Sierra Oncology (SRRA) 10-Q 8 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 06, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	SRRA
Entity Registrant Name	Sierra Oncology, Inc.
Entity Central Index Key	0001290149
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Emerging Growth Company	true
Entity Small Business	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		74,688,283

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
CURRENT ASSETS:
Cash and cash equivalents	$ 90,908	$ 106,046
Prepaid expenses and other current assets	3,699	2,706
Total current assets	94,607	108,752
Property and equipment, net	150	168
Operating lease right-of-use asset	716
Other assets	622	549
TOTAL ASSETS	96,095	109,469
CURRENT LIABILITIES:
Accrued and other liabilities	6,619	8,812
Accounts payable	619	1,287
Total current liabilities	7,238	10,099
Term loan	4,763	4,891
Operating lease liabilities	503
TOTAL LIABILITIES	12,504	14,990
Commitments and Contingencies (Note 8)
STOCKHOLDERS' EQUITY:
Preferred stock, $0.001 par value; 10,000,000 shares authorized as of March 31, 2019 and December 31, 2018; nil shares issued and outstanding as of March 31, 2019 and December 31, 2018
Common stock, $0.001 par value; 500,000,000 shares authorized as of March 31, 2019 and December 31, 2018; 74,688,283 and 74,365,965 shares issued and outstanding as of March 31, 2019 and December 31, 2018	74	74
Additional paid-in capital	773,961	771,817
Accumulated deficit	(690,444)	(677,412)
TOTAL STOCKHOLDERS' EQUITY	83,591	94,479
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 96,095	$ 109,469

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	74,688,283	74,365,965
Common stock, shares outstanding	74,688,283	74,365,965

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 10,137	$ 8,334
General and administrative	3,365	3,420
Total operating expenses	13,502	11,754
Loss from operations	(13,502)	(11,754)
Other income, net	325	272
Loss before provision for (benefit from) income taxes, net	(13,177)	(11,482)
Provision for (benefit from) income taxes, net	(145)	43
Net loss	$ (13,032)	$ (11,525)
Net loss per common share, basic and diluted	$ (0.17)	$ (0.19)
Weighted-average shares used in computing net loss per common share, basic and diluted	74,485,295	59,722,743

Condensed Consolidated Statem_2

Condensed Consolidated Statement of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]
Stockholders Equity, Beginning Balance at Dec. 31, 2017	$ 94,726	$ 52	$ 718,751	$ (624,077)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2017		52,395,223
Issuance of common stock for exercise of stock options	101		101
Issuance of common stock for exercise of stock options, Shares		64,458
Stock-based compensation	1,499		1,499
Issuance of common stock, net of offering costs	45,996	$ 22	45,974
Issuance of common stock, net of offering costs, Shares		21,850,000
Net loss	(11,525)			(11,525)
Stockholders Equity, Ending Balance at Mar. 31, 2018	130,797	$ 74	766,325	(635,602)
Stockholders Equity, Ending Balance, Shares at Mar. 31, 2018		74,309,681
Stockholders Equity, Beginning Balance at Dec. 31, 2018	94,479	$ 74	771,817	(677,412)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2018		74,365,965
Issuance of common stock for exercise of stock options	445		445
Issuance of common stock for exercise of stock options, Shares		322,318
Stock-based compensation	1,699		1,699
Net loss	(13,032)			(13,032)
Stockholders Equity, Ending Balance at Mar. 31, 2019	$ 83,591	$ 74	$ 773,961	$ (690,444)
Stockholders Equity, Ending Balance, Shares at Mar. 31, 2019		74,688,283

Condensed Consolidated Statem_3

Condensed Consolidated Statement of Stockholders' Equity (Parenthetical) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)
Net of offering costs	$ 3.2
Common Stock [Member]
Net of offering costs	3.2
Additional Paid-In Capital [Member]
Net of offering costs	$ 3.2

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (13,032)	$ (11,525)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	1,699	1,499
Depreciation and amortization	62	29
Other	(26)	41
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,003)	(795)
Accrued, other and operating lease liabilities	(2,599)	(1,261)
Accounts payable	(667)	(597)
Net cash used in operating activities	(15,566)	(12,609)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(14)
Net cash used in investing activities	(14)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon follow-on offering, net of offering costs		45,996
Proceeds from exercise of common stock options	445	101
Net cash provided by financing activities	445	46,097
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash	(3)	(20)
NET INCREASE / (DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(15,138)	33,468
CASH, CASH EQUIVALENTS AND RESTRICTED CASH - Beginning of period	106,346	100,536
CASH, CASH EQUIVALENTS AND RESTRICTED CASH - End of period	91,208	134,004
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for (refund of) income taxes, net	(29)	73
Cash paid for interest	81
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Right-of-use asset obtained in exchange for operating lease obligation	$ 771
Property and equipment purchases included in accrued and other liabilities		21
Offering costs included in accrued and other liabilities		$ 209

