Esperion Therapeutics (ESPR) 10-K 25 Feb 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Feb. 01, 2016	Jun. 30, 2015
Document and Entity Information
Entity Registrant Name	Esperion Therapeutics, Inc.
Entity Central Index Key	1,434,868
Document Type	10-K
Document Period End Date	Dec. 31, 2015
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	Large Accelerated Filer
Entity Public Float			$ 1,600
Entity Common Stock, Shares Outstanding		22,540,466
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 77,336	$ 85,038
Short-term investments	134,925	20,803
Prepaid clinical development costs	888	366
Other prepaid and current assets	1,245	492
Total current assets	214,394	106,699
Property and equipment, net	807	780
Intangible assets	56	56
Long-term investments	80,315	35,741
Total assets	295,572	143,276
Current liabilities:
Accounts payable	707	2,040
Current portion of long-term debt	1,604	638
Accrued clinical development costs	2,191	1,978
Other accrued liabilities	1,123	835
Total current liabilities	5,625	5,491
Long-term debt, net of discount and issuance costs	2,688	4,231
Total liabilities	$ 8,313	$ 9,722
Commitments and contingencies (Note 5)
Stockholders' equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized as of December 31, 2015 and December 31, 2014; no shares issued or outstanding at December 31, 2015 and December 31, 2014
Common stock, $0.001 par value; 120,000,000 shares authorized as of December 31, 2015 and December 31, 2014; 22,518,907 shares issued and 22,516,508 outstanding at December 31, 2015 and 20,352,876 shares issued and 20,343,325 outstanding at December 31, 2014	$ 23	$ 20
Additional paid-in capital	441,940	238,031
Accumulated other comprehensive income ( loss)	(482)	(59)
Accumulated deficit	(154,222)	(104,438)
Total stockholders' equity	287,259	133,554
Total liabilities and stockholders' equity	$ 295,572	$ 143,276

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2015	Dec. 31, 2014
Balance Sheets
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares issued	22,518,907	20,352,876
Common stock, shares outstanding	22,516,508	20,343,325

Statements of Operations and Co

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating expenses:
Research and development	$ 29,802	$ 25,302	$ 16,014
General and administrative	20,238	10,922	6,745
Total operating expenses	50,040	36,224	22,759
Loss from operations	(50,040)	(36,224)	(22,759)
Interest expense.	(520)	(270)	(936)
Change in fair value of warrant liability			(2,587)
Other income, net	776	119	194
Net loss	$ (49,784)	$ (36,375)	$ (26,088)
Net loss per common share (basic and diluted) (in dollars per share)	$ (2.26)	$ (2.22)	$ (3.31)
Weighted-average shares outstanding (basic and diluted)	22,019,818	16,374,102	7,885,921
Other comprehensive income (loss):
Unrealized ( loss) on investments	$ (423)	$ (56)	$ (3)
Total comprehensive loss	$ (50,207)	$ (36,431)	$ (26,091)

