0001564590-20-051456 | 10-Q | XBRL Viewer

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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2020	Oct. 30, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Trading Symbol	AVRO
Entity Registrant Name	AVROBIO, INC.
Entity Central Index Key	0001681087
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		36,450,950
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	Sep. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 219,546	$ 187,043
Prepaid expenses and other current assets	9,440	8,658
Total current assets	228,986	195,701
Property and equipment, net	3,312	3,696
Restricted cash	492	492
Other assets	465	625
Total assets	233,255	200,514
Current liabilities:
Accounts payable	2,311	3,949
Accrued expenses and other current liabilities	22,525	9,854
Deferred rent	216	214
Total current liabilities	25,052	14,017
Deferred rent, net of current portion	331	484
Total liabilities	25,383	14,501
Commitments and contingencies (Note 11)
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; 36,449 and 31,643 shares issued and outstanding as of September 30, 2020 and December 31, 2019, respectively	4	3
Additional paid-in capital	444,153	330,714
Accumulated deficit	(236,285)	(144,704)
Total stockholders’ equity	207,872	186,013
Total liabilities and stockholders’ equity	$ 233,255	$ 200,514

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, authorized	10,000,000	10,000,000
Preferred stock, issued	0	0
Preferred stock, outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, authorized	150,000,000	150,000,000
Common stock, issued	36,449,000	31,643,000
Common stock, outstanding	36,449,000	31,643,000

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating expenses:
Research and development	$ 28,509	$ 13,042	$ 67,649	$ 37,755
General and administrative	8,209	5,022	24,515	14,621
Total operating expenses	36,718	18,064	92,164	52,376
Loss from operations	(36,718)	(18,064)	(92,164)	(52,376)
Other income (expense):
Interest income	12	925	706	2,146
Other expense	(74)	(6)	(123)	(73)
Total other income (expense), net	(62)	919	583	2,073
Net loss	(36,780)	(17,145)	(91,581)	(50,303)
Comprehensive loss	$ (36,780)	$ (17,145)	$ (91,581)	$ (50,303)
Net loss per share attributable to common stockholders—basic and diluted (Note 10)	$ (1.01)	$ (0.57)	$ (2.59)	$ (1.93)
Weighted-average number of common shares outstanding used in computing net loss per share attributable to common stockholders—basic and diluted	36,444	30,297	35,409	26,019

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]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2018	$ 122,184			$ 2			$ 193,921			$ (71,739)
Beginning balance, shares at Dec. 31, 2018				23,807,000
Stock-based compensation expense	1,455						1,455
Exercise of stock options	252						252
Exercise of stock options, shares				117,000
Vesting of restricted stock awards and units, shares				30,000
Net loss	(17,103)									(17,103)
Ending balance at Mar. 31, 2019	106,788			$ 2			195,628			(88,842)
Ending balance, shares at Mar. 31, 2019				23,954,000
Beginning balance at Dec. 31, 2018	122,184			$ 2			193,921			(71,739)
Beginning balance, shares at Dec. 31, 2018				23,807,000
Net loss	(50,303)
Ending balance at Sep. 30, 2019	206,760			$ 3			328,799			(122,042)
Ending balance, shares at Sep. 30, 2019				31,607,000
Beginning balance at Mar. 31, 2019	106,788			$ 2			195,628			(88,842)
Beginning balance, shares at Mar. 31, 2019				23,954,000
Stock-based compensation expense	1,663						1,663
Exercise of stock options	238						238
Exercise of stock options, shares				110,000
Vesting of restricted stock awards and units, shares				31,000
Net loss	(16,055)									(16,055)
Ending balance at Jun. 30, 2019	92,634			$ 2			197,529			(104,897)
Ending balance, shares at Jun. 30, 2019				24,095,000
Stock-based compensation expense	1,797						1,797
Exercise of stock options	9						9
Exercise of stock options, shares				6,000
Vesting of restricted stock awards and units, shares				31,000
Issuance of common stock, net of offering costs		$ 129,465			$ 1			$ 129,464
Issuance of common stock, net of offering costs, shares					7,475,000
Net loss	(17,145)									(17,145)
Ending balance at Sep. 30, 2019	206,760			$ 3			328,799			(122,042)
Ending balance, shares at Sep. 30, 2019				31,607,000
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	2,822						2,822
Exercise of stock options	35						35
Exercise of stock options, shares				7,000
Vesting of restricted stock awards and units, shares				11,000
Issuance of common stock, net of offering costs		$ 93,628			$ 1			$ 93,627
Issuance of common stock, net of offering costs, shares					4,350,000
Net loss	(25,973)									(25,973)
Ending balance at Mar. 31, 2020	256,525			$ 4			427,198			(170,677)
Ending balance, shares at Mar. 31, 2020				36,011,000
Beginning balance at Dec. 31, 2019	$ 186,013			$ 3			330,714			(144,704)
Beginning balance, shares at Dec. 31, 2019				31,643,000
Exercise of stock options, shares	38,761
Net loss	$ (91,581)
Ending balance at Sep. 30, 2020	207,872			$ 4			444,153			(236,285)
Ending balance, shares at Sep. 30, 2020				36,449,000
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, 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
Stock-based compensation expense	4,805						4,805
Exercise of stock options	60						60
Exercise of stock options, shares				7,000
Vesting of restricted stock awards and units, shares				10,000
Net loss	(36,780)									(36,780)
Ending balance at Sep. 30, 2020	$ 207,872			$ 4			$ 444,153			$ (236,285)
Ending balance, shares at Sep. 30, 2020				36,449,000

Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019
Public Offering [Member]
Issuance of common stock upon public offering, offering costs		$ 419	$ 525
ATM Facility [Member]
Issuance of common stock upon public offering, offering costs	$ 67

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net loss			$ (91,581)	$ (50,303)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	$ 314	$ 215	898	605
Stock-based compensation expense			11,526	4,915
Deferred rent expense			(151)	(123)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets			(782)	(3,616)
Other assets			160	(13)
Accounts payable			(1,263)	(1,127)
Accrued expenses and other current liabilities			12,878	577
Net cash used in operating activities			(68,315)	(49,085)
Cash flows from investing activities:
Purchases of property and equipment			(1,096)	(819)
Net cash used in investing activities			(1,096)	(819)
Cash flows from financing activities:
Proceeds from exercise of stock options			156	499
Net cash provided by financing activities			101,914	129,964
Net increase in cash, cash equivalents and restricted cash			32,503	80,060
Cash, cash equivalents and restricted cash at beginning of period			187,535	126,794
Cash, cash equivalents and restricted cash at end of period	220,038	206,854	220,038	206,854
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses				80
Reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheets:
Cash and cash equivalents	219,546	206,362	219,546	206,362
Restricted cash	492	492	492	492
Cash, cash equivalents and restricted cash at end of period	$ 220,038	$ 206,854	220,038	206,854
Public Offering [Member]
Cash flows from financing activities:
Proceeds from issuance of common shares, net of offering costs			93,628	$ 129,465
ATM Facility [Member]
Cash flows from financing activities:
Proceeds from issuance of common shares, net of offering costs			$ 8,130

Nature of the Business

9 Months Ended

Sep. 30, 2020

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of the Business

1. Nature of the Business

AVROBIO, Inc. (the “Company” or “AVROBIO”) is a clinical-stage gene therapy company focused on developing potentially curative ex vivo lentiviral gene therapies to treat rare diseases following a single dose treatment regimen.

The Company is subject to risks and uncertainties common to 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 $91,581 and $50,303 for the nine months ended September 30, 2020 and 2019, respectively. In addition, as of September 30, 2020, the Company had an accumulated deficit of $236,285. 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 September 30, 2020 of $219,546 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 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

9 Months Ended

Sep. 30, 2020

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

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

The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the audited annual consolidated financial statements as of and for the year ended December 31, 2019, 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 September 30, 2020, and the results of its operations for the three and nine months ended September 30, 2020 and 2019, its statements of stockholders’ equity for the three and nine months ended September 30, 2020 and 2019 and its statement of cash flows for the nine months ended September 30, 2020 and 2019.

The results for the three and nine months ended September 30, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020, 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, 2019, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019, filed with the SEC on March 16, 2020.

The accompanying unaudited condensed consolidated financial statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the unaudited condensed consolidated financial statements. As of September 30, 2020, 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, 2019, except as it related to the adoption of new accounting standards during the first nine months of 2020 as discussed below.

Reclassifications

Certain reclassifications have been made to the prior year condensed consolidated balance sheet to conform to the current year presentation, which reflect a reclassification from other assets to restricted cash for $492 and from other current liabilities to deferred rent for $231. These reclassifications have no impact on the Company’s net loss or cash flows.

Use of Estimates

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

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 initial public offering (“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 Adopted Accounting Pronouncements

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808)—Clarifying the Interaction between Topic 808 and Topic 606, or ASU 2018-18. The amendments in ASU 2018-18 clarify that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606 when the collaborative arrangement participant is a customer in the context of a unit of account. The amendments under ASU 2018-18 are effective for interim and annual fiscal periods beginning after December 15, 2019, with early adoption permitted. The amendments in ASU 2018-18 should be applied retrospectively to the date of initial application of ASC 606. The Company adopted ASU 2018-18 during the quarter ended March 31, 2020. The adoption did not have a material impact on the condensed consolidated financial statements.

In August 2018, the FASB issued ASU No. 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40), or ASU 2018-15. ASU 2018-15 updates guidance regarding accounting for implementation costs associated with a cloud computing arrangement that is a service contract. The amendments under ASU 2018-15 are effective for interim and annual fiscal periods beginning after December 15, 2019, with early adoption permitted. The Company adopted ASU 2018-15 during the quarter ended March 31, 2020. The adoption did not have a material impact on the condensed consolidated financial statements.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, or ASU 2018-13. The amendments in ASU 2018-13 eliminate, add, and modify certain disclosure requirements for fair value measurements. The amendments are effective for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted for either the entire ASU or only the provisions that eliminate or modify requirements. The amendments with respect to changes in unrealized gains and losses, the range and weighted-average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty are to be applied prospectively. All other amendments are to be applied retrospectively to all periods presented. The Company adopted ASU 2018-13 during the quarter ended March 31, 2020. The adoption did not have a material impact on the condensed consolidated financial statements.

