aTyr Pharma (ATYR) 10-Q 11 May 17 2017 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2017	May 10, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1
Trading Symbol	LIFE
Entity Registrant Name	aTYR PHARMA INC
Entity Central Index Key	1,339,970
Current Fiscal Year End Date	--12-31
Entity Filer Category	Smaller Reporting Company
Entity Common Stock, Shares Outstanding		23,806,329

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 28,900	$ 38,388
Available-for sale investments, short-term	33,034	33,759
Prepaid expenses and other assets	1,848	2,621
Total current assets	63,782	74,768
Available-for sale investments, long-term		4,002
Property and equipment, net	1,623	1,421
Other assets	330	333
Total assets	65,735	80,524
Current liabilities:
Accounts payable	1,246	2,606
Accrued expenses	4,160	5,450
Current portion of deferred rent	49	130
Current portion of long-term debt	1,094	339
Total current liabilities	6,549	8,525
Long-term debt, net of current portion and issuance costs	8,480	9,198
Commitments and contingencies (Note 3)
Stockholders’ equity:
Common stock, $0.001 par value; authorized shares – 150,000,000 as of March 31, 2017 and December 31, 2016; issued and outstanding shares – 23,780,976 and 23,744,832 as of March 31, 2017 and December 31, 2016, respectively	24	24
Additional paid-in capital	280,134	278,832
Accumulated other comprehensive loss	(68)	(76)
Accumulated deficit	(229,384)	(215,979)
Total stockholders’ equity	50,706	62,801
Total liabilities and stockholders’ equity	$ 65,735	$ 80,524

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	23,780,976	23,744,832
Common stock, shares outstanding	23,780,976	23,744,832

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating expenses:
Research and development	$ 9,204	$ 12,000
General and administrative	4,007	4,115
Total operating expenses	13,211	16,115
Loss from operations	(13,211)	(16,115)
Other income (expense), net	(194)	28
Net loss	$ (13,405)	$ (16,087)
Net loss per share, basic and diluted	$ (0.56)	$ (0.68)
Weighted average common stock shares outstanding, basic and diluted	23,739,057	23,631,133

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (13,405)	$ (16,087)
Other comprehensive gain (loss):
Change in unrealized gain (loss) on available for sale investments, net of tax	8	152
Comprehensive loss	$ (13,397)	$ (15,935)

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (13,405)	$ (16,087)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	226	233
Stock-based compensation	1,277	1,280
Amortization of debt discount	(37)	54
Amortization of premium of available-for-sale investment securities	74	264
Deferred rent	(81)	(76)
Changes in operating assets and liabilities
Prepaid expenses and other assets	776	858
Accounts payable and accrued expenses	(2,742)	983
Net cash used in operating activities	(13,912)	(12,491)
Cash flows from investing activities:
Purchase of property and equipment	(262)	(314)
Purchases of available-for-sale investment securities	(11,489)	(9,994)
Maturities of available-for-sale investment securities	16,150	24,700
Net cash provided by investing activities	4,399	14,392
Cash flows from financing activities:
Proceeds from issuance of common stock through option exercises	25	11
Repayments on loans payable		(838)
Net cash provided by (used in) financing activities	25	(827)
Net change in cash and cash equivalents	(9,488)	1,074
Cash and cash equivalents at beginning of period	38,388	53,025
Cash and cash equivalents at the end of period	$ 28,900	$ 54,099