The Company and Basis of Presen

The Company and Basis of Presentation	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
The Company and Basis of Presentation	1. The Company and Basis of Presentation Organization and Description of Business Sierra Oncology, Inc. (together with its subsidiaries, collectively referred to as the “Company”), a Delaware corporation, is a clinical stage drug development company advancing targeted therapeutics for the treatment of patients with unmet medical needs in hematology and oncology. In August 2018, the Company acquired its lead drug candidate, momelotinib, a potent, selective and orally-bioavailable JAK1, JAK2 and ACVR1 inhibitor that has been investigated in two completed Phase 3 clinical trials for the treatment of myelofibrosis and has demonstrated a potentially differentiated therapeutic profile encompassing anemia-related clinical benefits, as well as achieving substantive splenic volume reduction and constitutional symptom control. The Company is also advancing SRA737 and SRA141. SRA737 is a potent, highly selective, orally bioavailable small molecule inhibitor of Checkpoint kinase 1 (Chk1), a key regulator of cell cycle progression and the DNA Damage Response (DDR) replication stress response. SRA141 is a potent, selective and orally bioavailable small molecule inhibitor of cell division cycle 7 kinase (Cdc7), a key regulator of DNA replication and involved in the DDR network. The Company’s primary activities since inception have been conducting research and development activities, conducting preclinical and clinical testing, recruiting personnel, performing business and financial planning, identifying and evaluating additional drug candidates for potential in-licensing The Company has not generated any product revenue related to its primary business purpose to date, nor has it generated any income, and is subject to a number of risks and uncertainties, which include dependence on key individuals, the need to identify and successfully develop commercially viable products, the need to obtain regulatory approval for its products and commercialize them, and the need to obtain adequate additional financing to fund the development of its product candidates. As of March 31, 2019, the Company had $90.9 million of cash and cash equivalents. The Company believes that its balance of cash and cash equivalents as of the date of the issuance of these condensed consolidated financial statements is sufficient to fund its current operational plan for at least the next twelve months though it may pursue raising additional capital through equity financings or other arrangements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) and applicable rules and regulations of the Securities and Exchange Commission regarding interim financial reporting. Accordingly, they do not include all the information and notes required for complete financial statements and should be read in conjunction with the Company’s audited consolidated financial statements included in the Company’s Annual Report on Form 10-K 10-K, These unaudited condensed consolidated financial statements and related disclosure have been prepared on the same basis as the annual consolidated financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair presentation of the Company’s condensed consolidated financial statements included in this report. The condensed consolidated results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019, or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2018 included herein was derived from the audited consolidated financial statements as of that date. Use of Estimates The preparation of the condensed consolidated financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying condensed consolidated financial statements include, but are not limited to the fair value of stock options and the warrant issued, accruals such as research and development costs, and recoverability of the Company’s net deferred tax assets and related valuation allowance. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could materially differ from those estimates. Operating Lease Right-of-Use On January 1, 2019, the Company adopted Financial Accounting Standards Board (FASB) Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) non-lease The Company recognizes operating leases with terms greater than one year as right-of-use Note 6, Leases Stock-Based Compensation The Company accounts for stock-based payments at fair value, which is measured using the Black-Scholes option-pricing model. For stock-based awards that vest subject to the satisfaction of a service requirement, the fair value measurement date for stock-based compensation awards is the date of grant and the expense is recognized on a straight-line basis over the vesting period, which is generally the service period. The Company accounts for forfeitures as they occur. On January 1, 2019, the Company adopted FASB ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	3. Net Loss Per Share Basic net loss per share is calculated by dividing net loss by the weighted-average number of common shares outstanding during the period without consideration for common stock equivalents. Diluted net loss per share is computed by dividing net loss by the weighted-average number of common share equivalents outstanding for the period determined using the treasury-stock method. For purposes of this calculation, stock options and the warrant for common stock 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 following shares of common stock equivalents were excluded from the calculation of diluted net loss per share for the periods presented because including them would have been antidilutive: March 31, 2019 March 31, 2018 (in thousands) Options to purchase common stock 13,198,385 10,202,831 Warrant for common stock 73,529 — Total potential dilutive shares 13,271,914 10,202,831

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements The Company measures and reports its cash equivalents and restricted cash at fair value. The following table sets forth the fair value of the Company’s financial assets measured on a recurring basis by level within the fair value hierarchy: March 31, 2019 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 90,238 $ — $ — $ 90,238 Restricted money market funds 300 — — 300 Total financial assets $ 90,538 $ — $ — $ 90,538 December 31, 2018 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 105,224 $ — $ — $ 105,224 Restricted money market funds 300 — — 300 Total financial assets $ 105,524 $ — $ — $ 105,524 Money market funds and restricted money market funds are measured at fair value on a recurring basis using quoted prices and are classified as a Level 1 input. There were no transfers between Levels 1, 2 or 3 during the three months ended March 31, 2019.

Balance Sheet Components

Balance Sheet Components	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Balance Sheet Components	5. Balance Sheet Components Cash and Cash Equivalents Cash and cash equivalents consist of the following: March 31, 2019 December 31, 2018 (in thousands) Cash $ 670 $ 822 Cash equivalents: Money market accounts 90,238 105,224 Total cash and cash equivalents $ 90,908 $ 106,046 The following table provides a reconciliation of cash, cash equivalents and restricted cash reported in the condensed consolidated balance sheets to the amounts shown in the condensed consolidated statements of cash flows. March 31, 2019 March 31, 2018 (in thousands) Cash and cash equivalents $ 90,908 $ 133,829 Restricted cash included in other assets 300 175 Total cash, cash equivalents and restricted cash shown in the condensed consolidated statement of cash flows $ 91,208 $ 134,004 Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consist of the following: March 31, 2019 December 31, 2018 (in thousands) Other receivables $ 1,487 $ 751 Prepaid research and development project costs 1,050 762 Prepaid insurance 371 555 Income taxes receivable 138 163 Other 653 475 Total prepaid expenses and other current assets $ 3,699 $ 2,706 Property and Equipment, net Property and equipment, net consists of the following: March 31, 2019 December 31, 2018 (in thousands) Software $ 327 $ 325 Leasehold improvements 112 112 Computer equipment 89 89 Furniture and fixtures 3 3 Property and equipment, gross 531 529 Less: accumulated depreciation (381 ) (361 ) Total property and equipment, net $ 150 $ 168 Depreciation related to the Company’s property and equipment was $21,000 and $29,000 for the three months ended March 31, 2019 and 2018, respectively. Accrued and Other Liabilities Accrued and other liabilities consist of the following: March 31, 2019 December 31, 2018 (in thousands) Accrued research and development costs $ 3,995 $ 4,485 Accrued employee-related costs 1,233 3,223 Accrued professional fees 704 357 Operating lease liabilities 171 — Current portion of term loan 168 — Accrued restructuring costs 30 33 Other 318 714 Total accrued and other liabilities $ 6,619 $ 8,812

