TRACON Pharmaceuticals (TCON) 10-K 1 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Feb. 09, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Entity Registrant Name	Tracon Pharmaceuticals, Inc.
Entity Central Index Key	1,394,319
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
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	Smaller Reporting Company
Entity Common Stock, Shares Outstanding		17,749,947
Entity Public Float			$ 32.7
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	TCON

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 29,467	$ 35,710
Short-term investments	4,999	8,703
Prepaid and other assets	1,591	1,235
Total current assets	36,057	45,648
Property and equipment, net	73	82
Total assets	36,130	45,730
Current liabilities:
Accounts payable and accrued expenses	6,800	6,213
Accrued compensation and related expenses	1,494	1,588
Current portion of deferred revenue	667	1,259
Long-term debt, current portion	2,837	333
Final payment due bank		850
Total current liabilities	11,798	10,243
Deferred revenue	2,333
Other long-term liabilities	409	21
Long-term debt, less current portion	4,603	7,130
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.001 par value, authorized shares — 10,000,000 at December 31, 2017 and December 31, 2016; issued and outstanding shares—none
Common stock, $0.001 par value; authorized shares — 200,000,000 at December 31, 2017 and December 31, 2016; issued and outstanding shares — 17,711,928 and 16,084,721 at December 31, 2017 and December 31, 2016, respectively	18	16
Additional paid-in capital	121,670	113,918
Accumulated deficit	(104,701)	(85,598)
Total stockholders’ equity	16,987	28,336
Total liabilities and stockholders’ equity	$ 36,130	$ 45,730

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred Stock, Par or Stated Value Per Share	$ 0.001	$ 0.001
Preferred Stock, Shares Authorized	10,000,000	10,000,000
Preferred Stock, Shares Issued	0	0
Preferred Stock, Shares Outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	17,711,928	16,084,721
Common stock, shares outstanding	17,711,928	16,084,721

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Collaboration revenue	$ 8,755	$ 3,449	$ 7,904
Operating expenses:
Research and development	19,355	21,566	25,680
General and administrative	7,610	7,859	5,691
Total operating expenses	26,965	29,425	31,371
Loss from operations	(18,210)	(25,976)	(23,467)
Other income (expense):
Interest expense, net	(886)	(1,119)	(923)
Other income (expense), net	(7)	87	(20)
Total other income (expense)	(893)	(1,032)	(943)
Net loss	(19,103)	(27,008)	(24,410)
Accretion to redemption value of redeemable convertible preferred stock			(31)
Net loss attributable to common stockholders	$ (19,103)	$ (27,008)	$ (24,441)
Net loss per share attributable to common stockholders, basic and diluted	$ (1.14)	$ (2.13)	$ (2.20)
Weighted-average shares outstanding, basic and diluted	16,806,094	12,677,910	11,115,651

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Balance at Dec. 31, 2014	$ 49,880
Balance (in Shares) at Dec. 31, 2014	24,650,273
Increase (Decrease) in Convertible Preferred Stock
Accretion to redemption value of redeemable convertible preferred stock	$ 31
Conversion of redeemable convertible preferred stock into common stock at initial public offering	$ (49,911)
Conversion of redeemable convertible preferred stock into common stock at initial public offering (in shares)	(24,650,273)
Balance at Dec. 31, 2014	$ (32,174)	$ 2	$ 2,004	$ (34,180)
Balance (in Shares) at Dec. 31, 2014		1,633,854
Increase (Decrease) in Shareholders' Equity
Initial public offering and private placement of common stock for cash of $10 per share, net of offering costs	34,954	$ 4	34,950
Initial public offering and private placement of common stock for cash of $10 per share, net of offering costs (in shares)		4,100,000
Accretion to redemption value of redeemable convertible preferred stock	(31)		(31)
Conversion of redeemable convertible preferred stock into common stock at initial public offering	49,911	$ 6	49,905
Conversion of redeemable convertible preferred stock into common stock at initial public offering (in shares)		6,369,567
Reclassification of redeemable convertible preferred stock warrant	311		311
Issuance of common stock under equity plans	179		179
Issuance of common stock under equity plans (in shares)		72,521
Stock-based compensation expense	2,088		2,088
Vested shares related to repurchase liability	12		12
Issuance of common stock warrants in connection with debt financing	138		138
Net loss	(24,410)			(24,410)
Balance at Dec. 31, 2015	30,978	$ 12	89,556	(58,590)
Balance (in Shares) at Dec. 31, 2015		12,175,942
Increase (Decrease) in Shareholders' Equity
Issuance of common stock under equity plans	178		178
Issuance of common stock under equity plans (in shares)		37,672
Stock-based compensation expense	3,083		3,083
Vested shares related to repurchase liability	13		13
Issuance of common stock in a public offering, net of offering costs	16,113	$ 3	16,110
Issuance of common stock in a public offering, net of offering costs (in shares)		3,018,750
Other issuances of common stock, net	4,956	$ 1	4,955
Other issuances of common stock, net (in shares)		840,022
Issuance of common stock in exchange for services	23		23
Issuance of common stock in exchange for services (in shares)		12,335
Net loss	(27,008)			(27,008)
Balance at Dec. 31, 2016	28,336	$ 16	113,918	(85,598)
Balance (in Shares) at Dec. 31, 2016		16,084,721
Increase (Decrease) in Shareholders' Equity
Issuance of common stock under equity plans	35		35
Issuance of common stock under equity plans (in shares)		172,120
Stock-based compensation expense	3,194		3,194
Vested shares related to repurchase liability	14		14
Issuance of common stock in a public offering, net of offering costs	4,308	$ 2	4,306
Issuance of common stock in a public offering, net of offering costs (in shares)		1,455,087
Issuance of common stock in exchange for services	29		29
Issuance of common stock warrants in connection with debt financing	174		174
Net loss	(19,103)			(19,103)
Balance at Dec. 31, 2017	$ 16,987	$ 18	$ 121,670	$ (104,701)
Balance (in Shares) at Dec. 31, 2017		17,711,928

Consolidated Statements of Red6

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical)	Dec. 31, 2015$ / shares
Common Stock | IPO and Private Placement
Share price (in dollar per share)	$ 10

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (19,103)	$ (27,008)	$ (24,410)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	3,194	3,083	2,088
Common stock issued for services	29	23
Depreciation and amortization	48	94	51
Amortization of debt discount	117	100	97
Amortization of premium/discount on short-term investments	(9)	3	8
Noncash interest	354	522	417
Change in fair value of preferred stock warrant liability			65
Deferred rent	60	(53)	(4)
Deferred revenue	1,741	(2,094)	(3,550)
Changes in assets and liabilities:
Prepaid expenses and other assets	(356)	(42)	(424)
Accounts payable and accrued expenses	776	(2,203)	6,144
Accrued compensation and related expenses	(94)	425	355
Net cash used in operating activities	(13,243)	(27,150)	(19,163)
Cash flows from investing activities
Purchase of property and equipment	(39)	(3)	(127)
Purchases of available-for-sale short-term investments	(13,992)	(17,506)	(12,790)
Proceeds from the maturity of available-for-sale short-term investments	17,705	19,582	2,000
Net cash provided by (used in) investing activities	3,674	2,073	(10,917)
Cash flows from financing activities
Proceeds from long-term debt	8,000		10,000
Repayment of long-term debt	(8,850)	(2,000)	(9,930)
Proceeds from sale of common stock, net of offering costs paid in the current period	4,141	21,236	36,204
Proceeds from issuance of common stock under equity plans	172	178	179
Payment of tax withholdings related to net share settlements of vested restricted stock awards	(137)
Net cash provided by financing activities	3,326	19,414	36,453
(Decrease) increase in cash and cash equivalents	(6,243)	(5,663)	6,373
Cash and cash equivalents at beginning of period	35,710	41,373	35,000
Cash and cash equivalents at end of period	29,467	35,710	41,373
Supplemental disclosure of cash flow information
Interest paid	664	$ 622	428
Supplemental schedule of noncash investing and financing activities
Issuance of common stock warrants in connection with long-term debt	174		$ 138
Issuance of common stock in connection with common stock purchase agreement	$ 793

