aTyr Pharma (ATYR) 10-Q 15 May 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	May 07, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Trading Symbol	LIFE
Entity Registrant Name	aTYR PHARMA INC
Entity Central Index Key	1,339,970
Current Fiscal Year End Date	--12-31
Entity Filer Category	Smaller Reporting Company
Entity Common Stock, Shares Outstanding		29,828,723

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 21,936	$ 21,091
Available-for-sale investments, short-term	52,164	64,028
Prepaid expenses and other assets	1,906	1,866
Total current assets	76,006	86,985
Property and equipment, net	2,374	2,280
Other assets	90	90
Total assets	78,470	89,355
Current liabilities:
Accounts payable	1,462	2,276
Accrued expenses	2,533	3,103
Current portion of long-term debt, net of issuance costs and discount	7,027	5,012
Total current liabilities	11,022	10,391
Long-term debt, net of current portion and issuance costs and discount	12,950	14,719
Commitments and contingencies (Note 3)
Stockholders’ equity:
Preferred stock, $0.001 par value; undesignated authorized shares – 5,000,000 at March 31, 2018 and December 31 2017; Class X Convertible Preferred Stock issued and outstanding shares – 2,285,952 as of March 31, 2018 and December 31, 2017	2	2
Common stock, $0.001 par value; authorized shares – 150,000,000 as of March 31, 2018 and December 31, 2017; issued and outstanding shares – 29,828,723 and 29,789,162 as of March 31, 2018 and December 31, 2017, respectively	30	30
Additional paid-in capital	329,455	328,519
Accumulated other comprehensive loss	(136)	(120)
Accumulated deficit	(274,853)	(264,186)
Total stockholders’ equity	54,498	64,245
Total liabilities and stockholders’ equity	$ 78,470	$ 89,355

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
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	29,828,723	29,789,162
Common stock, shares outstanding	29,828,723	29,789,162
Class X Convertible Preferred Stock [Member]
Preferred stock, shares issued	2,285,952	2,285,952
Preferred stock, shares outstanding	2,285,952	2,285,952

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating expenses:
Research and development	$ 6,150	$ 9,204
General and administrative	4,070	4,007
Total operating expenses	10,220	13,211
Loss from operations	(10,220)	(13,211)
Total other expense, net	(447)	(194)
Net loss	$ (10,667)	$ (13,405)
Net loss per share attributable to common stock holders, basic and diluted	$ (0.36)	$ (0.56)
Weighted average common stock shares outstanding, basic and diluted	29,795,466	23,739,057

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (10,667)	$ (13,405)
Other comprehensive gain (loss):
Change in unrealized gain (loss) on available for sale investments	(17)	8
Comprehensive loss	$ (10,684)	$ (13,397)

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net loss	$ (10,667)	$ (13,405)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	184	226
Stock-based compensation	928	1,277
Accretion of debt discount	81	37
Amortization (accretion) of premium (discount) of available-for-sale investment securities	(64)	74
Deferred rent		(81)
Changes in operating assets and liabilities
Prepaid expenses and other assets	(40)	776
Accounts payable and accrued expenses	(999)	(2,792)
Net cash used in operating activities	(10,577)	(13,888)
Cash flows from investing activities:
Purchases of property and equipment	(498)	(262)
Purchases of available-for-sale investment securities	(7,988)	(11,489)
Maturities of available-for-sale investment securities	19,900	16,150
Net cash provided by investing activities	11,414	4,399
Cash flows from financing activities:
Proceeds from issuance of common stock through option exercises	8	1
Net cash provided by financing activities	8	1
Net change in cash and cash equivalents	845	(9,488)
Cash and cash equivalents at beginning of period	21,091	38,388
Cash and cash equivalents at the end of period	$ 21,936	$ 28,900