Leases

Leases	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Leases	6. Leases In June 2017, the Company entered into an operating lease agreement to lease the office space in Vancouver, Canada commencing March 1, 2018. The lease expires on February 28, 2023 and can be extended for an additional term of 5 years. In January 2016, the Company entered into an operating lease agreement to lease office space near San Francisco, California. The operating lease agreement expires on April 30, 2019. In September 2017, the Company entered into a sublease agreement to sublet the premises to a third party until April 30, 2019. On January 1, 2019, with the adoption of ASU 2016-02, right-of-use The components of lease expense and related cash flows for the three months ended March 31, 2019 were as follows: March 31, 2019 (in thousands) Operating lease cost 51 Short-term lease cost 29 80 Operating cash flows used for operating leases $ 40 The total rent expense was $0.1 million for the three months ended March 31, 2018. As of March 31, 2019, the weighted average remaining lease term and discount rate for the operating leases are 3.9 years and 6.5%, respectively. As of March 31, 2019, maturities of lease liabilities due under the lease agreements are as follows: Years Ending December 31: Operating Leases (in thousands) Remainder of 2019 $ 157 2020 212 2021 216 2022 172 Total lease payments 757 Less imputed interest (83 ) Total $ 674 In addition to base rent, these leases require payment of non-lease non-component

Term Loan

Term Loan	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Term Loan	7. Term Loan On August 21, 2018, the Company entered into a Loan and Security Agreement (Loan Agreement) with Silicon Valley Bank (SVB), pursuant to which the Company may obtain a loan of aggregate principal amount of up to $15.0 million (Term Loans), which becomes available in three tranches, each of an aggregate principal amount of up to $5.0 million. Contemporaneously with executing the Loan Agreement, the Company drew down the first $5.0 million tranche. The second and third $5.0 million tranches may be drawn only upon the achievement of certain development milestones. Borrowings under the Loan Agreement bear interest at the greater of (i) 6.0% or (ii) a floating per annum rate 1.0% above the prime rate (for an interest rate of 6.50% at March 31, 2019), with interest only due and payable monthly, until March 1, 2020, at which time interest and principal will be due and payable monthly in equal monthly payments; are subject to a final payment fee equal to 6.75% of the aggregate principal amount. The Company may prepay all, but not less than all, of the loaned amounts subject to a prepayment fee. The Loan Agreement is secured by substantially all of the Company’s personal property, except for its intellectual property and requires the Company to maintain the lesser of $10 million or 80% of its cash and cash equivalents with SVB. The Loan Agreement contains customary covenants and customary events of default. The occurrence of an event of default could result in an increase to the applicable interest rate of 5.0%, and the consequent obligation for the Company to repay all amounts outstanding under the Loan Agreement. In connection with the Loan Agreement, the Company issued a warrant to SVB to purchase 73,529 of the Company’s common stock at a price per share of $1.87. The warrant was immediately exercisable, will expire on August 21, 2028, contains a cashless exercise provision and is classified as equity. If the Company is to draw the second or third tranche available under the Loan Agreement, the Company will grant an additional amount of common stock issuable upon exercise of the warrant based upon the principal amount advanced. In no event will the number of common stock issuable pursuant to the exercise of the warrant exceed 220,588 in aggregate. The fair value of the warrant and the debt issuance costs were recorded as debt discounts and together with the final payment fee are being amortized using the effective interest rate method over the term of the loan. As of March 31, 2019, the effective interest rate on the initial tranche of the loan was 9.89% and the unamortized debt discount was $0.1 million. Amortization of the debt discount and accrual of final payment was $0.1 million for the three months ended March 31, 2019. Scheduled payments due under the Loan Agreement, excluding the final payment fee of $0.3 million and interest payments, are as follows: March 31, 2019 (in thousands) 2020 $ 1,667 2021 2,000 2022 1,333 Total $ 5,000 During the three months ended March 31, 2019, the Company recognized $0.1 million of interest expense related to the Loan Agreement. The Company did not incur interest expense for the three months ended March 31, 2018.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. Commitments and Contingencies Asset Purchase Agreement In August 2018, the Company entered into an asset purchase agreement with Gilead Sciences, Inc. (Gilead) whereby the Company acquired worldwide rights to the pharmaceutical product momelotinib, an investigational inhibitor of Janus kinase, together with all related intellectual property rights and certain other related assets. The Company paid Gilead an upfront payment of $3.0 million in August 2018. The Company will be required to pay Gilead milestone payments of up to an aggregate of $195.0 million upon the achievement of certain development, regulatory and commercial milestones events, including a milestone payment of $5.0 million upon the dosing of the first patient in a registrational clinical trial. These milestones will be accrued once they are considered probable of occurring. In addition, the Company is required to pay Gilead mid-teen License Agreements In September 2016, the Company entered into an exclusive license agreement with CRT Pioneer Fund LP (CPF) for worldwide rights, know-how one-time product-by-product country-by-country In May 2016, the Company entered into an exclusive license agreement (Carna License Agreement) with Carna Biosciences, Inc. (Carna) for worldwide rights to develop and commercialize SRA141, a small molecule kinase inhibitor targeting Cdc7. In exchange for this exclusive right, the Company paid Carna an upfront payment of $0.9 million in June 2016. The Company will be required to pay Carna milestone payments of up to an aggregate of $270.0 million upon achievement of certain developmental, regulatory and commercial milestone events, including a milestone payment of $4.0 million upon dosing of the first patient in the first Phase 1 clinical trial for SRA141. These milestones will be accrued once they are considered probable of occurring. As of March 31, 2019, the Company had not recorded any milestone payments to Carna. In addition, the Company is required to pay Carna tiered single-digit royalties on net sales of product candidates (as defined under the Carna License Agreement). Legal On November 9, 2016, a purported securities class action lawsuit was filed in the United States District Court for the Southern District of New York against the Company and certain of its executive officers (the New York Lawsuit). The New York Lawsuit was brought by purported stockholders of the Company seeking to represent a class consisting of stockholders who purchased stock between July 15, 2015 and June 6, 2016. The New York Lawsuit asserts claims under Sections 10(b) and 20(a) of the Exchange Act and seeks unspecified damages and other relief. On March 13, 2018, the United States District Court for the Southern District of New York granted the defendants’ motion to dismiss and entered a final judgment dismissing the New York Lawsuit with prejudice. Plaintiffs thereafter filed an appeal. On December 3, 2018, the United States Court of Appeals for the Second Circuit affirmed the district court’s final judgment of dismissal. The plaintiffs did not file a notice or petition for further appellate review in this matter. On November 18, 2016, a purported securities class action lawsuit was filed in the Superior Court of the State of California for the County of San Mateo against the Company, certain of its executive officers and directors, and the underwriters for the Company’s initial public offering of its common stock. On February 9, 2017, a substantially identical putative class action suit was filed in the Superior Court of the State of California for the County of San Mateo asserting the same claims on behalf of the same putative class (the two lawsuits together, the California Lawsuits). The California Lawsuits were brought by purported stockholders of the Company seeking to represent a class consisting of stockholders who purchased stock pursuant to and/or traceable to the Company’s Registration Statement on Form S-1. From time to time, the Company may become subject to other legal proceedings, claims and litigation arising in the ordinary course of business. In addition, the Company may receive letters alleging infringement of patent or other intellectual property rights. The Company is not currently a party to any other material legal proceedings, nor is it aware of any pending or threatened litigation that, in the Company’s opinion, would have a material adverse effect on the business, operating results, cash flows or financial condition should such litigation be resolved unfavorably.