Statements of Convertible Prefe

Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Common Stock.	Additional Paid-In Capital	Deficit Accumulated During the Development Stage	Accumulated Other Comprehensive Loss	Series A preferred stock	Series A-1 preferred stock	Total
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued	346,478
Balance (in shares) at Dec. 31, 2012	346,478
Increase (Decrease) in Temporary Equity
Issuance of preferred stock in exchange for convertible promissory notes, net of issuance costs					$ 16,623	$ 7,750
Issuance of preferred stock in exchange for convertible promissory notes, net of issuance costs (in shares)					16,623,092	6,750,000
Issuance of Series A preferred stock, net of issuance costs					$ 16,880
Issuance of Series A preferred stock, net of issuance costs (in shares)					17,000,000
Preferred shares converted into common stock	$ 9	$ 65,216			$ (57,478)	$ (7,750)	$ 65,225
Preferred shares converted into common stock (in shares)	9,210,999				(57,598,092)	(6,750,000)
Balance (in shares) at Dec. 31, 2013	15,357,413
Balance at Dec. 31, 2012		610	$ (41,975)		$ 23,975		(41,365)
Balance (in shares) at Dec. 31, 2012	346,478
Increase (Decrease) in Stockholders' Equity
Early exercise of stock options and vesting of restricted stock		21					21
Early exercise of stock options and vesting of restricted stock (in shares)	25,765
Preferred shares converted into common stock	$ 9	65,216			$ (57,478)	$ (7,750)	65,225
Issuance of common stock from public offering, net of issuance costs	$ 6	72,188					72,194
Issuance of common stock from public offering, net of issuance costs (in shares)	5,750,000
Reclassification of warrants from liabilities to equity		2,852					2,852
Exercise of stock options		28					28
Exercise of stock options (in shares)	24,171
Stock-based compensation		1,227					1,227
Other comprehensive loss				$ (3)			(3)
Net loss			(26,088)				(26,088)
Balance at Dec. 31, 2013	$ 15	142,142	(68,063)	(3)			$ 74,091
Balance (in shares) at Dec. 31, 2013	15,357,413
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued	346,478
Common Stock, Shares, Issued							396,414
Balance (in shares) at Jun. 30, 2013							396,414
Balance (in shares) at Jul. 11, 2013							15,357,413
Balance (in shares) at Jun. 30, 2013							396,414
Balance (in shares) at Jul. 11, 2013							15,357,413
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued							396,414
Balance (in shares) at Jun. 30, 2013							396,414
Balance (in shares) at Jul. 01, 2013							14,607,413
Balance (in shares) at Jun. 30, 2013							396,414
Balance (in shares) at Jul. 01, 2013							14,607,413
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued							396,414
Common Stock, Shares, Issued							14,607,413
Common Stock, Shares, Issued							15,357,413
Common Stock, Shares, Issued	15,357,413
Balance (in shares) at Dec. 31, 2013	15,357,413
Balance (in shares) at Dec. 31, 2014	20,352,876						20,352,876
Balance (in shares) at Dec. 31, 2013	15,357,413
Increase (Decrease) in Stockholders' Equity
Early exercise of stock options and vesting of restricted stock		39					$ 39
Issuance of common stock from public offering, net of issuance costs	$ 5	91,620					91,625
Issuance of common stock from public offering, net of issuance costs (in shares)	4,887,500
Issued warrants in connection with debt issuance		78					78
Exercise of stock options		473					473
Exercise of stock options (in shares)	107,963
Stock-based compensation		3,679					3,679
Other comprehensive loss				(56)			(56)
Net loss			(36,375)				(36,375)
Balance at Dec. 31, 2014	$ 20	238,031	(104,438)	(59)			$ 133,554
Balance (in shares) at Dec. 31, 2014	20,352,876						20,352,876
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued	15,357,413						20,352,876
Common Stock, Shares, Issued	20,352,876						20,352,876
Balance (in shares) at Dec. 31, 2015	22,518,907						22,518,907
Balance (in shares) at Dec. 31, 2014	20,352,876						20,352,876
Increase (Decrease) in Stockholders' Equity
Early exercise of stock options and vesting of restricted stock		26					$ 26
Issuance of common stock from public offering, net of issuance costs	$ 3	189,980					189,983
Issuance of common stock from public offering, net of issuance costs (in shares)	2,012,500
Exercise of stock options		1,177					$ 1,177
Exercise of stock options (in shares)	128,086						128,086
Exercise of warrants (in shares)	25,445
Stock-based compensation		12,726					$ 12,726
Other comprehensive loss				(423)			(423)
Net loss			(49,784)				(49,784)
Balance at Dec. 31, 2015	$ 23	$ 441,940	$ (154,222)	$ (482)			$ 287,259
Balance (in shares) at Dec. 31, 2015	22,518,907						22,518,907
Increase (Decrease) in Stockholders' Equity
Common Stock, Shares, Issued	20,352,876						20,352,876
Common Stock, Shares, Issued	22,518,907						22,518,907

