Tectonic Therapeutic (TECX) 10-Q 7 Nov 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Nov. 05, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
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	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		31,667,901
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

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 206,362	$ 126,302
Prepaid expenses and other current assets	7,345	3,718
Total current assets	213,707	130,020
Property and equipment, net	2,673	2,634
Other assets	825	825
Total assets	217,205	133,479
Current liabilities:
Accounts payable	1,408	2,784
Accrued expenses and other current liabilities	8,502	7,822
Total current liabilities	9,910	10,606
Deferred rent, net of current portion	535	689
Total liabilities	10,445	11,295
Commitments and contingencies (Note 11)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized and no shares issued or outstanding as of September 30, 2019 and December 31, 2018
Common stock, $0.0001 par value; 150,000,000 shares authorized as of September 30, 2019 and December 31, 2018; 31,667,661 and 23,959,903 shares issued as of September 30, 2019 and December 31, 2018, respectively; 31,606,648 and 23,806,628 shares outstanding as of September 30, 2019 and December 31, 2018, respectively	3	2
Additional paid-in capital	328,799	193,921
Accumulated deficit	(122,042)	(71,739)
Total stockholders’ equity	206,760	122,184
Total liabilities and stockholders’ equity	$ 217,205	$ 133,479

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, authorized	10,000,000	10,000,000
Preferred stock, issued	0	0
Preferred stock, outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, authorized	150,000,000	150,000,000
Common stock, issued	31,667,661	23,959,903
Common stock, outstanding	31,606,648	23,806,628

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating expenses:
Research and development	$ 13,042	$ 9,232	$ 37,755	$ 22,286
General and administrative	5,022	3,000	14,621	7,281
Total operating expenses	18,064	12,232	52,376	29,567
Loss from operations	(18,064)	(12,232)	(52,376)	(29,567)
Other income (expense):
Interest income	925	646	2,146	1,038
Change in fair value of preferred stock warrant liability	0	0	0	(162)
Change in fair value of derivative liability	0	0	0	(1,629)
Other expense	(6)	(5)	(73)	(20)
Total other income (expense), net	919	641	2,073	(773)
Net loss	(17,145)	(11,591)	(50,303)	(30,340)
Comprehensive loss	(17,145)	(11,591)	(50,303)	(30,340)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(17,145)	(11,591)	(50,303)	(30,340)
Accretion of issuance costs on redeemable convertible preferred stock				(2,243)
Net loss attributable to common stockholders—basic and diluted	$ (17,145)	$ (11,591)	$ (50,303)	$ (32,583)
Net loss per share attributable to common stockholders—basic and diluted (Note 10)	$ (0.57)	$ (0.49)	$ (1.93)	$ (3.28)
Weighted-average number of common shares used in computing net loss per share attributable to common stockholders— basic and diluted	30,296,595	23,747,141	26,018,717	9,945,538

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) - USD ($) $ in Thousands	Total	July 2019 Follow-on Offering [Member]	IPO [Member]	Series Seed Redeemable Convertible Preferred Stock [Member]	Series A Redeemable Convertible Preferred Stock [Member]	Series B Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member]July 2019 Follow-on Offering [Member]	Common Stock [Member]IPO [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]July 2019 Follow-on Offering [Member]	Additional Paid-in Capital [Member]IPO [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017	$ (23,135)			$ 1,500	$ 25,000					$ 339			$ (23,474)
Beginning balance, shares at Dec. 31, 2017				3,333,333	31,450,499		2,305,173
Issuance of redeemable convertible preferred stock,						$ 58,257
Issuance of redeemable convertible preferred stock, shares						28,285,557
Issuance of series B redeemable convertible preferred stock to settled accrued liability of license cost						$ 500
Issuance of series B redeemable convertible preferred stock to settle accrued liability of license cost, shares						233,765
Accretion of issuance costs related to redeemable convertible preferred stock	(2,243)					$ 2,243				(339)			(1,904)
Stock-based compensation expense	109									109
Vesting of restricted stock awards							$ 30,753
Net loss	(8,242)												(8,242)
Ending balance at Mar. 31, 2018	(33,511)			$ 1,500	$ 25,000	$ 61,000				109			(33,620)
Ending balance, shares at Mar. 31, 2018				3,333,333	31,450,499	28,519,322	2,335,926
Beginning balance at Dec. 31, 2017	(23,135)			$ 1,500	$ 25,000					339			(23,474)
Beginning balance, shares at Dec. 31, 2017				3,333,333	31,450,499		2,305,173
Net loss	(30,340)
Ending balance at Sep. 30, 2018	137,109						$ 2			192,825			(55,718)
Ending balance, shares at Sep. 30, 2018							23,758,898
Beginning balance at Mar. 31, 2018	(33,511)			$ 1,500	$ 25,000	$ 61,000				109			(33,620)
Beginning balance, shares at Mar. 31, 2018				3,333,333	31,450,499	28,519,322	2,335,926
Issuance common stock on public offering, net of issuance or offering costs			$ 104,182						$ 1			$ 104,181
Issuance common stock on public offering, net of issuance or offering costs, shares									6,035,151
Stock-based compensation expense	457									457
Vesting of restricted stock awards	0			$ 0	$ 0	$ 0	$ 0			0			0
Vesting of restricted stock awards, shares							30,759
Net loss	(10,507)												(10,507)
Conversion of redeemable convertible preferred stock into common stock	87,500			$ (1,500)	$ (25,000)	$ (61,000)	$ 1			87,499
Conversion of redeemable convertible preferred stock into common stock, shares				(3,333,333)	(31,450,499)	(28,519,322)	15,320,213
Reclassification of warrants to purchase shares of redeemable convertible preferred stock into warrants to purchase common stock	197									197
Ending balance at Jun. 30, 2018	148,318						$ 2			192,443			(44,127)
Ending balance, shares at Jun. 30, 2018							23,722,049
Exercise of common stock warrant, shares							6,091
IPO issuance cost difference	(168)									(168)
Stock-based compensation expense	550									550
Vesting of restricted stock awards	0			$ 0	$ 0	$ 0	$ 0			0			0
Vesting of restricted stock awards, shares							30,758
Net loss	(11,591)												(11,591)
Ending balance at Sep. 30, 2018	137,109						$ 2			192,825			(55,718)
Ending balance, shares at Sep. 30, 2018							23,758,898
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	0						$ 0			0			0
Vesting of restricted stock awards, shares							30,753
Net loss	(17,103)												(17,103)
Ending balance at Mar. 31, 2019	106,788						$ 2			195,628			(88,842)
Ending balance, shares at Mar. 31, 2019							23,954,240
Beginning balance at Dec. 31, 2018	$ 122,184						$ 2			193,921			(71,739)
Beginning balance, shares at Dec. 31, 2018							23,806,628
Exercise of stock options, shares	232,758
Net loss	$ (50,303)
Ending balance at Sep. 30, 2019	206,760						$ 3			328,799			(122,042)
Ending balance, shares at Sep. 30, 2019							31,606,648
Beginning balance at Mar. 31, 2019	106,788						$ 2			195,628			(88,842)
Beginning balance, shares at Mar. 31, 2019							23,954,240
Stock-based compensation expense	1,663									1,663
Exercise of stock options	238									238
Exercise of stock options, shares							110,290
Vesting of restricted stock awards	0						$ 0			0			0
Vesting of restricted stock awards, shares							30,757
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							24,095,287
Issuance common stock on public offering, net of issuance or offering costs		$ 129,465						$ 1			$ 129,464
Issuance common stock on public offering, net of issuance or offering costs, shares								7,475,000
Stock-based compensation expense	1,797									1,797
Exercise of stock options	9									9
Exercise of stock options, shares							5,609
Vesting of restricted stock awards	0						$ 0			0			0
Vesting of restricted stock awards, shares							30,752
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							31,606,648

