aTyr Pharma (ATYR) 10-Q 14 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 10, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	LIFE
Entity Registrant Name	aTYR PHARMA INC
Entity Central Index Key	0001339970
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		45,742,332

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 12,965	$ 22,962
Available-for-sale investments, short-term	30,054	26,583
Collaboration receivable	630
Prepaid expenses and other assets	1,095	1,258
Total current assets	44,744	50,803
Property and equipment, net	1,692	1,853
Right-of-use assets	3,323
Other assets	156	90
Total assets	49,915	52,746
Current liabilities:
Accounts payable	834	1,040
Accrued expenses	1,239	2,026
Contract liability	630
Current portion of operating lease liability	681
Current portion of long-term debt, net of issuance costs and discount	7,791	7,767
Total current liabilities	11,175	10,833
Long-term operating lease liability, net of current portion	2,815
Long-term debt, net of current portion and issuance costs and discount	6,440	8,263
Commitments and contingencies (Note 4)
Stockholders’ equity:
Preferred stock, $0.001 par value; undesignated authorized shares – 5,000,000 at March 31, 2019 and December 31, 2018, respectively; Class X Convertible Preferred Stock issued and outstanding shares – 1,643,961 and 2,285,952 as of March 31, 2019 and December 31, 2018, respectively	2	2
Common stock, $0.001 par value; authorized shares – 150,000,000 as of March 31, 2019 and December 31, 2018, respectively; issued and outstanding shares – 36,500,189 and 30,579,076 as of March 31, 2019 and December 31, 2018, respectively	37	31
Additional paid-in capital	334,324	332,378
Accumulated other comprehensive loss	(40)	(60)
Accumulated deficit	(304,838)	(298,701)
Total stockholders’ equity	29,485	33,650
Total liabilities and stockholders’ equity	$ 49,915	$ 52,746

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Mar. 31, 2018
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	30,579,076	36,500,189
Common stock, shares outstanding	30,579,076	36,500,189
Class X Convertible Preferred Stock [Member]
Preferred stock, shares issued	2,285,952	1,643,961
Preferred stock, shares outstanding	2,285,952	1,643,961

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 3,345	$ 6,150
General and administrative	2,532	4,070
Total operating expenses	5,877	10,220
Loss from operations	(5,877)	(10,220)
Total other income (expense), net	(260)	(447)
Net loss	$ (6,137)	$ (10,667)
Net loss per share attributable to common stock holders, basic and diluted	$ (0.18)	$ (0.36)
Weighted average common stock shares outstanding, basic and diluted	33,823,909	29,795,466

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (6,137)	$ (10,667)
Other comprehensive gain (loss):
Change in unrealized gain (loss) on available-for-sale investments, net of tax	20	(16)
Comprehensive loss	$ (6,117)	$ (10,683)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Other Comprehensive Loss [Member]	Accumulated Deficit [Member]	Convertible Preferred Stock [Member]
Beginning balance at Dec. 31, 2017	$ 64,245	$ 30	$ 328,519	$ (120)	$ (264,186)	$ 2
Begining balance, Shares at Dec. 31, 2017		29,789,162				2,285,952
Issuance of common stock upon exercise of options and release of restricted stock units	8		8
Issuance of common stock upon exercise of options and release of restricted stock units, Share		39,561
Stock-based compensation	928		928
Net unrealized gain on investments, net of tax	(16)			(16)
Net loss	(10,667)				(10,667)
Ending balance at Mar. 31, 2018	54,498	$ 30	329,455	(136)	(274,853)	$ 2
Ending balance, Shares at Mar. 31, 2018		29,828,723				2,285,952
Beginning balance at Dec. 31, 2018	33,650	$ 31	332,378	(60)	(298,701)	$ 2
Begining balance, Shares at Dec. 31, 2018		30,579,076				2,285,952
Conversion of preferred stock to common stock		$ 3	(3)
Conversion of preferred stock to common stock, shares		3,209,955				(641,991)
Issuance of common stock from at the market offerings, net of offering costs	1,381	$ 3	1,378
Issuance of common stock from at the market offerings, net of offering costs, Shares		2,711,158
Stock-based compensation	571		571
Net unrealized gain on investments, net of tax	20			20
Net loss	(6,137)				(6,137)
Ending balance at Mar. 31, 2019	$ 29,485	$ 37	$ 334,324	$ (40)	$ (304,838)	$ 2
Ending balance, Shares at Mar. 31, 2019		36,500,189				1,643,961

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (6,137)	$ (10,667)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	167	184
Stock-based compensation	571	928
Debt discount accretion and non-cash interest expense	201	81
Accretion of discount of available-for-sale investment securities	(106)	(64)
Amortization of right-of-use asset	169
Changes in operating assets and liabilities
Collaboration receivable	(630)
Prepaid expenses and other assets	97	(40)
Accounts payable and accrued expenses	(985)	(999)
Contract liability	630
Net cash used in operating activities	(6,023)	(10,577)
Cash flows from investing activities:
Purchases of property and equipment	(10)	(498)
Purchases of available-for-sale investment securities	(13,995)	(7,988)
Maturities of available-for-sale investment securities	10,650	19,900
Net cash (used in) provided by investing activities	(3,355)	11,414
Cash flows from financing activities:
Proceeds from issuance of common stock through option exercises		8
Proceeds from issuance of common stock through at the market offerings, net of offering costs	1,381
Repayments on borrowing	(2,000)
Net cash (used in) provided by financing activities	(619)	8
Net change in cash and cash equivalents	(9,997)	845
Cash and cash equivalents at beginning of period	22,962	21,091
Cash and cash equivalents at the end of period	$ 12,965	$ 21,936