Statements of Convertible Pref6

Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Series A preferred stock
Payments of Stock Issuance Costs			$ (120)
Series A-1 preferred stock
Payments of Stock Issuance Costs			(53)
Common Stock.
Payments of Stock Issuance Costs	$ 199	$ 260	$ 2,671

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (49,784)	$ (36,375)	$ (26,088)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	236	160	71
Amortization of debt discount and beneficial conversion	29	15	459
Amortization of debt issuance costs	32	16	19
Amortization of premiums and discounts on investments	647	202	47
Revaluation of warrants			2,587
Noncash interest expense on convertible notes			459
Stock-based compensation expense	12,726	3,679	1,227
Loss related to assets held for sale		29	27
(Gain)/Loss on sale of assets	47	2	(148)
Changes in assets and liabilities:
Prepaids and other assets	(1,275)	(264)	27
Accounts payable	(1,333)	(299)	1,756
Other accrued liabilities	519	814	1,443
Net cash used in operating activities	(38,156)	(32,021)	(18,114)
Investing activities
Purchases of investments	(280,559)	(48,088)	(24,677)
Proceeds from sales/maturities of investments	120,792	12,351	3,505
Proceeds from sale of assets	24	12	201
Purchase of property and equipment	(325)	(873)	(31)
Net cash used in investing activities	(160,068)	(36,598)	(21,002)
Financing activities
Proceeds from issuance of common stock, net of issuance costs	189,983	91,731	72,194
Proceeds from issuance of preferred stock, net of issuance costs			16,824
Proceeds from exercise of common stock options	1,177	473	123
Proceeds from warrant issuance		78
Proceeds from debt issuance, net of issuance costs		4,838
Payments of long term debt	(638)
Net cash provided by financing activities	190,522	97,120	89,141
Net (decrease) increase in cash and cash equivalents	(7,702)	28,501	50,025
Cash and cash equivalents at beginning of period	85,038	56,537	6,512
Cash and cash equivalents at end of period	$ 77,336	$ 85,038	56,537
10% Convertible promissory notes
Supplemental disclosure of cash flow information:
Conversion of promissory notes, including accrued interest into Series A - 1 preferred stock			16,623
Pfizer note
Supplemental disclosure of cash flow information:
Conversion of promissory notes, including accrued interest into Series A - 1 preferred stock			$ 7,803

Statements of Cash Flows (Paren

Statements of Cash Flows (Parenthetical) $ in Thousands	Dec. 31, 2013USD ($)
10% Convertible promissory notes
Conversion of convertible promissory notes into preferred stock, accrued interest	$ 923
Pfizer note
Conversion of convertible promissory notes into preferred stock, accrued interest	$ 274

