Sierra Oncology (SRRA) 10-K 27 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Feb. 23, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	SRRA
Entity Registrant Name	Sierra Oncology, Inc.
Entity Central Index Key	1,290,149
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Smaller Reporting Company
Entity Common Stock, Shares Outstanding		52,454,181
Entity Public Float			$ 50.7

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
CURRENT ASSETS:
Cash and cash equivalents	$ 100,348	$ 109,007
Prepaid expenses and other current assets	1,377	1,343
Total current assets	101,725	110,350
Property and equipment, net	154	400
Other assets	319	223
TOTAL ASSETS	102,198	110,973
CURRENT LIABILITIES:
Accrued liabilities	6,133	5,121
Accounts payable	1,339	2,604
Total current liabilities	7,472	7,725
TOTAL LIABILITIES	7,472	7,725
Commitments and Contingencies (Note 6)
STOCKHOLDERS' EQUITY:
Preferred stock, $0.001 par value; 10,000,000 shares authorized as of December 31, 2017 and December 31, 2016; nil shares issued and outstanding as of December 31, 2017 and December 31, 2016
Common stock, $0.001 par value; 500,000,000 shares authorized as of December 31, 2017 and 2016; 52,395,223 and 30,370,946 shares issued and outstanding as of December 31, 2017 and 2016	52	30
Additional paid-in capital	718,751	685,272
Accumulated deficit	(624,077)	(582,054)
TOTAL STOCKHOLDERS' EQUITY	94,726	103,248
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 102,198	$ 110,973

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
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	52,395,223	30,370,946
Common stock, shares outstanding	52,395,223	30,370,946

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 30,157	$ 33,895	$ 26,356
General and administrative	12,462	14,180	9,472
Total operating expenses	42,619	48,075	35,828
Loss from operations	(42,619)	(48,075)	(35,828)
Other income (expense), net:
Other income	760	351	66
Change in fair value of preferred stock warrants			(17,443)
Total other income (expense), net	760	351	(17,377)
Loss before provision for income taxes	(41,859)	(47,724)	(53,205)
Provision for income taxes	156	143	55
Net loss and comprehensive loss	(42,015)	(47,867)	(53,260)
Adjustment to redemption value on redeemable convertible preferred stock			(374,015)
Net loss attributable to common stockholders	$ (42,015)	$ (47,867)	$ (453,184)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.84)	$ (1.58)	$ (31.47)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted	49,899,299	30,240,258	14,399,506
Series B and B-1 [Member]
Other income (expense), net:
Redeemable convertible preferred stock dividend			$ (5,543)
Series C and D [Member]
Other income (expense), net:
Redeemable convertible preferred stock dividend			$ (20,366)

Consolidated Statements of Conv

Consolidated Statements of Convertible and Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock [Member]	Redeemable Convertible Preferred Stock [Member]	Series C and D Redeemable Convertible Preferred Stock [Member]	Series B and B-1 Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member]Series C and D Redeemable Convertible Preferred Stock [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member]Series C and D Redeemable Convertible Preferred Stock [Member]	Additional Paid-In Capital [Member]Series B and B-1 Redeemable Convertible Preferred Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Stockholders Equity, Beginning Balance at Dec. 31, 2014	$ (107,815)	$ 2,543	$ 141,832			$ 2					$ (10)	$ (107,807)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2014		224,564	17,042,064			1,588,701
Issuance of common stock for exercise of stock options	18							$ 18
Issuance of common stock for exercise of stock options, Shares						41,505
Stock-based compensation	3,186							3,186
Vesting of early exercised stock options	90							90
Issuance of redeemable convertible preferred stock upon exercise of redeemable convertible preferred stock warrants			$ 8,976
Issuance of redeemable convertible preferred stock upon exercise of redeemable convertible preferred stock warrants, Shares			481,671
Issuance of redeemable convertible preferred stock upon net exercise of redeemable convertible preferred stock warrants upon initial public offering			$ 11,785
Issuance of redeemable convertible preferred stock upon net exercise of redeemable convertible preferred stock warrants and reclassification of preferred stock warrant liability upon initial public offering, Shares			390,032
Adjustment to redemption value on redeemable convertible preferred stock	(374,015)		$ 374,015					(895)				(373,120)
Conversion of convertible preferred stock to common stock upon initial public offering	2,543	$ (2,543)						2,543
Conversion of convertible preferred stock to common stock upon initial public offering, Shares		(224,564)				252,817
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	536,608		$ (536,608)			$ 18		536,590
Conversion of redeemable convertible preferred stock to common stock upon initial public offering, Shares			(17,913,767)			18,109,136
Issuance of stock, value	143,549					$ 9		143,540
Issuance of stock, shares						9,315,000
Redeemable convertible preferred stock dividend					$ (5,543)					$ (5,543)
Redeemable convertible preferred stock dividend, common stock issued							$ 1		$ (1)
Redeemable convertible preferred stock dividend				750,946			750,946
Reclassification of other-than-temporary losses on short-term investments to net loss	10										$ 10
Net loss	(53,260)											(53,260)
Stockholders Equity, Ending Balance at Dec. 31, 2015	145,371					$ 30		679,528				(534,187)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2015						30,058,105
Issuance of common stock for exercise of stock options	211							211
Issuance of common stock for exercise of stock options, Shares						312,841
Stock-based compensation	5,510							5,510
Vesting of early exercised stock options	23							23
Net loss	(47,867)											(47,867)
Stockholders Equity, Ending Balance at Dec. 31, 2016	103,248					$ 30		685,272				(582,054)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2016						30,370,946
Issuance of common stock for exercise of stock options	166							166
Issuance of common stock for exercise of stock options, Shares						176,641
Stock-based compensation	5,905							5,905
Issuance of stock, value	27,422					$ 22		27,400
Issuance of stock, shares						21,847,636
Net loss	(42,015)											(42,015)
Stockholders Equity, Ending Balance at Dec. 31, 2017	$ 94,726					$ 52		718,751				(624,077)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2017						52,395,223
Cumulative effect of adoption of new accounting standard								$ 8				$ (8)

Consolidated Statements of Con6

Consolidated Statements of Convertible and Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]
Issuance of common stock in connection with initial public offering, issuance costs	$ 2.1	$ 14.8

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (42,015)	$ (47,867)	$ (53,260)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	5,905	5,510	3,186
Depreciation	258	197	111
Non-cash restructuring charges		811
Change in fair value of preferred stock warrant liabilities			17,443
Other	31	15	34
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(115)	(362)	(1,087)
Accrued liabilities	990	(1,713)	5,302
Accounts payable	(1,217)	2,246	6
Net cash used in operating activities	(36,163)	(41,163)	(28,265)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(93)	(214)	(414)
Change in restricted cash	13	25	(150)
Proceeds from sale of property and equipment		43
Proceeds from sale of short-term investments			10,031
Purchase of short-term investments			(21)
Net cash (used in) provided by investing activities	(80)	(146)	9,446
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon follow-on offering, net of offering costs	27,422
Proceeds from exercise of common stock options	166	211	18
Payment of deferred offering costs		(15)	(3,389)
Proceeds from issuance of common stock upon initial public offering, net of issuance costs			147,270
Proceeds from exercise of redeemable convertible preferred stock warrants			1,507
Proceeds from early exercise of stock options			13
Net cash provided by financing activities	27,588	196	139,876
Effect of exchange rate changes on cash and cash equivalents	(4)	(60)	(31)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(8,659)	(41,173)	121,026
CASH AND CASH EQUIVALENTS-Beginning of period	109,007	150,180	29,154
CASH AND CASH EQUIVALENTS-End of period	100,348	109,007	150,180
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes	260	107	8
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Property and equipment purchases included in accounts payable and accrued liabilities	$ 4	$ 85	90
Change in redemption value of redeemable convertible preferred stock			(374,015)
Conversion of preferred stock to common stock			539,151
Issuance of redeemable convertible preferred stock on exercise of warrants			19,253
Deferred offering/financing costs included in accounts payable and accrued liabilities			154
Series B and B-1 Redeemable Convertible Preferred Stock [Member]
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment of Series B and B-1 redeemable convertible preferred stock cumulative dividend			(5,543)
Series C and D Redeemable Convertible Preferred Stock [Member]
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Fair value of common stock issued in settlement of Series C and Series D redeemable convertible preferred stock cumulative dividends			$ 20,366

