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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Mar. 07, 2018	Jun. 30, 2017
Document and Entity Information
Entity Registrant Name	CATABASIS PHARMACEUTICALS INC
Entity Central Index Key	0001454789
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	Non-accelerated Filer
Entity Public Float			$ 24,632,957
Entity Common Stock, Shares Outstanding		29,035,502
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 16,369	$ 23,596
Available-for-sale securities		14,931
Prepaid expenses and other current assets	1,094	1,001
Total current assets	17,463	39,528
Property and equipment, net	321	568
Restricted cash	113	113
Total assets	17,897	40,209
Current liabilities:
Accounts payable	773	1,405
Accrued expenses	2,432	3,677
Current portion of notes payable, net of discount	2,479	3,243
Other liability	332
Total current liabilities	6,016	8,325
Deferred rent, net of current portion	89	53
Notes payable, net of current portion and discount		2,479
Other liability		266
Total liabilities	6,105	11,123
Commitments (Note 9)
Stockholders' equity:
Preferred stock, $0.001 par value per share, 5,000,000 shares authorized and no shares issued and outstanding
Common stock, $0.001 par value per share, 150,000,000 shares authorized; 23,645,247 and 18,817,572 shares issued and outstanding at December 31, 2017 and 2016, respectively	24	19
Additional paid-in capital	183,202	173,141
Accumulated other comprehensive loss		(4)
Accumulated deficit	(171,434)	(144,070)
Total stockholders' equity	11,792	29,086
Total liabilities and stockholders' equity	$ 17,897	$ 40,209

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Consolidated Balance Sheets
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, authorized (in Shares)	5,000,000	5,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, authorized (in shares)	150,000,000	150,000,000
Common stock, issued (in shares)	23,645,247	18,817,572
Common stock, outstanding (in shares)	23,645,247	18,817,572

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements of Operations
Revenue	$ 500
Operating expenses:
Research and development	18,682	$ 25,450	$ 23,030
General and administrative	8,912	10,108	8,629
Total operating expenses	27,594	35,558	31,659
Loss from operations	(27,094)	(35,558)	(31,659)
Other (expense) income:
Interest expense	(462)	(837)	(978)
Interest and investment income	160	242
Other income, net	32	93	7
Total other expense, net	(270)	(502)	(971)
Net loss	$ (27,364)	$ (36,060)	$ (32,630)
Net loss per share - basic and diluted (in dollars per share)	$ (1.26)	$ (2.22)	$ (4.06)
Weighted-average common shares outstanding used in net loss per share - basic and diluted (in shares)	21,681,534	16,230,190	8,041,948

Consolidated Statements Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements Comprehensive Loss
Net loss	$ (27,364)	$ (36,060)	$ (32,630)
Other comprehensive income (loss):
Gain (loss) on available-for-sale securities	4	(4)
Total other comprehensive income (loss):	4	(4)
Comprehensive loss	$ (27,360)	$ (36,064)	$ (32,630)

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss	Total
Balance at Dec. 31, 2014	$ 47,898	$ 32,248
Balance (in shares) at Dec. 31, 2014	68,837,703	34,129,571
Increase (decrease) in Convertible Preferred Stock
Issuance of series B convertible preferred stock, net of issuance cost of $0.1 million		$ 12,331
Issuance of series B convertible preferred stock, net of issuance cost of $0.1 million (in shares)		13,062,965
Conversion of convertible preferred stock into common stock	$ (47,898)	$ (44,579)
Conversion of convertible preferred stock into common stock (in shares)	(68,837,703)	(47,192,536)
Balance at Dec. 31, 2014			$ 1	$ 2,326	$ (75,380)		$ (73,053)
Balance (in shares) at Dec. 31, 2014			493,200
Increase (Decrease) in Stockholders' Equity (Deficit)
Issuance of common stock from initial public offering/for registered direct and at-the-market offerings, net of issuance costs of $0.6 million, $1.1 million and $7.3 million for the years 2017, 2016 and 2015 respectively			$ 5	61,739			61,744
Issuance of common stock from initial public offering/for registered direct and at-the-market offerings, net of issuance costs of $0.6 million, $1.1 million and $7.3 million for the years 2017, 2016 and 2015 respectively (in shares)			5,750,000
Conversion of convertible preferred stock into common stock			$ 9	92,468			92,477
Conversion of convertible preferred stock into common stock (in shares)			9,029,549
Conversion of series B preferred stock warrants into warrants for the purchase of common stock				206			206
Proceeds from exercises of common stock options				91			91
Proceeds from exercises of common stock options (in shares)			40,548
Stock-based compensation expense				1,658			1,658
Net loss					(32,630)		(32,630)
Balance at Dec. 31, 2015			$ 15	158,488	(108,010)		50,493
Balance (in shares) at Dec. 31, 2015			15,313,297
Increase (Decrease) in Stockholders' Equity (Deficit)
Issuance of common stock from initial public offering/for registered direct and at-the-market offerings, net of issuance costs of $0.6 million, $1.1 million and $7.3 million for the years 2017, 2016 and 2015 respectively			$ 3	11,991			11,994
Issuance of common stock from initial public offering/for registered direct and at-the-market offerings, net of issuance costs of $0.6 million, $1.1 million and $7.3 million for the years 2017, 2016 and 2015 respectively (in shares)			3,243,015
Proceeds from exercises of common stock options and warrants			$ 1	502			503
Proceeds from exercises of common stock options and warrants (in shares)			261,260
Stock-based compensation expense				2,160			2,160
Unrealized losses on short-term investments						$ (4)	(4)
Net loss					(36,060)		(36,060)
Balance at Dec. 31, 2016			$ 19	173,141	(144,070)	(4)	$ 29,086
Balance (in shares) at Dec. 31, 2016			18,817,572				18,817,572
Increase (Decrease) in Stockholders' Equity (Deficit)
Issuance of common stock from initial public offering/for registered direct and at-the-market offerings, net of issuance costs of $0.6 million, $1.1 million and $7.3 million for the years 2017, 2016 and 2015 respectively			$ 5	8,031			$ 8,036
Issuance of common stock from initial public offering/for registered direct and at-the-market offerings, net of issuance costs of $0.6 million, $1.1 million and $7.3 million for the years 2017, 2016 and 2015 respectively (in shares)			4,804,796
Proceeds from exercises of common stock options				23			$ 23
Proceeds from exercises of common stock options (in shares)			22,879				22,879
Stock-based compensation expense				2,007			$ 2,007
Realized gain on short-term investments						$ 4	4
Net loss					(27,364)		(27,364)
Balance at Dec. 31, 2017			$ 24	$ 183,202	$ (171,434)		$ 11,792
Balance (in shares) at Dec. 31, 2017			23,645,247				23,645,247

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Series B Convertible Preferred Stock
Stock issuance costs			$ 0.1
Initial public offering
Stock issuance costs	$ 0.6	$ 1.1	$ 7.3

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (27,364)	$ (36,060)	$ (32,630)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	304	395	202
Stock-based compensation expense	2,007	2,160	1,658
Accretion of discount/premium on investment securities	26	148
Non-cash interest expense	155	277	293
Gain on sale of fixed assets	(30)	(52)
Changes in assets and liabilities:
Prepaid expenses and other current assets	(93)	(229)	(450)
Other assets			2
Accounts payable	(632)	77	196
Accrued expenses	(1,215)	418	954
Deferred rent	6	8	(18)
Net cash used in operating activities	(26,836)	(32,858)	(29,793)
Investing activities
Purchases of available-for-sale securities		(45,539)
Sales and maturities of available-for-sale securities	14,910	30,456
Purchases of property and equipment	(57)	(459)	(421)
Sale of property and equipment	30	52
Net cash provided by (used in) investing activities	14,883	(15,490)	(421)
Financing activities
Proceeds from initial public offering, net of issuance costs			61,744
Proceeds from issuance of preferred stock, net of issuance costs			12,331
Proceeds from registered direct offering, net of issuance costs		10,603
Proceeds from at-the-market offering, net of issuance costs	8,036	1,391
Proceeds from exercise of common stock options and warrants	23	503	91
Proceeds from borrowing			5,000
Payments on borrowing	(3,333)	(3,333)	(833)
Debt issuance costs			(7)
Net cash provided by financing activities	4,726	9,164	78,326
Net (decrease) increase in cash and cash equivalents	(7,227)	(39,184)	48,112
Cash and cash equivalents, beginning of period	23,596	62,780	14,668
Cash and cash equivalents, end of period	16,369	23,596	62,780
Supplemental disclosure of cash flow information
Cash paid for interest	$ 327	$ 582	684
Non-cash financing activities
Warrants for the purchase of series B preferred stock issued in conjunction with credit facility			107
Initial public offering costs in accounts payable and accrued liabilities			(492)
Reclassification of deferred IPO costs from non-current assets to additional paid-in capital			1,787
Reclassification of warrant liability to additional paid-in capital			$ 206

