aTyr Pharma (ATYR) 10-Q 14 Aug 17 2017 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2017	Aug. 04, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q2
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		23,839,007

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 35,915	$ 38,388
Available-for sale investments, short-term	21,306	33,759
Prepaid expenses and other assets	1,877	2,621
Total current assets	59,098	74,768
Available-for sale investments, long-term		4,002
Property and equipment, net	2,003	1,421
Other assets	371	333
Total assets	61,472	80,524
Current liabilities:
Accounts payable	2,669	2,606
Accrued expenses	4,194	5,450
Current portion of deferred rent		130
Current portion of long-term debt	2,786	339
Total current liabilities	9,649	8,525
Long-term debt, net of current portion and issuance costs	11,792	9,198
Commitments and contingencies (Note 3)
Stockholders’ equity:
Common stock, $0.001 par value; authorized shares – 150,000,000 as of June 30, 2017 and December 31, 2016; issued and outstanding shares – 23,837,204 and 23,744,832 as of June 30, 2017 and December 31, 2016, respectively	24	24
Additional paid-in capital	281,586	278,832
Accumulated other comprehensive loss	(57)	(76)
Accumulated deficit	(241,522)	(215,979)
Total stockholders’ equity	40,031	62,801
Total liabilities and stockholders’ equity	$ 61,472	$ 80,524

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 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	23,837,204	23,744,832
Common stock, shares outstanding	23,837,204	23,744,832

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Operating expenses:
Research and development	$ 8,420	$ 11,307	$ 17,624	$ 23,307
General and administrative	3,487	4,126	7,494	8,241
Total operating expenses	11,907	15,433	25,118	31,548
Loss from operations	(11,907)	(15,433)	(25,118)	(31,548)
Other income (expense), net	(231)	50	(425)	78
Net loss	$ (12,138)	$ (15,383)	$ (25,543)	$ (31,470)
Net loss per share, basic and diluted	$ (0.51)	$ (0.65)	$ (1.07)	$ (1.33)
Weighted average common shares outstanding, basic and diluted	23,810,112	23,672,527	23,774,736	23,655,366

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (12,138)	$ (15,383)	$ (25,543)	$ (31,470)
Other comprehensive gain:
Change in unrealized gain (loss) on available for sale investments, net of tax	11	38	19	190
Comprehensive loss	$ (12,127)	$ (15,345)	$ (25,524)	$ (31,280)

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net loss	$ (25,543)	$ (31,470)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	400	457
Stock-based compensation	2,460	2,653
Accretion of debt discount	76	97
Amortization of premium of available-for-sale investment securities	113	397
Deferred rent	(130)	(154)
Changes in operating assets and liabilities
Prepaid expenses and other assets	706	394
Accounts payable and accrued expenses	(1,246)	1,147
Net cash used in operating activities	(23,164)	(26,479)
Cash flows from investing activities:
Purchases of property and equipment	(711)	(490)
Purchases of available-for-sale investment securities	(11,489)	(21,924)
Maturities of available-for-sale investment securities	27,850	40,651
Net cash provided by investing activities	15,650	18,237
Cash flows from financing activities:
Proceeds from issuance of common stock through option exercises	23	19
Proceeds through employee stock purchase plan	88	75
Proceeds from borrowing, net	4,930
Repayments on loans payable		(1,686)
Net cash provided by (used in) financing activities	5,041	(1,592)
Net change in cash and cash equivalents	(2,473)	(9,834)
Cash and cash equivalents at beginning of period	38,388	53,025
Cash and cash equivalents at the end of period	$ 35,915	$ 43,191

Organization, Business, Basis o

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 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 June 30, 2017, we had an accumulated deficit of $241.5 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 $57.2 million as of June 30, 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. 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. If we are unable to obtain funding on a timely basis, we may be required to significantly curtail, delay or discontinue one or more of our research or development programs or the commercialization of any product candidates, or we may be unable to expand our operations, maintain our current organization and employee base or otherwise capitalize on our business opportunities, as desired, which could materially affect our business, financial condition and results of operations. 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 3,188 and 29,468 shares subject to repurchase from the weighted average number of common shares outstanding for the three months ended June 30, 2017 and 2016, respectively. We have excluded 6,142 and 33,265 shares subject to repurchase from the weighted average number of common shares outstanding for the six months ended June 30, 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): Three and Six Months Ended 2017 2016 Warrants for common stock 163,178 25,970 Common stock options and awards 4,826,424 3,949,219 Employee stock purchase plan 38,862 30,478 5,028,464 4,005,667 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. esults of operations. In May 2017, the FASB issued ASU 2017-09 , Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting,

