Adverum Biotechnologies (ADVM) 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	Oct. 31, 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	ADVM
Entity Registrant Name	Adverum Biotechnologies, Inc.
Entity Central Index Key	0001501756
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Small Business	true
Entity Common Stock Shares Outstanding		64,588,046
Entity Shell Company	false
Entity File Number	001-36579
Entity Tax Identification Number	20-5258327
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	1035 O’Brien Drive
Entity Address, City or Town	Menlo Park
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94025
City Area Code	650
Local Phone Number	272-6269
Document Quarterly Report	true
Document Transition Report	false
Title of 12(b) Security	Common Stock, $0.0001 par value
Security Exchange Name	NASDAQ

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 73,171	$ 154,949
Short-term investments	88,814	50,130
Prepaid expenses and other current assets	5,617	3,675
Total current assets	167,602	208,754
Operating lease right-of-use asset	21,511
Property and equipment, net	15,249	3,586
Restricted cash	999	999
Deposit and other long-term assets	164	156
Total assets	205,525	213,495
Current liabilities:
Accounts payable	3,382	1,707
Accrued expenses and other current liabilities	10,320	8,784
Lease liability, current portion	4,044
Deferred rent, current portion		228
Total current liabilities	17,746	10,719
Deferred rent, net of current portion		1,366
Lease liability, net of current portion	23,380
Other noncurrent liabilities	185	243
Total liabilities	41,311	12,328
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock
Common stock	7	6
Additional paid-in capital	531,056	522,503
Accumulated other comprehensive loss	(745)	(799)
Accumulated deficit	(366,104)	(320,543)
Total stockholders’ equity	164,214	201,167
Total liabilities and stockholders' equity	$ 205,525	$ 213,495

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Collaboration and license revenue	$ 250	$ 833	$ 250	$ 1,542
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 9,944	$ 14,480	$ 29,045	$ 38,491
General and administrative	7,389	4,826	20,097	19,373
Impairment of intangible asset		5,000		5,000
Total operating expenses	17,333	24,306	49,142	62,864
Operating loss	(17,083)	(23,473)	(48,892)	(61,322)
Other income, net	965	1,265	3,331	3,104
Net loss before income tax benefit	(16,118)	(22,208)	(45,561)	(58,218)
Income tax benefit		1,250		1,250
Net loss	(16,118)	(20,958)	(45,561)	(56,968)
Other comprehensive loss:
Net unrealized gain on marketable securities	4	53	27	129
Foreign currency translation adjustment	(11)		27	(21)
Comprehensive loss	$ (16,125)	$ (20,905)	$ (45,507)	$ (56,860)
Net loss per share — basic and diluted	$ (0.25)	$ (0.34)	$ (0.71)	$ (0.94)
Weighted-average common shares used to compute net loss per share - basic and diluted	64,484	62,454	63,764	60,856

