aTyr Pharma (ATYR) 10-K 26 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Mar. 15, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	LIFE
Entity Registrant Name	aTYR PHARMA INC
Entity Central Index Key	0001339970
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		34,767,133
Entity Public Float			$ 20,605,629

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 22,962	$ 21,091
Available-for-sale investments, short-term	26,583	64,028
Prepaid expenses and other assets	1,258	1,866
Total current assets	50,803	86,985
Property and equipment, net	1,853	2,280
Other assets	90	90
Total assets	52,746	89,355
Current liabilities:
Accounts payable	1,040	2,276
Accrued expenses	2,026	3,103
Current portion of long-term debt, net of issuance costs and discount	7,767	5,012
Total current liabilities	10,833	10,391
Long-term debt, net of current portion and issuance costs and discount	8,263	14,719
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.001 par value; undesignated authorized shares – 5,000,000 at December 31, 2018 and 2017, respectively; Class X Convertible Preferred Stock issued and outstanding shares – 2,285,952 as of December 31, 2018 and 2017, respectively	2	2
Common stock, $0.001 par value; authorized shares – 150,000,000 as of December 31, 2018 and 2017, respectively; issued and outstanding shares – 30,579,076 and 29,789,162 as of December 31, 2018 and 2017, respectively	31	30
Additional paid-in capital	332,378	328,519
Accumulated other comprehensive loss	(60)	(120)
Accumulated deficit	(298,701)	(264,186)
Total stockholders’ equity	33,650	64,245
Total liabilities and stockholders’ equity	$ 52,746	$ 89,355

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 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	30,579,076	29,789,162
Common stock, shares outstanding	30,579,076	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

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Research and development	$ 20,385	$ 30,067	$ 42,846
General and administrative	12,435	17,078	15,094
Total operating expenses	32,820	47,145	57,940
Loss from operations	(32,820)	(47,145)	(57,940)
Other income (expense), net
Other income (expense), net	(1,695)	(1,062)	65
Loss on extinguishment of debt			(29)
Total other income (expense), net	(1,695)	(1,062)	36
Loss before income taxes	(34,515)	(48,207)	(57,904)
Income tax benefit			49
Net loss	$ (34,515)	$ (48,207)	$ (57,855)
Net loss per share attributable to common stock holders, basic and diluted	$ (1.15)	$ (1.87)	$ (2.44)
Weighted average common stock shares outstanding, basic and diluted	29,957,102	25,799,853	23,681,019

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (34,515)	$ (48,207)	$ (57,855)
Other comprehensive gain (loss):
Change in unrealized gain (loss) on available for sale investments, net of tax	60	(44)	95
Comprehensive loss	$ (34,455)	$ (48,251)	$ (57,760)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - 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, 2015	$ 115,050	$ 24	$ 273,321	$ (171)	$ (158,124)
Beginning balance, Shares at Dec. 31, 2015		23,670,079
Exercise of common stock options and release of restricted stock units	20		20
Exercise of common stock options and release of restricted stock units, Shares		17,972
Issuance of common stock pursuant to employee stock purchase plan	143		143
Issuance of common stock pursuant to employee stock purchase plan, Shares		56,781
Issuance of warrants related to term loan	217		217
Changes in share repurchase liability	102		102
Stock-based compensation	5,029		5,029
Net unrealized gain (loss) on investments, net of tax	95			95
Net loss	(57,855)				(57,855)
Ending balance at Dec. 31, 2016	62,801	$ 24	278,832	(76)	(215,979)
Ending balance, Shares at Dec. 31, 2016		23,744,832
Exercise of common stock options and release of restricted stock units	186		186
Exercise of common stock options and release of restricted stock units, Shares		111,039
Issuance of common stock pursuant to employee stock purchase plan	175		175
Issuance of common stock pursuant to employee stock purchase plan, Shares		61,171
Issuance of common stock and preferred stock from private placement, net of offering costs	42,239	$ 6	42,231			$ 2
Issuance of common stock and preferred stock from private placement, net of offering costs, Shares		5,872,120				2,285,952
Issuance of warrants related to term loan	263		263
Changes in share repurchase liability	48		48
Stock-based compensation	6,784		6,784
Net unrealized gain (loss) on investments, net of tax	(44)			(44)
Net loss	(48,207)				(48,207)
Ending balance at Dec. 31, 2017	64,245	$ 30	328,519	(120)	(264,186)	$ 2
Ending balance, Shares at Dec. 31, 2017		29,789,162				2,285,952
Exercise of common stock options and release of restricted stock units	14		14
Exercise of common stock options and release of restricted stock units, Shares		51,442
Issuance of common stock pursuant to employee stock purchase plan	36		36
Issuance of common stock pursuant to employee stock purchase plan, Shares		42,405
Issuance of common stock from at the market offerings, net of offering costs	379	$ 1	378
Issuance of common stock from at the market offerings, net of offering costs, Shares		696,067
Stock-based compensation	3,431		3,431
Net unrealized gain (loss) on investments, net of tax	60			60
Net loss	(34,515)				(34,515)
Ending balance at Dec. 31, 2018	$ 33,650	$ 31	$ 332,378	$ (60)	$ (298,701)	$ 2
Ending balance, Shares at Dec. 31, 2018		30,579,076				2,285,952

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (34,515)	$ (48,207)	$ (57,855)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	746	713	900
Stock-based compensation	3,431	6,784	5,029
Debt discount accretion and non-cash interest expense	966	590	173
Loss on debt extinguishment			29
Amortization (accretion) of premium (discount) of available-for-sale investment securities	(261)	14	531
Loss on disposal of property and equipment	18
Deferred rent		(130)	(315)
Changes in operating assets and liabilities
Prepaid expenses and other assets	610	761	(421)
Accounts payable and accrued expenses	(2,058)	(2,889)	(932)
Net cash used in operating activities	(31,063)	(42,364)	(52,861)
Cash flows from investing activities:
Purchases of property and equipment	(594)	(1,312)	(600)
Purchases of available-for-sale investment securities	(40,299)	(77,672)	(28,089)
Maturities of available-for-sale investment securities	78,065	51,347	62,216
Net cash provided by (used in) investing activities	37,172	(27,637)	33,527
Cash flows from financing activities:
Proceeds from issuance of common stock through option exercises	14	186	20
Proceeds from issuance of common stock through at the market offerings, net of offering costs	379
Proceeds from issuance of common stock through employee purchase plan	36	175	143
Proceeds from borrowing, net		9,866	9,736
Repayment on borrowing	(4,667)		(5,202)
Proceeds from issuance of securities in the Private Placement, net of issuance cost		42,477
Net cash (used in) provided by financing activities	(4,238)	52,704	4,697
Net change in cash and cash equivalents	1,871	(17,297)	(14,637)
Cash and cash equivalents at beginning of period	21,091	38,388	53,025
Cash and cash equivalents at the end of period	22,962	21,091	38,388
Supplemental disclosure of cash flow information:
Interest paid	1,700	1,000	225
Purchase of fixed assets included in accounts payable	$ 4	260
Supplemental schedule of noncash investing and financing activities:
Issuance of warrants in connection with borrowings		263	217
Changes in share repurchase liability		$ 48	$ 102