Consolidated Statements of Re_2

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2019	Jun. 30, 2018	Mar. 31, 2018
Statement Of Stockholders Equity [Abstract]
Issuance of redeemable convertible preferred stock, net of issuance costs			$ 2,243
Issuance of common stock upon IPO, net of issuance costs	$ 525	$ 10,487

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss			$ (50,303)	$ (30,340)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense			605	196
Stock-based compensation expense			4,915	1,117
Amortization of deferred offering costs				33
Impairment loss of property and equipment				235
Deferred rent expense			(123)	(63)
Change in fair value of preferred stock warrant liability	$ 0	$ 0	0	162
Change in fair value of derivative liability	0	0	0	1,629
Changes in operating assets and liabilities:
Prepaid expenses and other current assets			(3,616)	(1,138)
Other assets			(13)	(383)
Accounts payable			(1,127)	829
Accrued expenses and other current liabilities			577	1,509
Net cash used in operating activities			(49,085)	(26,214)
Cash flows from investing activities:
Change in restricted cash				(259)
Purchases of property and equipment			(819)	(1,339)
Net cash used in investing activities			(819)	(1,598)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs				58,258
Payment of dilution liability				(2,000)
Proceeds from exercise of stock options			499
Proceeds from issuance of common shares upon completion of initial public offering costs, net of offering costs				104,168
Proceeds from offering			129,465
Net cash provided by financing activities			129,964	160,426
Net increase in cash, cash equivalents and restricted cash			80,060	132,614
Cash, cash equivalents and restricted cash at beginning of period			126,794	5,963
Cash, cash equivalents and restricted cash at end of period	206,854	138,577	206,854	138,577
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses			80	236
Property and equipment held for sale				19
IPO issuance costs included in accrued expenses				155
Purchase of property and equipment paid for by landlord				842
Accretion of issuance costs related to redeemable convertible preferred stock				2,240
Reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheets:
Cash and cash equivalents	206,362	138,553	206,362	138,553
Long-term restricted cash (included in other assets)	492	24	492	24
Cash, cash equivalents and restricted cash at end of period	$ 206,854	$ 138,577	$ 206,854	$ 138,577

Nature of the Business

Nature of the Business	9 Months Ended
	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business AVROBIO, Inc. (the “Company” or “AVROBIO”) is a clinical stage gene therapy company focused on developing potentially curative ex vivo The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including but not limited to, risks associated with completing preclinical studies and clinical trials, receiving regulatory approvals for product candidates, development by competitors of new biopharmaceutical products, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize revenue from product sales. On June 20, 2018, the Company’s registration statement on Form S-1 relating to its initial public offering (“IPO”) was declared effective by the Securities and Exchange Commission (the “SEC”). The IPO closed on June 25, 2018 and the Company issued and sold 5,247,958 common shares at a public offering price of $19.00 per share for net proceeds of $90,103 after deducting underwriting discounts and commissions of $6,980 and other offering expenses of approximately $2,628. Simultaneously, on June 25, 2018, the Company issued and sold 787,193 additional common shares, pursuant to the full exercise of the underwriters’ option to purchase additional shares, for net proceeds of $13,910 after deducting underwriting discounts and commissions of $1,047. Thus, the aggregate net proceeds to the Company from the IPO, after deducting underwriting discounts and commissions and other offering costs, were $104,013. Upon the closing of the IPO, all series Seed redeemable convertible preferred stock (the “Series Seed Preferred Stock”), series A redeemable convertible preferred stock (the “Series A Preferred Stock”) and series B redeemable convertible preferred stock (the “Series B Preferred Stock”), (the Series Seed Preferred Stock, the Series A Preferred Stock and the Series B Preferred Stock are collectively referred to as the “Preferred Stock”) then outstanding converted into an aggregate of 15,320,213 shares of common stock. In July 2019, the Company closed an underwritten public offering of 7,475,000 shares of its common stock at a public offering price of $18.50 per share, which included 975,000 shares of the Company’s common stock resulting from the full exercise of the underwriters’ option to purchase additional shares at the public offering price, less underwriting discounts and commissions (the “July 2019 Follow-on Offering”). The net proceeds to the Company from the July 2019 Follow-on Offering, after deducting underwriting discounts and commissions and other offering expenses payable by the Company, were approximately $129,500. Through September 30, 2019, the Company has funded its operations primarily with proceeds from the sale of the Preferred Stock and common stock through the Company’s IPO. The Company has incurred recurring losses since its inception, including net losses of $50,303 and $30,340 for the nine months ended September 30, 2019 and 2018, respectively. In addition, as of September 30, 2019, the Company had an accumulated deficit of $122,042. Although the Company has incurred recurring losses and expects to continue to incur losses for the foreseeable future, the Company expects that its existing cash, cash equivalents and marketable securities will be sufficient to fund current planned operations and capital expenditure requirements for at least the next twelve months from the date of issuance of the financial statements contained in this Form 10-Q. However, the future viability of the Company is dependent on its ability to raise additional capital to finance its operations. The Company’s inability to raise capital as and when needed could have a negative impact on its financial condition and ability to pursue its business strategies. There can be no assurance that the current operating plan will be achieved or that additional funding will be available on terms acceptable to the Company, or at all.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the audited annual consolidated financial statements as of and for the year ended December 31, 2018, and, in the opinion of management, reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of September 30, 2019, and the results of its operations for the three and nine months ended September 30, 2019 and 2018, its statements of stockholders’ equity for the three and nine months ended September 30, 2019 and 2018 and its statement of cash flows for the nine months ended September 30, 2019 and 2018. The results for the three and nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, filed with the Securities and Exchange Commission (“SEC”) on March 25, 2019. The accompanying Unaudited Condensed Consolidated Financial Statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the Unaudited Condensed Consolidated Financial Statements. As of September 30, 2019, there have been no changes to the Company’s significant accounting policies as described in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018, except as it related to the adoption of new accounting standards during the first nine months of 2019 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 an offering or such earlier time that it is no longer an “emerging growth company”. Subsequent Event Considerations The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the consolidated financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. See Note 13. Recently Issued Accounting Pronouncements In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808)—Clarifying the Interaction between Topic 808 and Topic 606 In August 2018, the FASB issued ASU No. 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40) 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 In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments In February 2016, the FASB issued ASU No. 2016-02 , Leases (Topic 842) Leases