Common Stock Reserved for Issua

Common Stock Reserved for Issuance	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Common Stock Reserved for Issuance	9. Common Stock Reserved for Issuance The Company is required to reserve and keep available out of its authorized but unissued shares of common stock a number of shares sufficient to effect the conversion of all outstanding options granted and available for grant under the incentive plans, shares reserved for issuance under the employee stock purchase plan and issued warrant. March 31, 2019 December 31, 2018 Outstanding stock options under equity incentive plans 13,198,385 10,504,412 Shares reserved for future option grants under equity incentive plans 1,903,372 1,945,025 Shares reserved under the 2015 employee stock purchase plan 700,000 700,000 Shares reserved under warrant upon contingent events 147,059 147,059 Outstanding warrant 73,529 73,529 Total common stock reserved for issuance 16,022,345 13,370,025

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	10. Stock-Based Compensation In the accompanying condensed consolidated statements of operations, the Company recognized stock-based compensation expense for its employees and non-employees Three Months Ended March 31, 2019 2018 Research and development $ 1,184 $ 998 General and administrative 515 501 Total stock-based compensation $ 1,699 $ 1,499 Determination of Fair Value The estimated grant-date fair values of all of the Company’s stock-based awards were calculated using the Black-Scholes option pricing model, based on assumptions as follows: Three Months Ended March 31, 2019 2018 Expected term (in years) 5.3 — 6.9 5.4 — 6.9 Expected volatility 91 — 94 % 88 — 91 % Risk-free interest rate 2.5 — 2.6 % 2.7 — 2.8 % Expected dividend rate — % — % Equity Incentive Plans 2018 Equity Inducement Plan In September 2018, the Company’s Compensation Committee approved the 2018 Equity Inducement Plan (2018 Plan). The number of shares available for awards under the 2018 Plan was set to 1,500,000. The exercise price of each stock-based award issued under the 2018 Plan is required to be no less than the fair value of the Company’s capital stock. The vesting and exercise provisions of options or restricted awards granted are determined individually with each grant. Stock options have a 10-year 2015 Plan The 2015 Equity Incentive Plan (2015 Plan) became effective on July 14, 2015. As of March 31, 2019, 11,617,575 shares were reserved for issuance under the 2015 Plan. The number of shares reserved for issuance under the 2015 Plan will increase automatically on January 1 of each calendar year 2016 through 2025 by the number of shares equal to 4% of the total outstanding shares of the Company’s common stock as of the immediately preceding December 31. The Company’s Board of Directors or Compensation Committee may reduce the amount of the increase in any particular year. The exercise price of each stock-based award issued under the 2015 Plan is required to be no less than the fair value of the Company’s capital stock. The vesting and exercise provisions of options or restricted awards granted are determined individually with each grant. Stock options have a 10-year 2008 Plan The Company granted options under the 2008 Stock Plan (2008 Plan) until July 2015 when it was terminated as to future awards, although it continues to govern the terms of options that remain outstanding under the 2008 Plan. The 2008 Plan provided for the granting of Incentive Stock Options (ISO), nonqualified stock options and stock purchase rights. In connection with the Board of Director’s approval of the 2015 Plan, all remaining shares available for future award under the 2008 Plan were transferred to the 2015 Plan, and the 2008 Plan was terminated. A summary of activity under the 2008 Plan, 2015 Plan and 2018 Plan and related information is as follows: Options Outstanding Shares Available for Grant Number of Shares Outstanding Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Term (Years) Aggregate Intrinsic Value of Outstanding Options (in thousands) Outstanding — December 31, 2018 1,945,025 10,504,412 $ 2.86 7.80 $ 545 Awards authorized 2,974,638 Options granted (3,093,300 ) 3,093,300 1.93 Options exercised — (322,318 ) 1.38 Options forfeited/cancelled 77,009 (77,009 ) 5.59 Outstanding — March 31, 2019 1,903,372 13,198,385 $ 2.66 8.13 $ 1,358 Exercisable — March 31, 2019 6,571,244 $ 3.09 7.06 $ 1,185 Vested and expected to vest — March 31, 2019 13,198,385 $ 2.66 8.13 $ 1,358 The weighted-average grant date fair values of options granted during the three months ended March 31, 2019 and 2018 was $1.47 and $1.76 per share, respectively. The aggregate intrinsic value of options exercised was $0.1 million for the three months ended March 31, 2019 and 2018. The total grant date fair value of options vested for the three months ended March 31, 2019 and 2018 was $2.7 million and $1.7 million, respectively. As of March 31, 2019, total unrecognized stock-based compensation related to unvested stock options was $10.8 million. These costs are expected to be recognized over a remaining weighted-average period of 2.7 years as of March 31, 2019. 