Sierra Oncology (SRRA) 10-Q 5 Nov 15 2015 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Nov. 04, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q3
Trading Symbol	DNAI
Entity Registrant Name	ProNAi Therapeutics Inc
Entity Central Index Key	1,290,149
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		30,058,105

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
CURRENT ASSETS:
Cash and cash equivalents (Note 5)	$ 157,308	$ 29,154
Short-term investments (Note 4)		10,010
Prepaid expenses and other current assets (Note 5)	1,280	561
Total current assets	158,588	39,725
Property and equipment, net (Note 5)	565	214
Other assets (Note 5)	203	626
TOTAL ASSETS	159,356	40,565
CURRENT LIABILITIES:
Accrued liabilities (Note 5)	2,913	1,473
Accounts payable	849	622
Other current liabilities	38
Total current liabilities	3,800	2,095
Preferred stock warrant liabilities (Note 9)		1,810
Other liabilities		100
TOTAL LIABILITIES	$ 3,800	$ 4,005
Commitments and Contingencies (Note 6)
Convertible preferred stock value		$ 144,375
STOCKHOLDERS' EQUITY (DEFICIT):
Preferred stock, $0.001 par value; 10,000,000 and nil shares authorized as of September 30, 2015 and December 31, 2014 and nil shares issued and outstanding as of September 30, 2015 and December 31, 2014
Common stock, $0.001 par value; 500,000,000 and 180,000,000 shares authorized as of September 30, 2015 and December 31, 2014 and 30,058,105 and 1,588,701 shares issued and outstanding as of September 30, 2015 and December 31, 2014	$ 30	$ 2
Additional paid-in capital	678,255
Accumulated other comprehensive loss		(10)
Accumulated deficit	(522,729)	(107,807)
TOTAL STOCKHOLDERS' EQUITY (DEFICIT)	155,556	(107,815)
TOTAL LIABILITIES, CONVERTIBLE AND REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT)	$ 159,356	40,565
Convertible Preferred Stock [Member]
CURRENT LIABILITIES:
Convertible preferred stock value		2,543
Redeemable Convertible Preferred Stock [Member]
CURRENT LIABILITIES:
Convertible preferred stock value		$ 141,832

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Convertible preferred stock, shares authorized		135,913,741
Convertible preferred stock, shares issued		17,266,628
Convertible preferred stock, shares outstanding		17,266,628
Preferred stock, par value	$ 0.001
Preferred stock, shares authorized	10,000,000
Preferred stock, shares issued
Preferred stock, shares outstanding
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	500,000,000	180,000,000
Common stock, shares issued	30,058,105	1,588,701
Common stock, shares outstanding	30,058,105	1,588,701
Convertible Preferred Stock [Member]
Convertible preferred stock, par value		$ 0.001
Convertible preferred stock, shares authorized		1,843,894
Convertible preferred stock, shares issued		224,564
Convertible preferred stock, shares outstanding		224,564
Convertible preferred stock, aggregate liquidation preference		$ 2.5
Redeemable Convertible Preferred Stock [Member]
Convertible preferred stock, par value		$ 0.001
Convertible preferred stock, shares authorized		134,069,847
Convertible preferred stock, shares issued		17,042,064
Convertible preferred stock, shares outstanding		17,042,064
Convertible preferred stock, aggregate liquidation preference		$ 103.5

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Operating expenses:
Research and development	$ 8,295	$ 2,437	$ 18,329	$ 16,981
General and administrative	2,738	1,043	6,053	2,327
Total operating expenses	11,033	3,480	24,382	19,308
Loss from operations	(11,033)	(3,480)	(24,382)	(19,308)
Other income (expense), net:
Change in fair value of preferred stock warrants	(7,487)	(503)	(17,443)	(999)
Other income (expense)	29	22	48	55
Total other income (expense), net	(7,458)	(481)	(17,395)	(944)
Loss before provision for income taxes	(18,491)	(3,961)	(41,777)	(20,252)
Provision for income tax	14		25
Net loss	(18,505)	(3,961)	(41,802)	(20,252)
Adjustment to redemption value on redeemable convertible preferred stock	(222,130)	(5,591)	(374,015)	(44,866)
Net loss attributable to common stockholders	$ (266,544)	$ (9,552)	$ (441,726)	$ (65,118)
Net loss per share attributable to common stockholders, basic and diluted (Note 3)	$ (11.03)	$ (9.37)	$ (48.36)	$ (69.95)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (Note 3)	24,167,238	1,019,553	9,133,978	930,922
Series B and B-1 [Member]
Other income (expense), net:
Redeemable convertible preferred stock dividend	$ (5,543)		$ (5,543)
Series C and D [Member]
Other income (expense), net:
Redeemable convertible preferred stock dividend	$ (20,366)		$ (20,366)

Condensed Consolidated Stateme5

Condensed Consolidated Statement of Convertible and Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - 9 months ended Sep. 30, 2015 - USD ($) $ in Thousands	Total	Convertible Preferred Stock [Member]	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					$ (10)	$ (107,807)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2014					1,588,701
Temporary Equity, Beginning Balance at Dec. 31, 2014	$ 144,375	$ 2,543	$ 141,832
Temporary Equity, Beginning Balance, Shares at Dec. 31, 2014	17,266,628	224,564	17,042,064
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	1,912						1,912
Vesting of early exercised stock options	75						75
Issuance of redeemable convertible preferred stock upon exercise of redeemable convertible preferred stock warrants for cash			$ 8,976
Issuance of redeemable convertible preferred stock upon exercise of redeemable convertible preferred stock warrants for cash, Shares			481,671
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			$ 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 preferred stock to common stock upon initial public offering	2,543	$ (2,543)					2,543
Conversion of 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 common stock in connection with initial public offering, net of issuance costs of $14.8 million	143,565				$ 9		143,556
Issuance of common stock in connection with initial public offering, net of issuance costs of $14.8 million, 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
Reclassification of other-than-temporary losses on short-term investments to net loss	10									$ 10
Net loss	(41,802)										(41,802)
Stockholders Equity, Ending Balance at Sep. 30, 2015	$ 155,556				$ 30		$ 678,255				$ (522,729)
Stockholders Equity, Ending Balance, Shares at Sep. 30, 2015					30,058,105
Temporary Equity, Ending Balance at Sep. 30, 2015
Temporary Equity, Ending Balance, Shares at Sep. 30, 2015

Condensed Consolidated Stateme6

Condensed Consolidated Statement of Convertible and Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Millions	9 Months Ended
	Sep. 30, 2015USD ($)
Statement of Stockholders' Equity [Abstract]
Issuance of common stock in connection with initial public offering, issuance costs	$ 14.8

Condensed Consolidated Stateme7

Condensed Consolidated Statements of Cash Flows - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (41,802,000)	$ (20,252,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of preferred stock warrant liabilities	17,443,000	999,000
Stock-based compensation	1,912,000	197,000
Depreciation and amortization	71,000	3,000
Unrealized foreign exchange gain	(8,000)
Impairment on short-term investments	10,000
Deferred income taxes	(1,000)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(694,000)	(501,000)
Accrued liabilities	1,397,000	528,000
Accounts payable	483,000	597,000
Net cash used in operating activities	(21,189,000)	(18,429,000)
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from sale of short-term investments	10,031,000
Purchase of short-term investments	(21,000)	(10,003,000)
Change in restricted cash	(150,000)	(75,000)
Purchase of property and equipment	(381,000)	(39,000)
Net cash provided by (used in) investing activities	9,479,000	(10,117,000)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon initial public offering, net of issuance costs	147,270,000
Payment of deferred offering costs	(3,389,000)	(197,000)
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		57,545,000
Repayment of stock subscription receivable		1,159,000
Proceeds from exercise of redeemable convertible preferred stock warrants	1,507,000
Proceeds from exercise of common stock options	18,000	165,000
Proceeds from early exercise of stock options	13,000
Net cash provided by financing activities	139,876,000	58,672,000
Effect of exchange rate changes on cash and cash equivalents	(12,000)
NET INCREASE IN CASH AND CASH EQUIVALENTS	128,154,000	30,126,000
CASH AND CASH EQUIVALENTS - Beginning of period	29,154,000	2,429,000
CASH AND CASH EQUIVALENTS - End of period	157,308,000	32,555,000
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes	8,000
Cash paid for interest	0	0
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Change in redemption value of redeemable convertible preferred stock	(374,015,000)	(44,866,000)
Conversion of preferred stock to common stock	539,151,000
Issuance of redeemable convertible preferred stock on exercise of warrants	19,253,000
Deferred offering costs included in accounts payable and accrued liabilities		269,000
Property and equipment purchases included in accounts payable and accrued liabilities	80,000	6,000
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	(5,543,000)
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	$ 20,366,000
Series C Redeemable Convertible Preferred Stock [Member]
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Conversion of long-term note payable and accrued interest into Series C redeemable convertible preferred stock		$ 433,000