Organization and Operations	12 Months Ended
Organization and Operations	Dec. 31, 2017
Organization and Operations
Organization and Operations	1. Organization and Operations The Company Catabasis Pharmaceuticals, Inc. (the "Company") is a clinical-stage biopharmaceutical company focused on the discovery, development and commercialization of novel therapeutics based on the Company's proprietary Safely Metabolized And Rationally Targeted, or SMART, linker, or SMART LinkerSM, drug discovery platform. The Company's SMART Linker technology platform enables the Company to engineer product candidates that can simultaneously modulate multiple targets in a disease. The Company's proprietary product candidates impact pathways that are central to diseases where efficacy may be optimized by a multiple target approach. The Company's primary focus is on treatments for rare diseases. The Company has applied its SMART Linker drug discovery platform to build an internal pipeline of product candidates for rare diseases and plans to pursue partnerships to develop additional product candidates. The Company was incorporated in the State of Delaware on June 26, 2008. Liquidity and Going Concern In August 2016, the Company entered into a sales agreement with Cowen and Company LLC, ("Cowen"), pursuant to which the Company could issue and sell shares of common stock for an aggregate maximum offering amount of $10.0 million under an at-the-market offering program (the "First ATM Program"). In March 2017, the Company reduced the maximum amount of the offering under the First ATM Program to $8.8 million, which was fully utilized as of September 30, 2017. In October 2017, the Company entered into a second sales agreement with Cowen pursuant to which the Company may issue and sell shares of common stock for an aggregate maximum offering amount of $10.0 million under a second at-the-market offering program (the "Second ATM Program"). Cowen is not required to sell any specific amount, but acts as the Company's sales agent using commercially reasonable efforts consistent with its normal trading and sales practices. Shares sold pursuant to the sales agreement have been sold pursuant to a shelf registration statement, which became effective on July 19, 2016. The Company pays Cowen 3% of the gross proceeds from any common stock sold through these sales agreements. During the year ended December 31, 2017, the Company sold an aggregate of 4,804,796 shares of common stock pursuant to the First ATM Program, at an average price of $1.79 per share, for gross proceeds of $8.6 million, resulting in net proceeds of $8.0 million after deducting sales commissions and offering. As of December 31, 2017, $8.6 million of common stock remained available for sale under the Second ATM Program. Subsequent to December 31, 2017, the Company sold an aggregate of 5,390,255 shares of common stock pursuant to the Second ATM Program, at an average price of $1.59 per share, for gross proceeds of $8.6 million, representing the remainder of the common stock that could be sold under the Second ATM Program, resulting in net proceeds of $8.3 million after deducting sales commissions and offering expenses, thus the Second ATM Program has been fully utilized. During the year ended December 31, 2016, the Company sold an aggregate of 3,243,015 shares of common stock either through a direct offering or an at-the-market offering at an average offering price of $4.04 per share. The shares sold in the offering were sold pursuant to the Shelf Registration Statement. The Company received aggregate gross proceeds from the sales of $13.1 million, resulting in net proceeds of $12.0 million after deducting underwriting discounts and commissions and offering expenses. As of December 31, 2017, the Company had an accumulated deficit of $171.4 million. The Company has been primarily involved with research and development activities and has incurred operating losses and negative cash flows from operations since its inception. The Company is subject to a number of risks including, but not limited to, the need to obtain adequate additional funding, including the resources necessary to fund a global Phase 3 clinical trial of edasalonexent in DMD, which the Company plans to begin in the first half of 2018. The Company is currently exploring various avenues to secure capital to advance edasalonexent into a Phase 3 clinical trial, and its Phase 3 development activities will be dependent on securing adequate capital resources. The Company is also subject to a number of risks similar to other life science companies, including, but not limited to, successful discovery and development of its drug candidates, raising additional capital, development by its competitors of new technological innovations, protection of proprietary technology and regulatory approval and market acceptance of the Company's products. The Company anticipates that it will continue to incur significant operating losses for the next several years as it continues to develop its product candidates. As of December 31, 2017, the Company had available cash and cash equivalents of $16.4 million. The Company's current operating plan provides for cash to fund operations through September 2018. Based on the Company's available cash resources, the Company does not have sufficient cash on hand to support current operations for at least the next twelve months from the date of filing this Annual Report on Form 10-K. This condition requires substantial doubt about the Company's ability to continue as a going concern. The Company's plans to address this condition include seeking additional funds through equity or debt financings or through collaboration or licensing transactions. The Company may be unable to obtain equity or debt financings or enter into collaboration or licensing transactions and, if necessary, the Company will be required to implement cost reduction strategies, including ceasing or delaying development of the Company's product candidates. Accordingly, the Company has concluded that substantial doubt exists regarding the Company's ability to continue as a going concern for a period of at least twelve months from the date of issuance of these consolidated financial statements. The accompanying consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and satisfaction of liabilities in the ordinary course of business. The consolidated financial statements do not include any adjustments relating to the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result from the outcome of this uncertainty. The Company will require substantial additional capital to fund operations. The Company has not generated any product revenues and has financed its operations primarily through public offerings and private placements of its equity securities. There can be no assurance that the Company will be able to obtain additional debt or equity financing or generate product revenue or revenues from collaborative partners, on terms acceptable to the Company, on a timely basis or at all. The failure of the Company to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the Company's business, results of operations, and financial condition.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation and Principles of Consolidation

The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiary, Catabasis Securities Corporation. All intercompany balances and transactions have been eliminated in consolidation. These consolidated financial statements have been prepared in accordance with United States generally accepted accounting principles ("U.S. GAAP") and include all adjustments necessary for the fair presentation of the Company's financial position for the periods presented.

Use of Estimates

The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from such estimates.

The Company utilizes certain estimates to record expenses relating to research and development contracts. These contract estimates, which are primarily related to the length each contract and the amount of service provided as of each measurement date, are determined by the Company based on input from internal project management, as well as from third-party service providers.

Off-Balance Sheet Risk and Concentrations of Credit Risk

The Company has no off-balance sheet risk, such as foreign exchange contracts, option contracts or other foreign hedging arrangements. Financial instruments that subject the Company to credit risk primarily consist of cash and cash equivalents and restricted cash. The primary objectives for the Company's investment portfolio are the preservation of capital and the maintenance of liquidity. The Company's investment policy includes guidelines on the quality of the institutions and financial instruments and defines allowable investments that the Company believes minimizes the exposure to concentration of credit risk.

Cash and Cash Equivalents

The Company considers highly liquid investments with a maturity of three months or less when purchased to be cash equivalents. Cash equivalents, which consist primarily of money market funds backed by U.S. government securities, are stated at fair value. Cash and cash equivalents consist of the following (in thousands):

	December 31,
	2017		2016
Cash	$	2,781	$	1,173
Money market fund		13,588		22,423

Total	$	16,369	$	23,596

Available-for-Sale Investments

The Company classifies all short-term investments with a remaining maturity of greater than three months at the time of purchase as available-for-sale. Available-for-sale securities are recorded at fair value, with the unrealized gains and losses reported in other comprehensive loss. The amortized cost of debt securities is adjusted for the amortization of premiums and accretion of discounts to maturity. Such amortization is included in interest and investment income. Realized gains and losses, interest, dividends and declines in value judged to be other-than-temporary are included in interest and investment income.

The cost of securities sold is based on the specific identification method for purposes of recording realized gains and losses. To determine whether an other-than-temporary impairment exists, the Company considers whether it has the ability and intent to hold the investment until a market price recovery, and whether evidence indicating the recoverability of the cost of the investment outweighs evidence to the contrary.

Fair Value of Financial Instruments

The fair value hierarchy prioritizes the inputs to valuation techniques used to measure fair value into three broad levels as follows: Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities; Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly; and Level 3 inputs are unobservable inputs that reflect the Company's own assumptions about the assumptions market participants would use in pricing the asset or liability. Financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement.

The carrying amounts reflected in the balance sheets for cash equivalents, restricted cash, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values at December 31, 2017 and 2016, due to their short-term nature. There have been no changes to the valuation methods during the years ended December 31, 2017 and 2016. The Company evaluates transfers between levels at the end of each reporting period. There were no transfers of assets or liabilities between levels during the year ended December 31, 2017 and 2016. At December 31, 2017, the carrying value of the Company's debt approximated fair value, which was determined using Level 3 inputs, including a quoted interest rate.

The Company's investment portfolio may include fixed income securities that do not always trade on a daily basis. As a result, the pricing services used by the Company apply other available information as applicable through processes such as benchmark yields, benchmarking of like securities, sector groupings and matrix pricing to prepare valuations. The Company validates the prices provided by its third party pricing services by obtaining market values from other pricing sources and analyzing pricing data in certain instances. The Company determines the fair value of available-for-sale securities (Note 4) using Level 2 inputs.

Deferred Financing Costs

Deferred financing costs include costs directly attributable to the Company's offerings of its equity securities and its debt financings. Costs attributable to equity offerings are charged against the proceeds of the offering once the offering is completed. Costs attributable to debt financings are deferred and amortized over the term of the debt using the effective interest rate method.

Property and Equipment

Property and equipment consist of laboratory equipment, computer equipment, leasehold improvements and furniture and fixtures. Property and equipment is stated at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets. Costs of major additions and betterments are capitalized; maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to expense as incurred. Upon retirement or sale, the cost of the disposed asset and the related accumulated depreciation are removed from the accounts and the resulting gain or loss is recognized.

Impairment of Long-Lived Assets

The Company continually evaluates whether events or circumstances have occurred that indicate that the estimated remaining useful life of its long-lived assets may warrant revision or that the carrying value of these assets may be impaired. The Company has not recognized any significant impairment charges from inception through December 31, 2017.

Research and Development Expenses

Research and development costs are expensed as incurred. Research and development costs include salaries and personnel-related costs, stock-based compensation, consulting fees, fees paid for contract research services, the costs of laboratory equipment and facilities and other external costs. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred. The deferred amounts are expensed as the related goods are delivered or the services are performed.

Revenue Recognition

Effective January 1, 2017, the Company adopted Accounting Standards Codification ("ASC"), Revenue from Contracts with Customers, ("ASC 606"). This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under the agreement, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that it will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied.

Stock-Based Compensation

The Company accounts for its stock-based compensation awards in accordance with ASC Topic 718, Compensation—Stock Compensation ("ASC 718"). ASC 718 requires all share-based payments to employees, including grants of employee stock options, to be recognized in the statements of operations based on their grant date fair values. For stock options granted to employees and to members of the board of directors for their services on the board of directors, the Company estimates the grant date fair value of each option award using the Black-Scholes option-pricing model. The use of the Black-Scholes option-pricing model requires management to make assumptions with respect to the expected term of the option, the expected volatility of the Common Stock consistent with the expected term of the option, risk-free interest rates and expected dividend yields of the Common Stock.

For awards subject to service-based vesting conditions, the Company recognizes stock-based compensation expense equal to the grant date fair value of stock options on a straight-line basis over the requisite service period.

The Company expenses restricted stock awards based on the fair value of the award on a straight-line basis over the associated service period of the award.