Organization, Business, Basis o

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2018
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 using our knowledge of tRNA synthetase biology. 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, 2017, contained in our Annual Report on Form 10-K filed with the SEC on March 20, 2018. 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, 2018, we had an accumulated deficit of $274.9 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 $74.1 million as of March 31, 2018 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. 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 no shares and 9,039 shares subject to repurchase from the weighted average number of common shares outstanding for the three months ended March 31, 2018 and 2017, 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 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 included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): March 31, 2018 2017 Class X Convertible Preferred Stock (if-converted) 11,429,760 — Warrants for common stock 6,682,708 121,512 Common stock options and restricted stock units 5,611,880 4,720,382 Employee stock purchase plan 31,086 36,837 23,755,434 4,878,731 Recent Accounting Pronouncements In January 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (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 May 2017, the FASB issued ASU No. 2017-09 , Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting, In February 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220) In February 2018, the FASB issued ASU No. 2018-03, Technical Corrections and Improvements to Financial Instruments – overall In March 2018, the FASB issued ASU 2018-04, Investments – Debt Securities (Topic 320) and Regulated Operation (Topic 980)

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2018
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 Loans 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 Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of March 31, 2018: Assets: Current: Cash equivalents $ 10,605 $ 10,605 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 5,095 — 5,095 — Commercial paper 9,675 — 9,675 — Corporate debt securities 20,025 — 20,025 — United States Treasury securities 17,369 17,369 — Sub-total short-term investments 52,164 17,369 34,795 — Total assets measured at fair value $ 62,769 $ 27,974 $ 34,795 $ — As of December 31, 2017: Assets: Current: Cash equivalents $ 9,070 $ 9,070 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 6,497 — 6,497 — Commercial paper 21,943 — 21,943 — Corporate debt securities 18,260 — 18,260 — United States Treasury securities 17,328 17,328 — Sub-total short-term investments 64,028 17,328 46,700 — Total assets measured at fair value $ 73,098 $ 26,398 $ 46,700 $ — As of March 31, 2018 and December 31, 2017 available-for-sale investments are detailed as follows (in thousands): March 31, 2018 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities $ 5,100 — (5 ) $ 5,095 Commercial paper $ 9,675 — — 9,675 Corporate debt securities 20,065 — (40 ) 20,025 United States Treasury securities 17,411 — (42 ) 17,369 $ 52,251 $ — $ (87 ) $ 52,164 December 31, 2017 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities $ 6,501 — (4 ) $ 6,497 Commercial paper 21,943 — — 21,943 Corporate debt securities 18,286 — (26 ) 18,260 United States Treasury securities 17,368 — (40 ) 17,328 $ 64,098 $ — $ (70 ) $ 64,028 As of March 31, 2018, all of our available-for-sale investments have a variety of effective maturity dates of less than one year. As of March 31, 2018, there are 20 available-for-sale investments in gross unrealized loss position, all of which had been in such position for less than twelve months. At each reporting date, we perform an evaluation of impairment to determine if the unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, and our intent and ability to hold the investment until recovery of its amortized cost basis. We intend, and have the ability, to hold our investments in unrealized loss positions until their amortized cost basis has been recovered. Based on our evaluation, we determined that the unrealized losses were not other-than-temporary as of March 31, 2018.