Recently Issued Accounting Pronouncements

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

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, or ASU 2016-13. ASU 2016-13 requires that credit losses be reported as an allowance using an expected losses model, representing the entity's current estimate of credit losses expected to be incurred. The accounting guidance currently in effect is based on an incurred loss model. For available-for-sale debt securities with unrealized losses, this standard now requires allowances to be recorded instead of reducing the amortized cost of the investment. ASU 2016-13 is effective for non-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, 2020, and interim periods within fiscal years beginning after December 15, 2021. The Company is currently evaluating ASU 2016-13 and ASU 2019-11 and their impact on its condensed 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. For public companies, the ASU is effective for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years, with early adoption permitted. The Company plans to adopt this guidance on January 1, 2021. The Company is currently evaluating the impact that the adoption of ASU 2019-12 will have on its consolidated financial statements.

Fair Value Measurement

9 Months Ended

Sep. 30, 2020

Fair Value Disclosures [Abstract]

Fair Value Measurement

3. 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 September 30, 2020 and December 31, 2019:

	Fair Value Measurements as of September 30, 2020
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	219,306	$	—	$	—	$	219,306
	$	219,306	$	—	$	—	$	219,306

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

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

During the three and nine months ended September 30, 2020, 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

Prepaid Expenses and Other Current Assets

9 Months Ended

Sep. 30, 2020

Text Block [Abstract]

Prepaid Expenses and Other Current Assets

4. Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:

	September 30, 2020		December 31, 2019
Tax incentive refund	$	2,366	$	1,916
Prepaid research and development costs		3,528		4,915
Prepaid insurance		1,762		897
Prepaid compensation and benefit costs		621		362
Other current assets		1,163		568
	$	9,440	$	8,658

Property and Equipment, Net

9 Months Ended

Sep. 30, 2020

Property Plant And Equipment [Abstract]

Property and Equipment, Net

5. Property and Equipment, Net

Property and equipment, net consisted of the following:

	September 30, 2020			December 31, 2019
Laboratory and office equipment	$	3,923		$	3,456
Leasehold improvements		1,378			1,340
Computer equipment and software		143			134
		5,444			4,930
Less: Accumulated depreciation and amortization		(2,132	)		(1,234	)
	$	3,312		$	3,696

Depreciation and amortization expense was $314 and $215 for the three months ended September 30, 2020 and 2019, respectively and $898 and $605 for the nine months ended September 30, 2020 and 2019.

Accrued Expenses and Other Current Liabilities

9 Months Ended

Sep. 30, 2020

Payables And Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

6. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:

	September 30, 2020		December 31, 2019
Compensation and benefit costs	$	5,761	$	3,028
Research and development costs		7,540		5,794
Consulting and professional fees		1,028		917
License agreement		8,000	—
Other liabilities		196		115
	$	22,525	$	9,854

Stockholders' Equity

9 Months Ended

Sep. 30, 2020

Equity [Abstract]

Stockholders' Equity

7. Stockholders’ Equity

Common Stock

As of September 30, 2020 and December 31, 2019, the authorized capital stock of the Company included 150,000,000 shares of common stock, $0.0001 par value and 10,000,000 shares of undesignated preferred stock. As of September 30, 2020 and December 31, 2019, no undesignated preferred stock was outstanding.

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

As of September 30, 2020, no cash dividends have been declared or paid.

Common Stock Reserved for Future Issuance

As of September 30, 2020 and December 31, 2019, the Company has reserved the following shares of common stock for future issuance:

	September 30, 2020		December 31, 2019
Shares reserved for vesting of restricted stock awards	—			30,252
Shares reserved for exercise of outstanding stock options		5,658,890		3,414,445
Shares reserved for vesting of restricted stock units		1,342		2,300
Shares reserved for issuance under the 2018 Stock Option and Grant Plan		3,541,042		332,513
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		774,088		459,595
Shares reserved for issuance under the 2019 Inducement Plan		831,550		1,800,000
Total shares of authorized common stock reserved for future issuance		10,806,912		6,039,105

Stock-Based Compensation

9 Months Ended

Sep. 30, 2020

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:

	Nine Months Ended September 30,
	2020			2019
Expected option life (years)		6.00			5.97
Risk-free interest rate		0.84	%		2.40	%
Expected volatility		75.85	%		81.25	%
Expected dividend yield		—	%		—	%

The following table summarizes the Company’s stock option activity for the nine months ended September 30, 2020:

	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2019		3,414,445		$	12.08		8.43	$	29,353
Granted		2,485,413		$	19.82
Exercised		(38,761	)	$	4.05
Cancelled or forfeited		(202,207	)	$	20.07
Outstanding as of September 30, 2020		5,658,890		$	15.25		8.43	$	13,481
Exercisable as of September 30, 2020		1,754,340		$	8.92		7.06	$	11,057

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 nine months ended September 30, 2020 and 2019 was $595 and $3,446, respectively.