Organization, Business, Basis o

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2019
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 based on immunological pathways. 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 follow 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, 2018, contained in our Annual Report on Form 10-K filed with the SEC on March 26, 2019. 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, 2019, we had an accumulated deficit of $304.8 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 $43.0 million as of March 31, 2019 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 to fund our operations. 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. Leases On January 1, 2019, we adopted Accounting Standards Update (ASU) No. 2016‑02, Leases We elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed us to exclude from our condensed consolidated balance sheets recognition of leases having a term of 12 months or less (short-term leases) and we elected to not separate lease components and non-lease components for our long-term leases. Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included in operating expenses in our condensed consolidated statements of operations. Prior period amounts continue to be reported in accordance with our historic accounting practices under previous lease guidance, Accounting Standards Codification (ASC) 840, Leases Revenue Recognition We have entered into a research collaboration and option agreement. The terms of this arrangement include payments to us for research and development services and potential development milestone payments. Performance of any obligations under the agreement will begin in the second quarter of 2019. We evaluate our agreements under ASU 2014-09, Revenue from Contracts with Customers (Topic 606) and ASC Topic 808, Collaborative Arrangements . . In determining the appropriate amount of revenue to be recognized as we fulfill our obligations under our agreement, we perform the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) we satisfy each performance obligation. As part of the accounting for these arrangements, we must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. We use key assumptions to determine the stand-alone selling price, which may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success. 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. 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 convertible preferred stock, warrants for common stock, options and restricted stock units outstanding under our stock option plan and estimated shares to be purchased under our 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 considered for the calculation of diluted net loss per share are as follows (in common share equivalents): March 31, 2019 2018 Class X convertible preferred stock (if-converted) 8,219,805 11,429,760 Warrants for common stock 6,682,708 6,682,708 Common stock options and restricted stock units 5,671,937 5,611,880 Employee stock purchase plan 22,548 31,086 20,596,998 23,755,434 Recent Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (FASB) issued ASU No. 2016-02, to increase transparency and comparability among organizations by requiring recognition of lease assets and lease liabilities on the balance sheet and disclosure of key information about leasing arrangements. The new standard is effective on December 15, 2018, including interim periods within those periods, using a modified retrospective approach. We adopted ASU No. 2016-02 on January 1, 2019 and recognized a $3.5 million right-of-use asset and $3.5 million lease liability in our condensed consolidated balance sheet for the discounted value of future lease payments from the adoption of this ASU. The adoption did not have any impact on our retained earnings. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses , to provide financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. To achieve this objective, the amendments in ASU No. 2016-13 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. ASU No. 2016-13 are effective for fiscal years beginning after December 15, 2020, including periods within those fiscal years. In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation n entity to apply the requirements of Topic 718 to nonemployee awards except for specific guidance on inputs to an option pricing model and the attribution of cost (that is, the period of time over which share-based payment awards vest and the pattern of cost recognition over that period). The amendments specify that Topic 718 applies to all share-based payment transactions in which a grantor acquires goods or services to be used or consumed in a grantor’s own operations by issuing share-based payment awards. The amendments also clarify that Topic 718 does not apply to share-based payments used to effectively provide (1) inancing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract accounted for under Topic 606, Revenue from Contracts with Customers . In July 2018, the FASB issued ASU No. 2018-09, Codification Improvements Amendments to Subtopic 718-40, Compensation–Stock Compensation–Income Taxes, deductions that are taken on the entity’s tax return in a different period from when the event that gives rise to the tax deduction occurs and the uncertainty about whether (1) the entity will receive a tax deduction and (2) the amount of the tax deduction is resolved. Some of the amendments in ASU No. 2018-09 do not require transition guidance and are effective immediately and others have transition guidance with effective dates for annual periods beginning after December 15, 2018 which we adopted in January 1, 2019. T In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements clarify the interaction between Topic 808 and Topic 606. A collaborative arrangement, as defined by the guidance in Topic 808, is a contractual arrangement under which two or more parties actively participate in a joint operating activity and are exposed to significant risks and rewards that depend on the activity’s commercial success. Topic 808 does not provide comprehensive recognition or measurement guidance for collaborative arrangements, and the accounting for those arrangements is often based on an analogy to other accounting literature or an accounting policy election. Some entities apply revenue guidance directly or by analogy to all or part of their arrangements, and others apply a different accounting method as an accounting policy. Those accounting differences result in diversity in practice on how entities account for transactions on the basis of their view of the economics of the collaborative arrangement. The amendments for ASU No. 2018-18 are effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2019
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 long-term debt approximates their 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 Total Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of March 31, 2019: Assets: Current: Cash equivalents $ 10,640 $ 10,640 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 6,631 — 6,631 — Commercial paper 14,137 — 14,137 — Corporate debt securities 9,286 — 9,286 — Sub-total short-term investments 30,054 — 30,054 — Total assets measured at fair value $ 40,694 $ 10,640 $ 30,054 $ — Fair Value Measurements Using Total Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of December 31, 2018: Assets: Current: Cash equivalents $ 16,019 $ 16,019 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 7,773 — 7,773 — Commercial paper 6,144 — 6,144 — Corporate debt securities 12,666 — 12,666 — Sub-total short-term investments 26,583 — 26,583 — Total assets measured at fair value $ 42,602 $ 16,019 $ 26,583 $ — As of March 31, 2019 and December 31, 2018, available-for-sale investments are detailed as follows (in thousands): March 31, 2019 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities $ 6,628 $ 3 $ — $ 6,631 Commercial paper 14,137 — — 14,137 Corporate debt securities 9,279 7 — 9,286 $ 30,044 $ 10 $ — $ 30,054 December 31, 2018 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities $ 7,777 $ — $ (4 ) $ 7,773 Commercial paper 6,144 — — 6,144 Corporate debt securities 12,672 — (6 ) 12,666 $ 26,593 $ — $ (10 ) $ 26,583 As of March 31, 2019, all of our available-for-sale investments have a variety of effective maturity dates of less than one year. As of March 31, 2019, all available-for-sale investments are in gross unrealized gain positions. At each reporting date, we perform an evaluation of impairment to determine if any 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, if any, until their amortized cost basis has been recovered.