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities The following table presents information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicates the level of the fair value hierarchy utilized to determine such fair values as of September 30, 2019 and December 31, 2018: Fair Value Measurements as of September 30, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ 206,121 $ — $ — $ 206,121 Restricted cash 492 — — 492 $ 206,613 $ — $ — $ 206,613 Fair Value Measurements as of December 31, 2018 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ 126,047 $ — $ — $ 126,047 Restricted cash 492 — — 492 $ 126,539 $ — $ — $ 126,539 There were no transfers within the hierarchy during the nine months ended September 30, 2019 or the year ended December 31, 2018. Valuation of the Warrant to Purchase Preferred Stock At June 30, 2018, the Company had outstanding a warrant to purchase shares of common stock that was issued to a lender in connection with a loan and security agreement entered into in 2017. The warrant was originally issued as a warrant to purchase shares of Series A Preferred Stock prior to the IPO. The fair value of the warrant to purchase preferred stock was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. Prior to the IPO, the Company classified the warrant as a liability on its consolidated balance sheets as the warrant is a free-standing financial instrument that may require the Company to transfer assets upon exercise. The preferred stock warrant liability was initially recorded at fair value upon the date of issuance and was subsequently remeasured to fair value at each reporting date. Changes in the fair value of the warrant to purchase preferred stock were recognized as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss. The Company utilized the Black-Scholes option-pricing model, which incorporates assumptions and estimates, to value the warrant. The Company assessed these assumptions and estimates on a quarterly basis as additional information impacting the assumptions was obtained. Estimates and assumptions impacting the fair value measurement included the fair value per share of the underlying redeemable convertible preferred stock issuable upon exercise of the warrant, the remaining contractual term of the warrant, the risk-free interest rate, the expected dividend yield and the expected volatility of the price of the underlying redeemable convertible preferred stock. Upon the IPO, the warrant to purchase preferred stock was converted to a warrant to purchase common stock. The carrying amount of the warrant to purchase preferred stock as of the date of IPO was transferred to additional paid in capital. The Company recognized a loss of $0 and Valuation of Derivative In January 2016, in connection with a license agreement entered into with University Health Network (“UHN”), and as part of the initial consideration for the license, the Company issued 1,161,665 shares of common stock to UHN pursuant to a stock purchase agreement (the “Stock Purchase Agreement”). The Stock Purchase Agreement contained a provision requiring the Company to make a cash payment to UHN of up to $2,000 if UHN’s fully diluted ownership was reduced within specified percentages as part of an IPO by the Company. The Company concluded the anti-dilution feature represented a derivative instrument and should be measured at fair value, with changes in fair value recognized as a gain or loss to other income (expense), net in the consolidated statements of operations and comprehensive loss. The initial fair value of the derivative was recorded as research and development expense in January 2016. On June 21, 2018, in connection with the Company’s IPO, the Company remeasured the fair value of the derivative to $2,000 as the Company was required to pay the dilution payment as mentioned above, which the Company paid in July 2018. An increase in fair value of $0 and $1,629 was recorded in other expense in the accompanying condensed consolidated statements of operations and comprehensive loss for the nine months ended September 30, 2019 and 2018, respectively. The Company did not recognize any expense for the three months ended September 30, 2019 and 2018.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	9 Months Ended
	Sep. 30, 2019
Text Block [Abstract]
Prepaid Expenses and Other Current Assets	4. Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following: September 30, 2019 December 31, 2018 Tax incentive refund $ 1,806 $ 1,325 Prepaid research and development costs 3,577 981 Prepaid insurance 1,309 316 Interest income receivable 314 220 Prepaid rent 92 81 Other current assets 247 795 $ 7,345 $ 3,718

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2019
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following: September 30, 2019 December 31, 2018 Laboratory and office equipment $ 2,188 $ 1,624 Leasehold improvements 1,260 1,260 Computer equipment and software 134 134 Construction in process 80 — 3,662 3,018 Less: Accumulated depreciation and amortization (989 ) (384 ) $ 2,673 $ 2,634 Depreciation and amortization expense for the three months ended September 30, 2019 and 2018 was $215 and $130, respectively. Depreciation and amortization expense for the nine months ended September 30, 2019 and 2018 was $605 and $196, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2019
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	6. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following: September 30, 2019 December 31, 2018 Compensation and benefit costs $ 2,860 $ 2,616 Research and development costs 4,508 3,969 Consulting and professional fees 850 320 Other liabilities 284 917 $ 8,502 $ 7,822