Organization and Summary of Sig

Organization and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Organization And Accounting Policies [Abstract]
Organization and Summary of Significant Accounting Policies	1. Organization and Summary of Significant Accounting Policies Organization and Business TRACON Pharmaceuticals, Inc. (formerly Lexington Pharmaceuticals, Inc.) (TRACON or the Company) was incorporated in the state of Delaware on October 28, 2004. TRACON is a clinical stage biopharmaceutical company focused on the development and commercialization of novel targeted therapeutics for cancer, wet age-related macular degeneration and fibrotic diseases. The Company’s lead product candidate is an antibody that binds to the endoglin receptor, which is essential to angiogenesis (the process of new blood vessel formation). The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiary, TRACON Pharma Limited, which was formed in September 2015 and is currently inactive. All significant intercompany accounts and transactions have been eliminated. Basis of Presentation As of December 31, 2017, the Company has devoted substantially all of its efforts to product development, raising capital, and building infrastructure and has not realized revenues from its planned principal operations. The Company has incurred operating losses since inception. As of December 31, 2017, the Company had an accumulated deficit of $104.7 million. The Company anticipates that it will continue to incur net losses into the foreseeable future as it continues the development and commercialization of its product candidates and works to develop additional product candidates through research and development programs. At December 31, 2017, the Company had cash, cash equivalents and short-term investments of $34.5 million. Based on the Company’s current business plan, management believes that existing cash, cash equivalents and short-term investments will be sufficient to fund the Company’s obligations into the third quarter of 2018. The Company’s ability to execute its operating plan beyond mid-2018 depends on its ability to obtain additional funding through equity offerings, debt financings or potential licensing and collaboration arrangements. The accompanying consolidated financial statements have been prepared assuming the Company will continue as a going concern, which contemplates the realization of assets and settlement of liabilities in the normal course of business. However, the Company’s current working capital, anticipated operating expenses and net losses and the uncertainties surrounding its ability to raise additional capital as needed, as discussed below, raise substantial doubt about its ability to continue as a going concern for a period of one year following the date that these financial statements are issued. The consolidated financial statements do not include any adjustments for the recovery and classification of assets or the amounts and classification of liabilities that might be necessary should the Company be unable to continue as a going concern. The Company plans to continue to fund its losses from operations through cash, cash equivalents and investments on hand, as well as through future equity offerings, debt financings, other third party funding, and potential licensing or collaboration arrangements, including equity financing through the common stock purchase agreement the Company entered into with Aspire Capital Fund, LLC in March 2017 for the purchase of up to $21.0 million of the Company’s stock over a 30 month period and/or the at-the-market equity offering sales agreement the Company entered into with Stifel, Nicolaus & Company, Incorporated in February 2016 for the sale of up to $25.0 million of the Company’s stock. There can be no assurance that additional funds will be available when needed from any source or, if available, will be available on terms that are acceptable to the Company. Even if the Company raises additional capital, it may also be required to modify, delay or abandon some of its plans which could have a material adverse effect on the Company’s business, operating results and financial condition and the Company’s ability to achieve its intended business objectives. Use of Estimates The Company’s consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States (GAAP). The preparation of the Company’s consolidated financial statements requires it to make estimates and assumptions that impact the reported amounts of assets, liabilities, revenue and expenses and the disclosure of contingent assets and liabilities in the Company’s financial statements and accompanying notes. The most significant estimates in the Company’s financial statements relate to revenue recognition, expenses incurred for clinical trials and the valuation of equity awards. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately materially differ from these estimates and assumptions. Cash and Cash Equivalents Cash and cash equivalents consist of cash and highly liquid investments with original maturities of three months or less at the date of purchase. The carrying amounts approximate fair value due to the short maturities of these investments. Cash and cash equivalents include cash in readily available checking and money market funds, U.S. treasury securities, as well as certificates of deposit. Concentration of Credit Risk Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash and cash equivalents. The Company maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and management believes that the Company is 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 is stated at cost and depreciated using the straight-line method over the estimated useful life of the related assets, which is generally five years. Leasehold improvements are amortized over the shorter of the lease term or estimated useful life of the related assets. Repairs and maintenance costs are charged to expense as incurred. Deferred Rent Rent expense is recorded on a straight-line basis over the term of the lease. The difference between rent expense and amounts paid under the lease agreements is recorded as deferred rent in the accompanying consolidated balance sheets. Tenant improvement allowances and other lease incentives are recorded as liabilities and are amortized on a straight-line basis over the term of the lease as reductions to rent expense. Revenue Recognition Through December 31, 2017, all of the Company’s revenue was derived from its license agreement with Santen Pharmaceutical Co., Ltd. (Santen) as described in Note 7. The Company recognizes revenue when all four of the following criteria are met: (1) there is persuasive evidence that an arrangement exists; (2) delivery of the products and/or services has occurred; (3) the selling price is fixed or determinable; and (4) collectability is reasonably assured. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as long-term deferred revenue. The Company evaluates multiple-element arrangements to determine: (1) the deliverables included in the arrangement and (2) whether the individual deliverables represent separate units of accounting or whether they must be accounted for as a combined unit of accounting. Deliverables are considered separate units of accounting provided that: (a) the delivered items have value to the customer on a standalone basis and (b) if the arrangement includes a general right of return relative to the delivered items, delivery or performance of the undelivered items is considered probable and substantially in the Company’s control. In assessing whether an item has standalone value, the Company considers factors such as the research, manufacturing and commercialization capabilities of the partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the partner can use the other deliverables for their intended purpose without the receipt of the remaining elements, whether the value of the deliverable is dependent on the undelivered items and whether there are other vendors that can provide the undelivered elements. Arrangement consideration that is fixed or determinable is allocated among the separate units of accounting using the relative selling price method. The Company uses the following hierarchy of values to estimate the selling price of each deliverable: (1) vendor-specific objective evidence of fair value; (2) third-party evidence of selling price; and (3) best estimate of selling price (BESP). The BESP reflects the Company’s best estimate of what the selling price would be if the Company regularly sold the deliverable on a standalone basis. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity-specific factors, including factors that are contemplated in negotiating an arrangement and estimated costs. The Company validates the BESP for units of accounting by evaluating whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration between multiple units of accounting. The Company then applies the applicable revenue recognition criteria to each of the separate units of accounting in determining the appropriate period and pattern of recognition. If there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement on a straight-line basis over the period the Company expects to complete its performance obligations. With respect to revenue derived from reimbursement of direct, out-of-pocket expenses for research and development costs associated with collaborations, where the Company acts as a principal with discretion to choose suppliers, bear credit risk and perform part of the services required in the transaction, the Company records revenue for the gross amount of the reimbursement. The costs associated with these reimbursements are reflected as a component of research and development expense in the consolidated statements of operations. Milestones The Company uses the milestone method of accounting and revenue is recognized when earned, as evidenced by written acknowledgment from the collaborator or other persuasive evidence that the milestone has been achieved and the payment is non-refundable, provided that the milestone event is substantive. A milestone event is defined as an event: (1) that can only be achieved based in whole or in part on either the Company’s performance or on the occurrence of a specific outcome resulting from the Company’s performance; (2) for which there is substantive uncertainty at the inception of the arrangement that the event will be achieved; and (3) that would result in additional payments being due to the Company. Events for which the occurrence is either contingent solely upon the passage of time or the result of a counterparty’s performance are not considered to be milestone events. A milestone event is substantive if all of the following conditions are met: (a) the consideration is commensurate with either the Company’s performance to achieve the milestone, or the enhancement of the value to the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone; (b) the consideration relates solely to past performance; and (c) the consideration is reasonable relative to all the deliverables and payment terms (including other potential milestone consideration) within the arrangement. The Company assesses whether a milestone is substantive at the inception of each arrangement. If a milestone is deemed non-substantive, the Company will account for that milestone payment in accordance with the multiple element arrangements guidance and recognize it consistent with the related units of accounting for the arrangement over the related performance period. Clinical Trial Expense Accruals As part of the process of preparing the Company’s financial statements, the Company is required to estimate expenses resulting from its obligations under contracts with vendors, clinical sites, contract research organizations (CROs), and consultants in connection with conducting clinical trials. The financial terms of these contracts vary and may result in payment flows that do not match the periods over which materials or services are provided under such contracts. The Company’s objective is to reflect the appropriate trial expenses in its financial statements by recording those expenses in the period in which services are performed and efforts are expended. The Company accounts for these expenses according to the progress of the clinical trial as measured by patient progression and the timing of various aspects of the trial. The Company determines accrual estimates through discussion with the clinical sites and applicable personnel and outside service providers as to the progress or state of consummation of trials. During the course of a clinical trial, the Company adjusts the clinical expense recognition if actual results differ from its estimates. The Company makes estimates of accrued expenses as of each balance sheet date based on the facts and circumstances known at that time. The Company’s clinical trial accruals are dependent upon accurate reporting by clinical sites, CROs and other third-party vendors. Although the Company does not expect its estimates to differ materially from amounts actually incurred, its understanding of the status and timing of services performed relative to the actual status and timing of services performed may vary and may result in reporting amounts that are too high or too low for any particular period. For the three years ended in the period December 31, 2017, there were no material adjustments to the Company’s prior period estimates of accrued expenses for clinical trials. Research and Development Costs Research and development costs, including license fees, are expensed as incurred. 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 grant date fair value of employee stock option grants, employee restricted stock unit grants (RSUs) and employee stock purchase plan (ESPP) rights recognized as expense over the requisite service period of the awards (usually the vesting period) on a straight-line basis. The Company estimates the fair value of stock option grants and ESPP rights using the Black-Scholes option pricing model. The fair value of RSUs is based on the stock price on the date of grant. The Company accounts for stock options granted to non-employees using the fair value approach. These option grants, if any, are subject to periodic revaluation over their vesting terms. Income Taxes The Company accounts 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. The Company recognizes net deferred tax assets to the extent that the Company believes these assets are more likely than not to be realized. In making such a determination, management considers 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 management determines that the Company would be able to realize its deferred tax assets in the future in excess of their net recorded amount, management would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. The Company records uncertain tax positions on the basis of a two-step process whereby (1) management determines 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, management recognizes the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority. The Company recognizes interest and penalties related to unrecognized tax benefits within income tax expense. Any accrued interest and penalties are included within the related tax liability. Comprehensive Loss Comprehensive loss is defined as a change in equity during a period from transactions and other events and circumstances from non-owner sources. Net loss and comprehensive loss were the same for all periods presented. Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average shares of common stock 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. The Company has excluded 5,617, 7,878, and 6,555 weighted-average shares subject to repurchase or forfeiture from the weighted‑average number of common shares outstanding for the years ended December 31, 2017, 2016, and 2015, respectively. Diluted net loss per share is calculated by dividing the net loss by the weighted-average number of common stock equivalents outstanding for the period determined using the treasury-stock method. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to the Company’s 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 stock equivalent shares): December 31, 2017 2016 2015 Warrants to purchase common stock 103,865 57,173 57,173 Common stock options and restricted stock units 2,516,246 2,023,478 1,788,149 ESPP shares 3,653 2,857 143 2,623,764 2,083,508 1,845,465 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. The Company views its operations and manages its business in one operating segment. Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2014-09, Revenue from Contracts with Customers plans on adopting ASU 2014-09 using the modified retrospective approach and does not expect the adoption to have a material impact on its financial position and results of operations. In February 2016, the FASB issued ASU 2016-02, Leases Recently Adopted Accounting Standards In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting

Short-Term Investments, Cash Eq

Short-Term Investments, Cash Equivalents and Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Short-Term Investments, Cash Equivalents and Fair Value Measurements	2. Short-Term Investments, Cash Equivalents and Fair Value Measurements At December 31, 2017, short-term investments consisted of U.S. treasury securities. The Company classifies all investments as available-for-sale, as the sale of such investments may be required prior to maturity to implement management strategies. These investments are carried at amortized cost which approximates fair value. A decline in the market value of any short-term investment below cost that is determined to be other-than-temporary will result in a revaluation of its carrying amount to fair value. The impairment is charged to earnings and a new cost basis for the security is established. No such impairment charges were recorded for any period presented. Realized gains and losses from the sale of short-term investments, if any, are determined on a specific identification basis. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on available-for-sale securities are included in other income or expense on the consolidated statements of operations. Realized and unrealized gains and losses during the periods presented were immaterial. 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 on the consolidated statements of operations. Interest and dividends on securities classified as available-for-sale are included in interest income on the consolidated statements of operations. At December 31, 2017, the remaining contractual maturities of all available-for-sale investments were less than one year. The carrying amounts of cash and 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 the Company for loans with similar terms, which is considered a Level 2 input, the Company believes that the fair value of long-term debt approximates its carrying 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. Assets and liabilities are classified based on the lowest level of input that is significant to the fair value measurements. None of the Company’s non-financial assets or liabilities are recorded at fair value on a non-recurring basis. No transfers between levels have occurred during the periods presented. Cash equivalents and short-term investments, all of which are classified as available-for-sale securities, consisted of the following (in thousands): December 31, 2017 Cost Unrealized Gain Unrealized (Loss) Estimated Fair Value Money market funds $ 5,488 $ — $ — $ 5,488 U.S. treasury securities 4,999 — — 4,999 $ 10,487 $ — $ — $ 10,487 Classified as: Cash equivalents $ 5,488 Short-term investments 4,999 Total Cash equivalents and Short-term investments $ 10,487 The fair values of the Company’s assets and liabilities, which are measured at fair value on a recurring basis, were determined using the following inputs (in thousands): Fair Value Measurements at Reporting Date Using Quoted Prices in Significant Active Markets Other Significant for Identical Observable Unobservable Assets Inputs Inputs Total (Level 1) (Level 2) (Level 3) At December 31, 2017 Money market funds and U.S. treasury securities, included in Cash equivalents and Short-term investments $ 10,487 $ — $ 10,487 $ — At December 31, 2016 Money market funds and U.S. treasury securities, included in Cash equivalents and Short-term investments $ 23,346 $ — $ 23,346 $ —

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Property and Equipment	3. Property and Equipment Property and equipment consisted of the following (in thousands): December 31, 2017 2016 Computer and office equipment $ 133 $ 115 Furniture and fixtures 19 19 Leasehold improvements 21 124 173 258 Less accumulated depreciation and amortization (100 ) (176 ) $ 73 $ 82 Depreciation expense related to property and equipment totaled approximately $48,000, $94,000 and $51,000 for the years ended December 31, 2017, 2016 and 2015, respectively.

Long-Term Debt

Long-Term Debt	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Long-Term Debt	4. Long-Term Debt Long-term debt and unamortized debt discount balances were as follows (in thousands): December 31, 2017 2016 Long-term debt $ 8,000 $ 8,000 Less debt discount, net of current portion (197 ) (537 ) Long-term debt, net of debt discount 7,803 7,463 Less current portion of long-term debt (3,200 ) (333 ) Long-term debt, net of current portion $ 4,603 $ 7,130 Current portion of long-term debt $ 3,200 $ 333 Current portion of debt discount (363 ) - Current portion of long-term debt, net $ 2,837 $ 333 In January 2017, the Company entered into a second amendment to its Amended and Restated Loan and Security Agreement with Silicon Valley Bank (SVB) (the 2017 Amended SVB Loan) under which the Company borrowed $8.0 million, all of which was immediately used to repay the Company’s existing loan with SVB (the 2015 Amended SVB Loan). In accordance with the terms of the 2015 Amended SVB Loan, the Company paid a final payment of $0.9 million associated with the payoff of the 2015 Amended SVB Loan. The transaction was accounted for as a debt modification. The 2017 Amended SVB Loan provides for interest to be paid at a rate of 8.55% per annum. Interest-only payments were due monthly through December 2017. Thereafter, in addition to interest accrued during such period, the monthly payments will include an amount equal to the outstanding principal at December 31, 2017 divided by 30 months. At maturity (or earlier prepayment), the Company is also required to make a final payment equal to 4.0% of the original principal amount borrowed. The 2017 Amended SVB Loan provides for prepayment fees of 2.0% of the amount prepaid if the prepayment occurs after January 25, 2018 but prior to January 25, 2019 and 1.0% of the amount prepaid if the prepayment occurs thereafter. Except as described above, the 2017 Amended SVB Loan is subject to the same material terms set forth in the 2015 Amended SVB Loan Agreement. Consistent with the terms of the 2015 Amended SVB Loan agreements, the 2017 Amended SVB Loan is collateralized by substantially all of the Company’s assets, other than the Company’s intellectual property, and contains customary conditions of borrowing, events of default and covenants, including covenants that restrict the Company’s ability to dispose of assets, merge with or acquire other entities, incur indebtedness and make distributions to holders of the Company’s capital stock. Should an event of default occur, including the occurrence of a material adverse change, the Company could be liable for immediate repayment of all obligations under the 2017 Amended SVB Loan. In connection with the 2017 Amended SVB Loan the Company issued SVB a warrant to purchase 46,692 shares of its common stock at an exercise price of $5.14 per share. The warrant is fully exercisable and expires on January 25, 2024. The fair value of the warrant and the final payment related to the 2017 Amended SVB Loan are being amortized to interest expense using the effective interest method over the term of the debt, in addition to the remaining unamortized discounts related to the 2015 Amended SVB Loan. At December 31, 2017, the Company had the following exercisable outstanding warrants for the purchase of common stock issued in connection with the Company’s loan agreements with SVB: Expiration Number of shares Exercise price May 13, 2022 18,415 $ 10.86 November 14, 2023 through June 4, 2024 38,758 7.74 January 25, 2024 46,692 5.14 103,865 Future minimum principal and interest payments under the 2017 Amended SVB Loan including the final payment, as of December 31, 2017 are as follows (in thousands): 2018 $ 3,766 2019 3,489 2020 1,961 9,216 Less interest and final payment (1,216 ) Long-term debt $ 8,000