Organization, Business and Basi

Organization, Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Organization, Business and Basis of Presentation	1. Organization, Business and Basis of Presentation Organization and Business We were incorporated in the state of Delaware on September 8, 2005. We are focused on the discovery and clinical development of innovative medicines based on novel immunological pathways. In May 2018, we implemented a corporate restructuring and program prioritization plan (Restructuring Plan) to streamline our operations and concentrate development efforts on the advancement of our therapeutic candidate, ATYR1923. In connection with the Restructuring Plan, we reduced our workforce by approximately 30% to 42 full-time employees. We completed the workforce reduction in June 2018. Principles of Consolidation Our consolidated financial statements include our accounts, our 98% majority-owned subsidiary in Hong Kong, Pangu BioPharma Limited (Pangu BioPharma). All intercompany transactions and balances are eliminated in consolidation. Use of Estimates Our consolidated financial statements are prepared in accordance with generally accepted accounting principles (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 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 expenses. 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. Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. We view our operations and manage our business in one operating segment. Reclassifications Certain reclassifications have been made to prior year amounts to conform to the current year presentation. The reclassifications were not material to the consolidated financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Cash and Cash Equivalents Cash and cash equivalents consist primarily of readily available checking, money market accounts and money market funds. We consider all highly liquid investments that mature in three months or less when purchased to be cash equivalents. Investment Securities Investment securities primarily consist of investment grade corporate debt securities, asset-backed securities, commercial paper and United States Treasury securities. We classify all investment securities as available-for-sale. Investment securities are carried at fair value, with the unrealized gains and losses, if any, reported as a component of other comprehensive income (loss) in stockholders’ equity until realized. Realized gains and losses from the sale of investment securities, if any, are determined on a specific identification basis. A decline in the market value of any investment security below cost that is determined to be other than temporary will result in an impairment charge to earnings and a new cost basis for the security is established. No such impairment charges were recorded for any period presented. Premiums and discounts are amortized or accreted over the life of the related security as an adjustment to yield using the straight-line method and are included in interest income. Interest income is recognized when earned. As of December 31, 2018, we held an aggregate total of $26.6 million of investment securities which consisted of corporate debt securities, asset-backed securities, and commercial paper all of which will mature in less than one year and there was approximately $10,000 difference between the amortized cost and fair value of these investment securities. As of December 31, 2017, we held $64.0 million of corporate debt securities, asset-backed securities and United States Treasury securities, all of which mature in less than one year, and there was $0.1 million difference between the amortized cost and fair value of these investment securities. Concentration of Credit Risk Financial instruments that potentially subject us to significant concentration of credit risk consist primarily of cash, cash equivalents and investment securities. We have established guidelines regarding diversification of investments and their maturities, which are designed to maintain principal and maximize liquidity. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have not experienced any losses in such accounts and we believe that we are not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. Property and Equipment Property and equipment are stated at cost and depreciated on a straight-line basis over the estimated useful life of the related assets (generally three to seven years). Leasehold improvements are stated at cost and amortized on a straight-line basis over the lesser of the remaining term of the related lease or the estimated useful life of the leasehold improvements. Repairs and maintenance costs are charged to expense as incurred . Impairment of Long-Lived Assets Long-lived assets consist primarily of property and equipment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. While our current and historical operating losses are indicators of impairment, we believe that future cash flows to be received support the carrying value of our long-lived assets and, accordingly, have not recognized any impairment losses since inception. Accrued Expenses As part of the process of preparing our consolidated financial statements, we are required to estimate accrued expenses, including accrued research and development expenses for fees paid to investigative sites and clinical research organizations (CROs) in connection with clinical trials; service providers in connection with preclinical development activities; service providers related to product manufacturing; and other professional services. The accrual process involves reviewing open contracts and purchase orders, communicating with our personnel to identify services that have been performed on our behalf and estimating the level of service performed and the associated cost incurred for the service when we have not yet been invoiced or otherwise notified of the actual cost. We make estimates of accrued expenses as of each balance sheet date in our consolidated financial statements based on facts and circumstances known to us at that time. Although we do not expect the estimates to be materially different from amounts actually incurred, if the estimates of the status and timing of services performed differs from the actual status and timing of services performed, we may report amounts that are too high or too low in any particular period. Historically, our estimated accrued liabilities have approximated actual expenses incurred. Subsequent changes in estimates may result in a material change in our accruals. Research and Development Costs Research and development costs are expensed as incurred. Research and development costs include: salaries and employee-related expenses, including stock-based compensation and benefits for personnel in research and product development functions; costs associated with conducting our preclinical, development and regulatory activities, including fees paid to third-party professional consultants, service providers and our scientific, therapeutic and clinical advisors; costs to acquire, develop and manufacture preclinical study and clinical trial materials; costs incurred under clinical trial agreements with CROs and investigative sites; costs for laboratory supplies; payments related to licensed products and technologies; allocated facilities and information technology costs; and depreciation. Patent Costs Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred since recoverability of such expenditures is uncertain. Stock-Based Compensation Stock-based compensation expense represents the cost of the grant date fair value of employee stock option and restricted stock unit grants recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis. We recognize forfeitures as they occur as a reduction of expense. For stock option grants with performance-based milestones, the expense is recorded over the service period after the achievement of the milestone is probable or the performance condition is achieved. For stock option grants with market-based conditions, the expense is recorded using the accelerated attribution method over the requisite service period for each vesting tranche. We account for stock options granted to non-employees using the fair value approach. These option grants are subject to periodic revaluation over their vesting terms. We estimate the fair value of employee and non-employee stock option grants using the Black-Scholes option pricing model. We estimate the fair value of the market-based stock option grants using a Monte Carlo simulation. The fair value of restricted stock units is determined by the closing price as of the grant date. Income Taxes We account for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized as income in the period that includes the enactment date. We recognize net deferred tax assets to the extent that we believe these assets are more likely than not to be realized. In making such a determination, we consider all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies and results of recent operations. If we determine that we would be able to realize the deferred tax assets in the future in excess of their net recorded amount, we would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. We record uncertain tax positions on the basis of a two-step process whereby (1) we determine whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, we recognize the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority. We recognize interest and penalties related to unrecognized tax benefits within income tax expense. Any accrued interest and penalties are included within the related tax liability. On December 22, 2017, the U.S. enacted the Tax Cuts and Jobs Act (TCJA), which among other provisions, reduced the U.S. corporate tax rate from 35% to 21%, effective January 1, 2018. Due to the timing of the enactment and the complexity involved in applying the provisions of the TCJA, we made reasonable estimates of the effects and recorded provisional amounts, offset with valuation allowances, in its financial statements as of December 31, 2017. At December 31, 2018, we had completed our assessment of the impact of the TCJA and has reflected the impact in the current year. At December 31, 2018, there were no material changes from the provisional amounts recorded for the year ended December 31, 2017. Net Loss Per Share Basic net loss per share is calculated by dividing the net loss attributable to common stockholders 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, 3,685 and 25,984 shares subject to repurchase from the weighted average number of common shares outstanding for the years ended December 31, 2018, 2017 and 2016, respectively. Diluted net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders 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): Year Ended December 31, 2018 2017 2016 Class X convertible preferred stock (if-converted) 11,429,760 11,429,760 — Warrants for common stock 6,682,708 6,682,708 121,512 Common stock options and restricted stock units 5,207,374 4,666,359 4,091,701 Employee stock purchase plan 22,548 31,086 36,836 23,342,390 22,809,913 4,250,049 The following table summarizes our net loss per share (in thousands, except per share data): Year Ended December 31, 2018 2017 2016 Numerator: Consolidated net loss $ (34,515 ) $ (48,207 ) $ (57,855 ) Denominator: Weighted average common shares outstanding 29,957,102 25,803,538 23,707,003 Weighted average common shares subject to repurchase — (3,685 ) (25,984 ) Weighted average common shares outstanding - basic and diluted 29,957,102 25,799,853 23,681,019 Net loss per share - basic and diluted $ (1.15 ) $ (1.87 ) $ (2.44 ) Convertible Preferred Stock We apply the relevant accounting standards to distinguish liabilities from equity when assessing the classification and measurement of preferred stock. Preferred shares subject to mandatory redemptions are considered liabilities and measured at fair value. Conditionally redeemable preferred shares are considered temporary equity. All other preferred shares are considered as stockholders’ equity. None of our outstanding preferred stock has redemption features. Derivative Financial Instruments We do not use derivative instruments to hedge exposures to cash flow, market, or foreign currency risks. We evaluate all of our financial instruments, including warrants, to determine if such instruments are derivatives or contain features that qualify as embedded derivatives. We generally use the Black-Scholes option-pricing model to value the derivative instruments at inception and subsequent valuation dates. The classification of derivative instruments, including whether such instruments should be recorded as liabilities or as equity, is re-assessed at the end of each reporting period. Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718) 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 for public business entities.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. 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 fair value of our Term Loans approximate its carrying values. 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 asset-backed securities, commercial paper, and corporate debt securities. 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 and liabilities 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 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 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 December 31, 2018 and 2017, available-for-sale investments are detailed as follows (in thousands): 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 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 December 31, 2018, all available-for-sale investments have contractual maturity dates less than one year. As of December 31, 2018, all available-for-sale investments are in a gross unrealized loss position, and they have 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 December 31, 2018.

Balance Sheet Details

Balance Sheet Details	12 Months Ended
	Dec. 31, 2018
Balance Sheet Details [Abstract]
Balance Sheet Details	4. Balance Sheet Details Property and equipment consist of the following (in thousands): December 31, 2018 2017 Computer and office equipment $ 543 $ 425 Scientific and laboratory equipment 5,631 5,494 Tenant improvements 1,703 1,706 7,877 7,625 Less accumulated depreciation and amortization (6,024 ) (5,345 ) $ 1,853 $ 2,280 As of December 31, 2018, 2017 and 2016, depreciation expense was $0.7 million, $0.7 million and $0.9 million, respectively. Accrued expenses consist of the following (in thousands): December 31, 2018 2017 Accrued salaries, wages and benefits $ 1,309 $ 1,920 Other accrued expenses (1) 717 1,183 $ 2,026 $ 3,103 (1) Other accrued expenses include expenses for clinical research organizations and contract manufacturing organizations.

Debt, Commitments and Contingen

Debt, Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Debt Instruments [Abstract]
Debt, Commitments and Contingencies	5. 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. 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, as well as 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 December 31, 2018, the carrying value of our Term Loans consists of $15.3 million principal outstanding, less the debt issuance costs of $0.3 million. The debt issuance costs have been recorded as a debt discount, and are being accreted to interest expense over the life of the Term Loans. 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): December 31, December 31, 2018 2017 Debt balance $ 15,333 $ 20,000 Less debt issuance costs and discount (115 ) (345 ) Long-term debt, net of issuance costs and discount 15,218 19,655 Less current portion of long-term debt (8,000 ) (5,333 ) Add accrual of final payment 1,045 397 Long-term debt, net of current portion and issuance costs and discount $ 8,263 $ 14,719 Current portion of long-term debt $ 8,000 $ 5,333 Less current portion of debt issuance costs and discount (233 ) (321 ) Current portion of long-term debt, net of issuance costs and discount $ 7,767 $ 5,012 Future principal payments for the Term Loans are as follows (in thousands): December 31, 2018 2019 $ 8,000 2020 7,333 $ 15,333 The final maturity payment of $1.8 million is accruing over the life of the Term Loans through interest expense. 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. 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. Rent expense for the years ended December 31, 2018, 2017 and 2016 was $1.0 million, $0.9 million and $0.5 million, respectively. As of December 31, 2018, future minimum payments under the non-cancelable operating lease are as follows (in thousands): Operating Lease 2019 $ 812 2020 1,002 2021 1,031 2022 1,062 Thereafter 403 $ 4,310 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. In May 2018, we provided TSRI with written notice of termination of our research funding and option agreement effective as of November 2018. During the years ended December 31, 2018, 2017 and 2016, we recognized expense under the agreement in the amount of $1.7 million, $1.8 million and $1.6 million, respectively. member of our board of directors, is a faculty member at TSRI and such payments fund a portion of his research activities conducted at TSRI. 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 year ended December 31, 2018, we recognized expenses under the Strategic Advisor Agreement in the amount of