The Company and Basis of Presen

The Company and Basis of Presentation	9 Months Ended
	Sep. 30, 2015
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 ProNAi Therapeutics, Inc. (together with its subsidiaries, collectively referred to as the “Company”), a Delaware corporation, is a clinical-stage oncology company pioneering a novel class of therapeutics based on its proprietary DNAi technology platform. The core of the Company’s scientific expertise is its understanding of DNAi oligonucleotides, which are rationally designed DNA sequences that modulate the transcription of oncogenes known to be involved in cancer cell survival and proliferation. The Company’s lead DNAi product candidate, PNT2258, targets BCL2, a widely overexpressed oncogene that is an important gatekeeper of the programmed cell death pathway known as apoptosis and has been linked to many forms of cancer. 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, and raising capital to support development activities. 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 for development of 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 (File No. 333-204921) relating to the initial public offering (IPO) of its common stock was declared effective by the Securities and Exchange Commission (SEC). Pursuant to such Registration Statement, the Company sold an aggregate of 9,315,000 shares of its common stock at a price of $17.00 per share for aggregate cash proceeds of approximately $143.6 million, net of underwriting discounts and commissions and offering costs. 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 on 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 on the cash exercise of 481,671 preferred stock warrants. The IPO closed on July 21, 2015. As discussed further in Note 8, on the closing of the IPO, the Company paid the holders of its Series B and B-1 redeemable convertible preferred stock $5.5 million in settlement of the cumulative dividends. In addition, the Company issued 750,946 shares of common stock to the holders of its Series C and D redeemable convertible preferred stock in settlement of the cumulative dividends. 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. 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 ratio (the “Reverse Stock Split”). The Reverse Stock Split became effective on July 2, 2015, upon the filing of the amendment to the Company’s amended and restated certificate of incorporation. The authorized shares and par value of the common, convertible preferred and redeemable convertible preferred stock were not adjusted as a result of the Reverse Stock Split. All issued and outstanding common stock, convertible preferred stock, redeemable convertible preferred stock, warrants for preferred stock, options for common stock and per share amounts contained in the condensed consolidated financial statements have been retroactively adjusted to reflect this Reverse Stock Split for all periods presented.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The condensed consolidated balance sheet as of September 30, 2015, the condensed consolidated statements of operations for the three and nine months September 30, 2015 and 2014 and the condensed consolidated statements of cash flows for the nine months ended September 30, 2015 and 2014 and the condensed consolidated statements of convertible and redeemable convertible preferred stock and stockholders’ equity (deficit) for the nine months ended September 30, 2015 are unaudited. The unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair presentation of the Company’s condensed consolidated financial statements included in this report. The condensed consolidated financial data disclosed in these notes to the condensed consolidated financial statements related to the three and nine month periods are also unaudited. The condensed consolidated results of operations for the three and nine months ended September 30, 2015 are not necessarily indicative of the results to be expected for the year ending December 31, 2015, or for any other future annual or interim period. The consolidated balance sheet as of December 31, 2014 included herein was derived from the audited consolidated financial statements as of that date. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements included in the prospectus dated July 15, 2015, filed with the SEC pursuant to Rule 424 promulgated under the Securities Act of 1933, as amended. Use of Estimates The preparation of the condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying condensed consolidated financial statements include, but are not limited to the fair value of common stock, the fair value of preferred stock, the fair value of preferred stock warrant liabilities, the fair value of stock options, recoverability of the Company’s net deferred tax assets, and related valuation allowance and certain accruals. 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 subsidiary is the U.S. Dollar. Transactions denominated in currencies other than the functional currency are remeasured to the functional currency at the average exchange rate in effect 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 assets and liabilities are remeasured at historical exchange rates. Gains and losses related to remeasurement are recorded in other income (expense) in the condensed consolidated statements of operations. Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less from the date or 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 a security deposit required for a facility lease. Restricted cash consists of funds invested in a money market fund. As of September 30, 2015, the current portion of restricted cash of $25,000 is included in prepaid expenses and other current assets and the long-term portion of restricted cash of $0.2 million is included in other assets in the accompanying condensed consolidated balance sheets. Investments The Company determines the appropriate designation of its investments as “trading,” “available-for-sale” or “held-to-maturity” based on management’s intent at the time of purchase and reevaluates such designation at each reporting date. During the three and nine months ended September 30, 2015 and for the year ended December 31, 2014, all of the Company’s short-term investments were designated as available-for-sale. Unrealized gains and losses, if any, are reported as a separate component of stockholders’ deficit, except for unrealized losses determined to be other-than-temporary which are recorded in other income (expense) in the accompanying condensed consolidated statements of operations. The Company determines any realized gains or losses on the sale of any investments on a specific identification method and records such gains and losses as a component of other income (expense) in the accompanying condensed consolidated statements of operations. The Company evaluates its short-term investments periodically for possible other-than-temporary impairment. A decline in fair value below the amortized cost of the investment is considered other-than-temporary impairment if the Company has the intent to sell the investment or it is more likely than not that the Company will be required to sell the investment before recovery of the entire amortized cost basis. In those instances, an impairment charge equal to the difference between the fair value and the amortized cost basis is recognized in other income (expense). Regardless of the Company’s intent or requirement to sell an investment, impairment is considered other-than-temporary if the Company does not expect to recover the entire amortized cost basis. Investments with original maturities beyond three months at the date of purchase and which mature at, or less than twelve months from, the balance sheet date are classified as current. Investments with a maturity beyond twelve months from the condensed consolidated balance sheet date are classified as long-term. Concentrations of Credit Risk Financial instruments that subject the Company to significant concentrations of credit risk consist of cash, cash equivalents, restricted cash and short-term investments. All of the Company’s cash, cash equivalents, restricted cash and short-term investments 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 with these financial institutions exceed federally insured limits. 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 or sector and establishing a minimum allowable credit rating. Comprehensive Income (Loss) The Company had no components of comprehensive income (loss) other than net loss for all periods presented. Fair Value of Financial Instruments The Company’s cash and cash equivalents, restricted cash, short-term investments, other current assets, accounts payable, and accrued liabilities approximate their fair value at September 30, 2015 and December 31, 2014, due to their short duration. The short-term investment maintains observable inputs, thus the carrying value of this instrument is carried at fair value and unrealized gains and losses, if any, are reported as a separate component of stockholders’ deficit. 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 and amortization. Depreciation and amortization 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 and amortization 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 balance sheet and the resulting gain or loss is reflected in the 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. Amortization on leasehold improvements are amortized over the term of the related property lease, for a period of three years. Deferred Offering Costs Deferred offering costs, consisting of legal, accounting and filing fees relating to an IPO were deferred until the completion of the IPO in July 2015, at which time they were reclassified to additional paid-in capital as a reduction of the gross proceeds. As of December 31, 2014, $0.5 million of deferred offering costs were capitalized and included in other long-term assets in the condensed consolidated balance sheets. Preferred Stock Warrant Liabilities The Company accounts 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 are recorded on the Company’s condensed 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 condensed consolidated statements of operations. The Company will continue to adjust the liability for changes in the fair value of these warrants until the earlier of the exercise of the warrants, the expiration of the warrants, or until such time as the warrants are no longer to be considered derivative instruments. 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 capital in stockholders’ equity (deficit), and was no longer subject to remeasurement. Research and Development Costs Research and development costs are expensed as incurred. The Company accounts for nonrefundable advance payments for goods and services that will be used in future research and development activities as expenses when the goods have been received or when the service has been performed rather than when the payment is made. Depending on the timing of payments to service providers of research and development costs, the Company recognizes prepaid expenses or accrued expenses related to these costs. These prepaid or accrued expenses are based on management’s estimates of the work performed under service agreements and milestones achieved. 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 preclinical studies and trials, regulatory activities, manufacturing activities to support clinical activities, license fees, 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 nonemployees are recognized at the grant date and remeasured to fair value at each reporting period. The expense is recognized over the vesting period which is generally the service period. 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 deferred tax assets have been offset by a 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.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2015
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, convertible and redeemable convertible preferred stock and warrants for preferred stock, 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 2015 2014 Options to purchase common stock 3,314,159 2,386,507 Common stock subject to repurchase 39,493 — Convertible preferred stock — 252,818 Redeemable convertible preferred stock — 17,236,784 Warrants for preferred stock — 984,529 Total potential dilutive shares 3,353,652 20,860,638 The Company computes diluted loss per common share after giving consideration to the dilutive effect of stock options, warrants and non-vested stock that are outstanding during the period, except where such non-participating securities would be anti-dilutive. To date, the Company has only incurred net losses and has not allocated any loss to participating securities.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2015
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements The Company measures and reports its cash equivalents, restricted cash, short-term investments and preferred stock warrant liabilities 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: September 30, 2015 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 155,766 $ — $ — $ 155,766 Restricted money market funds 225 — — 225 Total financial assets $ 155,991 $ — $ — $ 155,991 December 31, 2014 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 27,847 $ — $ — $ 27,847 Restricted money market funds 75 — — 75 Short-term investments — 10,010 — 10,010 Total financial assets $ 27,922 $ 10,010 $ — $ 37,932 Financial Liabilities Preferred stock warrant liabilities $ — $ — $ 1,810 $ 1,810 Money market funds are measured at fair value on a recurring basis using quoted prices and are classified as a Level 1 input. The short-term investments contain observable inputs. Accordingly, the carrying value of this instrument approximates its fair value and is classified as Level 2 inputs. The Company’s preferred stock warrant liabilities contained unobservable inputs that reflected the Company’s own assumptions in which there was little, if any, market activity for at the measurement date. Accordingly, the Company’s warrant liabilities at December 31, 2014 were measured at fair value on a recurring basis using unobservable inputs and were classified as Level 3 inputs. At December 31, 2014, the Company remeasured the preferred stock warrants to fair value using the OPM and/or the PWERM models with the following assumptions used in the OPM: As of December 31, Expected term (in years) 1.2 – 2.5 Expected volatility 69 – 86 % Risk-free interest rate 0.3 – 0.6 % Expected dividend rate — % Discount for lack of marketability 19.5 – 24.3 % During the nine months ended September 30, 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, the outstanding preferred stock warrants were exercised and the liability on the preferred stock warrants was reclassified to additional paid-in capital in stockholders’ equity (deficit), and is no longer subject to remeasurement. The following table provides a summary of the changes in the estimated fair value of the Company’s preferred stock warrants, which were measured at fair value on a recurring basis until their exercise (in thousands): Balance as of December 31, 2014 $ 1,810 Fair value of preferred stock warrants exercised (19,253 ) Change in fair value of preferred stock warrants 17,443 Balance as of September 30, 2015 $ — The change in fair value of the preferred stock warrant liabilities is attributable to the increase of the fair value of the underlying stock. The change in the fair value of preferred stock warrants for each presented period is recognized as a loss in the condensed consolidated statements of operations. There were no transfers between Levels 1, 2 or 3 for the year ended December 31, 2014. During the three and nine months ended September 30, 2015, $10.0 million of short-term investments transferred from being a Level 2 financial asset to a Level 1 financial asset when the short-term investments were sold in July 2015. The proceeds from the sale of the short-term investments were invested in money market funds, which is a Level 1 financial asset. During the three and nine months ended September 30, 2015, the Company realized a $0 and $10,000 other-than-temporary impairment loss on its short-term investments through other income (expense) in the accompanying condensed consolidated statements of operations. During the three and nine months ended September 30, 2014, the Company did not realize any other-than-temporary impairment on its short-term investments.