Organization, Business, Basis o

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies	1. Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies Organization and Business aTyr Pharma, Inc. (we, us, and our) was incorporated in the state of Delaware on September 8, 2005. We are focused on the discovery and clinical development of innovative medicines for patients suffering from severe rare diseases. Principles of Consolidation Our consolidated financial statements include our accounts and our 98% majority-owned subsidiary in Hong Kong, Pangu BioPharma Limited (Pangu BioPharma). All intercompany transactions and balances are eliminated in consolidation. Unaudited Interim Financial Information The accompanying interim condensed consolidated financial statements are unaudited. These unaudited interim financial statements have been prepared in accordance with United States generally accepted accounting principles (GAAP) and following the requirements of the United States Securities and Exchange Commission (SEC) for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by GAAP can be condensed or omitted. In our opinion, the unaudited interim financial statements have been prepared on the same basis as the audited financial statements and include all adjustments, which include only normal recurring adjustments, necessary for the fair presentation of our financial position and our results of operations and cash flows for periods presented. These statements do not include all disclosures required by GAAP and should be read in conjunction with our financial statements and accompanying notes for the fiscal year ended December 31, 2016, contained in our Annual Report on Form 10-K filed with the SEC on March 16, 2017. The results of the interim periods are not necessarily indicative of the results expected for the full fiscal year or any other interim period or any future year or period. Liquidity and Financial Condition We have incurred losses and negative cash flows from operations since our inception. As of March 31, 2017, we had an accumulated deficit of $229.4 million and we expect to continue to incur net losses for the foreseeable future. We believe that our existing cash, cash equivalents and available-for-sale investments, of $61.9 million as of March 31, 2017 will be sufficient to meet our anticipated cash requirements for a period of one year from the filing date of this Quarterly Report. We do not expect to generate any revenues from product sales unless and until we successfully complete development and obtain regulatory approval for one or more of our product candidates, which we expect will take a number of years at a minimum. If we obtain regulatory approval for any of our product candidates, we expect to incur significant commercialization expenses related to product sales, marketing, manufacturing and distribution. Accordingly, we will need to raise substantial additional capital beyond the net proceeds from our initial public offering (IPO). The amount and timing of our future funding requirements will depend on many factors, including the pace and results of our preclinical and clinical development efforts and the timing and nature of the regulatory approval process for our product candidates. We anticipate that we will seek to fund our operations through public or private equity or debt financings, collaborations, strategic partnerships or other sources. However, we may be unable to raise additional capital or enter into such other arrangements when needed on favorable terms or at all. Our failure to raise capital or enter into such other arrangements when needed would have a negative impact on our financial condition and ability to develop our product candidates. Use of Estimates Our consolidated financial statements are prepared in accordance with GAAP. The preparation of our consolidated financial statements requires us to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent assets and liabilities in our consolidated financial statements and accompanying notes. The most significant estimates in our consolidated financial statements relate to the fair value of equity issuances and awards, and clinical trials and research and development expense accruals. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ materially from these estimates and assumptions. Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents and adjusted for the weighted average number of common shares outstanding that are subject to repurchase. We have excluded 9,039 and 44,136 shares subject to repurchase from the weighted average number of common shares outstanding for the three months ended March 31, 2017 and 2016, respectively. Diluted net loss per share is calculated by dividing the net loss by the weighted average number of common stock equivalents outstanding for the period determined using the treasury-stock method. Dilutive common stock equivalents are comprised of warrants for common stock, restricted stock units and options outstanding under our stock option plan and employee stock purchase plan. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to our net loss position. Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): March 31, 2017 2016 Warrants for common stock 121,512 25,970 Common stock options and awards 4,720,382 3,742,770 Employee stock purchase plan 36,837 17,363 4,878,731 3,786,103 Recent Accounting Pronouncements In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments To achieve this objective, the amendments in this Update replace the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	2. Fair Value Measurements The carrying amounts of cash equivalents, prepaid and other assets, accounts payable and accrued liabilities are considered to be representative of their respective fair values because of the short-term nature of those instruments. Based on the borrowing rates currently available to us for loans with similar terms, which is considered a Level 2 input, we believe that the carrying value of our Term Loan approximates its fair value. Investment securities are recorded at fair value. The accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the accounting guidance establishes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows: Level 1: Observable inputs such as quoted prices in active markets. Level 2: Inputs, other than the quoted prices in active markets that are observable either directly or indirectly. Level 3: Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions. Financial assets measured at fair value on a recurring basis consist of investment securities. Investment securities are recorded at fair value, defined as the exit price in the principal market in which we would transact, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. Level 2 securities are valued using quoted market prices for similar instruments, non-binding market prices that are corroborated by observable market data, or discounted cash flow techniques and include our investments in corporate debt securities and commercial paper. We have no financial liabilities measured at fair value on a recurring basis. None of our non-financial assets and liabilities is recorded at fair value on a non-recurring basis. No transfers between levels have occurred during the periods presented. Assets measured at fair value on a recurring basis are as follows (in thousands): Fair Value Measurements Using Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of March 31, 2017: Assets: Current: Cash equivalents $ 24,125 $ 24,125 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 7,854 — 7,854 — Commercial paper 6,513 — 6,513 — Corporate debt securities 16,667 — 16,667 — United States Treasury securities 2,000 2,000 — — Sub-total short-term investments 33,034 2,000 31,034 - Total assets measured at fair value $ 57,159 $ 26,125 $ 31,034 $ — As of December 31, 2016: Assets: Current: Cash equivalents $ 29,251 $ 29,251 $ — $ — Available-for-sale investments, short-term: Commercial paper 7,843 — 7,843 — Corporate debt securities 20,913 — 20,913 — United States Treasury securities 5,003 5,003 — — Sub-total short-term investments 33,759 5,003 28,756 — Available-for-sale investments, long-term: Asset-backed securities 4,002 — 4,002 — Sub-total long-term investments 4,002 — 4,002 — Total assets measured at fair value $ 67,012 $ 34,254 $ 32,758 $ — As of March 31, 2017 and December 31, 2016 available-for-sale investments are detailed as follows (in thousands): March 31, 2017 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities 7,857 — (3 ) 7,854 Commercial paper $ 6,513 $ — $ — $ 6,513 Corporate debt securities 16,683 — (16 ) 16,667 United States Treasury securities 2,001 — (1 ) 2,000 $ 33,054 $ — $ (20 ) $ 33,034 December 31, 2016 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Commercial paper $ 7,843 $ — $ — $ 7,843 Corporate debt securities 20,942 — (29 ) 20,913 United States Treasury securities 5,002 1 — 5,003 $ 33,787 $ 1 $ (29 ) $ 33,759 Available-for-sale investments, long-term: Asset-backed securities $ 4,001 $ 1 $ — $ 4,002 $ 4,001 $ 1 $ — $ 4,002 Available-for-sale investments that are in an unrealized loss position as of March 31, 2017 are as follows (in thousands): Estimated Fair Value Gross Unrealized Losses Asset-backed securities $ 7,854 $ (3 ) Corporate debt securities 16,667 $ (16 ) United States Treasury securities 2,000 $ (1 ) $ 26,521 $ (20 ) As of March 31, 2017, all of our available-for-sale investments have a variety of effective maturity dates of less than one year. As of March 31, 2017, there are 15 available-for-sale investments in gross unrealized loss position, all of which had been in such position for less than twelve months. At each reporting date, we perform an evaluation of impairment to determine if the unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, and our intent and ability to hold the investment until recovery of its amortized cost basis. We intend, and have the ability, to hold our investments in unrealized loss positions until their amortized cost basis has been recovered. Based on our evaluation, we determined that the unrealized losses were not other-than-temporary as of March 31, 2017.