The Company and Basis of Presen

The Company and Basis of Presentation	12 Months Ended
	Dec. 31, 2017
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 cancer. Sierra Oncology’s lead drug candidate is SRA737, 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. Sierra Oncology is also advancing SRA141, a potent, selective and orally bioavailable small molecule inhibitor of cell division cycle 7 kinase (Cdc7) undergoing preclinical development. 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. Initial Public Offering On July 15, 2015, the Company’s Registration Statement on Form S-1 No. 333-204921) On July 21, 2015, immediately prior to the closing of the IPO, all outstanding shares of convertible and redeemable convertible preferred stock converted into 18,361,953 shares of common stock, including an aggregate of 390,680 shares of common stock that were issued pursuant to the net exercise of 493,648 preferred stock warrants at the IPO price of $17.00 per share and an aggregate of 481,671 shares of common stock that were issued pursuant to the cash exercise of 481,671 preferred stock warrants. The IPO closed on July 21, 2015. On the closing of the IPO, the Company paid $5.5 million to the holders of its Series B and B-1 Following the filing of the Restated Certificate of Incorporation of the Company on July 21, 2015, the number of shares of capital stock the Company is authorized to issue is 510,000,000 shares, of which 500,000,000 shares may be common stock and 10,000,000 shares may be preferred stock. Both the common stock and the preferred stock have a par value of $0.001 per share. Follow On Offering On February 14, 2017, the Company completed an underwritten public offering of 19,500,000 shares of common stock. As part of the underwritten public offering, on February 21, 2017 the Company issued an additional 2,347,636 shares of common stock representing the underwriters’ exercise of a majority of their over-allotment option. All shares were offered by the Company at a price to the public of $1.35 per share. The aggregate net proceeds received by the Company from the offering were $27.4 million, net of underwriting discounts and commissions and offering expenses of $2.1 million. Reverse Stock Split On June 29, 2015, the Company’s Board of Directors approved an amendment to the Company’s Amended and Restated Certificate of Incorporation to effect a reverse split of the Company’s common stock, convertible preferred stock and redeemable convertible preferred stock at a 7.45-to-1

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (U.S. GAAP). The accompanying consolidated financial statements include the accounts of Sierra Oncology, Inc. and its wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation. Use of Estimates The preparation of the 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 and disclosure of contingent assets and liabilities at the date of consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying consolidated financial statements include, but are not limited to the fair value of stock options issued, accruals such as research and development costs, the fair value of common stock, the fair value of preferred stock, the fair value of preferred stock warrant liabilities, 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. Foreign Currency The functional currency of the Company’s foreign subsidiaries is the U.S. Dollar. Transactions denominated in currencies other than the functional currency are recorded at prevailing exchange rates during the period. At the end of each reporting period, monetary assets and liabilities are remeasured to the functional currency using exchange rates in effect at the balance sheet date. Non-monetary Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents consist primarily of funds invested in readily available checking and savings accounts and highly liquid investments in money market funds. Restricted Cash Restricted cash represents collateral for a corporate credit card facility and security deposit required for a facility lease. Restricted cash consists of funds invested in a money market fund. As of December 31, 2017 and December 31, 2016, restricted cash of $0.2 million was included in other assets in the accompanying consolidated balance sheets. Concentrations of Credit Risk Financial instruments that subject the Company to significant concentrations of credit risk consist of cash, cash equivalents and restricted cash. All of the Company’s cash, cash equivalents and restricted cash are held at financial institutions in the United States and Canada that management believes to be of high credit quality. Deposits held in the United States and Canada with these financial institutions exceed federally insured limits. The Company’s cash, cash equivalents and restricted cash exceeded federally insured limits by $100.0 million and $108.7 million at December 31, 2017 and 2016, respectively. The primary focus of the Company’s investment strategy is to preserve capital and meet liquidity requirements. The Company’s investment policy addresses the level of credit exposure by limiting the concentration in any one corporate issuer and establishing a minimum allowable credit rating. Fair Value of Financial Instruments The Company’s cash and cash equivalents, restricted cash, other current assets, accounts payable, and accrued liabilities approximate their fair value at December 31, 2017 and 2016, due to their short duration. The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines the fair value of its financial instruments based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels: Level 1 Level 2 Level 3 Property and Equipment, Net Property and equipment, net are stated at cost, less accumulated depreciation. Depreciation on property and equipment, excluding leasehold improvements, is computed using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Depreciation begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statement of operations. Leasehold improvements are amortized on a straight-line basis over the shorter of the estimated useful lives of the assets or the remaining lease term. Other Assets Other assets consist primarily of restricted cash pledged as collateral for a corporate credit card facility, long-term prepaid rent and refundable security deposits required for facility leases. Preferred Stock Warrant Liabilities The Company accounted for its warrants issued in connection with its various financing transactions based upon the characteristics and provisions of the instrument. Warrants classified as derivative liabilities were recorded on the Company’s consolidated balance sheets at their fair value on the date of issuance and remeasured to fair value on each subsequent reporting period, with the changes in fair value recognized as a component of other income (expense), net in the accompanying consolidated statements of operations. On the closing of the IPO on July 21, 2015 (discussed in Note 1), all outstanding warrants were exercised and the liability on the preferred stock warrants was reclassified to additional paid-in Research and Development Costs Research and development costs are expensed as incurred. The Company accounts for non-refundable Upfront payments made in connection with license agreements are expensed as research and developments costs, as the assets acquired do not have alternative future use. Contingent milestone payment obligations due to third parties under license agreements are expensed when the milestones are considered probable of occurring. Research and development costs include fees incurred in connection with license agreements, compensation and other related costs for employees engaged in research and development, costs associated with research and preclinical studies, clinical trials, regulatory activities, manufacturing activities to support clinical activities, fees paid to external service providers that conduct certain research and development, clinical, and manufacturing activities on behalf of the Company and an allocation of overhead expenses. Stock-Based Compensation The Company accounts for share-based payments at fair value, which is measured using the Black-Scholes option-pricing model. For share-based awards that vest subject to the satisfaction of a service requirement, the fair value measurement date for employee stock-based compensation awards is the date of grant and the expense is recognized on a straight-line basis, over the vesting period. For share-based awards that vest subject to the satisfaction of a service requirement and a performance component, the fair value measurement date is the date of grant and is recognized over the requisite service period as achievement of the performance objective becomes probable. Stock-based compensation arrangements with non-employees Income Taxes The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Management makes an assessment of the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior fiscal periods, the net U.S. deferred tax assets have been offset by a full valuation allowance. The Company recognizes uncertain income tax positions at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. Changes in recognition or measurement are reflected in the period in which judgment occurs. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of provision for income taxes. Segment Information Operating segments are components of an enterprise for which separate financial information is available and is evaluated regularly by the Company’s chief operating decision maker in deciding how to allocate resources and assessing performance. The Company’s chief operating decision maker is its Chief Executive Officer. The Company’s Chief Executive Officer views the Company’s operations and manages its business in one operating segment, which is the business of researching, developing, and commercializing therapies for the treatment of patients with cancer. Accordingly, the Company has a single reporting segment. Recent Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (FASB) issued FASB Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) right-of-use

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Loss Per Share	3. Net Loss Per Share Basic net loss per share is calculated by dividing net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period, less common stock issued that is subject to repurchase, 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 common stock subject to repurchase 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 outstanding shares of common stock equivalents were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive: As of December 31, 2017 2016 2015 Options to purchase common stock 7,470,601 6,543,654 3,522,813 Common stock subject to repurchase — — 23,554 Total potential dilutive shares 7,470,601 6,543,654 3,546,367

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
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 and liabilities measured on a recurring basis by level within the fair value hierarchy: December 31, 2017 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 99,792 $ — $ — $ 99,792 Restricted money market funds 188 — — 188 Total financial assets $ 99,980 $ — $ — $ 99,980 December 31, 2016 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 107,043 $ — $ — $ 107,043 Restricted money market funds 200 — — 200 Total financial assets $ 107,243 $ — $ — $ 107,243 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 years ended December 31, 2017 and 2016.

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2017
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: December 31, 2017 2016 (in thousands) Cash $ 556 $ 1,964 Cash equivalents: Money market accounts 99,792 107,043 Total cash and cash equivalents $ 100,348 $ 109,007 Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consist of the following: December 31, 2017 2016 (in thousands) Prepaid research and development project costs $ 549 $ 515 Prepaid insurance 478 511 Other receivables 103 70 Other 247 247 Total prepaid expenses and other current assets $ 1,377 $ 1,343 Property and Equipment, net Property and equipment, net consists of the following: December 31, 2017 2016 (in thousands) Software $ 254 $ 242 Computer equipment 93 255 Leasehold improvements 79 110 Furniture and fixtures 3 45 Property and equipment, gross 429 652 Less: accumulated depreciation (275 ) (252 ) Total property and equipment, net $ 154 $ 400 Depreciation related to the Company’s property and equipment for the years ended December 31, 2017, 2016 and 2015 was $0.3 million, $0.2 million and $0.1 million, respectively. During the year ended December 31, 2016, as a result of the decision to halt investment in PNT2258 and the DNAi platform, the Company recorded impairment charges of $0.1 million on its lab equipment (see Note 12). Accrued Liabilities Accrued liabilities consist of the following: December 31, 2017 2016 (in thousands) Accrued research and development costs $ 2,763 $ 1,600 Accrued employee related costs 2,699 2,084 Accrued professional fees 317 381 Accrued restructuring costs (Note 12) 137 763 Other 217 293 Total accrued liabilities $ 6,133 $ 5,121