Research Collaboration

Research Collaboration	3 Months Ended
	Mar. 31, 2019
Research And Development [Abstract]
Research Collaboration	3. Research Collaboration In March 2019, we entered into a research collaboration and option agreement with CSL Behring (CSL) for the development of product candidates derived from up to four tRNA synthetases from our preclinical pipeline (CSL Agreement). Under the terms of the collaboration, CSL will fund all research and development activities related to the development of the applicable product candidates for the duration of the collaboration. CSL reimburses us for all research and development activities. The research and development activities will be performed in three phases by both parties. The first phase will be reimbursed prior to the start of the research activities and thereafter will be reimbursed on a quarterly basis. In March 2019, we recorded a receivable and related contract liability of $0.6 million per the research program funding for the first phase of the research and development activities described within the CSL Agreement. The first phase of research and development will begin in the second quarter of 2019. In addition, CSL will pay a total of up to $4.25 million per synthetase program ($17 million if all four synthetase programs advance) in option fees based on achievement of research milestones and CSL’s determination to continue development. As of March 31, 2019, no research milestone has been met. Moreover, aTyr will grant CSL an option to negotiate licenses for worldwide rights to each investigational new drug (IND) candidate that emerges from this research collaboration. Specific license terms will be negotiated during an exclusivity period following the exercise of each program option. CSL has the right to terminate the research collaboration and option agreement in its entirety or with respect to one or more synthetases upon 45 days notice. Either party has the right to terminate the agreement upon material breach of obligation or insolvency.

Debt, Commitments and Contingen

Debt, Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
Debt Instruments [Abstract]
Debt, Commitments and Contingencies	4. Debt, Commitments and Contingencies Term Loans In November 2016, we entered into a loan and security agreement and subsequently entered amendments (collectively, the Loan Agreement), for term loans with Silicon Valley Bank (SVB) and Solar Capital Ltd. (Solar, and together with SVB, the Lenders), to borrow up to $20.0 million issuable in three separate tranches (the Term Loans), $10.0 million of which was funded in November 2016, $5.0 million of which was funded in June 2017 and $5.0 million of which was funded in December 2017. Under the Loan Agreement, we are obligated to make interest only payments through June 1, 2018, followed by consecutive equal monthly payments of principal and interest in arrears through the maturity date of November 18, 2020. Accordingly, we started paying the Term Loans in June 2018. 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 obligations under the Term Loans are secured by liens on our tangible personal property and we agreed to not encumber any of our intellectual property. The Term Loans include a material adverse change clause, which enables the Lenders to require immediate repayment of the outstanding debt. The material adverse change clause covers a material impairment in the perfection or priority of the Lenders’ lien in the underlying collateral or in the value of such collateral, material adverse change in business operations or condition or material impairment of our prospects for repayment of any portion of the remaining debt obligation. As of March 31, 2019, the carrying value of our Term Loans consist of $13.3 million principal outstanding less the debt issuance costs of $0.3 million. The debt issuance costs have been recorded as a debt discount which are being accreted to interest expense over the life of the Term Loans. In connection with the first tranche, we issued warrants to the Lenders 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 second tranche, we issued warrants to the Lenders to purchase an aggregate of 20,833 shares of our common stock with an exercise price of $3.60 per share. In connection with the third tranche, we issued warrants to each of SVB and Solar to purchase an aggregate of 20,188 shares of our common stock with an exercise price of $3.72 per share. The warrants are immediately exercisable and have a maximum contractual term of seven years. The aggregate fair value of the warrants was determined to be $0.5 million using the Black-Scholes option pricing model and was recorded as a debt discount which is being accreted to interest expense over the life of Term Loans. Term Loans and unamortized discount balances are as follows (in thousands): March 31, 2019 Debt balance $ 13,333 Less debt issuance costs and discount (72 ) Long-term debt, net of issuance costs and discount 13,261 Less current portion of long-term debt (8,000 ) Add accrual of final payment 1,179 Long-term debt, net of current portion and issuance costs and discount $ 6,440 Current portion of long-term debt $ 8,000 Less current portion of debt issuance costs and discount (209 ) Current portion of long-term debt, net of issuance costs and discount $ 7,791 Future principal payments for the Term Loans are as follows (in thousands): March 31, 2019 2019 $ 6,000 2020 7,333 Principal payments balance $ 13,333 The final maturity payment of $1.8 million is accruing over the life of the Term Loans through interest expense. Leases W e adopted ASU No. 2016-02, utilizing the modified retrospective transition method at the beginning of the first quarter of 2019. We elected the package of practical expedients requiring no reassessment of whether any expired or existing contracts are or contain leases, the lease classification of any expired or existing leases, or initial direct costs for any existing leases. We did not elect the hindsight practical expedient. We also made accounting policy elections not to apply the recognition requirements under ASU No. 2016-02 to any of our short-term leases and to account for each separate lease and associated non-lease components as a single lease component for all of our leases. Under ASU No. 2016-02, we determine if an arrangement is a lease at inception. The adoption of the new lease standard had a material impact on the condensed consolidated balance sheets, but did not have a material impact on the condensed consolidated statements of operations. The impact on the condensed consolidated balance sheet resulted in the recording of a $3.5 million right-of-use asset and a corresponding operating lease liability for the same amount. Our right-of-use assets consist of an operating lease for our facility headquarters. We also have an immaterial amount of prepaid financing leases that are included within other assets in our condensed consolidated balance sheets We have a non-cancelable facility lease that is subject to base lease payments, which escalate over the term of the lease, additional charges for common area maintenance and other costs. In July 2018, we entered into a lease amendment that reduced the space we lease from 24,494 square feet to 20,508 square feet and extended the lease term to May 2023. With the lease amendment, we do not have an option to extend the lease. Future minimum payments under the non-cancelable facility lease and reconciliation to the operating lease liability as of March 31, 2019 were as follows (in thousands): Operating Lease 2019 $ 732 2020 1,002 2021 1,031 2022 1,062 Thereafter 403 Less: Amount representing interest (734 ) Present value of lease payments 3,496 Less: Current portion of operating lease liability (681 ) Long-term operating lease liability $ 2,815 Related Party Transactions Research Agreements and Funding Obligations We provided funding to The Scripps Research Institute (TSRI) pursuant to a research funding and option agreement to conduct certain research activities. We terminated our research funding and option agreement effective as of November 2018. For the three months ended March 31, 2018, we recognized expense under the agreement in the amount $0.5 million. Paul Schimmel, Ph.D., a member of our board of directors, is a faculty member at TSRI and such payments funded a portion of his research activities conducted at TSRI. Strategic Advisor Agreement In November 2017, John D. Mendlein, Ph.D., a member of our Board of Directors since July 2010 and our Chief Executive Officer from September 2011 to November 2017, began serving as a strategic advisor to us pursuant to the terms of a strategic advisor agreement entered with Dr. Mendlein on November 1, 2017 (Strategic Advisor Agreement). Pursuant to the terms of the Strategic Advisor Agreement, we agreed to, among other things, pay Dr. Mendlein as a strategic advisor to us for a period of up to four years, at a monthly rate of $42,500 for the first year and $7,500 per month for the rest of the term. Either party may terminate the Strategic Advisor Agreement after the first year, provided that payments under the Strategic Advisor Agreement and continued vesting of outstanding employee stock options are guaranteed through the second year of the Strategic Advisor Agreement in the event the Board terminates the Strategic Advisor Agreement for convenience or Dr. Mendlein terminates for our material breach of the Strategic Advisor Agreement. For the three months ended March 31, 2019 and 2018, we recognized expenses under the Strategic Advisor Agreement in the amount of