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Stockholders' Equity	6. Stockholders’ Equity Common Stock 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, par value $0.001 per share, at a price of $2.65 per share, 2,285,952 shares of our Class X Convertible Preferred Stock (Preferred Stock, and together with the common stock, 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 stocks, 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 stocks purchased by such Purchaser (rounded up to the nearest whole share). Gross proceeds from the private placement were $45.8 million. After giving effect to costs related to the private placement, net proceeds were $42.5 million. Each share of Preferred Stock is convertible into five shares of our common stock. VGO Fund is 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. 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 our common stock. 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 at the commitment date in August 2017 was $2.37, using the Black-Scholes valuation model. After the proceeds allocation, the Preferred Stock have 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 in September 2017. We evaluated the warrants for liability or equity classification under ASC 815, Derivative and Hedging Registration Statement on Form S-3 In June 2016, we filed a Registration Statement on Form S-3 (File No. 333-211998) containing two prospectuses: (i) a base prospectus which covers the offering, issuance and sale of up to $150.0 million in the aggregate of an indeterminate number of shares of common stock and preferred stock, an indeterminate principal amount of debt securities and such indeterminate number of warrants and units; and (ii) a sales agreement prospectus covering the offering, issuance and sale of up to a maximum aggregate offering price of up to $20.0 million of our common stock that may be sold from time to time under a sales agreement with Cowen and Company, LLC (Cowen) in at-the-market offerings (ATM Offering Program). In accordance with the terms of such sales agreement entered with Cowen, 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 through Cowen. We are required to file another prospectus supplement in the event we intend to offer more than $20.0 million in shares of our common stock in accordance with the sales agreement. The sales agreement prospectus amount of $20.0 million is included in the base prospectus amount of $150.0 million. In October 2018, we started utilizing the ATM Offering Program and sold an aggregate of 696,067 shares of common stock at an average price of $0.71 per common share for net proceeds of $0.4 million through December 31, 2018. 2014 Stock Plan We adopted a stock option plan in 2007 (the 2007 Plan), which was subsequently amended, restated and renamed in July 2014 (the 2014 Plan) to provide for the incentive stock options, nonstatutory stock options, stock and rights to purchase restricted stock to eligible recipients. Recipients of incentive stock options are eligible to purchase shares of our common stock at an exercise price equal to no less than the estimated fair market value of such stock on the date of grant. The maximum term of options under the 2014 Plan is ten years. Options granted generally vest over four years. Shares underlying any awards under the 2014 Plan that are forfeited, canceled, reacquired by us prior to vesting, satisfied without the issuance of stock or otherwise terminated (other than by exercise) will be added to shares available for issuance under the 2015 Plan. 2015 Stock Plan In April 2015, our board of directors adopted, and our stockholders approved, the 2015 Stock Plan (the 2015 Plan). The 2015 Plan became effective on May 6, 2015 and we ceased granting any new awards under our 2014 Plan. Awards granted under the 2014 Plan prior to our IPO that are forfeited, canceled, reacquired by us prior to vesting satisfied without the issuance of stock or otherwise terminated (other than by exercise) will be added to shares available for issuance under the 2015 Plan. A total of 1,574,566 shares of our common stock were initially reserved for issuance under the 2015 Plan. In addition, the number of shares reserved and available for issuance under the 2015 Plan automatically increased each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by the lesser of (i) 1,840,000 shares, (ii) 4% of the outstanding number of shares of our common stock on the immediately preceding December 31 or (iii) an amount determined by our board of directors. Pursuant to this provision, 1,223,163, 1,191,566 and 949,793 additional shares were reserved for issuance under the 2015 Plan on January 1, 2019, 2018 and 2017, respectively. Total shares available for issuance under the 2015 Plan as of January 1, 2019 were 2,724,262. Shares underlying any awards under the 2015 Plan that are forfeited, canceled, reacquired by us prior to vesting, satisfied without the issuance of stock or otherwise terminated (other than by exercise) will be added to shares available for issuance under the 2015 Plan. The maximum term of options granted under 2015 Plan is ten years. For an initial grant to an employee, 25% of the options generally vest on the first anniversary of the original vesting date, with the balance vesting monthly over the remaining three years. For subsequent grants to an employee, the options generally vest monthly over a four-year term. Inducement Grants In September 2016, we granted a non-qualified option to purchase 145,000 shares of our common stock at an exercise price of $3.29 per share as an inducement award in connection with the hiring of our Senior Vice President, Research (who was later promoted to Chief Scientific Officer) and filed a registration statement on Form S-8 on March 22, 2017 to register shares of common stock underlying this option. Upon resignation of our Chief Scientific Officer effective December 31, 2018, any unvested shares underlying this option were cancelled. Vested shares were exercisable within 90 days from termination date. We did not grant an inducement non-qualified option in 2017. In July 2018, we granted a non-qualified option to purchase 200,000 shares of our common stock at an exercise price of $0.82 per share as an inducement award in connection with the hiring of our Chief Financial Officer. Options under the inducement grants vest over a period of four years, with 25% vesting on the one year anniversary of the grant date and the remaining 75% vesting on a monthly basis over three years thereafter, subject to continuous employment. These options were inducement grants issued outside of the 2015 Plan in accordance with Nasdaq Listing Rule 5635(c)(4). We intend to file a registration statement on Form S-8 to register the shares of common stock underlying the options granted in July 2018 prior to the time at which this option becomes exercisable. In addition, from time to time, we may make inducement grants of stock options to new employees. Employee Stock Purchase Plan In April 2015, our board of directors adopted, and our stockholders approved, our 2015 Employee Stock Purchase Plan (the 2015 ESPP). The 2015 ESPP became effective on May 6, 2015. A total of 227,623 shares of our common stock were initially reserved for issuance under the 2015 ESPP. In addition, the number of shares reserved and available for purchase under the 2015 ESPP automatically increased each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by 1% of the outstanding number of shares of our common stock on the immediately preceding December 31 or such lesser number of shares as determined by the administrator of the 2015 ESPP. Pursuant to this provision, 305,790, 297,891 and 237,448 additional shares were reserved for issuance under the 2015 ESPP on January 1, 2019, 2018 and 2017, respectively. As of January 1, 2019, total shares reserved for issuance under the 2015 ESPP were 1,145,095. Stock-based Compensation Stock Options Stock option activity is summarized as follows: Number of Outstanding Options Weighted Average Exercise Price Weighted Remaining Contractual Term Aggregate Intrinsic Value Outstanding as of December 31, 2017 4,617,059 $ 5.52 Granted 2,091,361 $ 2.35 Exercised (12,141 ) $ 1.10 Canceled/forfeited/expired (1,705,605 ) $ 4.72 Outstanding as of December 31, 2018 4,990,674 $ 4.47 6.97 $ 427 Options vested and expected to vest as of December 31, 2018 4,990,674 $ 4.47 6.97 $ 427 Options exercisable as of December 31, 2018 2,856,255 $ 5.52 5.62 $ 427 The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows: Years Ended December 31, 2018 2017 2016 Expected term (in years) 5.00 – 6.08 5.50 – 6.08 5.50 – 6.08 Risk-free interest rate 2.3% – 3.0% 1.9% – 2.1% 1.2% – 2.1% Expected volatility 87.9% – 98.4% 99.1% – 124.4% 80.7% – 84.5% Expected dividend yield 0.0 % 0.0 % 0.0 % The assumptions used in the Black-Scholes option pricing model to determine the fair value of the ESPP offering were as follows: Years Ended December 31, 2018 2017 2016 Expected term (in years) 0.50 0.50 0.50 Risk-free interest rate 1.4% – 2.1% 0.6% – 1.0% 0.4% – 0.6% Expected volatility 71.5% – 99.7% 74.5% – 115.2% 75.5% – 80.8% Expected dividend yield 0.0 % 0.0 % 0.0 % Expected term . The expected term represents the period of time that options are expected to be outstanding. Because we do not have sufficient history of exercise behavior, we determine the expected life assumption using the simplified method, which is an average of the contractual term of the option and its vesting period. Risk-free interest rate. We base the risk-free interest rate assumption on the U.S. Treasury’s rates for U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term of the award being valued. Expected volatility. The expected volatility assumption is based on volatilities of a peer group of similar companies whose share prices are publicly available. The peer group was developed based on companies in the biotechnology industry. Expected dividend yield. We base the expected dividend yield assumption on the fact that we have never paid cash dividends and have no present intention to pay cash dividends. Performance Options with Market Conditions In October 2015, we granted to our executives, employees and certain consultants performance options with a market condition to purchase up to an aggregate 169,402 shares of common stock at an exercise price of $10.24. Upon achievement of specified market condition by October 2017, such performance options shall begin to vest over four years in equal monthly installments, otherwise the options will be subject to forfeiture. The fair value of the performance options with market conditions is estimated on the date of the grant using a Monte Carlo simulation, based on the market price of the underlying common stock, expected performance measurement period, expected peer group stock price volatility and expected risk-free interest rate. The weighted average grant date fair value was $4.23. The performance options with market conditions grants are expensed using the accelerated attribution method over the requisite service period of 4.8 years regardless of whether the market condition is achieved or earned and vested. As of October 2017, the market condition for these performance options were not met and therefore were forfeited. In January 2016, we granted to our executives, employees and certain consultants performance options with a market condition to purchase up to an aggregate 396,960 shares of common stock at an exercise price of $9.13. Upon achievement of specified market conditions by January 2018, such performance options shall begin to vest over four years in equal monthly installments, otherwise the options will be subject to forfeiture. The fair value of the performance options with a market condition is estimated on the date of the grant using a Monte Carlo simulation, based on the market price of the underlying common stock, expected performance measurement period, expected peer group stock price volatility and expected risk-free interest rate. The weighted average grant date fair value was $1.93. The performance options with market conditions grants are expensed using the accelerated attribution method over the requisite service period of 5.1 years regardless of whether the market condition is achieved or earned and vested. As of January 2018, the market condition for these performance options were not met and therefore were forfeited. There were no performance options with a market condition granted during 2017 and 2018. Restricted Stock Units Occasionally, we grant restricted stock units to employees. Restricted stock unit activity is summarized as follows: Number of Outstanding Restricted Stock Units Weighted Average Grant Date Fair Value Balance as of December 31, 2017 49,300 $ 4.28 Granted 270,300 $ 0.85 Released (39,301 ) $ 4.53 Forfeited (63,599 ) $ 1.24 Balance as of December 31, 2018 216,700 $ 0.85 The allocation of stock-based compensation for all options, including performance options with market condition and restricted stock units is as follows (in thousands): Twelve Months Ended December 31, 2018 2017 2016 Research and development $ 1,216 $ 1,399 $ 1,876 General and administrative 2,215 5,385 $ 3,153 Total share-based compensation expense $ 3,431 $ 6,784 $ 5,029 The weighted–average grant date fair value per share of stock options granted by us, excluding performance options with market conditions, during the years ended December 31, 2018, 2017 and 2016 was $1.84, $2.85 and $3.34, respectively. The total grant date fair value of restricted stock units vested during the years ended December 31, 2018, 2017 and 2016 was $0.2 million, $0.1 million and $13,000, respectively. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2018, 2017 and 2016 was $6,000, $0.3 million and $34,000, respectively. The aggregate intrinsic value of restricted stock units released during the years ended December 31, 2018, 2017 and 2016 was $19,000, $0.1 million and $4,000. As of December 31, 2018, total unrecognized share-based compensation expense related to unvested stock options and restricted stock units was approximately $3.9 million and $0.1 million, respectively. These unrecognized costs for options and restricted stock units are expected to be recognized ratably over a weighted-average period of approximately 2.6 years and 1.4 years, respectively. During the fourth quarter of 2017, in connection with the change of status of our then-Chief Executive Officer to an advisor consulting role, we modified certain terms of outstanding options granted to the executive. We recorded $1.9 million of share-based compensation expense related to the modifications. We determined that vesting of the shares underlying the options will occur whether or not our then-Chief Executive Officer provides substantive service. In addition, in connection with the departure of our then-Chief Business Officer, we modified certain terms of outstanding options previously granted to the executive. As a result, we recorded $0.3 million in share-based compensation expense related to the modification. Warrants Warrants outstanding as of December 31, 2018 are as follows: Number Exercise Price Expiration Outstanding Per Share Date 6,488,205 $ 4.64 December 2019 2,006 $ 7.48 March 2021 14,913 $ 20.12 July 2023 95,542 $ 3.14 November 2023 41,666 $ 3.60 June 2024 40,376 $ 3.72 December 2024 6,682,708 Common Stock Reserved for Future Issuance Common stock reserved for future issuance is as follows: Years Ended December 31, 2018 2017 Class X Preferred Stock (if-converted to common stock) 11,429,760 11,429,760 Common stock warrants 6,682,708 6,682,708 Common stock options and awards outstanding 5,207,374 4,666,359 Shares available under the 2015 Plan 1,501,099 765,427 Shares available under the 2015 ESPP 839,305 583,819 25,660,246 24,128,073