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 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 are 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 June 30, 2017: Assets: Current: Cash equivalents $ 21,100 $ 21,100 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 3,852 — 3,852 — Commercial paper 6,534 — 6,534 — Corporate debt securities 10,920 — 10,920 — Sub-total short-term investments 21,306 — 21,306 — Total assets measured at fair value $ 42,406 $ 21,100 $ 21,306 $ — 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 June 30, 2017 and December 31, 2016 available-for-sale investments are detailed as follows (in thousands): June 30, 2017 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities $ 3,853 — (1 ) $ 3,852 Commercial paper 6,534 — — 6,534 Corporate debt securities 10,927 — (7 ) 10,920 $ 21,314 $ — $ (8 ) $ 21,306 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 As of June 30, 2017, all of our available-for-sale investments have effective maturity dates of less than one year. As of June 30, 2017, there are 9 available-for-sale investments in a 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 June 30, 2017.

Debt, Commitments and Contingen

Debt, Commitments and Contingencies	6 Months Ended
	Jun. 30, 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 (the Loan Agreement) with Silicon Valley Bank (SVB) and Solar Capital Ltd. (Solar) to borrow up to $20.0 million, issuable in three separate tranches of $10.0 million, $5.0 million and $5.0 million, respectively. The first tranche of $10.0 million was funded on November 18, 2016 (Term A Loan). Under the Term A Loan, 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 (SVB Loan). We did not pay any termination or other fees in connection with the repayment of amounts due under the SVB Loan. In June 2017, we entered into an amendment of the Loan Agreement which modifies certain conditions under which we may receive and repay term loans under the Loan Agreement. The Loan Agreement, as amended, provides that (i) up to $5.0 million in the second tranche of term loans could be drawn down by us at any time before the earlier of June 30, 2017 or an event of default, at our discretion, and (ii) an additional $5.0 million in the third tranche of term loans may be drawn down by us at 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. In connection with the amendment to the Loan Agreement, the second tranche of $5.0 million was funded on June 30, 2017 (Term B Loan). We received cash proceeds of $4.9 million, net of debt issuance cost of $0.1 million. If we fail to meet certain milestones set forth in the Loan Agreement, as amended, by August 31, 2017, then we will be required to secure the Term B Loan with $5.0 million of cash until such time that those milestones are met. If such milestones are not met by October 31, 2017, we have the option to maintain the cash security for the Term B Loan or prepay the Term B Loan without the prepayment fee described below. Pursuant to the Loan Agreement, as amended, we are obligated to make interest only payments through December 1, 2017, followed by consecutive equal monthly payments of principal and interest in arrears through the maturity date of November 18, 2020. The interest only payments will be extended to June 1, 2018 upon achievement of certain milestones as set forth in the Loan Agreement, as amended. The Term A Loan and Term B Loan (collectively, the Term Loans) bear 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%. A final payment equal to 8.75% of the funded amounts is payable when the Term Loans become due or upon the prepayment of the respective outstanding balance. 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, as amended, provides for a 2.0% non-usage fee for any unfunded amount in the event we do not draw the third tranche, payable no later than the expiration date for the 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 June 30, 2017. As of June 30, 2017, the carrying value of our Term Loans consist of $15.0 million principal outstanding less the debt issuance costs of $0.7 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 Loans. The final maturity payment of $1.3 million is being accrued over the life of the Term Loans through interest expense. In connection with the Term A Loan, we issued warrants to each of SVB and Solar to purchase an aggregate of 47,771 shares of our common stock with an exercise price of $3.14 per share. In connection with the Term B Loan, we issued warrants to each of SVB and Solar to purchase an aggregate of 20,833 shares of our common stock with an exercise price of $3.60 per share. The warrants are immediately exercisable and have a maximum contractual term of seven years. Future principal payments for the Term Loan and the final payment are as follows (in thousands): June 30, 2017 2017 $ 370 2018 4,628 2019 5,023 2020 6,292 $ 16,313 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. 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. Rent expense for the three months ended June 30, 2017 and 2016 was $0.3 million and $0.1 million, respectively. Rent expense for the six months ended June 30, 2017 and 2016 was $0.4 million and $0.2 million, respectively. Future minimum payments under the non-cancelable operating lease as of June 30, 2017 were as follows (in thousands): Operating Lease 2017 $ 543 2018 1,108 2019 420 $ 2,071 Research Agreements and Funding Obligations (Related Party Transactions) 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 2016 and 2017 to provide additional funding to TSRI through March 31, 2017. For the three months ended June 30, 2017 and 2016, the additional funding expense recorded for TSRI was $0.2 million and $0.2 million, respectively. For the six months ended June 30, 2017 and 2016, the additional funding expense recorded for TSRI was $0.5 million and $0.4 million, respectively. Paul Schimmel, Ph.D., a member of our board of directors, is a board and faculty member at TSRI and such payments fund a portion of his research activities conducted at TSRI. During the three months ended June 30, 2017 and 2016, we provided charitable donations to the National Foundation for Cancer Research of $0.1 million and $0.1 million, respectively. During the six months ended June 30, 2017 and 2016, we provided charitable donations to the National Foundation for Cancer Research of $0.2 million and $0.2 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 Dr. Schimmel.