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock and Common Stock	9 Months Ended
	Sep. 30, 2019
Text Block [Abstract]
Redeemable Convertible Preferred Stock and Common Stock	7. Redeemable Convertible Preferred Stock and Common Stock Redeemable Convertible Preferred Stock Prior to the IPO, the authorized capital stock of the Company included 63,491,857 shares of $0.0001 par value preferred stock, of which 3,333,333 shares have been designated as Series Seed Preferred Stock, 31,639,202 shares have been designated as Series A Preferred Stock and 28,519,322 shares have been designated as Series B Preferred Stock. In January 2018, the Company issued and sold 28,519,322 shares of Series B Preferred Stock, at a price of $2.1389 per share, for total proceeds of $58,757, net of issuance costs of $2,243. Upon closing of the IPO, all outstanding shares of Preferred Stock were converted into 15,320,213 shares of common stock. The holders of the Company’s Preferred Stock had certain voting, dividend, and redemption rights, as well as liquidation preferences and conversion privileges. All rights, preferences, and privileges associated with the preferred stock were terminated at the time of the Company’s IPO in conjunction with the conversion of all outstanding shares of Preferred Stock into shares of common stock. Common Stock As of September 30, 2019 and December 31, 2018, the authorized capital stock of the Company included 150,000,000 shares of common stock, $0.0001 par value and 10,000,000 shares of undesignated preferred stock. As of September 30, 2019 and December 31, 2018, 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 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 September 30, 2019, no cash dividends have been declared or paid. Common Stock Reserved for Future Issuance At September 30, 2019 and December 31, 2018, the Company has reserved the following shares of common stock for future issuance: September 30, 2019 December 31, 2018 Shares reserved for vesting of restricted stock awards 63,313 153,276 Shares reserved for exercise of outstanding stock options 3,145,428 2,164,101 Shares reserved for issuance under the 2018 Stock Option and Grant Plan 474,326 385,561 Shares reserved for issuance under the 2018 Employee Stock Purchase Plan 461,266 223,200 Total shares of authorized common stock reserved for future issuance 4,144,333 2,926,138

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2019
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 were as follows, presented on a weighted-average basis: Nine Months Ended September 30, 2019 2018 Expected option life (years) 5.97 6.05 Risk-free interest rate 2.40 % 2.73 % Expected volatility 81.25 % 83.42 % Expected dividend yield — % — % The following table summarizes the Company’s stock option activity for the nine months ended September 30, 2019: Number of Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (Years) Aggregate Intrinsic Value Outstanding as of December 31, 2018 2,164,100 $ 7.75 8.77 $ 23,083 Granted 1,315,317 $ 16.76 Exercised (232,758 ) $ 2.14 Cancelled or forfeited (101,231 ) $ 10.26 Outstanding as of September 30, 2019 3,145,428 $ 11.86 8.63 $ 15,491 Exercisable as of September 30, 2019 790,343 $ 3.99 7.51 $ 8,759 Vested and expected to vest as of September 30, 2019 3,145,428 $ 11.86 8.63 $ 15,491 The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the underlying stock options and the estimated fair value of the Company’s common stock for those stock options that had exercise prices lower than the estimated fair value of the Company’s common stock. The weighted-average grant-date fair value of the Company’s stock options granted during the nine months ended September 30, 2019 and 2018 was $11.77 and $9.05, respectively. Restricted Common Stock The following table summarizes the Company’s restricted common stock activity for the nine months ended September 30, 2019: Number of Shares Weighted- Average Grant Date Fair Value Issued and unvested as of December 31, 2018 153,275 $ 0.42 Granted 2,300 $ 15.65 Vested (92,262 ) $ 0.42 Forfeited, canceled or expired — $ — Issued and unvested as of September 30, 2019 63,313 $ 0.42 The total fair value of restricted common stock vested during the nine months ended September 30, 2019 and 2018 was $44 and $39, respectively. Stock-Based Compensation Stock-based compensation expense was allocated as follows: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Research and development $ 889 $ 230 $ 2,649 $ 417 General and administrative 908 320 2,265 700 Total stock-based compensation expense $ 1,797 $ 550 $ 4,914 $ 1,117 As of September 30, 2019, total unrecognized compensation cost related to the unvested stock-based awards was $22,065, which is expected to be recognized over a weighted-average period of 3.24 years.