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2015
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: September 30, December 31, (in thousands) Cash $ 1,542 $ 1,307 Cash equivalents: Money market accounts 155,766 27,847 Total cash and cash equivalents $ 157,308 $ 29,154 Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consist of the following: September 30, December 31, (in thousands) Prepaid research and development project costs $ 444 $ 515 Prepaid insurance 642 23 Other 194 23 Total prepaid expenses and other current assets $ 1,280 $ 561 Property and Equipment, net Property and equipment, net consists of the following: September 30, December 31, (in thousands) Computer equipment $ 185 $ 56 Software 144 9 Lab equipment 241 163 Leasehold improvements 80 — Property and equipment, gross 650 228 Less: accumulated depreciation (85 ) (14 ) Total property and equipment, net $ 565 $ 214 Depreciation and amortization related to the Company’s property and equipment was $32,000 and $71,000 for the three and nine months ended September 30, 2015 and was $3,000 for the three and nine months ended September 30, 2014. Other Assets Other assets consist of the following: September 30, December 31, (in thousands) Deferred offering costs $ — $ 549 Restricted cash 200 75 Deferred tax assets 3 2 Total other assets $ 203 $ 626 Accrued Liabilities Accrued liabilities consist of the following: September 30, December 31, (in thousands) Accrued research and development costs $ 1,468 $ 526 Accrued employee related costs 1,003 759 Accrued professional fees 415 140 Other 27 48 Total accrued liabilities $ 2,913 $ 1,473

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies License Agreements In March 2007, the Company entered into an exclusive license agreement with Novosom AG (Novosom) to use certain patented LNP delivery technology for any of its DNAi drug candidates that target a region of the BCL2 gene (the Novosom License Agreement). In July 2010, the Novosom License Agreement was acquired by Marina Biotech, Inc. (Marina), but Novosom retained the right to receive all payments due from the Company under the Novosom License Agreement. In March 2012, the Company and Marina entered into another exclusive license agreement (the Marina License Agreement) for the use of certain of Marina’s patented delivery technology, including LNP technology, for any of the Company’s current or future DNAi product candidates that target any gene. In exchange for this exclusive right, the Company paid Marina an upfront payment of $0.3 million in 2012 to be applied to the first DNAi product candidate under this agreement. The Company will be required to pay Marina milestone payments of up to an aggregate of $14.5 million for each DNAi product candidate, other than PNT2258, upon successful completion of certain clinical and regulatory milestone events relating to each DNAi product candidate identified in the Marina License Agreement. In addition, for DNAi product candidates other than PNT2258, the Company is required to pay Marina a low single-digit royalty on net sales. In May 2013, the Company issued Novosom 2,108,870 shares of common stock with a fair market value of $0.1 million in settlement of the first milestone payment. Pursuant to the terms of the Novosom License Agreement, additional shares may have been issued if certain dilution events occurred through December 31, 2014. The Company evaluated the need to accrue an expense related to the future potential issuance of shares at December 31, 2013 and 2014, but determined that each of the terms requiring additional issuances was not likely to occur and thus the expense was not recorded. The dilution provisions expired on December 31, 2014. In April 2014, the Company entered into a Second License Amendment and Consent to Termination Agreement with Marina pursuant to which the Novosom License Agreement (which had been transferred to Marina by Novosom in July 2010) was terminated and the obligations previously set forth in the Novosom License Agreement were restated in the Marina License Agreement. In connection therewith, in April 2014, the Company also entered into a License Payment Agreement with Novosom under which the Company agreed to pay Novosom $11.0 million in cash upon the closing of a minimum $35.0 million financing. Also, the Company agreed to pay Novosom a $3.0 million milestone payment within 30 days of regulatory authority approval of PNT2258. Upon Novosom’s receipt of the cash payment of $11.0 million, all financial obligations of the Company to Novosom were terminated, other than the aforementioned milestone payment, historic manufacturing costs and a low-single digit royalty payment on net sales of PNT2258. The Company made the $11.0 million payment to Novosom in April 2014, upon the closing of the Series D redeemable convertible preferred stock financing and a payment of $0.1 million in July 2014 settling the obligation for the historic manufacturing costs. The April 2014 Second License Amendment and Consent to Termination Agreement serves as the second amendment to the Marina License Agreement. As of September 30, 2015, the Company has not accrued the $3.0 million milestone payment to Novosom as regulatory authority approval is not probable of occurring. As of September 30, 2015, the Company has not identified any product candidates that would pertain to the Marina License Agreement. Therefore, there is currently no potential expense or payment due under the Marina License Agreement. Lease Agreements The Company leases its Michigan facility under a short-term operating lease with a term of less than a year that provides for a fixed monthly rent for the term of the lease and also provides for certain rent adjustments covering the expenses and taxes of the facility. 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 Canadian Dollar $50,000 letter of credit to the sublessor on closing, which is collateralized by a restricted deposit of $50,000. As of September 30, 2015, the aggregate future non-cancelable minimum lease payments associated with this operating lease (based on the daily average exchange rate on September 30, 2015 quoted by the Bank of Canada) are as follows: Years Ending December 31: Operating Leases (in thousands) 2015 (remainder) $ 79 2016 315 2017 315 2018 53 Total $ 762 The total rent expense for all operating leases was $0.1 million and $0.2 million for the three and nine months ended September 30, 2015 and was $17,000 and $39,000 for the three and nine months ended September 30, 2014. Contract Research Organization Obligations As of September 30, 2015, the aggregate future minimum payments associated with expected material agreements the Company entered into with contract research organizations are as follows: Years Ending December 31: Contract Research (in thousands) 2015 (remainder) $ 761 2016 5,093 2017 4,434 2018 988 Total $ 11,276 For the three and nine months ended September 30, 2015, total payments made to the contract research organization related to these obligations were $0.9 million and $2.4 million.