Stockholders' Equity

Stockholders' Equity	6 Months Ended
	Jun. 30, 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: June 30, 2017 Common stock warrants 163,178 Common stock options and awards outstanding 4,826,424 Shares available under the 2015 Plan 655,253 Shares available under the 2015 ESPP 614,115 6,258,970 The following table summarizes our stock option activity under all equity incentive plans for the six months ended June 30, 2017: Number of Outstanding Options Weighted Average Exercise Price Outstanding as of December 31, 2016 4,014,988 $ 6.73 Granted 1,031,419 $ 3.27 Exercised (26,148 ) $ 0.88 Canceled/forfeited/expired (257,199 ) $ 6.44 Outstanding as of June 30, 2017 4,763,060 $ 6.03 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 June 30, Six Months Ended June 30, 2017 2016 2017 2016 Expected term (in years) 5.50 – 6.07 5.50 – 6.08 5.5 – 6.07 5.50 – 6.08 Risk-free interest rate 1.9% – 2.1% 1.4% – 1.6% 1.9% – 2.1% 1.4% – 1.9% Expected volatility 114.3% – 124.4% 81.5% – 82.5% 104.0% – 124.4% 81.2% – 82.5% Expected dividend yield 0.0% 0.0% 0.0% 0.0% The following table summarizes our restricted stock unit activity under all equity incentive plans for the six months ended June 30, 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 Balance as of June 30, 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 June 30, Six Months Ended June 30, 2017 2016 2017 2016 Research and development $ 423 $ 527 $ 867 $ 1,074 General and administrative 760 846 1,593 1,579 $ 1,183 $ 1,373 $ 2,460 $ 2,653

Organization, Business, Basis11

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 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 June 30, 2017, we had an accumulated deficit of $241.5 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 $57.2 million as of June 30, 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. 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. If we are unable to obtain funding on a timely basis, we may be required to significantly curtail, delay or discontinue one or more of our research or development programs or the commercialization of any product candidates, or we may be unable to expand our operations, maintain our current organization and employee base or otherwise capitalize on our business opportunities, as desired, which could materially affect our business, financial condition and results of operations.
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 3,188 and 29,468 shares subject to repurchase from the weighted average number of common shares outstanding for the three months ended June 30, 2017 and 2016, respectively. We have excluded 6,142 and 33,265 shares subject to repurchase from the weighted average number of common shares outstanding for the six months ended June 30, 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): Three and Six Months Ended 2017 2016 Warrants for common stock 163,178 25,970 Common stock options and awards 4,826,424 3,949,219 Employee stock purchase plan 38,862 30,478 5,028,464 4,005,667
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. esults of operations. In May 2017, the FASB issued ASU 2017-09 , Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting,

Organization, Business, Basis12

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 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): Three and Six Months Ended 2017 2016 Warrants for common stock 163,178 25,970 Common stock options and awards 4,826,424 3,949,219 Employee stock purchase plan 38,862 30,478 5,028,464 4,005,667

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 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 June 30, 2017: Assets: Current: Cash equivalents $ 21,100 $ 21,100 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 3,852 — 3,852 — Commercial paper 6,534 — 6,534 — Corporate debt securities 10,920 — 10,920 — Sub-total short-term investments 21,306 — 21,306 — Total assets measured at fair value $ 42,406 $ 21,100 $ 21,306 $ — 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 June 30, 2017 and December 31, 2016 available-for-sale investments are detailed as follows (in thousands): June 30, 2017 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities $ 3,853 — (1 ) $ 3,852 Commercial paper 6,534 — — 6,534 Corporate debt securities 10,927 — (7 ) 10,920 $ 21,314 $ — $ (8 ) $ 21,306 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