License Agreements

License Agreements	9 Months Ended
	Sep. 30, 2019
Text Block [Abstract]
License Agreements	9. License Agreements Agreements with UHN Fabry License Agreement— On January 27, 2016, the Company entered into an agreement with UHN, pursuant to which UHN granted the Company an option to enter into an exclusive license under the UHN intellectual property related to Fabry disease in accordance with the pre-negotiated licensing terms. On November 4, 2016, the Company exercised its option and entered into a license agreement with UHN, pursuant to which UHN granted the Company an exclusive worldwide license under certain intellectual property rights and a non-exclusive worldwide license under certain know-how, in each case subject to certain retained rights, to develop, commercialize and sell products for use in the treatment of Fabry disease. UHN also granted the Company an exclusive option, exercisable for a period of three years following the execution of the agreement, to obtain a license under certain improvements to the licensed intellectual property rights as well as an option to negotiate a license under certain other improvements. The Company elected not to exercise such option and the option has expired in accordance with its terms. 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, 2019 and 2018, the Company recorded research and development expense related to this agreement with UHN of $415 and $0, respectively, which consists of reimbursable funded study trial costs. For the nine months ended September 30, 2019 and 2018, the Company recorded research and development expense related to this agreement with UHN of $741 and $0, 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, 2019 and 2018. Interleukin 12 License Agreement— On January 27, 2016, the Company entered into an exclusive license agreement with UHN, pursuant to which UHN granted the Company a license to certain patent rights for the commercial development, manufacture, distribution and use of any products or processes resulting from development of those patent rights related to Interleukin 12. Upon execution of this agreement, the Company paid an upfront license fee of CAD $264. In addition, as part of the initial consideration for the license, the Company issued to UHN 1,161,665 shares of the Company’s common stock. The fair value of the shares issued to UHN of $480 and the upfront fee was expensed upon the execution of the agreement. In addition, the Company agreed to pay UHN up to $2,000 upon the closing of an IPO if certain criteria are met. This obligation was considered a derivative instrument and was initially recorded at fair value of $49. 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 nine months ended September 30, 2019 and 2018, the Company recorded research and development expense related to this agreement with UHN of $38 and $41, respectively, which consists of license maintenance fees. No license maintenance fees were recorded in the three months ended September 30, 2019 and 2018. No milestone fees were incurred related to the IL-12 license agreement in the nine months ended September 30, 2019 and 2018. Agreement with BioMarin Pharmaceutical Inc. (“BioMarin”) On August 31, 2017, the Company entered into a license agreement with BioMarin, pursuant to which BioMarin granted the Company an exclusive worldwide license under certain intellectual property rights owned or controlled by BioMarin to develop, commercialize and sell products for use in the treatment of Pompe disease. As consideration for this agreement, the Company paid an upfront license fee of $500 in cash and issued 233,765 shares of Series B Preferred Stock to BioMarin at the time of our Series B Preferred Stock financing in January 2018. Both the upfront cash payment of $500 and the value of the shares Series B Preferred Stock issued of $500 were recorded as research and development expense during the year ended December 31, 2017. The Company is also obligated to make future milestone payments of up to $13,000 upon the achievement of certain specified milestones and agreed to pay BioMarin royalties of a low single-digit percentage of net sales of licensed products sold by the Company or its affiliates covered by patent rights in a relevant country. No milestone fees related to the license were recorded for the nine months ended September 30, 2019 and 2018. Unless terminated earlier, the agreement expires upon the expiration of the Company’s royalty obligation for all licensed products throughout the world. BioMarin and the Company can terminate the agreement in the event of a material breach by the other party and failure to cure such breach within a certain period of time. The Company may terminate the agreement at will upon written notice to BioMarin. BioMarin has the right to terminate the agreement upon the Company’s bankruptcy or insolvency, or in the event of any challenge or opposition to the licensed patent rights or related actions brought by the Company or its affiliates or sublicensees, or if the Company, its affiliates or sublicensees knowingly assist a third-party in challenging or otherwise opposing the licensed patent rights, except as required under a court order or subpoena. Agreement with GenStem Therapeutics, Inc. (“GenStem”) On October 2, 2017, the Company entered into a license agreement with GenStem, pursuant to which GenStem granted the Company an exclusive worldwide license, subject to certain retained rights, under certain intellectual property rights owned or controlled by GenStem to develop, commercialize and sell products for use in the treatment of cystinosis. Under this agreement, the Company paid an upfront license fee of $1,000 and is required to make payments upon completion of certain milestones up to an aggregate of $16,000. 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, 2019 and 2018. Agreement with Lund University Rights Holders On November 17, 2016, the Company entered into a license agreement with affiliates of Lund University, along with certain other relevant rights holders that may be added from time to time, pursuant to which such rights holders granted to the Company an exclusive worldwide license, subject to certain retained rights, under certain intellectual property rights to develop, commercialize and sell products in any and all uses relevant to Gaucher disease. As consideration for the license, the Company is required to make payments in connection with the achievement of certain milestones up to an aggregate of $550. The agreement expires on the latest of (i) the twentieth anniversary of the end of a certain research project the Company is funding pursuant to an agreement with Lund University, (ii) the expiration of the term of any patent filed on the licensed rights that covers a licensed product, (iii) the expiration of any applicable marketing exclusivity right and (iv) such time that neither the Company nor any sublicensees, partners or contractors are commercializing a licensed product. Either the Company or the rights holders acting together may terminate the license agreement if the other such party commits a material breach and fails to cure such breach within a certain period of time, or if the other party enters into liquidation, becomes insolvent, or enters into composition or statutory reorganization proceedings. No milestone fees related to the license were recorded for the nine months ended September 30, 2019 and 2018.

Net Loss per Share Attributable

Net Loss per Share Attributable to Common Stockholders	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Net Loss per Share Attributable to Common Stockholders	10. Net Loss per Share Attributable to Common Stockholders For purposes of the diluted net loss per share calculation, stock options, unvested restricted stock, Preferred Stock and the warrant to purchase shares of Series A Preferred Stock are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, the weighted-average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The following potentially dilutive common stock equivalents, presented based on amounts outstanding at each period end, were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods indicated: Nine Months Ended September 30, 2019 2018 Options to purchase common stock 3,145,428 1,957,839 Restricted common stock 63,313 184,033 Warrants to purchase redeemable convertible preferred stock (as converted to common stock) — —

