 and 2020, the Company recorded expenses of $409 and $365, respectively, related to a sublease to rent lab space, provided by an entity affiliated with a member of the Company’s board of directors. For the six months ended June 30, 2021 and 2020, the Company recorded expenses of $1,050 and $725, respectively, related to a sublease to rent lab space, provided by an entity affiliated with a member of the Company’s board of directors

Summary of Significant Accounting Policies (Policies)	6 Months Ended
Summary of Significant Accounting Policies (Policies)	Jun. 30, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements (the “unaudited condensed consolidated financial statements”) have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). The unaudited condensed consolidated financial statements have been prepared on the same basis as the audited annual consolidated financial statements as of and for the year ended December 31, 2020, and, in the opinion of management, reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of June 30, 2021, and the results of its operations for the three and six months ended June 30, 2021 and 2020, its statements of stockholders’ equity for the three and six months ended June 30, 2021 and 2020 and its statement of cash flows for the six months ended June 30, 2021 and 2020. The results for the three and six months ended June 30, 2021 are not necessarily indicative of the results to be expected for the year ending December 31, 2021, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2020, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020, filed with the SEC on March 18, 2021. The unaudited condensed consolidated financial statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the unaudited condensed consolidated financial statements. As of June 30, 2021, there have been no changes to the Company’s significant accounting policies as described in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2020.
Segment Information	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.
Use of Estimates	Use of Estimates The preparation of the unaudited condensed consolidated 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 ongoing 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. Significant estimates relied upon in preparing the unaudited condensed consolidated financial statements include the determination of the fair value of share-based awards issued and the estimation of accrued research and development expenses.
Stock-Based Compensation	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. After adoption of ASU 2018-07, the measurement date for non-employee awards is the later of the adoption date of ASU 2018-07, or the date of grant, without change in the fair value of the award. For stock-based awards granted to nonemployees subject to graded vesting that only contain service conditions, the Company has elected to recognize stock-based compensation expense using the straight-line recognition method. The Company classifies stock-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s cash compensation costs are classified. Given the absence of an active market for the Company’s common stock prior to 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 up through December 31, 2020 based on an analysis of reported data for a group of guideline companies that issued options with substantially similar terms. Beginning with options granted in 2021, the Company estimates its expected stock volatility using a weighted-average calculation based on the historical volatility of the Company and publicly traded peer companies and 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.
Emerging Growth Company Status	Emerging Growth Company Status The Company is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act, or JOBS Act, and may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. The Company may take advantage of these exemptions until the Company is no longer an “emerging growth company.” Section 107 of the JOBS Act provides that an “emerging growth company” can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected to use the extended transition period for complying with new or revised accounting standards and as a result of this election, its consolidated financial statements may not be comparable to companies that comply with public company effective dates. The Company may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of its IPO or such earlier time that it is no longer an “emerging growth company”.
Subsequent Event Considerations	Subsequent Event Considerations The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the consolidated financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required.
Recently Issued Accounting Pronouncements	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-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.

- Definition

Disclosure of accounting policy for segment reporting.

+ References + Details

Fair Value Measurement (Tables)

6 Months Ended

Jun. 30, 2021

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured at Fair Value on Recurring Basis

	Fair Value Measurements as of June 30, 2021
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents — money market funds	$	225,191	$	—	$	—	$	225,191
	$	225,191	$	—	$	—	$	225,191

	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

- 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)

6 Months Ended

Jun. 30, 2021

Balance Sheet Related Disclosures [Abstract]

Summary of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:

	June 30, 2021		December 31, 2020
Tax incentive refund	$	2,332	$	2,558
Prepaid research and development expenses		3,492		3,145
Prepaid insurance		39		973
Prepaid compensation benefits		702		609
Other current assets		516		275
	$	7,081	$	7,560

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following:

	June 30, 2021			December 31, 2020
Laboratory and office equipment	$	5,266		$	3,965
Leasehold improvements		1,586			1,377
Computer equipment		149			143
		7,001			5,485
Less: Accumulated depreciation and amortization		(3,011	)		(2,421	)
Property and equipment, net	$	3,990		$	3,064

Summary of Accrued Expenses

Accrued expenses and other current liabilities consisted of the following:

	June 30, 2021		December 31, 2020
Compensation and benefit costs	$	5,084	$	6,402
Research and development expenses		8,491		5,847
Consulting and professional fees		897		1,096
Other liabilities		193		348
	$	14,665	$	13,693

Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2021

Equity [Abstract]

Summary of Common Stock Reserved for Future Issuance

As of June 30, 2021 and December 31, 2020, the Company has reserved the following shares of common stock for future issuance:

	June 30, 2021		December 31, 2020
Shares reserved for exercise of outstanding stock options		8,412,822		5,559,545
Shares reserved for vesting of restricted stock units		772,511		1,198
Shares reserved for issuance under the 2018 Stock Option and Grant Plan		1,863,992		3,877,478
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		1,151,010		762,900
Shares reserved for issuance under the 2019 Inducement Plan		352,549		365,050
Shares reserved for issuance under the 2020 Inducement Plan		1,567,000		1,700,000
Total shares of authorized common stock reserved for future issuance		14,119,884		12,266,171

- Definition

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

+ References

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

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21553-112644

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section S99
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65877616&loc=d3e177068-122764

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21538-112644

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21521-112644

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21488-112644

Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21506-112644

Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(27))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21484-112644

Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2021

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 of Directors

	Six Months Ended June 30,
	2021			2020
Expected option life (years)		6.07			6.00
Risk-free interest rate		0.76	%		0.88	%
Expected volatility		81.25	%		75.72	%
Expected dividend yield		—	%		—	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the six months ended June 30, 2021:

	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2020		5,559,545		$	14.97		7.65	$	13,511
Granted		3,743,232		$	13.52
Exercised		(196,872	)	$	4.32
Cancelled or forfeited		(693,083	)	$	17.28
Outstanding as of June 30, 2021		8,412,822		$	14.40		8.25	$	5,974
Exercisable as of June 30, 2021		2,622,442		$	13.36		6.36	$	5,575

Summary of Restricted Common Stock Units

The following table summarizes the Company’s restricted common stock units for the six months ended June 30, 2021:

	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2020		1,198		$	15.65
Granted		771,600		$	9.63
Vested		(287	)	$	15.65
Forfeited, cancelled or expired		—
Issued and unvested as of June 30, 2021		772,511		$	9.64

Schedule of Stock-based Compensation Expense

Stock-based compensation expense was allocated as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Research and development	$	1,909	$	1,966	$	4,024	$	3,409
General and administrative		2,782		1,933		5,302		3,312
Total stock-based compensation expense	$	4,691	$	3,899	$	9,326	$	6,721

Net Loss Per Share (Tables)

6 Months Ended

Jun. 30, 2021

Earnings Per Share [Abstract]

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

	Six Months Ended June 30,
	2021		2020
Options to purchase common stock		8,412,822		5,586,794
Restricted stock units		772,511		11,570

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ 31,387	$ 28,828	$ 58,286	$ 54,801
Accumulated deficit	322,702		322,702		$ 264,416
Cash and cash equivalents	$ 226,438	$ 244,380	$ 226,438	$ 244,380	$ 259,682

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669619-108580

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 31
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8924-108599

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(22))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721683-107760

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22595-107794

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669625-108580

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(18))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(20))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22499-107794

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22663-107794

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22583-107794

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(ii))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 20: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121326447&loc=SL5780133-109256

Reference 21: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

Reference 22: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 23: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22658-107794

+ Details

Summary of Significant Accounting Policies - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2021 Segment
Significant Accounting Policies [Abstract]
Number of operating segment	1