The Company and Basis of Presen

The Company and Basis of Presentation	12 Months Ended
	Dec. 31, 2015
The Company and Basis of Presentation
The Company and Basis of Presentation	1. The Company and Basis of Presentation The Company is a pharmaceutical company whose planned principal operations are focused on developing and commercializing first-in-class, oral, low-density lipoprotein cholesterol ("LDL-C") lowering therapies for the treatment of patients with elevated LDL-C. ETC-1002 ("bempedoic acid"), the Company's lead product candidate, is an inhibitor of ATP Citrate Lyase, a well-characterized enzyme on the cholesterol biosynthesis pathway. Bempedoic acid inhibits cholesterol synthesis in the liver, decreases intracellular cholesterol and up-regulates LDL-receptors, resulting in increased LDL-C clearance and reduced plasma levels of LDL-C. The Company held an End-of-Phase 2 meeting with the Food and Drug Administration in August 2015, and initiated a global Phase 3 long-term safety and tolerability study for bempedoic acid in January 2016, in patients with hyperlipidemia whose LDL-C is not adequately controlled with low- and moderate-dose statins. The Company owns the exclusive worldwide rights to bempedoic acid. The Company's primary activities since incorporation have been conducting research and development activities, including nonclinical, preclinical and clinical testing, performing business and financial planning, recruiting personnel, and raising capital. Accordingly, the Company has not commenced principal operations and is subject to risks and uncertainties which include the need to research, develop, and clinically test potential therapeutic products; obtain regulatory approvals for its products and commercialize them, if approved; expand its management and scientific staff; and finance its operations with an ultimate goal of achieving profitable operations. The Company has sustained operating losses since inception and expects such losses to continue over the foreseeable future. Management plans to continue to fund operations through public or private equity or debt financings or through other sources, which may include collaborations with third parties. If adequate funds are not available, the Company may not be able to continue the development of its current or future product candidates, or to commercialize its current or future product candidates, if approved. Reverse Stock Split On June 11, 2013, in connection with its initial public offering (the "IPO"), the Company effectuated a 1-for-6.986 reverse stock split of its outstanding common stock, which was approved by the Company's board of directors on June 5, 2013. The reverse stock split resulted in an adjustment to the Series A preferred stock and Series A-1 preferred stock conversion prices to reflect a proportional decrease in the number of shares of common stock to be issued upon conversion. The accompanying financial statements and notes to the financial statements give effect to the reverse stock split for all periods presented. The shares of common stock retained a par value of $0.001 per share. Accordingly, stockholders' equity reflects the reverse stock split by reclassifying from "Common stock" to "Additional paid-in capital" in an amount equal to the par value of the decreased shares resulting from the reverse stock split. Initial Public Offering On July 1, 2013, the Company completed its IPO whereby it sold 5,000,000 shares of common stock at a price of $14.00 per share. The shares began trading on the Nasdaq Global Market on June 26, 2013. On July 11, 2013, the underwriters exercised their over-allotment option in full and purchased an additional 750,000 shares of common stock at a price of $14.00 per share. The Company received approximately $72.2 million in net proceeds from the IPO, including proceeds from the exercise of the underwriters' over-allotment option, net of underwriting discounts and commissions and offering expenses. Upon closing of the IPO, all outstanding shares of preferred stock converted into 9,210,999 shares of common stock, and warrants exercisable for convertible preferred stock were automatically converted into warrants exercisable for 277,690 shares of common stock, resulting in the reclassification of the related convertible preferred stock warrant liability of $2.9 million to additional paid-in capital (see Note 4). The following table summarizes the Company's capitalization upon closing of its IPO: Total common stock issued as of June 30, 2013 Conversion of Series A preferred stock into common stock upon closing of IPO Conversion of Series A-1 preferred stock into common stock upon closing of IPO Sales of common stock through IPO ​ ​ ​ ​ ​ Common stock issued as of July 1, 2013 Issuance of common stock to underwriters due to exercise of over-allotment ​ ​ ​ ​ ​ Total common stock issued as of July 11, 2013 ​ ​ ​ ​ ​ Follow On Offerings On October 21, 2014, the Company completed an underwritten public offering of 4,887,500 shares of common stock, including 637,500 shares sold pursuant to the full exercise of an over-allotment option granted to the underwriters. All the shares were offered by the Company at a price to the public of $20.00 per share. The aggregate net proceeds received by the Company from the offering were $91.6 million, net of underwriting discounts and commissions and expenses payable by the Company. On March 24, 2015, the Company completed an underwritten public offering of 2,012,500 shares of common stock, including 262,500 shares sold pursuant to the full exercise of an over-allotment option granted to the underwriters. All the shares were offered by the Company at a price to the public of $100.00 per share. The aggregate net proceeds received by the Company from the offering were $190.0 million, net of underwriting discounts and commissions and expenses payable by the Company.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2015