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Stockholders Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive (Loss)/Income [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2017	$ 184,028	$ 5	$ 439,048	$ (963)	$ (254,062)
Balance, shares at Dec. 31, 2017		49,015,339
Stock-based compensation expense	3,429		3,429
Adoption of Topic 606	6,146				6,146
Issuance of common stock upon follow-on offerings, net of issuance costs	70,191	$ 2	70,189
Issuance of common stock upon follow-on offerings, net of issuance costs, shares		11,642,128
Issuance of common stock upon exercise of stock options	308		308
Issuance of common stock upon exercise of stock options, shares		1,361,546
Issuance of common stock upon release of restricted stock units, shares		328,952
Restricted stock surrendered for taxes, shares		(115,593)
Taxes paid for RSUs	(801)		(801)
Foreign currency translation adjustments	(75)			(75)
Unrealized gain on marketable securities, net	17			17
Net loss	(17,200)				(17,200)
Balance at Mar. 31, 2018	246,043	$ 7	512,173	(1,021)	(265,116)
Balance, shares at Mar. 31, 2018		62,232,372
Balance at Dec. 31, 2017	184,028	$ 5	439,048	(963)	(254,062)
Balance, shares at Dec. 31, 2017		49,015,339
Foreign currency translation adjustments	(21)
Unrealized gain on marketable securities, net	129
Net loss	(56,968)
Balance at Sep. 30, 2018	215,553	$ 6	521,286	(855)	(304,884)
Balance, shares at Sep. 30, 2018		62,826,416
Balance at Mar. 31, 2018	246,043	$ 7	512,173	(1,021)	(265,116)
Balance, shares at Mar. 31, 2018		62,232,372
Stock-based compensation expense	5,826		5,826
Issuance of common stock upon follow-on offerings, net of issuance costs	(2)		(2)
Issuance of common stock upon exercise of stock options	264		264
Issuance of common stock upon exercise of stock options, shares		127,554
Purchase and common stock issued under employee stock purchase plan	149		149
Purchase and common stock issued under employee stock purchase plan, shares		58,294
Issuance of common stock upon release of restricted stock units, shares		241,764
Restricted stock surrendered for taxes	(1)	$ (1)
Restricted stock surrendered for taxes, shares		(22,873)
Taxes paid for RSUs	(135)		(135)
Foreign currency translation adjustments	54			54
Unrealized gain on marketable securities, net	59			59
Net loss	(18,810)				(18,810)
Balance at Jun. 30, 2018	233,447	$ 6	518,275	(908)	(283,926)
Balance, shares at Jun. 30, 2018		62,637,111
Stock-based compensation expense	2,995		2,995
Issuance of common stock upon follow-on offerings, net of issuance costs	2		2
Issuance of common stock upon exercise of stock options	115		115
Issuance of common stock upon exercise of stock options, shares		107,037
Issuance of common stock upon release of restricted stock units, shares		101,250
Taxes paid for RSUs	(101)		(101)
Unrealized gain on marketable securities, net	53			53
Net loss	(20,958)				(20,958)
Balance at Sep. 30, 2018	215,553	$ 6	521,286	(855)	(304,884)
Balance, shares at Sep. 30, 2018		62,826,416
Restricted stock unit withholdings, shares		(18,982)
Balance at Dec. 31, 2018	201,167	$ 6	522,503	(799)	(320,543)
Balance, shares at Dec. 31, 2018		62,965,468
Stock-based compensation expense	1,762		1,762
Issuance of common stock upon exercise of stock options	162		162
Issuance of common stock upon exercise of stock options, shares		118,482
Issuance of common stock upon release of restricted stock units, shares		397,302
Restricted stock surrendered for taxes, shares		(145,603)
Taxes paid for RSUs	(504)		(504)
Foreign currency translation adjustments	42			42
Unrealized gain on marketable securities, net	3			3
Net loss	(14,489)				(14,489)
Balance at Mar. 31, 2019	188,143	$ 6	523,923	(754)	(335,032)
Balance, shares at Mar. 31, 2019		63,335,649
Balance at Dec. 31, 2018	$ 201,167	$ 6	522,503	(799)	(320,543)
Balance, shares at Dec. 31, 2018		62,965,468
Issuance of common stock upon exercise of stock options, shares	1,133,000
Foreign currency translation adjustments	$ 27
Unrealized gain on marketable securities, net	27
Net loss	(45,561)
Balance at Sep. 30, 2019	164,214	$ 7	531,056	(745)	(366,104)
Balance, shares at Sep. 30, 2019		64,581,538
Balance at Mar. 31, 2019	188,143	$ 6	523,923	(754)	(335,032)
Balance, shares at Mar. 31, 2019		63,335,649
Stock-based compensation expense	2,626		2,626
Issuance of common stock upon follow-on offerings, net of issuance costs	134		134
Issuance of common stock upon follow-on offerings, net of issuance costs, shares		20,000
Issuance of common stock upon exercise of stock options	2,312		2,312
Issuance of common stock upon exercise of stock options, shares		823,028
Purchase and common stock issued under employee stock purchase plan	163		163
Purchase and common stock issued under employee stock purchase plan, shares		51,112
Issuance of common stock upon release of restricted stock units, shares		219,704
Restricted stock surrendered for taxes, shares		(72,516)
Taxes paid for RSUs	(700)		(700)
Foreign currency translation adjustments	(4)			(4)
Unrealized gain on marketable securities, net	20			20
Net loss	(14,954)				(14,954)
Balance at Jun. 30, 2019	177,740	$ 6	528,458	(738)	(349,986)
Balance, shares at Jun. 30, 2019		64,376,977
Stock-based compensation expense	2,661		2,661
Issuance of common stock upon exercise of stock options	145	$ 1	144
Issuance of common stock upon exercise of stock options, shares		191,371
Issuance of common stock upon release of restricted stock units, shares		27,350
Restricted stock surrendered for taxes, shares		(14,160)
Taxes paid for RSUs	(207)		(207)
Foreign currency translation adjustments	(11)			(11)
Unrealized gain on marketable securities, net	4			4
Net loss	(16,118)				(16,118)
Balance at Sep. 30, 2019	$ 164,214	$ 7	$ 531,056	$ (745)	$ (366,104)
Balance, shares at Sep. 30, 2019		64,581,538

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (45,561)	$ (56,968)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,219	1,361
Stock-based compensation expense	7,049	12,250
Amortization of premium and accrued interest on marketable securities	(813)	199
Impairment of intangible asset		5,000
Amortization of operating lease right-of-use asset	1,620
Other	46	(15)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,868)	(1,518)
Accounts payable	(680)	5
Accrued expenses and other current liabilities	1,891	353
Deferred revenue		(884)
Deferred rent		233
Lease liability	2,762
Deferred tax liability		(1,250)
Net cash used in operating activities	(34,335)	(41,234)
Cash flows from investing activities:
Purchases of marketable securities	(133,144)	(55,924)
Maturities of marketable securities	95,155	117,993
Purchases of property and equipment	(10,874)	(652)
Net cash (used in) provided by investing activities	(48,863)	61,417
Cash flows from financing activities:
Proceeds from offerings of common stock, net of issuance costs		70,189
Proceeds from issuance of common stock	134
Proceeds from issuance of common stock pursuant to option exercises	2,618	687
Taxes paid related to net share settlement of restricted stock units	(1,411)	(1,037)
Proceeds from employee stock purchase plan	163	149
Proceeds from a financing arrangement		100
Repayment of loan	(84)	(118)
Net cash provided by financing activities	1,420	69,970
Net increase (decrease) in cash and cash equivalents and restricted cash	(81,778)	90,153
Cash and cash equivalents and restricted cash at beginning of period	155,948	70,519
Cash and cash equivalents and restricted cash at end of period	74,170	160,672
Supplemental schedule of noncash investing and financing information
Fixed assets in accounts payable, accrued expenses and other current liabilities	$ 3,625	$ 278