Common Stock Reserved for Issua

Common Stock Reserved for Issuance	9 Months Ended
	Sep. 30, 2015
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 shares of preferred stock (and preferred stock warrants), plus options granted and available for grant under the incentive plans. September 30, December 31, Conversion of outstanding Series A convertible preferred stock — 252,817 Conversion of outstanding Series B redeemable convertible preferred stock — 1,947,354 Conversion of outstanding Series B-1 redeemable convertible preferred stock — 1,392,300 Conversion of outstanding Series C redeemable convertible preferred stock — 2,487,770 Conversion of outstanding Series D redeemable convertible preferred stock — 11,409,360 Outstanding preferred stock warrants — 976,492 Outstanding and issued stock options 3,314,159 2,138,096 Shares reserved for future option grants 3,732,533 1,550,101 Shares reserved under the 2015 employee stock purchase plan 700,000 — Total common stock reserved for issuance 7,746,692 22,154,290

Convertible and Redeemable Conv

Convertible and Redeemable Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2015
Text Block [Abstract]
Convertible and Redeemable Convertible Preferred Stock	8. Convertible and Redeemable Convertible Preferred Stock During the three and nine months ended September 30, 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 and Series C redeemable convertible preferred stock warrants, consisting of 444,286 shares of Series B-1 redeemable convertible preferred stock and 37,385 shares of Series C redeemable convertible preferred stock. In addition, during the nine months ended September 30, 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 and Series C redeemable convertible preferred stock warrants, consisting of 5,850 shares of Series B redeemable convertible preferred stock, 291,165 shares of Series B-1 redeemable convertible preferred stock and 93,017 shares of Series C redeemable convertible preferred stock. 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, Series C and Series D redeemable convertible preferred stock converted into 18,109,136 shares of common stock. As of September 30, 2015, the Company had 10,000,000 shares of authorized preferred stock with a par value of $0.001. No preferred stock was outstanding as September 30, 2015. The convertible and redeemable convertible preferred stock as of December 31, 2014 consisted of the following: December 31, 2014 Shares Shares Issued Net Carrying Liquidation Aggregate Redemption (in thousands, except share and per share data) Convertible Preferred Stock: Series A 1,843,894 224,564 $ 2,543 $ 11.324 $ 2,543 $ — Redeemable Convertible Preferred Stock: Series B 13,134,880 1,752,634 19,504 12.268 21,502 19,504 Series B-1 16,122,618 1,392,300 6,979 3.656 5,091 6,979 Series C 19,812,349 2,487,770 21,052 5.642 14,035 21,052 Series D 85,000,000 11,409,360 94,297 5.510 62,865 94,297 Total redeemable convertible preferred stock 134,069,847 17,042,064 $ 141,832 $ 103,493 $ 141,832 Total preferred stock 135,913,741 17,266,628 $ 144,375 $ 106,036 $ 141,832 On issuance, the Company’s convertible and redeemable convertible preferred stock was recorded at fair value or the amount of allocated proceeds, net of issuance costs. The Company’s Series B, B-1, C and D redeemable convertible preferred stock (collectively, the “redeemable convertible preferred stock”) was classified outside of stockholders’ deficit because the stock contained redemption features that commenced at any time on or after December 31, 2018 at the election of the Series B, B-1, C and D redeemable convertible preferred stockholders. The Company adjusted the carrying amount of the redeemable convertible preferred stock to equal the redemption value at the end of each reporting period. Due to the absence of retained earnings, adjustments to redemption value were recorded against additional paid-in capital, if any, and then to accumulated deficit. The change in the redemption value of the convertible preferred stock from January 1, 2015 to July 21, 2015 (closing date of IPO) was as follows: Redeemable Convertible Preferred Stock 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 IPO), the fair value of redeemable convertible preferred stock was estimated based on the underlying fair value of the Company’s common stock. At December 31, 2014, the following assumptions were used in the OPM to determine fair value of the redeemable convertible preferred stock: As of December 31, Expected term (in years) 1.2 – 2.5 Expected volatility 69 – 86 % Risk-free interest rate 0.3 – 0.6 % Expected dividend rate — % 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 preferred stock were not modified. 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, $1.7 million for the Series C and $6.1 million for the Series D redeemable convertible preferred stock. During the three and nine months ended September 30, 2015, the Company paid in cash accruing dividends in the aggregate amount of $5.5 million to the Series B and B-1 redeemable convertible preferred stockholders, consisting of $4.7 million for the Series B and $0.8 million for the Series B-1 redeemable convertible preferred stockholders. During the three and nine months ended September 30, 2015, the Company issued an aggregate of 750,946 shares of common stock to the Series C and D redeemable convertible preferred stockholders with an aggregate fair value of $20.4 million in settlement of the Series C and Series D redeemable convertible preferred stock cumulative dividends in accordance with the June 11, 2015 amendment discussed above, consisting of 161,536 shares of common stock with a fair value of $4.4 million to the Series C redeemable convertible preferred stockholders and 589,410 shares of common stock with a fair value of $16.0 million to the Series D redeemable convertible preferred stockholders.

Preferred Stock Warrants

Preferred Stock Warrants	9 Months Ended
	Sep. 30, 2015
Text Block [Abstract]
Preferred Stock Warrants	9. Preferred Stock Warrants The Company classified its preferred stock warrants as liabilities on the accompanying condensed consolidated balance sheets. During the three and nine months ended September 30, 2015, $18.2 million and $19.3 million of the fair value of the warrant liability was reclassified to redeemable convertible preferred stock in mezzanine equity and then into additional paid-in capital in stockholders’ equity (deficit) on conversion of redeemable convertible preferred stock into common stock on the closing of the IPO. The fair value of the warrants was estimated to be nil and $1.8 million as of September 30, 2015 and December 31, 2014. Refer to Note 4 for the valuation technique and assumptions used in estimating the fair value of the warrants. During the three months ended September 30, 2015, 28,699 of the Series B-1 were cash exercised at an exercise price of $7.45 per share, 329,559 of the Series B-1 warrants were cash exercised at an exercise price of $2.6075 per share and 32,138 of the Series C warrants were cash exercised at an exercise price of $5.215 per share, resulting in total aggregate cash proceeds to the Company of $1.2 million. During the nine months ended September 30, 2015, 31,778 of the Series B-1 warrants were cash exercised at an exercise price of $7.45 per share, 412,508 of the Series B-1 warrants were cash exercised at an exercise price of $2.6075 per share and 37,385 of the Series C warrants were cash exercised at an exercise price of $5.215 per share, resulting in total aggregate cash proceeds to the Company of $1.5 million. 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 preferred stock and 93,017 shares of common stock being issued upon the net exercise of outstanding warrants to purchase 134,180 shares of Series C preferred stock.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	10. Stock-Based Compensation In the accompanying condensed consolidated statement of operations, the Company recognized stock-based compensation expense for its employees and non-employees as follows: Three Months Ended Nine Months Ended 2015 2014 2015 2014 (in thousands) Research and development $ 733 $ 11 $ 1,032 $ 29 General and administrative 579 105 880 168 Total stock-based compensation $ 1,312 $ 116 $ 1,912 $ 197 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 assumptions as follows: Three Months Ended Nine Months Ended 2015 2014 2015 2014 Expected term (in years) 6.1 5.9 – 6.3 5.2 – 10 5.0 – 6.3 Expected volatility 79% 82 – 83% 75 – 84% 81 – 83% Risk-free interest rate 1.7 – 1.9% 1.7 – 1.8% 1.5 – 2.4% 1.5 – 1.8% Expected dividend rate — % — % — % — % Equity Incentive Plans 2015 Plan The 2015 Equity Incentive Plan (the “2015 Plan”) became effective on July 14, 2015. Under the 2015 Plan, 3,400,000 shares of common stock were initially reserved for issuance pursuant to a variety of stock-based compensation awards, including stock options, restricted stock awards, stock appreciation rights, restricted stock units, performance awards, cash awards and stock bonuses. In addition, 365,535 shares that had been available for future awards under the 2008 Plan as of July 14, 2015, were added to the initial reserve available under the 2015 Plan, bringing the total number of shares reserved for issuance under the 2015 Plan upon its effective date to 3,765,535 shares. The number of shares initially reserved for issuance under the 2015 Plan will increase automatically on January 1 of each calendar year 2016 through 2025 by the number of shares equal to 4% of the total outstanding shares of the Company’s common stock as of the immediately preceding December 31. The Company’s Board of Directors or Compensation Committee may reduce the amount of the increase in any particular year. The exercise price of each stock-based award issued under the 2015 Plan is required to be no less than the fair value of the Company’s capital stock, as determined by the board of directors on the date of the grant. 2008 Plan The Company granted options under the 2008 Stock Plan (the “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. 2005 Stock Plan As of September 30, 2015, there were 940 awards issued and outstanding under the Company’s 2005 Plan (the “2005 Plan”). The terms of the 2005 Plan are similar to those of the 2008 Plan. On August 20, 2008, the board of directors canceled all shares available under the 2005 Plan with the intention to not issue any more shares under the plan. A summary of activity under the 2005 Plan, 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 (1,217,568 ) 1,217,568 5.96 Options exercised — (41,505 ) 0.76 Options cancelled — — — Outstanding — September 30, 2015 3,732,533 3,314,159 $ 2.77 8.96 $ 58,866 Exercisable — September 30, 2015 2,127,353 $ 0.98 8.59 $ 41,564 Vested and expected to vest — September 30, 2015 3,217,100 $ 2.72 8.95 $ 57,303 The weighted-average grant date fair values of options granted during the three and nine months ended September 30, 2015 was $23.60 and $10.19 per share and during the three and nine months ended September 30, 2014 was $1.12 and $0.96 per share. The aggregate intrinsic value of options exercised was $0.2 million for the three and nine months ended September 30, 2015 and $0.2 million for the three and nine months ended September 30, 2014. The total grant date fair value of options vested for the three and nine months ended September 30, 2015 was $0.7 million and $0.9 million and during the three and nine months ended September 30, 2014 was $0.1 million and $0.2 million. As of September 30, 2015, total unrecognized stock-based compensation related to unvested stock options was $11.9 million, net of estimated forfeitures, which the Company expects to recognize over a remaining weighted-average period of 3.3 years. Certain stock options are subject to occurrence of a performance condition. Performance-Based Stock Option Grant During the year ended December 31, 2014, the Company granted options to purchase 217,305 shares of common stock to an executive officer which contains both performance-based and service-based vesting criteria. Stock-based compensation associated with these performance-based stock options is recognized if the performance condition is achieved. On January 24, 2015, the Company amended the terms of the performance-based stock options to extend the date of the performance-based milestone. Upon the completion of the IPO in July 2015, the performance-based criteria of the stock options was achieved and the Company recorded additional stock-based compensation related to the achievement of these options of $0.1 million during the three and nine months ended September 30, 2015. Liability for Early Exercise of Stock Options The 2008 Plan allows for the granting of options that may be exercised before the options have vested. In December 2014, an executive officer early exercised 103,252 stock options. In February 2015, an executive officer early exercised 13,422 stock options. On early exercise, the awards became subject to a restricted stock agreement. The shares of restricted stock granted upon early exercise of the options are subject to the same vesting provisions as the original stock option awards. Shares issued as a result of early exercise that have not vested are subject to repurchase by the Company upon termination of the purchaser’s employment or services, at the price paid by the purchaser, and are not deemed to be issued for accounting purposes until those related shares vest. The liability is reclassified into common stock and additional paid-in capital as the shares vest and the repurchase right lapses. Accordingly, the Company has recorded the unvested portion of the exercise proceeds of $38,000 as a current liability as of September 30, 2015 and $0.1 million as a long-term liability as of December 31, 2014 from the early exercise in the accompanying condensed consolidated balance sheet. As of September 30, 2015 and December 31, 2014, 39,493 and 103,252 shares held by the employee remain unvested and subject to repurchase. 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. The ESPP will not become effective until such time as the Compensation Committee determines in the future, and as of September 30, 2015, the initial offering periods had not commenced. As of September 30, 2015, 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	9 Months Ended
	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes The Company did not record a provision for federal income taxes for the three and nine months ended September 30, 2015 because it expects to generate a loss for the year ended December 31, 2015. The income tax expense of $14,000 and $25,000 for the three and nine months ended September 30, 2015 represents foreign taxes. The Company’s deferred tax assets continue to be offset by a valuation allowance.