Debt, Commitments and Contingen

Debt, Commitments and Contingencies	3 Months Ended
	Mar. 31, 2018
Debt Instruments [Abstract]
Debt, Commitments and Contingencies	3. 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), 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. 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, 2018, the carrying value of our Term Loans consist of $20.0 million principal outstanding less the debt issuance costs of $0.9 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. The final maturity payment of $1.8 million is accruing over the life of the Term Loans through interest expense. In connection with the first tranche, 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 second tranche, 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. 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 debt discount which are being accreted to interest expense over the life of Term Loans. Term loans and unamortized discount balances are as follows (in thousands): March 31, December 31, 2018 2017 Long-term debt $ 20,000 $ 20,000 Less debt issuance costs and discount (279 ) (345 ) Long-term debt, net of issuance costs and discount 19,721 19,655 Less current portion of long-term debt (7,333 ) (5,333 ) Add accrual of final payment 562 397 Long-term debt, net of current portion and issuance costs and discount $ 12,950 $ 14,719 Current portion of long-term debt $ 7,333 $ 5,333 Less current portion of debt issuance costs and discount (306 ) (321 ) Current portion of long-term debt, net of issuance costs and discount $ 7,027 $ 5,012 Future principal payments for the Term Loans are as follows (in thousands): March 31, 2018 2018 $ 5,333 2019 8,000 2020 6,667 $ 20,000 Facility Lease We have 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. The lease expires in 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 March 31, 2018 and 2017 was $0.3 million and $0.2 million, respectively. Future minimum payments under the non-cancelable operating lease as of March 31, 2018 were as follows (in thousands): Operating Lease 2018 $ 836 2019 420 $ 1,256 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 to provide additional funding to TSRI. For the three months ended March 31, 2018 and 2017, we recognized expense under the agreement in the amount of $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. Refer to Note 5, Subsequent Events for further discussion on this agreement .

Stockholders' Equity

Stockholders' Equity	3 Months Ended
	Mar. 31, 2018
Equity [Abstract]
Stockholders' Equity	4. Stockholders’ Equity Private Placement of Common Stock, Convertible Preferred Shares and Common Stock Warrants On August 27, 2017, we entered into a Securities Purchase Agreement (Securities Purchase Agreement) for a private placement (Private Placement) with 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). Pursuant to the Securities Purchase Agreement, (i) VGO Fund purchased 1,777,784 shares of our common stock, par value $0.001 per share (the Common Shares), at a price of $2.65 per share, 2,285,952 shares of our Class X Convertible Preferred Stock (the Preferred Shares or Preferred Stock, and together with the Common Shares, the Shares), par value $0.001 per share, at a price of $13.25 per share, and warrants to purchase up to that number of additional shares of Common Stock equal to thirty seven and one half percent (37.5%) of the number of Shares purchased by VGO Fund on an if-converted to common stock basis (rounded up to the nearest whole share), and (ii) the remaining Purchasers purchased an aggregate of 4,094,336 shares of our Common Shares, at a price of $2.65 per share, and warrants to purchase up to that number of additional shares of Common Stock equal to thirty-seven and one half percent (37.5%) of the number of Common Shares purchased by such Purchaser (rounded up to the nearest whole share). The Private Placement closed on August 31, 2017 for gross proceeds of $45.8 million, and after giving effect to costs related to the Private Placement, net proceeds of $42.5 million. Each share of Preferred Stock is convertible into five shares of our common stock. VGO Fund will be prohibited from converting the Preferred Stock into shares of our common stock if, as a result of such conversion, VGO Fund, together with its affiliates, would own more than 9.50% of the shares of our common stock then issued and outstanding, which percentage may change at VGO Fund’s election upon 61 days’ notice to us to (i) any other number less than or equal to 19.99% or (ii) subject to approval of our stockholders to the extent required in accordance with the NASDAQ Global Market rules, any number in excess of 19.99%. Holders of outstanding Preferred Stock are entitled to receive a dividend (on an if-converted to common stock basis), if we at any time pay a stock dividend equal to and in the same form as a dividend paid to holders of Common Shares. In the event of our liquidation, dissolution or winding up, holders of Preferred Stock will participate in any distribution of proceeds, pro rata based on the number of shares held by each such holder on an if-converted basis. The Preferred Shares have no voting rights. We evaluated the Preferred Stock for liability or equity classification under ASC 480, Distinguishing Liabilities from Equity , We also evaluated the Preferred Stock in accordance with the provisions of ASC 815, Derivatives and Hedging The issuance of convertible preferred stock could generate a beneficial conversion feature (BCF), which arises when a debt or equity security is issued with an embedded conversion option that is beneficial to the investor (or in-the-money) at inception because the conversion option has an effective strike price that is less than the market price of the underlying stock on the commitment date. The fair value of our common stock was $2.37 on August 31, 2017, the commitment date, using the Black-Scholes valuation model. After the proceeds allocation, the Preferred Stock had an effective conversion price of $2.37 per common share, which was equal to the fair value of our common stock on the commitment date. Therefore, no BCF is present. The warrants 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. We also entered into a registration rights agreement (Registration Rights Agreement) with certain of the Purchasers, excluding those Purchasers affiliated with our directors and officers, requiring us to register for the resale of the relevant securities. We registered all of the relevant securities issued in the Private Placement for resale on a Form S-3 filed with the SEC, as required under the Registration Rights Agreement, and the registration statement was declared effective on September 27, 2017. We evaluated the warrants for liability or equity classification under ASC 815, Derivative and Hedging 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), 1,191,566 additional shares were reserved for future issuance under the 2015 Plan on January 1, 2018 and 297,891 additional shares were reserved for future issuances under the 2015 ESPP on January 1, 2018. Common stock reserved for future issuance is as follows: March 31, 2018 Class X Preferred Stock (if-converted to common stock) 11,429,760 Common stock warrants 6,682,708 Common stock options and awards outstanding 5,611,880 Shares available under the 2015 Plan 971,911 Shares available under the 2015 ESPP 881,710 25,577,969 The following table summarizes our stock option activity under all equity incentive plans for the three months ended March 31, 2018: Number of Outstanding Options Weighted Average Exercise Price Outstanding as of December 31, 2017 4,617,059 $ 5.52 Granted 1,250,761 $ 3.31 Exercised (3,593 ) $ 2.02 Canceled/forfeited/expired (259,013 ) $ 6.21 Outstanding as of March 31, 2018 5,605,214 $ 5.00 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, 2018 2017 Expected term (in years) 5.77 – 6.08 6.02 – 6.06 Risk-free interest rate 2.3% – 2.7% 2.0% – 2.1% Expected volatility 89.2% – 98.4% 104.0% – 105.2% 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, 2018: Number of Outstanding Restricted Stock Units Weighted Average Grant Date Fair Value Balance as of December 31, 2017 49,300 $ 4.28 Granted — $ — Released (35,968 ) $ 4.64 Forfeited (6,666 ) $ 3.30 Balance as of March 31, 2018 6,666 $ 3.30 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, 2018 2017 Research and development $ 324 $ 444 General and administrative 604 833 $ 928 $ 1,277