Stockholders' Equity

Stockholders' Equity	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Stockholders' Equity	5. Stockholders’ Equity At the Market Offering Program In June 2016, we entered into a sales agreement with Cowen and Company, LLC (Cowen) for at the market offerings (ATM Offering Program), under which we may offer and sell shares of our common stock having an aggregate offering price of up to $35.0 million from time to time. During the three months ended March 31, 2019, we sold an aggregate of 2,711,158 shares of common stock at an average price of $0.53 per common share for Private Placement of Common Stock, Convertible Preferred Shares and Common Stock Warrants In August 2017, we completed a private placement of common and preferred stock in which a select group of institutional investors, including Viking Global Opportunities Illiquid Investments Sub-Master, LP (VGO Fund) and other accredited investors, certain of whom are affiliated with our directors and officers (collectively, the Purchasers), purchased preferred stock and common stock. We issued to VGO Fund 1,777,784 shares of our common stock, at a price of $2.65 per share, 2,285,952 shares of our Class X Convertible Preferred Stock, at a price of $13.25 per share, and warrants to purchase up to 4,952,829 of additional shares of common stock. The remaining Purchasers purchased an aggregate of 4,094,336 shares of our common stock, at a price of $2.65 per share, and warrants to purchase up to 1,535,376 additional shares of our common stock. Gross proceeds from the private placement were $45.8 million. The warrants to purchase 6,488,205 shares of our common stock are exercisable at an exercise price of $4.64 per share, subject to adjustments as provided under the terms of the warrants. The warrants are immediately exercisable and expire on December 31, 2019. Each share of preferred stock is convertible into five shares of our common stock. In January 2019, the VGO Fund converted 641,991 shares of its preferred stock into 3,209,955 shares of common stock. 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), 1,223,163 additional shares were reserved for future issuance under the 2015 Plan on January 1, 2019 and 305,790 additional shares were reserved for future issuances under the 2015 ESPP on January 1, 2019. Common stock reserved for future issuance is as follows: March 31, 2019 Class X Preferred Stock (if-converted to common stock) 8,219,805 Common stock warrants 6,682,708 Common stock options and awards outstanding 5,671,937 Shares available under the 2015 Plan 2,178,136 Shares available under the 2015 ESPP 1,145,095 23,897,681 The following table summarizes our stock option activity under all equity incentive plans for the three months ended March 31, 2019: Number of Outstanding Options Weighted Average Exercise Price Outstanding as of December 31, 2018 4,990,674 $ 4.47 Granted 856,493 $ 0.52 Canceled/forfeited/expired (465,430 ) $ 5.38 Outstanding as of March 31, 2019 5,381,737 $ 3.77 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, 2019 2018 Expected term (in years) 6.02 – 6.07 5.77 – 6.08 Risk-free interest rate 2.5% – 2.6% 2.3% – 2.7% Expected volatility 100.0% – 101.0% 89.2% – 98.4% 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, 2019: Number of Outstanding Restricted Stock Units Weighted Average Grant Date Fair Value Balance as of December 31, 2018 216,700 $ 0.85 Granted 75,000 $ 0.52 Forfeited (1,500 ) $ 0.85 Balance as of March 31, 2019 290,200 $ 0.76 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, 2019 2018 Research and development $ 116 $ 324 General and administrative 455 604 Total stock-based compensation expense $ 571 $ 928

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	6. Subsequent Events In April 2019, we entered into a securities purchase agreement (Purchase Agreement) with an institutional investor, The Federated Kaufmann Small Cap Fund . In April 2019, we suspended and terminated the prospectus supplement related to our ATM Offering Program. We will not make any future sales of our securities pursuant to the ATM Offering Program, unless and until a new prospectus supplement is filed. In May 2019, we terminated the ATM Offering Program.