Debt, Commitments and Conting14

Debt, Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2017
Debt Instruments [Abstract]
Schedule of Future Principal Payments for Term Loan and Final Payment	Future principal payments for the Term Loan and the final payment are as follows (in thousands): June 30, 2017 2017 $ 370 2018 4,628 2019 5,023 2020 6,292 $ 16,313
Schedule of Future Minimum Payments under Non-cancelable Operating Lease	Future minimum payments under the non-cancelable operating lease as of June 30, 2017 were as follows (in thousands): Operating Lease 2017 $ 543 2018 1,108 2019 420 $ 2,071

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	6 Months Ended
	Jun. 30, 2017
Summary of Common Stock Reserved for Future Issuance	Common stock reserved for future issuance is as follows: June 30, 2017 Common stock warrants 163,178 Common stock options and awards outstanding 4,826,424 Shares available under the 2015 Plan 655,253 Shares available under the 2015 ESPP 614,115 6,258,970
Summary of Stock Option Activity	The following table summarizes our stock option activity under all equity incentive plans for the six months ended June 30, 2017: Number of Outstanding Options Weighted Average Exercise Price Outstanding as of December 31, 2016 4,014,988 $ 6.73 Granted 1,031,419 $ 3.27 Exercised (26,148 ) $ 0.88 Canceled/forfeited/expired (257,199 ) $ 6.44 Outstanding as of June 30, 2017 4,763,060 $ 6.03
Schedule of Restricted Stock Unit Activity	The following table summarizes our restricted stock unit activity under all equity incentive plans for the six months ended June 30, 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 Balance as of June 30, 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 June 30, Six Months Ended June 30, 2017 2016 2017 2016 Research and development $ 423 $ 527 $ 867 $ 1,074 General and administrative 760 846 1,593 1,579 $ 1,183 $ 1,373 $ 2,460 $ 2,653
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 June 30, Six Months Ended June 30, 2017 2016 2017 2016 Expected term (in years) 5.50 – 6.07 5.50 – 6.08 5.5 – 6.07 5.50 – 6.08 Risk-free interest rate 1.9% – 2.1% 1.4% – 1.6% 1.9% – 2.1% 1.4% – 1.9% Expected volatility 114.3% – 124.4% 81.5% – 82.5% 104.0% – 124.4% 81.2% – 82.5% Expected dividend yield 0.0% 0.0% 0.0% 0.0%

Organization, Business, Basis16

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Significant Accounting Policies [Line Items]
Accumulated deficit	$ 241,522		$ 241,522		$ 215,979
Cash, cash equivalents and available-for-sale investments	$ 57,200		$ 57,200
Weighted average shares subject to repurchase	3,188	29,468	6,142	33,265
Pangu BioPharma [Member] | Hong Kong [Member]
Significant Accounting Policies [Line Items]
Majority-owned subsidiary percentage	98.00%		98.00%

Organization, Business, Basis17

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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 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	5,028,464	4,005,667	5,028,464	4,005,667
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	163,178	25,970	163,178	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	4,826,424	3,949,219	4,826,424	3,949,219
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	38,862	30,478	38,862	30,478

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	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 21,100	$ 29,251
Total assets measured at fair value	42,406	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	21,306	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	6,534	7,843
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	10,920	20,913
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		5,003
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	3,852
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	21,100	29,251
Total assets measured at fair value	21,100	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		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		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	21,306	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	21,306	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	6,534	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	10,920	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	$ 3,852
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 - Sch19

Fair Value Measurements - Schedule of Available-for-sale Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	$ 21,314	$ 33,787
Gross Unrealized Gains		1
Gross Unrealized Losses	(8)	(29)
Market Value	21,306	33,759
Short-term Investments [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	6,534	7,843
Market Value	6,534	7,843
Short-term Investments [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	10,927	20,942
Gross Unrealized Losses	(7)	(29)
Market Value	10,920	20,913
Short-term Investments [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	3,853
Gross Unrealized Losses	(1)
Market Value	$ 3,852
Short-term Investments [Member] | United States Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost		5,002
Gross Unrealized Gains		1
Market Value		5,003
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 - Addit

Fair Value Measurements - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2017Security
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments in gross unrealized loss position	9
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 Conting21