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	5. Subsequent Events On May 10, 2018, we approved the implementation of a corporate restructuring and program prioritization plan (the Restructuring Plan) to streamline our operations and concentrate our development efforts on the advancement of ATYR1923. In connection with the Restructuring Plan, we committed to a reduction in our total workforce by approximately 30% to 42 full-time employees. The Restructuring Plan was approved by our management team (with authorization delegated by our Board of Directors), and affected employees were informed on May 11, 2018. We estimate that we will record charges of approximately $0.9 million for employee severance and other related termination benefits and approximately $0.4 million in one-time, non-cash stock-based compensation charges due to the acceleration of time-based vesting provisions of outstanding equity awards in accordance with our Executive Severance and Change in Control Policy. Severance payments are expected to be paid in full by the end of July 2018. On May 10, 2018, we provided TSRI with written notice of termination of our research funding and option agreement effective as of November 10, 2018.

Organization, Business, Basis12

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2018
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 using our knowledge of tRNA synthetase biology.
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, 2017, contained in our Annual Report on Form 10-K filed with the SEC on March 20, 2018. 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, 2018, we had an accumulated deficit of $274.9 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 $74.1 million as of March 31, 2018 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.
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 no shares and 9,039 shares subject to repurchase from the weighted average number of common shares outstanding for the three months ended March 31, 2018 and 2017, 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 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 included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): March 31, 2018 2017 Class X Convertible Preferred Stock (if-converted) 11,429,760 — Warrants for common stock 6,682,708 121,512 Common stock options and restricted stock units 5,611,880 4,720,382 Employee stock purchase plan 31,086 36,837 23,755,434 4,878,731
Recent Accounting Pronouncements	Recent Accounting Pronouncements In January 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (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 May 2017, the FASB issued ASU No. 2017-09 , Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting, In February 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220) In February 2018, the FASB issued ASU No. 2018-03, Technical Corrections and Improvements to Financial Instruments – overall In March 2018, the FASB issued ASU 2018-04, Investments – Debt Securities (Topic 320) and Regulated Operation (Topic 980)