Organization, Business, Basis_2

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
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 based on immunological pathways.
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 follow 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, 2018, contained in our Annual Report on Form 10-K filed with the SEC on March 26, 2019. 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, 2019, we had an accumulated deficit of $304.8 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 $43.0 million as of March 31, 2019 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 to fund our operations. 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.
Leases	Leases On January 1, 2019, we adopted Accounting Standards Update (ASU) No. 2016‑02, Leases We elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed us to exclude from our condensed consolidated balance sheets recognition of leases having a term of 12 months or less (short-term leases) and we elected to not separate lease components and non-lease components for our long-term leases. Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included in operating expenses in our condensed consolidated statements of operations. Prior period amounts continue to be reported in accordance with our historic accounting practices under previous lease guidance, Accounting Standards Codification (ASC) 840, Leases
Revenue Recognition	Revenue Recognition We have entered into a research collaboration and option agreement. The terms of this arrangement include payments to us for research and development services and potential development milestone payments. Performance of any obligations under the agreement will begin in the second quarter of 2019. We evaluate our agreements under ASU 2014-09, Revenue from Contracts with Customers (Topic 606) and ASC Topic 808, Collaborative Arrangements . . In determining the appropriate amount of revenue to be recognized as we fulfill our obligations under our agreement, we perform the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) we satisfy each performance obligation. As part of the accounting for these arrangements, we must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. We use key assumptions to determine the stand-alone selling price, which may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success.
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. 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 convertible preferred stock, warrants for common stock, options and restricted stock units outstanding under our stock option plan and estimated shares to be purchased under our 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 considered for the calculation of diluted net loss per share are as follows (in common share equivalents): March 31, 2019 2018 Class X convertible preferred stock (if-converted) 8,219,805 11,429,760 Warrants for common stock 6,682,708 6,682,708 Common stock options and restricted stock units 5,671,937 5,611,880 Employee stock purchase plan 22,548 31,086 20,596,998 23,755,434
Recent Accounting Pronouncements	Recent Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (FASB) issued ASU No. 2016-02, to increase transparency and comparability among organizations by requiring recognition of lease assets and lease liabilities on the balance sheet and disclosure of key information about leasing arrangements. The new standard is effective on December 15, 2018, including interim periods within those periods, using a modified retrospective approach. We adopted ASU No. 2016-02 on January 1, 2019 and recognized a $3.5 million right-of-use asset and $3.5 million lease liability in our condensed consolidated balance sheet for the discounted value of future lease payments from the adoption of this ASU. The adoption did not have any impact on our retained earnings. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses , to provide financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. To achieve this objective, the amendments in ASU No. 2016-13 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. ASU No. 2016-13 are effective for fiscal years beginning after December 15, 2020, including periods within those fiscal years. In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation n entity to apply the requirements of Topic 718 to nonemployee awards except for specific guidance on inputs to an option pricing model and the attribution of cost (that is, the period of time over which share-based payment awards vest and the pattern of cost recognition over that period). The amendments specify that Topic 718 applies to all share-based payment transactions in which a grantor acquires goods or services to be used or consumed in a grantor’s own operations by issuing share-based payment awards. The amendments also clarify that Topic 718 does not apply to share-based payments used to effectively provide (1) inancing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract accounted for under Topic 606, Revenue from Contracts with Customers . In July 2018, the FASB issued ASU No. 2018-09, Codification Improvements Amendments to Subtopic 718-40, Compensation–Stock Compensation–Income Taxes, deductions that are taken on the entity’s tax return in a different period from when the event that gives rise to the tax deduction occurs and the uncertainty about whether (1) the entity will receive a tax deduction and (2) the amount of the tax deduction is resolved. Some of the amendments in ASU No. 2018-09 do not require transition guidance and are effective immediately and others have transition guidance with effective dates for annual periods beginning after December 15, 2018 which we adopted in January 1, 2019. T In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements clarify the interaction between Topic 808 and Topic 606. A collaborative arrangement, as defined by the guidance in Topic 808, is a contractual arrangement under which two or more parties actively participate in a joint operating activity and are exposed to significant risks and rewards that depend on the activity’s commercial success. Topic 808 does not provide comprehensive recognition or measurement guidance for collaborative arrangements, and the accounting for those arrangements is often based on an analogy to other accounting literature or an accounting policy election. Some entities apply revenue guidance directly or by analogy to all or part of their arrangements, and others apply a different accounting method as an accounting policy. Those accounting differences result in diversity in practice on how entities account for transactions on the basis of their view of the economics of the collaborative arrangement. The amendments for ASU No. 2018-18 are effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years.