Share-based payments issued to non-employees are recorded at their fair values, and are periodically revalued as the equity instruments vest and are recognized as expense over the related service period in accordance with the provisions of ASC Topic 505, Equity. For equity instruments granted to non-employees, the Company recognizes stock-based compensation expense on a straight-line basis.

During the years ended December 31, 2017, 2016 and 2015, the Company recorded stock-based compensation expense for employee and non-employee stock options and restricted stock, which was allocated as follows in the statements of operations (in thousands):

	Year Ended December 31,
	2017		2016		2015
Research and development	$	784	$	723	$	681
General and administrative		1,223		1,437		977

Total	$	2,007	$	2,160	$	1,658

No related tax benefits were recognized for the years ended December 31, 2017, 2016 and 2015.

Grant Awards

In the years ended December 31, 2017, 2016 and 2015, the Company received $124,000, $376,000, $100,000, respectively, in grants from the Muscular Dystrophy Association, Friedreich's Ataxia Research Alliance and Parent Project for Muscular Dystrophy. In the years ended December 31, 2017, 2016, 2015 the Company utilized $205,000, $224,000 and $100,000 of the grants received to offset related expenses incurred in the Company's statements of operations.

Net Loss Per Share

Basic net loss per share is calculated by dividing net loss by the weighted average shares outstanding during the period, without consideration for Common Stock equivalents. Diluted net loss per share is calculated by adjusting weighted average shares outstanding for the dilutive effect of Common Stock equivalents outstanding for the period, determined using the treasury-stock method. For purposes of the dilutive net loss per share calculation, Preferred Stock, stock options, warrants to purchase Common Stock and warrants to purchase Preferred Stock are considered to be Common Stock equivalents but are excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive; therefore, basic and diluted net loss per share were the same for all periods presented.

The following Common Stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect:

	Year Ended December 31,
	2017		2016		2015
Stock options		2,845,626		2,270,169		1,723,554
Common stock warrants		24,566		24,566		59,405

		2,870,192		2,294,735		1,782,959

Income Taxes

The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company's financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more likely than not be realized.

The Company accounts for uncertain tax positions in accordance with the provisions of ASC Topic 740, Expenses—Income Taxes. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. The Company did not have any significant uncertain tax positions for any periods presented.

Segment Information

Operating segments are identified as components of an enterprise for which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision making group, in making decisions on how to allocate resources and assess performance. The Company views its operations and manages its business in one operating segment. The Company operates in one geographic segment.

Comprehensive Loss

Comprehensive loss is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. For the years ended December 31, 2017 and 2016, amounts in accumulated other comprehensive loss were comprised of unrealized gains and losses on available-for-sale securities. For the year ended December 31, 2015, comprehensive loss was equal to net loss.

Recent Accounting Pronouncements—Adopted

In May 2014, the Financial Accounting Standards Board ("FASB") issued ASU 2014-09, Revenue from Contracts with Customers, ("ASU 2014-09"). The standard replaced existing revenue recognition standards and significantly expanded the disclosure requirements for revenue arrangements. The Company adopted the standard on January 1, 2017 using the full retrospective method. The adoption of the new standard did not have an effect on previously issued consolidated financial statements as the Company had not recognized any revenue prior to 2017.

In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting, ("ASU 2016-09"). This standard amended the existing guidance in an attempt to simplify several aspects of accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures, and statutory tax withholding requirements, as well as classification in the statement of cash flows. The Company adopted the standard on January 1, 2017. The amended guidance eliminates the requirement that excess tax benefits be realized as a reduction in current taxes payable before the associated tax benefit can be recognized in additional paid-in capital. This created approximately $0.2 million of deferred tax assets relating to federal and state net operating losses that were fully offset by a corresponding increase in the valuation allowance. As a result, there was no cumulative effect adjustment to accumulated deficit. As part of the adoption of ASU 2016-09, the Company elected to record forfeitures as they occur, which did not have a material impact on the consolidated financial statements upon adoption.

In May 2017, the FASB issued ASU 2017-09, Scope of Modification Accounting, ("ASU 2017-09"). This standard provides clarity on whether the change in existing terms and conditions of a share-based award require application of the guidance in ASC Topic 718, Compensation—Stock Compensation. The Company early adopted the standard in the period ending June 30, 2017. As of December 31, 2017, there was no material impact on the Company's consolidated financial statements, and any future impact from adoption will be dependent on future transactions involving modifications of share-based awards.

Recent Accounting Pronouncements—Not Yet Adopted

From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption.

In February 2016, the FASB issued ASU 2016-02, Leases, ("ASU 2016-02"). This standard amends the existing guidance to require lessees to present most leases on their balance sheets but recognize corresponding expenses on their statements of operations. It is effective for annual reporting periods beginning after December 15, 2018, but early adoption is permitted. The Company is currently evaluating the impact that this standard will have on its consolidated financial statements.

Financial Instruments

12 Months Ended

Dec. 31, 2017

Financial Instruments

Financial Instruments

3. Financial Instruments

The following tables present information about the Company's financial assets and liabilities that have been measured at fair value, and indicates the fair value hierarchy of the valuation inputs utilized to determine such fair value. Below is a summary of assets and liabilities measured at fair value on a recurring basis (in thousands):

	As of December 31, 2017
	Quoted Prices in Active Markets (Level 1)		Significant Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Assets:
Cash and cash equivalents:
Money market funds	$	13,588	$	—	$	—	$	13,588

Total assets	$	13,588	$	—	$	—	$	13,588

	As of December 31, 2016
	Quoted Prices in Active Markets (Level 1)		Significant Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Assets:
Cash and cash equivalents:
Money market funds	$	22,423	$	—	$	—	$	22,423
Available-for-sale securities:
Corporate debt securities		—		13,930		—		13,930
U.S. government-sponsored securities		—		1,001		—		1,001

Total assets	$	22,423	$	14,931	$	—	$	37,354

As of December 31, 2017 and December 31, 2016, the Company's cash equivalents consisted principally of money market funds, which approximated their fair value due to their short-term nature.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Available-for-Sale Securities

12 Months Ended

Dec. 31, 2017

Available-for-Sale Securities

Available-for-Sale Securities

4. Available-for-Sale Securities

As of December 31, 2017, the Company held no available-for-sale securities. The following table summarizes the available-for-sale securities held at December 31, 2016 (in thousands):

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
December 31, 2016
Corporate debt securities	$	13,934	$	—	$	(4	)	$	13,930
U.S. government-sponsored securities		1,001		—		—			1,001

Total	$	14,935	$	—	$	(4	)	$	14,931

The contractual maturities of all available-for-sale securities held at December 31, 2016 were one year or less. There were fifteen available-for-sale securities in an unrealized loss position at December 31, 2016, none of which had been in an unrealized loss position for more than 12 months. The aggregate fair value of these securities at December 31, 2016 was approximately $13.0 million. The Company did not hold any securities with other-than-temporary impairment at December 31, 2016.

Gross realized gains and losses on the sales of available-for-sale securities are included in other income, net. Unrealized holding gains or losses for the period that have been included in accumulated other comprehensive income, as well as gains and losses reclassified out of accumulated other comprehensive income into other income, net were not material to the Company's condensed consolidated results of operations. The cost of securities sold or the amount reclassified out of the accumulated other comprehensive income into other income, net is based on the specific identification method for purposes of recording realized gains and losses. During the year ended December 31, 2016 the Company received $7.8 million in proceeds from sales of available-for-sale securities. There were no proceeds from sales of available-for-sale securities in the year ended December 31, 2017, as all proceeds related to maturities of underlying securities. The gains on proceeds from sales and maturities of available-for-sale securities were not material to the Company's condensed consolidated results of operations for the years ended December 31, 2017 and 2016.

Restricted Cash	12 Months Ended
Restricted Cash	Dec. 31, 2017
Restricted Cash
Restricted Cash	5. Restricted Cash At December 31, 2017 and 2016, the Company had an outstanding letter of credit for $0.1 million as a security deposit for its operating lease agreement for office space (Note 9). The Company is required to maintain this deposit for the duration of the lease agreement.

Property and Equipment

12 Months Ended

Dec. 31, 2017

Property and Equipment

Property and Equipment

6. Property and Equipment

Property and equipment and related accumulated depreciation were as follows (in thousands):

		December 31,
	Estimated Useful Life (Years)	2017			2016
Lab equipment	3	$	1,470		$	1,565
Computer equipment	3		165			166
Furniture and fixtures	5		77			77
Leasehold improvements	Lesser of useful life or remaining lease term		264			259

			1,976			2,067
Less accumulated depreciation and amortization			(1,655	)		(1,499	)

Total property and equipment, net		$	321		$	568

Depreciation and amortization expense was $0.3 million, $0.4 million and $0.2 million for the years ended December 31, 2017, 2016 and 2015, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2017

Accrued Expenses

Accrued Expenses

7. Accrued Expenses

Accrued expenses consisted of the following (in thousands):

	December 31,
	2017		2016
Accrued compensation	$	632	$	1,252
Accrued contracted research costs		1,357		1,850
Accrued professional fees		298		215
Accrued other		145		360

Total	$	2,432	$	3,677

Notes Payable

12 Months Ended

Dec. 31, 2017

Notes Payable

Notes Payable

8. Notes Payable

On August 27, 2014, the Company entered into a credit facility with MidCap Financial Trust, Flexpoint MCLS SPV LLC and Square 1 Bank, which was subsequently amended in March and December 2015 (as amended, the "Credit Facility"). The Credit Facility provided for initial borrowings of $5.0 million under a term loan ("Term Loan A") and additional borrowings of up to $20.0 million under other term loans, for a maximum of $25.0 million. On August 27, 2014, the Company received proceeds of $5.0 million from the issuance of promissory notes under Term Loan A. On March 31, 2015, the Company received proceeds of $5.0 million from the issuance of promissory notes under another term loan ("Term Loan B"). The remaining amounts available for borrowing under this arrangement expired unused as of July 31, 2015, leaving total borrowings under the Credit Facility at $10.0 million. All amounts outstanding under the Credit Facility are due on October 1, 2018 and are collateralized by substantially all of the Company's personal property, other than its intellectual property.