The weighted-average grant-date fair value of the Company’s stock options granted during the nine months ended September 30, 2020 and 2019 was $12.98 and $11.77, respectively.

Restricted Common Stock

The following table summarizes the Company’s restricted common stock activity for the nine months ended September 30, 2020:

	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2019		32,552		$	1.50
Granted		—
Vested		(31,210	)	$	0.89
Forfeited, cancelled or expired		—
Issued and unvested as of September 30, 2020		1,342		$	15.65

The total fair value of restricted common stock vested during the nine months ended September 30, 2020 and 2019 was $28 and $44, respectively.

Stock-Based Compensation

Stock-based compensation expense was allocated as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Research and development	$	2,686	$	889	$	6,095	$	2,649
General and administrative		2,119		908		5,431		2,265
Total stock-based compensation expense	$	4,805	$	1,797	$	11,526	$	4,914

As of September 30, 2020, total unrecognized compensation cost related to the unvested stock-based awards was $41,143, which is expected to be recognized over a weighted-average period of 3.08 years.

During the three months ended September 30, 2020, the Company modified certain awards in conjunction with an employee’s termination. The modification provided for the extension of the post-employment exercise period. The modifications resulted in approximately $274 of additional research and development expenses during the three and nine months ended September 30, 2020.

License Agreements

9 Months Ended

Sep. 30, 2020

Text Block [Abstract]

License Agreements

9. 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 for the three and nine months ended September 30, 2020.

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

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

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

As of September 30, 2020, the Company recognized no costs related to the CRFA.

Agreements with UHN

Fabry License Agreement—

On January 27, 2016, the Company entered into an agreement with the University Health Network (“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.

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 September 30, 2020 and 2019, the Company recorded research and development expense related to this agreement with UHN of $65 and $415, respectively, which consists of reimbursable funded study trial costs. For the nine months ended September 30, 2020 and 2019, the Company recorded research and development expense related to this agreement with UHN of $174 and $741, respectively, which consists of reimbursable funded study trial costs. No milestone or maintenance fees were incurred related to the Fabry license agreement in the nine months ended September 30, 2020 and 2019.

Interleukin 12 License Agreement—

On January 27, 2016, the Company entered into an exclusive license agreement with UHN, pursuant to which UHN granted the Company a license to certain patent rights for the commercial development, manufacture, distribution and use of any products or processes resulting from development of those patent rights related to Interleukin 12. Upon execution of this agreement, the Company paid an upfront license fee of CAD $264. In addition, as part of the initial consideration for the license, the Company issued to UHN 1,161,665 shares of the Company’s common stock. The fair value of the shares issued to UHN of $480 and the upfront fee was expensed upon the execution of the agreement. In addition, the Company agreed to pay UHN up to $2,000 upon the closing of its IPO if certain criteria were met. This obligation was considered a derivative instrument and was initially recorded at fair value. 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 September 30, 2020 and 2019, the Company did not record research and development expense related to this agreement with UHN. For the nine months ended September 30, 2020 and 2019, the Company recorded research and development expense related to this agreement with UHN of $38 and $38, respectively, which consists of license maintenance fees. No milestone fees were incurred related to the IL-12 license agreement in the nine months ended September 30, 2020 and 2019.

Agreement with BioMarin Pharmaceutical Inc. (“BioMarin”)

On August 31, 2017, the Company entered into a license agreement with BioMarin, pursuant to which BioMarin granted the Company an exclusive worldwide license under certain intellectual property rights owned or controlled by BioMarin to develop, commercialize and sell products for use in the treatment of Pompe disease. As consideration for this agreement, the Company paid an upfront license fee of $500 in cash and issued 233,765 shares of preferred stock to BioMarin in January 2018. The Company is also obligated to make future milestone payments of up to $13,000 upon the achievement of certain specified milestones and agreed to pay BioMarin royalties of a low single-digit percentage of net sales of licensed products sold by the Company or its affiliates covered by patent rights in a relevant country No milestone fees related to the license were recorded for the nine months ended September 30, 2020 and 2019.

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

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

On October 2, 2017, the Company entered into a license agreement with GenStem, pursuant to which GenStem granted the Company an exclusive worldwide license, subject to certain retained rights, under certain intellectual property rights owned or controlled by GenStem to develop, commercialize and sell products for use in the treatment of cystinosis. Under this agreement, the Company paid an upfront license fee of $1,000 and is required to make payments upon completion of certain milestones up to an aggregate of $16,000. In November 2019, the Company made a payment of $2,000 in connection with the dosing of the first patient in the investigator-sponsored Phase 1/2 clinical trial of AVR-RD-04 in cystinosis in the United States. The next anticipated payment under this Agreement is $2,000, which would become due following the dosing of the first patient in the first pivotal clinical trial of AVR-RD-04 in cystinosis in the United States. The Company also agreed to pay GenStem a tiered mid to high single-digit royalty percentage on annual net sales of licensed products as well as a low double-digit percentage of sublicense income received from certain third-party licensees. The Company’s royalty obligation expires on a licensed product-by-licensed product and country-by-country basis on the eleventh anniversary of the first commercial sale of such licensed product in such country or the expiration of the last valid claim under the licensed patent rights covering such licensed product in such country, whichever is later. Unless terminated earlier, the agreement expires upon the expiration of the Company’s royalty obligation for all licensed products throughout the world. GenStem and the Company can terminate the agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. The Company may terminate the agreement at will upon the specified prior written notice to GenStem. No milestone fees related to the license were recorded for the nine months ended September 30, 2020 and 2019.