Summary of Significant Accoun19

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The condensed consolidated balance sheet as of September 30, 2015, the condensed consolidated statements of operations for the three and nine months September 30, 2015 and 2014 and the condensed consolidated statements of cash flows for the nine months ended September 30, 2015 and 2014 and the condensed consolidated statements of convertible and redeemable convertible preferred stock and stockholders’ equity (deficit) for the nine months ended September 30, 2015 are unaudited. The unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for the fair presentation of the Company’s condensed consolidated financial statements included in this report. The condensed consolidated financial data disclosed in these notes to the condensed consolidated financial statements related to the three and nine month periods are also unaudited. The condensed consolidated results of operations for the three and nine months ended September 30, 2015 are not necessarily indicative of the results to be expected for the year ending December 31, 2015, or for any other future annual or interim period. The consolidated balance sheet as of December 31, 2014 included herein was derived from the audited consolidated financial statements as of that date. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements included in the prospectus dated July 15, 2015, filed with the SEC pursuant to Rule 424 promulgated under the Securities Act of 1933, as amended.
Use of Estimates	Use of Estimates The preparation of the condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying condensed consolidated financial statements include, but are not limited to the fair value of common stock, the fair value of preferred stock, the fair value of preferred stock warrant liabilities, the fair value of stock options, recoverability of the Company’s net deferred tax assets, and related valuation allowance and certain accruals. 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 subsidiary is the U.S. Dollar. Transactions denominated in currencies other than the functional currency are remeasured to the functional currency at the average exchange rate in effect 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 assets and liabilities are remeasured at historical exchange rates. Gains and losses related to remeasurement are recorded in other income (expense) in the condensed consolidated statements of operations.
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 or 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 a security deposit required for a facility lease. Restricted cash consists of funds invested in a money market fund. As of September 30, 2015, the current portion of restricted cash of $25,000 is included in prepaid expenses and other current assets and the long-term portion of restricted cash of $0.2 million is included in other assets in the accompanying condensed consolidated balance sheets.
Investments	Investments The Company determines the appropriate designation of its investments as “trading,” “available-for-sale” or “held-to-maturity” based on management’s intent at the time of purchase and reevaluates such designation at each reporting date. During the three and nine months ended September 30, 2015 and for the year ended December 31, 2014, all of the Company’s short-term investments were designated as available-for-sale. Unrealized gains and losses, if any, are reported as a separate component of stockholders’ deficit, except for unrealized losses determined to be other-than-temporary which are recorded in other income (expense) in the accompanying condensed consolidated statements of operations. The Company determines any realized gains or losses on the sale of any investments on a specific identification method and records such gains and losses as a component of other income (expense) in the accompanying condensed consolidated statements of operations. The Company evaluates its short-term investments periodically for possible other-than-temporary impairment. A decline in fair value below the amortized cost of the investment is considered other-than-temporary impairment if the Company has the intent to sell the investment or it is more likely than not that the Company will be required to sell the investment before recovery of the entire amortized cost basis. In those instances, an impairment charge equal to the difference between the fair value and the amortized cost basis is recognized in other income (expense). Regardless of the Company’s intent or requirement to sell an investment, impairment is considered other-than-temporary if the Company does not expect to recover the entire amortized cost basis. Investments with original maturities beyond three months at the date of purchase and which mature at, or less than twelve months from, the balance sheet date are classified as current. Investments with a maturity beyond twelve months from the condensed consolidated balance sheet date are classified as long-term.
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, restricted cash and short-term investments. All of the Company’s cash, cash equivalents, restricted cash and short-term investments 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 with these financial institutions exceed federally insured limits. 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 or sector and establishing a minimum allowable credit rating.
Comprehensive Income (Loss)	Comprehensive Income (Loss) The Company had no components of comprehensive income (loss) other than net loss for all periods presented.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s cash and cash equivalents, restricted cash, short-term investments, other current assets, accounts payable, and accrued liabilities approximate their fair value at September 30, 2015 and December 31, 2014, due to their short duration. The short-term investment maintains observable inputs, thus the carrying value of this instrument is carried at fair value and unrealized gains and losses, if any, are reported as a separate component of stockholders’ deficit. 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 and amortization. Depreciation and amortization 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 and amortization 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 balance sheet and the resulting gain or loss is reflected in the 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. Amortization on leasehold improvements are amortized over the term of the related property lease, for a period of three years.
Deferred Offering Costs	Deferred Offering Costs Deferred offering costs, consisting of legal, accounting and filing fees relating to an IPO were deferred until the completion of the IPO in July 2015, at which time they were reclassified to additional paid-in capital as a reduction of the gross proceeds. As of December 31, 2014, $0.5 million of deferred offering costs were capitalized and included in other long-term assets in the condensed consolidated balance sheets.
Preferred Stock Warrant Liabilities	Preferred Stock Warrant Liabilities The Company accounts 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 are recorded on the Company’s condensed 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 condensed consolidated statements of operations. The Company will continue to adjust the liability for changes in the fair value of these warrants until the earlier of the exercise of the warrants, the expiration of the warrants, or until such time as the warrants are no longer to be considered derivative instruments. 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 capital in stockholders’ equity (deficit), and was no longer subject to remeasurement.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. The Company accounts for nonrefundable advance payments for goods and services that will be used in future research and development activities as expenses when the goods have been received or when the service has been performed rather than when the payment is made. Depending on the timing of payments to service providers of research and development costs, the Company recognizes prepaid expenses or accrued expenses related to these costs. These prepaid or accrued expenses are based on management’s estimates of the work performed under service agreements and milestones achieved. 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 preclinical studies and trials, regulatory activities, manufacturing activities to support clinical activities, license fees, 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 nonemployees are recognized at the grant date and remeasured to fair value at each reporting period. The expense is recognized over the vesting period which is generally the service period.
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 deferred tax assets have been offset by a 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.