Debt, Commitments and Contingencies - Additional Information (Detail)	1 Months Ended				3 Months Ended		6 Months Ended
	Jun. 30, 2017USD ($)$ / sharesshares	Apr. 30, 2017USD ($)ft²	Jan. 31, 2017	Nov. 30, 2016USD ($)Tranche$ / sharesshares	Jun. 30, 2017USD ($)$ / sharesshares	Jun. 30, 2016USD ($)	Jun. 30, 2017USD ($)$ / sharesshares	Jun. 30, 2016USD ($)	Dec. 31, 2017USD ($)
Debt Instrument [Line Items]
Net cash proceeds from borrowings							$ 4,930,000
Loan and security agreement, payment term							The first tranche of $10.0 million was funded on November 18, 2016 (Term A Loan). Under the Term A Loan, 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 (SVB Loan). We did not pay any termination or other fees in connection with the repayment of amounts due under the SVB Loan
Extended term of lease			2 years
Noncancelable operating lease extension period		2019-05	2019-05
Additional lease facility | ft²		7,411
Noncancelable operating lease total additional commitment		$ 700,000
Rent expense					$ 300,000	$ 100,000	$ 400,000	$ 200,000
Research Funding and Option Agreement [Member] | The Scripps Research Institute [Member]
Debt Instrument [Line Items]
Additional funding expense for research conducted					200,000	200,000	500,000	400,000
Research Funding and Option Agreement [Member] | National Foundation for Cancer Research [Member]
Debt Instrument [Line Items]
Charitable donations					100,000	$ 100,000	200,000	$ 200,000
Silicon Valley Bank and Solar Capital, Ltd. [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement				$ 20,000,000
Number of tranches | Tranche				3
Term loan, debt issuance costs	$ 700,000				700,000		700,000
Term loan, principal outstanding before deducting debt issuance cost	15,000,000				15,000,000		15,000,000
Final maturity payment accrued over life of term loan through interest expense	$ 1,300,000				$ 1,300,000		$ 1,300,000
Warrants expiration year							7 years
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term A Loan [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement				$ 10,000,000
Net cash proceeds from borrowings				7,300,000
Repayment of debt, principal including accrued interest				2,600,000
Debt instrument, final payment				$ 500,000
Exercise price of warrant per share | $ / shares				$ 3.14
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term A Loan [Member] | Common Stock [Member]
Debt Instrument [Line Items]
Warrants to purchase number of common stock, shares | shares				47,771
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term B Loan [Member]
Debt Instrument [Line Items]
Exercise price of warrant per share | $ / shares	$ 3.60				$ 3.60		$ 3.60
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term B Loan [Member] | Common Stock [Member]
Debt Instrument [Line Items]
Warrants to purchase number of common stock, shares | shares	20,833				20,833		20,833
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Tranche One [Member]
Debt Instrument [Line Items]
Loan and security agreement funded date							Nov. 18, 2016
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Tranche One [Member] | Term A Loan [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement				$ 10,000,000
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Tranche Two [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]
Debt Instrument [Line Items]
Available credit under loan and security agreement				$ 5,000,000
Loan Amendment Agreement with SVB and Solar [Member]
Debt Instrument [Line Items]
Loan and security agreement, payment term							Pursuant to the Loan Agreement, as amended, we are obligated to make interest only payments through December 1, 2017, followed by consecutive equal monthly payments of principal and interest in arrears through the maturity date of November 18, 2020. The interest only payments will be extended to June 1, 2018 upon achievement of certain milestones as set forth in the Loan Agreement, as amended.
Loan amendment, description							In June 2017, we entered into an amendment of the Loan Agreement which modifies certain conditions under which we may receive and repay term loans under the Loan Agreement. The Loan Agreement, as amended, provides that (i) up to $5.0 million in the second tranche of term loans could be drawn down by us at any time before the earlier of June 30, 2017 or an event of default, at our discretion, and (ii) an additional $5.0 million in the third tranche of term loans may be drawn down by us at 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. In connection with the amendment to the Loan Agreement, the second tranche of $5.0 million was funded on June 30, 2017 (Term B Loan). We received cash proceeds of $4.9 million, net of debt issuance cost of $0.1 million. If we fail to meet certain milestones set forth in the Loan Agreement, as amended, by August 31, 2017, then we will be required to secure the Term B Loan with $5.0 million of cash until such time that those milestones are met. If such milestones are not met by October 31, 2017, we have the option to maintain the cash security for the Term B Loan or prepay the Term B Loan without the prepayment fee described below.
Required cash security	$ 5,000,000				$ 5,000,000		$ 5,000,000
Maturity date							Nov. 18, 2020
Percentage of funded amounts for final payment				8.75%
Non-usage fee percentage				2.00%
Loan Amendment Agreement with SVB and Solar [Member] | Prime Rate
Debt Instrument [Line Items]
Interest rate spread on variable rate				4.10%
Loan Amendment Agreement with SVB and Solar [Member] | Minimum [Member]
Debt Instrument [Line Items]
Prepayment fee percentage				1.00%
Loan Amendment Agreement with SVB and Solar [Member] | Maximum [Member]
Debt Instrument [Line Items]
Prepayment fee percentage				3.00%
Loan Amendment Agreement with SVB and Solar [Member] | Term B Loan [Member]
Debt Instrument [Line Items]
Net cash proceeds from borrowings	4,900,000
Term loan, debt issuance costs	100,000				100,000		$ 100,000
Loan Amendment Agreement with SVB and Solar [Member] | Tranche Two [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement	5,000,000				5,000,000		$ 5,000,000
Loan Amendment Agreement with SVB and Solar [Member] | Tranche Two [Member] | Maximum [Member]
Debt Instrument [Line Items]
Loan and security agreement funded date							Jun. 30, 2017
Loan Amendment Agreement with SVB and Solar [Member] | Tranche Two [Member] | Term B Loan [Member]
Debt Instrument [Line Items]
Current borrowing capacity under loan and security agreement	$ 5,000,000				$ 5,000,000		$ 5,000,000
Loan Amendment Agreement with SVB and Solar [Member] | Tranche Three [Member]
Debt Instrument [Line Items]
Loan and security agreement funded date, description							after June 30, 2017 and before the earlier of December 31, 2017
Loan Amendment Agreement with SVB and Solar [Member] | Tranche Three [Member] | Scenario Forecast [ Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement									$ 5,000,000