Interest-only payments were due monthly on amounts outstanding under the Credit Facility until September 1, 2015 and, thereafter, interest and principal payments are due in 36 equal monthly installments from October 1, 2015 through September 1, 2018. Amounts due under the Credit Facility bear interest at an annual rate of 7.49%. In addition, a final payment equal to 3.48% of any amounts drawn under the Credit Facility is due upon the earlier of the maturity date, acceleration of the term loans or prepayment of all or part of the term loans. The final payment is being accrued as additional interest expense using the effective-interest method from the date of issuance through the maturity date, and is recorded within other current liabilities. In the event of prepayment, the Company is obligated to pay 1% to 3% of the amount of the outstanding principal depending upon the timing of the prepayment. As of December 31, 2017, the Company would be obligated to pay 1% of the amount of the outstanding principal in the event of a prepayment. The effective interest rate as of December 31, 2017 was 11.2%.

In conjunction with Term Loan A, the Company issued warrants (the "2014 Warrants") to purchase 157,844 shares of series B convertible preferred stock. In conjunction with Term Loan B, the Company issued warrants (the "2015 Warrants") to purchase an additional 157,844 shares of series B convertible preferred stock. Upon the closing of the Company's initial public offering (the "IPO") in June 2015, the warrants to purchase shares of convertible preferred stock to the lenders were automatically converted into warrants to purchase an aggregate of 24,566 shares of Common Stock at an exercise price of $12.21 per share. The warrants were exercisable immediately and expire after seven-years. At issuance, the warrants were valued at $0.2 million using the Black-Scholes option-pricing model. They were recorded as debt discounts of $0.2 million and are being accreted as interest expense using the effective-interest method over the remaining term of the loans.

There are no financial covenants associated with the Credit Facility; however, there are negative covenants restricting the Company's activities, including limitations on asset dispositions, mergers or acquisitions; encumbering or granting a security interest in its intellectual property; incurring indebtedness or liens; paying dividends; making certain investments; and entering into certain other business transactions.

Upon the occurrence and continuation of an event of default, the lenders have the right to exercise certain remedies against the Company and the collateral securing the loans under the Credit Facility, including cash. Events of default include, among other things, failure to pay amounts due under the Credit Facility, insolvency, the occurrence of a material adverse event, which includes a material adverse change in the business, operations or conditions (financial or otherwise) of the Company or a material impairment of the prospect of repayment of any portion of the obligations, the occurrence of any default under certain other indebtedness and a final judgment against the Company in an amount greater than $250,000. The occurrence of a material adverse event could result in acceleration of the payment of the debt. At December 31, 2016, the Company concluded that the likelihood of the acceleration of the debt was remote, as a material adverse event had not occurred and was unlikely to occur and therefore the debt was classified in current and long-term liabilities based on scheduled principal payments. At December 31, 2017, the maturity date of the debt was within twelve months and therefore all debt was classified as current. Following the occurrence and during the continuance of an event of default, borrowings under the Credit Facility shall bear interest at a rate per annum, which is five hundred basis points, or 5.00%, above the rate that is otherwise applicable.

The Company assessed all terms and features of the Credit Facility in order to identify any potential embedded features that would require bifurcation or any beneficial conversion features. As part of this analysis, the Company assessed the economic characteristics and risks of the Credit Facility, including put and call features. The Company determined that all features of the Credit Facility were clearly and closely associated with a debt host and did not require bifurcation as a derivative liability, or the fair value of the feature was immaterial to the Company's financial statements. The Company reassesses the features on a quarterly basis to determine if they require separate accounting.

Estimated future principal payments at December 31, 2017 are as follows (in thousands):

Year Ending December 31,	Amount
2018		2,500

Total	$	2,500
Less: discount for warrants and costs paid to lenders		(21	)
Less: current portion		(2,479	)

Note payable, net of current portion and discount	$	—

During the years ended December 31, 2017, 2016 and 2015, the Company recognized $0.5 million, $0.8 million and $1.0 million, respectively, of interest expense related to the Credit Facility.

Commitments

12 Months Ended

Dec. 31, 2017

Commitments

Commitments

9. Commitments

In November 2010, the Company entered into an operating lease for office and laboratory space, which has been amended multiple times. Based on the latest amendment, the lease agreement includes escalating rent payments and is effective through June 30, 2020. The Company is recognizing rent expense on a straight-line basis over the lease term.

Future minimum payments required under the non-cancelable operating lease as of December 31, 2017 are summarized as follows (in thousands):

Period Ending December 31,	Amount
2018		1,379
2019		1,421
2020		721

Total minimum lease payments	$	3,521

Rent expense for the years ended December 31, 2017, 2016 and 2015 was $1.3 million, $0.9 million and $0.8 million, respectively.

Revenue Recognition	12 Months Ended
Revenue Recognition	Dec. 31, 2017
Revenue Recognition
Revenue Recognition	10. Revenue Recognition In August 2017, the Company entered into an option agreement ("Option Agreement") with an unaffiliated party ("Recipient"), which is within the scope of ASC 606. Under the terms of the Option Agreement, the Company agreed to provide compound material for certain of their product candidates and the right for the Recipient to use the material to perform research during the term of the Option Agreement. The Option Agreement included a non-refundable up-front payment of $250 thousand. The term of the Option Agreement commenced in August 2017 and could be extended upon additional payments of $250 thousand made by the Recipient, at its option, at the beginning of each of the next two quarterly periods. In October 2017, the Company received an additional $250 thousand payment to extend the term for an additional quarterly period. In December, 2017, the Recipient terminated the agreement. In the future, the Company will seek to generate additional revenue primarily from a combination of product sales and collaborations with strategic partners. During the year ended December 31, 2017, the Company recognized $500 thousand in revenue related to the Option Agreement.

Preferred Stock	12 Months Ended
Preferred Stock	Dec. 31, 2017
Preferred Stock
Preferred Stock	11. Preferred Stock Convertible Preferred Stock In March, 2015, the Company issued 13,062,965 shares of Series B Preferred Stock at $0.9503 per share, and received net proceeds of $12.3 million. Prior to the IPO, the holders of the Company's convertible Preferred Stock had certain voting and dividend rights, as well as liquidation preferences and conversion privileges. All rights, preferences, and privileges associated with the convertible Preferred Stock were terminated at the time of the Company's IPO in conjunction with the conversion of all outstanding shares of convertible Preferred Stock into shares of Common Stock. Upon the closing of the Company's IPO on June 30, 2015, all outstanding shares of the Company's Preferred Stock were automatically converted into 9,029,549 shares of Common Stock. As of December 31, 2016, the Company had 5,000,000 shares of Preferred Stock authorized for issuance, $0.001 par value per share, with none issued or outstanding. Preferred Stock As of December 31, 2017, the Company had 5,000,000 shares of preferred stock authorized for issuance, $0.001 par value per share, with none issued or outstanding. Preferred stock may be issued from time to time in one or more series, each series to have such terms as stated or expressed in the resolutions providing for the issue of such series adopted by the board of directors of the Company. Preferred stock which may be redeemed, purchased or acquired by the Company may be reissued except as otherwise provided by law.

Common Stock

12 Months Ended

Dec. 31, 2017

Common Stock

Common Stock

12. Common Stock

As of December 31, 2017, the Company had 150,000,000 shares of Common Stock authorized for issuance, $0.001 par value per share, with 23,645,247 shares issued and outstanding. The voting, dividend and liquidation rights of holders of Common Stock are subject to and qualified by the rights, powers and preferences of the holders of any outstanding Preferred Stock. The Company's Common Stock has the following characteristics:

Voting

The holders of Common Stock are entitled to one vote for each share of Common Stock held at all meetings of stockholders and written actions in lieu of meetings.

Dividends

The holders of Common Stock are entitled to receive dividends, if and when declared by the board of directors. Cash dividends may not be declared or paid to holders of Common Stock until paid on each series of outstanding Preferred Stock in accordance with their respective terms. No dividends have been declared or paid from the Company's inception through December 31, 2017.

Liquidation

In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company, the holders of Common Stock are entitled to share ratably in the Company's assets available for distribution to stockholders, subject to any preferential or other rights of any then-outstanding Preferred Stock.

Reserved for Future Issuance

The Company has reserved for future issuance the following shares of Common Stock:

	As of December 31,
	2017		2016
Warrants for the purchase of Common Stock		24,566		24,566
Options to purchase Common Stock		3,669,791		2,939,968
Employee Stock Purchase Plan		523,659		335,484

Total		4,218,016		3,300,018

Stock Incentive Plans

12 Months Ended

Dec. 31, 2017

Stock Incentive Plans

Stock Incentive Plans

13. Stock Incentive Plans

Prior to the IPO, the Company granted awards to eligible participants under its 2008 Equity Incentive Plan ("2008 Plan"). In May 2015, the Company's board of directors adopted and, in June 2015, the Company's stockholders approved the 2015 Stock Incentive Plan ("2015 Plan"), which became effective immediately prior to the effectiveness of the IPO. Subsequent to the IPO, option grants are awarded to eligible participants only under the 2015 Plan.

The 2015 Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock awards, restricted stock units, stock appreciation rights and other stock-based awards. The Company's employees, officers, directors and consultants and advisors are eligible to receive awards under the 2015 Plan. The maximum number of shares of Common Stock that may be delivered in satisfaction of awards under the 2015 Plan is 1,068,287 shares, plus (1) 25,942 shares that were available for grant under the 2008 Plan immediately prior to the closing of the IPO, (2) the number of shares of Common Stock subject to outstanding awards under the 2008 Plan upon closing of the IPO that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right and (3) an annual increase, to be added the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2025, equal to the lowest of 1,297,334 shares of Common Stock, 4% of the number of shares of Common Stock outstanding on the first day of the fiscal year and an amount determined by the Company's board of directors. The January 1, 2018 and 2017 increases to the 2015 Plan added 945,810 and 752,702 authorized shares, respectively.