2015 Employee Stock Purchase Plan The Company adopted the 2015 Employee Stock Purchase Plan (ESPP) and initially reserved 700,000 shares of common stock as of its effective date of July 15, 2015. The number of shares initially reserved for issuance under the ESPP will increase automatically on January 1 for nine years from the first offering date by the number of shares equal to 1% of the total outstanding shares of the Company’s common stock as of the immediately preceding December 31. The aggregate number of shares issued over the term of the 2015 Employee Stock Purchase Plan will not exceed 3,400,000 shares of common stock. Under the ESPP, participants are offered the option to purchase shares of the Company’s common stock at a 15% discount during a series of discrete offering periods, subject to any plan limitations. The ESPP will not become effective until such time as the Compensation Committee determines in the future, and as of March 31, 2019, the initial offering periods had not commenced. As of March 31, 2019, no shares of common stock have been issued to employees participating in the ESPP and 700,000 shares were available for issuance under the ESPP.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes The Company did not record a provision for U.S. federal income taxes for the three months ended March 31, 2019 because it expects to generate a loss for the year ended December 31, 2019. The net income tax benefit of $0.1 million for the three months ended March 31, 2019 primarily represented income tax benefit resulting from foreign research and development tax credits. Income tax expense of $43,000 for the three months ended March 31, 2018 represented foreign taxes. The Company’s net U.S. deferred tax assets continue to be offset by a full valuation allowance.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) and applicable rules and regulations of the Securities and Exchange Commission regarding interim financial reporting. Accordingly, they do not include all the information and notes required for complete financial statements and should be read in conjunction with the Company’s audited consolidated financial statements included in the Company’s Annual Report on Form 10-K 10-K, These unaudited condensed consolidated financial statements and related disclosure have been prepared on the same basis as the annual consolidated financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair presentation of the Company’s condensed consolidated financial statements included in this report. The condensed consolidated results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019, or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2018 included herein was derived from the audited consolidated financial statements as of that date.
Use of Estimates	Use of Estimates The preparation of the condensed consolidated financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying condensed consolidated financial statements include, but are not limited to the fair value of stock options and the warrant issued, accruals such as research and development costs, and recoverability of the Company’s net deferred tax assets and related valuation allowance. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could materially differ from those estimates.
Operating Lease Right-of-Use Asset and Lease Liabilities	Operating Lease Right-of-Use On January 1, 2019, the Company adopted Financial Accounting Standards Board (FASB) Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) non-lease The Company recognizes operating leases with terms greater than one year as right-of-use Note 6, Leases
Stock-Based Compensation	Stock-Based Compensation The Company accounts for stock-based payments at fair value, which is measured using the Black-Scholes option-pricing model. For stock-based awards that vest subject to the satisfaction of a service requirement, the fair value measurement date for stock-based compensation awards is the date of grant and the expense is recognized on a straight-line basis over the vesting period, which is generally the service period. The Company accounts for forfeitures as they occur. On January 1, 2019, the Company adopted FASB ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Schedule of Outstanding Shares of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	The following shares of common stock equivalents were excluded from the calculation of diluted net loss per share for the periods presented because including them would have been antidilutive: March 31, 2019 March 31, 2018 (in thousands) Options to purchase common stock 13,198,385 10,202,831 Warrant for common stock 73,529 — Total potential dilutive shares 13,271,914 10,202,831

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Fair Value of Financial Assets Measured on Recurring Basis	The following table sets forth the fair value of the Company’s financial assets measured on a recurring basis by level within the fair value hierarchy: March 31, 2019 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 90,238 $ — $ — $ 90,238 Restricted money market funds 300 — — 300 Total financial assets $ 90,538 $ — $ — $ 90,538 December 31, 2018 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 105,224 $ — $ — $ 105,224 Restricted money market funds 300 — — 300 Total financial assets $ 105,524 $ — $ — $ 105,524