Organization, Business, Basis13

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Schedule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share	Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): March 31, 2018 2017 Class X Convertible Preferred Stock (if-converted) 11,429,760 — Warrants for common stock 6,682,708 121,512 Common stock options and restricted stock units 5,611,880 4,720,382 Employee stock purchase plan 31,086 36,837 23,755,434 4,878,731

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Schedule of Assets Measured at Fair Value on Recurring Basis	Assets measured at fair value on a recurring basis are as follows (in thousands): Fair Value Measurements Using Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of March 31, 2018: Assets: Current: Cash equivalents $ 10,605 $ 10,605 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 5,095 — 5,095 — Commercial paper 9,675 — 9,675 — Corporate debt securities 20,025 — 20,025 — United States Treasury securities 17,369 17,369 — Sub-total short-term investments 52,164 17,369 34,795 — Total assets measured at fair value $ 62,769 $ 27,974 $ 34,795 $ — As of December 31, 2017: Assets: Current: Cash equivalents $ 9,070 $ 9,070 $ — $ — Available-for-sale investments, short-term: Asset-backed securities 6,497 — 6,497 — Commercial paper 21,943 — 21,943 — Corporate debt securities 18,260 — 18,260 — United States Treasury securities 17,328 17,328 — Sub-total short-term investments 64,028 17,328 46,700 — Total assets measured at fair value $ 73,098 $ 26,398 $ 46,700 $ —
Schedule of Available-for-sale Investments	As of March 31, 2018 and December 31, 2017 available-for-sale investments are detailed as follows (in thousands): March 31, 2018 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities $ 5,100 — (5 ) $ 5,095 Commercial paper $ 9,675 — — 9,675 Corporate debt securities 20,065 — (40 ) 20,025 United States Treasury securities 17,411 — (42 ) 17,369 $ 52,251 $ — $ (87 ) $ 52,164 December 31, 2017 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Available-for-sale investments, short-term: Asset-backed securities $ 6,501 — (4 ) $ 6,497 Commercial paper 21,943 — — 21,943 Corporate debt securities 18,286 — (26 ) 18,260 United States Treasury securities 17,368 — (40 ) 17,328 $ 64,098 $ — $ (70 ) $ 64,028

Debt, Commitments and Conting15

Debt, Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2018
Debt Instruments [Abstract]
Schedule of Term Loans and Unamortized Discount Balances	Term loans and unamortized discount balances are as follows (in thousands): March 31, December 31, 2018 2017 Long-term debt $ 20,000 $ 20,000 Less debt issuance costs and discount (279 ) (345 ) Long-term debt, net of issuance costs and discount 19,721 19,655 Less current portion of long-term debt (7,333 ) (5,333 ) Add accrual of final payment 562 397 Long-term debt, net of current portion and issuance costs and discount $ 12,950 $ 14,719 Current portion of long-term debt $ 7,333 $ 5,333 Less current portion of debt issuance costs and discount (306 ) (321 ) Current portion of long-term debt, net of issuance costs and discount $ 7,027 $ 5,012
Schedule of Future Principal Payments for Term Loans	Future principal payments for the Term Loans are as follows (in thousands): March 31, 2018 2018 $ 5,333 2019 8,000 2020 6,667 $ 20,000
Schedule of Future Minimum Payments under Non-cancelable Operating Lease	Future minimum payments under the non-cancelable operating lease as of March 31, 2018 were as follows (in thousands): Operating Lease 2018 $ 836 2019 420 $ 1,256