Debt, Commitments and Conting22

Debt, Commitments and Contingencies - Schedule of Future Principal Payments for Term Loan and Final Payment (Detail) - Silicon Valley Bank and Solar Capital, Ltd. [Member] $ in Thousands	Jun. 30, 2017USD ($)
Debt Instrument [Line Items]
2,017	$ 370
2,018	4,628
2,019	5,023
2,020	6,292
Future principal payments for term loan and final payment, total	$ 16,313

Debt, Commitments and Conting23

Debt, Commitments and Contingencies - Schedule of Future Minimum Payments under Non-cancelable Operating Lease (Detail) $ in Thousands	Jun. 30, 2017USD ($)
Leases Operating [Abstract]
2,017	$ 543
2,018	1,108
2,019	420
Operating Lease, total	$ 2,071

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)	Jun. 30, 2017shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	6,258,970
Common Stock Warrants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	163,178
Shares Available Under the 2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	655,253
Shares Available Under the 2015 ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	614,115
Common Stock Options and Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	4,826,424

Stockholders' Equity - Summar26

Stockholders' Equity - Summary of Stock Option Activity (Detail)	6 Months Ended
	Jun. 30, 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	1,031,419
Number of Options, Exercised | shares	(26,148)
Number of Options, Canceled/forfeited/expired | shares	(257,199)
Number of Outstanding Options, Ending Balance | shares	4,763,060
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 6.73
Weighted Average Exercise Price, Granted | $ / shares	3.27
Weighted Average Exercise Price, Exercised | $ / shares	0.88
Weighted Average Exercise Price, Canceled/forfeited/expired | $ / shares	6.44
Weighted Average Exercise Price, Ending Balance | $ / shares	$ 6.03

Stockholders' Equity - Summar27

Stockholders' Equity - Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants (Detail) - Employee Stock Option [Member]	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	1.90%	1.40%	1.90%	1.40%
Risk-free interest rate, maximum	2.10%	1.60%	2.10%	1.90%
Expected volatility, minimum	114.30%	81.50%	104.00%	81.20%
Expected volatility, maximum	124.40%	82.50%	124.40%	82.50%
Expected dividend yield	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 6 months	5 years 6 months	5 years 6 months	5 years 6 months
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 26 days	6 years 29 days	6 years 26 days	6 years 29 days

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Restricted Stock Unit Activity (Detail) - Restricted Stock Unit [Member]	6 Months Ended
	Jun. 30, 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 - Schedu29

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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,183	$ 1,373	$ 2,460	$ 2,653
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	423	527	867	1,074
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 760	$ 846	$ 1,593	$ 1,579