As of December 31, 2017, the Company had reserved 832,092 shares of Common Stock under the 2008 Plan, of which none remained available for future issuance. As of December 31, 2017, the Company had reserved 2,837,699 shares of Common Stock under the 2015 Plan, of which 824,165 shares remained available for future issuance. Under the 2015 Plan, stock options may not be granted with exercise prices at less than fair value on the date of the grant.

Terms of stock option agreements, including vesting requirements, are determined by the Company's board of directors, subject to the provisions of the applicable stock incentive plan. Options granted by the Company generally vest ratably over four years, with a one-year cliff, and options are exercisable from the date of grant for a period of ten years. For options granted through December 31, 2017, the exercise price or purchase price, as applicable, equaled the estimated fair value of the Common Stock as determined by the Company's board of directors on the date of grant.

A summary of the Company's stock option activity and related information for employees and nonemployees follows:

	Shares			Weighted- Average Exercise Price		Weighted Average Remaining Contractual Term (years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2016		2,270,169		$	6.11		8.14	$	613
Granted		890,740		$	1.26
Exercised		(22,879	)	$	1.01
Cancelled or forfeited		(292,404	)	$	7.13

Outstanding at December 31, 2017		2,845,626		$	4.53		7.75	$	216


Vested and Exercisable at December 31, 2017		1,265,411		$	5.91		6.53	$	—

The total intrinsic value of options exercised for the years ended December 31, 2017, 2016 and 2015 was $40 thousand, $0.5 million and $0.3 million, respectively. The total adjusted fair value of employee options vested for the years ended December 31, 2017, 2016 and 2015 was $2.2 million, $2.3 million and $1.4 million, respectively. The weighted-average grant date fair value of options granted to employees and non-employees for the years ended December 31, 2017, 2016, and 2015 was $0.85, $2.89 and $7.68, respectively.

At December 31, 2017, the total unrecognized compensation expense related to unvested stock option awards was $3.2 million. The Company expects to recognize that cost over a weighted-average period of approximately 2.0 years.

Stock-Based Compensation Expense

The fair value of stock options granted to employees and non-employees was estimated using the Black-Scholes option-pricing model based on the following assumptions:

	Year Ended December 31,
	2017			2016			2015
Weighted-average expected volatility		76.1 - 81.9	%		72.0 - 73.9	%		73.6 - 86.8	%
Expected term (in years)		6.25 - 9.77			6.25			6.17 - 10.00
Risk-free interest rate		2.00 - 2.38	%		1.24 - 2.00	%		0.92 - 2.45	%
Expected dividend yield		0	%		0	%		0	%

Volatility

Due to the lack of company-specific historical and implied volatility data of its Common Stock, the Company does not have relevant historical data to support its expected volatility. As such, the Company has used a weighted average of expected volatility based on the volatilities of a representative group of publicly traded biopharmaceutical companies. For purposes of identifying representative companies, the Company considered characteristics such as number of product candidates in early stages of product development, area of therapeutic focus, and length of trading history. The expected volatility was determined using a weighted average of the historical volatilities of the representative group of companies for a period equal to the expected term of the option grant. The Company intends to continue to consistently apply this process using the same representative companies until a sufficient amount of historical information regarding the volatility of the Company's own share price becomes available or until circumstances change, such that the identified entities are no longer representative companies. In the latter case, more suitable, similar entities whose share prices are publicly available would be utilized in the calculation.

Expected Term

The Company uses the "simplified method" to estimate the expected term of stock option grants. Under this approach, the weighted-average expected life is presumed to be the average of the contractual term (ten years) and the vesting term (generally four years) of the Company's stock options, taking into consideration multiple vesting tranches. The Company utilizes this method due to lack of historical exercise data and the plain-vanilla nature of the Company's share-based awards.

Risk-Free Rate

The risk-free rate was based on the yield curve of U.S. Treasury securities with periods commensurate with the expected term of the options being valued.

Employee Stock Purchase Plan

In June 2015, the Company's board of directors adopted and the Company's stockholders approved the 2015 Employee Stock Purchase Plan (the "2015 ESPP") which became effective upon closing of the IPO. The 2015 ESPP initially authorized the issuance of up to a total of 182,352 shares of Common Stock to participating eligible employees. The number of authorized shares increases each January 1, commencing on January 1, 2016 and ending on December 31, 2026, by an amount equal to the lesser of one percent of the Company's outstanding shares as of the first day of the applicable year, 364,705 shares and any lower amount determined by the Company's board of directors. The January 1, 2018 and 2017 increases to the 2015 ESPP added 236,452 and 188,175 authorized shares, respectively. As of December 31, 2017, there had been no shares issued under the 2015 ESPP.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Taxes

Income Taxes

14. Income Taxes

For the years ended December 31, 2017, 2016 and 2015, the Company did not record a provision for federal or state income taxes as it has incurred cumulative net operating losses since inception.

On December 22, 2017, the Tax Cuts and Jobs Act (the "Act") was enacted in the United States. The Act reduces the U.S. federal corporate tax rate from a graduated rate of 35% to a flat rate of 21%, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and creates new taxes on certain foreign sourced earnings. At December 31, 2017, the Company has not completed its accounting for the tax effects of enactment of the Act, including the effects on its existing deferred tax balances and the one-time transition tax. For the year ended December 31, 2017, the Company recognized no transition tax as it has no foreign subsidiaries.

As a result of the Act, the Company remeasured certain deferred tax assets and liabilities based on the rates at which they are anticipated to reverse in the future, which is generally 21%. This resulted in a decrease to the Company's tax effected deferred tax assets of $19.8 million. As the deferred tax asset is offset in full by a valuation allowance, this enacted legislation had no impact on the Company's financial position or results of operations.

Any items reported are done so using provisional amounts until the initial accounting required by the Act is complete. Additional time and resources are needed to ensure the correct implementation of the Act.

A reconciliation of the U.S. statutory income tax rate to the Company's effective tax rate is as follows for the years ended December 31, 2017, 2016 and 2015:

	Year Ended December 31,
	2017			2016			2015
Federal income tax (benefit) at statutory rate		34.00	%		34.00	%		34.00	%
Permanent differences		(1.16	)		(0.85	)		(0.86	)
Federal research and development credits and adjustments		2.36			2.43			2.64
State income tax, net of federal benefit		5.80			5.70			5.77
Tax reform deferred rate change		(72.21	)		—			—
Other		(0.36	)		(0.40	)		0.28
Change in valuation allowance		31.57			(40.88	)		(41.82	)

Effective income tax rate		—	%		—	%		—	%

The Company's deferred tax assets consisted of the following (in thousands):

	Year Ended December 31,
	2017			2016
Deferred tax assets
Net operating loss carryforwards	$	41,436		$	48,682
Tax credit carryforwards		5,896			4,833
Capitalized research and development		1,879			3,394
Capitalized legal expenses		1,156			1,597
Other differences		1,048			1,335

Total deferred tax assets		51,415			59,841
Valuation allowance		(51,415	)		(59,841	)

Net deferred tax assets	$	—		$	—

The Company recorded a decrease to the valuation allowance of $8.4 million during the year ended December 31, 2017 primarily due to the federal rate reduction from 34% to 21% as a result of the Act. The Company recorded an increase to the valuation allowance of $14.7 million and $13.6 million during the years ended December 31, 2016 and 2015, respectively, due primarily to an increase in the net operating loss carryforwards and tax credits.

In assessing the realizability of deferred tax assets, the Company considers whether it is more likely than not that some portion or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which the temporary differences representing net future deductible amounts become deductible. Due to the Company's history of losses and expectation of future losses, the deferred tax assets were fully offset by a valuation allowance at December 31, 2017 and 2016.

As of December 31, 2017, the Company had approximately $152.1 million of federal and $150.4 million of state net operating loss carryforwards to offset future taxable income, if any. Such net operating loss carryforwards expire at varying times through the year 2037, if not utilized. The Company had approximately $4.5 million of federal and $1.8 million of state tax credit carryforwards available to reduce future tax liabilities as of December 31, 2017, which will expire at varying times through the year 2037.

The Internal Revenue Code of 1986, as amended (the "Code"), provides for a limitation of the annual use of net operating losses and other tax attributes (such as research and development tax credit carryforwards) following certain ownership changes (as defined by the Code) that could limit the Company's ability to utilize these carryforwards. At this time, the Company has not completed a study to assess whether an ownership change under Section 382 of the Code has occurred, or whether there have been multiple ownership changes since the Company's formation, due to the costs and complexities associated with such a study. The Company may have experienced various ownership changes, as defined by the Code, as a result of past financing transactions. Accordingly, the Company's ability to utilize the aforementioned carryforwards may be limited. Additionally, U.S. tax laws limit the time during which these carryforwards may be applied against future taxes. Therefore, the Company may not be able to take full advantage of these carryforwards for federal or state income tax purposes.

The Company's tax returns for the years ended December 31, 2009 through December 31, 2017 are still subject to examination by major tax jurisdictions.

Defined Contribution Benefit Plan	12 Months Ended
Defined Contribution Benefit Plan	Dec. 31, 2017
Defined Contribution Benefit Plan
Defined Contribution Benefit Plan	15. Defined Contribution Benefit Plan The Company sponsors a 401(k) retirement plan, in which substantially all of its full-time employees are eligible to participate. Participants may contribute a percentage of their annual compensation to this plan, subject to statutory limitations. The Company did not provide any contributions to this plan during the years ended December 31, 2017, 2016 or 2015.

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2017
Subsequent Events
Subsequent Events	16. Subsequent Events The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence for certain estimates and to identify matters that require additional disclosure. Subsequent events have been evaluated as required. Sales Agreement Subsequent to December 31, 2017, the Company sold an aggregate of 5,390,255 shares of common stock pursuant to the Second ATM Program, at an average price of $1.59 per share, for gross proceeds of $8.6 million, resulting in net proceeds of $8.3 million after deducting sales commissions and offering expenses. The Second ATM Program was fully utilized as of the date of issuance of these financial statements.

