0001564590-20-022815 | 10-Q | XBRL Viewer

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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2020	Apr. 30, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Trading Symbol	AVRO
Entity Registrant Name	AVROBIO, INC.
Entity Central Index Key	0001681087
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		36,035,459
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	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 257,695	$ 187,043
Prepaid expenses and other current assets	7,652	8,658
Total current assets	265,347	195,701
Property and equipment, net	3,407	3,696
Other assets	974	1,117
Total assets	269,728	200,514
Current liabilities:
Accounts payable	2,135	3,949
Accrued expenses and other current liabilities	10,638	10,068
Total current liabilities	12,773	14,017
Deferred rent, net of current portion	430	484
Total liabilities	13,203	14,501
Commitments and contingencies (Note 11)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized and no shares issued or outstanding as of March 31, 2020 and December 31, 2019
Common stock, $0.0001 par value; 150,000,000 shares authorized as of March 31, 2020 and December 31, 2019; 36,031,166 and 31,673,058 shares issued as of March 31, 2020 and December 31, 2019, respectively; 36,010,998 and 31,642,806 shares outstanding as of March 31, 2020 and December 31, 2019, respectively	4	3
Additional paid-in capital	427,198	330,714
Accumulated deficit	(170,677)	(144,704)
Total stockholders’ equity	256,525	186,013
Total liabilities and stockholders’ equity	$ 269,728	$ 200,514

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, authorized	10,000,000	10,000,000
Preferred stock, issued	0	0
Preferred stock, outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, authorized	150,000,000	150,000,000
Common stock, issued	36,031,166	31,673,058
Common stock, outstanding	36,010,998	31,642,806

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating expenses:
Research and development	$ 18,274	$ 12,446
General and administrative	8,315	5,254
Total operating expenses	26,589	17,700
Loss from operations	(26,589)	(17,700)
Other income (expense):
Interest income	632	657
Other expense	(16)	(60)
Total other income (expense), net	616	597
Net loss	(25,973)	(17,103)
Comprehensive loss	$ (25,973)	$ (17,103)
Net loss per share attributable to common stockholders—basic and diluted (Note 10)	$ (0.77)	$ (0.72)
Weighted-average number of common shares used in computing net loss per share attributable to common stockholders—basic and diluted	33,666,801	23,893,696

Condensed Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2018	$ 122,184	$ 2	$ 193,921	$ (71,739)
Beginning balance, shares at Dec. 31, 2018		23,806,628
Stock-based compensation expense	1,455		1,455
Exercise of stock options	252		252
Exercise of stock options, shares		116,859
Vesting of restricted stock awards and units, shares		30,753
Net loss	(17,103)			(17,103)
Ending balance at Mar. 31, 2019	106,788	$ 2	195,628	(88,842)
Ending balance, shares at Mar. 31, 2019		23,954,241
Beginning balance at Dec. 31, 2019	186,013	$ 3	330,714	(144,704)
Beginning balance, shares at Dec. 31, 2019		31,642,806
Stock-based compensation expense	2,822		2,822
Exercise of stock options	$ 35		35
Exercise of stock options, shares	7,438	7,438
Vesting of restricted stock awards and units, shares		10,754
Issuance of common stock upon public offering, net of offering costs	$ 93,628	$ 1	93,627
Issuance of common stock upon public offering, 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		36,010,998

- Definition

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

+ References + Details

Condensed Consolidated Statements of Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Statement Of Stockholders Equity [Abstract]
Issuance of common stock upon public offering, offering costs	$ 419

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (25,973)	$ (17,103)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	283	190
Stock-based compensation expense	2,822	1,455
Deferred rent expense	(53)	(40)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	976	(1,485)
Other assets	319
Accounts payable	(1,443)	671
Accrued expenses and other current liabilities	635	(1,426)
Net cash used in operating activities	(22,434)	(17,738)
Cash flows from investing activities:
Purchases of property and equipment	(577)	(340)
Net cash used in investing activities	(577)	(340)
Cash flows from financing activities:
Proceeds from exercise of stock options	35	252
Proceeds from issuance of common shares upon completion of public offering, net of offering costs	93,628
Net cash provided by financing activities	93,663	252
Net increase (decrease) in cash, cash equivalents and restricted cash	70,652	(17,826)
Cash, cash equivalents and restricted cash at beginning of period	187,535	126,794
Cash, cash equivalents and restricted cash at end of period	258,187	108,968
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment held for sale		336
Offering costs included in accrued expenses and accounts payable	249
Reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheets:
Cash and cash equivalents	257,695	108,476
Long-term restricted cash (included in other assets)	492	492
Cash, cash equivalents and restricted cash at end of period	$ 258,187	$ 108,968