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Lease Agreement On January 12, 2018, the Company entered into a lease agreement for office space located in Cambridge, Massachusetts. The lease agreement expires in January 2023, with a landlord who is an affiliate of the landlord of the Company’s prior lease facility. The annual lease payments are subject to a 3% increase each year. The Company recognizes rent expense on a straight-line basis over the lease period and has recorded deferred rent for rent expense incurred but not yet paid. The Company received a tenant incentive allowance of $842 in 2018. Such incentive allowance is being amortized as a reduction of rent expense on a straight-line basis over the lease period. In accordance with the lease agreement, the Company is required to maintain a security deposit of $209, which was recorded in other assets. In contemplation of this agreement, the Company terminated its prior lease agreement. On August 31, 2018, the Company entered into a sub-lease agreement for lab space located in Cambridge, Massachusetts, United States, which expires in October 2020. The annual lease payments are subject to a 3% increase each year. In accordance with the lease agreement, the Company is required to maintain a security deposit of $283, which was recorded in other assets as of September 30, 2018. Legal Proceedings The Company, from time to time, may be party to litigation arising in the ordinary course of business. The Company was not subject to any material legal proceedings during the nine months ended September 30, 2019 and 2018 and to the best of its knowledge, no material legal proceedings are currently pending or threatened. Other The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at September 30, 2019 and December 31, 2018, or royalties on future sales. No milestone or royalty payments under these agreements are expected to be payable in the immediate future, except as disclosed in Note 9. See Note 9 for discussion of these arrangements. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company agrees to indemnify, hold harmless, and to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners, in connection with any U.S. patent or any copyright or other intellectual property infringement claim by any third-party with respect to the Company’s products. Further, the Company indemnifies its directors and officers who are, or were, serving at the Company’s request in such capacities. The Company’s maximum exposure under these arrangements is unknown at September 30, 2019. The Company does not anticipate recognizing any significant losses relating to these arrangements. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2019
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions UHN In connection with the Company’s entry into a license agreement with UHN on January 27, 2016, the Company issued UHN 1,161,665 shares of its common stock. Upon the closing of the IPO, as UHN’s fully-diluted percentage ownership of the Company was reduced within a range of specified percentages, the Company was obligated to pay UHN an amount up to $2,000, which was paid in July 2018. See Note 3 for further discussion on the accounting treatment for this provision. For the three months ended September 30, 2019 and 2018, the Company recognized $415 and $0, respectively, of research and development expense related to the license agreements with UHN (Note 9). For the nine months ended September 30, 2019 and 2018, the Company recognized $779 and $41, respectively, of research and development expense related to the license agreements with UHN (Note 9). Others For the three and nine months ended September 30, 2019, the Company recorded expenses of $369 and $1,090, respectively, related to a sublease to rent lab space, provided by an entity affiliated with a member of the Board. No such expense was recorded for the three and nine months ended September 30, 2018. For the three nine months ended September 30, 2019 and 2018, the Company recognized $0 and $2, respectively, related to consulting services provided by an entity affiliated with an officer of the Company and a member of the Board. For the nine months ended September 30, 2019 and 2018, the Company recognized $0 and $42, respectively, related to consulting services provided by an entity affiliated with an officer of the Company and a member of the Board

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2019
Subsequent Events [Abstract]
Subsequent Events	13. Subsequent Events In November 2019, the Company made a payment of $2,000 to GenStem 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 (Note 9).

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the audited annual consolidated financial statements as of and for the year ended December 31, 2018, and, in the opinion of management, reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of September 30, 2019, and the results of its operations for the three and nine months ended September 30, 2019 and 2018, its statements of stockholders’ equity for the three and nine months ended September 30, 2019 and 2018 and its statement of cash flows for the nine months ended September 30, 2019 and 2018. The results for the three and nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, filed with the Securities and Exchange Commission (“SEC”) on March 25, 2019. The accompanying Unaudited Condensed Consolidated Financial Statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the Unaudited Condensed Consolidated Financial Statements. As of September 30, 2019, there have been no changes to the Company’s significant accounting policies as described in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018, except as it related to the adoption of new accounting standards during the first nine months of 2019 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 an offering or such earlier time that it is no longer an “emerging growth company”.
Subsequent Event Considerations	Subsequent Event Considerations The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the consolidated financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. See Note 13.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808)—Clarifying the Interaction between Topic 808 and Topic 606 In August 2018, the FASB issued ASU No. 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40) 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 In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments In February 2016, the FASB issued ASU No. 2016-02 , Leases (Topic 842) Leases

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following table presents information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicates the level of the fair value hierarchy utilized to determine such fair values as of September 30, 2019 and December 31, 2018: Fair Value Measurements as of September 30, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ 206,121 $ — $ — $ 206,121 Restricted cash 492 — — 492 $ 206,613 $ — $ — $ 206,613 Fair Value Measurements as of December 31, 2018 Using: Level 1 Level 2 Level 3 Total Assets: Money market funds $ 126,047 $ — $ — $ 126,047 Restricted cash 492 — — 492 $ 126,539 $ — $ — $ 126,539

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 2019
Text Block [Abstract]
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following: September 30, 2019 December 31, 2018 Tax incentive refund $ 1,806 $ 1,325 Prepaid research and development costs 3,577 981 Prepaid insurance 1,309 316 Interest income receivable 314 220 Prepaid rent 92 81 Other current assets 247 795 $ 7,345 $ 3,718

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2019
Property Plant And Equipment [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consisted of the following: September 30, 2019 December 31, 2018 Laboratory and office equipment $ 2,188 $ 1,624 Leasehold improvements 1,260 1,260 Computer equipment and software 134 134 Construction in process 80 — 3,662 3,018 Less: Accumulated depreciation and amortization (989 ) (384 ) $ 2,673 $ 2,634

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2019
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: September 30, 2019 December 31, 2018 Compensation and benefit costs $ 2,860 $ 2,616 Research and development costs 4,508 3,969 Consulting and professional fees 850 320 Other liabilities 284 917 $ 8,502 $ 7,822

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock and Common Stock (Tables)	9 Months Ended
	Sep. 30, 2019
Text Block [Abstract]
Summary of Common Stock Reserved for Future Issuance	At September 30, 2019 and December 31, 2018, the Company has reserved the following shares of common stock for future issuance: September 30, 2019 December 31, 2018 Shares reserved for vesting of restricted stock awards 63,313 153,276 Shares reserved for exercise of outstanding stock options 3,145,428 2,164,101 Shares reserved for issuance under the 2018 Stock Option and Grant Plan 474,326 385,561 Shares reserved for issuance under the 2018 Employee Stock Purchase Plan 461,266 223,200 Total shares of authorized common stock reserved for future issuance 4,144,333 2,926,138

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Assumptions that Used to Determine Grant-Date Fair Value of Stock Options Granted to Employees and Members of Board	The assumptions that the Company used to determine the grant-date fair value of stock options granted to employees and members of the Board were as follows, presented on a weighted-average basis: Nine Months Ended September 30, 2019 2018 Expected option life (years) 5.97 6.05 Risk-free interest rate 2.40 % 2.73 % Expected volatility 81.25 % 83.42 % 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, 2019: Number of Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (Years) Aggregate Intrinsic Value Outstanding as of December 31, 2018 2,164,100 $ 7.75 8.77 $ 23,083 Granted 1,315,317 $ 16.76 Exercised (232,758 ) $ 2.14 Cancelled or forfeited (101,231 ) $ 10.26 Outstanding as of September 30, 2019 3,145,428 $ 11.86 8.63 $ 15,491 Exercisable as of September 30, 2019 790,343 $ 3.99 7.51 $ 8,759 Vested and expected to vest as of September 30, 2019 3,145,428 $ 11.86 8.63 $ 15,491
Summary of Restricted Stock Unit Activity	The following table summarizes the Company’s restricted common stock activity for the nine months ended September 30, 2019: Number of Shares Weighted- Average Grant Date Fair Value Issued and unvested as of December 31, 2018 153,275 $ 0.42 Granted 2,300 $ 15.65 Vested (92,262 ) $ 0.42 Forfeited, canceled or expired — $ — Issued and unvested as of September 30, 2019 63,313 $ 0.42
Schedule of Stock-based Compensation Expense	Stock-based compensation expense was allocated as follows: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Research and development $ 889 $ 230 $ 2,649 $ 417 General and administrative 908 320 2,265 700 Total stock-based compensation expense $ 1,797 $ 550 $ 4,914 $ 1,117