Quarterly Financial Information (Unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information (unaudited)

Quarterly Financial Information (unaudited)

17. Quarterly Financial Information (unaudited, in thousands, except share and per share data)

	Three Months Ended
	March 31, 2017			June 30, 2017			September 30, 2017			December 31, 2017
Revenue	$	—		$	—		$	250		$	250
Operating expenses		7,761			6,919			7,202			5,712
Net loss		(7,876	)		(6,974	)		(7,017	)		(5,497	)
Net loss per share:
Basic and Diluted	$	(0.41	)	$	(0.32	)	$	(0.31	)	$	(0.24	)
Weighted-average common shares outstanding used in net loss per share:
Basic and Diluted		19,093,273			21,796,194			22,563,174			23,218,476

	Three Months Ended
	March 31, 2016			June 30, 2016			September 30, 2016			December 31, 2016
Operating expenses	$	9,206		$	9,396		$	8,283		$	8,673
Net loss		(9,418	)		(9,445	)		(8,419	)		(8,778	)
Net loss per share:
Basic and Diluted	$	(0.61	)	$	(0.61	)	$	(0.54	)	$	(0.47	)
Weighted-average common shares outstanding used in net loss per share:
Basic and Diluted		15,335,516			15,373,964			15,512,608			18,699,480

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

Summary of Significant Accounting Policies

Basis of Presentation and Principles of Consolidation

Use of Estimates

Off-Balance Sheet Risk and Concentrations of Credit Risk

Cash and Cash Equivalents

	December 31,
	2017		2016
Cash	$	2,781	$	1,173
Money market fund		13,588		22,423

Total	$	16,369	$	23,596

Available-for-Sale Investments

Fair Value of Financial Instruments

Deferred Financing Costs

Property and Equipment

Impairment of Long-Lived Assets

Research and Development Expenses

Revenue Recognition

Stock-Based Compensation

The Company expenses restricted stock awards based on the fair value of the award on a straight-line basis over the associated service period of the award.

	Year Ended December 31,
	2017		2016		2015
Research and development	$	784	$	723	$	681
General and administrative		1,223		1,437		977

Total	$	2,007	$	2,160	$	1,658

No related tax benefits were recognized for the years ended December 31, 2017, 2016 and 2015.

Grant Awards

Net Loss Per Share

The following Common Stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect:

	Year Ended December 31,
	2017		2016		2015
Stock options		2,845,626		2,270,169		1,723,554
Common stock warrants		24,566		24,566		59,405

		2,870,192		2,294,735		1,782,959

Income Taxes

Segment Information

Comprehensive Loss

Recent Accounting Pronouncements

Recent Accounting Pronouncements—Adopted

Recent Accounting Pronouncements—Not Yet Adopted

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Summary of Significant Accounting Policies

Schedule of cash and cash equivalents

Cash and cash equivalents consist of the following (in thousands):

	December 31,
	2017		2016
Cash	$	2,781	$	1,173
Money market fund		13,588		22,423

Total	$	16,369	$	23,596

Schedule of stock-based compensation expense

	Year Ended December 31,
	2017		2016		2015
Research and development	$	784	$	723	$	681
General and administrative		1,223		1,437		977

Total	$	2,007	$	2,160	$	1,658

Schedule of anti-dilutive common stock equivalents excluded from calculation of diluted net loss per share

	Year Ended December 31,
	2017		2016		2015
Stock options		2,845,626		2,270,169		1,723,554
Common stock warrants		24,566		24,566		59,405

		2,870,192		2,294,735		1,782,959

Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2017

Financial Instruments

Schedule of assets and liabilities measures at fair value on a recurring basis

Below is a summary of assets and liabilities measured at fair value on a recurring basis (in thousands):

	As of December 31, 2017
	Quoted Prices in Active Markets (Level 1)		Significant Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Assets:
Cash and cash equivalents:
Money market funds	$	13,588	$	—	$	—	$	13,588

Total assets	$	13,588	$	—	$	—	$	13,588

	As of December 31, 2016
	Quoted Prices in Active Markets (Level 1)		Significant Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Assets:
Cash and cash equivalents:
Money market funds	$	22,423	$	—	$	—	$	22,423
Available-for-sale securities:
Corporate debt securities		—		13,930		—		13,930
U.S. government-sponsored securities		—		1,001		—		1,001

Total assets	$	22,423	$	14,931	$	—	$	37,354

Available-for-Sale Securities (Tables)

12 Months Ended

Dec. 31, 2017

Available-for-Sale Securities

Summary of available-for-sale securities

The following table summarizes the available-for-sale securities held at December 31, 2016 (in thousands):

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
December 31, 2016
Corporate debt securities	$	13,934	$	—	$	(4	)	$	13,930
U.S. government-sponsored securities		1,001		—		—			1,001

Total	$	14,935	$	—	$	(4	)	$	14,931

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2017

Property and Equipment

Summary of property and equipment and related accumulated depreciation

Property and equipment and related accumulated depreciation were as follows (in thousands):

		December 31,
	Estimated Useful Life (Years)	2017			2016
Lab equipment	3	$	1,470		$	1,565
Computer equipment	3		165			166
Furniture and fixtures	5		77			77
Leasehold improvements	Lesser of useful life or remaining lease term		264			259

			1,976			2,067
Less accumulated depreciation and amortization			(1,655	)		(1,499	)

Total property and equipment, net		$	321		$	568

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2017

Accrued Expenses

Schedule of components of accrued expenses

Accrued expenses consisted of the following (in thousands):

	December 31,
	2017		2016
Accrued compensation	$	632	$	1,252
Accrued contracted research costs		1,357		1,850
Accrued professional fees		298		215
Accrued other		145		360

Total	$	2,432	$	3,677

Notes Payable (Tables)

12 Months Ended

Dec. 31, 2017

Notes Payable

Schedule of estimated future principal payments

Estimated future principal payments at December 31, 2017 are as follows (in thousands):

Year Ending December 31,	Amount
2018		2,500

Total	$	2,500
Less: discount for warrants and costs paid to lenders		(21	)
Less: current portion		(2,479	)

Note payable, net of current portion and discount	$	—

Commitments (Tables)

12 Months Ended

Dec. 31, 2017

Commitments

Schedule of future minimum payments under non-cancelable operating lease

Future minimum payments required under the non-cancelable operating lease as of December 31, 2017 are summarized as follows (in thousands):

Period Ending December 31,	Amount
2018		1,379
2019		1,421
2020		721

Total minimum lease payments	$	3,521

Common Stock (Tables)

12 Months Ended

Dec. 31, 2017

Common Stock

Schedule of Common Stock reserved for future issuance

	As of December 31,
	2017		2016
Warrants for the purchase of Common Stock		24,566		24,566
Options to purchase Common Stock		3,669,791		2,939,968
Employee Stock Purchase Plan		523,659		335,484

Total		4,218,016		3,300,018

Stock Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2017

Stock Incentive Plans

Summary of stock option activity

	Shares			Weighted- Average Exercise Price		Weighted Average Remaining Contractual Term (years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2016		2,270,169		$	6.11		8.14	$	613
Granted		890,740		$	1.26
Exercised		(22,879	)	$	1.01
Cancelled or forfeited		(292,404	)	$	7.13

Outstanding at December 31, 2017		2,845,626		$	4.53		7.75	$	216


Vested and Exercisable at December 31, 2017		1,265,411		$	5.91		6.53	$	—

Schedule of assumptions made based on the Black-Scholes option pricing model

	Year Ended December 31,
	2017			2016			2015
Weighted-average expected volatility		76.1 - 81.9	%		72.0 - 73.9	%		73.6 - 86.8	%
Expected term (in years)		6.25 - 9.77			6.25			6.17 - 10.00
Risk-free interest rate		2.00 - 2.38	%		1.24 - 2.00	%		0.92 - 2.45	%
Expected dividend yield		0	%		0	%		0	%

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Taxes

Schedule of reconciliation of U.S. statutory income tax rate to effective tax rate

	Year Ended December 31,
	2017			2016			2015
Federal income tax (benefit) at statutory rate		34.00	%		34.00	%		34.00	%
Permanent differences		(1.16	)		(0.85	)		(0.86	)
Federal research and development credits and adjustments		2.36			2.43			2.64
State income tax, net of federal benefit		5.80			5.70			5.77
Tax reform deferred rate change		(72.21	)		—			—
Other		(0.36	)		(0.40	)		0.28
Change in valuation allowance		31.57			(40.88	)		(41.82	)

Effective income tax rate		—	%		—	%		—	%

Schedule of components of deferred tax assets

The Company's deferred tax assets consisted of the following (in thousands):

	Year Ended December 31,
	2017			2016
Deferred tax assets
Net operating loss carryforwards	$	41,436		$	48,682
Tax credit carryforwards		5,896			4,833
Capitalized research and development		1,879			3,394
Capitalized legal expenses		1,156			1,597
Other differences		1,048			1,335

Total deferred tax assets		51,415			59,841
Valuation allowance		(51,415	)		(59,841	)

Net deferred tax assets	$	—		$	—

Quarterly Financial Information (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information (unaudited)

Summary of unaudited quarterly results of operations

Quarterly Financial Information (unaudited, in thousands, except share and per share data)

	Three Months Ended
	March 31, 2017			June 30, 2017			September 30, 2017			December 31, 2017
Revenue	$	—		$	—		$	250		$	250
Operating expenses		7,761			6,919			7,202			5,712
Net loss		(7,876	)		(6,974	)		(7,017	)		(5,497	)
Net loss per share:
Basic and Diluted	$	(0.41	)	$	(0.32	)	$	(0.31	)	$	(0.24	)
Weighted-average common shares outstanding used in net loss per share:
Basic and Diluted		19,093,273			21,796,194			22,563,174			23,218,476

	Three Months Ended
	March 31, 2016			June 30, 2016			September 30, 2016			December 31, 2016
Operating expenses	$	9,206		$	9,396		$	8,283		$	8,673
Net loss		(9,418	)		(9,445	)		(8,419	)		(8,778	)
Net loss per share:
Basic and Diluted	$	(0.61	)	$	(0.61	)	$	(0.54	)	$	(0.47	)
Weighted-average common shares outstanding used in net loss per share:
Basic and Diluted		15,335,516			15,373,964			15,512,608			18,699,480