Debt, Commitments and Contingen

Debt, Commitments and Contingencies	3 Months Ended
	Mar. 31, 2017
Debt Instruments [Abstract]
Debt, Commitments and Contingencies	3. Debt, Commitments and Contingencies Term Loan In November 2016, we entered into a loan and security agreement with Silicon Valley Bank (SVB) and Solar Capital Ltd. (Solar), which we refer to as the Term Loan. SVB and Solar agreed to lend us up to $20.0 million, issuable in three separate tranches of: (i) $10.0 million which was funded on November 18, 2016; (ii) $5.0 million may be drawn down by us at any time before the earlier of June 30, 2017 or an event of default, at our discretion, subject to achievement of certain financial and clinical milestones; and (iii) $5.0 million may be drawn down by us any time after June 30, 2017 and before the earlier of December 31, 2017 or an event of default, at our discretion, subject to achievement of certain milestones specified for the second tranche and additional financial and clinical milestones. Pursuant to the Term Loan agreement, we have interest only payments through December 1, 2017 or June 1, 2018, if we draw the second tranche, followed by consecutive equal monthly payments of principal and interest in arrears through the maturity date of November 18, 2020. The Term Loan bears interest at the prime rate, as reported in The Wall Street Journal on the last date of the month preceding the month in which interest will accrue, plus 4.10%. The Term Loan also provides for a final payment equal to 8.75% of the funded amount, which is due when the Term Loan becomes due or upon the prepayment of the Term Loan. We have the option to prepay the outstanding balance of the loan in full, subject to a prepayment fee ranging from 1.0% to 3.0% depending upon when the prepayment occurs, including any non-usage fees. The loan agreement provides for a 2.0% non-usage fee for any unfunded amount in the event we do not draw the second tranche and third tranche, payable no later than the expiration date for the second tranche or third tranche, as applicable, or the date of cancellation of the loan due to prepayment or an event of default. No such events have occurred or are anticipated as of March 31, 2017. We received cash proceeds of $7.3 million, net of a $2.6 million repayment of the principal, accrued interest and the $0.5 million final payment under our previous $10.0 million loan and security agreement with SVB, which we refer to as the SVB Loan. We did not pay any termination or other fees in connection with the repayment of amounts due under the SVB Loan. As of March 31, 2017, the carrying value of our Term Loan consists of $10.0 million principal outstanding less the debt issuance costs of $0.4 million. The debt issuance costs have been recorded as a debt discount of long-term debt in our balance sheet, which are being accreted to interest expense over the life of the Term Loan using an effective interest rate of 12.28%. The final maturity payment of $0.9 million is being accrued over the life of the Term Loan through interest expense. Future principal payments for the Term Loan, including the final payment, are as follows (in thousands): March 31, 2017 2017 $ 248 2018 3,093 2019 3,348 2020 4,186 $ 10,875 Facility Lease In December 2011, we entered into a noncancelable operating lease that included certain tenant improvement allowances and is subject to base lease payments, which escalate over the term of the lease, additional charges for common area maintenance and other costs. In January 2017, we extended the lease for two years to May 2019. Rent expense for the three months ended March 31, 2017 and 2016 was $0.2 million and $0.1 million, respectively. In June 2016, we entered into a sublease agreement with a tenant of our landlord for additional facility space in our existing building that commenced in August 2016 and will expire in June 2017. Future minimum payments under the non-cancelable operating lease as of March 31, 2017 were as follows (in thousands): Operating Lease 2017 $ 683 2018 773 2019 293 $ 1,749 Research Agreements and Funding Obligations We provide funding to The Scripps Research Institute (TSRI) pursuant to a research funding and option agreement to conduct certain research activities. We have entered into additional amendments to our research funding and option agreement in 2017 to provide additional funding to TSRI through March 31, 2017. For the three months ended March 31, 2017 and 2016, the additional funding provided to TSRI was $0.2 million and $0.2 million, respectively. A member of our board of directors is a faculty member at TSRI and such payments fund a portion of his research activities conducted at TSRI. During the three months ended March 31, 2017 and 2016, we provided charitable donations to the National Foundation for Cancer Research of $0.1 million and $0.1 million, respectively. We have requested that the donations be restricted to certain basic research in cancer biology and therapeutics, a portion of which funds research activities conducted at TSRI in the laboratory of a member of our board of directors.