Nature of the Business

3 Months Ended

Mar. 31, 2020

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of the Business

1. Nature of the Business

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

The Company is subject to risks and uncertainties common to 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 $25,973 and $17,103 for the three months ended March 31, 2020 and 2019, respectively. In addition, as of March 31, 2020, the Company had an accumulated deficit of $170,677. 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 March 31, 2020 of $257,695 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

3 Months Ended

Mar. 31, 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 March 31, 2020, and the results of its operations for the three months ended March 31, 2020 and 2019, its statements of stockholders’ equity for the three months ended March 31, 2020 and 2019 and its statement of cash flows for the three months ended March 31, 2020 and 2019.

The results for the three months ended March 31, 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 March 31, 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 three months of 2020 as discussed below.

Use of Estimates

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

Emerging Growth Company Status

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

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. The guidance is effective for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years, with early adoption permitted. The Company is currently evaluating the impact that the adoption of ASU 2019-12 will have on its consolidated financial statements.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808)—Clarifying the Interaction between Topic 808 and Topic 606, 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.

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.

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 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, 2021, 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.

Fair Value of Financial Assets and Liabilities

3 Months Ended

Mar. 31, 2020

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

3. Fair Value of Financial Assets and Liabilities

The following table presents information about the Company’s financial assets measured at fair value on a recurring basis and indicates the level of the fair value hierarchy utilized to determine such fair values as of March 31, 2020 and December 31, 2019:

	Fair Value Measurements as of March 31, 2020 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	253,588	$	—	$	—	$	253,588
Restricted cash		492		—		—		492
	$	254,080	$	—	$	—	$	254,080

	Fair Value Measurements as of December 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	186,797	$	—	$	—	$	186,797
Restricted cash		492		—		—		492
	$	187,289	$	—	$	—	$	187,289

There were no transfers within the hierarchy during the three months ended March 31, 2020 or the year ended December 31, 2019.

- Definition

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

+ References + Details

Prepaid Expenses and Other Current Assets

3 Months Ended

Mar. 31, 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:

	March 31, 2020		December 31, 2019
Tax incentive refund	$	2,034	$	1,916
Prepaid research and development costs		4,006		4,915
Prepaid insurance		410		897
Interest income receivable		149		224
Prepaid rent		87		86
Other current assets		966		620
	$	7,652	$	8,658

Property and Equipment, Net

3 Months Ended

Mar. 31, 2020

Property Plant And Equipment [Abstract]

Property and Equipment, Net

5. Property and Equipment, Net

Property and equipment, net consisted of the following:

	March 31, 2020			December 31, 2019
Laboratory and office equipment	$	3,450		$	3,456
Leasehold improvements		1,340			1,340
Computer equipment and software		134			134
		4,924			4,930
Less: Accumulated depreciation and amortization		(1,517	)		(1,234	)
	$	3,407		$	3,696

Depreciation and amortization expense for the three months ended March 31, 2020 and 2019 was $283 and $190, respectively.

Accrued Expenses and Other Current Liabilities

3 Months Ended

Mar. 31, 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:

	March 31, 2020		December 31, 2019
Compensation and benefit costs	$	1,931	$	3,028
Research and development costs		6,873		5,794
Consulting and professional fees		974		917
Other liabilities		860		329
	$	10,638	$	10,068

Stockholders' Equity

3 Months Ended

Mar. 31, 2020

Equity [Abstract]

Stockholders' Equity

7. Stockholders’ Equity

Common Stock

As of March 31, 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 March 31, 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.

Through March 31, 2020, no cash dividends have been declared or paid.