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2015
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 2015 2014 Options to purchase common stock 3,314,159 2,386,507 Common stock subject to repurchase 39,493 — Convertible preferred stock — 252,818 Redeemable convertible preferred stock — 17,236,784 Warrants for preferred stock — 984,529 Total potential dilutive shares 3,353,652 20,860,638

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2015
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: September 30, 2015 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 155,766 $ — $ — $ 155,766 Restricted money market funds 225 — — 225 Total financial assets $ 155,991 $ — $ — $ 155,991 December 31, 2014 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 27,847 $ — $ — $ 27,847 Restricted money market funds 75 — — 75 Short-term investments — 10,010 — 10,010 Total financial assets $ 27,922 $ 10,010 $ — $ 37,932 Financial Liabilities Preferred stock warrant liabilities $ — $ — $ 1,810 $ 1,810
Schedule of Remeasured Preferred Stock Warrants to Fair Value using OPM and/or PWERM Models	At December 31, 2014, the Company remeasured the preferred stock warrants to fair value using the OPM and/or the PWERM models with the following assumptions used in the OPM: As of December 31, Expected term (in years) 1.2 – 2.5 Expected volatility 69 – 86 % Risk-free interest rate 0.3 – 0.6 % Expected dividend rate — % Discount for lack of marketability 19.5 – 24.3 %
Summary of Changes in Estimated Fair Value of Preferred Stock Warrants Measured at Fair Value on Recurring Basis	The following table provides a summary of the changes in the estimated fair value of the Company’s preferred stock warrants, which were measured at fair value on a recurring basis until their exercise (in thousands): Balance as of December 31, 2014 $ 1,810 Fair value of preferred stock warrants exercised (19,253 ) Change in fair value of preferred stock warrants 17,443 Balance as of September 30, 2015 $ —

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Cash and Cash Equivalents	Cash and cash equivalents consist of the following: September 30, December 31, (in thousands) Cash $ 1,542 $ 1,307 Cash equivalents: Money market accounts 155,766 27,847 Total cash and cash equivalents $ 157,308 $ 29,154
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consist of the following: September 30, December 31, (in thousands) Prepaid research and development project costs $ 444 $ 515 Prepaid insurance 642 23 Other 194 23 Total prepaid expenses and other current assets $ 1,280 $ 561
Summary of Property and Equipment, Net	Property and equipment, net consists of the following: September 30, December 31, (in thousands) Computer equipment $ 185 $ 56 Software 144 9 Lab equipment 241 163 Leasehold improvements 80 — Property and equipment, gross 650 228 Less: accumulated depreciation (85 ) (14 ) Total property and equipment, net $ 565 $ 214
Schedule of Other Assets	Other assets consist of the following: September 30, December 31, (in thousands) Deferred offering costs $ — $ 549 Restricted cash 200 75 Deferred tax assets 3 2 Total other assets $ 203 $ 626
Summary of Accrued Liabilities	Accrued liabilities consist of the following: September 30, December 31, (in thousands) Accrued research and development costs $ 1,468 $ 526 Accrued employee related costs 1,003 759 Accrued professional fees 415 140 Other 27 48 Total accrued liabilities $ 2,913 $ 1,473

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of Aggregate Future Non-Cancelable Minimum Lease Payments	As of September 30, 2015, the aggregate future non-cancelable minimum lease payments associated with this operating lease (based on the daily average exchange rate on September 30, 2015 quoted by the Bank of Canada) are as follows: Years Ending December 31: Operating Leases (in thousands) 2015 (remainder) $ 79 2016 315 2017 315 2018 53 Total $ 762
Summary of Aggregate Future Minimum Payments	As of September 30, 2015, the aggregate future minimum payments associated with expected material agreements the Company entered into with contract research organizations are as follows: Years Ending December 31: Contract Research (in thousands) 2015 (remainder) $ 761 2016 5,093 2017 4,434 2018 988 Total $ 11,276

Common Stock Reserved for Iss24

Common Stock Reserved for Issuance (Tables)	9 Months Ended
	Sep. 30, 2015
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 shares of preferred stock (and preferred stock warrants), plus options granted and available for grant under the incentive plans. September 30, December 31, Conversion of outstanding Series A convertible preferred stock — 252,817 Conversion of outstanding Series B redeemable convertible preferred stock — 1,947,354 Conversion of outstanding Series B-1 redeemable convertible preferred stock — 1,392,300 Conversion of outstanding Series C redeemable convertible preferred stock — 2,487,770 Conversion of outstanding Series D redeemable convertible preferred stock — 11,409,360 Outstanding preferred stock warrants — 976,492 Outstanding and issued stock options 3,314,159 2,138,096 Shares reserved for future option grants 3,732,533 1,550,101 Shares reserved under the 2015 employee stock purchase plan 700,000 — Total common stock reserved for issuance 7,746,692 22,154,290

Convertible and Redeemable Co25

Convertible and Redeemable Convertible Preferred Stock (Tables)	9 Months Ended
	Sep. 30, 2015
Text Block [Abstract]
Components of Convertible and Redeemable Convertible Preferred Stock	The convertible and redeemable convertible preferred stock as of December 31, 2014 consisted of the following: December 31, 2014 Shares Shares Issued Net Carrying Liquidation Aggregate Redemption (in thousands, except share and per share data) Convertible Preferred Stock: Series A 1,843,894 224,564 $ 2,543 $ 11.324 $ 2,543 $ — Redeemable Convertible Preferred Stock: Series B 13,134,880 1,752,634 19,504 12.268 21,502 19,504 Series B-1 16,122,618 1,392,300 6,979 3.656 5,091 6,979 Series C 19,812,349 2,487,770 21,052 5.642 14,035 21,052 Series D 85,000,000 11,409,360 94,297 5.510 62,865 94,297 Total redeemable convertible preferred stock 134,069,847 17,042,064 $ 141,832 $ 103,493 $ 141,832 Total preferred stock 135,913,741 17,266,628 $ 144,375 $ 106,036 $ 141,832
Change in Redemption Value of Convertible Preferred Stock	The change in the redemption value of the convertible preferred stock from January 1, 2015 to July 21, 2015 (closing date of IPO) was as follows: Redeemable Convertible Preferred Stock 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
Summary of Assumptions used to Determine Fair Value of Temporary Equity	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 IPO), the fair value of redeemable convertible preferred stock was estimated based on the underlying fair value of the Company’s common stock. At December 31, 2014, the following assumptions were used in the OPM to determine fair value of the redeemable convertible preferred stock: As of December 31, Expected term (in years) 1.2 – 2.5 Expected volatility 69 – 86 % Risk-free interest rate 0.3 – 0.6 % Expected dividend rate — %

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Stock-based Compensation Expense for Employees and Non-employees	In the accompanying condensed consolidated statement of operations, the Company recognized stock-based compensation expense for its employees and non-employees as follows: Three Months Ended Nine Months Ended 2015 2014 2015 2014 (in thousands) Research and development $ 733 $ 11 $ 1,032 $ 29 General and administrative 579 105 880 168 Total stock-based compensation $ 1,312 $ 116 $ 1,912 $ 197
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 assumptions as follows: Three Months Ended Nine Months Ended 2015 2014 2015 2014 Expected term (in years) 6.1 5.9 – 6.3 5.2 – 10 5.0 – 6.3 Expected volatility 79% 82 – 83% 75 – 84% 81 – 83% Risk-free interest rate 1.7 – 1.9% 1.7 – 1.8% 1.5 – 2.4% 1.5 – 1.8% Expected dividend rate — % — % — % — %
Summary of Stock Options Activity	A summary of activity under the 2005 Plan, 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 (1,217,568 ) 1,217,568 5.96 Options exercised — (41,505 ) 0.76 Options cancelled — — — Outstanding — September 30, 2015 3,732,533 3,314,159 $ 2.77 8.96 $ 58,866 Exercisable — September 30, 2015 2,127,353 $ 0.98 8.59 $ 41,564 Vested and expected to vest — September 30, 2015 3,217,100 $ 2.72 8.95 $ 57,303

The Company and Basis of Pres27

The Company and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Millions	Jul. 21, 2015$ / sharesshares	Jul. 15, 2015USD ($)$ / sharesshares	Jun. 29, 2015	Sep. 30, 2015USD ($)$ / sharesshares	Dec. 31, 2014$ / sharesshares
Class of Stock [Line Items]
Common stock issued and sold, 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 on 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 on 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	180,000,000
Preferred stock, shares authorized	10,000,000			10,000,000
Preferred stock, par value | $ / shares	$ 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			7.45
Reverse stock split, effective date				Jul. 2, 2015
Common Stock [Member]
Class of Stock [Line Items]
Redeemable convertible preferred stock converted into common stock	18,361,953
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
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]
Common stock issued and sold		9,315,000
Common stock issued and sold, price per share | $ / shares		$ 17
Proceeds from issuance of common stock, net | $		$ 143.6