Stockholders' Equity

Stockholders' Equity	3 Months Ended
	Mar. 31, 2017
Equity [Abstract]
Stockholders' Equity	4. Stockholders’ Equity Common Stock Reserved for Future Issuance Pursuant to the automatic increase provisions of our 2015 Stock Option and Incentive Plan (2015 Plan) and 2015 Employee Stock Purchase Plan (2015 ESPP), 949,793 additional shares were reserved for future issuance under the 2015 Plan on January 1, 2017 and 237,448 additional shares were reserved for future issuances under the 2015 ESPP on January 1, 2017. Common stock reserved for future issuance is as follows: March 31, 2017 Common stock warrants 121,512 Common stock options and awards outstanding 4,720,382 Shares available under the 2015 Plan 795,728 Shares available under the 2015 ESPP 644,990 6,282,612 The following table summarizes our stock option activity under all equity incentive plans for the three months ended March 31, 2017: Number of Outstanding Options Weighted Average Exercise Price Outstanding as of December 31, 2016 4,014,988 $ 6.73 Granted 765,719 $ 3.29 Exercised (795 ) $ 1.01 Canceled/forfeited/expired (122,894 ) $ 6.98 Outstanding as of March 31, 2017 4,657,018 $ 6.16 The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows: Three Months Ended March 31, 2017 2016 Expected term (in years) 6.02 – 6.06 5.77 – 6.08 Risk-free interest rate 2.0% – 2.1% 1.4% – 1.9% Expected volatility 104.0% – 105.2% 81.2% – 82.1% Expected dividend yield 0.0% 0.0% The following table summarizes our restricted stock unit activity under all equity incentive plans for the three months ended March 31, 2017: Number of Outstanding Restricted Stock Units Weighted Average Grant Date Fair Value Balance as of December 31, 2016 76,713 $ 4.95 Granted 22,000 $ 3.30 Released (35,349 ) $ 5.00 Forfeited — $ — Balance as of March 31, 2017 63,364 $ 4.35 Stock-based Compensation The allocation of stock-based compensation for all options, including performance options with a market condition, 2015 ESPP and restricted stock units is as follows (in thousands): Three Months Ended March 31, 2017 2016 Research and development $ 444 $ 547 General and administrative 833 733 $ 1,277 $ 1,280

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	5. Subsequent Events In April 2017, we amended our facility lease to include an additional 7,411 square feet through May 2019 for a total additional commitment of $0.7 million.