Common Stock Reserved for Future Issuance

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

	March 31, 2020		December 31, 2019
Shares reserved for vesting of restricted stock awards		20,168		30,252
Shares reserved for exercise of outstanding stock options		5,310,482		3,414,445
Shares reserved for vesting of restricted stock units		1,630		2,300
Shares reserved for issuance under the 2018 Stock Option and Grant Plan		202,260		332,513
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		776,325		459,595
Shares reserved for issuance under the 2019 Inducement Plan		1,296,000		1,800,000
Total shares of authorized common stock reserved for future issuance		7,606,865		6,039,105

Stock-Based Compensation

3 Months Ended

Mar. 31, 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:

	Three Months Ended March 31,
	2020			2019
Expected option life (years)		6.00			6.00
Risk-free interest rate		0.96	%		2.53	%
Expected volatility		75.49	%		80.57	%
Expected dividend yield		—	%		—	%

The following table summarizes the Company’s stock option activity for the three months ended March 31, 2020:

	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2019		3,414,445		$	12.08		8.43	$	29,353
Granted		1,932,713
Exercised		(7,438	)
Cancelled or forfeited		(29,238	)
Outstanding as of March 31, 2020		5,310,482		$	15.25		8.83	$	17,718
Exercisable as of March 31, 2020		1,319,413		$	7.66		7.45	$	11,932

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 three months ended March 31, 2020 and 2019 was $133 and $1,363, respectively.

The weighted-average grant-date fair value of the Company’s stock options granted during the three months ended March 31, 2020 and 2019 was $13.68 and $11.41, respectively.

Restricted Common Stock

The following table summarizes the Company’s restricted common stock activity for the three months ended March 31, 2020:

	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2019		32,552		$	1.50
Granted		—
Vested		(10,754	)	$	1.37
Forfeited, canceled or expired		—
Issued and unvested as of March 31, 2020		21,798		$	1.56

The total fair value of restricted common stock vested during the three months ended March 31, 2020 and 2019 was $15 and $13, respectively.

Stock-Based Compensation

Stock-based compensation expense was allocated as follows:

	Three Months Ended March 31,
	2020		2019
Research and development	$	1,443	$	860
General and administrative		1,379		595
Total stock-based compensation expense	$	2,822	$	1,455

As of March 31, 2020, total unrecognized compensation cost related to the unvested stock-based awards was $46,313, which is expected to be recognized over a weighted-average period of 3.46 years.

License Agreements

3 Months Ended

Mar. 31, 2020

Text Block [Abstract]

License Agreements

9. License Agreements

Agreements with UHN

Fabry License Agreement—

On January 27, 2016, the Company entered into an agreement with UHN, pursuant to which UHN granted the Company an option to enter into an exclusive license under the UHN intellectual property related to Fabry disease in accordance with the pre-negotiated licensing terms. On November 4, 2016, the Company exercised its option and entered into a license agreement with UHN, pursuant to which UHN granted the Company an exclusive worldwide license under certain intellectual property rights and a non-exclusive worldwide license under certain know-how, in each case subject to certain retained rights, to develop, commercialize and sell products for use in the treatment of Fabry disease.

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 March 31, 2020 and 2019, the Company recorded research and development expense related to this agreement with UHN of $0 and $194, respectively, which consists of reimbursable funded study trial costs. No milestone or maintenance fees were incurred related to the Fabry license agreement in the three months ended March 31, 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 March 31, 2020 and 2019, the Company recorded research and development expense related to this agreement with UHN of $35 and $38, respectively, which consists of license maintenance fees. No milestone fees were incurred related to the IL-12 license agreement in the three months ended March 31, 2020 and 2019.

Agreement with BioMarin Pharmaceutical Inc. (“BioMarin”)

On August 31, 2017, the Company entered into a license agreement with BioMarin, pursuant to which BioMarin granted the Company an exclusive worldwide license under certain intellectual property rights owned or controlled by BioMarin to develop, commercialize and sell products for use in the treatment of Pompe disease. 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 three months ended March 31, 2020 and 2019.

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

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

On October 2, 2017, the Company entered into a license agreement with GenStem, pursuant to which GenStem granted the Company an exclusive worldwide license, subject to certain retained rights, under certain intellectual property rights owned or controlled by GenStem to develop, commercialize and sell products for use in the treatment of cystinosis. Under this agreement, the Company paid an upfront license fee of $1,000 and is required to make payments upon completion of certain milestones up to an aggregate of $16,000. 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 three months ended March 31, 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 three months ended March 31, 2020 and 2019.