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies 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, which will be accrued once they are considered probable of occurring. As at December 31, 2017, 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). Lease Agreements In February 2015, the Company entered into an operating lease agreement to sublease office space in Vancouver, Canada. The operating lease agreement expires on February 27, 2018. Under the terms of the agreement, the Company issued a letter of credit to the sublessor on closing, which was collateralized by a restricted deposit of $13,000 at December 31, 2017. In June 2017, the Company entered into a new operating lease agreement to continue leasing the office space in Vancouver, Canada commencing March 1, 2018. The new 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. The fair value of the remaining contractual obligation, net of sublease income, as of the cease-use In addition to base rent, these leases require payment of taxes and other operating costs. These operating costs are not included in the table below. As of December 31, 2017, the aggregate future non-cancelable Years Ending December 31: Operating (in thousands) 2018 $ 424 2019 272 2020 225 2021 229 2022 183 Total (a) $ 1,333 (a) Minimum lease payments have not been reduced by total minimum sublease rentals of $0.2 million due in the future under sublease. The total rent expense for the years ended December 31, 2017, 2016 and 2015 was $0.5 million, $0.5 million and $0.3 million, respectively. 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 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 lawsuit asserts claims under Sections 10(b) and 20(a) of the Securities Exchange Act of 1934 and seeks unspecified damages and other relief. The Company believes that the claims are without merit and intends to defend the lawsuit vigorously. It is possible that additional similar lawsuits could be filed. Due to the early stage of the litigation, the Company is unable to predict the outcome of this matter and is unable to make a meaningful estimate of the amount or range of loss, if any, that could result from an unfavorable outcome. 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 IPO 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 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	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Common Stock Reserved for Issuance	7. 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 and shares reserved for issuance under the employee stock purchase plan. December 31, 2017 2016 Outstanding stock options 7,470,601 6,543,654 Shares reserved for future option grants 1,503,770 1,392,521 Shares reserved under the 2015 employee stock purchase plan 700,000 700,000 Total common stock reserved for issuance 9,674,371 8,636,175

Preferred Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Preferred Stock	8. Preferred Stock As of December 31, 2017 and December 31, 2016, the Company had 10,000,000 shares of preferred stock authorized with a par value of $0.001. No shares of preferred stock were outstanding as of December 31, 2017 and December 31, 2016. Convertible and Redeemable Convertible Preferred Stock During the year ended December 31, 2015, the Company issued an aggregate of 481,671 shares of redeemable convertible preferred stock upon the cash exercise of the Company’s Series B-1 B-1 In addition, during the year ended December 31, 2015, the Company issued an aggregate of 390,032 shares of redeemable convertible preferred stock upon the net exercise of the Company’s Series B, Series B-1 B-1 On July 21, 2015 (closing date of the IPO), all outstanding shares of Series A convertible preferred stock converted into 252,817 shares of common stock and all outstanding shares of Series B, Series B-1, The Company’s Series B, B-1, B-1, paid-in Year Ended December 31, Redeemable Convertible Preferred Stock (in thousands) Series B $ 46,976 Series B-1 36,961 Series C 52,832 Series D 237,246 Total adjustment to redemption value on redeemable convertible preferred stock $ 374,015 As the redemption value for the redeemable convertible preferred stock was at times based on fair market value, the Company determined the fair value of the redeemable convertible preferred stock using a combination of the OPM and/or the PWERM models, or the fair value of the Company’s common stock. At July 21, 2015 (closing date of the IPO), the fair value of redeemable convertible preferred stock was estimated based on the underlying value of the Company’s common stock. Amendment to Dividend Rights On June 11, 2015, the Company’s stockholders approved an amendment to the Company’s Certificate of Incorporation to modify the terms of the cumulative accruing dividends on the outstanding shares of the Company’s Series C and Series D redeemable convertible preferred stock. Under the terms of the Amended Certificate of Incorporation, upon conversion of the Company’s redeemable convertible preferred stock into common stock in connection with an IPO, the Company was required to pay 50% of the then accrued but unpaid accruing dividends on shares of the Series C and Series D redeemable convertible preferred stock in shares of the Company’s common stock instead of payment in cash and the remaining 50% of the then accrued but unpaid accruing dividends was to be forfeited. The settlement of the accrued but unpaid accruing dividends in shares of common stock was required to be at the original issue price of the Series C and Series D redeemable convertible preferred stock of $5.215 per share. The terms of the dividends payable on the Series B and Series B-1 Settlement of Cumulative Dividends On July 21, 2015 (closing date of the IPO), the Company had total cumulative unpaid dividends in arrears of $18.9 million, which consisted of $9.4 million for the Series B, $1.7 million for the Series B-1, B-1 B-1