Organization, Business, Basis12

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Organization and Business	Organization and Business aTyr Pharma, Inc. (we, us, and our) was incorporated in the state of Delaware on September 8, 2005. We are focused on the discovery and clinical development of innovative medicines for patients suffering from severe rare diseases.
Principles of Consolidation	Principles of Consolidation Our consolidated financial statements include our accounts and our 98% majority-owned subsidiary in Hong Kong, Pangu BioPharma Limited (Pangu BioPharma). All intercompany transactions and balances are eliminated in consolidation.
Unaudited Interim Financial Information	Unaudited Interim Financial Information The accompanying interim condensed consolidated financial statements are unaudited. These unaudited interim financial statements have been prepared in accordance with United States generally accepted accounting principles (GAAP) and following the requirements of the United States Securities and Exchange Commission (SEC) for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by GAAP can be condensed or omitted. In our opinion, the unaudited interim financial statements have been prepared on the same basis as the audited financial statements and include all adjustments, which include only normal recurring adjustments, necessary for the fair presentation of our financial position and our results of operations and cash flows for periods presented. These statements do not include all disclosures required by GAAP and should be read in conjunction with our financial statements and accompanying notes for the fiscal year ended December 31, 2016, contained in our Annual Report on Form 10-K filed with the SEC on March 16, 2017. The results of the interim periods are not necessarily indicative of the results expected for the full fiscal year or any other interim period or any future year or period.
Liquidity and Financial Condition	Liquidity and Financial Condition We have incurred losses and negative cash flows from operations since our inception. As of March 31, 2017, we had an accumulated deficit of $229.4 million and we expect to continue to incur net losses for the foreseeable future. We believe that our existing cash, cash equivalents and available-for-sale investments, of $61.9 million as of March 31, 2017 will be sufficient to meet our anticipated cash requirements for a period of one year from the filing date of this Quarterly Report. We do not expect to generate any revenues from product sales unless and until we successfully complete development and obtain regulatory approval for one or more of our product candidates, which we expect will take a number of years at a minimum. If we obtain regulatory approval for any of our product candidates, we expect to incur significant commercialization expenses related to product sales, marketing, manufacturing and distribution. Accordingly, we will need to raise substantial additional capital beyond the net proceeds from our initial public offering (IPO). The amount and timing of our future funding requirements will depend on many factors, including the pace and results of our preclinical and clinical development efforts and the timing and nature of the regulatory approval process for our product candidates. We anticipate that we will seek to fund our operations through public or private equity or debt financings, collaborations, strategic partnerships or other sources. However, we may be unable to raise additional capital or enter into such other arrangements when needed on favorable terms or at all. Our failure to raise capital or enter into such other arrangements when needed would have a negative impact on our financial condition and ability to develop our product candidates.
Use of Estimates	Use of Estimates Our consolidated financial statements are prepared in accordance with GAAP. The preparation of our consolidated financial statements requires us to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent assets and liabilities in our consolidated financial statements and accompanying notes. The most significant estimates in our consolidated financial statements relate to the fair value of equity issuances and awards, and clinical trials and research and development expense accruals. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ materially from these estimates and assumptions.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents and adjusted for the weighted average number of common shares outstanding that are subject to repurchase. We have excluded 9,039 and 44,136 shares subject to repurchase from the weighted average number of common shares outstanding for the three months ended March 31, 2017 and 2016, respectively. Diluted net loss per share is calculated by dividing the net loss by the weighted average number of common stock equivalents outstanding for the period determined using the treasury-stock method. Dilutive common stock equivalents are comprised of warrants for common stock, restricted stock units and options outstanding under our stock option plan and employee stock purchase plan. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to our net loss position. Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): March 31, 2017 2016 Warrants for common stock 121,512 25,970 Common stock options and awards 4,720,382 3,742,770 Employee stock purchase plan 36,837 17,363 4,878,731 3,786,103
Recent Accounting Pronouncements	Recent Accounting Pronouncements In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments To achieve this objective, the amendments in this Update replace the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates.

Organization, Business, Basis13

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Schedule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share	Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): March 31, 2017 2016 Warrants for common stock 121,512 25,970 Common stock options and awards 4,720,382 3,742,770 Employee stock purchase plan 36,837 17,363 4,878,731 3,786,103