Organization and Operations - Liquidity and Going Concern (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Jan. 01, 2018	Aug. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2017	Mar. 31, 2017	Dec. 31, 2014
Common Stock
Share price (in dollars per share)			$ 0.001
Additional common stock issued			$ 8,036	$ 11,994	$ 61,744
Proceeds from registered direct offering, net of issuance costs				10,603
Accumulated deficit			(171,434)	(144,070)
Cash and cash equivalents			$ 16,369	$ 23,596	$ 62,780			$ 14,668
Common Stock
Common Stock
Number of shares issued			4,804,796	3,243,015	5,750,000
Additional common stock issued			$ 5	$ 3	$ 5
Common Stock \| Cowen
Common Stock
Percentage of commission payable to underwriter based on proceeds from common stock		3.00%
Common Stock \| First ATM Program
Common Stock
Number of shares issued			4,804,796
Share price (in dollars per share)			$ 1.79
Additional common stock issued			$ 8,600
Net proceeds			8,000
Common Stock \| First ATM Program \| Cowen
Common Stock
Aggregate maximum offering amount		$ 10,000					$ 8,800
Common Stock \| Second ATM Program
Common Stock
Remaining common stock available for sale under offering program			$ 8,600
Common Stock \| Second ATM Program \| Cowen
Common Stock
Aggregate maximum offering amount						$ 10,000
Common Stock \| Second ATM Program \| Subsequent Events
Common Stock
Number of shares issued	5,390,255
Share price (in dollars per share)	$ 1.59
Additional common stock issued	$ 8,600
Net proceeds	$ 8,300
Common Stock \| Second ATM Program \| Subsequent Events \| Cowen
Common Stock
Number of shares issued	5,390,255
Share price (in dollars per share)	$ 1.59
Additional common stock issued	$ 8,600
Net proceeds	$ 8,300
Common Stock \| Registered Direct Offering
Common Stock
Number of shares issued				3,243,015
Share price (in dollars per share)				$ 4.04
Additional common stock issued				$ 13,100
Proceeds from registered direct offering, net of issuance costs				$ 12,000

Summary of Significant Accounting Polices - Cash and Cash Equivalents and Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents
Cash	$ 2,781	$ 1,173
Money market fund	13,588	22,423
Total	16,369	23,596	$ 62,780	$ 14,668
Fair Value of Financial Instruments
Transfers of assets or liabilities between levels	$ 0	$ 0

Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-Based Compensation
Stock-based compensation expense	$ 2,007	$ 2,160	$ 1,658
Tax benefits recognized from stock-based compensation expense	0	0	0
Research and development
Stock-Based Compensation
Stock-based compensation expense	784	723	681
General and administrative
Stock-Based Compensation
Stock-based compensation expense	$ 1,223	$ 1,437	$ 977

Summary of Significant Accounting Polices - Grant Awards (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of Significant Accounting Policies
Grants from Muscular Dystrophy Association	$ 124,000	$ 376,000	$ 100,000
Grants utilized to offset related expenses	$ 205,000	$ 224,000	$ 100,000

Summary of Significant Accounting Policies - Net Loss Per Share and Segment Information (Details)	12 Months Ended
	Dec. 31, 2017 segment shares	Dec. 31, 2016 shares	Dec. 31, 2015 shares
Antidilutive common stock equivalents excluded from computation of diluted net loss per share
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)	2,870,192	2,294,735	1,782,959
Segment Information
Number of operating segments \| segment	1
Stock options
Antidilutive common stock equivalents excluded from computation of diluted net loss per share
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)	2,845,626	2,270,169	1,723,554
Common stock warrants
Antidilutive common stock equivalents excluded from computation of diluted net loss per share
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)	24,566	24,566	59,405

Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements
Increase in valuation allowance	$ (8.4)	$ 14.7	$ 13.6
Accounting Standards Update 2016-09
New Accounting Pronouncements
Deferred tax assets related to federal and state net operating losses	0.2
Increase in valuation allowance	$ 0.2

Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and cash equivalents:
Cash and cash equivalents	$ 16,369	$ 23,596	$ 62,780	$ 14,668
Available-for-sale securities:
Available-for-sale securities	0	14,931
Corporate debt securities
Available-for-sale securities:
Available-for-sale securities		13,930
U.S. government-sponsored securities
Available-for-sale securities:
Available-for-sale securities		1,001
Recurring
Assets:
Total assets	13,588	37,354
Recurring \| Money market funds
Cash and cash equivalents:
Cash and cash equivalents	13,588	22,423
Recurring \| Corporate debt securities
Available-for-sale securities:
Available-for-sale securities		13,930
Recurring \| U.S. government-sponsored securities
Available-for-sale securities:
Available-for-sale securities		1,001
Recurring \| Level 1
Assets:
Total assets	13,588	22,423
Recurring \| Level 1 \| Money market funds
Cash and cash equivalents:
Cash and cash equivalents	$ 13,588	22,423
Recurring \| Level 2
Assets:
Total assets		14,931
Recurring \| Level 2 \| Corporate debt securities
Available-for-sale securities:
Available-for-sale securities		13,930
Recurring \| Level 2 \| U.S. government-sponsored securities
Available-for-sale securities:
Available-for-sale securities		$ 1,001

Available-for-Sale Securities (Details) $ in Thousands	12 Months Ended
Available-for-Sale Securities (Details) $ in Thousands	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) item
Available-for-Sale Securities
Amortized Cost		$ 14,935
Gross Unrealized Losses		(4)
Fair Value	$ 0	$ 14,931
Number of available-for-sale securities in an unrealized loss position \| item		15
Number of available-for-sale securities in an unrealized loss position more than 12 months \| item		0
Unrealized loss position, available-for-sale securities		$ 13,000
Proceeds from sales of available-for-sale securities	$ 0	7,800
Corporate debt securities
Available-for-Sale Securities
Amortized Cost		13,934
Gross Unrealized Losses		(4)
Fair Value		13,930
U.S. government-sponsored securities
Available-for-Sale Securities
Amortized Cost		1,001
Fair Value		$ 1,001

Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Restricted Cash
Security deposit held for operating lease agreement	$ 0.1	$ 0.1

Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property and Equipment
Property and equipment, gross	$ 1,976	$ 2,067
Less accumulated depreciation and amortization	(1,655)	(1,499)
Total property and equipment, net	321	568
Depreciation and amortization expense	304	395	$ 202
Lab equipment
Property and Equipment
Property and equipment, gross	$ 1,470	$ 1,565
Estimated Useful Life	3 years	3 years
Computer equipment
Property and Equipment
Property and equipment, gross	$ 165	$ 166
Estimated Useful Life	3 years	3 years
Furniture and fixtures
Property and Equipment
Property and equipment, gross	$ 77	$ 77
Estimated Useful Life	5 years	5 years
Leasehold improvements
Property and Equipment
Property and equipment, gross	$ 264	$ 259

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Expenses
Accrued compensation	$ 632	$ 1,252
Accrued contracted research costs	1,357	1,850
Accrued professional fees	298	215
Accrued other	145	360
Total	$ 2,432	$ 3,677

Notes Payable - (Details) - USD ($)			1 Months Ended	12 Months Ended
Notes Payable - (Details) - USD ($)	Mar. 31, 2015	Aug. 27, 2014	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2015
Line of Credit Facility
Proceeds from issuance of promissory notes					$ 5,000,000
Term Loan A
Line of Credit Facility
Initial borrowings		$ 5,000,000
Proceeds from issuance of promissory notes		5,000,000
Term Loan B
Line of Credit Facility
Proceeds from issuance of promissory notes	$ 5,000,000
Other Term Loan
Line of Credit Facility
Maximum borrowing capacity		20,000,000
Credit Facility
Line of Credit Facility
Initial borrowings				$ 10,000,000
Maximum borrowing capacity		$ 25,000,000
Number of monthly payments (in number of payments)				36
Interest rate (as a percent)				7.49%
Percentage of amount outstanding due as a payment (as a percent)				3.48%
Effective interest rate (as a percent)				11.20%
Debt Instruments
Financial covenants				0
Final judgment amount that would trigger lender rights for remedy				$ 250,000
Basis spread				5.00%
Interest rate increase in the event of a default (as a percent)				5.00%
Minimum \| Credit Facility
Line of Credit Facility
Percentage of amount outstanding due as a payment (as a percent)				1.00%
Maximum \| Credit Facility
Line of Credit Facility
Percentage of amount outstanding due as a payment (as a percent)				3.00%
Warrant Liability
Debt Instruments
Exercise period			7 years
2014 and 2015 Warrants
Debt Instruments
Value of warrants			$ 200,000
Debt discounts			$ 200,000
2014 Warrants \| Term Loan A
Debt Instruments
Number of shares after conversion of issued warrants (in shares)				157,844
2015 Warrants \| Term Loan B
Debt Instruments
Number of shares after conversion of issued warrants (in shares)				157,844
Common stock warrants \| Initial public offering
Debt Instruments
Number of shares after conversion of issued warrants (in shares)			24,566
Exercise price of warrants (in dollars per share)			$ 12.2100

Notes Payable - Future Principal Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Estimated future principal payments
2018	$ 2,500
Total	2,500
Less: discount for warrants and costs paid to lenders	(21)
Less: current portion	(2,479)
Notes payable, net of current portion and discount		$ 2,479
Credit Facility
Estimated future principal payments
Interest expense	$ 500	$ 800	$ 1,000

Commitments (Details) - USD ($) $ in Thousands	12 Months Ended
Commitments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating leases
Rent expense	$ 1,300	$ 900	$ 800
Future minimum payments required under the non-cancelable operating lease
2018	1,379
2019	1,421
2020	721
Total minimum lease payments	$ 3,521

Revenue Recognition (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		12 Months Ended
Revenue Recognition (Details) - USD ($) $ in Thousands	Oct. 31, 2017	Aug. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2017
Revenue recognition
Revenue recognized related to a Option agreement			$ 250	$ 250	$ 500
Option agreement
Revenue recognition
Non-refundable up-front payment		$ 250
Quarterly payments		$ 250
Quarterly payment received	$ 250
Revenue recognized related to a Option agreement					$ 500