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	5. Commitments and Contingencies Lonza Biologics Tuas Pte Ltd (Lonza) On February 22, 2017, the Company entered into a long-term manufacturing agreement, or the Manufacturing Agreement, with Lonza for the long term manufacture and supply of registration and commercial batches of TRC105, the Company’s lead drug product candidate. Under the Manufacturing Agreement, Lonza has agreed to manufacture TRC105 pursuant to purchase orders and in accordance with the manufacturing specifications agreed upon between the Company and Lonza. The Company is required to purchase certain batches of TRC105 prior to regulatory approval with a total estimated cost of approximately $15.0 million. The Manufacturing Agreement has an initial term beginning on the effective date and ending on the seventh anniversary of the date of first regulatory approval of TRC105 by the FDA or EMA. The Manufacturing Agreement may be renewed for an additional three years upon the written agreement of both parties no later than the fifth anniversary of the date of first approval of TRC105 by the FDA or EMA. Either party may terminate the Manufacturing Agreement due to a material breach of the Manufacturing Agreement by the other party, subject to prior written notice and a cure period, due to the insolvency or bankruptcy of the other party, or due to a force majeure event that prevents performance under the Manufacturing Agreement for at least six months. The Company may terminate the Manufacturing Agreement, subject to 60 days’ written notice, if the Company discontinues the TRC105 program, whether due to a notice of non-approval or withdrawal of marketing approval by a regulatory agency or otherwise. In the event of a termination by the Company due to discontinuation of the TRC105 program or a termination by Lonza due to the Company’s material breach or insolvency or bankruptcy, the Company would be obligated to pay to Lonza certain batch cancellation and/or early termination fees. Facility Lease The Company leases its office space under a non‑cancelable operating lease that expires in April 2022 that may be extended for an additional term of 60 months. The lease is subject to base lease payments and additional charges for common area maintenance and other costs and includes certain lease incentives and tenant improvement allowances. Rent expense for each of the years ended December 31, 2017, 2016 and 2015 was $0.4 million, $0.4 million and $0.2 million, respectively. Under the terms of the lease agreement, the Company provided the lessor with an irrevocable letter of credit in the amount of $175,000. The lessor is entitled to draw on the letter of credit in the event of any default by the Company under the terms of the lease. Future minimum payments under the non‑cancelable operating lease as of December 31, 2017 were as follows (in thousands): 2018 $ 405 2019 423 2020 442 2021 461 2022 156 $ 1,887 License Agreements The Company has entered into various license agreements pursuant to which the Company acquired licenses to certain intellectual property. The agreements generally required an upfront license fee and, in some cases, reimbursement of patent costs. Additionally, under each agreement, the Company may be required to pay annual maintenance fees, royalties, milestone payments and sublicensing fees. Each of the license agreements is generally cancelable by the Company, given appropriate prior written notice. At December 31, 2017, potential future milestone payments under these agreements, including future milestone payments associated with assets acquired from Janssen Pharmaceutica N.V. should they not exercise their option to regain their rights to certain assets as discussed in Note 7, totaled an aggregate of approximately $126.0 million.

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit)	12 Months Ended
	Dec. 31, 2017
Stockholders Equity Note [Abstract]
Stockholders' Equity (Deficit)	6. Stockholders’ Equity (Deficit) Redeemable Convertible Preferred Stock In connection with the completion of the Company’s initial public offering on February 4, 2015, all of the outstanding shares of redeemable convertible preferred stock were converted into 6,369,567 shares of the Company’s common stock; outstanding warrants to purchase 150,000 shares of Series A redeemable convertible preferred stock were converted into warrants to purchase 38,758 shares of the Company’s common stock, and the Company’s certificate of incorporation was amended and restated to authorize 200,000,000 shares of common stock and 10,000,000 shares of undesignated preferred stock. No preferred stock dividends were paid or declared by the Company. Sales of Common Stock In March 2017, the Company entered into a Common Stock Purchase Agreement (the Purchase Agreement) with Aspire Capital Fund, LLC (Aspire Capital) which provides that, upon the terms and subject to the conditions and limitations, Aspire Capital is committed to purchase up to an aggregate of $21.0 million of shares of the Company’s common stock. Under the terms of the Purchase Agreement, the Company sold 222,222 shares of the Company’s common stock to Aspire Capital at $4.50 per share for net proceeds of approximately $0.9 million upon execution of the Purchase Agreement and Aspire Capital is committed to purchase up to $20.0 million of additional shares of its common stock solely at TRACON’s request from time to time during a 30 month period that began on May 1, 2017 and at prices based on the market price at the time of each sale, subject to certain conditions. In February 2016, the Company entered into an At-the-Market Equity Offering Sales Agreement (Sales Agreement) with Stifel, Nicolaus & Company, Incorporated (Stifel), pursuant to which it may sell from time to time, at its option, up to an aggregate of $25.0 million of the Company’s shares of its common stock through Stifel, as sales agent. The Company is required to pay Stifel 2.5% of gross proceeds for the common stock sold through the Sales Agreement. During the year ended December 31, 2017, the Company sold approximately 1,037,000 shares of common stock through the Sales Agreement with Stifel for gross proceeds of approximately $3.5 million, and approximately $21.5 million of common stock remains available for sale under the Sales Agreement, subject to limitations on the amount of securities the Company may sell under its effective registration statement on Form S-3 within any 12 month period. During November 2016, the Company completed an underwritten public offering of 3,018,750 shares of its common stock at an offering price of $5.75 per share. The Company received net proceeds from this offering of approximately $16.1 million, after deducting underwriting discounts, commissions and offering-related expenses of $1.3 million. In September 2016, concurrent with its License and Option Agreement with Janssen Pharmaceutica N.V. (Janssen) and its affiliate, Johnson & Johnson Innovation-JJDC, Inc. (JJDC) at a purchase price of $5.95 per share (determined by the average of the daily volume weighted average closing prices of the common stock as reported on NASDAQ for the five days prior to the date of the purchase) The Company also entered into an Investor Rights Agreement, pursuant to which the Company granted JJDC certain rights to require the Company to register the shares for resale under the Securities Act. Stock Compensation Plans 2011 Equity Incentive Plan The Company granted awards under the TRACON Pharmaceuticals, Inc. 2011 Equity Incentive Plan until January 2015. The 2011 Plan provides for the grant of incentive stock options, non-statutory stock options, stock appreciation rights (SARs), restricted stock grants and restricted stock units to eligible recipients. Recipients of incentive stock options are eligible to purchase shares of the Company’s 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 granted under the 2011 Plan is no more than ten years. Grants made under the 2011 Plan generally vest on the last day of each month over 48 months from the vesting commencement date subject to continuous service. In connection with the adoption of the 2015 Equity Incentive Plan (the 2015 Plan), the Company terminated the 2011 Plan and no additional awards will be granted under the 2011 Plan. 2015 Equity Incentive Plan Effective January 1, 2015, the Company’s board of directors adopted the 2015 Equity Incentive Plan (the 2015 Plan). Under the 2015 Plan, the Company may grant stock options, stock appreciation rights, restricted stock, restricted stock units and other awards to individuals who are then employees, officers, non-employee directors or consultants of the Company or its subsidiaries. Initially, a total of 801,033 shares of common stock were reserved for issuance under the 2015 Plan. In addition, the number of shares of common stock available for issuance under the 2015 Plan will be annually increased on the first day of each fiscal year during the term of the 2015 Plan, beginning with the 2016 fiscal year, by an amount equal to 4% of the total number of shares of common stock outstanding on December 31st of the preceding calendar year or such other amount as the Company’s board of directors may determine. The maximum term of the options granted under the 2015 Plan is no more than ten years. Grants generally vest at 25% one year from the vesting commencement date and ratably each month thereafter for a period of 36 months, subject to continuous service. In December 2015, the 2015 Plan was amended to allow an additional 500,000 shares of common stock to be used exclusively for the grant of equity awards as a material inducement for individuals to commence employment at the Company in compliance with NASDAQ Listing Rule 5635(c)(4). Restricted Stock Units In 2016, the Company issued RSUs to employees and members of the Board of Directors under the 2015 Equity Incentive Plan. The total grant-date fair value of RSUs that vested during the years ended December 31, 2017, and 2016 was $0.8 million and $0, respectively. The aggregate intrinsic value of outstanding RSUs at December 31, 2017 was $0.6 million and is based on the Company’s closing market price per share on December 31, 2017 of $3.35. As of December 31, 2017, there was approximately $1.0 million of unrecognized compensation costs related to outstanding RSUs, which is expected to be recognized over a weighted average remaining period of 2.1 years. Restricted stock unit activity under the 2015 Plan is summarized as follows: Weighted Average Number of Grant Date Shares Fair Value Outstanding at December 31, 2016 306,780 $ 7.70 Granted - - Vested (102,378 ) 7.47 Forfeited (11,438 ) 6.11 Outstanding at December 31, 2017 192,964 $ 7.92 Stock Options Stock option activity under all Plans is summarized as follows: Weighted- Number of Average Options Exercise Price Balance at December 31, 2016 1,716,698 $ 7.54 Granted 689,527 4.76 Exercised (53,756 ) 0.83 Forfeited (29,187 ) 6.39 Balance at December 31, 2017 2,323,282 $ 6.88 Information about the Company’s outstanding stock options is as follows (in thousands, except share and per share data and contractual term): Weighted- Average Weighted- Remaining Average Contractual Aggregate Number of Exercise Term Intrinsic Shares Price (in years) Value December 31, 2017: Options outstanding 2,323,282 $ 6.88 7.19 $ 1,293 Options vested and expected to vest 2,323,282 $ 6.88 7.19 $ 1,293 Options exercisable 1,359,286 $ 7.07 6.20 $ 1,198 The weighted-average grant date fair value per share of employee option grants during the years ended December 31, 2017, 2016 and 2015 was $3.45, $6.68 and $12.97, respectively. The aggregate intrinsic value used in the above table of options at December 31, 2017 is based on the Company’s closing market price per common share on December 31, 2017 of $3.35. The Company received approximately $44,900, $6,500 and $54,200 in proceeds from the exercise of stock options during the years ended December 31, 2017, 2016 and 2015, respectively. The total intrinsic value of options exercised was approximately $0.2 million, $78,000 and $0.8 million during the years ended December 31, 2017, 2016 and 2015, respectively. The total grant-date fair value of options that vested during the years ended December 31, 2017, 2016 and 2015 was $1.9 million, $3.4 million and $0.8 million, respectively. Employee Stock Purchase Plan On January 1, 2015, the Company’s board of directors adopted the ESPP, which became effective upon the pricing of the Company’s initial public offering on January 29, 2015. The ESPP permits participants to purchase common stock through payroll deductions of up to 15% of their eligible compensation. Initially, a total of 183,462 shares of common stock was reserved for issuance under the ESPP. In addition, the number of shares of common stock available for issuance under the ESPP will be annually increased on the first day of each fiscal year during the term of the ESPP, beginning with the 2016 fiscal year, by an amount equal to the lessor of: (i) 366,925 shares; (ii) 1% of the total number of shares of common stock outstanding on December 31st of the preceding calendar year; or (iii) such other amount as the Company’s board of directors may determine. Stock compensation expense for the years ended December 31, 2017 and 2016 related to the ESPP was immaterial. Stock-Based Compensation Expense The weighted-average 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, 2017 2016 2015 Risk-free interest rate 2.1 % 1.6 % 1.7 % Expected volatility 83.0 % 80.0 % 74.0 % Expected term (in years) 6.2 6.3 6.3 Expected dividend yield — % — % — % Risk-free interest rate. The Company bases 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 term. The expected term represents the period of time that options are expected to be outstanding. Because the Company does not have historical exercise behavior, it determines the expected life assumption using the simplified method, which is an average of the contractual term of the option and its vesting period. Expected dividend yield. The Company bases the expected dividend yield assumption on the fact that it has never paid cash dividends and has no present intention to pay cash dividends. The allocation of stock-based compensation was as follows (in thousands): Years Ended December 31, 2017 2016 2015 Research and development $ 1,482 $ 1,090 $ 1,038 General and administrative 1,712 1,993 1,050 $ 3,194 $ 3,083 $ 2,088 As of December 31, 2017 and 2016, the unrecognized compensation cost related to outstanding time-based options was $3.7 million and $4.0 million, respectively, and is expected to be recognized as expense over approximately 2.1 years and 2.1 years, respectively. Common Stock Reserved for Future Issuance Common stock reserved for future issuance was as follows: December 31, 2017 2016 Common stock warrants 103,865 57,173 Common stock options and restricted stock units granted and outstanding 2,516,246 2,023,478 Awards available under the 2015 Plan 772,573 749,753 Shares available under the Employee Stock Purchase Plan 378,367 261,840 3,771,051 3,092,244