Organization, Business, Basis_3

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Schedule of Potentially Dilutive Securities Not Considered for Calculation of Diluted Net Loss Per Share	Potentially dilutive securities not considered for the calculation of diluted net loss per share are as follows (in common share equivalents): March 31, 2019 2018 Class X convertible preferred stock (if-converted) 8,219,805 11,429,760 Warrants for common stock 6,682,708 6,682,708 Common stock options and restricted stock units 5,671,937 5,611,880 Employee stock purchase plan 22,548 31,086 20,596,998 23,755,434

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2019
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 Total Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of March 31, 2019: Assets: Current: Cash equivalents $ 10,640 $ 10,640 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 6,631 — 6,631 — Commercial paper 14,137 — 14,137 — Corporate debt securities 9,286 — 9,286 — Sub-total short-term investments 30,054 — 30,054 — Total assets measured at fair value $ 40,694 $ 10,640 $ 30,054 $ — Fair Value Measurements Using Total Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of December 31, 2018: Assets: Current: Cash equivalents $ 16,019 $ 16,019 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 7,773 — 7,773 — Commercial paper 6,144 — 6,144 — Corporate debt securities 12,666 — 12,666 — Sub-total short-term investments 26,583 — 26,583 — Total assets measured at fair value $ 42,602 $ 16,019 $ 26,583 $ —
Schedule of Available-for-sale Investments	As of March 31, 2019 and December 31, 2018, available-for-sale investments are detailed as follows (in thousands): March 31, 2019 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities $ 6,628 $ 3 $ — $ 6,631 Commercial paper 14,137 — — 14,137 Corporate debt securities 9,279 7 — 9,286 $ 30,044 $ 10 $ — $ 30,054 December 31, 2018 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities $ 7,777 $ — $ (4 ) $ 7,773 Commercial paper 6,144 — — 6,144 Corporate debt securities 12,672 — (6 ) 12,666 $ 26,593 $ — $ (10 ) $ 26,583

Debt, Commitments and Conting_2

Debt, Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2019
Debt Instruments [Abstract]
Schedule of Term Loans and Unamortized Discount Balances	Term Loans and unamortized discount balances are as follows (in thousands): March 31, 2019 Debt balance $ 13,333 Less debt issuance costs and discount (72 ) Long-term debt, net of issuance costs and discount 13,261 Less current portion of long-term debt (8,000 ) Add accrual of final payment 1,179 Long-term debt, net of current portion and issuance costs and discount $ 6,440 Current portion of long-term debt $ 8,000 Less current portion of debt issuance costs and discount (209 ) Current portion of long-term debt, net of issuance costs and discount $ 7,791
Schedule of Future Principal Payments for Term Loans	Future principal payments for the Term Loans are as follows (in thousands): March 31, 2019 2019 $ 6,000 2020 7,333 Principal payments balance $ 13,333
Schedule of Future Minimum Payments under Non-cancelable Operating Lease and Reconciliation to Operating Lease Liability	Future minimum payments under the non-cancelable facility lease and reconciliation to the operating lease liability as of March 31, 2019 were as follows (in thousands): Operating Lease 2019 $ 732 2020 1,002 2021 1,031 2022 1,062 Thereafter 403 Less: Amount representing interest (734 ) Present value of lease payments 3,496 Less: Current portion of operating lease liability (681 ) Long-term operating lease liability $ 2,815

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	3 Months Ended
	Mar. 31, 2019
Summary of Common Stock Reserved for Future Issuance	Common stock reserved for future issuance is as follows: March 31, 2019 Class X Preferred Stock (if-converted to common stock) 8,219,805 Common stock warrants 6,682,708 Common stock options and awards outstanding 5,671,937 Shares available under the 2015 Plan 2,178,136 Shares available under the 2015 ESPP 1,145,095 23,897,681
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, 2019: Number of Outstanding Options Weighted Average Exercise Price Outstanding as of December 31, 2018 4,990,674 $ 4.47 Granted 856,493 $ 0.52 Canceled/forfeited/expired (465,430 ) $ 5.38 Outstanding as of March 31, 2019 5,381,737 $ 3.77
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, 2019: Number of Outstanding Restricted Stock Units Weighted Average Grant Date Fair Value Balance as of December 31, 2018 216,700 $ 0.85 Granted 75,000 $ 0.52 Forfeited (1,500 ) $ 0.85 Balance as of March 31, 2019 290,200 $ 0.76
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, 2019 2018 Research and development $ 116 $ 324 General and administrative 455 604 Total stock-based compensation expense $ 571 $ 928
Employee Stock Option [Member]
Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants and Performance Options with Market Condition	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, 2019 2018 Expected term (in years) 6.02 – 6.07 5.77 – 6.08 Risk-free interest rate 2.5% – 2.6% 2.3% – 2.7% Expected volatility 100.0% – 101.0% 89.2% – 98.4% Expected dividend yield 0.0 % 0.0 %

Organization, Business, Basis_4

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Significant Accounting Policies [Line Items]
Accumulated deficit	$ 304,838		$ 298,701
Cash, cash equivalents and available-for-sale investments	43,000
Operating lease, right-of-use asset	3,323	$ 3,500
Operating lease, liability	$ 3,496	3,500
Accounting Standards Update 2016-02 [Member]
Significant Accounting Policies [Line Items]
Operating lease, right-of-use asset		3,500
Operating lease, liability		$ 3,500
Pangu BioPharma [Member] | Hong Kong [Member]
Significant Accounting Policies [Line Items]
Majority-owned subsidiary percentage	98.00%

Organization, Business, Basis_5

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Not Considered for Calculation of Diluted Net Loss Per Share (Detail) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not considered for calculation of diluted net loss per share	20,596,998	23,755,434
Class X Convertible Preferred Stock (if-converted) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not considered for calculation of diluted net loss per share	8,219,805	11,429,760
Warrants for Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not considered for calculation of diluted net loss per share	6,682,708	6,682,708
Common Stock Options and Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not considered for calculation of diluted net loss per share	5,671,937	5,611,880
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not considered for calculation of diluted net loss per share	22,548	31,086

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, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 10,640	$ 16,019
Total assets measured at fair value	40,694	42,602
Available-for-sale [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	30,054	26,583
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	6,631	7,773
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	14,137	6,144
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	9,286	12,666
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	10,640	16,019
Total assets measured at fair value	10,640	16,019
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	30,054	26,583
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	30,054	26,583
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	6,631	7,773
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	14,137	6,144
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	$ 9,286	$ 12,666