Balance Sheet Components (Table

Balance Sheet Components (Tables)	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Cash and Cash Equivalents	Cash and cash equivalents consist of the following: March 31, 2019 December 31, 2018 (in thousands) Cash $ 670 $ 822 Cash equivalents: Money market accounts 90,238 105,224 Total cash and cash equivalents $ 90,908 $ 106,046
Summary of Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported in the condensed consolidated balance sheets to the amounts shown in the condensed consolidated statements of cash flows. March 31, 2019 March 31, 2018 (in thousands) Cash and cash equivalents $ 90,908 $ 133,829 Restricted cash included in other assets 300 175 Total cash, cash equivalents and restricted cash shown in the condensed consolidated statement of cash flows $ 91,208 $ 134,004
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consist of the following: March 31, 2019 December 31, 2018 (in thousands) Other receivables $ 1,487 $ 751 Prepaid research and development project costs 1,050 762 Prepaid insurance 371 555 Income taxes receivable 138 163 Other 653 475 Total prepaid expenses and other current assets $ 3,699 $ 2,706
Summary of Property and Equipment, Net	Property and equipment, net consists of the following: March 31, 2019 December 31, 2018 (in thousands) Software $ 327 $ 325 Leasehold improvements 112 112 Computer equipment 89 89 Furniture and fixtures 3 3 Property and equipment, gross 531 529 Less: accumulated depreciation (381 ) (361 ) Total property and equipment, net $ 150 $ 168
Summary of Accrued and Other Liabilities	Accrued and other liabilities consist of the following: March 31, 2019 December 31, 2018 (in thousands) Accrued research and development costs $ 3,995 $ 4,485 Accrued employee-related costs 1,233 3,223 Accrued professional fees 704 357 Operating lease liabilities 171 — Current portion of term loan 168 — Accrued restructuring costs 30 33 Other 318 714 Total accrued and other liabilities $ 6,619 $ 8,812

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Schedule of Components of Lease Expense and Related Cash Flows	The components of lease expense and related cash flows for the three months ended March 31, 2019 were as follows: March 31, 2019 (in thousands) Operating lease cost 51 Short-term lease cost 29 80 Operating cash flows used for operating leases $ 40
Schedule of Maturities Lease Liabilities under Leases Agreement	As of March 31, 2019, maturities of lease liabilities due under the lease agreements are as follows: Years Ending December 31: Operating Leases (in thousands) Remainder of 2019 $ 157 2020 212 2021 216 2022 172 Total lease payments 757 Less imputed interest (83 ) Total $ 674

Term Loan (Tables)

Term Loan (Tables)	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Schedule of Estimated Future Principal Payments Due Under Loan Agreement	Scheduled payments due under the Loan Agreement, excluding the final payment fee of $0.3 million and interest payments, are as follows: March 31, 2019 (in thousands) 2020 $ 1,667 2021 2,000 2022 1,333 Total $ 5,000

Common Stock Reserved for Iss_2

Common Stock Reserved for Issuance (Tables)	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Schedule of Common Stock Reserved for Issuance	The Company is required to reserve and keep available out of its authorized but unissued shares of common stock a number of shares sufficient to effect the conversion of all outstanding options granted and available for grant under the incentive plans, shares reserved for issuance under the employee stock purchase plan and issued warrant. March 31, 2019 December 31, 2018 Outstanding stock options under equity incentive plans 13,198,385 10,504,412 Shares reserved for future option grants under equity incentive plans 1,903,372 1,945,025 Shares reserved under the 2015 employee stock purchase plan 700,000 700,000 Shares reserved under warrant upon contingent events 147,059 147,059 Outstanding warrant 73,529 73,529 Total common stock reserved for issuance 16,022,345 13,370,025

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Stock-based Compensation Expense for Employees and Non-employees	In the accompanying condensed consolidated statements of operations, the Company recognized stock-based compensation expense for its employees and non-employees Three Months Ended March 31, 2019 2018 Research and development $ 1,184 $ 998 General and administrative 515 501 Total stock-based compensation $ 1,699 $ 1,499
Schedule of Estimated Grant-date Fair Value of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions	The estimated grant-date fair values of all of the Company’s stock-based awards were calculated using the Black-Scholes option pricing model, based on assumptions as follows: Three Months Ended March 31, 2019 2018 Expected term (in years) 5.3 — 6.9 5.4 — 6.9 Expected volatility 91 — 94 % 88 — 91 % Risk-free interest rate 2.5 — 2.6 % 2.7 — 2.8 % Expected dividend rate — % — %
Summary of Stock-Based Compensation Activity	A summary of activity under the 2008 Plan, 2015 Plan and 2018 Plan and related information is as follows: Options Outstanding Shares Available for Grant Number of Shares Outstanding Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Term (Years) Aggregate Intrinsic Value of Outstanding Options (in thousands) Outstanding — December 31, 2018 1,945,025 10,504,412 $ 2.86 7.80 $ 545 Awards authorized 2,974,638 Options granted (3,093,300 ) 3,093,300 1.93 Options exercised — (322,318 ) 1.38 Options forfeited/cancelled 77,009 (77,009 ) 5.59 Outstanding — March 31, 2019 1,903,372 13,198,385 $ 2.66 8.13 $ 1,358 Exercisable — March 31, 2019 6,571,244 $ 3.09 7.06 $ 1,185 Vested and expected to vest — March 31, 2019 13,198,385 $ 2.66 8.13 $ 1,358

The Company and Basis of Pres_2

The Company and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash and cash equivalents	$ 90,908	$ 106,046	$ 133,829

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Outstanding Shares of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	13,271,914	10,202,831
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	13,198,385	10,202,831
Warrants for Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	73,529

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Fair Value of Financial Assets Measured on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial Assets
Total financial assets	$ 90,538	$ 105,524
Money Market Funds [Member]
Financial Assets
Total financial assets	90,238	105,224
Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	300	300
Level 1 [Member]
Financial Assets
Total financial assets	90,538	105,524
Level 1 [Member] | Money Market Funds [Member]
Financial Assets
Total financial assets	90,238	105,224
Level 1 [Member] | Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	$ 300	$ 300

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2019USD ($)
Fair Value Disclosures [Abstract]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	0
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	$ 0

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Cash and Cash Equivalents [Abstract]
Cash	$ 670	$ 822
Cash equivalents:
Money market accounts	90,238	105,224
Total cash and cash equivalents	$ 90,908	$ 106,046	$ 133,829