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	3 Months Ended
	Mar. 31, 2018
Summary of Common Stock Reserved for Future Issuance	Common stock reserved for future issuance is as follows: March 31, 2018 Class X Preferred Stock (if-converted to common stock) 11,429,760 Common stock warrants 6,682,708 Common stock options and awards outstanding 5,611,880 Shares available under the 2015 Plan 971,911 Shares available under the 2015 ESPP 881,710 25,577,969
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, 2018: Number of Outstanding Options Weighted Average Exercise Price Outstanding as of December 31, 2017 4,617,059 $ 5.52 Granted 1,250,761 $ 3.31 Exercised (3,593 ) $ 2.02 Canceled/forfeited/expired (259,013 ) $ 6.21 Outstanding as of March 31, 2018 5,605,214 $ 5.00
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, 2018: Number of Outstanding Restricted Stock Units Weighted Average Grant Date Fair Value Balance as of December 31, 2017 49,300 $ 4.28 Granted — $ — Released (35,968 ) $ 4.64 Forfeited (6,666 ) $ 3.30 Balance as of March 31, 2018 6,666 $ 3.30
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, 2018 2017 Research and development $ 324 $ 444 General and administrative 604 833 $ 928 $ 1,277
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, 2018 2017 Expected term (in years) 5.77 – 6.08 6.02 – 6.06 Risk-free interest rate 2.3% – 2.7% 2.0% – 2.1% Expected volatility 89.2% – 98.4% 104.0% – 105.2% Expected dividend yield 0.0 % 0.0 %

Organization, Business, Basis17

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Accumulated deficit	$ 274,853		$ 264,186
Cash, cash equivalents and available-for-sale investments	$ 74,100
Weighted average shares subject to repurchase	0	9,039
Pangu BioPharma [Member] | Hong Kong [Member]
Significant Accounting Policies [Line Items]
Majority-owned subsidiary percentage	98.00%

Organization, Business, Basis18

Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share (Detail) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	23,755,434	4,878,731
Class X Convertible Preferred Stock (if-converted) [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	11,429,760
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	6,682,708	121,512
Common Stock Options And Restricted Stock Units [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	5,611,880	4,720,382
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	31,086	36,837

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, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 10,605	$ 9,070
Total assets measured at fair value	62,769	73,098
Available-for-sale [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	52,164	64,028
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	9,675	21,943
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	17,369	17,328
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	20,025	18,260
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	5,095	6,497
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	10,605	9,070
Total assets measured at fair value	27,974	26,398
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	17,369	17,328
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	17,369	17,328
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	34,795	46,700
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	34,795	46,700
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	9,675	21,943
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	20,025	18,260
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	$ 5,095	$ 6,497

Fair Value Measurements - Sch20

Fair Value Measurements - Schedule of Available-for-sale Investments (Detail) - Short-term Investments [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	$ 52,251	$ 64,098
Gross Unrealized Losses	(87)	(70)
Market Value	52,164	64,028
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	9,675	21,943
Market Value	9,675	21,943
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	20,065	18,286
Gross Unrealized Losses	(40)	(26)
Market Value	20,025	18,260
United States Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	17,411	17,368
Gross Unrealized Losses	(42)	(40)
Market Value	17,369	17,328
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	5,100	6,501
Gross Unrealized Losses	(5)	(4)
Market Value	$ 5,095	$ 6,497

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018Security
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments in gross unrealized loss position	20
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 Conting22

Debt, Commitments and Contingencies - Additional Information (Detail)	1 Months Ended				3 Months Ended
	Dec. 31, 2017USD ($)$ / sharesshares	Jun. 30, 2017USD ($)$ / sharesshares	Apr. 30, 2017USD ($)ft²	Nov. 30, 2016USD ($)Tranche$ / sharesshares	Mar. 31, 2018USD ($)	Mar. 31, 2017USD ($)
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.
Noncancelable operating lease term of expiration					2019-05
Additional lease facility | ft²			7,411
Noncancelable operating lease extension period			2019-05
Noncancelable operating lease total additional commitment			$ 700,000
Rent expense					$ 300,000	$ 200,000
Research and development expenses					6,150,000	9,204,000
Research Funding and Option Agreement [Member] | The Scripps Research Institute [Member]
Debt Instrument [Line Items]
Research and development expenses					500,000	$ 400,000
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 loans, principal outstanding before deducting debt issuance cost					20,000,000
Term loans, debt issuance costs					900,000
Final maturity payment accrued over life of term loans through interest expense					$ 1,800,000
Warrants expiration year					7 years
Aggregate fair value of warrants using black scholes option pricing model	$ 500,000
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 Conting23