Net Loss per Share Attributable to Common Stockholders

3 Months Ended

Mar. 31, 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:

	Three Months Ended March 31,
	2020		2019
Options to purchase common stock		5,310,482		2,986,048
Restricted common stock		21,798		124,821

Commitments and Contingencies

3 Months Ended

Mar. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. Commitments and Contingencies

Lease Agreement

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

On August 31, 2018, the Company entered into a sub-lease agreement for lab space located in Cambridge, Massachusetts, United States, which expires in October 2020. The annual lease payments are subject to a 3% increase each year. In accordance with the lease agreement, the Company is required to maintain a security deposit of $283, which was recorded in other assets as of December 31, 2019 and March 31, 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 three months ended March 31, 2020 and 2019 and to the best of its knowledge, no material legal proceedings are currently pending or threatened.

Other

The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at March 31, 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 March 31, 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

3 Months Ended

Mar. 31, 2020

Related Party Transactions

12. Related Party Transactions

UHN

In connection with the Company’s entry into a license agreement with UHN on January 27, 2016, the Company issued UHN 1,161,665 shares of its common stock. Upon the closing of the IPO, as UHN’s fully-diluted percentage ownership of the Company was reduced within a range of specified percentages, the Company was obligated to pay UHN an amount up to $2,000, which was paid in July 2018.

For the three months ended March 31, 2020 and 2019, the Company recognized $35 and $232, respectively, of research and development expense related to the license agreements with UHN (Note 9).

Others

For the three months ended March 31, 2020 and 2019, the Company recorded expenses of $360 and $356, 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)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 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 March 31, 2020, and the results of its operations for the three months ended March 31, 2020 and 2019, its statements of stockholders’ equity for the three months ended March 31, 2020 and 2019 and its statement of cash flows for the three months ended March 31, 2020 and 2019. The results for the three months ended March 31, 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 March 31, 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 three months of 2020 as discussed below.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, stock-based compensation expense, the valuation of equity and derivative instruments and the recoverability of the Company’s net deferred tax assets and related valuation allowance. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ materially from those estimates.
Emerging Growth Company Status	Emerging Growth Company Status The Company is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act, or JOBS Act, and may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. The Company may take advantage of these exemptions until the Company is no longer an “emerging growth company.” Section 107 of the JOBS Act provides that an “emerging growth company” can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected to use the extended transition period for complying with new or revised accounting standards and as a result of this election, its consolidated financial statements may not be comparable to companies that comply with public company effective dates. The Company may take advantage of these exemptions up until the last day of the fiscal year following the fifth anniversary of 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 Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements 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. The guidance is effective for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years, with early adoption permitted. The Company is currently evaluating the impact that the adoption of ASU 2019-12 will have on its consolidated financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808)—Clarifying the Interaction between Topic 808 and Topic 606, 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. 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. 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 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, 2021, 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. .

Fair Value of Financial Assets and Liabilities (Tables)

3 Months Ended

Mar. 31, 2020

Fair Value Disclosures [Abstract]

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

	Fair Value Measurements as of March 31, 2020 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	253,588	$	—	$	—	$	253,588
Restricted cash		492		—		—		492
	$	254,080	$	—	$	—	$	254,080

	Fair Value Measurements as of December 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	186,797	$	—	$	—	$	186,797
Restricted cash		492		—		—		492
	$	187,289	$	—	$	—	$	187,289

- Definition

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

+ References + Details

Prepaid Expenses and Other Current Assets (Tables)

3 Months Ended

Mar. 31, 2020

Text Block [Abstract]

Summary of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:

	March 31, 2020		December 31, 2019
Tax incentive refund	$	2,034	$	1,916
Prepaid research and development costs		4,006		4,915
Prepaid insurance		410		897
Interest income receivable		149		224
Prepaid rent		87		86
Other current assets		966		620
	$	7,652	$	8,658

Property and Equipment, Net (Tables)

3 Months Ended

Mar. 31, 2020

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following:

	March 31, 2020			December 31, 2019
Laboratory and office equipment	$	3,450		$	3,456
Leasehold improvements		1,340			1,340
Computer equipment and software		134			134
		4,924			4,930
Less: Accumulated depreciation and amortization		(1,517	)		(1,234	)
	$	3,407		$	3,696

Accrued Expenses and Other Current Liabilities (Tables)

3 Months Ended

Mar. 31, 2020

Payables And Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:

	March 31, 2020		December 31, 2019
Compensation and benefit costs	$	1,931	$	3,028
Research and development costs		6,873		5,794
Consulting and professional fees		974		917
Other liabilities		860		329
	$	10,638	$	10,068

Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2020

Equity [Abstract]

Summary of Common Stock Reserved for Future Issuance

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

	March 31, 2020		December 31, 2019
Shares reserved for vesting of restricted stock awards		20,168		30,252
Shares reserved for exercise of outstanding stock options		5,310,482		3,414,445
Shares reserved for vesting of restricted stock units		1,630		2,300
Shares reserved for issuance under the 2018 Stock Option and Grant Plan		202,260		332,513
Shares reserved for issuance under the 2018 Employee Stock Purchase Plan		776,325		459,595
Shares reserved for issuance under the 2019 Inducement Plan		1,296,000		1,800,000
Total shares of authorized common stock reserved for future issuance		7,606,865		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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-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

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

	Three Months Ended March 31,
	2020			2019
Expected option life (years)		6.00			6.00
Risk-free interest rate		0.96	%		2.53	%
Expected volatility		75.49	%		80.57	%
Expected dividend yield		—	%		—	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the three months ended March 31, 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		1,932,713
Exercised		(7,438	)
Cancelled or forfeited		(29,238	)
Outstanding as of March 31, 2020		5,310,482		$	15.25		8.83	$	17,718
Exercisable as of March 31, 2020		1,319,413		$	7.66		7.45	$	11,932

Summary of Restricted Stock Unit Activity

The following table summarizes the Company’s restricted common stock activity for the three months ended March 31, 2020:

	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2019		32,552		$	1.50
Granted		—
Vested		(10,754	)	$	1.37
Forfeited, canceled or expired		—
Issued and unvested as of March 31, 2020		21,798		$	1.56

Schedule of Stock-based Compensation Expense

Stock-based compensation expense was allocated as follows:

	Three Months Ended March 31,
	2020		2019
Research and development	$	1,443	$	860
General and administrative		1,379		595
Total stock-based compensation expense	$	2,822	$	1,455

Net Loss per Share Attributable to Common Stockholders (Tables)

3 Months Ended

Mar. 31, 2020

Earnings Per Share [Abstract]

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

	Three Months Ended March 31,
	2020		2019
Options to purchase common stock		5,310,482		2,986,048
Restricted common stock		21,798		124,821

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ (25,973)	$ (17,103)
Accumulated deficit	(170,677)		$ (144,704)
Cash and cash equivalents	$ 257,695	$ 108,476	$ 187,043

- Definition

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

+ References + Details

Fair Value of Financial Assets and Liabilities - Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 254,080	$ 187,289
Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	253,588	186,797
Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	492	492
Level 1 [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	254,080	187,289
Level 1 [Member] \| Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	253,588	186,797
Level 1 [Member] \| Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 492	$ 492

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

Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Prepaid Expense And Other Assets Current [Abstract]
Tax incentive refund	$ 2,034	$ 1,916
Prepaid research and development costs	4,006	4,915
Prepaid insurance	410	897
Interest income receivable	149	224
Prepaid rent	87	86
Other current assets	966	620
Total	$ 7,652	$ 8,658

Property and Equipment, Net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 4,924	$ 4,930
Less: Accumulated depreciation and amortization	(1,517)	(1,234)
Property plant and equipment, net	3,407	3,696
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	3,450	3,456
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	1,340	1,340
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 134	$ 134

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 283	$ 190

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Compensation and benefit costs	$ 1,931	$ 3,028
Research and development costs	6,873	5,794
Consulting and professional fees	974	917
Other liabilities	860	329
Accrued expenses	$ 10,638	$ 10,068

Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 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	Mar. 31, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	7,606,865	6,039,105
Restricted Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	20,168	30,252
Employee Stock Option [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	5,310,482	3,414,445
Restricted Stock Units [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	1,630	2,300
2018 Stock Option and Incentive Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	202,260	332,513
2018 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	776,325	459,595
2019 Inducement Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	1,296,000	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)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Abstract]
Expected option life (years)	6 years	6 years
Risk-free interest rate	0.96%	2.53%
Expected volatility	75.49%	80.57%