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 90,908	$ 106,046	$ 133,829
Restricted cash included in other assets	300		175
Total cash, cash equivalents and restricted cash shown in the condensed consolidated statement of cash flows	$ 91,208	$ 106,346	$ 134,004	$ 100,536

Balance Sheet Components - Su_3

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Other receivables	$ 1,487	$ 751
Prepaid research and development project costs	1,050	762
Prepaid insurance	371	555
Income taxes receivable	138	163
Other	653	475
Total prepaid expenses and other current assets	$ 3,699	$ 2,706

Balance Sheet Components - Su_4

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 531	$ 529
Less: accumulated depreciation	(381)	(361)
Total property and equipment, net	150	168
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	327	325
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	112	112
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	89	89
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3	$ 3

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Property, Plant and Equipment [Abstract]
Depreciation related to property and equipment	$ 62	$ 29

Balance Sheet Components - Su_5

Balance Sheet Components - Summary of Accrued and Other Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued research and development costs	$ 3,995	$ 4,485
Accrued employee-related costs	1,233	3,223
Accrued professional fees	704	357
Operating lease liabilities	171
Current portion of term loan	168
Accrued restructuring costs	30	33
Other	318	714
Total accrued and other liabilities	$ 6,619	$ 8,812

Leases - Additional Information

Leases - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2016	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019
Loss Contingencies [Line Items]
Operating lease right-of-use asset		$ 716		$ 800
Operating lease liability		$ 757		$ 700
Rent expense			$ 100
Weighted-average remaining lease term, operating leases (Year)		3 years 10 months 24 days
Weighted-average discount rate, Operating leases		6.50%
Operating Lease Agreement To Lease Office Space [Member] | San Francisco [Member]
Loss Contingencies [Line Items]
Operating lease agreement expiration date	Apr. 30, 2019
Vancouver Lease [Member] | Vancouver [Member]
Loss Contingencies [Line Items]
Operating lease agreement expiration date		Feb. 28, 2023
Additional lease term to be extended		5 years

Leases - Schedule of Components

Leases - Schedule of Components of Lease Expense and Related Cash Flows (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Leases [Abstract]
Operating lease cost	$ 51
Short-term lease cost	29
Total Lease Cost	80
Operating cash flows used for operating leases	$ 40

Leases - Schedule of Maturities

Leases - Schedule of Maturities Lease Liabilities under Leases Agreement (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Leases [Abstract]
Remainder of 2019	$ 157
2020	212
2021	216
2022	172
Total lease payments	757	$ 700
Less imputed interest	(83)
Total	$ 674

Term Loan - Additional Informat

Term Loan - Additional Information (Detail)	Aug. 21, 2018USD ($)Tranches	Mar. 31, 2019USD ($)$ / sharesshares	Mar. 31, 2018USD ($)
Debt Instrument [Line Items]
Amount borrowed		$ 5,000,000
Silicon Valley Bank [Member] | Maximum [Member]
Debt Instrument [Line Items]
Number of common stock issuable | shares		220,588
Silicon Valley Bank [Member] | Term Loans [Member]
Debt Instrument [Line Items]
Aggregate principal amount of loans	$ 15,000,000
Number of tranches | Tranches	3
Loan agreement, description		Contemporaneously with executing the Loan Agreement, the Company drew down the first $5.0 million tranche. The second and third $5.0 million tranches may be drawn only upon the achievement of certain development milestones.
Loan agreement, frequency of payments		Monthly
Interest payment description		Borrowings under the Loan Agreement bear interest at the greater of (i) 6.0% or (ii) a floating per annum rate 1.0% above the prime rate (for an interest rate of 6.50% at March 31, 2019), with interest only due and payable monthly, until March 1, 2020, at which time interest and principal will be due and payable monthly in equal monthly payments; are subject to a final payment fee equal to 6.75% of the aggregate principal amount.
Final payment fee as percent of aggregate principal amount of borrowing	6.75%
Minimum requirement of cash and cash equivalents held with bank		$ 10,000,000
Minimum percentage requirement of cash and cash equivalents held with bank		80.00%
Applicable interest rate of loan		5.00%
Loan payment fee		$ 300,000
Interest expense		$ 100,000	$ 0
Silicon Valley Bank [Member] | Term Loans [Member] | Prime Rate [Member]
Debt Instrument [Line Items]
Loan agreement bear interest rate		6.50%
Variable rate on borrowings above prime rate, percentage	1.00%
Silicon Valley Bank [Member] | Term Loans [Member] | Minimum [Member]
Debt Instrument [Line Items]
Loan agreement bear interest rate	6.00%
Silicon Valley Bank [Member] | Term Loans [Member] | Tranche One [Member]
Debt Instrument [Line Items]
Aggregate principal amount of loans	$ 5,000,000
Amount borrowed	5,000,000
Effective interest rate of loan		9.89%
Unamortized debt discount		$ 100,000
Amortization of debt discount and accrual of final payment		$ 100,000
Silicon Valley Bank [Member] | Term Loans [Member] | Tranche Two [Member]
Debt Instrument [Line Items]
Aggregate principal amount of loans	5,000,000
Silicon Valley Bank [Member] | Term Loans [Member] | Tranche Three [Member]
Debt Instrument [Line Items]
Aggregate principal amount of loans	$ 5,000,000
Silicon Valley Bank [Member] | Warrants to Purchase Common Stock [Member]
Debt Instrument [Line Items]
Warrant expiration date		Aug. 21, 2028
Silicon Valley Bank [Member] | Warrants to Purchase Common Stock [Member] | Term Loans [Member]
Debt Instrument [Line Items]
Warrants issued in connection with loan | shares		73,529
Price per share of issued warrants | $ / shares		$ 1.87