Preferred Stock Warrants

Preferred Stock Warrants	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Preferred Stock Warrants	9. Preferred Stock Warrants The Company classified its preferred stock warrants as liabilities. During the year ended December 31, 2015, $19.3 million of the fair value of the warrant liability was reclassified to redeemable convertible preferred stock and then into additional paid-in During the year ended December 31, 2015, 31,778 of the Series B-1 B-1 On the closing of the Company’s IPO, all of the remaining outstanding preferred stock warrants were net exercised at the IPO price of $17.00 per share, which resulted in 6,499 shares of common stock being issued upon the net exercise of outstanding warrants to purchase 10,414 shares of Series B preferred stock, 291,164 shares of common stock being issued upon the net exercise of outstanding warrants to purchase 349,054 shares of Series B-1 During the year ended December 31, 2015, the Company remeasured the preferred stock warrants using the OPM and/or PWERM models, or based on the fair value of the underlying stock. On the closing of the IPO in July 2015, the outstanding preferred stock warrants were exercised and the liability on the preferred stock warrants was reclassified to additional paid-in The change in fair value of the preferred stock warrant liabilities was attributable to the increase of the fair value of the underlying stock. The change in the fair value of preferred stock warrants for the year ended December 31, 2015 was recognized as a component of other income (expense), net in the condensed consolidated statements of operations.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	10. Stock-Based Compensation In the accompanying consolidated statement of operations, the Company recognized stock-based compensation expense for its employees and non-employees Year Ended December 31, 2017 2016 2015 (in thousands) Research and development $ 3,966 $ 3,635 $ 1,846 General and administrative 1,939 1,875 1,340 Total stock-based compensation $ 5,905 $ 5,510 $ 3,186 Determination of Fair Value The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on the following assumptions: Year Ended December 31, 2017 2016 2015 Expected term (in years) 5.3 – 7.0 5.1 – 9.9 5.2 – 10.0 Expected volatility 86 – 96 % 77 – 87 % 75 – 84 % Risk-free interest rate 1.8 – 2.3 % 1.1 – 2.4 % 1.5 – 2.4 % Expected dividend rate — % — % — % The fair value of each stock option grant was determined by the Company using the methods and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment and estimation by management. Expected Term— non-employees Expected Volatility— Risk-Free Interest Rate— zero-coupon Expected Dividend Rate— Forfeiture Rate— Fair Value of Common Stock— On January 1, 2017, the Company adopted FASB ASU No. 2016-09 2016-09), Compensation—Stock Compensation (Topic 718) , Equity Incentive Plans 2015 Plan The 2015 Equity Incentive Plan (2015 Plan) became effective on July 14, 2015. As of December 31, 2017, 6,493,998 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 from 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 Directors 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 and 2015 Plan and related information is as follows: Options Outstanding Shares Number Weighted- Weighted- Aggregate Outstanding—December 31, 2014 1,550,101 2,138,096 $0.91 9.29 $ 3,153 Awards authorized 3,400,000 Options granted (1,471,664 ) 1,471,664 8.06 Options exercised — (41,505 ) 0.76 Options cancelled 45,442 (45,442 ) 7.76 Outstanding—December 31, 2015 3,523,879 3,522,813 3.81 8.78 40,425 Awards authorized 1,202,324 Options granted (3,710,980 ) 3,710,980 3.22 Options exercised — (312,841 ) 0.67 Options cancelled 377,298 (377,298 ) 7.04 Outstanding—December 31, 2016 1,392,521 6,543,654 3.44 8.82 888 Awards authorized 1,214,837 Options granted (1,502,025 ) 1,502,025 1.45 Options exercised — (176,641 ) 0.94 Options cancelled 398,437 (398,437 ) 3.55 Outstanding—December 31, 2017 1,503,770 7,470,601 $3.09 8.12 $ 12,363 Exercisable—December 31, 2017 3,995,823 $3.10 7.61 $ 6,749 Vested and expected to vest—December 31, 2017 7,470,601 $3.09 8.12 $ 12,363 The weighted-average grant date fair values of options granted during the years ended December 31, 2017, 2016 and 2015 was $1.09, $2.19 and $10.51 per share. The aggregate intrinsic value of options exercised was $0.2 million, $1.2 million and $0.2 million for the years ended December 31, 2017, 2016 and 2015. The total grant date fair value of options vested for the years ended December 31, 2017, 2016 and 2015 was $6.5 million, $6.1 million and $1.4 million. As of December 31, 2017, total unrecognized stock-based compensation related to unvested stock options was $9.2 million, which the Company expects to recognize over a remaining weighted-average period of 1.8 years. 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 options to purchase shares of 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 December 31, 2017, the initial offering periods had not commenced. As of December 31, 2017, 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	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes On December 22, 2017, the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (Tax Act). The Tax Act significantly revises U.S. tax law by, among other provisions, lowering the U.S. federal statutory income tax rate from 35% to 21%, changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017 and eliminating or reducing certain income tax deductions. The effects of changes in tax laws are required to be recognized in the period in which the legislation is enacted. However, due to the complexity and significance of the Tax Act’s provisions, the SEC staff issued Staff Accounting Bulletin No. 118 (SAB 118), which allows companies to record the tax effects of the Tax Act on a provisional basis based on a reasonable estimate, and then, if necessary, subsequently adjust such amounts during a limited measurement period as more information becomes available. The measurement period ends when a company has obtained, prepared, and analyzed the information necessary to finalize its accounting, but cannot extend beyond one year from enactment. In connection with the Company’s initial analysis of the Tax Act, it recorded a decrease of its net deferred tax assets of $7.2 million for the period ended December 31, 2017, to account for the rate reduction. This did not have an impact on the Company’s financial statements since its U.S. deferred tax assets are fully offset by a valuation allowance. However, given the significant complexity of the Tax Act, these estimates may be adjusted during the measurement period. The geographical breakdown of loss before provision for income taxes is as follows: Year Ended December 31, 2017 2016 2015 (in thousands) United States $ (42,425 ) $ (48,244 ) $ (53,376 ) International 566 520 171 Loss before provision for income taxes $ (41,859 ) $ (47,724 ) $ (53,205 ) The components of the provision for income taxes are as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Current tax provision: Federal $ — $ — $ — State — — — Foreign 183 170 59 Total current tax provision 183 170 59 Deferred tax provision (benefit): Foreign (27 ) (27 ) (4 ) Total deferred tax provision (benefit) $ (27 ) $ (27 ) $ (4 ) Total provision for income taxes $ 156 $ 143 $ 55 The reconciliation between income taxes computed at the federal statutory income tax rate and the provision for income taxes is as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Federal statutory rate 34.0 % 34.0 % 34.0 % Effect of: Effect of ownership change on deferred tax assets (84.8 ) — — Change in valuation allowance 69.5 (33.5 ) (27.8 ) US tax reform deferred impact of tax rate change (17.3 ) — — Federal tax credit (0.9 ) 1.6 2.6 State income tax benefit, net of federal benefit 0.1 0.1 2.9 Change in fair value of preferred stock warrants — — (11.1 ) Other permanent items (1.0 ) (2.5 ) (0.7 ) Total provision for income taxes (0.4 )% (0.3 )% (0.1 )% The components of the deferred tax assets are as follows: December 31, 2017 2016 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 9,183 $ 35,244 Stock Based Compensation 2,444 2,291 59 (e) expenditures and amortization 1,435 — License fee 1,346 3,339 Research and development credits 365 2,597 Other 698 1,017 Gross deferred tax assets 15,471 44,488 Valuation allowance (15,395 ) (44,440 ) Total deferred tax assets 76 48 Deferred tax liabilities: Other 13 12 Total deferred tax liabilities 13 12 Total net deferred tax assets $ 63 $ 36 Recognition of deferred tax assets is appropriate when realization of these assets is more likely than not. Based upon the weight of available evidence, which includes historical operating performance and the recorded cumulative net losses in prior fiscal periods, the Company recorded a full valuation allowance of $15.4 million and $44.4 million against the net U.S. deferred tax assets as of December 31, 2017 and 2016. The net valuation allowance decreased by $29.0 million for the year ended December 31, 2017 and increased by $16.0 million for the year ended December 31, 2016. Management assesses the available positive and negative evidence to estimate if sufficient future taxable income will be generated to use the existing deferred tax assets. Based on the weight of all evidence, including a history of operating losses and the Company’s ability to generate future taxable income to realize the assets, management has determined that it is more likely than not that the deferred tax assets will not be realized. Utilization of the Company’s net operating loss and research and development credit carryforwards to offset taxable income are subject to an annual limitation, pursuant to Internal Revenue Code (IRC) Sections 382 and 383. As a result of stock offerings that occurred prior to 2016, subsequent changes in the stock ownership, and the stock offering in February 2017, an ownership change under Section 382 is deemed to have occurred. As such, certain of the Company’s tax attributes existing as of the date of the ownership change are not be available for future use. The loss of these attributes does not have any impact on the financial statements since the net U.S. deferred tax assets are offset by a full valuation allowance. As of December 31, 2017, the Company has federal tax net operating loss carryforwards of $31.4 million expiring in 2037 and state operating loss carryforwards of $54.0 million expiring in years ranging from 2022 to 2037. The Company also had net tax credit carryforwards of $0.4 million which begin to expire in 2032. Uncertain Tax Positions The activity related to the gross amount of unrecognized tax benefits is as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Beginning balance $ 311 $ 232 $ 162 Increases based on tax positions related to prior years — — — Decreases based on tax positions related to prior years (79 ) — (77 ) Decreases due to ownership change (232 ) — — Increases based on tax positions in current year 43 79 147 Settlement — — — Lapse of statute of limitations — — — Ending balance $ 43 $ 311 $ 232 If recognized, gross unrecognized tax benefits would not have an impact on the Company’s effective tax rate due to the Company’s full valuation allowance position. While it is often difficult to predict the final outcome of any particular uncertain tax position, the Company does not believe that the amount of gross unrecognized tax benefits will change significantly in the next twelve months. The Company recognizes interest and penalties related to unrecognized tax benefits in the provision for income taxes in the accompanying consolidated statement of operations. Accrued interest and penalties, if applicable, are included in accrued liabilities in the consolidated balance sheet. For the years ended December 31, 2017 and 2016, the Company did not recognize any accrued interest and penalties. The Company is subject to taxation in the United States, various states, Canada and Australia. Tax years 2014 through 2016 remain open to examination by the United States, various state jurisdictions and Canada. The tax year ended December 31, 2016 remains open to examination in Australia. The Company is currently under examination by the Internal Revenue Service for the 2015 tax year. There are currently no other examinations in any other jurisdictions for any other year.

Restructuring Costs

Restructuring Costs	12 Months Ended
	Dec. 31, 2017
Restructuring and Related Activities [Abstract]
Restructuring Costs	12. Restructuring Costs In June 2016, the Company halted investment in PNT2258 and the DNAi platform and closed the related Phase 2 clinical trials to further enrollment. As a result, the Company closed its research facility in Plymouth, Michigan, renegotiated and terminated certain contracts, and implemented staff reductions in the United States and Canada. The following table summarizes restructuring activities for the years ended December 31, 2017 and December 31, 2016: Contract Employee Asset Total (in thousands) Accrual balance at December 31, 2015 $ — $ — $ — $ — Restructuring costs charged to research and development expense 2,249 — — 2,249 Restructuring costs charged to general and administrative expense 5 332 130 467 Non-cash (681 ) — (130 ) (811 ) Cash payments (909 ) (233 ) — (1,142 ) Accrual balance at December 31, 2016 $ 664 $ 99 $ — $ 763 Adjustments to research and development expense (69 ) — — (69 ) Cash payments (458 ) (99 ) — (557 ) Accrual balance at December 31, 2017 $ 137 $ — $ — $ 137 The accrual balance, which was included in accrued liabilities in the accompanying balance sheet, is currently expected to be substantially paid by the end of 2018.

Selected Quarterly Financial Da

Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data	13. Selected Quarterly Financial Data (Unaudited) The following tables present certain selected unaudited consolidated quarterly financial information for each of the eight quarters ended December 31, 2017. This consolidated quarterly information has been prepared on the same basis as the consolidated financial statements and includes all adjustments necessary to state fairly the information for the periods presented. The selected consolidated quarterly financial results from operations for the years ended December 31, 2017 and 2016 are set forth therein. Fiscal 2017 Quarter Ended March 31, June 30, September 30, December 31, (in thousands, except per share amounts) Operating expenses $ 11,154 $ 10,477 $ 10,183 $ 10,805 Net loss $ (11,092 ) $ (10,329 ) $ (9,995 ) $ (10,599 ) Basic and diluted net loss per share $ (0.26 ) $ (0.20 ) $ (0.19 ) $ (0.20 ) Fiscal 2016 Quarter Ended March 31, June 30, September 30, December 31, (in thousands, except per share amounts) Operating expenses $ 10,613 $ 12,946 $ 15,283 $ 9,233 Net loss $ (10,538 ) $ (12,872 ) $ (15,209 ) $ (9,248 ) Basic and diluted net loss per share $ (0.35 ) $ (0.43 ) $ (0.50 ) $ (0.30 )