Organization and Basis of Prese

Organization and Basis of Presentation	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Organization and Basis of Presentation	1. Organization and Basis of Presentation Adverum Biotechnologies, Inc. (the “Company”) is a clinical-stage gene therapy company targeting unmet medical needs in ocular and rare diseases. The Company develops gene therapy product candidates designed to provide durable efficacy by inducing sustained expression of a therapeutic protein. The Company’s core capabilities include clinical development, novel vector discovery, and in‑house manufacturing expertise, specifically in scalable process development, assay development, and current Good Manufacturing Practices (“cGMP”) quality control. Since the Company’s inception, it has devoted its efforts to performing research and development activities, filing patent applications, hiring personnel and raising capital to support these activities. The Company has not generated any revenue from the sale of products since its inception. The Company has experienced net losses since its inception and had an accumulated deficit of $366.1 million as of September 30, 2019. The Company expects to incur losses and have negative net cash flows from operating activities as it engages in further research and development activities. The Company believes that it has sufficient funds to continue operations into 2021. Basis of Presentation —The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) and follow the requirements of the Securities and Exchange Commission (“SEC”) for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP can be condensed or omitted. These unaudited condensed consolidated financial statements have been prepared on the same basis as the Company’s annual consolidated financial statements and, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, which are necessary for a fair statement of the Company’s consolidated financial information. The results of operations for the three and nine months ended September 30, 2019, are not necessarily indicative of the results to be expected for the full year or any other future period. The balance sheet as of December 31, 2018 is derived from the audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete consolidated financial statements. The accompanying unaudited condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 filed with the SEC.

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 Use of Estimates The accompanying condensed consolidated financial statements have been prepared in accordance with U.S. GAAP. The preparation of the condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent liabilities and the reported amounts of expenses in the condensed consolidated financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates. Recently Adopted Accounting Pronouncements Leases The Company adopted Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) (“Topic 842”) on January 1, 2019. For its long-term operating leases, the Company recognizes a right-of-use asset and a lease liability on its condensed consolidated balance sheets. The Company adopted the new standard using the modified retrospective approach and recorded a lease liability of $24.7 million, and a right‑to‑use asset of $23.1 million, and made no adjustment to the accumulated deficit. In connection with the adoption of the lease standard, the Company also derecognized deferred rent of $1.5 million. The adoption of Topic 842 did not have an impact on the condensed consolidated statement of operations. The lease liability is determined as the present value of future lease payments using an estimated rate of interest that the Company would pay to borrow equivalent funds on a collateralized basis at the lease commencement date. In order to estimate the incremental borrowing rate, management estimated its credit rating, adjusted the credit rating for the nature of the collateral, and benchmarked the borrowing rate against observable yields on comparable securities with a similar term. As of the adoption date, the Company estimated the incremental borrowing rate to be 8.5%. The Company based the right-of-use asset on the liability adjusted for any prepaid or deferred rent. The Company determined the lease term at the commencement date by considering whether renewal options and termination options are reasonably assured of exercise. Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included it in operating expenses on the statements of operations and comprehensive loss. Variable lease payments include lease operating expenses. The Company elected the practical expedients permitted under Topic 842, which among other things, allowed the Company to carry forward the historical lease classification of those leases in place as of January 1, 2019. The Company elected to exclude from its condensed consolidated balance sheets recognition of leases having a term of 12 months or less (short-term leases) and elected to not separate lease components and non-lease components for its long-term real-estate leases. Share-based payment to nonemployees In June 2018, the FASB issued ASU 2018-07, “Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting” (“Topic 718”) that expands the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. The Company adopted ASU 2018–07 on January 1, 2019 and the impact of the adoption resulted in lower stock-based compensation of $1.4 million in the nine months ended September 30, 2019. Under the new standard, our share-based payment transactions with non-employees have a fixed measurement and are not remeasured over the vesting period.

Fair Value Measurements and Fai

Fair Value Measurements and Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements and Fair Value of Financial Instruments	3. Fair Value Measurements and Fair Value of Financial Instruments The authoritative guidance on the fair value hierarchy for disclosure of fair value measurements is as follows: Level 1 : Quoted prices in active markets for identical assets or liabilities. Level 2 : Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 : Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. The fair value of Level 1 securities is determined using quoted prices in active markets for identical assets. Level 1 securities consist of highly liquid money market funds. Financial assets and liabilities are considered Level 2 when their fair values are determined using inputs that are observable in the market or can be derived principally from or corroborated by observable market data such as pricing for similar securities, recently executed transactions, cash flow models with yield curves, and benchmark securities. In addition, Level 2 financial instruments are valued using comparisons to like-kind financial instruments and models that use readily observable market data as their basis. U.S. government and agency securities, commercial paper, corporate bond and certificates of deposit are valued primarily using market prices of comparable securities, bid/ask quotes, interest rate yields and prepayment spreads and are included in Level 2. In certain cases, where there is limited activity or less transparency around inputs to valuation, securities are classified as Level 3 within the valuation hierarchy. The following is a summary of the Company’s cash equivalents and short-term investments: September 30, 2019 Amortized Cost Basis Unrealized Gains Unrealized Loses Estimated Fair Value (In thousands) Level 1: Money market funds $ 145 $ — $ — $ 145 Level 2: U.S. government and agency securities 31,704 7 (1 ) 31,710 Commercial paper 103,129 2 (6 ) 103,125 Corporate bonds 23,844 11 (1 ) 23,854 Total cash equivalents and short-term investments 158,822 20 (8 ) 158,834 Less: cash equivalents (70,023 ) (1 ) 4 (70,020 ) Total short-term investments $ 88,799 $ 19 $ (4 ) $ 88,814 December 31, 2018 Amortized Cost Basis Unrealized Gains Unrealized Loses Estimated Fair Value (In thousands) Level 1: Money market funds $ 126 $ — $ — $ 126 Level 2: U.S. government and agency securities 25,792 1 (4 ) 25,789 Commercial paper 147,606 — — 147,606 Corporate bonds 27,778 5 (17 ) 27,766 Certificates of deposit 1,420 — — 1,420 Total cash equivalents and short-term investments 202,722 6 (21 ) 202,707 Less: cash equivalents (152,577 ) — — (152,577 ) Total short-term investments $ 50,145 $ 6 $ (21 ) $ 50,130 There were no transfers within the fair value hierarchy during the three and nine months ended September 30, 2019 or 2018. The Company’s marketable securities as of September 30, 2019 mature within one year. Management regularly reviews all of the Company’s investments for other-than-temporary declines in estimated fair value. Management’s review includes the consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether management has the intent to sell the securities and whether it is more likely than not that the Company will be required to sell the securities before the recovery of their amortized cost basis. Management determined that the gross unrealized losses on the Company’s marketable securities as of September 30, 2019 were temporary in nature. Therefore, none of the Company’s marketable securities were other-than-temporarily impaired as of September 30, 2019.