Term Loan - Schedule of Estimat

Term Loan - Schedule of Estimated Future Principal Payments Due Under Loan Agreement (Detail) $ in Thousands	Mar. 31, 2019USD ($)
Debt Disclosure [Abstract]
2020	$ 1,667
2021	2,000
2022	1,333
Total	$ 5,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Millions	1 Months Ended			3 Months Ended
	Aug. 31, 2018USD ($)	Oct. 31, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2019USD ($)Payments	Jan. 31, 2017USD ($)
Carna License Agreement [Member]
Loss Contingencies [Line Items]
Aggregate milestone payment				$ 270
Upfront payment paid			$ 0.9
Number of milestone payments recorded | Payments				0
Milestone payment				$ 4
CPF License Agreements [Member]
Loss Contingencies [Line Items]
Milestone payment					$ 2
Upfront payment paid		$ 7
Additional milestone payment payable				319.5
Asset Purchase Agreement [Member]
Loss Contingencies [Line Items]
Upfront payment paid	$ 3
Aggregate milestone payment				195
Milestone payment				$ 5

Common Stock Reserved for Iss_3

Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Mar. 31, 2019	Dec. 31, 2018	Jul. 15, 2015
Class of Stock [Line Items]
Total common stock reserved for future issuance	16,022,345	13,370,025
Equity Incentive Plans [Member]
Class of Stock [Line Items]
Awards outstanding	13,198,385	10,504,412
Total common stock reserved for future issuance	1,903,372	1,945,025
2015 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	700,000	700,000	700,000
Warrants to Purchase Common Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	147,059	147,059
Outstanding warrant	73,529	73,529

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-based Compensation Expense for Employees and Non-employees (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 1,699	$ 1,499
Research and development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	1,184	998
General and administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 515	$ 501

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Estimated Grant-date Fair Values of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions (Detail)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, Minimum	91.00%	88.00%
Expected volatility, Maximum	94.00%	91.00%
Risk-free interest rate, Minimum	2.50%	2.70%
Risk-free interest rate, Maximum	2.60%	2.80%
Expected dividend rate	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 3 months 18 days	5 years 4 months 24 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 10 months 24 days	6 years 10 months 24 days

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Jul. 15, 2015	Jul. 14, 2015	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance			16,022,345		13,370,025
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance	700,000		700,000		700,000
Stock option grants description			The number of shares initially reserved for issuance under the ESPP will increase automatically on January 1 for nine years from the first offering date by the number of shares equal to 1% of the total outstanding shares of the Company's common stock as of the immediately preceding December 31.
Percentage threshold of outstanding shares increased annually under the plan	1.00%
Common shares authorized to be issued during the period	3,400,000
Options to purchase shares of common stock, discount percentage	15.00%
Number of common stock issued			0
2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance			11,617,575
Stock option life in years			10 years
Stock option grants description			The number of shares reserved for issuance under the 2015 Plan will increase automatically on January 1 of each calendar year 2016 through 2025 by the number of shares equal to 4% of the total outstanding shares of the Company's common stock as of the immediately preceding December 31.
Percentage threshold of outstanding shares increased annually under the plan		4.00%
2008 Plan, 2015 Plan and 2018 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards issued			3,093,300
Awards outstanding			13,198,385		10,504,412
Weighted-average grant date fair values of options granted			$ 1.47	$ 1.76
Aggregate intrinsic value of options exercised			$ 0.1	$ 0.1
Total grant date fair value of options vested			2.7	$ 1.7
Total unrecognized stock-based compensation related to unvested stock options			$ 10.8
Weighted-average period			2 years 8 months 12 days
Common shares authorized to be issued during the period			2,974,638
2018 Equity Inducement Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance			1,500,000
Stock option life in years			10 years
Awards issued			0
Awards outstanding			0

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - 2008 Plan, 2015 Plan and 2018 Plan [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Options Outstanding, Shares Available for Grant
Shares Available for Grant, Beginning balance	1,945,025
Shares Available for Grant, Awards authorized	2,974,638
Shares Available for Grant, Granted	(3,093,300)
Shares Available for Grant, Exercised	0
Shares Available for Grant, forfeited/cancelled	77,009
Shares Available for Grant, Ending balance	1,903,372	1,945,025
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	10,504,412
Number of Shares Outstanding, Options granted	3,093,300
Number of Shares Outstanding, Options exercised	(322,318)
Number of Shares Outstanding, Options forfeited/cancelled	(77,009)
Number of Shares Outstanding, Ending balance	13,198,385	10,504,412
Number of Shares Outstanding, Exercisable	6,571,244
Number of Shares Outstanding, Vested and expected to vest	13,198,385
Options Outstanding, Weighted-Average Exercise Price Per Share
Weighted-Average Exercise Price Per Share, Beginning balance	$ 2.86
Weighted-Average Exercise Price Per Share, Options granted	1.93
Weighted-Average Exercise Price Per Share, Options exercised	1.38
Weighted-Average Exercise Price Per Share, Options forfeited/cancelled	5.59
Weighted-Average Exercise Price Per Share, Ending balance	2.66	$ 2.86
Weighted-Average Exercise Price Per Share, Exercisable	3.09
Weighted-Average Exercise Price Per Share, Vested and expected to vest	$ 2.66
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)
Weighted-Average Remaining Contractual Term (Years)	8 years 1 month 17 days	7 years 9 months 18 days
Weighted-Average Remaining Contractual Term (Year), Exercisable	7 years 21 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest	8 years 1 month 17 days
Options Outstanding, Aggregate Intrinsic Value of Outstanding Options
Aggregate Intrinsic Value of Outstanding Options	$ 1,358	$ 545
Aggregate Intrinsic Value of Outstanding Options, Exercisable	1,185
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest	$ 1,358

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Provision for federal income taxes	$ 0
Income tax expense (benefit) for foreign taxes	$ 100,000	$ 43,000