Collaborations

Collaborations	12 Months Ended
	Dec. 31, 2017
Collaborations Disclosure [Abstract]
Collaborations	7. Collaborations Santen In March 2014, the Company entered into a license agreement with Santen, under which the Company granted Santen an exclusive, worldwide license to certain patents, information and know-how related to TRC105. Under the agreement, Santen is permitted to use, develop, manufacture and commercialize TRC105 products for ophthalmology indications, excluding systemic treatment of ocular tumors. Santen also has the right to grant sublicenses to affiliates and third party collaborators. In the event Santen sublicenses any of its rights under the agreement, Santen will be obligated to pay the Company a portion of any upfront and certain milestone payments received under such sublicense. Santen has sole responsibility for funding, developing, seeking regulatory approval for and commercializing TRC105 products in the field of ophthalmology. In the event that Santen fails to meet certain commercial diligence obligations, the Company will have the option to co-promote TRC105 products in the field of ophthalmology in the United States with Santen. If the Company exercises this option, the Company will pay Santen a percentage of certain development expenses, and the Company will receive a percentage of profits from sales of the licensed products in the ophthalmology field in the United States, but will not also receive royalties on such sales. In consideration of the rights granted to Santen under the agreement, the Company received a one-time upfront fee of $10.0 million. The license agreement provides for various types of payments, including the upfront payment, payments for various technical and regulatory support, payments for delivery of drug substance, reimbursement of certain development costs, milestone payments, and royalties on net product sales. The Company has identified multiple deliverables, which include at inception: (1) a license to patents, information and know-how related to TRC105, (2) technology transfer, (3) collaboration, including technical and regulatory support provided by the Company, (4) manufacturing and supply obligations, and (5) shared chemistry, manufacturing and controls (CMC) development activities. Deliverables 1 and 2 above were substantially delivered at the inception of the agreement, and deliverables 3 through 5 were delivered during the 41-month period over which the Company provided technical and regulatory support to Santen. At inception and through December 31, 2017, the Company has identified one single unit of accounting for all the deliverables under the agreement since the delivered elements do not have standalone value. The Company’s technical and regulatory expertise, including manufacturing and CMC activities, in the development of biologic therapeutics, specifically TRC105, is a significant component of Santen’s ability to utilize the license and know-how related to TRC105. Given the early stage of development of TRC105 for ophthalmology, the Company is the only party capable of performing the level and type of technical and regulatory collaboration services required by Santen under the agreement. As a result, the Company has determined that the license, including the ability to sublicense, and know-how related to TRC105 do not have standalone value to a licensee. Accordingly, the Company recognized revenue for the fixed or determinable collaboration consideration on a straight-line basis over the 41-month period over which it delivered its technical and regulatory support. In addition, the Company is eligible to receive up to a total of $155.0 million in milestone payments upon the achievement of certain milestones, of which $20.0 million relates to the initiation of certain development activities, $52.5 million relates to the submission of certain regulatory filings and receipt of certain regulatory approvals and $82.5 million relates to commercialization activities and the achievement of specified levels of product sales. The Company has determined that $10.0 million related to the initiation of certain clinical development activities will be based upon its efforts and meet the criteria of substantive milestones and therefore will be recognized as revenue upon achievement of the milestone in accordance with the milestone method of accounting. The remaining $145.0 million of potential milestone payments are not substantive milestones as they do not require the efforts of the Company. During the years ended December 31, 2017 and 2015, a development milestone that was deemed a substantive milestone at the inception of the arrangement was achieved, and accordingly, the milestone payments of $7.0 million and $3.0 million, respectively, were recognized as revenue. If TRC105 products are successfully commercialized in the field of ophthalmology, Santen will be required to pay the Company tiered royalties on net sales ranging from high single digits to low teens, depending on the volume of sales, subject to adjustments in certain circumstances. In addition, Santen will reimburse the Company for all royalties due by the Company under certain third party agreements with respect to the use, manufacture or commercialization of TRC105 products in the field of ophthalmology by Santen and its affiliates and sublicensees. Royalties will continue on a country-by-country basis through the later of the expiration of the Company’s patent rights applicable to the TRC105 products in a given country or 12 years after the first commercial sale of the first TRC105 product commercially launched in such country. Santen may unilaterally terminate this agreement in its entirety, or on a country-by-country basis, upon written notice to the Company. Either party may terminate the agreement in the event of the other party’s bankruptcy or dissolution or for the other party’s material breach of the agreement that remains uncured 90 days (or 30 days with respect to a payment breach) after receiving notice from the non-breaching party. Unless earlier terminated, the agreement continues in effect until the termination of Santen’s payment obligations. In connection with the collaboration with Santen, the Company recognized revenue of $8.8 million, $3.4 million and $7.9 million for the years ended December 31, 2017, 2016 and 2015, respectively, and had deferred revenue of $0.0 million and $1.3 million as of December 31, 2017 and 2016, respectively. Janssen In September 2016, the Company entered into a license and option agreement with Janssen (the License and Option Agreement) under which Janssen granted the Company a license to technology and intellectual property to develop, manufacture and commercialize two compounds: a small molecule inhibitor of androgen receptor and androgen receptor mutations (the AR Mutant Program or TRC253) which is intended for the treatment of men with prostate cancer, and an inhibitor of NF-kB inducing kinase (the NIK Program or TRC694, and, together with the AR Mutant Program, the Programs). With respect to the AR Mutant Program, Janssen maintains an option, which is exercisable until 90 days after the Company demonstrates clinical proof of concept, to regain the rights to the licensed intellectual property and to obtain an exclusive license to commercialize the compounds and certain other specified intellectual property developed under the AR Mutant Program. If Janssen exercises the option, Janssen will be obligated to pay the Company (i) a one-time option exercise fee of $45.0 million; (ii) regulatory and commercial based milestone payments totaling up to $137.5 million upon achievement of specified events; and (iii) royalties in the low single digits on annual net sales of AR Mutant Program products. If Janssen does not exercise the option, the Company would then have the right to retain worldwide development and commercialization rights to the AR Mutant Program, in which case, the Company would be obligated to pay to Janssen (x) development and regulatory based milestone payments totaling up to $45.0 million upon achievement of specified events, and (y) royalties in the low single digits based on annual net sales of AR Mutant Program products, subject to certain specified reductions. With respect to the NIK Program, Janssen maintains a right, which is exercisable within 90 days following the date on which the Company demonstrates clinical proof of concept with respect to the NIK Program, to negotiate exclusively for a period of six months for a reversion of the related rights in the licensed intellectual property and to obtain an exclusive license to commercialize the compounds and certain other specified intellectual property developed under the NIK Program. If Janssen does not exercise its right of first negotiation, or, if after exercise of such right, the Company and Janssen are unable to reach an agreement on the terms of a reversion and exclusive license, and, in either case, the Company continues the development of the NIK Program, then the Company would be obligated to pay Janssen (i) development and regulatory based milestone payments totaling up to $60.0 million upon achievement of specified events, and (ii) royalties in the low single digits based on annual net sales of NIK Program products, subject to certain specified reductions. No consideration was exchanged for these assets on the acquisition date. Given the early preclinical stage of development of these assets and the low likelihood of success of development through regulatory approval on the acquisition date, no value was assigned to these assets in the accompanying consolidated balance sheet. The Company is obligated to use diligent efforts to develop the Programs according to agreed upon development plans, timelines and budgets. For each Program that the Company retains, the Company is further obligated to use commercially reasonable efforts to develop, obtain marketing approval for, and commercialize licensed products. Until the expiration or earlier termination of the development term of the AR Mutant Program or the NIK Program, as applicable, under the License and Option Agreement, subject to specified exceptions, the Company has agreed not to research, develop or commercialize any compounds or products related to the AR Mutant Program or the NIK Program, as applicable, other than pursuant to the collaboration with Janssen. The License and Option Agreement may be terminated for uncured breach, bankruptcy, or the failure or inability to demonstrate clinical proof of concept with respect to a particular Program during specified timeframes. In addition, the License and Option Agreement will automatically terminate (a) with respect to the AR Mutant Program, upon Janssen exercising its option in respect of the AR Mutant Program and making payment of the option exercise fee to the Company or, if Janssen does not exercise the option, upon the expiration of all payment obligations of the Company to Janssen with respect of the AR Mutant Program, and (b) with respect to the NIK Program, upon the Company and Janssen entering into an exclusive license agreement following Janssen’s exercise of its right of first negotiation or, if Janssen’s right of first negotiation with respect to the NIK Program expires and the Company and Janssen have not entered into an exclusive license agreement, upon the expiration of all payment obligations of the Company to Janssen with respect of the NIK Program. The Company may also terminate a Program or the License and Option Agreement in its entirety without cause, subject to specified conditions. Ambrx, Inc. In December 2017, the Company entered into a license agreement with Ambrx Inc. (Ambrx), for the development and commercialization of TRC105 in China. The license grants Ambrx the exclusive rights to use, develop, manufacture and commercialize TRC105 products in all indications (excluding ophthalmology which are held by Santen) in China (including Hong Kong and Macau) and Taiwan, or the Ambrx Territory. Ambrx also has the right to grant sublicenses to affiliates and third party collaborators, provided such sublicenses are consistent with the terms of the Company’s agreement and excluding the rights licensed to the Company under the license with Lonza. Ambrx has sole responsibility for funding, developing, seeking regulatory approval for and commercializing TRC105 products in the Ambrx Territory. Ambrx has the option to either pursue a China only development strategy at its sole expense, or upon mutual agreement of the Company and Ambrx, participate in the Company’s ongoing global Phase 3 TAPPAS clinical trial in angiosarcoma by enrolling patients in this trial, and the Company may participate in an Ambrx-sponsored clinical trial in hepatocellular carcinoma, or any other indication Ambrx pursues in the Ambrx Territory. In consideration of the rights granted to Ambrx under the agreement, the Company received a one-time upfront fee of $3.0 million. The license agreement provides for various types of payments, including the upfront payment, payments for various technical and regulatory support, payments for delivery of drug product, milestone payments, and royalties on net sales. The Company has identified multiple deliverables, which include at inception: (1) a license to patents, information and know-how related to TRC105, (2) collaboration, including technical and regulatory support provided by the Company, and (3) manufacturing and supply obligations. The Company anticipates that the first deliverable will be substantially delivered within the first three months of the agreement, the second deliverable will be delivered within the first nine months of the agreement, and the third deliverable will be delivered during the estimated 54-month period over which the Company will supply TRC105 to Ambrx. At inception and through December 31, 2017, the Company has identified one single unit of accounting for all the deliverables under the agreement since the delivered elements do not have standalone value. As the Company is the only manufacturer of TRC105 (through its contract manufacturer Lonza), a significant component of Ambrx’s ability to utilize the license and know-how related to TRC105 lies in access to TRC105 drug supplies. As a result, the Company has determined that the license, including the ability to sublicense, and know-how related to TRC105 do not have standalone value to a licensee. Accordingly, the Company will recognize revenue for the fixed or determinable collaboration consideration on a straight-line basis over the 54-month period over which it expects to deliver its manufacturing and supply obligations. In connection with the Ambrx agreement, at December 31, 2017, the Company had recorded $3.0 million of deferred revenue. In addition, the Company is eligible to receive up to a total of $140.5 million in milestone payments upon the achievement of certain milestones, of which $10.5 million relates to the submission of certain regulatory filings and receipt of certain regulatory approvals and $130.0 million relates to the achievement of specified levels of product sales. The Company has determined that $0.5 million related to the initiation of certain clinical development activities will be based upon its efforts and meet the criteria of substantive milestones and therefore will be recognized as revenue upon the achievement of the milestone in accordance with the milestone method of accounting. As of December 31, 2017, none of the development milestones had been achieved. The remaining $140.0 million of potential milestone payments are not substantive milestones as they do not require the efforts of the Company. If TRC105 products are successfully commercialized in the territory, Ambrx will be required to pay the Company tiered royalties on net sales ranging from high single digits to low teens, depending on the volume of sales, subject to adjustments in certain circumstances. Royalties will continue on a country-by-country basis through the later of the expiration of our patent rights applicable to the TRC105 products in a given country or 12 years after the first commercial sale of the first TRC105 product commercially launched in such country. Ambrx may unilaterally terminate this agreement in its entirety for any reason or for no reason upon at least 90 days’ notice to the Company. Either party may terminate the agreement in the event of the other party’s bankruptcy or dissolution or for the other party’s material breach of the agreement that remains uncured 60 days (or 30 days with respect to a payment breach) after receiving notice from the non-breaching party. Unless earlier terminated, the agreement continues in effect until the termination of Ambrx’s payment obligations.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	8. Income Taxes A reconciliation of the Company’s effective tax rate and federal statutory tax rate is summarized as follows (in thousands): Years Ended December 31, 2017 2016 2015 Federal income taxes $ (6,686 ) $ (9,453 ) $ (8,300 ) State income taxes, net of federal benefit (1,404 ) — — Permanent items 1,170 717 277 Uncertain tax positions (1,158 ) 1,644 749 Research and development credits (2,719 ) (2,078 ) (881 ) California net operating loss carryforwards (2,208 ) (1,054 ) — Rate change — (489 ) — Tax Cuts and Jobs Act 11,478 — — Other, net (126 ) 5 82 Stock-based compensation 123 — — Change in valuation allowance 1,530 10,708 8,073 Provision for income taxes $ — $ — $ — Significant components of the Company’s deferred tax assets are summarized as follows (in thousands): December 31, 2017 2016 Deferred tax assets: Net operating loss carryforwards $ 23,198 $ 24,484 Research and development credits and Orphan Drug credits 6,518 3,262 Deferred revenue — 441 Depreciation and amortization 414 229 Other, net 1,802 1,753 Total deferred tax assets 31,932 30,169 Valuation allowance (31,932 ) (30,169 ) Net deferred tax assets $ — $ — The Company has net deferred tax assets relating primarily to net operating loss (NOL) carryforwards and research and development credit carryforwards. Subject to certain limitations, the Company may use these deferred tax assets to offset taxable income in future periods. Due to the Company’s history of losses and uncertainty regarding future earnings, a full valuation allowance has been recorded against the Company’s deferred tax assets, as it is more likely than not that such assets will not be realized. The net change in the total valuation allowance for the years ended December 31, 2017, 2016 and 2015 was $1.5 million, $10.7 million and $8.1 million, respectively. At December 31, 2017, the Company had federal and California NOL carryforwards of approximately $83.1 million and $85.3 million, respectively. The federal and California NOL carryforwards will begin to expire in 2030, unless previously utilized. At December 31, 2017, the Company also had federal and California research and development and Orphan Drug credit carryforwards of approximately $7.0 million and $1.6 million, respectively. The federal research and development and Orphan Drug credit carryforwards will begin expiring in 2031 unless previously utilized. The California research credit will carry forward indefinitely under current law. Pursuant to Sections 382 and 383 of the Code, the annual use of the Company’s NOL and research and development credit carryforwards may be limited in the event that a cumulative change in ownership of more than 50% occurs within a three-year period. The Company completed a Section 382/383 analysis regarding the limitation of NOL and research and development credit carryforwards as of December 31, 2015 and as a result of the analysis, an ownership change was determined to have occurred at the time of the Company’s initial public offering in January 2015. Future ownership changes may further limit the Company’s ability to utilize the remaining NOL and research and development credit carryforwards. On December 22, 2017, President Trump signed into law the Tax Cuts and Jobs Act (the Tax Act). The Act amends the Internal Revenue Code to reduce tax rates and modify policies, credits, and deductions for individuals and businesses. For businesses, the Act reduces the corporate income tax rate from a maximum of 35% to a flat 21% rate. The rate reduction is effective on January 1, 2018. As a result of the rate reduction, the Company has reduced the deferred tax asset balance as of December 31, 2017 by $11.5 million. Due to the Company's full valuation allowance position, the Company has also reduced the valuation allowance by the same amount. In accordance with Staff Accounting Bulletin 118, as of December 31, 2017, the Company has not completed its accounting for the tax effects of the enactment of the Tax Act; however, the Company has made a reasonable estimate of the effects on its existing deferred tax balances. Due to uncertainties which currently exist in the interpretation of the provisions of the Tax Act regarding Internal Revenue Code Section 162(m), the Company has not completed its evaluation of the potential impacts of IRC Section 162(m) as amended by the Tax Act on its financial statements. The changes in the Company’s unrecognized tax benefits are summarized as follows (in thousands): Balance at December 31, 2014 $ 369 Increase related to prior year positions 1,135 Increase related to current year positions 318 Balance at December 31, 2015 1,822 Increase related to prior year positions 1,902 Increase related to current year positions 453 Balance at December 31, 2016 4,177 Decrease related to prior year positions (2,701 ) Increase related to current year positions 690 Balance at December 31, 2017 $ 2,166 The Company’s policy is to include interest and penalties related to unrecognized income tax benefits as a component of income tax expense. The Company has no accruals for interest or penalties in the accompanying consolidated balance sheets as of December 31, 2017 and 2016 and has not recognized interest or penalties in the accompanying consolidated statements of operations for the three years in the period ended December 31, 2017. Due to the valuation allowance recorded against the Company’s deferred tax assets, future changes in unrecognized tax benefits will not impact the Company’s effective tax rate. The Company does not expect its unrecognized tax benefits to change significantly in the next 12 months. The Company is subject to taxation in the United States and California. Due to the net operating loss carryforwards, the U.S. federal and California returns are open to examination for all years since inception. The Company has not been, nor is it currently, under examination by the federal or any state tax authority.