Summary of Significant Accoun21

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (U.S. GAAP). The accompanying consolidated financial statements include the accounts of Sierra Oncology, Inc. and its wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of the 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 and disclosure of contingent assets and liabilities at the date of consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying consolidated financial statements include, but are not limited to the fair value of stock options issued, accruals such as research and development costs, the fair value of common stock, the fair value of preferred stock, the fair value of preferred stock warrant liabilities, 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.
Foreign Currency	Foreign Currency The functional currency of the Company’s foreign subsidiaries is the U.S. Dollar. Transactions denominated in currencies other than the functional currency are recorded at prevailing exchange rates during the period. At the end of each reporting period, monetary assets and liabilities are remeasured to the functional currency using exchange rates in effect at the balance sheet date. Non-monetary
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents consist primarily of funds invested in readily available checking and savings accounts and highly liquid investments in money market funds.
Restricted Cash	Restricted Cash Restricted cash represents collateral for a corporate credit card facility and security deposit required for a facility lease. Restricted cash consists of funds invested in a money market fund. As of December 31, 2017 and December 31, 2016, restricted cash of $0.2 million was included in other assets in the accompanying consolidated balance sheets.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments that subject the Company to significant concentrations of credit risk consist of cash, cash equivalents and restricted cash. All of the Company’s cash, cash equivalents and restricted cash are held at financial institutions in the United States and Canada that management believes to be of high credit quality. Deposits held in the United States and Canada with these financial institutions exceed federally insured limits. The Company’s cash, cash equivalents and restricted cash exceeded federally insured limits by $100.0 million and $108.7 million at December 31, 2017 and 2016, respectively. The primary focus of the Company’s investment strategy is to preserve capital and meet liquidity requirements. The Company’s investment policy addresses the level of credit exposure by limiting the concentration in any one corporate issuer and establishing a minimum allowable credit rating.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s cash and cash equivalents, restricted cash, other current assets, accounts payable, and accrued liabilities approximate their fair value at December 31, 2017 and 2016, due to their short duration. The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines the fair value of its financial instruments based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels: Level 1 Level 2 Level 3
Property and Equipment, Net	Property and Equipment, Net Property and equipment, net are stated at cost, less accumulated depreciation. Depreciation on property and equipment, excluding leasehold improvements, is computed using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Depreciation begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statement of operations. Leasehold improvements are amortized on a straight-line basis over the shorter of the estimated useful lives of the assets or the remaining lease term.
Other Assets	Other Assets Other assets consist primarily of restricted cash pledged as collateral for a corporate credit card facility, long-term prepaid rent and refundable security deposits required for facility leases.
Preferred Stock Warrant Liabilities	Preferred Stock Warrant Liabilities The Company accounted for its warrants issued in connection with its various financing transactions based upon the characteristics and provisions of the instrument. Warrants classified as derivative liabilities were recorded on the Company’s consolidated balance sheets at their fair value on the date of issuance and remeasured to fair value on each subsequent reporting period, with the changes in fair value recognized as a component of other income (expense), net in the accompanying consolidated statements of operations. On the closing of the IPO on July 21, 2015 (discussed in Note 1), all outstanding warrants were exercised and the liability on the preferred stock warrants was reclassified to additional paid-in
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. The Company accounts for non-refundable Upfront payments made in connection with license agreements are expensed as research and developments costs, as the assets acquired do not have alternative future use. Contingent milestone payment obligations due to third parties under license agreements are expensed when the milestones are considered probable of occurring. Research and development costs include fees incurred in connection with license agreements, compensation and other related costs for employees engaged in research and development, costs associated with research and preclinical studies, clinical trials, regulatory activities, manufacturing activities to support clinical activities, fees paid to external service providers that conduct certain research and development, clinical, and manufacturing activities on behalf of the Company and an allocation of overhead expenses.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for share-based payments at fair value, which is measured using the Black-Scholes option-pricing model. For share-based awards that vest subject to the satisfaction of a service requirement, the fair value measurement date for employee stock-based compensation awards is the date of grant and the expense is recognized on a straight-line basis, over the vesting period. For share-based awards that vest subject to the satisfaction of a service requirement and a performance component, the fair value measurement date is the date of grant and is recognized over the requisite service period as achievement of the performance objective becomes probable. Stock-based compensation arrangements with non-employees
Income Taxes	Income Taxes The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Management makes an assessment of the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior fiscal periods, the net U.S. deferred tax assets have been offset by a full valuation allowance. The Company recognizes uncertain income tax positions at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. Changes in recognition or measurement are reflected in the period in which judgment occurs. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of provision for income taxes.
Segment Information	Segment Information Operating segments are components of an enterprise for which separate financial information is available and is evaluated regularly by the Company’s chief operating decision maker in deciding how to allocate resources and assessing performance. The Company’s chief operating decision maker is its Chief Executive Officer. The Company’s Chief Executive Officer views the Company’s operations and manages its business in one operating segment, which is the business of researching, developing, and commercializing therapies for the treatment of patients with cancer. Accordingly, the Company has a single reporting segment.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (FASB) issued FASB Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) right-of-use

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Schedule of Outstanding Shares of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share Attributable to Common Stockholders	The following outstanding shares of common stock equivalents were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive: As of December 31, 2017 2016 2015 Options to purchase common stock 7,470,601 6,543,654 3,522,813 Common stock subject to repurchase — — 23,554 Total potential dilutive shares 7,470,601 6,543,654 3,546,367

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Schedule of Fair Value Financial Assets and Liabilities Measured on Recurring Basis	The following table sets forth the fair value of the Company’s financial assets and liabilities measured on a recurring basis by level within the fair value hierarchy: December 31, 2017 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 99,792 $ — $ — $ 99,792 Restricted money market funds 188 — — 188 Total financial assets $ 99,980 $ — $ — $ 99,980 December 31, 2016 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 107,043 $ — $ — $ 107,043 Restricted money market funds 200 — — 200 Total financial assets $ 107,243 $ — $ — $ 107,243

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Cash and Cash Equivalents	Cash and cash equivalents consist of the following: December 31, 2017 2016 (in thousands) Cash $ 556 $ 1,964 Cash equivalents: Money market accounts 99,792 107,043 Total cash and cash equivalents $ 100,348 $ 109,007
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consist of the following: December 31, 2017 2016 (in thousands) Prepaid research and development project costs $ 549 $ 515 Prepaid insurance 478 511 Other receivables 103 70 Other 247 247 Total prepaid expenses and other current assets $ 1,377 $ 1,343
Summary of Property and Equipment, Net	Property and equipment, net consists of the following: December 31, 2017 2016 (in thousands) Software $ 254 $ 242 Computer equipment 93 255 Leasehold improvements 79 110 Furniture and fixtures 3 45 Property and equipment, gross 429 652 Less: accumulated depreciation (275 ) (252 ) Total property and equipment, net $ 154 $ 400
Summary of Accrued Liabilities	Accrued liabilities consist of the following: December 31, 2017 2016 (in thousands) Accrued research and development costs $ 2,763 $ 1,600 Accrued employee related costs 2,699 2,084 Accrued professional fees 317 381 Accrued restructuring costs (Note 12) 137 763 Other 217 293 Total accrued liabilities $ 6,133 $ 5,121

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Schedule of Aggregate Future Non-Cancelable Minimum Lease Payments	As of December 31, 2017, the aggregate future non-cancelable Years Ending December 31: Operating (in thousands) 2018 $ 424 2019 272 2020 225 2021 229 2022 183 Total (a) $ 1,333 (a) Minimum lease payments have not been reduced by total minimum sublease rentals of $0.2 million due in the future under sublease.

Common Stock Reserved for Iss26

Common Stock Reserved for Issuance (Tables)	12 Months Ended
	Dec. 31, 2017
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 and shares reserved for issuance under the employee stock purchase plan. December 31, 2017 2016 Outstanding stock options 7,470,601 6,543,654 Shares reserved for future option grants 1,503,770 1,392,521 Shares reserved under the 2015 employee stock purchase plan 700,000 700,000 Total common stock reserved for issuance 9,674,371 8,636,175

Preferred Stock (Tables)

Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Changes in Redemption Value of Convertible Preferred Stock	The changes in the redemption value of the redeemable convertible preferred stock for the year ended December 31, 2015 was as follows: Year Ended December 31, Redeemable Convertible Preferred Stock (in thousands) Series B $ 46,976 Series B-1 36,961 Series C 52,832 Series D 237,246 Total adjustment to redemption value on redeemable convertible preferred stock $ 374,015

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Stock-based Compensation Expense for Employees and Non-employees	In the accompanying consolidated statement of operations, the Company recognized stock-based compensation expense for its employees and non-employees Year Ended December 31, 2017 2016 2015 (in thousands) Research and development $ 3,966 $ 3,635 $ 1,846 General and administrative 1,939 1,875 1,340 Total stock-based compensation $ 5,905 $ 5,510 $ 3,186
Schedule of Estimated Grant-date Fair Value of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions	The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on the following assumptions: Year Ended December 31, 2017 2016 2015 Expected term (in years) 5.3 – 7.0 5.1 – 9.9 5.2 – 10.0 Expected volatility 86 – 96 % 77 – 87 % 75 – 84 % Risk-free interest rate 1.8 – 2.3 % 1.1 – 2.4 % 1.5 – 2.4 % Expected dividend rate — % — % — %
Summary of Stock-Based Compensation Activity	A summary of activity under the 2008 Plan and 2015 Plan and related information is as follows: Options Outstanding Shares Number Weighted- Weighted- Aggregate Outstanding—December 31, 2014 1,550,101 2,138,096 $0.91 9.29 $ 3,153 Awards authorized 3,400,000 Options granted (1,471,664 ) 1,471,664 8.06 Options exercised — (41,505 ) 0.76 Options cancelled 45,442 (45,442 ) 7.76 Outstanding—December 31, 2015 3,523,879 3,522,813 3.81 8.78 40,425 Awards authorized 1,202,324 Options granted (3,710,980 ) 3,710,980 3.22 Options exercised — (312,841 ) 0.67 Options cancelled 377,298 (377,298 ) 7.04 Outstanding—December 31, 2016 1,392,521 6,543,654 3.44 8.82 888 Awards authorized 1,214,837 Options granted (1,502,025 ) 1,502,025 1.45 Options exercised — (176,641 ) 0.94 Options cancelled 398,437 (398,437 ) 3.55 Outstanding—December 31, 2017 1,503,770 7,470,601 $3.09 8.12 $ 12,363 Exercisable—December 31, 2017 3,995,823 $3.10 7.61 $ 6,749 Vested and expected to vest—December 31, 2017 7,470,601 $3.09 8.12 $ 12,363