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Schedule of Assets Measured at Fair Value on Recurring Basis	Assets measured at fair value on a recurring basis are as follows (in thousands): Fair Value Measurements Using Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of March 31, 2017: Assets: Current: Cash equivalents $ 24,125 $ 24,125 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 7,854 — 7,854 — Commercial paper 6,513 — 6,513 — Corporate debt securities 16,667 — 16,667 — United States Treasury securities 2,000 2,000 — — Sub-total short-term investments 33,034 2,000 31,034 - Total assets measured at fair value $ 57,159 $ 26,125 $ 31,034 $ — As of December 31, 2016: Assets: Current: Cash equivalents $ 29,251 $ 29,251 $ — $ — Available-for-sale investments, short-term: Commercial paper 7,843 — 7,843 — Corporate debt securities 20,913 — 20,913 — United States Treasury securities 5,003 5,003 — — Sub-total short-term investments 33,759 5,003 28,756 — Available-for-sale investments, long-term: Asset-backed securities 4,002 — 4,002 — Sub-total long-term investments 4,002 — 4,002 — Total assets measured at fair value $ 67,012 $ 34,254 $ 32,758 $ —
Schedule of Available-for-sale Investments	As of March 31, 2017 and December 31, 2016 available-for-sale investments are detailed as follows (in thousands): March 31, 2017 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities 7,857 — (3 ) 7,854 Commercial paper $ 6,513 $ — $ — $ 6,513 Corporate debt securities 16,683 — (16 ) 16,667 United States Treasury securities 2,001 — (1 ) 2,000 $ 33,054 $ — $ (20 ) $ 33,034 December 31, 2016 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Commercial paper $ 7,843 $ — $ — $ 7,843 Corporate debt securities 20,942 — (29 ) 20,913 United States Treasury securities 5,002 1 — 5,003 $ 33,787 $ 1 $ (29 ) $ 33,759 Available-for-sale investments, long-term: Asset-backed securities $ 4,001 $ 1 $ — $ 4,002 $ 4,001 $ 1 $ — $ 4,002
Schedule of Available-for-sale Investments in Unrealized Loss Position	Available-for-sale investments that are in an unrealized loss position as of March 31, 2017 are as follows (in thousands): Estimated Fair Value Gross Unrealized Losses Asset-backed securities $ 7,854 $ (3 ) Corporate debt securities 16,667 $ (16 ) United States Treasury securities 2,000 $ (1 ) $ 26,521 $ (20 )

Debt, Commitments and Conting15

Debt, Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2017
Debt Instruments [Abstract]
Schedule of Future Principal Payments for Term Loan Including Final Payment	Future principal payments for the Term Loan, including the final payment, are as follows (in thousands): March 31, 2017 2017 $ 248 2018 3,093 2019 3,348 2020 4,186 $ 10,875
Schedule of Future Minimum Payments under Non-cancelable Operating Lease	Future minimum payments under the non-cancelable operating lease as of March 31, 2017 were as follows (in thousands): Operating Lease 2017 $ 683 2018 773 2019 293 $ 1,749

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	3 Months Ended
	Mar. 31, 2017
Summary of Common Stock Reserved for Future Issuance	Common stock reserved for future issuance is as follows: March 31, 2017 Common stock warrants 121,512 Common stock options and awards outstanding 4,720,382 Shares available under the 2015 Plan 795,728 Shares available under the 2015 ESPP 644,990 6,282,612
Summary of Stock Option Activity	The following table summarizes our stock option activity under all equity incentive plans for the three months ended March 31, 2017: Number of Outstanding Options Weighted Average Exercise Price Outstanding as of December 31, 2016 4,014,988 $ 6.73 Granted 765,719 $ 3.29 Exercised (795 ) $ 1.01 Canceled/forfeited/expired (122,894 ) $ 6.98 Outstanding as of March 31, 2017 4,657,018 $ 6.16
Schedule of Restricted Stock Unit Activity	The following table summarizes our restricted stock unit activity under all equity incentive plans for the three months ended March 31, 2017: Number of Outstanding Restricted Stock Units Weighted Average Grant Date Fair Value Balance as of December 31, 2016 76,713 $ 4.95 Granted 22,000 $ 3.30 Released (35,349 ) $ 5.00 Forfeited — $ — Balance as of March 31, 2017 63,364 $ 4.35
Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition, 2015 ESPP and Restricted Stock Units	The allocation of stock-based compensation for all options, including performance options with a market condition, 2015 ESPP and restricted stock units is as follows (in thousands): Three Months Ended March 31, 2017 2016 Research and development $ 444 $ 547 General and administrative 833 733 $ 1,277 $ 1,280
Employee Stock Option [Member]
Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants	The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows: Three Months Ended March 31, 2017 2016 Expected term (in years) 6.02 – 6.06 5.77 – 6.08 Risk-free interest rate 2.0% – 2.1% 1.4% – 1.9% Expected volatility 104.0% – 105.2% 81.2% – 82.1% Expected dividend yield 0.0% 0.0%

Organization, Business, Basis17

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Significant Accounting Policies [Line Items]
Accumulated deficit	$ 229,384		$ 215,979
Cash, cash equivalents and available-for-sale investments	$ 61,900
Weighted average shares subject to repurchase	9,039	44,136
Pangu BioPharma [Member] | Hong Kong [Member]
Significant Accounting Policies [Line Items]
Majority-owned subsidiary percentage	98.00%

Organization, Business, Basis18

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share (Detail) - shares	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	4,878,731	3,786,103
Warrants for Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	121,512	25,970
Common Stock Options and Awards [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	4,720,382	3,742,770
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	36,837	17,363