401(k) Plan

401(k) Plan	12 Months Ended
	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
401(k) Plan	9. 401(k) Plan The Company maintains 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. The Company, at its discretion, may make certain matching contributions to the 401(k) plan. Matching contributions for the years ended December 31, 2017, 2016 and 2015 totaled approximately $181,000, $172,000 and $107,000, respectively.

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data	10. Quarterly Financial Data (Unaudited) The following financial information reflects all normal recurring adjustments, which are, in the opinion of management, necessary for a fair statement of the results of the interim periods. Summarized quarterly data for the years ended December 31, 2017 and 2016 are as follows (in thousands, except per share data): First Second Third Fourth Quarter Quarter Quarter Quarter 2017 Revenue $ 626 $ 631 $ 7,498 $ - Total operating expenses $ 7,546 $ 6,961 $ 6,104 $ 6,354 Consolidated net income (loss) $ (7,147 ) $ (6,566 ) $ 1,170 $ (6,560 ) Basic and diluted net income (loss) attributable to common stockholders $ (0.44 ) $ (0.40 ) $ 0.07 $ (0.37 ) 2016 Revenue $ 1,210 $ 807 $ 815 $ 617 Total operating expenses $ 7,504 $ 8,817 $ 6,412 $ 6,692 Consolidated net loss $ (6,526 ) $ (8,297 ) $ (5,871 ) $ (6,314 ) Basic and diluted net loss attributable to common stockholders $ (0.54 ) $ (0.68 ) $ (0.48 ) $ (0.45 )

Organization and Summary of S18

Organization and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Organization And Accounting Policies [Abstract]
Organization and Business	Organization and Business TRACON Pharmaceuticals, Inc. (formerly Lexington Pharmaceuticals, Inc.) (TRACON or the Company) was incorporated in the state of Delaware on October 28, 2004. TRACON is a clinical stage biopharmaceutical company focused on the development and commercialization of novel targeted therapeutics for cancer, wet age-related macular degeneration and fibrotic diseases. The Company’s lead product candidate is an antibody that binds to the endoglin receptor, which is essential to angiogenesis (the process of new blood vessel formation). The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiary, TRACON Pharma Limited, which was formed in September 2015 and is currently inactive. All significant intercompany accounts and transactions have been eliminated.
Basis of Presentation	Basis of Presentation As of December 31, 2017, the Company has devoted substantially all of its efforts to product development, raising capital, and building infrastructure and has not realized revenues from its planned principal operations. The Company has incurred operating losses since inception. As of December 31, 2017, the Company had an accumulated deficit of $104.7 million. The Company anticipates that it will continue to incur net losses into the foreseeable future as it continues the development and commercialization of its product candidates and works to develop additional product candidates through research and development programs. At December 31, 2017, the Company had cash, cash equivalents and short-term investments of $34.5 million. Based on the Company’s current business plan, management believes that existing cash, cash equivalents and short-term investments will be sufficient to fund the Company’s obligations into the third quarter of 2018. The Company’s ability to execute its operating plan beyond mid-2018 depends on its ability to obtain additional funding through equity offerings, debt financings or potential licensing and collaboration arrangements. The accompanying consolidated financial statements have been prepared assuming the Company will continue as a going concern, which contemplates the realization of assets and settlement of liabilities in the normal course of business. However, the Company’s current working capital, anticipated operating expenses and net losses and the uncertainties surrounding its ability to raise additional capital as needed, as discussed below, raise substantial doubt about its ability to continue as a going concern for a period of one year following the date that these financial statements are issued. The consolidated financial statements do not include any adjustments for the recovery and classification of assets or the amounts and classification of liabilities that might be necessary should the Company be unable to continue as a going concern. The Company plans to continue to fund its losses from operations through cash, cash equivalents and investments on hand, as well as through future equity offerings, debt financings, other third party funding, and potential licensing or collaboration arrangements, including equity financing through the common stock purchase agreement the Company entered into with Aspire Capital Fund, LLC in March 2017 for the purchase of up to $21.0 million of the Company’s stock over a 30 month period and/or the at-the-market equity offering sales agreement the Company entered into with Stifel, Nicolaus & Company, Incorporated in February 2016 for the sale of up to $25.0 million of the Company’s stock. There can be no assurance that additional funds will be available when needed from any source or, if available, will be available on terms that are acceptable to the Company. Even if the Company raises additional capital, it may also be required to modify, delay or abandon some of its plans which could have a material adverse effect on the Company’s business, operating results and financial condition and the Company’s ability to achieve its intended business objectives.
Use of Estimates	Use of Estimates The Company’s consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States (GAAP). The preparation of the Company’s consolidated financial statements requires it to make estimates and assumptions that impact the reported amounts of assets, liabilities, revenue and expenses and the disclosure of contingent assets and liabilities in the Company’s financial statements and accompanying notes. The most significant estimates in the Company’s financial statements relate to revenue recognition, expenses incurred for clinical trials and the valuation of equity awards. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately materially differ from these estimates and assumptions.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents consist of cash and highly liquid investments with original maturities of three months or less at the date of purchase. The carrying amounts approximate fair value due to the short maturities of these investments. Cash and cash equivalents include cash in readily available checking and money market funds, U.S. treasury securities, as well as certificates of deposit.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash and cash equivalents. The Company maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and management believes that the Company is 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 is stated at cost and depreciated using the straight-line method over the estimated useful life of the related assets, which is generally five years. Leasehold improvements are amortized over the shorter of the lease term or estimated useful life of the related assets. Repairs and maintenance costs are charged to expense as incurred.
Deferred Rent	Deferred Rent Rent expense is recorded on a straight-line basis over the term of the lease. The difference between rent expense and amounts paid under the lease agreements is recorded as deferred rent in the accompanying consolidated balance sheets. Tenant improvement allowances and other lease incentives are recorded as liabilities and are amortized on a straight-line basis over the term of the lease as reductions to rent expense.
Revenue Recognition	Revenue Recognition Through December 31, 2017, all of the Company’s revenue was derived from its license agreement with Santen Pharmaceutical Co., Ltd. (Santen) as described in Note 7. The Company recognizes revenue when all four of the following criteria are met: (1) there is persuasive evidence that an arrangement exists; (2) delivery of the products and/or services has occurred; (3) the selling price is fixed or determinable; and (4) collectability is reasonably assured. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as long-term deferred revenue. The Company evaluates multiple-element arrangements to determine: (1) the deliverables included in the arrangement and (2) whether the individual deliverables represent separate units of accounting or whether they must be accounted for as a combined unit of accounting. Deliverables are considered separate units of accounting provided that: (a) the delivered items have value to the customer on a standalone basis and (b) if the arrangement includes a general right of return relative to the delivered items, delivery or performance of the undelivered items is considered probable and substantially in the Company’s control. In assessing whether an item has standalone value, the Company considers factors such as the research, manufacturing and commercialization capabilities of the partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the partner can use the other deliverables for their intended purpose without the receipt of the remaining elements, whether the value of the deliverable is dependent on the undelivered items and whether there are other vendors that can provide the undelivered elements. Arrangement consideration that is fixed or determinable is allocated among the separate units of accounting using the relative selling price method. The Company uses the following hierarchy of values to estimate the selling price of each deliverable: (1) vendor-specific objective evidence of fair value; (2) third-party evidence of selling price; and (3) best estimate of selling price (BESP). The BESP reflects the Company’s best estimate of what the selling price would be if the Company regularly sold the deliverable on a standalone basis. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity-specific factors, including factors that are contemplated in negotiating an arrangement and estimated costs. The Company validates the BESP for units of accounting by evaluating whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration between multiple units of accounting. The Company then applies the applicable revenue recognition criteria to each of the separate units of accounting in determining the appropriate period and pattern of recognition. If there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement on a straight-line basis over the period the Company expects to complete its performance obligations. With respect to revenue derived from reimbursement of direct, out-of-pocket expenses for research and development costs associated with collaborations, where the Company acts as a principal with discretion to choose suppliers, bear credit risk and perform part of the services required in the transaction, the Company records revenue for the gross amount of the reimbursement. The costs associated with these reimbursements are reflected as a component of research and development expense in the consolidated statements of operations. Milestones The Company uses the milestone method of accounting and revenue is recognized when earned, as evidenced by written acknowledgment from the collaborator or other persuasive evidence that the milestone has been achieved and the payment is non-refundable, provided that the milestone event is substantive. A milestone event is defined as an event: (1) that can only be achieved based in whole or in part on either the Company’s performance or on the occurrence of a specific outcome resulting from the Company’s performance; (2) for which there is substantive uncertainty at the inception of the arrangement that the event will be achieved; and (3) that would result in additional payments being due to the Company. Events for which the occurrence is either contingent solely upon the passage of time or the result of a counterparty’s performance are not considered to be milestone events. A milestone event is substantive if all of the following conditions are met: (a) the consideration is commensurate with either the Company’s performance to achieve the milestone, or the enhancement of the value to the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone; (b) the consideration relates solely to past performance; and (c) the consideration is reasonable relative to all the deliverables and payment terms (including other potential milestone consideration) within the arrangement. The Company assesses whether a milestone is substantive at the inception of each arrangement. If a milestone is deemed non-substantive, the Company will account for that milestone payment in accordance with the multiple element arrangements guidance and recognize it consistent with the related units of accounting for the arrangement over the related performance period.
Clinical Trial Expense Accruals	Clinical Trial Expense Accruals As part of the process of preparing the Company’s financial statements, the Company is required to estimate expenses resulting from its obligations under contracts with vendors, clinical sites, contract research organizations (CROs), and consultants in connection with conducting clinical trials. The financial terms of these contracts vary and may result in payment flows that do not match the periods over which materials or services are provided under such contracts. The Company’s objective is to reflect the appropriate trial expenses in its financial statements by recording those expenses in the period in which services are performed and efforts are expended. The Company accounts for these expenses according to the progress of the clinical trial as measured by patient progression and the timing of various aspects of the trial. The Company determines accrual estimates through discussion with the clinical sites and applicable personnel and outside service providers as to the progress or state of consummation of trials. During the course of a clinical trial, the Company adjusts the clinical expense recognition if actual results differ from its estimates. The Company makes estimates of accrued expenses as of each balance sheet date based on the facts and circumstances known at that time. The Company’s clinical trial accruals are dependent upon accurate reporting by clinical sites, CROs and other third-party vendors. Although the Company does not expect its estimates to differ materially from amounts actually incurred, its understanding of the status and timing of services performed relative to the actual status and timing of services performed may vary and may result in reporting amounts that are too high or too low for any particular period. For the three years ended in the period December 31, 2017, there were no material adjustments to the Company’s prior period estimates of accrued expenses for clinical trials.
Research and Development Costs	Research and Development Costs Research and development costs, including license fees, are expensed as incurred.
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 grant date fair value of employee stock option grants, employee restricted stock unit grants (RSUs) and employee stock purchase plan (ESPP) rights recognized as expense over the requisite service period of the awards (usually the vesting period) on a straight-line basis. The Company estimates the fair value of stock option grants and ESPP rights using the Black-Scholes option pricing model. The fair value of RSUs is based on the stock price on the date of grant. The Company accounts for stock options granted to non-employees using the fair value approach. These option grants, if any, are subject to periodic revaluation over their vesting terms.
Income Taxes	Income Taxes The Company accounts 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. The Company recognizes net deferred tax assets to the extent that the Company believes these assets are more likely than not to be realized. In making such a determination, management considers 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 management determines that the Company would be able to realize its deferred tax assets in the future in excess of their net recorded amount, management would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. The Company records uncertain tax positions on the basis of a two-step process whereby (1) management determines 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, management recognizes the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority. The Company recognizes interest and penalties related to unrecognized tax benefits within income tax expense. Any accrued interest and penalties are included within the related tax liability.
Comprehensive Loss	Comprehensive Loss Comprehensive loss is defined as a change in equity during a period from transactions and other events and circumstances from non-owner sources. Net loss and comprehensive loss were the same for all periods presented.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average shares of common stock 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. The Company has excluded 5,617, 7,878, and 6,555 weighted-average shares subject to repurchase or forfeiture from the weighted‑average number of common shares outstanding for the years ended December 31, 2017, 2016, and 2015, respectively. Diluted net loss per share is calculated by dividing the net loss by the weighted-average number of common stock equivalents outstanding for the period determined using the treasury-stock method. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to the Company’s 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 stock equivalent shares): December 31, 2017 2016 2015 Warrants to purchase common stock 103,865 57,173 57,173 Common stock options and restricted stock units 2,516,246 2,023,478 1,788,149 ESPP shares 3,653 2,857 143 2,623,764 2,083,508 1,845,465
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. The Company views its operations and manages its business in one operating segment.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2014-09, Revenue from Contracts with Customers plans on adopting ASU 2014-09 using the modified retrospective approach and does not expect the adoption to have a material impact on its financial position and results of operations. In February 2016, the FASB issued ASU 2016-02, Leases
Recently Adopted Accounting Standards	Recently Adopted Accounting Standards In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting