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of Geographical Breakdown of Loss Before Provision for Income Taxes	The geographical breakdown of loss before provision for income taxes is as follows: Year Ended December 31, 2017 2016 2015 (in thousands) United States $ (42,425 ) $ (48,244 ) $ (53,376 ) International 566 520 171 Loss before provision for income taxes $ (41,859 ) $ (47,724 ) $ (53,205 )
Components of Provision for Income Taxes	The components of the provision for income taxes are as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Current tax provision: Federal $ — $ — $ — State — — — Foreign 183 170 59 Total current tax provision 183 170 59 Deferred tax provision (benefit): Foreign (27 ) (27 ) (4 ) Total deferred tax provision (benefit) $ (27 ) $ (27 ) $ (4 ) Total provision for income taxes $ 156 $ 143 $ 55
Reconciliation Between Income Taxes Computed at the Federal Statutory Income Tax Rate and the Provision for Income Taxes	The reconciliation between income taxes computed at the federal statutory income tax rate and the provision for income taxes is as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Federal statutory rate 34.0 % 34.0 % 34.0 % Effect of: Effect of ownership change on deferred tax assets (84.8 ) — — Change in valuation allowance 69.5 (33.5 ) (27.8 ) US tax reform deferred impact of tax rate change (17.3 ) — — Federal tax credit (0.9 ) 1.6 2.6 State income tax benefit, net of federal benefit 0.1 0.1 2.9 Change in fair value of preferred stock warrants — — (11.1 ) Other permanent items (1.0 ) (2.5 ) (0.7 ) Total provision for income taxes (0.4 )% (0.3 )% (0.1 )%
Components of the Deferred Tax Assets	The components of the deferred tax assets are as follows: December 31, 2017 2016 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 9,183 $ 35,244 Stock Based Compensation 2,444 2,291 59 (e) expenditures and amortization 1,435 — License fee 1,346 3,339 Research and development credits 365 2,597 Other 698 1,017 Gross deferred tax assets 15,471 44,488 Valuation allowance (15,395 ) (44,440 ) Total deferred tax assets 76 48 Deferred tax liabilities: Other 13 12 Total deferred tax liabilities 13 12 Total net deferred tax assets $ 63 $ 36
Unrecognized Tax Benefits	The activity related to the gross amount of unrecognized tax benefits is as follows: Year Ended December 31, 2017 2016 2015 (in thousands) Beginning balance $ 311 $ 232 $ 162 Increases based on tax positions related to prior years — — — Decreases based on tax positions related to prior years (79 ) — (77 ) Decreases due to ownership change (232 ) — — Increases based on tax positions in current year 43 79 147 Settlement — — — Lapse of statute of limitations — — — Ending balance $ 43 $ 311 $ 232

Restructuring Costs (Tables)

Restructuring Costs (Tables)	12 Months Ended
	Dec. 31, 2017
Restructuring and Related Activities [Abstract]
Summary of Restructuring Activities	The following table summarizes restructuring activities for the years ended December 31, 2017 and December 31, 2016: Contract Employee Asset Total (in thousands) Accrual balance at December 31, 2015 $ — $ — $ — $ — Restructuring costs charged to research and development expense 2,249 — — 2,249 Restructuring costs charged to general and administrative expense 5 332 130 467 Non-cash (681 ) — (130 ) (811 ) Cash payments (909 ) (233 ) — (1,142 ) Accrual balance at December 31, 2016 $ 664 $ 99 $ — $ 763 Adjustments to research and development expense (69 ) — — (69 ) Cash payments (458 ) (99 ) — (557 ) Accrual balance at December 31, 2017 $ 137 $ — $ — $ 137

Selected Quarterly Financial 31

Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Summary of Selected Unaudited Consolidated Quarterly Financial Information	The selected consolidated quarterly financial results from operations for the years ended December 31, 2017 and 2016 are set forth therein. Fiscal 2017 Quarter Ended March 31, June 30, September 30, December 31, (in thousands, except per share amounts) Operating expenses $ 11,154 $ 10,477 $ 10,183 $ 10,805 Net loss $ (11,092 ) $ (10,329 ) $ (9,995 ) $ (10,599 ) Basic and diluted net loss per share $ (0.26 ) $ (0.20 ) $ (0.19 ) $ (0.20 ) Fiscal 2016 Quarter Ended March 31, June 30, September 30, December 31, (in thousands, except per share amounts) Operating expenses $ 10,613 $ 12,946 $ 15,283 $ 9,233 Net loss $ (10,538 ) $ (12,872 ) $ (15,209 ) $ (9,248 ) Basic and diluted net loss per share $ (0.35 ) $ (0.43 ) $ (0.50 ) $ (0.30 )

The Company and Basis of Pres32

The Company and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Millions	Feb. 21, 2017shares	Feb. 14, 2017USD ($)$ / sharesshares	Jul. 21, 2015USD ($)$ / sharesshares	Jul. 15, 2015USD ($)$ / sharesshares	Jun. 29, 2015	Dec. 31, 2017$ / sharesshares	Dec. 31, 2015shares	Dec. 31, 2016$ / sharesshares
Class of Stock [Line Items]
Shares issued, price per share | $ / shares			$ 17
Convertible preferred stock, conversion basis						On July 21, 2015, immediately prior to the closing of the IPO, all outstanding shares of convertible and redeemable convertible preferred stock converted into 18,361,953 shares of common stock, including an aggregate of 390,680 shares of common stock that were issued pursuant to the net exercise of 493,648 preferred stock warrants at the IPO price of $17.00 per share and an aggregate of 481,671 shares of common stock that were issued pursuant to the cash exercise of 481,671 preferred stock warrants. The IPO closed on July 21, 2015.
Number of common stock shares issued upon exercise of preferred stock warrants			390,680				481,671
Preferred stock warrants exercised			493,648
Capital stock, authorized			510,000,000
Common stock, shares authorized			500,000,000			500,000,000		500,000,000
Preferred stock, shares authorized			10,000,000			10,000,000		10,000,000
Preferred stock, par value | $ / shares			$ 0.001			$ 0.001		$ 0.001
Common stock, par value | $ / shares			$ 0.001			$ 0.001		$ 0.001
Reverse stock split, description						7.45-to-1
Reverse Stock Split, Conversion Ratio					0.13422818
Reverse stock split, effective date						Jul. 2, 2015
Series B and B-1 Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Cumulative dividends | $			$ 5.5
Series C and D Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Issuable common stock to redeemable convertible preferred stockholders			750,946				750,946
Common Stock [Member]
Class of Stock [Line Items]
Stock issued, shares						21,847,636	9,315,000
Redeemable convertible preferred stock converted into common stock			18,361,953
Common Stock [Member] | Series C and D Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Issuable common stock to redeemable convertible preferred stockholders							750,946
Cash Exercise [Member]
Class of Stock [Line Items]
Number of common stock shares issued upon exercise of preferred stock warrants			481,671
Preferred stock warrants exercised			481,671
Initial Public Offering [Member]
Class of Stock [Line Items]
Stock issued, shares				9,315,000
Shares issued, price per share | $ / shares				$ 17
Proceeds from issuance of common stock, net of deferred financing costs | $				$ 143.6
Follow on Offering [Member]
Class of Stock [Line Items]
Stock issued, shares	2,347,636	19,500,000
Shares issued, price per share | $ / shares		$ 1.35
Proceeds from issuance of common stock, net of deferred financing costs | $		$ 27.4
Underwriting discounts and commissions and offering expenses | $		$ 2.1

Summary of Significant Accoun33

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)Segment	Dec. 31, 2016USD ($)
Summary Of Significant Accounting Policy [Line Items]
Net foreign exchange transaction loss	$ 0.1	$ 0.1
Deposits in excess of federally-insured limits	$ 100	108.7
Number of operating segment | Segment	1
Minimum [Member] | Property Plant and Equipment Other than Leasehold Improvement [Member]
Summary Of Significant Accounting Policy [Line Items]
Estimated useful lives of assets	3 years
Maximum [Member] | Property Plant and Equipment Other than Leasehold Improvement [Member]
Summary Of Significant Accounting Policy [Line Items]
Estimated useful lives of assets	5 years
Other Assets [Member]
Summary Of Significant Accounting Policy [Line Items]
Long-term portion of restricted cash	$ 0.2	$ 0.2

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 Attributable to Common Stockholders (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	7,470,601	6,543,654	3,546,367
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	7,470,601	6,543,654	3,522,813
Common Stock Subject to Repurchase [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares			23,554

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Fair Value Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Assets
Total financial assets	$ 99,980	$ 107,243
Money Market Funds [Member]
Financial Assets
Total financial assets	99,792	107,043
Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	188	200
Level 1 [Member]
Financial Assets
Total financial assets	99,980	107,243
Level 1 [Member] | Money Market Funds [Member]
Financial Assets
Total financial assets	99,792	107,043
Level 1 [Member] | Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	$ 188	$ 200