Income Tax

Income Tax	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Tax	7. Income Tax Pretax earnings (loss) were generated by both domestic and foreign operations as follows (in thousands): Years Ended December 31, 2018 2017 2016 United States $ (34,021 ) $ (47,712 ) $ (57,096 ) Foreign (494 ) (495 ) (808 ) $ (34,515 ) $ (48,207 ) $ (57,904 ) A reconciliation of the expected statutory federal income tax provision to the actual income tax provision is summarized as follows (in thousands): Years Ended December 31, (1) 2018 2017 2016 Expected income taxes benefit at federal statutory rate $ (7,248 ) $ (16,390 ) $ (19,687 ) State income taxes, net of federal benefit (14 ) (13 ) — Permanent items and other 770 1,311 675 Research credits (1,222 ) (2,286 ) (6,800 ) Unrecognized tax benefits 489 914 2,720 Foreign rate differential 22 87 141 Change in tax rate (11 ) (25 ) — Tax cuts and Jobs Act — 27,933 — Change in valuation allowance 7,214 (11,531 ) 22,902 Income tax (benefit) expense $ — $ — $ (49 ) (1) For the years ended December 31, 2017 and 2016, the statutory tax rate was 34%. For the year ended December 31, 2018, as a result of the TJCA, the statutory tax rate was decreased to 21%. The TJCA was enacted on December 22, 2017. The TJCA reduces the US federal corporate tax rate from 35 percent to 21 percent, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred, and creates new taxes on certain foreign sourced earnings. At December 31, 2017, we made a reasonable estimate of the effects on their existing deferred tax balances. At December 31, 2017, we recognized a provisional amount of $27.9 million, which was included as a component of income tax expense from continuing operations offset with valuation allowances. As of December 31, 2018, we had completed our assessment of the impact of the TCJA and has reflected the impact in the current year. Deferred income taxes are provided for temporary differences in recognizing certain income and expense items for financial and tax reporting purposes. The deferred tax assets consisted primarily of the income tax benefits from net operating loss (NOLs) carryforwards, research and development credits and capitalized research and development expenses, along with other accruals and reserves. Valuation allowances of $66.9 million and $59.7 million as of December 31, 2018 and 2017, respectively, have been recorded to offset deferred tax assets as realization of such assets does not meet the more-likely-than-not threshold under ASC 740, Accounting for Income Taxes Significant components of our deferred tax assets are summarized as follows (in thousands): December 31, 2018 2017 Net operating loss carryforwards $ 32,997 $ 27,226 Capitalized research and development expenses 17,279 16,218 Research credits and other state credits 11,962 11,229 Intangible assets 2,024 2,210 Reserve and accruals 2,667 2,843 Valuation allowance (66,929 ) (59,726 ) Net deferred tax assets $ — $ — As of December 31, 2018, we had federal net operating loss carryforwards of approximately $139.5 million, with $27.0 million of net operating losses generated after December 31, 2017 carrying forward indefinitely and $112.5 million of net operating losses that will begin to expire in 2025. We had state net operating loss carryforwards of approximately $148.2 million, and foreign net operating loss carryforwards of $7.5 million. The state net operating losses will begin to expire in 2021. The foreign net operating losses carry over indefinitely. As of December 31, 2018, we had federal and state research and development credit carryforwards of approximately $4.4 million and $3.8 million, respectively, which begin to expire in 2026 for federal purposes and carry over indefinitely for state purposes. We had $12.5 million of federal Orphan Drug Credits as of December 31, 2018, which will begin to expire in 2035. Utilization of the domestic NOLs and research and development credit carryforwards may be subject to a substantial annual limitation due to ownership change limitations that may have occurred or that could occur in the future, as required by Section 382 and 383 of the Internal Revenue Code of 1986, as amended (the Code), as well as similar state and foreign provisions. These ownership changes may limit the amount of NOLs and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. In general, an “ownership change” as defined by Section 382 of the Code results from a transaction or series of transactions over a three-year period resulting in an ownership change of more than 50 percentage points of the outstanding stock of a company by certain stockholders. Since the Company’s formation, we raised capital through the issuance of capital stock on several occasions which on its own or combined with the purchasing stockholders’ subsequent disposition of those shares, has resulted in such an ownership change, and could result in an ownership change in the future. Upon the occurrence of an ownership change under Section 382 as outlined above, utilization of the NOLs and research and development credit carryforwards become subject to an annual limitation under Section 382 of the Code, which is determined by first multiplying the value of our stock at the time of the ownership change by the applicable long-term, tax-exempt rate, which could be subject to additional adjustments. Any limitation may result in expiration of a portion of the NOLs or research and development credit carryforwards before utilization. We completed analyses through December 31, 2017, and are in the process of analyzing the impact to our NOLs and research and development tax credit carryforwards. During 2018, we decided to postpone completing another Section 382 study until we start utilizing our NOLs. Due to the existence of the valuation allowance, any impact to the NOLs and research and development tax credit carryforwards from Section 382 analysis will be offset by a corresponding adjustment to valuation allowance, resulting in no tax provision impact. Ownership changes that may have occurred subsequent to December 31, 2017, and future ownership changes, including any ownership change resulting from this offering, may further limit our ability to utilize its remaining tax attributes. We recognize a tax benefit from an uncertain tax position when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, based on the technical merits. Income tax positions must meet a more-likely-than-not recognition threshold to be recognized. Our practice is to recognize interest and penalties related to income tax matters in income tax expense. We had no accrual for interest and penalties on our balance sheet and had not recognized interest or penalties in the consolidated statements of operations for the years ended December 31, 2018, 2017 and 2016. Due to the existence of the valuation allowance, future changes in unrecognized tax benefits will not impact our effective tax rate. Uncertain tax positions are evaluated based upon the facts and circumstances that exist at each reporting period. Subsequent changes in judgment based upon new information may lead to changes in recognition, derecognition, and measurement. Adjustments may result, for example, upon resolution of an issue with the taxing authorities, or expiration of a statute of limitations barring an assessment for an issue. We are subject to taxation in the United States, Hong Kong and state jurisdictions. Our tax years from inception are subject to examination by the United States, Hong Kong and California authorities due to the carry forward of unutilized NOLs and research and development credits. The activity related to our unrecognized tax benefits is summarized as follows (in thousands): December 31, 2018 2017 2016 Balance as of beginning of year $ 16,558 $ 13,000 $ 5,033 Increase (decrease) related to prior year tax positions 2 (189 ) 1,890 Increase related to current year tax positions 3,083 3,747 6,077 Balance as of end of year $ 19,643 $ 16,558 $ 13,000 We do not anticipate that the amount of unrecognized tax benefits as of December 31, 2018 will change within the next twelve months.

Employee Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Employee Benefits	8. Employee Benefits 401(k) Plan We maintain a defined contribution 401(k) plan available to eligible employees. Employee contributions are voluntary and are determined on an individual basis, limited to the maximum amount allowable under federal tax regulations. In April 2015, our Board of Directors approved a policy, beginning on June 1, 2015, to match employee contributions equal to 50% of the participant’s contribution of up to a maximum of 6% of the participant’s annual salary. We made discretionary contributions totaling $0.2 million during each of the years ended December 31, 2018, 2017 and 2016, respectively.