Fair Value Measurements - Sch_2

Fair Value Measurements - Schedule of Available-for-sale Investments (Detail) - Short-term Investments [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	$ 30,044	$ 26,593
Gross Unrealized Gains	10
Gross Unrealized Losses		(10)
Market Value	30,054	26,583
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	14,137	6,144
Market Value	14,137	6,144
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	6,628	7,777
Gross Unrealized Gains	3
Gross Unrealized Losses		(4)
Market Value	6,631	7,773
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	9,279	12,672
Gross Unrealized Gains	7
Gross Unrealized Losses		(6)
Market Value	$ 9,286	$ 12,666

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2019
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments effective maturity period	1 year

Research Collaboration - Additi

Research Collaboration - Additional Information (Detail)	1 Months Ended
	Mar. 31, 2019USD ($)Program
Research And Development Arrangement Contract To Perform For Others [Line Items]
Contract liability	$ 630,000
Research Collaboration and Option Agreement [Member] | CSL Behring [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Number of programs | Program	4
Maximum option fees receivable based on achievement of research milestones per program	$ 4,250,000
Maximum option fees receivable based on achievement of research milestones	17,000,000
Contract liability	$ 600,000
Termination notice period	45 days

Debt, Commitments and Conting_3

Debt, Commitments and Contingencies - Additional Information (Detail)	1 Months Ended					3 Months Ended
	Jul. 30, 2018ft²	Dec. 31, 2017USD ($)$ / sharesshares	Nov. 30, 2017USD ($)	Jun. 30, 2017USD ($)$ / sharesshares	Nov. 30, 2016USD ($)Tranche$ / sharesshares	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Jan. 01, 2019USD ($)	Apr. 30, 2017ft²
Debt Instrument [Line Items]
Loan and security agreement, payment term						Issuable in three separate tranches (the Term Loans), $10.0 million of which was funded in November 2016, $5.0 million of which was funded in June 2017 and $5.0 million of which was funded in December 2017.
Lease, practical expedients, package						true
Operating lease, right-of-use asset						$ 3,323,000		$ 3,500,000
Operating lease, liability						$ 3,496,000		$ 3,500,000
Operating lease, discount rate						9.60%
Operating lease cost						$ 200,000
Operating lease, weighted average remaining lease term						4 years 1 month 6 days
Operating lease, weighted average discount rate						9.60%
Lease facility | ft²	20,508								24,494
Non-cancelable operating leases termination term	2023-05
Non-cancelable operating lease, existence of option to extend	false
Non-cancelable operating lease, option to extend	With the lease amendment, we do not have an option to extend the lease
Research and development expenses						$ 3,345,000	$ 6,150,000
Strategic Advisor Agreement [Member] | John D. Mendlein [Member]
Debt Instrument [Line Items]
Strategic advisor agreement period, maximum			4 years
Strategic advisor fee monthly rate for first year			$ 42,500
Strategic advisor fee monthly rate for rest of term			$ 7,500
Strategic advisor agreement expenses						$ 22,500	100,000
The Scripps Research Institute [Member]
Debt Instrument [Line Items]
Termination of research funding and option agreement effective month and year						2018-11
Research Funding and Option Agreement [Member] | The Scripps Research Institute [Member]
Debt Instrument [Line Items]
Research and development expenses							$ 500,000
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, principal outstanding before deducting debt issuance cost						$ 13,300,000
Term loan, debt issuance costs						$ 300,000
Warrants expiration year						7 years
Aggregate fair value of warrants using black scholes option pricing model		$ 500,000
Final maturity payment accrued over life of term loan through interest expense						$ 1,800,000
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term Loans Tranche One [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement					$ 10,000,000
Loan and security agreement funded date					Nov. 30, 2016
Exercise price of warrant per share | $ / shares					$ 3.14
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term Loans Tranche One [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 Loans Tranche Two [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement				$ 5,000,000
Loan and security agreement funded date				Jun. 30, 2017
Exercise price of warrant per share | $ / shares				$ 3.60
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term Loans Tranche Two [Member] | Common Stock [Member]
Debt Instrument [Line Items]
Warrants to purchase number of common stock, shares | shares				20,833
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term Loans Tranche Three [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement		$ 5,000,000
Loan and security agreement funded date		Dec. 31, 2017
Exercise price of warrant per share | $ / shares		$ 3.72
Silicon Valley Bank and Solar Capital, Ltd. [Member] | Term Loans Tranche Three [Member] | Common Stock [Member]
Debt Instrument [Line Items]
Warrants to purchase number of common stock, shares | shares		20,188
Loan Amendment Agreement with SVB and Solar [Member]
Debt Instrument [Line Items]
Loan and security agreement, payment term						Under the Loan Agreement, we are obligated to make interest only payments through June 1, 2018, 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 amounts for final payment					8.75%
Loan Amendment Agreement with SVB and Solar [Member] | Prime Rate [Member]
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%

Debt, Commitments and Conting_4

Debt, Commitments and Contingencies - Schedule of Term Loans and Unamortized Discount Balances (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt, net of current portion and issuance costs and discount	$ 6,440	$ 8,263
Current portion of long-term debt, net of issuance costs and discount	7,791	$ 7,767
Term Loans [Member]
Debt Instrument [Line Items]
Debt balance	13,333
Less debt issuance costs and discount	(72)
Long-term debt, net of issuance costs and discount	13,261
Less current portion of long-term debt	(8,000)
Add accrual of final payment	1,179
Long-term debt, net of current portion and issuance costs and discount	6,440
Current portion of long-term debt	8,000
Less current portion of debt issuance costs and discount	(209)
Current portion of long-term debt, net of issuance costs and discount	$ 7,791