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 0	$ 0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	0	0
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	0	0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	$ 0	$ 0

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Abstract]
Cash	$ 556	$ 1,964
Cash equivalents:
Money market accounts	99,792	107,043
Total cash and cash equivalents	$ 100,348	$ 109,007	$ 150,180	$ 29,154

Balance Sheet Components - Su38

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid research and development project costs	$ 549	$ 515
Prepaid insurance	478	511
Other receivables	103	70
Other	247	247
Total prepaid expenses and other current assets	$ 1,377	$ 1,343

Balance Sheet Components - Su39

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 429	$ 652
Less: accumulated depreciation	(275)	(252)
Total property and equipment, net	154	400
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	254	242
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	93	255
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	79	110
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3	$ 45

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation related to property and equipment	$ 258	$ 197	$ 111
Lab Equipment [Member]
Property, Plant and Equipment [Line Items]
Impairment charges		$ 100

Balance Sheet Components - Su41

Balance Sheet Components - Summary of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Accrued research and development costs	$ 2,763	$ 1,600
Accrued employee related costs	2,699	2,084
Accrued professional fees	317	381
Accrued restructuring costs	137	763
Other	217	293
Total accrued liabilities	$ 6,133	$ 5,121

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended					12 Months Ended
	Dec. 31, 2017USD ($)Payments	Jan. 31, 2017USD ($)	Oct. 31, 2016USD ($)	Jun. 30, 2016USD ($)	Jan. 31, 2016	Dec. 31, 2017USD ($)Payments	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Carna License Agreement [Member]
Loss Contingencies [Line Items]
Upfront payment paid				$ 900,000
Aggregate milestone payment						$ 270,000,000
Number of milestone payments recorded | Payments	0					0
CPF License Agreements [Member]
Loss Contingencies [Line Items]
Upfront payment paid			$ 7,000,000
Aggregate milestone payment		$ 2,000,000
Additional milestone payment payable						$ 319,500,000
Operating Lease Agreement To Lease Office Space [Member] | San Francisco [Member]
Loss Contingencies [Line Items]
Operating lease agreement expiration date					Apr. 30, 2019
Operating Lease Agreement To Sublease Office Space [Member] | Vancouver [Member]
Loss Contingencies [Line Items]
Operating lease agreement expiration date						Feb. 27, 2018
Collateralized restricted deposit	$ 13,000					$ 13,000
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
Sublease Agreement [Member] | San Francisco [Member]
Loss Contingencies [Line Items]
Operating lease agreement expiration date						Apr. 30, 2019
Fair value of remaining contractual obligation, net income	$ 100,000					$ 100,000
Lease Agreements [Member]
Loss Contingencies [Line Items]
Total rent expense						$ 500,000	$ 500,000	$ 300,000

Commitments and Contingencies43

Commitments and Contingencies - Schedule of Aggregate Future Non-Cancelable Minimum Lease Payments (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,018	$ 424
2,019	272
2,020	225
2,021	229
2,022	183
Total	$ 1,333

Commitments and Contingencies44

Commitments and Contingencies - Schedule of Aggregate Future Non-Cancelable Minimum Lease Payments (Parenthetical) (Detail) $ in Millions	Dec. 31, 2017USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Minimum future sublease rentals	$ 0.2

Common Stock Reserved for Iss45

Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 15, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Outstanding stock options	7,470,601	6,543,654
Total common stock reserved for future issuance	9,674,371	8,636,175
2008 Plan and 2015 Plan [Member]
Class of Stock [Line Items]
Outstanding stock options	7,470,601	6,543,654	3,522,813		2,138,096
Total common stock reserved for future issuance	1,503,770	1,392,521
2015 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	700,000	700,000		700,000

Preferred Stock - Additional In

Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Jul. 21, 2015	Jun. 11, 2015	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Temporary Equity [Line Items]
Preferred stock, shares authorized	10,000,000			10,000,000	10,000,000
Preferred stock, par value	$ 0.001			$ 0.001	$ 0.001
Preferred stock, shares outstanding				0	0
Number of shares issued upon exercise of stock warrants	390,680		481,671
Number of convertible preferred stock issued upon exercise of stock warrants			390,032
Percentage of accrued but unpaid dividends paid in shares		50.00%
Percentage of accrued but unpaid dividends forfeited		50.00%
Share price		$ 5.215
Cumulative unpaid dividends in arrears	$ 18.9
Series C Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Number of shares issued upon exercise of stock warrants	93,017		37,385
Number of convertible preferred stock issued upon exercise of stock warrants			93,017
Cumulative unpaid dividends in arrears	$ 1.7
Issuable common stock to redeemable convertible preferred stockholders			161,536
Fair value of common stock to redeemable convertible preferred stockholders			$ 4.4
Series D Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Cumulative unpaid dividends in arrears	$ 6.1
Issuable common stock to redeemable convertible preferred stockholders			589,410
Fair value of common stock to redeemable convertible preferred stockholders			$ 16
Series B-1 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Number of shares issued upon exercise of stock warrants	291,164		444,286
Number of convertible preferred stock issued upon exercise of stock warrants			291,165
Cumulative unpaid dividends in arrears	$ 1.7
Accruing dividends			$ 0.8
Series B Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Number of shares issued upon exercise of stock warrants	6,499
Number of convertible preferred stock issued upon exercise of stock warrants			5,850
Cumulative unpaid dividends in arrears	$ 9.4
Accruing dividends			$ 4.7
Series A Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock converted into common stock shares	252,817
Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock converted into common stock shares	18,109,136		(17,913,767)
Series B and B-1 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Accruing dividends			$ 5.5
Series C and D Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Issuable common stock to redeemable convertible preferred stockholders	750,946		750,946
Fair value of common stock to redeemable convertible preferred stockholders			$ 20.4

Preferred Stock - Changes in Re

Preferred Stock - Changes in Redemption Value of Convertible Preferred Stock (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)
Temporary Equity [Line Items]
Adjustment to redemption value	$ 374,015
Series B Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Adjustment to redemption value	46,976
Series B-1 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Adjustment to redemption value	36,961
Series C Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Adjustment to redemption value	52,832
Series D Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Adjustment to redemption value	237,246
Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Adjustment to redemption value	$ (374,015)

Preferred Stock Warrants - Addi

Preferred Stock Warrants - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jul. 21, 2015	Dec. 31, 2015
Class of Warrant or Right [Line Items]
Fair value of warrant liability reclassified to redeemable convertible preferred stock and additional paid-in capital		$ 19,300
Warrants exercised	493,648
Warrants exercise price	$ 17
Cash proceeds from warrants exercised		$ 1,507
Number of common stock shares issued upon exercise of preferred stock warrants	390,680	481,671
Series B Redeemable Convertible Preferred Stock [Member]
Class of Warrant or Right [Line Items]
Warrants exercised	10,414
Number of common stock shares issued upon exercise of preferred stock warrants	6,499
Series B-1 Redeemable Convertible Preferred Stock [Member]
Class of Warrant or Right [Line Items]
Warrants exercised	349,054
Number of common stock shares issued upon exercise of preferred stock warrants	291,164	444,286
Series C Redeemable Convertible Preferred Stock [Member]
Class of Warrant or Right [Line Items]
Warrants exercised	134,180
Number of common stock shares issued upon exercise of preferred stock warrants	93,017	37,385
Series B-1 Redeemable Convertible Preferred Stock Cash Exercised at Exercise Price of $7.45 [Member]
Class of Warrant or Right [Line Items]
Warrants exercised		31,778
Warrants exercise price		$ 7.45
Series B-1 Redeemable Convertible Preferred Stock Warrants Cash Exercised at Exercise Price of $2.6075 [Member]
Class of Warrant or Right [Line Items]
Warrants exercised		412,508
Warrants exercise price		$ 2.6075
Series C Redeemable Convertible Preferred Stock Warrants Cash Exercised at Exercise Price of $5.215 [Member]
Class of Warrant or Right [Line Items]
Warrants exercised		37,385
Warrants exercise price		$ 5.215

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-based Compensation Expense for Employees and Non-employees (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 5,905	$ 5,510	$ 3,186
Research and development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	3,966	3,635	1,846
General and administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 1,939	$ 1,875	$ 1,340

Stock-Based Compensation - Sc50

Stock-Based Compensation - Schedule of Estimated Grant-date Fair Value of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, Minimum	86.00%	77.00%	75.00%
Expected volatility, Maximum	96.00%	87.00%	84.00%
Risk-free interest rate, Minimum	1.80%	1.10%	1.50%
Risk-free interest rate, Maximum	2.30%	2.40%	2.40%
Expected dividend rate	0.00%	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 19 days	5 years 1 month 6 days	5 years 2 months 12 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	7 years	9 years 10 months 25 days	10 years