Organization and Summary of S19

Organization and Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Organization And Accounting Policies [Abstract]
Schedule of potentially dilutive securities not included in the 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 stock equivalent shares): December 31, 2017 2016 2015 Warrants to purchase common stock 103,865 57,173 57,173 Common stock options and restricted stock units 2,516,246 2,023,478 1,788,149 ESPP shares 3,653 2,857 143 2,623,764 2,083,508 1,845,465

Short-Term Investments, Cash 20

Short-Term Investments, Cash Equivalents and Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Schedule of Available-for-sale Securities	Cash equivalents and short-term investments, all of which are classified as available-for-sale securities, consisted of the following (in thousands): December 31, 2017 Cost Unrealized Gain Unrealized (Loss) Estimated Fair Value Money market funds $ 5,488 $ — $ — $ 5,488 U.S. treasury securities 4,999 — — 4,999 $ 10,487 $ — $ — $ 10,487 Classified as: Cash equivalents $ 5,488 Short-term investments 4,999 Total Cash equivalents and Short-term investments $ 10,487
Schedule of assets and liabilities measured at fair value on a recurring basis	The fair values of the Company’s assets and liabilities, which are measured at fair value on a recurring basis, were determined using the following inputs (in thousands): Fair Value Measurements at Reporting Date Using Quoted Prices in Significant Active Markets Other Significant for Identical Observable Unobservable Assets Inputs Inputs Total (Level 1) (Level 2) (Level 3) At December 31, 2017 Money market funds and U.S. treasury securities, included in Cash equivalents and Short-term investments $ 10,487 $ — $ 10,487 $ — At December 31, 2016 Money market funds and U.S. treasury securities, included in Cash equivalents and Short-term investments $ 23,346 $ — $ 23,346 $ —

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Schedule of property and equipment	Property and equipment consisted of the following (in thousands): December 31, 2017 2016 Computer and office equipment $ 133 $ 115 Furniture and fixtures 19 19 Leasehold improvements 21 124 173 258 Less accumulated depreciation and amortization (100 ) (176 ) $ 73 $ 82

Long-Term Debt (Tables)

Long-Term Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of Long-term debt and unamortized debt discount balances	Long-term debt and unamortized debt discount balances were as follows (in thousands): December 31, 2017 2016 Long-term debt $ 8,000 $ 8,000 Less debt discount, net of current portion (197 ) (537 ) Long-term debt, net of debt discount 7,803 7,463 Less current portion of long-term debt (3,200 ) (333 ) Long-term debt, net of current portion $ 4,603 $ 7,130 Current portion of long-term debt $ 3,200 $ 333 Current portion of debt discount (363 ) - Current portion of long-term debt, net $ 2,837 $ 333
Schedule of exercisable outstanding warrants for purchase of common stock issued	At December 31, 2017, the Company had the following exercisable outstanding warrants for the purchase of common stock issued in connection with the Company’s loan agreements with SVB: Expiration Number of shares Exercise price May 13, 2022 18,415 $ 10.86 November 14, 2023 through June 4, 2024 38,758 7.74 January 25, 2024 46,692 5.14 103,865
Schedule of future minimum principal and interest payments	Future minimum principal and interest payments under the 2017 Amended SVB Loan including the final payment, as of December 31, 2017 are as follows (in thousands): 2018 $ 3,766 2019 3,489 2020 1,961 9,216 Less interest and final payment (1,216 ) Long-term debt $ 8,000

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Schedule of future minimum payments under the non-cancelable operating lease	Future minimum payments under the non‑cancelable operating lease as of December 31, 2017 were as follows (in thousands): 2018 $ 405 2019 423 2020 442 2021 461 2022 156 $ 1,887

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit) (Tables)	12 Months Ended
	Dec. 31, 2017
Stockholders Equity Note [Abstract]
Schedule of restricted stock unit activity under 2015 plan	Restricted stock unit activity under the 2015 Plan is summarized as follows: Weighted Average Number of Grant Date Shares Fair Value Outstanding at December 31, 2016 306,780 $ 7.70 Granted - - Vested (102,378 ) 7.47 Forfeited (11,438 ) 6.11 Outstanding at December 31, 2017 192,964 $ 7.92
Schedule of stock option activity	Stock option activity under all Plans is summarized as follows: Weighted- Number of Average Options Exercise Price Balance at December 31, 2016 1,716,698 $ 7.54 Granted 689,527 4.76 Exercised (53,756 ) 0.83 Forfeited (29,187 ) 6.39 Balance at December 31, 2017 2,323,282 $ 6.88
Schedule of outstanding stock options	Information about the Company’s outstanding stock options is as follows (in thousands, except share and per share data and contractual term): Weighted- Average Weighted- Remaining Average Contractual Aggregate Number of Exercise Term Intrinsic Shares Price (in years) Value December 31, 2017: Options outstanding 2,323,282 $ 6.88 7.19 $ 1,293 Options vested and expected to vest 2,323,282 $ 6.88 7.19 $ 1,293 Options exercisable 1,359,286 $ 7.07 6.20 $ 1,198
Summary of weighted-average assumptions used Black-Scholes option pricing model to determine the fair value	The weighted-average 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, 2017 2016 2015 Risk-free interest rate 2.1 % 1.6 % 1.7 % Expected volatility 83.0 % 80.0 % 74.0 % Expected term (in years) 6.2 6.3 6.3 Expected dividend yield — % — % — %
Summary of allocation of stock-based compensation	The allocation of stock-based compensation was as follows (in thousands): Years Ended December 31, 2017 2016 2015 Research and development $ 1,482 $ 1,090 $ 1,038 General and administrative 1,712 1,993 1,050 $ 3,194 $ 3,083 $ 2,088
Schedule of common stock reserved for future issuance	Common stock reserved for future issuance was as follows: December 31, 2017 2016 Common stock warrants 103,865 57,173 Common stock options and restricted stock units granted and outstanding 2,516,246 2,023,478 Awards available under the 2015 Plan 772,573 749,753 Shares available under the Employee Stock Purchase Plan 378,367 261,840 3,771,051 3,092,244

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of reconciliation of effective tax rate and federal statutory tax rate	A reconciliation of the Company’s effective tax rate and federal statutory tax rate is summarized as follows (in thousands): Years Ended December 31, 2017 2016 2015 Federal income taxes $ (6,686 ) $ (9,453 ) $ (8,300 ) State income taxes, net of federal benefit (1,404 ) — — Permanent items 1,170 717 277 Uncertain tax positions (1,158 ) 1,644 749 Research and development credits (2,719 ) (2,078 ) (881 ) California net operating loss carryforwards (2,208 ) (1,054 ) — Rate change — (489 ) — Tax Cuts and Jobs Act 11,478 — — Other, net (126 ) 5 82 Stock-based compensation 123 — — Change in valuation allowance 1,530 10,708 8,073 Provision for income taxes $ — $ — $ —
Schedule of components of deferred tax assets	Significant components of the Company’s deferred tax assets are summarized as follows (in thousands): December 31, 2017 2016 Deferred tax assets: Net operating loss carryforwards $ 23,198 $ 24,484 Research and development credits and Orphan Drug credits 6,518 3,262 Deferred revenue — 441 Depreciation and amortization 414 229 Other, net 1,802 1,753 Total deferred tax assets 31,932 30,169 Valuation allowance (31,932 ) (30,169 ) Net deferred tax assets $ — $ —
Schedule of changes in unrecognized tax benefits	The changes in the Company’s unrecognized tax benefits are summarized as follows (in thousands): Balance at December 31, 2014 $ 369 Increase related to prior year positions 1,135 Increase related to current year positions 318 Balance at December 31, 2015 1,822 Increase related to prior year positions 1,902 Increase related to current year positions 453 Balance at December 31, 2016 4,177 Decrease related to prior year positions (2,701 ) Increase related to current year positions 690 Balance at December 31, 2017 $ 2,166