Agreement with Lund University Rights Holders

On November 17, 2016, the Company entered into a license agreement with affiliates of Lund University, along with certain other relevant rights holders that may be added from time to time, pursuant to which such rights holders granted to the Company an exclusive worldwide license, subject to certain retained rights, under certain intellectual property rights to develop, commercialize and sell products in any and all uses relevant to Gaucher disease. As consideration for the license, the Company is required to make payments in connection with the achievement of certain milestones up to an aggregate of $550. The agreement expires on the latest of (i) the twentieth anniversary of the end of a certain research project the Company is funding pursuant to an agreement with Lund University, (ii) the expiration of the term of any patent filed on the licensed rights that covers a licensed product, (iii) the expiration of any applicable marketing exclusivity right and (iv) such time that neither the Company nor any sublicensees, partners or contractors are commercializing a licensed product. Either the Company or the rights holders acting together may terminate the license agreement if the other such party commits a material breach and fails to cure such breach within a certain period of time, or if the other party enters into liquidation, becomes insolvent, or enters into composition or statutory reorganization proceedings. No milestone fees related to the license were recorded for the nine months ended September 30, 2020 and 2019.

Net Loss per Share Attributable to Common Stockholders

9 Months Ended

Sep. 30, 2020

Earnings Per Share [Abstract]

Net Loss per Share Attributable to Common Stockholders

10. Net Loss per Share Attributable to Common Stockholders

For purposes of the diluted net loss per share calculation, stock options 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 attributable to common stockholders is the same.

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

	Nine Months Ended September 30,
	2020		2019
Options to purchase common stock		5,658,890		3,145,428
Restricted common stock		1,342		63,313

Commitments and Contingencies

9 Months Ended

Sep. 30, 2020

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. Commitments and Contingencies

Lease Agreement

On June 1, 2020, the Company entered into a lease agreement for office space located in Toronto, Ontario, Canada, which is set to expire in June 2025. The annual lease payments are fixed for years 1 and 2, and then subject to a 6.67% increase for years 2 through 5. As a result, the Company’s lease obligations through 2025 increased by CAD $865 during the nine months ended September 30, 2020. In accordance with the lease agreement, the Company is required to maintain a security deposit of CAD $27, which was recorded in other long-term assets as of September 30, 2020.

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 nine months ended September 30, 2020 and 2019 and to the best of its knowledge, no material legal proceedings are currently pending or threatened.

Other

The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at September 30, 2020 and December 31, 2019, or royalties on future sales. No milestone or royalty payments under these agreements are expected to be payable in the immediate future, except as disclosed in Note 9. See Note 9 for discussion of these arrangements.

The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company agrees to indemnify, hold harmless, and to reimburse the indemnified party for losses suffered or incurred by the

indemnified party, generally the Company’s business partners, in connection with any U.S. patent or any copyright or other intellectual property infringement claim by any third-party with respect to the Company’s products. Further, the Company indemnifies its directors and officers who are, or were, serving at the Company’s request in such capacities. The Company’s maximum exposure under these arrangements is unknown at September 30, 2020. The Company does not anticipate recognizing any significant losses relating to these arrangements. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

Related Party Transactions

9 Months Ended

Sep. 30, 2020

Related Party Transactions

12. Related Party Transactions

UHN

For the three months ended September 30, 2020 and 2019, the Company recognized $65 and $415, respectively, of research and development expense related to the license agreements with UHN. For the nine months ended September 30, 2020 and 2019, the Company recognized $212 and $779, respectively, of research and development expense related to the license agreements with UHN (Note 9).