Debt, Commitments and Conting_5

Debt, Commitments and Contingencies - Schedule of Future Principal Payments for Term Loans (Detail) - Silicon Valley Bank and Solar Capital, Ltd. [Member] $ in Thousands	Mar. 31, 2019USD ($)
Debt Instrument [Line Items]
2019	$ 6,000
2020	7,333
Long-term debt, net of issuance costs and discount	$ 13,333

Debt, Commitments and Conting_6

Debt, Commitments and Contingencies - Schedule of Future Minimum Payments under Non-cancelable Operating Lease and Reconciliation to Operating Lease Liability (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Leases Operating [Abstract]
2019	$ 732
2020	1,002
2021	1,031
2022	1,062
Thereafter	403
Less: Amount representing interest	(734)
Present value of lease payments	3,496	$ 3,500
Less: Current portion of operating lease liability	(681)
Long-term operating lease liability	$ 2,815

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	Aug. 31, 2017	Jan. 31, 2019	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Dec. 31, 2018	Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Net proceeds from issuance of common stock				$ 8,000
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					1,223,163
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					305,790
Class X Convertible Preferred Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Preferred stock, shares issued				1,643,961		2,285,952
ATM Offering Program [Member] | Cowen Company, LLC (Cowen) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Net proceeds from issuance of common stock			$ 1,400,000
ATM Offering Program [Member] | Cowen Company, LLC (Cowen) [Member] | Sale Agreement [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Agreed upon value of sale of common stock per transaction							$ 35,000,000
ATM Offering Program [Member] | Common Stock [Member] | Cowen Company, LLC (Cowen) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period			2,711,158
Shares issued, price per share			$ 0.53
Private Placement [Member] | VGO Fund [Member] | Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Convertible preferred stock, common stock issued upon conversion		3,209,955
Private Placement [Member] | VGO Fund [Member] | Securities Purchase Agreement [Member] | Class X Convertible Preferred Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued, price per share	$ 13.25
Preferred stock, shares issued	2,285,952
Convertible preferred stock, terms of conversion			Each share of preferred stock is convertible into five shares of our common stock. In January 2019, the VGO Fund converted 641,991 shares of its preferred stock into 3,209,955 shares of common stock.
Convertible preferred stock, common stock issued upon conversion	5
Converted shares of preferred stock		641,991
Private Placement [Member] | VGO Fund [Member] | Maximum [Member] | Securities Purchase Agreement [Member] | Class X Convertible Preferred Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Warrants to purchase number of common stock, shares	4,952,829
Private Placement [Member] | Remaining Purchasers [Member] | Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Warrants to purchase number of common stock, shares	6,488,205
Gross proceeds from transaction	$ 45,800,000
Private Placement [Member] | Common Stock [Member] | VGO Fund [Member] | Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period	1,777,784
Shares issued, price per share	$ 2.65
Private Placement [Member] | Common Stock [Member] | Remaining Purchasers [Member] | Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period	4,094,336
Shares issued, price per share	$ 2.65
Private Placement [Member] | Common Stock [Member] | Remaining Purchasers [Member] | Maximum [Member] | Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Warrants to purchase number of common stock, shares	1,535,376
Private Placement [Member] | Warrants [Member] | Remaining Purchasers [Member] | Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price of warrant per share	$ 4.64
Warrants expiration date	Dec. 31, 2019

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail)	Mar. 31, 2019shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	23,897,681
Class X Preferred Stock (if-Converted to Common Stock) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	8,219,805
Common Stock Warrants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	6,682,708
Common Stock Options and Awards Outstanding [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	5,671,937
Shares Available Under the 2015 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	2,178,136
Shares Available Under the 2015 ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	1,145,095

Stockholders' Equity - Summar_2

Stockholders' Equity - Summary of Stock Option Activity (Detail)	3 Months Ended
	Mar. 31, 2019$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Outstanding Options, Beginning Balance | shares	4,990,674
Number of Outstanding Options, Granted | shares	856,493
Number of Outstanding Options, Canceled/forfeited/expired | shares	(465,430)
Number of Outstanding Options, Ending Balance | shares	5,381,737
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 4.47
Weighted Average Exercise Price, Granted | $ / shares	0.52
Weighted Average Exercise Price, Canceled/forfeited/expired | $ / shares	5.38
Weighted Average Exercise Price, Ending Balance | $ / shares	$ 3.77

Stockholders' Equity - Summar_3

Stockholders' Equity - Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants, Employee Stock Purchase Plan and Performance Options with Market Condition (Detail) - Employee Stock Option [Member]	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	2.50%	2.30%
Risk-free interest rate, maximum	2.60%	2.70%
Expected volatility, minimum	100.00%	89.20%
Expected volatility, maximum	101.00%	98.40%
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 25 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, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Number of Outstanding Restricted Stock Units, Beginning Balance | shares	216,700
Number of Outstanding Restricted Stock Units, Granted | shares	75,000
Number of Outstanding Restricted Stock Units, Forfeited | shares	(1,500)
Number of Outstanding Restricted Stock Units, Ending Balance | shares	290,200
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 0.85
Weighted Average Grant Date Fair Value, Granted | $ / shares	0.52
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	0.85
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 0.76

Stockholders' Equity - Schedu_2

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, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 571	$ 928
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	116	324
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 455	$ 604

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Apr. 30, 2019	Mar. 31, 2018
Subsequent Event [Line Items]
Proceeds from issuance of common stock through option exercises		$ 8
Subsequent Event | Securities Purchase Agreement [Member] | Institutional Investor [Member] | Federated Kaufmann Small Cap Fund | Common Stock [Member]
Subsequent Event [Line Items]
Shares issued during the period	9,242,143
Shares issued, price per share	$ 0.541
Proceeds from issuance of common stock through option exercises	$ 5,000