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 24,125	$ 29,251
Total assets measured at fair value	57,159	67,012
Available-for-sale [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	33,034	33,759
Available-for-sale [Member] | Short-term Investments [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	6,513	7,843
Available-for-sale [Member] | Short-term Investments [Member] | United States Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,000	5,003
Available-for-sale [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	16,667	20,913
Available-for-sale [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	7,854
Available-for-sale [Member] | Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		4,002
Available-for-sale [Member] | Long-term Investments [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		4,002
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	24,125	29,251
Total assets measured at fair value	26,125	34,254
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Available-for-sale [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,000	5,003
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Available-for-sale [Member] | Short-term Investments [Member] | United States Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,000	5,003
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	31,034	32,758
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	31,034	28,756
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Short-term Investments [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	6,513	7,843
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	16,667	20,913
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	$ 7,854
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		4,002
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Long-term Investments [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		$ 4,002

Fair Value Measurements - Sch20

Fair Value Measurements - Schedule of Available-for-sale Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	$ 33,054	$ 33,787
Gross Unrealized Gains		1
Gross Unrealized Losses	(20)	(29)
Market Value	33,034	33,759
Short-term Investments [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	6,513	7,843
Market Value	6,513	7,843
Short-term Investments [Member] | United States Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	2,001	5,002
Gross Unrealized Gains		1
Gross Unrealized Losses	(1)
Market Value	2,000	5,003
Short-term Investments [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	16,683	20,942
Gross Unrealized Losses	(16)	(29)
Market Value	16,667	20,913
Short-term Investments [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	7,857
Gross Unrealized Losses	(3)
Market Value	$ 7,854
Long-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost		4,001
Gross Unrealized Gains		1
Market Value		4,002
Long-term Investments [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost		4,001
Gross Unrealized Gains		1
Market Value		$ 4,002

Fair Value Measurements - Sch21

Fair Value Measurements - Schedule of Available-for-sale Investments in Unrealized Loss Position (Detail) $ in Thousands	Mar. 31, 2017USD ($)
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	$ 26,521
Gross Unrealized Losses	(20)
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	7,854
Gross Unrealized Losses	(3)
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	16,667
Gross Unrealized Losses	(16)
United States Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,000
Gross Unrealized Losses	$ (1)

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2017Security
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments in gross unrealized loss position	15
Available-for-sale investments in gross unrealized loss position	12 months
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments effective maturity period	1 year

Debt, Commitments and Conting23

Debt, Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			3 Months Ended
	Jan. 31, 2017	Nov. 30, 2016USD ($)Tranche	Jun. 30, 2016	Mar. 31, 2017USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2017USD ($)	Jun. 30, 2017USD ($)	Nov. 18, 2016USD ($)
Debt Instrument [Line Items]
Loan and security agreement, payment term				(i) $10.0 million which was funded on November 18, 2016; (ii) $5.0 million may be drawn down by us at any time before the earlier of June 30, 2017 or an event of default, at our discretion, subject to achievement of certain financial and clinical milestones; and (iii) $5.0 million may be drawn down by us any time after June 30, 2017 and before the earlier of December 31, 2017 or an event of default, at our discretion, subject to achievement of certain milestones specified for the second tranche and additional financial and clinical milestones.
Extended term of lease	2 years
Noncancelable operating lease extension period				2019-05
Rent expense				$ 200,000	$ 100,000
Sublease agreement, additional facility space, commenced period			2016-08
Sublease agreement, additional facility space, expiration period			2017-06
Research Funding and Option Agreement [Member] | The Scripps Research Institute [Member]
Debt Instrument [Line Items]
Additional funding for research conducted				200,000	200,000
Research Funding and Option Agreement [Member] | National Foundation for Cancer Research [Member]
Debt Instrument [Line Items]
Charitable donations				100,000	$ 100,000
Silicon Valley Bank and Solar Capital, Ltd. [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement		$ 20,000,000		$ 10,000,000
Number of tranches | Tranche		3
Loan and security agreement, payment term				Pursuant to the Term Loan agreement, we have interest only payments through December 1, 2017 or June 1, 2018, if we draw the second tranche, followed by consecutive equal monthly payments of principal and interest in arrears through the maturity date of November 18, 2020.
Maturity date				Nov. 18, 2020
Percentage of funded amount for final payment		8.75%
Non-usage fee percentage		2.00%
Net cash proceeds from borrowings				$ 7,300,000
Repayment of debt, principal including accrued interest				2,600,000
Debt instrument, final payment				$ 500,000
Term loan effective interest rate				12.28%
Term loan, principal outstanding before deducting debt issuance cost				$ 10,000,000
Term loan, debt issuance costs				400,000
Final maturity payment accrued over life of term loan through interest expense				$ 900,000
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Prime Rate
Debt Instrument [Line Items]
Interest rate spread on variable rate		4.10%
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Tranche One [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement								$ 10,000,000
Loan and security agreement funded date				Nov. 18, 2016
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Tranche Two [Member] | Scenario Forecast [ Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement							$ 5,000,000
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Tranche Three [Member] | Scenario Forecast [ Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement						$ 5,000,000
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Minimum [Member]
Debt Instrument [Line Items]
Prepayment fee percentage		1.00%
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Minimum [Member] | Tranche Three [Member]
Debt Instrument [Line Items]
Loan and security agreement funded date				Jun. 30, 2017
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Maximum [Member]
Debt Instrument [Line Items]
Prepayment fee percentage		3.00%
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Maximum [Member] | Tranche Two [Member]
Debt Instrument [Line Items]
Loan and security agreement funded date				Jun. 30, 2017
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Maximum [Member] | Tranche Three [Member]
Debt Instrument [Line Items]
Loan and security agreement funded date				Dec. 31, 2017