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Jul. 15, 2015	Jul. 14, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend rate			0.00%	0.00%	0.00%
Number of common stock shares reserved for issuance			9,674,371	8,636,175
Accounting Standards Update 2016-09 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Retrospective adjustment to retained earnings			$ 8,000
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			6,493,998
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 from 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%
Stock option life in years			10 years
2008 Plan and 2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance			1,503,770	1,392,521
Weighted-average grant date fair values of options granted			$ 1.09	$ 2.19	$ 10.51
Aggregate intrinsic value of options exercised			$ 200,000	$ 1,200,000	$ 200,000
Total grant date fair value of options vested			6,500,000	$ 6,100,000	$ 1,400,000
Total unrecognized stock-based compensation related to unvested stock options			$ 9,200,000
Weighted-average period			1 year 9 months 18 days
Common shares authorized to be issued during the period			1,214,837	1,202,324	3,400,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	6,543,654
Number of Shares Outstanding, Ending balance	7,470,601	6,543,654
2008 Plan and 2015 Plan [Member]
Options Outstanding, Shares Available for Grant
Shares Available for Grant, Beginning balance	1,392,521	3,523,879	1,550,101
Shares Available for Grant, Awards authorized	1,214,837	1,202,324	3,400,000
Shares Available for Grant, Granted	(1,502,025)	(3,710,980)	(1,471,664)
Shares Available for Grant, Exercised	0	0	0
Shares Available for Grant, Cancelled	398,437	377,298	45,442
Shares Available for Grant, Ending balance	1,503,770	1,392,521	3,523,879	1,550,101
Shares Available for Grant, Awards authorized	1,214,837	1,202,324	3,400,000
Number of Shares Outstanding, Exercisable	3,995,823
Shares Available for Grant, Granted	(1,502,025)	(3,710,980)	(1,471,664)
Number of Shares Outstanding, Vested and expected to vest	7,470,601
Shares Available for Grant, Exercised	0	0	0
Shares Available for Grant, Cancelled	398,437	377,298	45,442
Shares Available for Grant, Ending balance	1,503,770	1,392,521	3,523,879	1,550,101
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	6,543,654	3,522,813	2,138,096
Number of Shares Outstanding, Options granted	1,502,025	3,710,980	1,471,664
Number of Shares Outstanding, Options exercised	(176,641)	(312,841)	(41,505)
Number of Shares Outstanding, Options cancelled	(398,437)	(377,298)	(45,442)
Number of Shares Outstanding, Ending balance	7,470,601	6,543,654	3,522,813	2,138,096
Options Outstanding, Weighted-Average Exercise Price Per Share
Weighted-Average Exercise Price Per Share, Beginning balance	$ 3.44	$ 3.81	$ 0.91
Weighted-Average Exercise Price Per Share, Options granted	1.45	3.22	8.06
Weighted-Average Exercise Price Per Share, Options exercised	0.94	0.67	0.76
Weighted-Average Exercise Price Per Share, Options cancelled	3.55	7.04	7.76
Weighted-Average Exercise Price Per Share, Ending balance	3.09	$ 3.44	$ 3.81	$ 0.91
Weighted-Average Exercise Price Per Share, Exercisable	3.10
Weighted-Average Exercise Price Per Share, Vested and expected to vest	$ 3.09
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)
Weighted-Average Remaining Contractual Term (Years)	8 years 1 month 13 days	8 years 9 months 25 days	8 years 9 months 11 days	9 years 3 months 15 days
Weighted-Average Remaining Contractual Term (Year), Exercisable	7 years 7 months 10 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest	8 years 1 month 13 days
Options Outstanding, Aggregate Intrinsic Value of Outstanding Options
Aggregate Intrinsic Value of Outstanding Options	$ 12,363	$ 888	$ 40,425	$ 3,153
Aggregate Intrinsic Value of Outstanding Options, Exercisable	6,749
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest	$ 12,363

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Asset [Line Items]
Federal statutory rate		34.00%	34.00%	34.00%
Decrease of net deferred tax assets		$ 7,200,000
Income Tax Disclosure [Abstract]
Deferred tax assets, valuation allowance		15,395,000	$ 44,440,000
Deferred tax assets, valuation allowance, change in amount		(29,000,000)	16,000,000
Deferred tax assets, federal operating loss carry forwards		$ 31,400,000
Federal operating loss carry forwards, expiry year		2,037
Deferred tax assets, state operating loss carry forwards		$ 54,000,000
Operating loss carry forwards, expiry year start		2,022
Operating loss carry forwards, expiry year end		2,037
Deferred tax credit carry forwards		$ 400,000
Deferred tax credit carry forwards, year begin to expiration		2,032
Unrecognized tax benefits, accrued interest and penalties		$ 0	$ 0
Scenario, Forecast [Member]
Deferred Tax Asset [Line Items]
Federal statutory rate	21.00%

Income Taxes - Schedule of Geog

Income Taxes - Schedule of Geographical Breakdown of Loss Before Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
United States	$ (42,425)	$ (48,244)	$ (53,376)
International	566	520	171
Loss before provision for income taxes	$ (41,859)	$ (47,724)	$ (53,205)

Income Taxes - Components of Pr

Income Taxes - Components of Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current tax provision:
Federal	$ 0	$ 0	$ 0
State	0	0	0
Foreign	183	170	59
Total current tax provision	183	170	59
Deferred tax provision (benefit):
Foreign	(27)	(27)	(4)
Total deferred tax provision (benefit)	(27)	(27)	(4)
Total provision for income taxes	$ 156	$ 143	$ 55

Income Taxes - Reconciliation B

Income Taxes - Reconciliation Between Income Taxes Computed at the Federal Statutory Income Tax Rate and the Provision for Income Taxes (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory rate	34.00%	34.00%	34.00%
Effect of ownership change on deferred tax assets	(84.80%)
Change in valuation allowance	69.50%	(33.50%)	(27.80%)
US tax reform deferred impact of tax rate change	(17.30%)
Federal tax credit	(0.90%)	1.60%	2.60%
State income tax benefit, net of federal benefit	0.10%	0.10%	2.90%
Change in fair value of preferred stock warrants			(11.10%)
Other permanent items	(1.00%)	(2.50%)	(0.70%)
Total provision for income taxes	(0.40%)	(0.30%)	(0.10%)

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 9,183	$ 35,244
Stock Based Compensation	2,444	2,291
59 (e) expenditures and amortization	1,435
License fee	1,346	3,339
Research and development credits	365	2,597
Other	698	1,017
Gross deferred tax assets	15,471	44,488
Valuation allowance	(15,395)	(44,440)
Total deferred tax assets	76	48
Deferred tax liabilities:
Other	13	12
Total deferred tax liabilities	13	12
Total net deferred tax assets	$ 63	$ 36

Income Taxes - Unrecognized Tax

Income Taxes - Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Beginning balance	$ 311	$ 232	$ 162
Increases based on tax positions related to prior years	0	0	0
Decreases based on tax positions related to prior years	(79)		(77)
Decreases due to ownership change	(232)
Increases based on tax positions in current year	43	79	147
Settlement	0	0	0
Lapse of statute of limitations	0	0	0
Ending balance	$ 43	$ 311	$ 232

Income Taxes - Additional Inf59

Income Taxes - Additional Information - Uncertain Tax Positions (Detail)	12 Months Ended
	Dec. 31, 2017
Earliest Tax Year [Member]
Income Tax Contingency [Line Items]
Open tax year	2,014
Latest Tax Year [Member]
Income Tax Contingency [Line Items]
Open tax year	2,016

Restructuring Costs - Summary o

Restructuring Costs - Summary of Restructuring Activities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance	$ 763
Non-cash charges		$ (811)
Accrual, Ending balance	137	763
PNT2258 [Member]
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance	763
Adjustment to research and development expense	(69)
Non-cash charges		(811)
Cash payments	(557)	(1,142)
Accrual, Ending balance	137	763
PNT2258 [Member] | Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance	664
Adjustment to research and development expense	(69)
Non-cash charges		(681)
Cash payments	(458)	(909)
Accrual, Ending balance	137	664
PNT2258 [Member] | Employee Termination [Member]
Restructuring Cost and Reserve [Line Items]
Accrual, Beginning balance	99
Cash payments	$ (99)	(233)
Accrual, Ending balance		99
PNT2258 [Member] | Asset Impairment [Member]
Restructuring Cost and Reserve [Line Items]
Non-cash charges		(130)
Research and development [Member] | PNT2258 [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		2,249
Research and development [Member] | PNT2258 [Member] | Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		2,249
General and administrative [Member] | PNT2258 [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		467
General and administrative [Member] | PNT2258 [Member] | Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		5
General and administrative [Member] | PNT2258 [Member] | Employee Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		332
General and administrative [Member] | PNT2258 [Member] | Asset Impairment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring cost		$ 130

Selected Quarterly Financial 61

Selected Quarterly Financial Data - Summary of Selected Unaudited Consolidated Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$ 10,805	$ 10,183	$ 10,477	$ 11,154	$ 9,233	$ 15,283	$ 12,946	$ 10,613	$ 42,619	$ 48,075	$ 35,828
Net loss	$ (10,599)	$ (9,995)	$ (10,329)	$ (11,092)	$ (9,248)	$ (15,209)	$ (12,872)	$ (10,538)	$ (42,015)	$ (47,867)	$ (53,260)
Basic and diluted net loss per share	$ (0.20)	$ (0.19)	$ (0.20)	$ (0.26)	$ (0.30)	$ (0.50)	$ (0.43)	$ (0.35)	$ (0.84)	$ (1.58)	$ (31.47)