Quarterly Financial Data (Una26

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Schedule of quarterly data	Summarized quarterly data for the years ended December 31, 2017 and 2016 are as follows (in thousands, except per share data): First Second Third Fourth Quarter Quarter Quarter Quarter 2017 Revenue $ 626 $ 631 $ 7,498 $ - Total operating expenses $ 7,546 $ 6,961 $ 6,104 $ 6,354 Consolidated net income (loss) $ (7,147 ) $ (6,566 ) $ 1,170 $ (6,560 ) Basic and diluted net income (loss) attributable to common stockholders $ (0.44 ) $ (0.40 ) $ 0.07 $ (0.37 ) 2016 Revenue $ 1,210 $ 807 $ 815 $ 617 Total operating expenses $ 7,504 $ 8,817 $ 6,412 $ 6,692 Consolidated net loss $ (6,526 ) $ (8,297 ) $ (5,871 ) $ (6,314 ) Basic and diluted net loss attributable to common stockholders $ (0.54 ) $ (0.68 ) $ (0.48 ) $ (0.45 )

Organization and Summary of S27

Organization and Summary of Significant Accounting Policies - Additional Information (Details)	1 Months Ended	12 Months Ended
	Mar. 31, 2017USD ($)	Dec. 31, 2017USD ($)segmentshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015shares	Feb. 29, 2016USD ($)
Basis of Presentation
Accumulated deficit		$ (104,701,000)	$ (85,598,000)
Cash, cash equivalents and short-term investments		$ 34,500,000
Property and Equipment
Estimated useful life		5 years
Weighted-average shares subject to repurchase or forfeiture (in shares) | shares		5,617	7,878	6,555
Segment reporting
Number of reportable segments | segment		1
Stifel, Nicolaus & Company, Incorporated | At-the-Market Equity Offering Sales Agreement | Common Stock
Basis of Presentation
Maximum aggregate value of stock to be sold					$ 25,000,000
Aspire Capital | Common Stock
Basis of Presentation
Maximum aggregate value of stock to be purchase	$ 21,000,000
Sale duration for common stock under purchase agreement	30 months

Organization and Summary of S28

Organization and Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Not Included in the Calculation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive securities
Antidilutive securities	2,623,764	2,083,508	1,845,465
Warrants to Purchase Common Stock
Antidilutive securities
Antidilutive securities	103,865	57,173	57,173
Common Stock Options and Restricted Stock Units
Antidilutive securities
Antidilutive securities	2,516,246	2,023,478	1,788,149
ESPP Shares
Antidilutive securities
Antidilutive securities	3,653	2,857	143

Short-Term Investments, Cash 29

Short-Term Investments, Cash Equivalents and Fair Value Measurements - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017USD ($)
Fair Value Disclosures [Abstract]
Maximum period for remaining contractual maturities of available-for-sale investments	1 year
Amount of transfers between levels	$ 0

Short-Term Investments, Cash 30

Short-Term Investments, Cash Equivalents and Fair Value Measurements - Schedule of Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short-Term Investments, Cash Equivalents and Fair Value Measurements
Cost	$ 10,487
Estimated Fair Value	10,487
Short-term investments	4,999	$ 8,703
Estimated Fair Value
Short-Term Investments, Cash Equivalents and Fair Value Measurements
Cash equivalents	5,488
Short-term investments	4,999
Money market funds
Short-Term Investments, Cash Equivalents and Fair Value Measurements
Cost	5,488
Estimated Fair Value	5,488
U.S. treasury securities
Short-Term Investments, Cash Equivalents and Fair Value Measurements
Cost	4,999
Estimated Fair Value	$ 4,999

Short-Term Investments, Cash 31

Short-Term Investments, Cash Equivalents and Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring - Money market funds and U.S. treasury securities, included in Cash and cash equivalents and Short-term investments - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Fair value, assets	$ 10,487	$ 23,346
Level 2
Assets:
Fair value, assets	$ 10,487	$ 23,346

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property and Equipment
Property and equipment, gross	$ 173	$ 258
Less accumulated depreciation and amortization	(100)	(176)
Property and equipment, net	73	82
Computer and office equipment
Property and Equipment
Property and equipment, gross	133	115
Furniture and fixtures
Property and Equipment
Property and equipment, gross	19	19
Leasehold improvements
Property and Equipment
Property and equipment, gross	$ 21	$ 124

Property and Equipment - Additi

Property and Equipment - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation and amortization	$ 48	$ 94	$ 51

Long-Term Debt - Schedule of Lo

Long-Term Debt - Schedule of Long-Term Debt and Unamortized Debt Discount Balances (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Long-term debt
Long-term debt	$ 8,000	$ 8,000
Less debt discount, net of current portion	(197)	(537)
Long-term debt, net of debt discount	7,803	7,463
Less current portion of long-term debt	(3,200)	(333)
Long-term debt, net of current portion	4,603	7,130
Current portion of long-term debt	3,200	333
Current portion of debt discount	(363)
Current portion of long-term debt, net	$ 2,837	$ 333

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2015
Debt Instrument [Line Items]
Proceeds from long-term debt		$ 8,000,000	$ 10,000,000
Silicon Valley Bank
Debt Instrument [Line Items]
Warrants issued		103,865
2017 SVB Loan | Silicon Valley Bank
Debt Instrument [Line Items]
Proceeds from long-term debt	$ 8,000,000
Final payment on payoff	$ 900,000
Interest rate	8.55%
Repayment period	30 months
Additional fee on final payment due (as a percent)	4.00%
Warrants issued	46,692
Exercise price (per share)	$ 5.14
2017 SVB Loan | Silicon Valley Bank | After January 25, 2018 Prior to January 25, 2019
Debt Instrument [Line Items]
Prepayment fee (as a percent)	2.00%
2017 SVB Loan | Silicon Valley Bank | After January 25, 2019
Debt Instrument [Line Items]
Prepayment fee (as a percent)	1.00%

Long-Term Debt - Schedule of Ex

Long-Term Debt - Schedule of Exercisable Outstanding Warrants for Purchase of Common Stock Issued (Details) - Silicon Valley Bank	Dec. 31, 2017$ / sharesshares
Debt Instrument [Line Items]
Warrants issued	103,865
May 13, 2022
Debt Instrument [Line Items]
Warrants issued	18,415
Exercise price (per share) | $ / shares	$ 10.86
November 14, 2023 Through June 4, 2024
Debt Instrument [Line Items]
Warrants issued	38,758
Exercise price (per share) | $ / shares	$ 7.74
January 25, 2024
Debt Instrument [Line Items]
Warrants issued	46,692
Exercise price (per share) | $ / shares	$ 5.14

Long-Term Debt - Schedule of Fu

Long-Term Debt - Schedule of Future Minimum Principal and Interest Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt	$ 8,000	$ 8,000
2015 Amended SVB Loan
Debt Instrument [Line Items]
2,018	3,766
2,019	3,489
2,020	1,961
Long Term Debt Including Final Payment	9,216
Less interest and final payment	(1,216)
Long-term debt	$ 8,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	Feb. 22, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies
Non-cancelable purchase obligations		$ 9,600
Long-term manufacturing agreement description		Either party may terminate the Manufacturing Agreement due to a material breach of the Manufacturing Agreement by the other party, subject to prior written notice and a cure period, due to the insolvency or bankruptcy of the other party, or due to a force majeure event that prevents performance under the Manufacturing Agreement for at least six months. The Company may terminate the Manufacturing Agreement, subject to 60 days’ written notice, if the Company discontinues the TRC105 program, whether due to a notice of non-approval or withdrawal of marketing approval by a regulatory agency or otherwise. In the event of a termination by the Company due to discontinuation of the TRC105 program or a termination by Lonza due to the Company’s material breach or insolvency or bankruptcy, the Company would be obligated to pay to Lonza certain batch cancellation and/or early termination fees.
Prior written notice for termination on the breach of agreement period	60 days
Rent expense		$ 400	$ 400	$ 200
Non‑cancelable operating lease expiration term		2022-04
Non‑cancelable operating lease additional term to be extended		60 months
Irrevocable letter of credit provided to lessor		$ 175,000
Research and development arrangement
Commitments and Contingencies
Potential milestone payable		$ 126,000
Prior to Approval
Commitments and Contingencies
Long-Term manufacturing agreement for lead purchase of product candidate batches estimated annual cost	$ 15,000
Regulatory Approval
Commitments and Contingencies
Long-Term manufacturing agreement for lead purchase of product candidate batches estimated annual cost	$ 22,000

Commitments and Contingencies39

Commitments and Contingencies - Schedule of Future Minimum Payments Under the Non-Cancelable Operating Lease (Details) $ in Thousands	Dec. 31, 2017USD ($)
Future minimum payments under non-cancelable operating lease
2,018	$ 405
2,019	423
2,020	442
2,021	461
2,022	156
Total	$ 1,887

Stockholders' Equity (Deficit40

Stockholders' Equity (Deficit) - Additional Information (Details) - USD ($)	May 01, 2017	Feb. 04, 2015	Mar. 31, 2017	Nov. 30, 2016	Sep. 30, 2016	Feb. 29, 2016	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2016
Class Of Stock [Line Items]
Common stock, shares authorized		200,000,000					200,000,000		200,000,000
Undesignated preferred stock authorized		10,000,000					10,000,000		10,000,000
Preferred stock dividends paid		$ 0
Common Stock
Class Of Stock [Line Items]
Number of redeemable convertible preferred stock converted into common stock		6,369,567
Conversion of warrants to purchase shares		38,758
Stock issued during period, shares, new issues								4,100,000
Proceeds from sale of common stock				$ 16,100,000
Payments of stock issuance costs				$ 1,300,000
Common Stock | License and Option Agreement
Class Of Stock [Line Items]
Stock issued during period, shares, new issues					840,022
Shares issued, price per share					$ 5.95
Proceeds from sale of common stock					$ 5,000,000
Number of consecutive trading days					5 days
Common Stock | IPO
Class Of Stock [Line Items]
Stock issued during period, shares, new issues				3,018,750
Shares issued, price per share				$ 5.75
Common Stock | Aspire Capital
Class Of Stock [Line Items]
Maximum aggregate value of stock to be purchase			$ 21,000,000
Stock issued during period, shares, new issues	195,726		222,222
Shares issued, price per share			$ 4.50
Proceeds from sale of common stock			$ 900,000
Maximum value of additional shares committed to purchase	$ 20,000,000
Sale duration for common stock under purchase agreement	30 months
Common Stock | Stifel, Nicolaus & Company, Incorporated | At-the-Market Equity Offering Sales Agreement
Class Of Stock [Line Items]
Stock issued during period, shares, new issues							1,037,000
Proceeds from sale of common stock							$ 3,500,000
Maximum aggregate value of stock to be sold						$ 25,000,000
Percentage of gross proceeds, required to pay for common stock sold through sales agreement						2.50%
Common stock remains available for sale							$ 21,500,000
Period over which limited value of shares subjected to sell under form S-3							12 months
Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Conversion of warrants to purchase shares		150,000

Stockholders' Equity (Deficit41

Stockholders' Equity (Deficit) - (SBC) - Additional Information (Details) - USD ($)	Jan. 02, 2015	Dec. 31, 2015	Jan. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Plans
Shares of common stock reserved for future issuance				3,771,051	3,092,244
Stock Option
Share-based compensation
Share price (in dollar per share)				$ 3.35
Unrecognized compensation cost, period of recognition				2 years 1 month 6 days	2 years 1 month 6 days
Unrecognized compensation cost				$ 3,700,000	$ 4,000,000
Stock options, additional disclosures
Granted (in dollars per share)				$ 3.45	$ 6.68	$ 12.97
Share price (in dollar per share)				$ 3.35
Proceeds from exercise of stock options				$ 44,900	$ 6,500	$ 54,200
Total intrinsic value of options exercised				200,000	78,000	800,000
Total fair value of options vested during the year				$ 1,900,000	$ 3,400,000	$ 800,000
Stock Option | Year One
Stock Plans
Grants vesting period				1 year
Grants vesting (as a percent)				25.00%
Stock Option | Thereafter
Stock Plans
Grants vesting period				36 months
ESPP
Stock Plans
Shares of common stock reserved for future issuance	183,462			378,367	261,840
Percentage of total number of shares of common stock outstanding	1.00%
Share-based compensation
Percentage of eligible compensation	15.00%
Stock options, additional disclosures
Number of shares that could potentially be issued	366,925
2011 Equity Incentive Plan | Stock Option
Stock Plans
Maximum term of options granted			10 years
Grants vesting period			48 months
2015 Equity Incentive Plan
Stock Plans
Maximum term of options granted				10 years
Shares of common stock reserved for future issuance	801,033			772,573	749,753
Percentage of total number of shares of common stock outstanding	4.00%
Additional shares to be used exclusively for grant for Inducement awards		500,000
2015 Equity Incentive Plan | Restricted Stock Units
Share-based compensation
Aggregate intrinsic value of outstanding units				$ 600,000
Share price (in dollar per share)				$ 3.35
Unrecognized compensation cost related to outstanding units				$ 1,000,000
Unrecognized compensation cost, period of recognition				2 years 1 month 6 days
Stock options, additional disclosures
Share price (in dollar per share)				$ 3.35
2015 Equity Incentive Plan | Restricted Stock Units | Board of Directors
Share-based compensation
Total fair value of the units				$ 800,000	$ 0