Debt, Commitments and Contingencies - Schedule of Term Loans and Unamortized Discount Balances (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt, net of current portion and issuance costs and discount	$ 12,950	$ 14,719
Current portion of long-term debt, net of issuance costs and discount	7,027	5,012
Term Loans [Member]
Debt Instrument [Line Items]
Long-term debt	20,000	20,000
Less debt issuance costs and discount	(279)	(345)
Long-term debt, net of issuance costs and discount	19,721	19,655
Less current portion of long-term debt	(7,333)	(5,333)
Add accrual of final payment	562	397
Long-term debt, net of current portion and issuance costs and discount	12,950	14,719
Current portion of long-term debt	7,333	5,333
Less current portion of debt issuance costs and discount	(306)	(321)
Current portion of long-term debt, net of issuance costs and discount	$ 7,027	$ 5,012

Debt, Commitments and Conting24

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, 2018USD ($)
Debt Instrument [Line Items]
2,018	$ 5,333
2,019	8,000
2,020	6,667
Long-term debt, net of issuance costs and discount	$ 20,000

Debt, Commitments and Conting25

Debt, Commitments and Contingencies - Schedule of Future Minimum Payments under Non-cancelable Operating Lease (Detail) $ in Thousands	Mar. 31, 2018USD ($)
Leases Operating [Abstract]
2,018	$ 836
2,019	420
Operating Lease, total	$ 1,256

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) $ / shares in Units, $ in Millions	Aug. 31, 2017USD ($)$ / shares	Aug. 27, 2017Day$ / sharesshares	Mar. 31, 2018$ / sharesshares	Jan. 01, 2018shares	Dec. 31, 2017$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, par value			$ 0.001		$ 0.001
Preferred stock, par value			$ 0.001		$ 0.001
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 | shares				1,191,566
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 | shares				297,891
Class X Convertible Preferred Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Preferred stock, shares issued | shares			2,285,952		2,285,952
Securities Purchase Agreement [Member] | Private Placement [Member] | Common Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock price per share	$ 2.37
Preferred stock conversion price per share	$ 2.37
Securities Purchase Agreement [Member] | Private Placement [Member] | Warrants [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
Securities Purchase Agreement [Member] | Private Placement [Member] | VGO Fund [Member] | Common Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period | shares		1,777,784
Shares issued, price per share		$ 2.65
Common stock, par value		$ 0.001
Securities Purchase Agreement [Member] | Private Placement [Member] | VGO Fund [Member] | Warrants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of number of shares purchased on as-converted basis		37.50%
Securities Purchase Agreement [Member] | Private Placement [Member] | VGO Fund [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 | shares		2,285,952
Preferred stock, par value		$ 0.001
Convertible preferred stock, terms of conversion			Each share of Preferred Stock is convertible into five shares of our common stock. VGO Fund will be prohibited from converting the Preferred Stock into shares of our common stock if, as a result of such conversion, VGO Fund, together with its affiliates, would own more than 9.50% of the shares of our common stock then issued and outstanding, which percentage may change at VGO Fund’s election upon 61 days’ notice to us to (i) any other number less than or equal to 19.99% or (ii) subject to approval of our stockholders to the extent required in accordance with the NASDAQ Global Market rules, any number in excess of 19.99%.
Convertible preferred stock, common stock issued upon conversion | shares		5
Maximum ownership percentage prohibiting conversion		9.50%
Number of days of notice | Day		61
Maximum ownership percentage upon conversion, as per NASDAQ Global Market rules		19.99%
Securities Purchase Agreement [Member] | Private Placement [Member] | Remaining Purchasers [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Gross proceeds from transaction | $	$ 45.8
Net proceeds from transaction, after giving effect to cost related to placement | $	$ 42.5
Securities Purchase Agreement [Member] | Private Placement [Member] | Remaining Purchasers [Member] | Common Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period | shares		4,094,336
Shares issued, price per share		$ 2.65
Securities Purchase Agreement [Member] | Private Placement [Member] | Remaining Purchasers [Member] | Warrants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of number of shares purchased on as-converted basis		37.50%