Significant Agreements

Significant Agreements	9 Months Ended
	Sep. 30, 2019
Research And Development [Abstract]
Significant Agreements	4. Significant Agreements Editas— In January 2018, the Company entered into an agreement to amend its collaboration, option and license agreement with Editas Medicine, Inc. (“Editas”). The Company originally entered into an agreement with Editas in August 2016 pursuant to which the Company and Editas collaborated on certain studies using AAV vectors in connection with Editas’ genome editing technology and the Company granted to Editas an exclusive option to obtain certain exclusive rights to use the Company’s proprietary vectors in up to five ophthalmic indications. Under the terms of the agreement, as amended, Editas had until November 2018 to exercise the option with respect to a designated initial indication, which Editas declined to do. With respect to the four other indications, Editas had until August 2019 to exercise the option, otherwise all options would expire. Editas did not exercise the option, and the agreement terminated on August 8, 2019. Under Topic 606, the transaction price is $1.5 million, which includes the $1.0 million non-refundable upfront payment for license and research services at contract inception and the one-time, non-refundable cash payment of $0.5 million made by Editas in February 2018 in consideration for extending the agreement. The arrangement provides for additional payments to the Company when certain development and regulatory milestones are achieved. Because these milestone payments are not within the control of the Company and are not considered probable of being achieved until the events occur, the Company did not include them in the transaction price. The transaction price of $1.5 million was allocated to a single performance obligation: research and development. During the three and nine months ended September 30, 2018, the Company recognized revenue of $0.5 million and $0.7 million associated with Editas. The remaining performance obligations for Editas were completed during 2018. During the three and nine months ended September 30, 2019, the Company had no recognized revenue from the Editas collaboration agreement. GenSight - In February 2014, the Company entered into an agreement with GenSight Biologics, S.A, in which the Company granted GenSight a non-exclusive license to its proprietary AAV.7m8 vector to develop gene therapy products to deliver certain therapeutic transgenes. Under the agreement, the Company is eligible to receive development, regulatory and commercial milestones. Also, the Company is eligible to receive low to mid-single digit royalties on sales of GenSight’s licensed products. During the three and nine month ended September 30, 2019, GenSight achieved a clinical development milestone pursuant to the agreement. This milestone was previously constrained under Topic 606. The Company earned a $250,000 milestone payment, which was recognized as revenue in the condensed consolidated statement of operations and comprehensive loss for the three and nine month ended September 30, 2019

Leases

Leases	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Leases	5. Leases In June 2018, the Company entered into an operating lease agreement for new office and laboratory space which consists of approximately 81,000 square feet located in Redwood City, California. The lease term is 10 years beginning September 2018 through February 2029 with two options to extend the lease term for a period of seven years each. The Company has the right to make tenant improvements, including the addition of laboratory space, with a lease incentive allowance of $8.1 million. The rent payments began on March 1, 2019. The lease agreement provides for an escalation of rent payments each year after an abatement period. In connection with the lease, the Company has provided the landlord with a letter of credit in the amount of $1.0 million. The security for the letter of credit of $1.0 million is classified as restricted cash under long term assets on the condensed consolidated balance sheet. The Company also has an operating lease agreement for its Menlo Park office building which expires on May 8, 2020. The Company may extend this lease for up to four years. The lease agreement provides for an escalation of rent payments each year. The Company adopted Accounting Standards Update (ASU) No. 2016-02, Leases (See Note 2) and recorded right-of-use assets of $23.1 million and a lease liability of $24.7 million as of January 1, 2019. The estimated incremental borrowing rate used to measure the lease liability is 8.5%. Rent expense for the three months ended September 30, 2019 and 2018 was $1.7 million, and $0.8 million, respectively, which includes variable lease costs for utilities, parking, maintenance, and real estate taxes. Variable lease expenses for the three months ended September 30, 2019 and 2018 were $0.4 million and $0.2 million, respectively. Rent expense for the nine months ended September 30, 2019 and 2018 was $4.8 million, and $1.8 million, respectively, including variable lease cost of $0.9 million and $0.7 million, respectively. The Company recognizes rent expense on a straight‑line basis over the lease period. The undiscounted future non-cancellable lease payments under the Company’s operating leases as of September 30, 2019 is as follows: Year ending December 31, (In thousands) 2019 (remaining 3 months) $ 1,142 2020 4,221 2021 4,683 2022 4,846 2023 5,016 Thereafter 28,837 Total undiscounted lease payments 48,745 Less: Present value adjustment (16,046 ) Less: Tenant improvement allowance (5,275 ) Total $ 27,424