License Agreements - Additional Information (Detail) £ in Thousands, $ in Thousands							3 Months Ended		6 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2020 GBP (£)	Oct. 02, 2017 USD ($)	Aug. 31, 2017 USD ($) shares	Jan. 27, 2016 USD ($) shares	Jan. 27, 2016 CAD ($)	Jun. 30, 2021 USD ($) shares	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) shares	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) shares	Nov. 17, 2016 USD ($)	Jan. 27, 2016 CAD ($) shares
License Agreement [Line Items]
Collaborative agreement expenses							$ 825,000	$ 0	$ 825,000	$ 0
Common stock, issued \| shares							43,623,000		43,623,000		41,569,000
Research and development							$ 22,544,000	20,866,000	$ 41,024,000	39,140,000
Preferred stock, issued \| shares							0		0		0
University of Manchester Agreement [Member] \| MPSII License Agreement [Member]
License Agreement [Line Items]
Upfront license fee											$ 8,000,000
Milestone payments payable	$ 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 patent
Research and development expense \| £		£ 9,275
UHN Agreement [Member]
License Agreement [Line Items]
Research and development expense							$ 24,000	147,000	$ 68,000	147,000
Milestone fees							0	0	0	0
UHN Agreement [Member] \| Fabry License Agreement [Member]
License Agreement [Line Items]
Upfront license fee						$ 75
Milestone payments payable													$ 2,450
Option fee						20
UHN Agreement [Member] \| Interleukin 12 License Agreement [Member]
License Agreement [Line Items]
Upfront license fee						264
Milestone payments payable					$ 2,000,000								$ 19,275
Milestone fees							0	0	0	0
Annual maintenance fees						$ 50
Common stock, issued \| shares					1,161,665								1,161,665
Fair value of shares issued					$ 480,000
Payments upon closing of an initial public offering					$ 2,000,000
Research and development							0	0	39,000	38,000
BioMarin Pharmaceutical Inc [Member]
License Agreement [Line Items]
Upfront license fee				$ 500,000
Milestone payments payable				$ 13,000,000
Preferred stock, issued \| shares				233,765
Expenses related to license							0	0	0	0
GenStem Therapeutics Inc [Member]
License Agreement [Line Items]
Upfront license fee			$ 1,000,000
Milestone payments payable			$ 16,000,000
Expenses related to license							0	0	0	0
Lund University Rights Holders Agreement [Member]
License Agreement [Line Items]
Milestone payments payable												$ 550,000
Expenses related to license							$ 0	$ 0	$ 0	$ 0

Fair Value Measurement - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 225,191	$ 44,416
Money Market Funds [Member] \| Cash Equivalents [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	225,191	44,416
Level 1 [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	225,191	44,416
Level 1 [Member] \| Money Market Funds [Member] \| Cash Equivalents [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 225,191	$ 44,416

Fair Value Measurement - Additional Information (Detail)	Jun. 30, 2021 USD ($)
Fair Value Disclosures [Abstract]
Fair value input transfer between levels	$ 0

Supplemental Balance Sheet Information - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Supplemental Balance Sheet Information Disclosures [Abstract]
Tax incentive refund	$ 2,332	$ 2,558
Prepaid research and development expenses	3,492	3,145
Prepaid insurance	39	973
Prepaid compensation benefits	702	609
Other current assets	516	275
Total	$ 7,081	$ 7,560

Supplemental Balance Sheet Information - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Property plant and equipment, gross	$ 7,001	$ 5,485
Less: Accumulated depreciation and amortization	(3,011)	(2,421)
Property and equipment, net	3,990	3,064
Laboratory and Office Equipment [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment, gross	5,266	3,965
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment, gross	1,586	1,377
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment, gross	$ 149	$ 143

Supplemental Balance Sheet Information - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Supplemental Balance Sheet Information Disclosures [Abstract]
Depreciation and amortization expense	$ 299	$ 301	$ 589	$ 584
Restricted cash	$ 492		$ 492		$ 492