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of options, Outstanding beginning balance	3,414,445
Number of options, Granted	1,932,713
Number of options, Exercised	(7,438)
Number of options, Cancelled or forfeited	(29,238)
Number of options, Outstanding ending balance	5,310,482	3,414,445
Number of options, Exercisable	1,319,413
Weighted average exercise price, Outstanding beginning balance	$ 12.08
Weighted average exercise price, Outstanding ending balance	15.25	$ 12.08
Weighted average exercise price, Exercisable	$ 7.66
Weighted average remaining contractual term, Outstanding balance	8 years 9 months 29 days	8 years 5 months 4 days
Weighted average remaining contractual term, Exercisable	7 years 5 months 12 days
Aggregate intrinsic value, Outstanding balance	$ 17,718	$ 29,353
Aggregate intrinsic value, Exercisable	$ 11,932

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Aggregate intrinsic value of options exercised	$ 133	$ 1,363
Options Granted, Weighted-average grant-date fair value	$ 13.68	$ 11.41
Total fair value of restricted common stock vested	$ 15	$ 13
Unrecognized stock-based compensation expenses	$ 46,313
Unrecognized stock-based compensation expense, period for recognition	3 years 5 months 15 days

Stock-Based Compensation - Summary of Restricted Stock Unit Activity (Detail)	3 Months Ended
	Mar. 31, 2020 $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of shares, Issued and unvested beginning balance \| shares	32,552
Number of shares, Vested \| shares	(10,754)
Number of shares, Issued and unvested ending balance \| shares	21,798
Weighted average grant date fair value, Issued and unvested, beginning balance \| $ / shares	$ 1.50
Weighted average grant date fair value, Vested \| $ / shares	1.37
Weighted average grant date fair value, Issued and unvested, ending balance \| $ / shares	$ 1.56

Stock-Based Compensation - Schedule of Stock-based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,822	$ 1,455
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	1,443	860
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,379	$ 595

License Agreements - Additional Information (Detail) $ in Thousands					1 Months Ended		3 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	Mar. 31, 2020 USD ($) shares	Mar. 31, 2019 USD ($)	Dec. 31, 2019 shares
License Agreement [Line Items]
Common stock, shares issued \| shares							36,031,166		31,673,058
Fair value of shares issued							$ 93,628,000
Research and development							$ 18,274,000	$ 12,446,000
Preferred stock issued \| shares							0		0
UHN Agreement [Member]
License Agreement [Line Items]
Milestone fees							$ 0	0
Annual maintenance fees							0	0
Research and development expense							0	194,000
UHN Agreement [Member] \| Fabry License Agreement [Member]
License Agreement [Line Items]
Option fee				$ 20
Upfront license fee				75
Milestone payments				2,450
UHN Agreement [Member] \| Interleukin 12 License Agreement [Member]
License Agreement [Line Items]
Upfront license fee				264
Milestone payments				19,275
Milestone fees							0	0
Annual maintenance fees				$ 50
Common stock, shares issued \| shares				1,161,665
Fair value of shares issued			$ 480
Payments upon closing of an initial public offering			$ 2,000,000
Research and development							35,000	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
Expenses related to license							0	0
GenStem Therapeutics Inc [Member]
License Agreement [Line Items]
Upfront license fee	$ 1,000,000
Expenses related to license							0	0
Milestone payments	16,000,000
Milestone payment due	$ 2,000,000
Milestone payment paid					$ 2,000,000
Lund University Rights Holders Agreement [Member]
License Agreement [Line Items]
Expenses related to license							$ 0	$ 0
Milestone payments		$ 550

Net Loss per Share Attributable to Common Stockholders - Schedule of Dilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Detail) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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	5,310,482	2,986,048
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	21,798	124,821

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

Related Party Transactions - Additional information (Detail) - USD ($)		1 Months Ended	3 Months Ended
	Jan. 27, 2016	Jul. 31, 2018	Mar. 31, 2020	Mar. 31, 2019
Related Party Transaction [Line Items]
Research and development expense			$ 18,274,000	$ 12,446,000
License Agreement [Member]
Related Party Transaction [Line Items]
Provision for maximum cash payment		$ 2,000,000
Research and development expense			35,000	232,000
License Agreement [Member] \| Stock Purchase Agreement [Member]
Related Party Transaction [Line Items]
Common stock issued for initial consideration for license	1,161,665
Sub-lease Agreement [Member] \| Officers and Board Members [Member]
Related Party Transaction [Line Items]
Expense related to related party			$ 360,000	$ 356,000