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	6. Balance Sheet Components Property and Equipment, Net Property and equipment, net consists of the following: September 30, 2019 December (In thousands) Computer equipment and software $ 697 $ 646 Laboratory equipment 6,170 5,470 Furniture and fixtures 678 678 Leasehold improvements 1,602 1,602 Construction in progress 13,735 1,612 Total property and equipment 22,882 10,008 Less accumulated depreciation and amortization (7,633 ) (6,422 ) Property and equipment, net $ 15,249 $ 3,586 Depreciation and amortization expense related to property and equipment for the three months ended September 30, 2019 and 2018 was $0.4 million, and $0.4 million, respectively, and for the nine months ended September 30, 2019 and 2018 was $1.2 million, and $1.4 million, respectively. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consist of the following: September 30, 2019 December (In thousands) Accrued professional services $ 4,623 $ 2,291 Employee compensation 3,037 2,944 Accrued preclinical, clinical and process development costs 1,248 1,850 Other 1,412 1,699 Total accrued expenses and other current liabilities $ 10,320 $ 8,784

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies The Company was a party to a master services agreement (“MSA”) with Cornell University (“Cornell”) originally established in August 2014 and amended in December 2015. Under the MSA, Cornell provided assistance in regulatory affairs, overall project management, and parameter development. This MSA included services relating to gene therapy programs directed to A1AT deficiency, HAE and severe allergy. The MSA, as amended, provided for the Company to pay Cornell $13.3 million ratably over four years for these services as services were performed. In December 2016, the Company informed Cornell that the Company decided to terminate the MSA for material breach, effective January 6, 2017. Subsequently, Cornell informed the Company that it disputed the validity of the Company’s termination of the MSA. In June 2019, Cornell and the Company entered into a settlement agreement, as a result of which the Company paid Cornell a $2.0 million settlement payment. There was no current period expense from the settlement, as the estimated costs associated with the termination of the MSA were previously accrued during the year ended December 31, 2017. The Company’s license agreements with Cornell for A1AT deficiency and HAE remain in effect. Legal Proceedings From time to time, the Company may become involved in litigation and other legal actions. The Company estimates the range of liability related to any pending litigation where the amount and range of loss can be estimated. The Company records its best estimate of a loss when the loss is considered probable. Where a liability is probable and there is a range of estimated loss with no best estimate in the range, the Company records a charge equal to at least the minimum estimated liability for a loss contingency when both of the following conditions are met: (i) information available prior to issuance of the financial statements indicates that it is probable that a liability had been incurred at the date of the financial statements and (ii) the range of loss can be reasonably estimated. There have been no material changes from the legal proceedings described in the Company’s annual report on Form 10-K for the year ended December 31, 2018.

Equity Incentive Awards

Equity Incentive Awards	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Equity Incentive Awards	8. Equity Incentive Awards The following table summarizes the Company’s option activity and related information: Number of Options (in thousands) Weighted- Average Exercise Price Balance at December 31, 2018 6,447 $ 5.83 Options granted 4,265 7.13 Options exercised (1,133 ) 2.31 Options cancelled (924 ) 3.77 Balance at September 30, 2019 8,655 $ 7.15 Exercisable as of September 30, 2019 3,563 $ 7.59 Restricted Stock Units (“RSUs”) The following table summarizes the Company’s RSUs activity and related information: Number of Units (in thousands) Weighted- Average Grant- Date Fair Value Outstanding at December 31, 2018 2,397 $ 9.23 Granted 250 3.95 Vested and released (676 ) 4.88 Forfeited (834 ) 4.75 Outstanding at September 30, 2019 1,137 $ 4.66 On July 30, 2019, the Company’s Board of Directors amended the 2017 Inducement Plan to increase the number of shares issuable thereunder by 1,500,000 to 3,100,000 shares. Stock-Based Compensation Expense The following table presents, by operating expense, the Company’s stock-based compensation expense: Three Months Ended Nine Months Ended September 30, September 30, 2019 2018 2019 2018 (In thousands) Research and development $ 1,352 $ 1,693 $ 2,829 $ 4,892 General and administrative 1,309 1,303 4,220 7,358 Total stock-based compensation expense $ 2,661 $ 2,996 $ 7,049 $ 12,250 On May 1, 2019, the Chair of the Board of Directors of the Company resigned. In connection with the resignation, the Company approved the modification of stock options to accelerate vesting and to extend the exercise period of certain awards. As a result of the modification, the Company recorded additional stock-based compensation expense of $0.9 million in the nine months ended September 30, 2019.

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Net Loss per Share	9. Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding for the period. Diluted net loss per share is computed by giving effect to all potential dilutive common stock equivalents outstanding for the period using the treasury stock method. Outstanding stock options, RSUs, rights under the employee stock purchase plan (“ESPP”) and warrants are considered to be common stock equivalents and are only included in the calculation of diluted net loss per share when their effect is dilutive. The Company excluded approximately 10.0 million and 9.1 million shares of potentially dilutive securities as of September 30, 2019 and 2018, respectively, from the computations of diluted weighted-average shares outstanding because their effect would be anti-dilutive.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2019
Related Party Transactions [Abstract]
Related Party Transactions	10. Related Party Transactions In May 2019, the Company entered into a common stock purchase agreement with James Scopa, a member of the Board, and Anne Kenner, as Trustees for the James P. Scopa and Anne E. Kenner Family Trust (the “Trust”), pursuant to which the Trust purchased an aggregate of 20,000 shares of the Company’s common stock at a price of $6.71 per share, for an aggregate cash purchase price of $0.1 million.