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail)	Mar. 31, 2018shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	25,577,969
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	11,429,760
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	5,611,880
Shares Available Under the 2015 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	971,911
Shares Available Under the 2015 ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	881,710

Stockholders' Equity - Summar28

Stockholders' Equity - Summary of Stock Option Activity (Detail)	3 Months Ended
	Mar. 31, 2018$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Outstanding Options, Beginning Balance | shares	4,617,059
Number of Outstanding Options, Granted | shares	1,250,761
Number of Outstanding Options, Exercised | shares	(3,593)
Number of Outstanding Options, Canceled/forfeited/expired | shares	(259,013)
Number of Outstanding Options, Ending Balance | shares	5,605,214
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 5.52
Weighted Average Exercise Price, Granted | $ / shares	3.31
Weighted Average Exercise Price, Exercised | $ / shares	2.02
Weighted Average Exercise Price, Canceled/forfeited/expired | $ / shares	6.21
Weighted Average Exercise Price, Ending Balance | $ / shares	$ 5

Stockholders' Equity - Summar29

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, 2018	Mar. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	2.30%	2.00%
Risk-free interest rate, maximum	2.70%	2.10%
Expected volatility, minimum	89.20%	104.00%
Expected volatility, maximum	98.40%	105.20%
Expected dividend yield	0.00%	0.00%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 9 months 7 days	6 years 7 days
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 21 days

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Restricted Stock Unit Activity (Detail) - Restricted Stock Unit [Member]	3 Months Ended
	Mar. 31, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Number of Outstanding Restricted Stock Units, Beginning Balance | shares	49,300
Number of Outstanding Restricted Stock Units, Released | shares	(35,968)
Number of Outstanding Restricted Stock Units, Forfeited | shares	(6,666)
Number of Outstanding Restricted Stock Units, Ending Balance | shares	6,666
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 4.28
Weighted Average Grant Date Fair Value, Released | $ / shares	4.64
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	3.30
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 3.30

Stockholders' Equity - Schedu31

Stockholders' Equity - Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition, 2015 ESPP and Restricted Stock Units (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 928	$ 1,277
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	324	444
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 604	$ 833

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] $ in Millions	May 10, 2018USD ($)Employee
The Scripps Research Institute [Member]
Subsequent Event [Line Items]
Termination of research funding and option agreement effective date	Nov. 10, 2018
ATYR1923 Manufacturer [Member] | Restructuring Plan [Member]
Subsequent Event [Line Items]
Restructuring plan, percentage of reduction in total workforce	30.00%
Restructuring plan, description	On May 10, 2018, we approved the implementation of a corporate restructuring and program prioritization plan (the Restructuring Plan) to streamline our operations and concentrate our development efforts on the advancement of ATYR1923. In connection with the Restructuring Plan, we committed to a reduction in our total workforce by approximately 30% to 42 full-time employees. The Restructuring Plan was approved by our management team (with authorization delegated by our Board of Directors), and affected employees were informed on May 11, 2018.
Number of full-time employees | Employee	42
ATYR1923 Manufacturer [Member] | Restructuring Plan [Member] | Employee Severance and Other Related Termination Benefits [Member]
Subsequent Event [Line Items]
Restructuring charges	$ 0.9
ATYR1923 Manufacturer [Member] | Restructuring Plan [Member] | One-time Termination Benefits [Member]
Subsequent Event [Line Items]
Non-cash stock-based compensation charges	$ 0.4