Others

For three months ended September 30, 2020 and 2019, the Company recorded expenses of $355 and $369, 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 nine months ended September 30, 2020 and 2019, the Company recorded expenses of $1,081 and $1,090, 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)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the audited annual consolidated financial statements as of and for the year ended December 31, 2019, 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 September 30, 2020, and the results of its operations for the three and nine months ended September 30, 2020 and 2019, its statements of stockholders’ equity for the three and nine months ended September 30, 2020 and 2019 and its statement of cash flows for the nine months ended September 30, 2020 and 2019. The results for the three and nine months ended September 30, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020, 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, 2019, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019, filed with the SEC on March 16, 2020. The accompanying unaudited condensed consolidated financial statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the unaudited condensed consolidated financial statements. As of September 30, 2020, 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, 2019, except as it related to the adoption of new accounting standards during the first nine months of 2020 as discussed below.
Reclassifications	Reclassifications Certain reclassifications have been made to the prior year condensed consolidated balance sheet to conform to the current year presentation, which reflect a reclassification from other assets to restricted cash for $492 and from other current liabilities to deferred rent for $231. These reclassifications have no impact on the Company’s net loss or cash flows.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, stock-based compensation expense, the valuation of equity and derivative instruments and the recoverability of the Company’s net deferred tax assets and related valuation allowance. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results may differ from these estimates under different assumptions or conditions. Changes in estimates are recorded in the period in which they become known.
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 initial public offering (“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 Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808)—Clarifying the Interaction between Topic 808 and Topic 606, or ASU 2018-18. The amendments in ASU 2018-18 clarify that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606 when the collaborative arrangement participant is a customer in the context of a unit of account. The amendments under ASU 2018-18 are effective for interim and annual fiscal periods beginning after December 15, 2019, with early adoption permitted. The amendments in ASU 2018-18 should be applied retrospectively to the date of initial application of ASC 606. The Company adopted ASU 2018-18 during the quarter ended March 31, 2020. The adoption did not have a material impact on the condensed consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40), or ASU 2018-15. ASU 2018-15 updates guidance regarding accounting for implementation costs associated with a cloud computing arrangement that is a service contract. The amendments under ASU 2018-15 are effective for interim and annual fiscal periods beginning after December 15, 2019, with early adoption permitted. The Company adopted ASU 2018-15 during the quarter ended March 31, 2020. The adoption did not have a material impact on the condensed consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, or ASU 2018-13. The amendments in ASU 2018-13 eliminate, add, and modify certain disclosure requirements for fair value measurements. The amendments are effective for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted for either the entire ASU or only the provisions that eliminate or modify requirements. The amendments with respect to changes in unrealized gains and losses, the range and weighted-average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty are to be applied prospectively. All other amendments are to be applied retrospectively to all periods presented. The Company adopted ASU 2018-13 during the quarter ended March 31, 2020. The adoption did not have a material impact on the condensed consolidated financial statements.
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, 2020, and interim periods within fiscal years beginning after December 15, 2021. The Company is currently evaluating ASU 2016-13 and ASU 2019-11 and their impact on its condensed 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. For public companies, the ASU is effective for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years, with early adoption permitted. The Company plans to adopt this guidance on January 1, 2021. The Company is currently evaluating the impact that the adoption of ASU 2019-12 will have on its consolidated financial statements.

Fair Value Measurement (Tables)

9 Months Ended

Sep. 30, 2020

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured at Fair Value on Recurring Basis

	Fair Value Measurements as of September 30, 2020
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	219,306	$	—	$	—	$	219,306
	$	219,306	$	—	$	—	$	219,306

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

- Definition

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

+ References + Details

Prepaid Expenses and Other Current Assets (Tables)

9 Months Ended

Sep. 30, 2020

Text Block [Abstract]

Summary of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:

	September 30, 2020		December 31, 2019
Tax incentive refund	$	2,366	$	1,916
Prepaid research and development costs		3,528		4,915
Prepaid insurance		1,762		897
Prepaid compensation and benefit costs		621		362
Other current assets		1,163		568
	$	9,440	$	8,658

Property and Equipment, Net (Tables)

9 Months Ended

Sep. 30, 2020

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following:

	September 30, 2020			December 31, 2019
Laboratory and office equipment	$	3,923		$	3,456
Leasehold improvements		1,378			1,340
Computer equipment and software		143			134
		5,444			4,930
Less: Accumulated depreciation and amortization		(2,132	)		(1,234	)
	$	3,312		$	3,696

Accrued Expenses and Other Current Liabilities (Tables)

9 Months Ended

Sep. 30, 2020

Payables And Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:

	September 30, 2020		December 31, 2019
Compensation and benefit costs	$	5,761	$	3,028
Research and development costs		7,540		5,794
Consulting and professional fees		1,028		917
License agreement		8,000	—
Other liabilities		196		115
	$	22,525	$	9,854

Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2020

Equity [Abstract]

Summary of Common Stock Reserved for Future Issuance

As of September 30, 2020 and December 31, 2019, the Company has reserved the following shares of common stock for future issuance:

	September 30, 2020		December 31, 2019
Shares reserved for vesting of restricted stock awards	—			30,252
Shares reserved for exercise of outstanding stock options		5,658,890		3,414,445
Shares reserved for vesting of restricted stock units		1,342		2,300
Shares reserved for issuance under the 2018 Stock Option and Grant Plan		3,541,042		332,513
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		774,088		459,595
Shares reserved for issuance under the 2019 Inducement Plan		831,550		1,800,000
Total shares of authorized common stock reserved for future issuance		10,806,912		6,039,105

- Definition

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

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+ Details

Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

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

	Nine Months Ended September 30,
	2020			2019
Expected option life (years)		6.00			5.97
Risk-free interest rate		0.84	%		2.40	%
Expected volatility		75.85	%		81.25	%
Expected dividend yield		—	%		—	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the nine months ended September 30, 2020:

	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2019		3,414,445		$	12.08		8.43	$	29,353
Granted		2,485,413		$	19.82
Exercised		(38,761	)	$	4.05
Cancelled or forfeited		(202,207	)	$	20.07
Outstanding as of September 30, 2020		5,658,890		$	15.25		8.43	$	13,481
Exercisable as of September 30, 2020		1,754,340		$	8.92		7.06	$	11,057

Summary of Restricted Stock Unit Activity

The following table summarizes the Company’s restricted common stock activity for the nine months ended September 30, 2020:

	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2019		32,552		$	1.50
Granted		—
Vested		(31,210	)	$	0.89
Forfeited, cancelled or expired		—
Issued and unvested as of September 30, 2020		1,342		$	15.65

Schedule of Stock-based Compensation Expense

Stock-based compensation expense was allocated as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Research and development	$	2,686	$	889	$	6,095	$	2,649
General and administrative		2,119		908		5,431		2,265
Total stock-based compensation expense	$	4,805	$	1,797	$	11,526	$	4,914

Net Loss per Share Attributable to Common Stockholders (Tables)

9 Months Ended

Sep. 30, 2020

Earnings Per Share [Abstract]

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

	Nine Months Ended September 30,
	2020		2019
Options to purchase common stock		5,658,890		3,145,428
Restricted common stock		1,342		63,313

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ (36,780)	$ (28,828)	$ (25,973)	$ (17,145)	$ (16,055)	$ (17,103)	$ (91,581)	$ (50,303)
Accumulated deficit	(236,285)						(236,285)		$ (144,704)
Cash and cash equivalents	$ 219,546			$ 206,362			$ 219,546	$ 206,362	$ 187,043

- Definition

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

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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
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Deferred rent		$ 231
ASU No. 2018-18 [Member]
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ASU No. 2018-13 [Member]
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+ Details

Fair Value Measurement - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 219,306	$ 186,797
Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	219,306	186,797
Level 1 [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	219,306	186,797
Level 1 [Member] \| Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 219,306	$ 186,797

Fair Value Measurement - Additional Information (Detail)	Sep. 30, 2020 USD ($)
Fair Value Disclosures [Abstract]
Fair value input transfer between levels	$ 0

Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Prepaid Expense And Other Assets Current [Abstract]
Tax incentive refund	$ 2,366	$ 1,916
Prepaid research and development costs	3,528	4,915
Prepaid insurance	1,762	897
Prepaid compensation and benefit costs	621	362
Other current assets	1,163	568
Total	$ 9,440	$ 8,658

Property and Equipment, Net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 5,444	$ 4,930
Less: Accumulated depreciation and amortization	(2,132)	(1,234)
Property plant and equipment, net	3,312	3,696
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	3,923	3,456
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	1,378	1,340
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 143	$ 134

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 314	$ 215	$ 898	$ 605

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Compensation and benefit costs	$ 5,761	$ 3,028
Research and development costs	7,540	5,794
Consulting and professional fees	1,028	917
License agreement	8,000
Other liabilities	196	115
Accrued expenses	$ 22,525	$ 9,854

Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Equity [Abstract]
Common stock, shares authorized	150,000,000	150,000,000
Common stock, par value	$ 0.0001	$ 0.0001
Undesignated preferred stock	10,000,000	10,000,000
Undesignated preferred stock outstanding	0	0
Cash dividends	$ 0

Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Sep. 30, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	10,806,912	6,039,105
Restricted Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance		30,252
Employee Stock Option [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	5,658,890	3,414,445
Restricted Stock Units [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	1,342	2,300
2018 Stock Option and Incentive Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	3,541,042	332,513
2018 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	774,088	459,595
2019 Inducement Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	831,550	1,800,000

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)	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Abstract]
Expected option life (years)	6 years	5 years 11 months 19 days
Risk-free interest rate	0.84%	2.40%
Expected volatility	75.85%	81.25%

Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of options, Outstanding beginning balance \| shares	3,414,445
Number of options, Granted \| shares	2,485,413
Number of options, Exercised \| shares	(38,761)
Number of options, Cancelled or forfeited \| shares	(202,207)
Number of options, Outstanding ending balance \| shares	5,658,890	3,414,445
Number of options, Exercisable \| shares	1,754,340
Weighted average exercise price, Outstanding beginning balance \| $ / shares	$ 12.08
Weighted average exercise price, Granted \| $ / shares	19.82
Weighted average exercise price, Exercised \| $ / shares	4.05
Weighted average exercise price, Cancelled or forfeited \| $ / shares	20.07
Weighted average exercise price, Outstanding ending balance \| $ / shares	15.25	$ 12.08
Weighted average exercise price, Exercisable \| $ / shares	$ 8.92
Weighted average remaining contractual term, Outstanding balance	8 years 5 months 4 days	8 years 5 months 4 days
Weighted average remaining contractual term, Exercisable	7 years 21 days
Aggregate intrinsic value, Outstanding balance \| $	$ 13,481	$ 29,353
Aggregate intrinsic value, Exercisable \| $	$ 11,057