Summary of significant accoun_2

Summary of significant accounting policies (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The accompanying condensed consolidated financial statements have been prepared in accordance with U.S. GAAP. The preparation of the condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent liabilities and the reported amounts of expenses in the condensed consolidated financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Leases	Recently Adopted Accounting Pronouncements Leases The Company adopted Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) (“Topic 842”) on January 1, 2019. For its long-term operating leases, the Company recognizes a right-of-use asset and a lease liability on its condensed consolidated balance sheets. The Company adopted the new standard using the modified retrospective approach and recorded a lease liability of $24.7 million, and a right‑to‑use asset of $23.1 million, and made no adjustment to the accumulated deficit. In connection with the adoption of the lease standard, the Company also derecognized deferred rent of $1.5 million. The adoption of Topic 842 did not have an impact on the condensed consolidated statement of operations. The lease liability is determined as the present value of future lease payments using an estimated rate of interest that the Company would pay to borrow equivalent funds on a collateralized basis at the lease commencement date. In order to estimate the incremental borrowing rate, management estimated its credit rating, adjusted the credit rating for the nature of the collateral, and benchmarked the borrowing rate against observable yields on comparable securities with a similar term. As of the adoption date, the Company estimated the incremental borrowing rate to be 8.5%. The Company based the right-of-use asset on the liability adjusted for any prepaid or deferred rent. The Company determined the lease term at the commencement date by considering whether renewal options and termination options are reasonably assured of exercise. Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included it in operating expenses on the statements of operations and comprehensive loss. Variable lease payments include lease operating expenses. The Company elected the practical expedients permitted under Topic 842, which among other things, allowed the Company to carry forward the historical lease classification of those leases in place as of January 1, 2019. The Company elected to exclude from its condensed consolidated balance sheets recognition of leases having a term of 12 months or less (short-term leases) and elected to not separate lease components and non-lease components for its long-term real-estate leases.
Accounting Standard Update Recently Adopted	Share-based payment to nonemployees In June 2018, the FASB issued ASU 2018-07, “Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting” (“Topic 718”) that expands the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. The Company adopted ASU 2018–07 on January 1, 2019 and the impact of the adoption resulted in lower stock-based compensation of $1.4 million in the nine months ended September 30, 2019. Under the new standard, our share-based payment transactions with non-employees have a fixed measurement and are not remeasured over the vesting period.

Fair Value Measurements and F_2

Fair Value Measurements and Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Summary of Cash Equivalents and Short-term Investments	The following is a summary of the Company’s cash equivalents and short-term investments: September 30, 2019 Amortized Cost Basis Unrealized Gains Unrealized Loses Estimated Fair Value (In thousands) Level 1: Money market funds $ 145 $ — $ — $ 145 Level 2: U.S. government and agency securities 31,704 7 (1 ) 31,710 Commercial paper 103,129 2 (6 ) 103,125 Corporate bonds 23,844 11 (1 ) 23,854 Total cash equivalents and short-term investments 158,822 20 (8 ) 158,834 Less: cash equivalents (70,023 ) (1 ) 4 (70,020 ) Total short-term investments $ 88,799 $ 19 $ (4 ) $ 88,814 December 31, 2018 Amortized Cost Basis Unrealized Gains Unrealized Loses Estimated Fair Value (In thousands) Level 1: Money market funds $ 126 $ — $ — $ 126 Level 2: U.S. government and agency securities 25,792 1 (4 ) 25,789 Commercial paper 147,606 — — 147,606 Corporate bonds 27,778 5 (17 ) 27,766 Certificates of deposit 1,420 — — 1,420 Total cash equivalents and short-term investments 202,722 6 (21 ) 202,707 Less: cash equivalents (152,577 ) — — (152,577 ) Total short-term investments $ 50,145 $ 6 $ (21 ) $ 50,130

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Schedule of Undiscounted Future Lease Payments under Operating Lease	The undiscounted future non-cancellable lease payments under the Company’s operating leases as of September 30, 2019 is as follows: Year ending December 31, (In thousands) 2019 (remaining 3 months) $ 1,142 2020 4,221 2021 4,683 2022 4,846 2023 5,016 Thereafter 28,837 Total undiscounted lease payments 48,745 Less: Present value adjustment (16,046 ) Less: Tenant improvement allowance (5,275 ) Total $ 27,424

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consists of the following: September 30, 2019 December (In thousands) Computer equipment and software $ 697 $ 646 Laboratory equipment 6,170 5,470 Furniture and fixtures 678 678 Leasehold improvements 1,602 1,602 Construction in progress 13,735 1,612 Total property and equipment 22,882 10,008 Less accumulated depreciation and amortization (7,633 ) (6,422 ) Property and equipment, net $ 15,249 $ 3,586
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following: September 30, 2019 December (In thousands) Accrued professional services $ 4,623 $ 2,291 Employee compensation 3,037 2,944 Accrued preclinical, clinical and process development costs 1,248 1,850 Other 1,412 1,699 Total accrued expenses and other current liabilities $ 10,320 $ 8,784