Summary of Significant Accoun28

Summary of Significant Accounting Policies - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2015USD ($)Segment	Dec. 31, 2014USD ($)
Summary Of Significant Accounting Policy [Line Items]
Deferred offering costs		$ 549,000
Number of operating segment | Segment	1
Leasehold Improvements [Member]
Summary Of Significant Accounting Policy [Line Items]
Estimated useful lives of assets	3 years
Minimum [Member]
Summary Of Significant Accounting Policy [Line Items]
Estimated useful lives of assets	3 years
Maximum [Member]
Summary Of Significant Accounting Policy [Line Items]
Estimated useful lives of assets	5 years
Other Noncurrent Assets [Member]
Summary Of Significant Accounting Policy [Line Items]
Long-term portion of restricted cash	$ 200,000
Prepaid Expenses and Other Current Assets [Member]
Summary Of Significant Accounting Policy [Line Items]
Current portion of restricted cash	$ 25,000

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	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	3,353,652	20,860,638
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares		252,818
Redeemable Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares		17,236,784
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	3,314,159	2,386,507
Common Stock Subject to Repurchase [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	39,493
Warrants for Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares		984,529

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Fair Value Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financial Assets
Total financial assets	$ 155,991	$ 37,932
Preferred Stock Warrant Liabilities [Member]
Financial Liabilities
Total financial Liabilities		1,810
Money Market Funds [Member]
Financial Assets
Total financial assets	155,766	27,847
Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	225	75
Short-Term Investments [Member]
Financial Assets
Total financial assets		10,010
Level 1 [Member]
Financial Assets
Total financial assets	155,991	27,922
Level 1 [Member] | Money Market Funds [Member]
Financial Assets
Total financial assets	155,766	27,847
Level 1 [Member] | Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	$ 225	75
Level 2 [Member]
Financial Assets
Total financial assets		10,010
Level 2 [Member] | Short-Term Investments [Member]
Financial Assets
Total financial assets		10,010
Level 3 [Member] | Preferred Stock Warrant Liabilities [Member]
Financial Liabilities
Total financial Liabilities		$ 1,810

Fair Value Measurements - Sch31

Fair Value Measurements - Schedule of Remeasured Preferred Stock Warrants to Fair Value using OPM and/or PWERM Models (Detail)	Dec. 31, 2014	Dec. 31, 2014
Class of Stock [Line Items]
Expected dividend rate		0.00%
Outstanding Preferred Stock Warrants [Member]
Class of Stock [Line Items]
Expected dividend rate	0.00%
Minimum [Member]
Class of Stock [Line Items]
Expected term (in years)		1 year 2 months 12 days
Expected volatility		69.00%
Risk-free interest rate		0.30%
Minimum [Member] | Outstanding Preferred Stock Warrants [Member]
Class of Stock [Line Items]
Expected term (in years)	1 year 2 months 12 days
Expected volatility	69.00%
Risk-free interest rate	0.30%
Expected dividend rate	0.00%
Discount for lack of marketability	19.50%
Maximum [Member]
Class of Stock [Line Items]
Expected term (in years)		2 years 6 months
Expected volatility		86.00%
Risk-free interest rate		0.60%
Maximum [Member] | Outstanding Preferred Stock Warrants [Member]
Class of Stock [Line Items]
Expected term (in years)	2 years 6 months
Expected volatility	86.00%
Risk-free interest rate	0.60%
Expected dividend rate	0.00%
Discount for lack of marketability	24.30%

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Changes in Estimated Fair Value of Preferred Stock Warrants Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value Disclosures [Abstract]
Balance as of December 31, 2014			$ 1,810
Fair value of preferred stock warrants exercised	$ (18,200)		(19,253)
Change in fair value of preferred stock warrants	7,487	$ 503	17,443	$ 999
Balance as of September 30, 2015	$ 0		$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfer of assets from level 1 to 2					$ 0
Transfer of assets from level 2 to 1	$ 10,000,000		$ 10,000,000		0
Transfer of liabilities from level 1 to 2					0
Transfer of liabilities from level 2 to 1					0
Transfer of assets into level 3					0
Transfer of assets out of level 3					0
Transfer of liabilities into level 3					0
Transfer of liabilities out of level 3					$ 0
Other Income (Expense) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recognized other-than-temporary impairment loss	$ 0	$ 0	$ 10,000	$ 0

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Cash and Cash Equivalents [Abstract]
Cash	$ 1,542	$ 1,307
Cash equivalents:
Money market accounts	155,766	27,847
Total cash and cash equivalents	$ 157,308	$ 29,154	$ 32,555	$ 2,429

Balance Sheet Components - Su35

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid research and development project costs	$ 444	$ 515
Prepaid insurance	642	23
Other	194	23
Total prepaid expenses and other current assets	$ 1,280	$ 561

Balance Sheet Components - Su36

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 650	$ 228
Less: accumulated depreciation	(85)	(14)
Total property and equipment, net	565	214
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	185	56
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	144	9
Lab Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	241	$ 163
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 80

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Property, Plant and Equipment [Abstract]
Depreciation and amortization related to property and equipment	$ 32,000	$ 3,000	$ 71,000	$ 3,000

Balance Sheet Components - Sche

Balance Sheet Components - Schedule of Other Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Deferred offering costs		$ 549
Restricted cash	$ 200	75
Deferred tax assets	3	2
Total other assets	$ 203	$ 626

Balance Sheet Components - Su39

Balance Sheet Components - Summary of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Payables and Accruals [Abstract]
Accrued research and development costs	$ 1,468	$ 526
Accrued employee related costs	1,003	759
Accrued professional fees	415	140
Other	27	48
Total accrued liabilities	$ 2,913	$ 1,473

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 31, 2014USD ($)	Apr. 30, 2014USD ($)	May. 31, 2013USD ($)shares	Sep. 30, 2015USD ($)	Sep. 30, 2014USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2014USD ($)	Dec. 31, 2012USD ($)	Feb. 28, 2015CAD	Feb. 28, 2015USD ($)	Dec. 31, 2014USD ($)
Loss Contingencies [Line Items]
Upfront payment paid								$ 300,000
Payment in cash upon closing of financing		$ 11,000,000
Minimum financing		35,000,000
Milestone payment upon regulatory authority approval		$ 3,000,000
Manufacturing costs obligation	$ 100,000
Milestone payment not accrued				$ 3,000,000		$ 3,000,000
Collateralized restricted deposit				200,000		200,000					$ 75,000
Payments to contract research organization				900,000		$ 2,400,000
Novosom License Agreement [Member]
Loss Contingencies [Line Items]
Shares of common stock issued in settlement of first milestone payment | shares			2,108,870
Fair marker value of common stock issued			$ 100,000
Operating Lease Agreement To Sublease Office Space [Member] | Vancouver [Member]
Loss Contingencies [Line Items]
Operating lease agreement expiration date						Feb. 27, 2018
Letter of credit issued | CAD									CAD 50,000
Collateralized restricted deposit										$ 50,000
Vancouver Facility [Member]
Loss Contingencies [Line Items]
Total rent expense for operating leases				$ 100,000	$ 17,000	$ 200,000	$ 39,000
Maximum [Member]
Loss Contingencies [Line Items]
Aggregate milestone payment								$ 14,500,000

Commitments and Contingencies41

Commitments and Contingencies - Schedule of Aggregate Future Non-Cancelable Minimum Lease Payments (Detail) $ in Thousands	Sep. 30, 2015USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2015 (remainder)	$ 79
2,016	315
2,017	315
2,018	53
Total	$ 762

Commitments and Contingencies42

Commitments and Contingencies - Summary of Aggregate Future Minimum Payments (Detail) $ in Thousands	Sep. 30, 2015USD ($)
Commitments and Contingencies Disclosure [Abstract]
Contract Research Organization Obligations, 2015 (remainder)	$ 761
Contract Research Organization Obligations, 2016	5,093
Contract Research Organization Obligations, 2017	4,434
Contract Research Organization Obligations, 2018	988
Contract Research Organization Obligations, Total	$ 11,276

Common Stock Reserved for Iss43

Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Sep. 30, 2015	Jul. 15, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Total common stock reserved for future issuance	7,746,692		22,154,290
2015 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	700,000	700,000
Outstanding Preferred Stock Warrants [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			976,492
Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			252,817
Series B Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			1,947,354
Series B-1 Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			1,392,300
Series C Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			2,487,770
Series D Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			11,409,360
Outstanding And Issued Stock Options [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	3,314,159		2,138,096
Shares Reserved For Future Option Grants [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	3,732,533		1,550,101

Convertible and Redeemable Co44

Convertible and Redeemable Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Jul. 21, 2015	Jun. 11, 2015	Sep. 30, 2015	Dec. 31, 2014
Temporary Equity [Line Items]
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
Preferred stock, shares authorized	10,000,000		10,000,000
Preferred stock, par value	$ 0.001		$ 0.001
Preferred stock, shares outstanding
Percentage of accrued but unpaid dividends paid in shares		50.00%
Percentage of accrued but unpaid dividends forfeited		50.00%
Share price, accrued dividend in shares		$ 5.215
Cumulative unpaid dividends in arrears	$ 18.9
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 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 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 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 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
Fair value of common stock to redeemable convertible preferred stockholders			$ 20.4