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Data [Abstract]
Quarterly Financial Data (Unaudited)	9. Quarterly Financial Data (Unaudited) The following financial information reflects all normal recurring adjustments, which are, in our opinion, necessary for a fair statement of the results of the interim periods. Summarized quarterly data for 2018 and 2017 are as follows (in thousands, except per share data): For the Quarters Ended March 31 June 30 September 30 December 31 2018: Operating expenses $ 10,220 $ 9,960 $ 6,677 $ 5,963 Net loss (10,667 ) (10,412 ) (7,114 ) (6,322 ) Basic and diluted net loss per share $ (0.36 ) $ (0.35 ) $ (0.24 ) (0.21 ) 2017: Operating expenses $ 13,211 $ 11,907 $ 10,827 $ 11,200 Net loss (13,405 ) (12,138 ) (11,190 ) (11,474 ) Basic and diluted net loss per share $ (0.56 ) $ (0.51 ) $ (0.43 ) (0.39 ) Net loss per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per-share calculations will not necessarily equal the annual per share calculation.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	10. Subsequent Events In January 2019, the VGO Fund converted 641,991 shares of its Preferred Stock into 3,209,955 shares of common stock. After the conversion, VGO Fund owns 9.50% of our common stock. In March 2019, we entered into a research collaboration and option agreement with CSL Behring for the development of product candidates derived from up to four tRNA synthetases from our preclinical pipeline. Under the terms of the collaboration, CSL Behring will fund all research and development activities related to the development of the applicable product candidates for the duration of the collaboration. CSL Behring 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 Behring’s determination to continue development. In addition, aTyr will grant CSL Behring an option to negotiate licenses for worldwide rights to each 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. As of March 22, 2019, we issued and sold 2,580,839 shares of common stock at a weighted average price of $0.52 per share through our ATM Offering Program and received total net proceeds of $1.3 million.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Organization and Business	Organization and Business We were incorporated in the state of Delaware on September 8, 2005. We are focused on the discovery and clinical development of innovative medicines based on novel immunological pathways. In May 2018, we implemented a corporate restructuring and program prioritization plan (Restructuring Plan) to streamline our operations and concentrate development efforts on the advancement of our therapeutic candidate, ATYR1923. In connection with the Restructuring Plan, we reduced our workforce by approximately 30% to 42 full-time employees. We completed the workforce reduction in June 2018.
Principles of Consolidation	Principles of Consolidation Our consolidated financial statements include our accounts, our 98% majority-owned subsidiary in Hong Kong, Pangu BioPharma Limited (Pangu BioPharma). All intercompany transactions and balances are eliminated in consolidation.
Use of Estimates	Use of Estimates Our consolidated financial statements are prepared in accordance with generally accepted accounting principles (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 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 expenses. 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.
Segment Reporting	Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. We view our operations and manage our business in one operating segment.
Reclassifications	Reclassifications Certain reclassifications have been made to prior year amounts to conform to the current year presentation. The reclassifications were not material to the consolidated financial statements.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents consist primarily of readily available checking, money market accounts and money market funds. We consider all highly liquid investments that mature in three months or less when purchased to be cash equivalents.
Investment Securities	Investment Securities Investment securities primarily consist of investment grade corporate debt securities, asset-backed securities, commercial paper and United States Treasury securities. We classify all investment securities as available-for-sale. Investment securities are carried at fair value, with the unrealized gains and losses, if any, reported as a component of other comprehensive income (loss) in stockholders’ equity until realized. Realized gains and losses from the sale of investment securities, if any, are determined on a specific identification basis. A decline in the market value of any investment security below cost that is determined to be other than temporary will result in an impairment charge to earnings and a new cost basis for the security is established. No such impairment charges were recorded for any period presented. Premiums and discounts are amortized or accreted over the life of the related security as an adjustment to yield using the straight-line method and are included in interest income. Interest income is recognized when earned. As of December 31, 2018, we held an aggregate total of $26.6 million of investment securities which consisted of corporate debt securities, asset-backed securities, and commercial paper all of which will mature in less than one year and there was approximately $10,000 difference between the amortized cost and fair value of these investment securities. As of December 31, 2017, we held $64.0 million of corporate debt securities, asset-backed securities and United States Treasury securities, all of which mature in less than one year, and there was $0.1 million difference between the amortized cost and fair value of these investment securities.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject us to significant concentration of credit risk consist primarily of cash, cash equivalents and investment securities. We have established guidelines regarding diversification of investments and their maturities, which are designed to maintain principal and maximize liquidity. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have not experienced any losses in such accounts and we believe that we are not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held.
Property and Equipment	Property and Equipment Property and equipment are stated at cost and depreciated on a straight-line basis over the estimated useful life of the related assets (generally three to seven years). Leasehold improvements are stated at cost and amortized on a straight-line basis over the lesser of the remaining term of the related lease or the estimated useful life of the leasehold improvements. Repairs and maintenance costs are charged to expense as incurred .
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist primarily of property and equipment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. While our current and historical operating losses are indicators of impairment, we believe that future cash flows to be received support the carrying value of our long-lived assets and, accordingly, have not recognized any impairment losses since inception.
Accrued Expenses	Accrued Expenses As part of the process of preparing our consolidated financial statements, we are required to estimate accrued expenses, including accrued research and development expenses for fees paid to investigative sites and clinical research organizations (CROs) in connection with clinical trials; service providers in connection with preclinical development activities; service providers related to product manufacturing; and other professional services. The accrual process involves reviewing open contracts and purchase orders, communicating with our personnel to identify services that have been performed on our behalf and estimating the level of service performed and the associated cost incurred for the service when we have not yet been invoiced or otherwise notified of the actual cost. We make estimates of accrued expenses as of each balance sheet date in our consolidated financial statements based on facts and circumstances known to us at that time. Although we do not expect the estimates to be materially different from amounts actually incurred, if the estimates of the status and timing of services performed differs from the actual status and timing of services performed, we may report amounts that are too high or too low in any particular period. Historically, our estimated accrued liabilities have approximated actual expenses incurred. Subsequent changes in estimates may result in a material change in our accruals.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development costs include: salaries and employee-related expenses, including stock-based compensation and benefits for personnel in research and product development functions; costs associated with conducting our preclinical, development and regulatory activities, including fees paid to third-party professional consultants, service providers and our scientific, therapeutic and clinical advisors; costs to acquire, develop and manufacture preclinical study and clinical trial materials; costs incurred under clinical trial agreements with CROs and investigative sites; costs for laboratory supplies; payments related to licensed products and technologies; allocated facilities and information technology costs; and depreciation.
Patent Costs	Patent Costs Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred since recoverability of such expenditures is uncertain.
Stock-Based Compensation	Stock-Based Compensation Stock-based compensation expense represents the cost of the grant date fair value of employee stock option and restricted stock unit grants recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis. We recognize forfeitures as they occur as a reduction of expense. For stock option grants with performance-based milestones, the expense is recorded over the service period after the achievement of the milestone is probable or the performance condition is achieved. For stock option grants with market-based conditions, the expense is recorded using the accelerated attribution method over the requisite service period for each vesting tranche. We account for stock options granted to non-employees using the fair value approach. These option grants are subject to periodic revaluation over their vesting terms. We estimate the fair value of employee and non-employee stock option grants using the Black-Scholes option pricing model. We estimate the fair value of the market-based stock option grants using a Monte Carlo simulation. The fair value of restricted stock units is determined by the closing price as of the grant date.
Income Taxes	Income Taxes We account for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized as income in the period that includes the enactment date. We recognize net deferred tax assets to the extent that we believe these assets are more likely than not to be realized. In making such a determination, we consider all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies and results of recent operations. If we determine that we would be able to realize the deferred tax assets in the future in excess of their net recorded amount, we would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. We record uncertain tax positions on the basis of a two-step process whereby (1) we determine whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, we recognize the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority. We recognize interest and penalties related to unrecognized tax benefits within income tax expense. Any accrued interest and penalties are included within the related tax liability. On December 22, 2017, the U.S. enacted the Tax Cuts and Jobs Act (TCJA), which among other provisions, reduced the U.S. corporate tax rate from 35% to 21%, effective January 1, 2018. Due to the timing of the enactment and the complexity involved in applying the provisions of the TCJA, we made reasonable estimates of the effects and recorded provisional amounts, offset with valuation allowances, in its financial statements as of December 31, 2017. At December 31, 2018, we had completed our assessment of the impact of the TCJA and has reflected the impact in the current year. At December 31, 2018, there were no material changes from the provisional amounts recorded for the year ended December 31, 2017.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing the net loss attributable to common stockholders 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, 3,685 and 25,984 shares subject to repurchase from the weighted average number of common shares outstanding for the years ended December 31, 2018, 2017 and 2016, respectively. Diluted net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders 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): Year Ended December 31, 2018 2017 2016 Class X convertible preferred stock (if-converted) 11,429,760 11,429,760 — Warrants for common stock 6,682,708 6,682,708 121,512 Common stock options and restricted stock units 5,207,374 4,666,359 4,091,701 Employee stock purchase plan 22,548 31,086 36,836 23,342,390 22,809,913 4,250,049 The following table summarizes our net loss per share (in thousands, except per share data): Year Ended December 31, 2018 2017 2016 Numerator: Consolidated net loss $ (34,515 ) $ (48,207 ) $ (57,855 ) Denominator: Weighted average common shares outstanding 29,957,102 25,803,538 23,707,003 Weighted average common shares subject to repurchase — (3,685 ) (25,984 ) Weighted average common shares outstanding - basic and diluted 29,957,102 25,799,853 23,681,019 Net loss per share - basic and diluted $ (1.15 ) $ (1.87 ) $ (2.44 )
Convertible Preferred Stock	Convertible Preferred Stock We apply the relevant accounting standards to distinguish liabilities from equity when assessing the classification and measurement of preferred stock. Preferred shares subject to mandatory redemptions are considered liabilities and measured at fair value. Conditionally redeemable preferred shares are considered temporary equity. All other preferred shares are considered as stockholders’ equity. None of our outstanding preferred stock has redemption features.
Derivative Financial Instruments	Derivative Financial Instruments We do not use derivative instruments to hedge exposures to cash flow, market, or foreign currency risks. We evaluate all of our financial instruments, including warrants, to determine if such instruments are derivatives or contain features that qualify as embedded derivatives. We generally use the Black-Scholes option-pricing model to value the derivative instruments at inception and subsequent valuation dates. The classification of derivative instruments, including whether such instruments should be recorded as liabilities or as equity, is re-assessed at the end of each reporting period.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718) 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 for public business entities.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 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): Year Ended December 31, 2018 2017 2016 Class X convertible preferred stock (if-converted) 11,429,760 11,429,760 — Warrants for common stock 6,682,708 6,682,708 121,512 Common stock options and restricted stock units 5,207,374 4,666,359 4,091,701 Employee stock purchase plan 22,548 31,086 36,836 23,342,390 22,809,913 4,250,049
Summary of Net Loss Per Share	The following table summarizes our net loss per share (in thousands, except per share data): Year Ended December 31, 2018 2017 2016 Numerator: Consolidated net loss $ (34,515 ) $ (48,207 ) $ (57,855 ) Denominator: Weighted average common shares outstanding 29,957,102 25,803,538 23,707,003 Weighted average common shares subject to repurchase — (3,685 ) (25,984 ) Weighted average common shares outstanding - basic and diluted 29,957,102 25,799,853 23,681,019 Net loss per share - basic and diluted $ (1.15 ) $ (1.87 ) $ (2.44 )

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis	Assets and liabilities 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 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 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 December 31, 2018 and 2017, available-for-sale investments are detailed as follows (in thousands): 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 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

Balance Sheet Details (Tables)

Balance Sheet Details (Tables)	12 Months Ended
	Dec. 31, 2018
Balance Sheet Details [Abstract]
Summary of Property and Equipment	Property and equipment consist of the following (in thousands): December 31, 2018 2017 Computer and office equipment $ 543 $ 425 Scientific and laboratory equipment 5,631 5,494 Tenant improvements 1,703 1,706 7,877 7,625 Less accumulated depreciation and amortization (6,024 ) (5,345 ) $ 1,853 $ 2,280
Summary of Accrued Expenses	Accrued expenses consist of the following (in thousands): December 31, 2018 2017 Accrued salaries, wages and benefits $ 1,309 $ 1,920 Other accrued expenses (1) 717 1,183 $ 2,026 $ 3,103 (1) Other accrued expenses include expenses for clinical research organizations and contract manufacturing organizations.