Preferred Stock (Details) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Jun. 30, 2015 shares	Mar. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2017 series $ / shares shares	Dec. 31, 2015 USD ($)	Dec. 31, 2016 $ / shares shares
Preferred Stock
Preferred stock, authorized (in Shares)			5,000,000		5,000,000
Preferred stock, par value (in dollars per share) \| $ / shares			$ 0.001		$ 0.001
Preferred Stock, shares issued (in shares)			0		0
Share price (in dollars per share) \| $ / shares			$ 0.001
Preferred stock, outstanding (in shares)			0		0
Minimum number of series used to issue preferred stock \| series			1
Net proceeds from issuance of preferred stock \| $				$ 12,331
Initial public offering
Preferred Stock
Conversion of convertible preferred stock into common stock (in shares)	9,029,549
Series B Convertible Preferred Stock
Preferred Stock
Preferred Stock, shares issued (in shares)		13,062,965
Share price (in dollars per share) \| $ / shares		$ 0.9503
Net proceeds from issuance of preferred stock \| $		$ 12,300

Common Stock (Details)	12 Months Ended
Common Stock (Details)	Dec. 31, 2017 Vote $ / shares shares	Dec. 31, 2016 $ / shares shares
Common Stock
Common stock, authorized (in shares)	150,000,000	150,000,000
Par value of common stock (in dollars per share) \| $ / shares	$ 0.001	$ 0.001
Common stock, issued (in shares)	23,645,247	18,817,572
Common Stock, outstanding (in shares)	23,645,247	18,817,572
Number of votes entitled to each common stock share \| Vote	1
Common stock dividends, declared or paid (in dollars per share) \| $ / shares	$ 0
Shares reserved for future issuance (in shares)	4,218,016	3,300,018
Common stock warrants
Common Stock
Shares reserved for future issuance (in shares)	24,566	24,566
Stock options
Common Stock
Shares reserved for future issuance (in shares)	3,669,791	2,939,968
Employee Stock Purchase Plan
Common Stock
Shares reserved for future issuance (in shares)	523,659	335,484

Stock Incentive Plans - Plan Information and Valuation Assumptions (Details) - shares			12 Months Ended
	Jan. 01, 2018	Jan. 01, 2017	Dec. 31, 2017	Dec. 31, 2016
Stock-Based Compensation
Shares reserved for future issuance (in shares)			4,218,016	3,300,018
2015 Plan
Stock-Based Compensation
Shares available for grant (in shares)			1,068,287
Shares reserved for future issuance (in shares)			2,837,699
Shares available for future issuance (in shares)			824,165
2015 Plan \| Annual minimum increase
Stock-Based Compensation
Shares available for grant (in shares)			1,297,334
Percentage of outstanding Common Stock available for issuance (as a percent)			4.00%
Potential increase in shares available for issuance as determined by the Board of Directors (in shares)	945,810	752,702
2008 Plan
Stock-Based Compensation
Shares available for grant (in shares)			25,942
Shares reserved for future issuance (in shares)			832,092
Shares available for future issuance (in shares)			0
Stock options
Stock-Based Compensation
Vesting period			4 years
Cliff period for vesting			1 year
Exercisable period			10 years

Stock Incentive Plans - Stock Option Plan Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock options
Outstanding at beginning of year (in shares)	2,270,169
Granted (in shares)	890,740
Exercised (in shares)	(22,879)
Cancelled or forfeited (in shares)	(292,404)
Outstanding at end of year (in shares)	2,845,626	2,270,169
Vested and Exercisable to vest at end of year (in shares)	1,265,411
Weighted-Average Exercise Price
Outstanding at beginning of year (in dollars per share)	$ 6.11
Granted (in dollars per share)	1.26
Exercised (in dollars per share)	1.01
Cancelled or forfeited (in dollars per share)	7.13
Outstanding at end of year (in dollars per share)	4.53	$ 6.11
Vested and Exercisable at end of year (in dollars per share)	$ 5.91
Weighted Average Remaining Contractual Term (years)
Outstanding at end of year	7 years 9 months	8 years 1 month 21 days
Vested and Exercisable at end of year	6 years 6 months 11 days
Aggregate Intrinsic Value
Outstanding at the beginning of the year	$ 613
Outstanding at the end of the year	$ 216	$ 613
Weighted average of the assumptions used to compute employee stock based compensation
Expected term (in years)		6 years 3 months
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Stock options
Unrecognized stock-based compensation expense
Intrinsic value of options exercised	$ 40	$ 500	$ 300
Adjusted fair value of options vested	$ 2,200	$ 2,300	$ 1,400
Weighted average grant date fair value of options granted (in dollars per share)	$ 0.85	$ 2.89	$ 7.68
Unrecognized compensation expense related to unvested stock option awards	$ 3,200
Weighted-average amortization period over which cost is expected to be recognized	2 years
Minimum
Weighted average of the assumptions used to compute employee stock based compensation
Weighted-average expected volatility (as a percent)	76.10%	72.00%	73.60%
Expected term (in years)	6 years 3 months		6 years 2 months 1 day
Risk-free interest rate (as a percent)	2.00%	1.24%	0.92%
Maximum
Weighted average of the assumptions used to compute employee stock based compensation
Weighted-average expected volatility (as a percent)	81.90%	73.90%	86.80%
Expected term (in years)	9 years 9 months 7 days		10 years
Risk-free interest rate (as a percent)	2.38%	2.00%	2.45%

Stock Incentive Plans - Employee Stock Purchase Plan (Details) - shares			1 Months Ended	12 Months Ended
	Jan. 01, 2018	Jan. 01, 2017	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016
Stock-Based Compensation
Common Stock, Issued, (in shares)				23,645,247	18,817,572
2015 Plan
Stock-Based Compensation
Shares available for grant (in shares)				1,068,287
2015 Plan \| Annual minimum increase
Stock-Based Compensation
Shares available for grant (in shares)				1,297,334
Percentage of outstanding Common Stock available for issuance (as a percent)				4.00%
Potential increase in shares available for issuance as determined by the Board of Directors (in shares)	945,810	752,702
Common Stock \| 2015 ESPP
Stock-Based Compensation
Common Stock, Issued, (in shares)				0
Shares available for grant (in shares)			182,352
Common Stock \| 2015 ESPP \| Annual minimum increase
Stock-Based Compensation
Percentage of outstanding Common Stock available for issuance (as a percent)			1.00%
Potential increase in shares available for issuance as determined by the Board of Directors (in shares)	236,452	188,175	364,705

Income Taxes - Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Transition tax on foreign subsidiaries		$ 0
Amount of decrease in deferred tax assets		$ (19,800)
Reconciliation of the U.S. statutory income tax rate to the effective income tax rate
Federal income tax (benefit) at statutory rate		34.00%	34.00%	34.00%
Permanent differences		(1.16%)	(0.85%)	(0.86%)
Federal research and development credits and adjustments		2.36%	2.43%	2.64%
State income tax benefit, net of federal benefit		5.80%	5.70%	5.77%
Tax reform deferred rate change		(72.21)
Other (as a percent)		(0.36%)	(0.40%)	0.28%
Change in valuation allowance		31.57%	(40.88%)	(41.82%)
Deferred tax assets
Net operating loss carryforwards		$ 41,436	$ 48,682
Tax credit carryforwards		5,896	4,833
Capitalized research and development		1,879	3,394
Capitalized legal expenses		1,156	1,597
Other differences		1,048	1,335
Total deferred tax assets		51,415	59,841
Valuation allowance		(51,415)	(59,841)
Valuation allowance
Recorded increase (decrease) in valuation allowance		(8,400)	$ 14,700	$ 13,600
Federal
Valuation allowance
Net operating loss carryforwards		152,100
Tax credit carryforwards		4,500
State
Valuation allowance
Net operating loss carryforwards		150,400
Tax credit carryforwards		1,800
Accounting Standards Update 2016-09
Valuation allowance
Recorded increase (decrease) in valuation allowance		$ 200
Forecast
Reconciliation of the U.S. statutory income tax rate to the effective income tax rate
Federal income tax (benefit) at statutory rate	21.00%

Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Events
Share price (in dollars per share)		$ 0.001
Additional common stock issued		$ 8,036	$ 11,994	$ 61,744
Common Stock
Subsequent Events
Number of shares issued		4,804,796	3,243,015	5,750,000
Additional common stock issued		$ 5	$ 3	$ 5
Second ATM Program \| Common Stock \| Subsequent Events
Subsequent Events
Number of shares issued	5,390,255
Share price (in dollars per share)	$ 1.59
Additional common stock issued	$ 8,600
Net proceeds	$ 8,300
Second ATM Program \| Cowen \| Common Stock \| Subsequent Events
Subsequent Events
Number of shares issued	5,390,255
Share price (in dollars per share)	$ 1.59
Additional common stock issued	$ 8,600
Net proceeds	$ 8,300

Quarterly Financial Information (Unaudited) (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
Quarterly Financial Information (Unaudited)
Revenue	$ 250	$ 250							$ 500
Operating expenses	5,712	7,202	$ 6,919	$ 7,761	$ 8,673	$ 8,283	$ 9,396	$ 9,206	27,594	$ 35,558	$ 31,659
Net loss	$ (5,497)	$ (7,017)	$ (6,974)	$ (7,876)	$ (8,778)	$ (8,419)	$ (9,445)	$ (9,418)	$ (27,094)	$ (35,558)	$ (31,659)
Net loss per share:
Basic and Diluted (in dollars per share)	$ (0.24)	$ (0.31)	$ (0.32)	$ (0.41)	$ (0.47)	$ (0.54)	$ (0.61)	$ (0.61)	$ (1.26)	$ (2.22)	$ (4.06)
Weighted-average common shares outstanding used in net loss per share:
Basic and Diluted (in shares)	23,218,476	22,563,174	21,796,194	19,093,273	18,699,480	15,512,608	15,373,964	15,335,516	21,681,534	16,230,190	8,041,948