Convertible and Redeemable Co45

Convertible and Redeemable Convertible Preferred Stock - Components of Convertible and Redeemable Convertible Preferred Stock (Detail) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Temporary Equity [Line Items]
Shares Authorized		135,913,741
Shares Issued		17,266,628
Net Carrying Value		$ 144,375
Aggregate Liquidation Preference		106,036
Redemption Value		$ 141,832
Shares Outstanding		17,266,628
Series A Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Shares Authorized		1,843,894
Shares Issued		224,564
Net Carrying Value		$ 2,543
Liquidation Price Per Share		$ 11.324
Aggregate Liquidation Preference		$ 2,543
Shares Outstanding		224,564
Series B Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Shares Authorized		13,134,880
Shares Issued		1,752,634
Net Carrying Value		$ 19,504
Liquidation Price Per Share		$ 12.268
Aggregate Liquidation Preference		$ 21,502
Redemption Value		$ 19,504
Shares Outstanding		1,752,634
Series B-1 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Shares Authorized		16,122,618
Shares Issued		1,392,300
Net Carrying Value		$ 6,979
Liquidation Price Per Share		$ 3.656
Aggregate Liquidation Preference		$ 5,091
Redemption Value		$ 6,979
Shares Outstanding		1,392,300
Series C Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Shares Authorized		19,812,349
Shares Issued		2,487,770
Net Carrying Value		$ 21,052
Liquidation Price Per Share		$ 5.642
Aggregate Liquidation Preference		$ 14,035
Redemption Value		$ 21,052
Shares Outstanding		2,487,770
Series D Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Shares Authorized		85,000,000
Shares Issued		11,409,360
Net Carrying Value		$ 94,297
Liquidation Price Per Share		$ 5.510
Aggregate Liquidation Preference		$ 62,865
Redemption Value		$ 94,297
Shares Outstanding		11,409,360
Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Shares Authorized		134,069,847
Shares Issued		17,042,064
Net Carrying Value		$ 141,832
Aggregate Liquidation Preference		103,493
Redemption Value		$ 141,832
Shares Outstanding		17,042,064

Convertible and Redeemable Co46

Convertible and Redeemable Convertible Preferred Stock - Change in Redemption Value of Convertible Preferred Stock (Detail) - USD ($) $ in Thousands	3 Months Ended		7 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Jul. 21, 2015	Sep. 30, 2015	Sep. 30, 2014
Temporary Equity [Line Items]
Adjustment to redemption value	$ 222,130	$ 5,591		$ 374,015	$ 44,866
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

Convertible and Redeemable Co47

Convertible and Redeemable Convertible Preferred Stock - Summary of Assumptions used to Determine Fair Value of Temporary Equity (Detail)	12 Months Ended
	Dec. 31, 2014
Temporary Equity [Line Items]
Expected dividend rate	0.00%
Minimum [Member]
Temporary Equity [Line Items]
Expected term (in years)	1 year 2 months 12 days
Expected volatility	69.00%
Risk-free interest rate	0.30%
Maximum [Member]
Temporary Equity [Line Items]
Expected term (in years)	2 years 6 months
Expected volatility	86.00%
Risk-free interest rate	0.60%

Preferred Stock Warrants - Addi

Preferred Stock Warrants - Additional Information (Detail) - USD ($)	Jul. 21, 2015	Sep. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Class of Warrant or Right [Line Items]
Estimated fair value of warrant		$ 0	$ 0	$ 1,800,000
Fair value of warrant liability reclassified to redeemable convertible preferred stock and additional paid-in capital		18,200,000	19,253,000
Warrants exercised	493,648
Warrants exercise price	$ 17
Cash proceeds from warrants exercised		$ 1,200,000	$ 1,500,000
Number of common stock shares issued upon exercise of preferred stock warrants	390,680	481,671	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	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	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		28,699	31,778
Warrants exercise price		$ 7.45	$ 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		329,559	412,508
Warrants exercise price		$ 2.6075	$ 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		32,138	37,385
Warrants exercise price		$ 5.215	$ 5.215

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-based Compensation Expense for Employees and Non-employees (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 1,312	$ 116	$ 1,912	$ 197
Research and development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	733	11	1,032	29
General and administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 579	$ 105	$ 880	$ 168

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)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days
Expected volatility	79.00%
Expected volatility, Minimum		82.00%	75.00%	81.00%
Expected volatility, Maximum		83.00%	84.00%	83.00%
Risk-free interest rate, Minimum	1.70%	1.70%	1.50%	1.50%
Risk-free interest rate, Maximum	1.90%	1.80%	2.40%	1.80%
Expected dividend rate	0.00%	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 10 months 24 days	5 years 2 months 12 days	5 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		6 years 3 months 18 days	10 years	6 years 3 months 18 days

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Sep. 30, 2015	Jul. 15, 2015	Jul. 14, 2015	Dec. 31, 2014	Feb. 28, 2015	Dec. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance	7,746,692			22,154,290		22,154,290	7,746,692		7,746,692		22,154,290
Proceeds from early exercise of stock options of unvested portion									$ 13,000
Performance-Based Stock Option Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional stock-based compensation related to stock options							$ 100,000		$ 100,000
Performance-Based Stock Option Grant [Member] | Executive officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted options to purchase shares of common stock											217,305
2008 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance			365,535
Number of shares held by employees subject to repurchase	39,493			103,252		103,252	39,493		39,493		103,252
2008 Plan [Member] | Executive officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options early exercised					13,422	103,252
Proceeds from early exercise of stock options of unvested portion	$ 38,000			$ 100,000
2005 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards issued	940
Awards outstanding	940						940		940
2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares initially reserved for issuance			3,400,000
Number of common stock shares reserved for issuance			3,765,535
Stock option grants description									The number of shares initially reserved for issuance under the 2015 Plan will increase automatically on January 1 of each calendar year 2016 through 2025 by the number of shares equal to 4% of the total outstanding shares of the Company's common stock as of the immediately preceding December 31.
Percentage threshold of outstanding shares increased annually under the plan			4.00%
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		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%
Maximum number of common stock shares allowed to issue under employee stock purchase plan		3,400,000
Options to purchase shares of common stock, discount percentage		15.00%
Number of common stock issued									0
2005 Plan, 2008 Plan and 2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards issued									1,217,568
Awards outstanding	3,314,159			2,138,096		2,138,096	3,314,159		3,314,159		2,138,096
Weighted-average grant date fair values of options granted							$ 23.60	$ 1.12	$ 10.19	$ 0.96
Aggregate intrinsic value of options exercised							$ 200,000	$ 200,000	$ 200,000	$ 200,000
Total grant date fair value of options vested							700,000	$ 100,000	900,000	$ 200,000
Total unrecognized stock-based compensation related to unvested stock options	$ 11,900,000						$ 11,900,000		$ 11,900,000
Weighted-average period									3 years 3 months 18 days
Maximum number of common stock shares allowed to issue under employee stock purchase plan	3,400,000						3,400,000		3,400,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Options Activity (Detail) - 2005 Plan, 2008 Plan and 2015 Plan [Member] $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015USD ($)$ / sharesshares	Dec. 31, 2014USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Available for Grant, Beginning balance	1,550,101
Shares Available for Grant, Awards authorized	3,400,000
Shares Available for Grant, Granted	(1,217,568)
Shares Available for Grant, Exercised	0
Shares Available for Grant, Cancelled	0
Shares Available for Grant, Ending balance	3,732,533	1,550,101
Number of Shares Outstanding, Beginning balance	2,138,096
Number of Shares Outstanding, Options granted	1,217,568
Number of Shares Outstanding, Options exercised	(41,505)
Number of Shares Outstanding, Options cancelled	0
Number of Shares Outstanding, Ending balance	3,314,159	2,138,096
Number of Shares Outstanding, Exercisable	2,127,353
Number of Shares Outstanding, Vested and expected to vest	3,217,100
Weighted-Average Exercise Price Per Share, Beginning balance | $ / shares	$ 0.91
Weighted-Average Exercise Price Per Share, Options granted | $ / shares	5.96
Weighted-Average Exercise Price Per Share, Options exercised | $ / shares	0.76
Weighted-Average Exercise Price Per Share, Options cancelled | $ / shares	0
Weighted-Average Exercise Price Per Share, Ending balance | $ / shares	2.77	$ 0.91
Weighted-Average Exercise Price Per Share, Exercisable | $ / shares	0.98
Weighted-Average Exercise Price Per Share, Vested and expected to vest | $ / shares	$ 2.72
Weighted-Average Remaining Contractual Term (Years)	8 years 11 months 16 days	9 years 3 months 15 days
Weighted-Average Remaining Contractual Term (Year), Exercisable	8 years 7 months 2 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest	8 years 11 months 12 days
Aggregate Intrinsic Value of Outstanding Options | $	$ 58,866	$ 3,153
Aggregate Intrinsic Value of Outstanding Options, Exercisable | $	41,564
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest | $	$ 57,303

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Provision for federal income taxes	$ 0	$ 0
Income tax expense for foreign taxes	$ 14,000	$ 25,000