Debt, Commitments and Conting_2

Debt, Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Instruments [Abstract]
Schedule of Term Loans and Unamortized Discount Balances	Term loans and unamortized discount balances are as follows (in thousands): December 31, December 31, 2018 2017 Debt balance $ 15,333 $ 20,000 Less debt issuance costs and discount (115 ) (345 ) Long-term debt, net of issuance costs and discount 15,218 19,655 Less current portion of long-term debt (8,000 ) (5,333 ) Add accrual of final payment 1,045 397 Long-term debt, net of current portion and issuance costs and discount $ 8,263 $ 14,719 Current portion of long-term debt $ 8,000 $ 5,333 Less current portion of debt issuance costs and discount (233 ) (321 ) Current portion of long-term debt, net of issuance costs and discount $ 7,767 $ 5,012
Schedule of Future Principal Payments for Term Loans	Future principal payments for the Term Loans are as follows (in thousands): December 31, 2018 2019 $ 8,000 2020 7,333 $ 15,333
Schedule of Future Minimum Payments under Non-cancelable Operating Lease	As of December 31, 2018, future minimum payments under the non-cancelable operating lease are as follows (in thousands): Operating Lease 2019 $ 812 2020 1,002 2021 1,031 2022 1,062 Thereafter 403 $ 4,310

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Summary of Stock Option Activity	Stock option activity is summarized as follows: Number of Outstanding Options Weighted Average Exercise Price Weighted Remaining Contractual Term Aggregate Intrinsic Value Outstanding as of December 31, 2017 4,617,059 $ 5.52 Granted 2,091,361 $ 2.35 Exercised (12,141 ) $ 1.10 Canceled/forfeited/expired (1,705,605 ) $ 4.72 Outstanding as of December 31, 2018 4,990,674 $ 4.47 6.97 $ 427 Options vested and expected to vest as of December 31, 2018 4,990,674 $ 4.47 6.97 $ 427 Options exercisable as of December 31, 2018 2,856,255 $ 5.52 5.62 $ 427
Summary of Assumptions Used to Determine Fair Value of Employee Stock Purchase Plan	The assumptions used in the Black-Scholes option pricing model to determine the fair value of the ESPP offering were as follows: Years Ended December 31, 2018 2017 2016 Expected term (in years) 0.50 0.50 0.50 Risk-free interest rate 1.4% – 2.1% 0.6% – 1.0% 0.4% – 0.6% Expected volatility 71.5% – 99.7% 74.5% – 115.2% 75.5% – 80.8% Expected dividend yield 0.0 % 0.0 % 0.0 %
Schedule of Restricted Stock Units Activity	Occasionally, we grant restricted stock units to employees. Restricted stock unit activity is summarized as follows: Number of Outstanding Restricted Stock Units Weighted Average Grant Date Fair Value Balance as of December 31, 2017 49,300 $ 4.28 Granted 270,300 $ 0.85 Released (39,301 ) $ 4.53 Forfeited (63,599 ) $ 1.24 Balance as of December 31, 2018 216,700 $ 0.85
Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition and Restricted Stock Units	The allocation of stock-based compensation for all options, including performance options with market condition and restricted stock units is as follows (in thousands): Twelve Months Ended December 31, 2018 2017 2016 Research and development $ 1,216 $ 1,399 $ 1,876 General and administrative 2,215 5,385 $ 3,153 Total share-based compensation expense $ 3,431 $ 6,784 $ 5,029
Summary of Warrants Outstanding	Warrants outstanding as of December 31, 2018 are as follows: Number Exercise Price Expiration Outstanding Per Share Date 6,488,205 $ 4.64 December 2019 2,006 $ 7.48 March 2021 14,913 $ 20.12 July 2023 95,542 $ 3.14 November 2023 41,666 $ 3.60 June 2024 40,376 $ 3.72 December 2024 6,682,708
Summary of Common Stock Reserved for Future Issuance	Common stock reserved for future issuance is as follows: Years Ended December 31, 2018 2017 Class X Preferred Stock (if-converted to common stock) 11,429,760 11,429,760 Common stock warrants 6,682,708 6,682,708 Common stock options and awards outstanding 5,207,374 4,666,359 Shares available under the 2015 Plan 1,501,099 765,427 Shares available under the 2015 ESPP 839,305 583,819 25,660,246 24,128,073
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: Years Ended December 31, 2018 2017 2016 Expected term (in years) 5.00 – 6.08 5.50 – 6.08 5.50 – 6.08 Risk-free interest rate 2.3% – 3.0% 1.9% – 2.1% 1.2% – 2.1% Expected volatility 87.9% – 98.4% 99.1% – 124.4% 80.7% – 84.5% Expected dividend yield 0.0 % 0.0 % 0.0 %

Income Tax (Tables)

Income Tax (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Pretax Earnings (Loss) for Domestic and Foreign Operations	Pretax earnings (loss) were generated by both domestic and foreign operations as follows (in thousands): Years Ended December 31, 2018 2017 2016 United States $ (34,021 ) $ (47,712 ) $ (57,096 ) Foreign (494 ) (495 ) (808 ) $ (34,515 ) $ (48,207 ) $ (57,904 )
Schedule of Reconciliations of the Expected Statutory Federal Income Tax	A reconciliation of the expected statutory federal income tax provision to the actual income tax provision is summarized as follows (in thousands): Years Ended December 31, (1) 2018 2017 2016 Expected income taxes benefit at federal statutory rate $ (7,248 ) $ (16,390 ) $ (19,687 ) State income taxes, net of federal benefit (14 ) (13 ) — Permanent items and other 770 1,311 675 Research credits (1,222 ) (2,286 ) (6,800 ) Unrecognized tax benefits 489 914 2,720 Foreign rate differential 22 87 141 Change in tax rate (11 ) (25 ) — Tax cuts and Jobs Act — 27,933 — Change in valuation allowance 7,214 (11,531 ) 22,902 Income tax (benefit) expense $ — $ — $ (49 ) (1) For the years ended December 31, 2017 and 2016, the statutory tax rate was 34%. For the year ended December 31, 2018, as a result of the TJCA, the statutory tax rate was decreased to 21%.
Schedule of Significant Components of Deferred Tax Assets	Significant components of our deferred tax assets are summarized as follows (in thousands): December 31, 2018 2017 Net operating loss carryforwards $ 32,997 $ 27,226 Capitalized research and development expenses 17,279 16,218 Research credits and other state credits 11,962 11,229 Intangible assets 2,024 2,210 Reserve and accruals 2,667 2,843 Valuation allowance (66,929 ) (59,726 ) Net deferred tax assets $ — $ —
Schedule of Activity Related to Unrecognized Tax Benefits	We are subject to taxation in the United States, Hong Kong and state jurisdictions. Our tax years from inception are subject to examination by the United States, Hong Kong and California authorities due to the carry forward of unutilized NOLs and research and development credits. The activity related to our unrecognized tax benefits is summarized as follows (in thousands): December 31, 2018 2017 2016 Balance as of beginning of year $ 16,558 $ 13,000 $ 5,033 Increase (decrease) related to prior year tax positions 2 (189 ) 1,890 Increase related to current year tax positions 3,083 3,747 6,077 Balance as of end of year $ 19,643 $ 16,558 $ 13,000

Quarterly Financial Data (Una_2

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Data [Abstract]
Summary of Quarterly Financial Data	The following financial information reflects all normal recurring adjustments, which are, in our opinion, necessary for a fair statement of the results of the interim periods. Summarized quarterly data for 2018 and 2017 are as follows (in thousands, except per share data): For the Quarters Ended March 31 June 30 September 30 December 31 2018: Operating expenses $ 10,220 $ 9,960 $ 6,677 $ 5,963 Net loss (10,667 ) (10,412 ) (7,114 ) (6,322 ) Basic and diluted net loss per share $ (0.36 ) $ (0.35 ) $ (0.24 ) (0.21 ) 2017: Operating expenses $ 13,211 $ 11,907 $ 10,827 $ 11,200 Net loss (13,405 ) (12,138 ) (11,190 ) (11,474 ) Basic and diluted net loss per share $ (0.56 ) $ (0.51 ) $ (0.43 ) (0.39 )

Organization, Business and Ba_2

Organization, Business and Basis of Presentation - Additional Information (Detail) $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2018USD ($)Employee	Dec. 31, 2018Segment
Description Of Business [Line Items]
Number of operating segment | Segment		1
Pangu BioPharma [Member] | Hong Kong [Member]
Description Of Business [Line Items]
Majority-owned subsidiary percentage		98.00%
ATYR1923 Manufacturer [Member] | Restructuring Plan [Member]
Description Of Business [Line Items]
Restructuring plan, percentage of reduction in total workforce	30.00%
Number of full-time employees | Employee	42
Restructuring plan, description	In May 2018, we implemented a corporate restructuring and program prioritization plan (Restructuring Plan) to streamline our operations and concentrate development efforts on the advancement of our therapeutic candidate, ATYR1923. In connection with the Restructuring Plan, we reduced our workforce by approximately 30% to 42 full-time employees. We completed the workforce reduction in June 2018.
ATYR1923 Manufacturer [Member] | Restructuring Plan [Member] | Employee Severance and Other Related Termination Benefits [Member]
Description Of Business [Line Items]
Restructuring charges	$ 0.9
ATYR1923 Manufacturer [Member] | Restructuring Plan [Member] | One-time Termination Benefits [Member]
Description Of Business [Line Items]
Non-cash stock-based compensation charges	$ 0.4

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Significant Accounting Policies [Line Items]
Impairment charges on investment securities	$ 0
Aggregate value of investment securities	26,600,000	$ 64,000,000
Adjustment between amortized cost and fair value of investment securities	10,000	$ 100,000
Impairment of long lived assets	$ 0
U.S. federal corporate tax rate	21.00%	34.00%	34.00%
Weighted average shares subject to repurchase	0	3,685	25,984
Preferred stock redemption value	$ 0
Maximum [Member]
Significant Accounting Policies [Line Items]
Maturity period of investment securities	1 year	1 year
Estimated useful life of property and equipment	7 years
U.S. federal corporate tax rate		35.00%
Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment	3 years
Percentage of tax benefit to be realized upon settlement	50.00%

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	23,342,390	22,809,913	4,250,049
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	11,429,760
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	5,207,374	4,666,359	4,091,701
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	6,682,708	121,512
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	22,548	31,086	36,836

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Summary of Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Consolidated net loss	$ (6,322)	$ (7,114)	$ (10,412)	$ (10,667)	$ (11,474)	$ (11,190)	$ (12,138)	$ (13,405)	$ (34,515)	$ (48,207)	$ (57,855)
Denominator:
Weighted average common shares outstanding									29,957,102	25,803,538	23,707,003
Weighted average common shares subject to repurchase									0	(3,685)	(25,984)
Weighted average common shares outstanding - basic and diluted									29,957,102	25,799,853	23,681,019
Net loss per share - basic and diluted	$ (0.21)	$ (0.24)	$ (0.35)	$ (0.36)	$ (0.39)	$ (0.43)	$ (0.51)	$ (0.56)	$ (1.15)	$ (1.87)	$ (2.44)

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 16,019	$ 9,070
Total assets measured at fair value	42,602	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	26,583	64,028
Available-for-sale [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	7,773	6,497
Available-for-sale [Member] | Short-term Investments [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	6,144	21,943
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	12,666	18,260
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		17,328
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	16,019	9,070
Total assets measured at fair value	16,019	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		17,328
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		17,328
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	26,583	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	26,583	46,700
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	7,773	6,497
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Short-term Investments [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	6,144	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	$ 12,666	$ 18,260