Debt, Commitments and Conting24

Debt, Commitments and Contingencies - Schedule of Future Principal Payments for Term Loan Including Final Payment (Detail) - Silicon Valley Bank and Solar Capital, Ltd. [Member] $ in Thousands	Mar. 31, 2017USD ($)
Debt Instrument [Line Items]
2,017	$ 248
2,018	3,093
2,019	3,348
2,020	4,186
Future principal payments for term loan including final payment, total	$ 10,875

Debt, Commitments and Conting25

Debt, Commitments and Contingencies - Schedule of Future Minimum Payments under Non-cancelable Operating Lease (Detail) $ in Thousands	Mar. 31, 2017USD ($)
Leases Operating [Abstract]
2,017	$ 683
2,018	773
2,019	293
Operating Lease, total	$ 1,749

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail)	Jan. 01, 2017shares
2015 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional number of common stock shares reserved for issuance	949,793
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional number of common stock shares reserved for issuance	237,448

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail)	Mar. 31, 2017shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	6,282,612
Common Stock Warrants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	121,512
Shares Available Under the 2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	795,728
Shares Available Under the 2015 ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	644,990
Common Stock Options and Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	4,720,382

Stockholders' Equity - Summar28

Stockholders' Equity - Summary of Stock Option Activity (Detail)	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Outstanding Options, Beginning Balance | shares	4,014,988
Number of Options, Granted | shares	765,719
Number of Options, Exercised | shares	(795)
Number of Options, Canceled/forfeited/expired | shares	(122,894)
Number of Outstanding Options, Ending Balance | shares	4,657,018
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 6.73
Weighted Average Exercise Price, Granted | $ / shares	3.29
Weighted Average Exercise Price, Exercised | $ / shares	1.01
Weighted Average Exercise Price, Canceled/forfeited/expired | $ / shares	6.98
Weighted Average Exercise Price, Ending Balance | $ / shares	$ 6.16

Stockholders' Equity - Summar29

Stockholders' Equity - Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants (Detail) - Employee Stock Option [Member]	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	2.00%	1.40%
Risk-free interest rate, maximum	2.10%	1.90%
Expected volatility, minimum	104.00%	81.20%
Expected volatility, maximum	105.20%	82.10%
Expected dividend yield	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 7 days	5 years 9 months 7 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 22 days	6 years 29 days

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Restricted Stock Unit Activity (Detail) - Restricted Stock Unit [Member]	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Number of Outstanding Restricted Stock Units, Beginning Balance | shares	76,713
Number of Outstanding Restricted Stock Units, Granted | shares	22,000
Number of Outstanding Restricted Stock Units, Released | shares	(35,349)
Number of Outstanding Restricted Stock Units, Ending Balance | shares	63,364
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 4.95
Weighted Average Grant Date Fair Value, Granted | $ / shares	3.30
Weighted Average Grant Date Fair Value, Released | $ / shares	5
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 4.35

Stockholders' Equity - Schedu31

Stockholders' Equity - Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition, 2015 ESPP and Restricted Stock Units (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,277	$ 1,280
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	444	547
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 833	$ 733

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) $ in Millions	1 Months Ended	3 Months Ended
	Apr. 30, 2017USD ($)ft²	Mar. 31, 2017
Subsequent Event [Line Items]
Noncancelable operating lease extension period		2019-05
Subsequent Event [Member]
Subsequent Event [Line Items]
Additional lease facility | ft²	7,411
Noncancelable operating lease extension period	2019-05
Noncancelable operating lease total additional commitment | $	$ 0.7