Net Loss per Share Attributab_2

Net Loss per Share Attributable to Common Stockholders (Tables)	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Schedule of Dilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share	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, 2019 2018 Options to purchase common stock 3,145,428 1,957,839 Restricted common stock 63,313 184,033 Warrants to purchase redeemable convertible preferred stock (as converted to common stock) — —

Nature of the Business - Additi

Nature of the Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jun. 25, 2018	Jul. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Nature Of Organization And Operations [Line Items]
Net proceeds from issuance of common stock	$ 104,013								$ 129,465
Convertible preferred stock converted into common stock	15,320,213
Net loss			$ (17,145)	$ (16,055)	$ (17,103)	$ (11,591)	$ (10,507)	$ (8,242)	(50,303)	$ (30,340)
Accumulated deficit			$ (122,042)						$ (122,042)		$ (71,739)
IPO [Member]
Nature Of Organization And Operations [Line Items]
Common stock, shares issued and sold	5,247,958
Issuance price per shares	$ 19
Net proceeds from issuance of common stock	$ 90,103
Underwriting discounts and commissions and offering costs	6,980
Other offering expenses	$ 2,628
Over-Allotment Option [Member]
Nature Of Organization And Operations [Line Items]
Common stock, shares issued and sold	787,193
Net proceeds from issuance of common stock	$ 13,910
Underwriting discounts and commissions and offering costs	$ 1,047
July 2019 Follow-on Offering [Member]
Nature Of Organization And Operations [Line Items]
Net proceeds from issuance of common stock		$ 129,500
Common Stock [Member] | Over-Allotment Option [Member]
Nature Of Organization And Operations [Line Items]
Common stock, shares issued and sold		975,000
Common Stock [Member] | July 2019 Follow-on Offering [Member]
Nature Of Organization And Operations [Line Items]
Common stock, shares issued and sold		7,475,000
Issuance price per shares		$ 18.50

Fair Value of Financial Asset_3

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 ($)	Sep. 30, 2019	Dec. 31, 2018
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 206,613	$ 126,539
Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	206,121	126,047
Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	492	492
Level 1 [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	206,613	126,539
Level 1 [Member] | Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	206,121	126,047
Level 1 [Member] | Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 492	$ 492

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	Jan. 27, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jul. 31, 2018
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Fair value input transfer between levels		$ 0		$ 0		$ 0
Change in fair value of preferred stock warrant liability		0	$ 0	$ 0	$ 162,000
Common stock issued for initial consideration for license				2,300
Increase in fair value of derivative		$ 0	$ 0	$ 0	$ (1,629,000)
IPO [Member]
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Dilution payment							$ 2,000,000
License Agreement [Member]
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Provision for maximum cash payment	$ 2,000,000
License Agreement [Member] | Stock Purchase Agreement [Member]
Fair Value of Assets and Liabilities Measured on Non Recurring Basis [Line Items]
Common stock issued for initial consideration for license	1,161,665

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Prepaid Expense And Other Assets Current [Abstract]
Tax incentive refund	$ 1,806	$ 1,325
Prepaid research and development costs	3,577	981
Prepaid insurance	1,309	316
Interest income receivable	314	220
Prepaid rent	92	81
Other current assets	247	795
Total	$ 7,345	$ 3,718

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 3,662	$ 3,018
Less: Accumulated depreciation and amortization	(989)	(384)
Property plant and equipment, net	2,673	2,634
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	2,188	1,624
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	1,260	1,260
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	134	$ 134
Construction in Process [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	$ 80

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 215	$ 130	$ 605	$ 196

Accrued Expenses - Summary of A

Accrued Expenses - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accrued Liabilities Current [Abstract]
Compensation and benefit costs	$ 2,860	$ 2,616
Research and development costs	4,508	3,969
Consulting and professional fees	850	320
Other liabilities	284	917
Accrued expenses	$ 8,502	$ 7,822

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock and Common Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Jan. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jun. 25, 2018
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		63,491,857
Redeemable convertible preferred stock, par value		$ 0.0001
Proceeds from issuance of redeemable convertible preferred stock			$ 58,258
Common stock, shares authorized		150,000,000		150,000,000
Common stock, par value		$ 0.0001		$ 0.0001
Undesignated preferred stock		10,000,000		10,000,000
Undesignated preferred stock outstanding		0		0
Cash dividends		$ 0
IPO [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, sales price per share					$ 19
Preferred stock converted into common stock		15,320,213
Series Seed Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		3,333,333
Series A Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		31,639,202
Series B Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized		28,519,322
Redeemable convertible preferred stock, shares issued and sold	28,519,322
Redeemable convertible preferred stock, sales price per share	$ 2.1389
Proceeds from issuance of redeemable convertible preferred stock	$ 58,757
Redeemable convertible preferred stock, net of issuance costs	$ 2,243

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Sep. 30, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	4,144,333	2,926,138
2018 Stock Option and Incentive Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	474,326	385,561
2018 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	461,266	223,200
Restricted Stock [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	63,313	153,276
Employee Stock Option [Member]
Class of Stock [Line Items]
Total shares of authorized common stock reserved for future issuance	3,145,428	2,164,101

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Assumptions that Used to Determine Grant-Date Fair Value of Stock Options Granted to Employees and Members of Board (Detail)	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Abstract]
Expected option life (years)	5 years 11 months 19 days	6 years 18 days
Risk-free interest rate	2.40%	2.73%
Expected volatility	81.25%	83.42%