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Aggregate intrinsic value of options exercised		$ 595	$ 3,446
Options Granted, Weighted-average grant-date fair value		$ 12.98	$ 11.77
Total fair value of restricted common stock vested		$ 28	$ 44
Unrecognized stock-based compensation expenses	$ 41,143	$ 41,143
Unrecognized stock-based compensation expense, period for recognition		3 years 29 days
Additional research and development expenses	$ 274	$ 274

Stock-Based Compensation - Summary of Restricted Stock Unit Activity (Detail)	9 Months Ended
	Sep. 30, 2020 $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of shares, Issued and unvested beginning balance \| shares	32,552
Number of shares, Vested \| shares	(31,210)
Number of shares, Issued and unvested ending balance \| shares	1,342
Weighted average grant date fair value, Issued and unvested, beginning balance \| $ / shares	$ 1.50
Weighted average grant date fair value, Vested \| $ / shares	0.89
Weighted average grant date fair value, Issued and unvested, ending balance \| $ / shares	$ 15.65

Stock-Based Compensation - Schedule of Stock-based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 4,805	$ 1,797	$ 11,526	$ 4,914
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	2,686	889	6,095	2,649
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,119	$ 908	$ 5,431	$ 2,265

License Agreements - Additional Information (Detail) $ in Thousands					1 Months Ended		3 Months Ended		9 Months Ended
	Oct. 02, 2017 USD ($)	Nov. 17, 2016 USD ($)	Jan. 27, 2016 USD ($)	Jan. 27, 2016 CAD ($) shares	Nov. 30, 2019 USD ($)	Aug. 31, 2017 USD ($) shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2020 GBP (£)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 shares
License Agreement [Line Items]
Accrued upfront license							$ 0		$ 0
Common stock, shares issued \| shares							36,449,000		36,449,000			31,643,000
Research and development							$ 28,509,000	$ 13,042,000	$ 67,649,000		$ 37,755,000
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		$ 8,000
Milestone payments									80,000
Milestone payment paid									2,000
Milestone payment due									$ 4,000
Agreement Expiration Period									15 years	15 years
Agreement Expiration Description									upon the later of 15 years from the effective date or the expiration of the last valid claim of the licensed patents	upon the later of 15 years from the effective date or the expiration of the last valid claim of the licensed patents
Research and development expense \| £										£ 9,100
UHN Agreement [Member]
License Agreement [Line Items]
Research and development expense							65,000	415,000	$ 174,000		741,000
Milestone fees									0		0
Annual maintenance fees									0		0
UHN Agreement [Member] \| Fabry License Agreement [Member]
License Agreement [Line Items]
Upfront license fee				$ 75
Milestone payments				2,450
Option fee				20
UHN Agreement [Member] \| Interleukin 12 License Agreement [Member]
License Agreement [Line Items]
Upfront license fee				264
Milestone payments				19,275
Milestone fees									0		0
Annual maintenance fees				$ 50
Common stock, shares issued \| shares				1,161,665,000
Fair value of shares issued			$ 480,000
Payments upon closing of an initial public offering			$ 2,000,000
Research and development							$ 0	$ 0	38,000		38,000
BioMarin Pharmaceutical Inc [Member]
License Agreement [Line Items]
Upfront license fee						$ 500,000
Milestone payments									13,000,000
Preferred stock issued \| shares						233,765,000
Expenses related to license									0		0
GenStem Therapeutics Inc [Member]
License Agreement [Line Items]
Upfront license fee	$ 1,000,000
Milestone payment paid					$ 2,000,000
Milestone payment due	2,000,000
Expenses related to license									0		0
Milestone payments	$ 16,000,000
Lund University Rights Holders Agreement [Member]
License Agreement [Line Items]
Expenses related to license									$ 0		$ 0
Milestone payments		$ 550,000

Net Loss per Share Attributable to Common Stockholders - Schedule of Dilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Detail) - shares	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	5,658,890	3,145,428
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	1,342	63,313

Commitments and Contingencies - Additional Information (Detail) - CAD ($) $ in Thousands		9 Months Ended
	Jun. 01, 2020	Sep. 30, 2020
Commitments And Contingencies Disclosure [Line Items]
Lease agreement expiration month and year	2025-06
Annual lease payments, description		The annual lease payments are fixed for years 1 and 2, and then subject to a 6.67% increase for years 2 through 5
Percentage of annual increase in rent	6.67%
Increase in lease obligations		$ 865
Other Long-term Assets [Member]
Commitments And Contingencies Disclosure [Line Items]
Security deposit in connection with lease		$ 27

Related Party Transactions - Additional information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Related Party Transaction [Line Items]
Research and development expense	$ 28,509	$ 13,042	$ 67,649	$ 37,755
License Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense	65	415	212	779
Sub-lease Agreement [Member] \| Officers and Board Members [Member]
Related Party Transaction [Line Items]
Expense related to related party	$ 355	$ 369	$ 1,081	$ 1,090