Equity Incentive Awards (Tables

Equity Incentive Awards (Tables)	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Options Activity	The following table summarizes the Company’s option activity and related information: Number of Options (in thousands) Weighted- Average Exercise Price Balance at December 31, 2018 6,447 $ 5.83 Options granted 4,265 7.13 Options exercised (1,133 ) 2.31 Options cancelled (924 ) 3.77 Balance at September 30, 2019 8,655 $ 7.15 Exercisable as of September 30, 2019 3,563 $ 7.59
Summary of Restricted Stock Units Activity	The following table summarizes the Company’s RSUs activity and related information: Number of Units (in thousands) Weighted- Average Grant- Date Fair Value Outstanding at December 31, 2018 2,397 $ 9.23 Granted 250 3.95 Vested and released (676 ) 4.88 Forfeited (834 ) 4.75 Outstanding at September 30, 2019 1,137 $ 4.66
Stock-Based Compensation Expense	The following table presents, by operating expense, the Company’s stock-based compensation expense: Three Months Ended Nine Months Ended September 30, September 30, 2019 2018 2019 2018 (In thousands) Research and development $ 1,352 $ 1,693 $ 2,829 $ 4,892 General and administrative 1,309 1,303 4,220 7,358 Total stock-based compensation expense $ 2,661 $ 2,996 $ 7,049 $ 12,250

Organization and Basis of Pre_2

Organization and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ (366,104)	$ (320,543)

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019
Summary Of Significant Accounting Policies [Line Items]
Lease liability	$ 27,424		$ 27,424
Right-to-use asset	21,511		21,511
Stock-based compensation	$ (2,661)	$ (2,996)	$ (7,049)	$ (12,250)
ASU No. 2016-2, Leases [Member]
Summary Of Significant Accounting Policies [Line Items]
Lease liability					$ 24,700
Right-to-use asset					23,100
Adjustment to accumulated deficit					$ 0
Estimated incremental borrowing rate	8.50%		8.50%		8.50%
Restatement Adjustment [Member] | ASU No. 2016-2, Leases [Member]
Summary Of Significant Accounting Policies [Line Items]
Derecognized deferred rent					$ 1,500
Restatement Adjustment [Member] | ASU 2018-07 [Member]
Summary Of Significant Accounting Policies [Line Items]
Stock-based compensation			$ 1,400

Fair Value Measurements and F_3

Fair Value Measurements and Fair Value of Financial Instruments - Summary of Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$ 88,799	$ 50,145
Unrealized Gains	19	6
Unrealized Loses	(4)	(21)
Estimated Fair Value	88,814	50,130
Less: Cash Equivalents, Amortized Cost Basis	(70,023)	(152,577)
Less: Cash Equivalents, Unrealized Gains	(1)	0
Less: Cash Equivalents, Unrealized Loses	4	0
Less: Cash Equivalents, Estimated Fair Value	(70,020)	(152,577)
Money Market Funds [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	145	126
Estimated Fair Value	145	126
U.S. Government Agency Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	31,704	25,792
Unrealized Gains	7	1
Unrealized Loses	(1)	(4)
Estimated Fair Value	31,710	25,789
Commercial Paper [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	103,129	147,606
Unrealized Gains	2
Unrealized Loses	(6)
Estimated Fair Value	103,125	147,606
Certificates of Deposit [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		1,420
Estimated Fair Value		1,420
Corporate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	23,844	27,778
Unrealized Gains	11	5
Unrealized Loses	(1)	(17)
Estimated Fair Value	23,854	27,766
Total Cash Equivalents and Short-term Investments [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	158,822	202,722
Unrealized Gains	20	6
Unrealized Loses	(8)	(21)
Estimated Fair Value	$ 158,834	$ 202,707

Fair Value Measurements and F_4

Fair Value Measurements and Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Available For Sale Securities [Abstract]
Liabilities transferred within Level 3	$ 0	$ 0	$ 0	$ 0
Marketable securities other-than-temporarily impaired	$ 0		$ 0

Significant Agreements - Additi

Significant Agreements - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended
	Feb. 28, 2018	Aug. 31, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
License Agreement [Line Items]
Revenue recognized			$ 250,000	$ 833,000	$ 250,000	$ 1,542,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Editas Medicine Inc [Member]
License Agreement [Line Items]
Revenue recognized			$ 0	$ 500,000	$ 0	$ 700,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Editas Medicine Inc [Member] | Topic 606 [Member]
License Agreement [Line Items]
Collaboration, option and license agreement fee receivable per indication		$ 1,500,000
Non-refundable upfront payment received		$ 1,000,000
Initial payments received	$ 500,000
GenSight Biologics [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
License Agreement [Line Items]
Revenue recognized			$ 250,000,000		$ 250,000,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember

Leases - Additional Information

Leases - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019USD ($)ft²	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)ft²	Sep. 30, 2018USD ($)	Jan. 01, 2019USD ($)	Dec. 31, 2018USD ($)
Lessee Lease Description [Line Items]
Restricted cash	$ 999		$ 999			$ 999
Right-of-use assets	21,511		21,511
Lease liability	27,424		27,424
Rent expense	1,700	$ 800	4,800	$ 1,800
Variable lease costs	$ 400	$ 200	$ 900	$ 700
ASU No. 2016-2, Leases [Member]
Lessee Lease Description [Line Items]
Right-of-use assets					$ 23,100
Lease liability					$ 24,700
Estimated incremental borrowing rate	8.50%		8.50%		8.50%
Redwood City [Member]
Lessee Lease Description [Line Items]
Lease entered date			Jun. 30, 2018
Lease, area of land | ft²	81,000		81,000
Lease term	10 years		10 years
Lease renewal term	7 years		7 years
Lease description			The lease term is 10 years beginning September 2018 through February 2029 with two options to extend the lease term for a period of seven years each.
Lease, option to extend			true
Lease incentive allowance			$ 8,100
Restricted cash	$ 1,000		$ 1,000
Menlo Park [Member]
Lessee Lease Description [Line Items]
Lease renewal term	4 years		4 years
Lease, option to extend			true
Lease expiration date			May 8, 2020
Letter of Credit [Member] | Redwood City [Member]
Lessee Lease Description [Line Items]
Security deposit	$ 1,000		$ 1,000

Leases - Schedule of Undiscount

Leases - Schedule of Undiscounted Future Lease Payments under Operating Lease (Detail) $ in Thousands	Sep. 30, 2019USD ($)
Leases [Abstract]
2019 (remaining 3 months)	$ 1,142
2020	4,221
2021	4,683
2022	4,846
2023	5,016
Thereafter	28,837
Total undiscounted lease payments	48,745
Less: Present value adjustment	(16,046)
Less: Tenant improvement allowance	(5,275)
Total	$ 27,424

Balance Sheet Components - Sche

Balance Sheet Components - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 22,882	$ 10,008
Less accumulated depreciation and amortization	(7,633)	(6,422)
Property and equipment, net	15,249	3,586
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	697	646
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	6,170	5,470
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	678	678
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,602	1,602
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 13,735	$ 1,612

Balance Sheet Components - Addi

Balance Sheet Components - 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	$ 400	$ 400	$ 1,219	$ 1,361

Balance Sheet Components - Sc_2

Balance Sheet Components - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accrued Expenses And Other Current Liabilities [Abstract]
Accrued professional services	$ 4,623	$ 2,291
Employee compensation	3,037	2,944
Accrued preclinical, clinical and process development costs	1,248	1,850
Other	1,412	1,699
Total accrued expenses and other current liabilities	$ 10,320	$ 8,784

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - Cornell University [Member] - Master Service Agreement [Member] - USD ($)	1 Months Ended		9 Months Ended
	Jun. 30, 2019	Dec. 31, 2015	Sep. 30, 2019
Other Commitments [Line Items]
Service purchase commitment amount		$ 13,300,000
Service expenses payment period		4 years
Payment for settlement agreement	$ 2,000,000
Settlement agreement current period expense			$ 0

Equity Incentive Awards - Summa

Equity Incentive Awards - Summary of Stock Options Activity (Detail) - $ / shares shares in Thousands	9 Months Ended
	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Options Outstanding [Roll Forward]
Number of Options, Beginning Balance	6,447
Number of Options, Options granted	4,265
Number of Options, Options exercised	(1,133)
Number of Options, Options cancelled	(924)
Number of Options, Ending Balance	8,655
Number of Options, Exercisable	3,563
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price, Beginning Balance	$ 5.83
Weighted-Average Exercise Price, Options granted	7.13
Weighted-Average Exercise Price, Options exercised	2.31
Weighted-Average Exercise Price, Options cancelled	3.77
Weighted-Average Exercise Price, Ending Balance	7.15
Weighted-Average Exercise Price, Exercisable	$ 7.59

Equity Incentive Awards - Sum_2

Equity Incentive Awards - Summary of Restricted Stock Units Activity (Detail) - Restricted Stock Units [Member] shares in Thousands	9 Months Ended
	Sep. 30, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Beginning Balance | shares	2,397
Number of Units, Granted | shares	250
Number of Units, Vested and released | shares	(676)
Number of Units, Forfeited | shares	(834)
Number of Units, Ending Balance | shares	1,137
Weighted-Average Grant-Date Fair Value, Beginning Balance | $ / shares	$ 9.23
Weighted-Average Grant-Date Fair Value, Granted | $ / shares	3.95
Weighted-Average Grant-Date Fair Value, Vested and released | $ / shares	4.88
Weighted-Average Grant-Date Fair Value, Forfeited | $ / shares	4.75
Weighted-Average Grant-Date Fair Value, Ending Balance | $ / shares	$ 4.66

Equity Incentive Awards - Addit

Equity Incentive Awards - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Jul. 30, 2019	Jul. 29, 2019
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Additional stock-based compensation expense	$ 0.9
Restricted Stock Units [Member] | 2017 Inducement Plan [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Number of shares issuable under Inducement plan		3,100,000	1,500,000

Equity Incentive Awards - Stock

Equity Incentive Awards - 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
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 2,661	$ 2,996	$ 7,049	$ 12,250
Research and Development [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	1,352	1,693	2,829	4,892
General and Administrative [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 1,309	$ 1,303	$ 4,220	$ 7,358

Net Loss per Share - Additional

Net Loss per Share - Additional Information (Detail) - shares shares in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Antidilutive common stock equivalents excluded from calculation of diluted net loss per share	10	9.1

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2019	Jun. 30, 2019	Mar. 31, 2018	Sep. 30, 2019
Related Party Transaction [Line Items]
Cash purchase price				$ 134
Common Stock [Member]
Related Party Transaction [Line Items]
Issuance of common stock		20,000	11,642,128
Common Stock [Member] | Trust [Member]
Related Party Transaction [Line Items]
Issuance of common stock	20,000
Share price	$ 6.71
Cash purchase price	$ 100