Fair Value Measurements - Sch_2

Fair Value Measurements - Schedule of Available-for-sale Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Market Value	$ 26,600	$ 64,000
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	26,593	64,098
Gross Unrealized Losses	(10)	(70)
Market Value	26,583	64,028
Short-term Investments [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	6,144	21,943
Market Value	6,144	21,943
Short-term Investments [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	7,777	6,501
Gross Unrealized Losses	(4)	(4)
Market Value	7,773	6,497
Short-term Investments [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	12,672	18,286
Gross Unrealized Losses	(6)	(26)
Market Value	$ 12,666	18,260
Short-term Investments [Member] | United States Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost		17,368
Gross Unrealized Losses		(40)
Market Value		$ 17,328

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
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 contractual maturity period	1 year

Balance Sheet Details - Summary

Balance Sheet Details - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, Gross	$ 7,877	$ 7,625
Less accumulated depreciation and amortization	(6,024)	(5,345)
Property and equipment, Net	1,853	2,280
Computer and Office Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	543	425
Scientific and Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	5,631	5,494
Tenant Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	$ 1,703	$ 1,706

Balance Sheet Details - Additio

Balance Sheet Details - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation expense	$ 746	$ 713	$ 900

Balance Sheet Details - Summa_2

Balance Sheet Details - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Accrued salaries, wages and benefits	$ 1,309	$ 1,920
Other accrued expenses	717	1,183
Accrued expenses	$ 2,026	$ 3,103

Debt, Commitments and Conting_3

Debt, Commitments and Contingencies - Additional Information (Detail)	1 Months Ended					12 Months Ended
	Jul. 30, 2018ft²	Dec. 31, 2017USD ($)$ / sharesshares	Nov. 30, 2017USD ($)	Jun. 30, 2017USD ($)$ / sharesshares	Nov. 30, 2016USD ($)Tranche$ / sharesshares	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)	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 facility | ft²	20,508								24,494
Noncancelable operating leases termination term	2023-05
Rent expense						$ 1,000,000	$ 900,000	$ 500,000
Research and development expenses						20,385,000	30,067,000	42,846,000
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						$ 500,000
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						$ 1,700,000	1,800,000	$ 1,600,000
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						15,300,000
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					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					$ 5,000,000
Loan and security agreement funded date		Dec. 31, 2017
Exercise price of warrant per share | $ / shares		$ 3.72					$ 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					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. Accordingly, we started paying the Term Loans in June 2018.
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	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt, net of current portion and issuance costs and discount	$ 8,263	$ 14,719
Current portion of long-term debt, net of issuance costs and discount	7,767	5,012
Term Loans [Member]
Debt Instrument [Line Items]
Debt balance	15,333	20,000
Less debt issuance costs and discount	(115)	(345)
Long-term debt, net of issuance costs and discount	15,218	19,655
Less current portion of long-term debt	(8,000)	(5,333)
Add accrual of final payment	1,045	397
Long-term debt, net of current portion and issuance costs and discount	8,263	14,719
Current portion of long-term debt	8,000	5,333
Less current portion of debt issuance costs and discount	(233)	(321)
Current portion of long-term debt, net of issuance costs and discount	$ 7,767	$ 5,012

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	Dec. 31, 2018USD ($)
Debt Instrument [Line Items]
2019	$ 8,000
2020	7,333
Long-term debt, net of issuance costs and discount	$ 15,333

Debt, Commitments and Conting_6

Debt, Commitments and Contingencies - Schedule of Future Minimum Payments under Non-cancelable Operating Lease (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Leases Operating [Abstract]
2019	$ 812
2020	1,002
2021	1,031
2022	1,062
Thereafter	403
Operating Lease, total	$ 4,310

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail)

Mar. 22, 2019USD ($)$ / sharesshares

Aug. 31, 2017USD ($)Day$ / sharesshares

Jul. 31, 2018$ / sharesshares

Sep. 30, 2016$ / sharesshares

Jan. 31, 2016$ / sharesshares

Oct. 31, 2015$ / sharesshares

Apr. 30, 2015shares

Dec. 31, 2017USD ($)$ / sharesshares

Dec. 31, 2018USD ($)$ / sharesshares

Dec. 31, 2017USD ($)$ / sharesshares

Dec. 31, 2016USD ($)$ / shares

Jan. 31, 2019shares

Jan. 01, 2019shares

Jan. 01, 2018shares

Jan. 01, 2017shares

Jun. 30, 2016USD ($)

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Common stock, par value | $ / shares

$ 0.001

$ 0.001

$ 0.001

Preferred stock, par value | $ / shares

$ 0.001

$ 0.001

$ 0.001

Net proceeds from transaction, after giving effect to cost related to placement | $

$ 42,477,000

Registration Statement filing date

2016-06

Net proceeds from issuance of common stock | $

$ 14,000

$ 186,000

$ 20,000

Number of common stock shares reserved for issuance

24,128,073

25,660,246

24,128,073

Stock options granted

2,091,361

Weighted average exercise price, granted | $ / shares

$ 2.35

Weighted average grant date fair value of options granted | $ / shares

$ 1.84

$ 2.85

$ 3.34

Total grant date fair value of restricted stock units vested | $

$ 200,000

$ 100,000

$ 13,000

Aggregate intrinsic value of stock options exercised | $

6,000

300,000

34,000

Aggregate intrinsic value of restricted stock units | $

$ 100,000

19,000

$ 100,000

$ 4,000

Unrecognized share based compensation expense related to unvested stock options | $

3,900,000

Unrecognized share based compensation expense related to restricted stock units | $

$ 100,000

Employee Stock Option [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Number of common stock shares reserved for issuance

4,666,359

5,207,374

4,666,359

Unrecognized cost expected to be recognized over a weighted average period

2 years 7 months 6 days

Restricted Stock Unit [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Unrecognized cost expected to be recognized over a weighted average period

1 year 4 months 24 days

Chief Executive Officer [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Stock compensation expense | $

$ 1,900,000

Chief Business Officer [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Stock compensation expense | $

$ 300,000

2014 Plan [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Maximum term for stock option plan grant

10 years

Options vesting period

4 years

2015 Stock Option and Incentive Plan [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Maximum term for stock option plan grant

10 years

Options vesting period

4 years

Options vesting period, description

For an initial grant to an employee, 25% of the options generally vest on the first anniversary of the original vesting date, with the balance vesting monthly over the remaining three years. For subsequent grants to an employee, the options generally vest monthly over a four-year term.

2015 Stock Option and Incentive Plan [Member] | One Year Anniversary [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Option vesting percentage

25.00%

2015 Employee Stock Purchase Plan [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Number of common stock shares reserved for issuance

583,819

839,305

583,819

Maximum [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Amount of offering, issuance and sale covered in base prospectus | $

$ 150,000,000

Common Stock [Member] | Senior Vice President [Member] | Inducement Pool Non-Qualified Option [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Stock options granted

145,000

0

Weighted average exercise price, granted | $ / shares

$ 3.29

Vested shares exercisable period from termination date

90 days

Common Stock [Member] | Chief Financial Officer [Member] | Inducement Pool Non-Qualified Option [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Options vesting period

4 years

Stock options granted

200,000

Weighted average exercise price, granted | $ / shares

$ 0.82

Common Stock [Member] | Executives, Employees and Consultants [Member] | Performance Options with Market Condition [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Options vesting period

4 years

4 years

Stock options granted

396,960

169,402

0

0

Weighted average exercise price, granted | $ / shares

$ 9.13

$ 10.24

Achievement of specified market condition year and month

2018-01

2017-10

Weighted average grant date fair value of options granted | $ / shares

$ 1.93

$ 4.23

Requisite service period

5 years 1 month 6 days

4 years 9 months 18 days

Common Stock [Member] | One Year Anniversary [Member] | Chief Financial Officer [Member] | Inducement Pool Non-Qualified Option [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Options vesting period

1 year

Option vesting percentage

25.00%

Common Stock [Member] | Monthly Basis Over Three Years [Member] | Chief Financial Officer [Member] | Inducement Pool Non-Qualified Option [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Options vesting period

3 years

Option vesting percentage

75.00%

Class X Convertible Preferred Stock [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Preferred stock, shares issued

2,285,952

2,285,952

2,285,952

VGO Fund [Member] | Subsequent Event [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Convertible preferred stock, common stock issued upon conversion

3,209,955

Maximum ownership percentage prohibiting conversion

9.50%

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]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Net proceeds from issuance of common stock | $

$ 400,000

ATM Offering Program [Member] | Subsequent Event [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Net proceeds from issuance of common stock | $

$ 1,300,000

ATM Offering Program [Member] | Common Stock [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Shares issued during the period

696,067

Shares issued, price per share | $ / shares

$ 0.71

ATM Offering Program [Member] | Common Stock [Member] | Subsequent Event [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Shares issued during the period

2,580,839

Shares issued, price per share | $ / shares

$ 0.52

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]

Amount of offering, issuance and sale covered in sales agreement prospectus | $

$ 20,000,000

IPO [Member] | 2015 Stock Option and Incentive Plan [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Effective date of plan

May 6, 2015

Number of common stock shares reserved for issuance

1,574,566

Stock option grants description

The number of shares reserved and available for issuance under the 2015 Plan will automatically increase each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by the lesser of (i) 1,840,000 shares, (ii) 4% of the outstanding number of shares of our common stock on the immediately preceding December 31 or (iii) an amount determined by our board of directors.

Percentage threshold of outstanding shares as of December 31 of each year for calculation of annual increase in authorized shares under the plan

4.00%

Additional number of common stock shares reserved for issuance

1,191,566

949,793

IPO [Member] | 2015 Stock Option and Incentive Plan [Member] | Subsequent Event [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Number of common stock shares reserved for issuance

2,724,262

Additional number of common stock shares reserved for issuance

1,223,163

IPO [Member] | 2015 Employee Stock Purchase Plan [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Effective date of plan

May 6, 2015

Number of common stock shares reserved for issuance

227,623

Percentage threshold of outstanding shares as of December 31 of each year for calculation of annual increase in authorized shares under the plan

1.00%

Additional number of common stock shares reserved for issuance

297,891

237,448

IPO [Member] | 2015 Employee Stock Purchase Plan [Member] | Subsequent Event [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Number of common stock shares reserved for issuance

1,145,095

Additional number of common stock shares reserved for issuance

305,790

IPO [Member] | Maximum [Member] | 2015 Stock Option and Incentive Plan [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Annual increase in shares authorized under plan, shares threshold

1,840,000

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 | $ / shares

$ 2.37

Preferred stock conversion price per share | $ / shares

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 | $ / shares

$ 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

1,777,784

Shares issued, price per share | $ / shares

$ 2.65

Common stock, par value | $ / shares

$ 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 | $ / shares

$ 13.25

Preferred stock, shares issued

2,285,952

Preferred stock, par value | $ / shares

$ 0.001

Convertible preferred stock, terms of conversion

Each share of Preferred Stock is convertible into five shares of our common stock. VGO Fund is 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.