Stockholders' Equity (Deficit42

Stockholders' Equity (Deficit) - Summary of Restricted Stock Unit Activity (Details) - Restricted Stock Units - 2015 Equity Incentive Plan	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Number of Options
Balance (in shares) | shares	306,780
Vested (in shares) | shares	(102,378)
Forfeited (in shares) | shares	(11,438)
Balance (in shares) | shares	192,964
Weighted Average Grant Date Fair Value
Balance (in dollars per share) | $ / shares	$ 7.70
Vested (in dollars per share) | $ / shares	7.47
Forfeited (in dollars per share) | $ / shares	6.11
Balance (in dollars per share) | $ / shares	$ 7.92

Stockholders' Equity (Deficit43

Stockholders' Equity (Deficit) - Summary of Stock Option Activity (Details) - Stock Option	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Number of Options
Balance (in shares) | shares	1,716,698
Granted (in shares) | shares	689,527
Exercised (in shares) | shares	(53,756)
Forfeited (in shares) | shares	(29,187)
Balance (in shares) | shares	2,323,282
Weighted-Average Exercise Price
Balance (in dollars per share) | $ / shares	$ 7.54
Granted (in dollars per share) | $ / shares	4.76
Exercised (in dollars per share) | $ / shares	0.83
Forfeited (in dollars per share) | $ / shares	6.39
Balance (in dollars per share) | $ / shares	$ 6.88

Stockholders' Equity (Deficit44

Stockholders' Equity (Deficit) - Summary of Company's Outstanding Stock Option (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Number of Options
Number of shares, options vested and expected to vest	2,516,246	2,023,478
Stock Option
Number of Options
Balance (in shares)	1,716,698
Number of shares, options vested and expected to vest	2,323,282
Number of shares, options exercisable	1,359,286
Aggregate intrinsic value, options outstanding	$ 1,293
Aggregate intrinsic value, options vested and expected to vest	1,293
Aggregate intrinsic value, options exercisable	$ 1,198
Weighted-Average Exercise Price
Balance (in shares)	$ 6.88	$ 7.54
Weighted-average exercise price, options vested and expected to vest (in dollars per share)	6.88
Weighted-average exercise price, options exercisable (in dollars per share)	$ 7.07
Weighted-average remaining contractual term, options outstanding (in years)	7 years 2 months 8 days
Weighted-average remaining contractual term, options vested and expected to vest (in years)	7 years 2 months 8 days
Weighted-average remaining contractual term, options exercisable (in years)	6 years 2 months 12 days

Stockholders' Equity (Deficit45

Stockholders' Equity (Deficit) - Summary of Weighted-Average Assumptions Fair Value of the Employee Stock Option Grants (Details) - Stock Option	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class Of Stock [Line Items]
Risk-free interest rate	2.10%	1.60%	1.70%
Expected volatility	83.00%	80.00%	74.00%
Expected term (in years)	6 years 2 months 12 days	6 years 3 months 18 days	6 years 3 months 18 days
Expected dividend yield	0.00%	0.00%	0.00%

Stockholders' Equity (Deficit46

Stockholders' Equity (Deficit) - Summary of Allocation of Stock-Based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class Of Stock [Line Items]
Stock based compensation expense	$ 3,194	$ 3,083	$ 2,088
Research and development
Class Of Stock [Line Items]
Stock based compensation expense	1,482	1,090	1,038
General and administrative
Class Of Stock [Line Items]
Stock based compensation expense	$ 1,712	$ 1,993	$ 1,050

Stockholders' Equity (Deficit47

Stockholders' Equity (Deficit) - Summary of Common Stock Reserved For Future Issuance (Details) - shares	Dec. 31, 2017	Dec. 31, 2016	Jan. 02, 2015
Stock Plans
Shares of common stock reserved for future issuance	3,771,051	3,092,244
Number of shares, options vested and expected to vest	2,516,246	2,023,478
2015 Equity Incentive Plan
Stock Plans
Shares of common stock reserved for future issuance	772,573	749,753	801,033
Common Stock. | Warrants
Stock Plans
Shares of common stock reserved for future issuance	103,865	57,173
ESPP
Stock Plans
Shares of common stock reserved for future issuance	378,367	261,840	183,462

Collaborations - Additional Inf

Collaborations - Additional Information (Details) - USD ($)

1 Months Ended

3 Months Ended

12 Months Ended

Dec. 31, 2017

Mar. 31, 2014

Sep. 30, 2017

Jun. 30, 2017

Mar. 31, 2017

Dec. 31, 2016

Sep. 30, 2016

Jun. 30, 2016

Mar. 31, 2016

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Contracts Revenue

$ 7,498,000

$ 631,000

$ 626,000

$ 617,000

$ 815,000

$ 807,000

$ 1,210,000

$ 8,755,000

$ 3,449,000

$ 7,904,000

Janssen

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

License and Option Agreement termination description

The License and Option Agreement may be terminated for uncured breach, bankruptcy, or the failure or inability to demonstrate clinical proof of concept with respect to a particular Program during specified timeframes. In addition, the License and Option Agreement will automatically terminate (a) with respect to the AR Mutant Program, upon Janssen exercising its option in respect of the AR Mutant Program and making payment of the option exercise fee to the Company or, if Janssen does not exercise the option, upon the expiration of all payment obligations of the Company to Janssen with respect of the AR Mutant Program, and (b) with respect to the NIK Program, upon the Company and Janssen entering into an exclusive license agreement following Janssen’s exercise of its right of first negotiation or, if Janssen’s right of first negotiation with respect to the NIK Program expires and the Company and Janssen have not entered into an exclusive license agreement, upon the expiration of all payment obligations of the Company to Janssen with respect of the NIK Program. The Company may also terminate a Program or the License and Option Agreement in its entirety without cause, subject to specified conditions.

Janssen | License and Option Agreement | AR Mutant Program

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Exercisable period

90 days

License and option agreement terms

Janssen will be obligated to pay the Company (i) a one-time option exercise fee of $45.0 million; (ii) regulatory and commercial based milestone payments totaling up to $137.5 million upon achievement of specified events; and (iii) royalties in the low single digits on annual net sales of AR Mutant Program products.

One-time option exercise fee

$ 45,000,000

Regulatory and commercial based milestone payments

137,500,000

Development and regulatory based milestone payments

$ 45,000,000

Janssen | License and Option Agreement | NIK Program

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Exercisable period

90 days

License and option agreement terms

The Company would be obligated to pay Janssen (i)?development and regulatory based milestone payments totaling up to $60.0 million upon achievement of specified events, and (ii) royalties in the low single digits based on annual net sales of NIK Program products, subject to certain specified reductions.

Development and regulatory based milestone payments

$ 60,000,000

Consideration exchanged for assets acquired

0

Value given to assets acquired

0

0

Collaborative Arrangement | Santen

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

One-time upfront fee

$ 10,000,000

Period for recognition of revenue

41 months

Potential milestone that could be received

155,000,000

Balance of potential milestone that could be received

$ 145,000,000

$ 145,000,000

Royalty continuation term

12 years

Cure period for bankruptcy or dissolution

90 days

Cure period for breach of payment

30 days

Contracts Revenue

$ 8,800,000

3,400,000

7,900,000

Deferred revenue

0

$ 1,300,000

0

$ 1,300,000

Collaborative Arrangement | Santen | Development Milestones

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone that could be received

20,000,000

Collaborative Arrangement | Santen | Regulatory Milestones

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone that could be received

52,500,000

Collaborative Arrangement | Santen | Commercialization Milestones

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone that could be received

$ 82,500,000

Revenue recognized upon achievement of milestone

10,000,000

Aggregate milestone revenue recognized

7,000

$ 3,000,000

Collaborative Arrangement | Ambrx, Inc.

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

One-time upfront fee

3,000,000

Potential milestone that could be received

140,500,000

140,500,000

Balance of potential milestone that could be received

140,000,000

$ 140,000,000

Royalty continuation term

12 years

Cure period for bankruptcy or dissolution

60 days

Cure period for breach of payment

30 days

Deferred revenue

3,000,000

$ 3,000,000

Collaborative Arrangement | Ambrx, Inc. | Regulatory Milestones

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone that could be received

10,500,000

10,500,000

Collaborative Arrangement | Ambrx, Inc. | Commercialization Milestones

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized upon achievement of milestone

500,000

Aggregate milestone revenue recognized

0

Collaborative Arrangement | Ambrx, Inc. | Product Sales Milestones

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone that could be received

$ 130,000,000

$ 130,000,000

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Effective Tax Rate and Federal Statutory Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of effective tax rate and federal statutory tax rate
Federal income taxes	$ (6,686)	$ (9,453)	$ (8,300)
State income taxes, net of federal benefit	(1,404)
Permanent items	1,170	717	277
Uncertain tax positions	(1,158)	1,644	749
Research and development credits	(2,719)	(2,078)	(881)
California net operating loss carryforwards	(2,208)	(1,054)
Rate change		(489)
Tax Cuts and Jobs Act	11,478
Other, net	(126)	5	82
Stock-based compensation	123
Change in valuation allowance	$ 1,530	$ 10,708	$ 8,073

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 23,198	$ 24,484
Research and development credits and Orphan Drug credits	6,518	3,262
Deferred revenue		441
Depreciation and amortization	414	229
Other, net	1,802	1,753
Total deferred tax assets	31,932	30,169
Valuation allowance	$ (31,932)	$ (30,169)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Carryforwards
Net change in the total valuation allowance		$ 1,500,000	$ 10,700,000	$ 8,100,000
Corporate tax rate		35.00%
Tax Cuts and Jobs Act of 2017 reduction in rate, reduced the deferred tax asset		$ 11,500,000
Accrual for interest or penalties for unrecognized tax benefits		0	$ 0
Income tax interest or penalties expense for unrecognized tax benefits		$ 0
Scenario, Forecast
Carryforwards
Corporate tax rate	21.00%
Minimum
Carryforwards
Percent of ownership		50.00%
Federal
Carryforwards
NOL carryforward		$ 83,100,000
NOL carryforward begin to expire		2,030
Federal | Research and Development and Orphan Drug Credit Carryforwards
Carryforwards
Credit carryforward		$ 7,000,000
Credit carryforward begin to expire		2,031
California
Carryforwards
NOL carryforward		$ 85,300,000
NOL carryforward begin to expire		2,030
California | Research and Development and Orphan Drug Credit Carryforwards
Carryforwards
Credit carryforward		$ 1,600,000

Income Taxes - Schedule of Chan

Income Taxes - Schedule of Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized tax benefits
Balance, beginning of period	$ 4,177	$ 1,822	$ 369
Decrease related to prior year positions	(2,701)
Increase related to prior year positions		1,902	1,135
Increase related to current year positions	690	453	318
Balance, end of period	$ 2,166	$ 4,177	$ 1,822

401(k) Plan - Additional Inform

401(k) Plan - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Pension and Other Postretirement Plans Disclosure
Defined contribution amount	$ 181,000	$ 172,000	$ 107,000

Quarterly Financial Data (Una54

Quarterly Financial Data (Unaudited) - Schedule of Quarterly Data - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information
Revenue		$ 7,498	$ 631	$ 626	$ 617	$ 815	$ 807	$ 1,210	$ 8,755	$ 3,449	$ 7,904
Total operating expenses	$ 6,354	6,104	6,961	7,546	6,692	6,412	8,817	7,504	26,965	29,425	31,371
Consolidated net income (loss)	$ (6,560)	$ 1,170	$ (6,566)	$ (7,147)	$ (6,314)	$ (5,871)	$ (8,297)	$ (6,526)	$ (19,103)	$ (27,008)	$ (24,410)
Basic and diluted net income (loss) attributable to common stockholders	$ (0.37)	$ 0.07	$ (0.40)	$ (0.44)	$ (0.45)	$ (0.48)	$ (0.68)	$ (0.54)	$ (1.14)	$ (2.13)	$ (2.20)