Supplemental Balance Sheet Information - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Accrued Liabilities Current [Abstract]
Compensation and benefit costs	$ 5,084	$ 6,402
Research and development expenses	8,491	5,847
Consulting and professional fees	897	1,096
Other liabilities	193	348
Accrued expenses	$ 14,665	$ 13,693

Stockholders' Equity - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Equity [Abstract]
Common stock, 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

Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Jun. 30, 2021	Dec. 31, 2020
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	14,119,884	12,266,171
Employee Stock Option [Member]
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	8,412,822	5,559,545
Restricted Stock Units [Member]
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	772,511	1,198
2018 Stock Option and Incentive Plan [Member]
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	1,863,992	3,877,478
2018 Employee Stock Purchase Plan [Member]
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	1,151,010	762,900
2019 Inducement Plan [Member]
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	352,549	365,050
2020 Inducement Plan [Member]
Class Of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	1,567,000	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 of Directors (Detail)	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Abstract]
Expected option life (years)	6 years 25 days	6 years
Risk-free interest rate	0.76%	0.88%
Expected volatility	81.25%	75.72%

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of options, Outstanding beginning balance	5,559,545
Number of options, Granted	3,743,232
Number of options, Exercised	(196,872)
Number of options, Cancelled or forfeited	(693,083)
Number of options, Outstanding ending balance	8,412,822	5,559,545
Number of options, Exercisable	2,622,442
Weighted average exercise price, Outstanding beginning balance	$ 14.97
Weighted average exercise price, Granted	13.52
Weighted average exercise price, Exercised	4.32
Weighted average exercise price, Cancelled or forfeited	17.28
Weighted average exercise price, Outstanding ending balance	14.40	$ 14.97
Weighted average exercise price, Exercisable	$ 13.36
Weighted average remaining contractual term, Outstanding balance	8 years 3 months	7 years 7 months 24 days
Weighted average remaining contractual term, Exercisable	6 years 4 months 9 days
Aggregate intrinsic value, Outstanding balance	$ 5,974	$ 13,511
Aggregate intrinsic value, Exercisable	$ 5,575

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Aggregate intrinsic value of options exercised	$ 1,729	$ 546
Options Granted, Weighted-average grant-date fair value	$ 9.31	$ 13.16
Total fair value of restricted stock units vested	$ 4	$ 21
Unrecognized stock-based compensation expenses	$ 62,167
Unrecognized stock-based compensation expense, period for recognition	3 years 1 month 17 days

Stock-Based Compensation - Summary of Restricted Common Stock Units (Detail) - Restricted Stock Units [Member]	6 Months Ended
	Jun. 30, 2021 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares, Issued and unvested beginning balance \| shares	1,198
Number of shares, Granted \| shares	771,600
Number of shares, Vested \| shares	(287)
Number of shares, Issued and unvested ending balance \| shares	772,511
Weighted average grant date fair value, Issued and unvested, beginning balance \| $ / shares	$ 15.65
Weighted average grant date fair value, Granted \| $ / shares	9.63
Weighted average grant date fair value, Vested \| $ / shares	15.65
Weighted average grant date fair value, Issued and unvested, ending balance \| $ / shares	$ 9.64

Stock-Based Compensation - Schedule of Stock-based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 4,691	$ 3,899	$ 9,326	$ 6,721
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	1,909	1,966	4,024	3,409
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,782	$ 1,933	$ 5,302	$ 3,312

Net Loss Per Share - Schedule of Dilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Detail) - shares	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
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	8,412,822	5,586,794
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	772,511	11,570

Related Party Transactions - Additional information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Related Party Transaction [Line Items]
Research and development expense	$ 22,544	$ 20,866	$ 41,024	$ 39,140
License Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense	24	147	107	185
Sub-lease Agreement [Member] \| Officers and Board Members [Member]
Related Party Transaction [Line Items]
Expense related to related party	$ 409	$ 365	$ 1,050	$ 725