Convertible preferred stock, common stock issued upon conversion

5

Maximum ownership percentage prohibiting conversion

9.50%

Number of days of notice | Day

61

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 | $

$ 45,800,000

Net proceeds from transaction, after giving effect to cost related to placement | $

$ 42,500,000

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

4,094,336

Shares issued, price per share | $ / shares

$ 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 Stock Option Activity (Detail)	12 Months Ended
	Dec. 31, 2018USD ($)$ / 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	2,091,361
Number of Outstanding Options, Exercised | shares	(12,141)
Number of Outstanding Options, Canceled/forfeited/expired | shares	(1,705,605)
Number of Outstanding Options, Ending Balance | shares	4,990,674
Number of Outstanding Options, Vested and Expected to Vest, Ending Balance | shares	4,990,674
Number of Outstanding Options, Exercisable, Ending Balance | shares	2,856,255
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 5.52
Weighted Average Exercise Price, Granted | $ / shares	2.35
Weighted Average Exercise Price, Exercised | $ / shares	1.10
Weighted Average Exercise Price, Canceled/forfeited/expired | $ / shares	4.72
Weighted Average Exercise Price, Ending Balance | $ / shares	4.47
Weighted Average Exercise Price, Options Vested and Expected to Vest, Ending Balance | $ / shares	4.47
Weighted Average Exercise Price, Options Exercisable, Ending Balance | $ / shares	$ 5.52
Weighted Average Contractual Term, Outstanding	6 years 11 months 19 days
Weighted Average Contractual Term, Options Vested and Expected to Vest	6 years 11 months 19 days
Weighted Average Contractual Term, Options Exercisable	5 years 7 months 13 days
Aggregate Intrinsic Value, Outstanding | $	$ 427
Aggregate Intrinsic Value, Options Vested and Expected to Vest | $	427
Aggregate Intrinsic Value, Options Exercisable | $	$ 427

Stockholders' Equity - Summar_2

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)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 months	6 months	6 months
Risk-free interest rate, minimum	1.40%	0.60%	0.40%
Risk-free interest rate, maximum	2.10%	1.00%	0.60%
Expected volatility, minimum	71.50%	74.50%	75.50%
Expected volatility, maximum	99.70%	115.20%	80.80%
Expected dividend yield	0.00%	0.00%	0.00%
Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	2.30%	1.90%	1.20%
Risk-free interest rate, maximum	3.00%	2.10%	2.10%
Expected volatility, minimum	87.90%	99.10%	80.70%
Expected volatility, maximum	98.40%	124.40%	84.50%
Expected dividend yield	0.00%	0.00%	0.00%
Employee Stock Option [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years	5 years 6 months	5 years 6 months
Employee Stock Option [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days	6 years 29 days

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Restricted Stock Unit Activity (Detail) - Restricted Stock Unit [Member]	12 Months Ended
	Dec. 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, Granted | shares	270,300
Number of Outstanding Restricted Stock Units, Released | shares	(39,301)
Number of Outstanding Restricted Stock Units, Forfeited | shares	(63,599)
Number of Outstanding Restricted Stock Units, Ending Balance | shares	216,700
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 4.28
Weighted Average Grant Date Fair Value, Granted | $ / shares	0.85
Weighted Average Grant Date Fair Value, Released | $ / shares	4.53
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	1.24
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 0.85

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition and Restricted Stock Units (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 3,431	$ 6,784	$ 5,029
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	1,216	1,399	1,876
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,215	$ 5,385	$ 3,153

Stockholders' Equity - Summar_3

Stockholders' Equity - Summary of Warrants Outstanding (Detail)	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Class Of Warrant Or Right [Line Items]
Number Outstanding	6,682,708
Warrant One [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	6,488,205
Exercise Price Per Share | $ / shares	$ 4.64
Expiration Date	2019-12
Warrant Two [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	2,006
Exercise Price Per Share | $ / shares	$ 7.48
Expiration Date	2021-03
Warrant Three [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	14,913
Exercise Price Per Share | $ / shares	$ 20.12
Expiration Date	2023-07
Warrant Four [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	95,542
Exercise Price Per Share | $ / shares	$ 3.14
Expiration Date	2023-11
Warrant Five [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	41,666
Exercise Price Per Share | $ / shares	$ 3.60
Expiration Date	2024-06
Warrant Six [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	40,376
Exercise Price Per Share | $ / shares	$ 3.72
Expiration Date	2024-12

Stockholders' Equity - Summar_4

Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	25,660,246	24,128,073
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	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	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	5,207,374	4,666,359
Shares Available Under the 2015 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	1,501,099	765,427
Shares Available Under the 2015 ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	839,305	583,819

Income Tax - Schedule of Pretax

Income Tax - Schedule of Pretax Earnings (Loss) for Domestic and Foreign Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
United States	$ (34,021)	$ (47,712)	$ (57,096)
Foreign	(494)	(495)	(808)
Loss before income taxes	$ (34,515)	$ (48,207)	$ (57,904)

Income Tax - Schedule of Reconc

Income Tax - Schedule of Reconciliations of the Expected Statutory Federal Income Tax (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Expected income taxes benefit at federal statutory rate	$ (7,248)	$ (16,390)	$ (19,687)
State income taxes, net of federal benefit	(14)	(13)
Permanent items and other	770	1,311	675
Research credits	(1,222)	(2,286)	(6,800)
Unrecognized tax benefits	489	914	2,720
Foreign rate differential	22	87	141
Change in tax rate	(11)	(25)
Tax cuts and Jobs Act		27,933
Change in valuation allowance	$ 7,214	$ (11,531)	22,902
Income tax (benefit) expense			$ (49)

Income Tax - Schedule of Reco_2

Income Tax - Schedule of Reconciliations of the Expected Statutory Federal Income Tax (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.S. federal corporate tax rate	21.00%	34.00%	34.00%

Income Tax - Additional Informa

Income Tax - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax [Line Items]
U.S. federal corporate tax rate	21.00%	34.00%	34.00%
Remeasurement of deferred tax balance		$ 27,900,000
Valuation allowance	$ 66,929,000	59,726,000
Period of change in ownership	3 years
Percentage of change in ownership	50.00%
Unrecognized tax benefits, income tax penalties and interest accrued	$ 0	$ 0	$ 0
Change in unrecognized tax benefits	0
Federal [Member]
Income Tax [Line Items]
Net operating loss carryforwards	139,500,000
Net operating loss carryforwards not subject to expiration	27,000,000
Net operating loss carryforwards subject to expiration	$ 112,500,000
Net operating loss carryforwards expiration year	2025
Research and development credit carryforward	$ 4,400,000
Research and development credit carryforward expiration year	2026
Federal [Member] | Orphan Drug Credits [Member]
Income Tax [Line Items]
Research and development credit carryforward	$ 12,500,000
Research and development credit carryforward expiration year	2035
State [Member]
Income Tax [Line Items]
Net operating loss carryforwards subject to expiration	$ 148,200,000
Net operating loss carryforwards expiration year	2021
Research and development credit carryforward	$ 3,800,000
Foreign [Member]
Income Tax [Line Items]
Net operating loss carryforwards not subject to expiration	$ 7,500,000
Maximum [Member]
Income Tax [Line Items]
U.S. federal corporate tax rate		35.00%

Income Tax - Schedule of Signif

Income Tax - Schedule of Significant Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 32,997	$ 27,226
Capitalized research and development expenses	17,279	16,218
Research credits and other state credits	11,962	11,229
Intangible assets	2,024	2,210
Reserve and accruals	2,667	2,843
Valuation allowance	$ (66,929)	$ (59,726)

Income Tax - Schedule of Activi

Income Tax - Schedule of Activity Related to Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Balance as of beginning of year	$ 16,558	$ 13,000	$ 5,033
Increase (decrease) related to prior year tax positions	2	(189)	1,890
Increase related to current year tax positions	3,083	3,747	6,077
Balance as of end of year	$ 19,643	$ 16,558	$ 13,000

Employee Benefits - Additional

Employee Benefits - Additional Information (Detail) - 401 (k) Plan [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Policy approved month and year	2015-04
Date of Policy beginning	Jun. 1, 2015
Percentage of match employee contribution	50.00%
Percentage of participants's annual salary	6.00%
Discretionary contribution	$ 0.2	$ 0.2	$ 0.2

Quarterly Financial Data (Una_3

Quarterly Financial Data (Unaudited) - Schedule of Quarterly Data for Interim Periods (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$ 5,963	$ 6,677	$ 9,960	$ 10,220	$ 11,200	$ 10,827	$ 11,907	$ 13,211	$ 32,820	$ 47,145	$ 57,940
Net loss	$ (6,322)	$ (7,114)	$ (10,412)	$ (10,667)	$ (11,474)	$ (11,190)	$ (12,138)	$ (13,405)	$ (34,515)	$ (48,207)	$ (57,855)
Basic and diluted net loss per share	$ (0.21)	$ (0.24)	$ (0.35)	$ (0.36)	$ (0.39)	$ (0.43)	$ (0.51)	$ (0.56)	$ (1.15)	$ (1.87)	$ (2.44)

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail)	Mar. 22, 2019USD ($)$ / sharesshares	Mar. 26, 2019USD ($)Program	Jan. 31, 2019shares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Subsequent Event [Line Items]
Net proceeds from sale of common stock				$ 14,000	$ 186,000	$ 20,000
ATM Offering Program [Member]
Subsequent Event [Line Items]
Net proceeds from sale of common stock				$ 400,000
ATM Offering Program [Member] | Common Stock [Member]
Subsequent Event [Line Items]
Number of shares issued and sold | shares				696,067
Weighted average price per share | $ / shares				$ 0.71
Subsequent Event [Member] | ATM Offering Program [Member]
Subsequent Event [Line Items]
Net proceeds from sale of common stock	$ 1,300,000
Subsequent Event [Member] | ATM Offering Program [Member] | Common Stock [Member]
Subsequent Event [Line Items]
Number of shares issued and sold | shares	2,580,839
Weighted average price per share | $ / shares	$ 0.52
VGO Fund [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Convertible preferred stock, common stock issued upon conversion | shares			3,209,955
Maximum ownership percentage prohibiting conversion			9.50%
CSL Behring [Member] | Subsequent Event [Member] | Research Collaboration and Option Agreement [Member]
Subsequent Event [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
Class X Convertible Preferred Stock [Member] | VGO Fund [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Converted shares of preferred stock | shares			641,991