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of options, Outstanding beginning balance | shares	2,164,100
Number of options, Granted | shares	1,315,317
Number of options, Exercised | shares	(232,758)
Number of options, Cancelled or forfeited | shares	(101,231)
Number of options, Outstanding ending balance | shares	3,145,428	2,164,100
Number of options, Exercisable | shares	790,343
Number of options, Vested and expected to vest | shares	3,145,428
Weighted average exercise price, Outstanding beginning balance | $ / shares	$ 7.75
Weighted average exercise price, Granted | $ / shares	16.76
Weighted average exercise price, Exercised | $ / shares	2.14
Weighted average exercise price, Cancelled or forfeited | $ / shares	10.26
Weighted average exercise price, Outstanding ending balance | $ / shares	11.86	$ 7.75
Weighted average exercise price, Exercisable | $ / shares	3.99
Weighted average exercise price, Vested and expected to vest | $ / shares	$ 11.86
Weighted average remaining contractual term, Outstanding balance	8 years 7 months 17 days	8 years 9 months 7 days
Weighted average remaining contractual term, Exercisable	7 years 6 months 3 days
Weighted average remaining contractual term, vested and expected to vest	8 years 7 months 17 days
Aggregate intrinsic value, Outstanding balance | $	$ 15,491	$ 23,083
Aggregate intrinsic value, Exercisable | $	8,759
Aggregate intrinsic value, vested and expected to vest | $	$ 15,491

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options Granted, Weighted-average grant-date fair value	$ 11.77	$ 9.05
Total fair value of restricted common stock vested	$ 44	$ 39
Unrecognized stock-based compensation expenses	$ 22,065
Unrecognized stock-based compensation expense, period for recognition	3 years 2 months 26 days

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Restricted Stock Unit Activity (Detail)	9 Months Ended
	Sep. 30, 2019$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of shares, Issued and unvested beginning balance	153,275
Number of shares, Granted	2,300
Number of shares, Vested	(92,262)
Number of shares, Forfeited, cancelled or expired	0
Number of shares, Issued and unvested ending balance	63,313
Weighted average grant date fair value, Issued and unvested, beginning balance | $ / shares	$ 0.42
Weighted average grant date fair value, Granted | $ / shares	15.65
Weighted average grant date fair value, Vested | $ / shares	0.42
Weighted average grant date fair value, Issued and unvested, ending balance | $ / shares	$ 0.42

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,797	$ 550	$ 4,914	$ 1,117
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	889	230	2,649	417
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 908	$ 320	$ 2,265	$ 700

License Agreements - Additional

License Agreements - Additional Information (Detail) $ in Thousands	Nov. 08, 2019USD ($)	Oct. 02, 2017USD ($)	Nov. 17, 2016USD ($)	Jan. 27, 2016USD ($)shares	Jan. 27, 2016CAD ($)	Aug. 31, 2017USD ($)shares	Sep. 30, 2019USD ($)shares	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)shares	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2018shares	Jan. 27, 2016CAD ($)shares
License Agreement [Line Items]
Common stock, shares issued | shares							31,667,661		31,667,661			23,959,903
Research and development							$ 13,042,000	$ 9,232,000	$ 37,755,000	$ 22,286,000
Preferred stock issued | shares							0		0			0
UHN Agreement [Member]
License Agreement [Line Items]
Milestone fees									$ 0	0
Annual maintenance fees									0	0
Research and development expense							$ 415,000	0	741,000	0
UHN Agreement [Member] | Fabry License Agreement [Member]
License Agreement [Line Items]
Option fee													$ 20
Upfront license fee					$ 75
Milestone payments					2,450
UHN Agreement [Member] | Interleukin 12 License Agreement [Member]
License Agreement [Line Items]
Upfront license fee					264
Milestone payments					19,275
Annual maintenance fees					$ 50		$ 0	$ 0
Common stock, shares issued | shares				1,161,665									1,161,665
Fair value of shares issued				$ 480
Payments upon closing of an initial public offering				2,000,000
Fair value of derivative liability				$ 49,000
Research and development									38,000	41,000
BioMarin Pharmaceutical Inc [Member]
License Agreement [Line Items]
Upfront license fee						$ 500,000
Milestone payments									13,000,000
Upfront cash payment											$ 500,000
Expenses related to license									0	0
BioMarin Pharmaceutical Inc [Member] | Series B Preferred Stock [Member]
License Agreement [Line Items]
Research and development											$ 500,000
Preferred stock issued | shares						233,765
GenStem Therapeutics Inc [Member]
License Agreement [Line Items]
Upfront license fee		$ 1,000,000
Expenses related to license									0	0
Milestone payments		16,000,000
Milestone payment due		$ 2,000,000
GenStem Therapeutics Inc [Member] | Subsequent Event [Member]
License Agreement [Line Items]
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 - Schedule o

Net Loss per Share - Schedule of Dilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Detail) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	3,145,428	1,957,839
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share amount	63,313	184,033

Commitments and Contingencies -

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

Related Party Transactions - Ad

Related Party Transactions - Additional information (Detail) - USD ($)	Jan. 27, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Related Party Transaction [Line Items]
Common stock issued for initial consideration for license				2,300
Research and development expense related to license agreements		$ 13,042,000	$ 9,232,000	$ 37,755,000	$ 22,286,000
Officers and Board Members [Member] | Consulting Services Agreement [Member]
Related Party Transaction [Line Items]
Expense related to related party		0	2,000	0	42,000
License Agreement [Member]
Related Party Transaction [Line Items]
Provision for maximum cash payment	$ 2,000,000
Research and development expense related to license agreements		415,000	$ 0	779,000	$ 41,000
License Agreement [Member] | Stock Purchase Agreement [Member]
Related Party Transaction [Line Items]
Common stock issued for initial consideration for license	1,161,665
Sub-lease Agreement [Member] | Officers and Board Members [Member]
Related Party Transaction [Line Items]
Expense related to related party		$ 369,000		$ 1,090,000

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) $ in Thousands	Nov. 08, 2019USD ($)
GenStem Therapeutics Inc [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Milestone payment paid	$ 2,000