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in accordance with generally accepted accounting principles in the United States requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, expenses and related disclosures. Actual results could differ from those estimates. Prior to the completion of the IPO on July 1, 2013, the Company utilized significant estimates and assumptions in determining the fair value of its common stock. The Company utilized valuation methodologies in accordance with the framework of the 2004 American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation , to estimate the fair value of its common stock. Each valuation methodology includes estimates and assumptions that require the Company's judgment. These estimates and assumptions include a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector, the prices at which the Company sold shares of its preferred stock, the superior rights and preferences of securities senior to its common stock at the time and the likelihood of achieving a liquidity event, such as an IPO or sale. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock at each valuation date. Cash and Cash Equivalents The Company invests its excess cash in bank deposits, money market accounts, and short-term investments. The Company considers all highly liquid investments with an original maturity of 90 days or less at the time of purchase to be cash equivalents. Cash equivalents are reported at fair value. Investments Investments are considered to be available-for-sale and are carried at fair value. Unrealized gains and losses, if any, are reported as a separate component of stockholders' equity. The cost of investments classified as available-for-sale are adjusted for the amortization of premiums and accretion of discounts to maturity and recorded in other income, net. Realized gains and losses, if any, are determined using the specific identification method and recorded in other income, net. Investments with original maturities beyond 90 days at the date of purchase and which mature at, or less than twelve months from, the balance sheet date are classified as current. Investments with a maturity beyond twelve months from the balance sheet date are classified as long-term. Concentration of Credit Risk Cash, cash equivalents, and marketable securities consist of financial instruments that potentially subject the Company to concentrations of credit risk. The Company has established guidelines for investment of its excess cash and believes the guidelines maintain safety and liquidity through diversification of counterparties and maturities. Segment Information The Company views its operations and manages its business in one operating segment, which is the business of researching, developing and commercializing therapies for the treatment of patients with elevated LDL-C. Fair Value of Financial Instruments The Company's cash, cash equivalents and investments are carried at fair value. Financial instruments, including other prepaid and current assets, accounts payable and accrued liabilities are carried at cost, which approximates fair value. Debt is carried at amortized cost, which approximates fair value. Property and Equipment, Net Property and equipment are recorded at cost, less accumulated depreciation. Depreciation is provided using the straight-line method over the estimated useful lives of the respective assets, generally three to ten years. Leasehold improvements are amortized over the lesser of the lease term or the estimated useful lives of the related assets. Impairment of Long-Lived Assets The Company reviews long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition are less than its carrying amount. The impairment loss, if recognized, would be based on the excess of the carrying value of the impaired asset over its respective fair value. Excluding impairment losses recorded on assets held for sale, no other impairment losses have been recorded through December 31, 2015. Research and Development Research and development expenses consist of costs incurred to further the Company's research and development activities and include salaries and related benefits, costs associated with clinical activities, nonclinical activities, regulatory activities, manufacturing activities to support clinical activities, research-related overhead expenses and fees paid to external service providers that conduct certain research and development, clinical, and manufacturing activities on behalf of the Company. Research and development costs are expensed as incurred. Accrued Clinical Development Costs Outside research costs are a component of research and development expense. These expenses include fees paid to clinical research organizations and other service providers that conduct certain clinical and product development activities on behalf of the Company. Depending upon the timing of payments to the service providers, the Company recognizes prepaid expenses or accrued expenses related to these costs. These accrued or prepaid expenses are based on management's estimates of the work performed under service agreements, milestones achieved and experience with similar contracts. The Company monitors each of these factors and adjusts estimates accordingly. Income Taxes The Company utilizes the liability method of accounting for income taxes as required by ASC 740, Income Taxes. Under this method, deferred tax assets and liabilities are determined based on differences between financial reporting and the tax basis of assets and liabilities and are measured using enacted tax rates and laws that will be in effect when the differences are expected to reverse. The Company has incurred operating losses since inception. Accordingly, it is not more likely than not that the Company will realize deferred tax assets and as such, it has recorded a full valuation allowance. Warrants The Company accounts for its warrants issued in connection with its various financing transactions based upon the characteristics and provisions of the instrument. Warrants classified as liabilities are recorded on the Company's balance sheet at their fair value on the date of issuance and are marked-to-market on each subsequent reporting period, with the fair value changes recognized in the statement of operations. Warrants classified as additional-paid-in-capital are recorded on the Company's balance sheet at their fair value on the date of issuance. The warrants are measured using the Black-Scholes option-pricing model subsequent to the pricing of the Company's IPO and a Monte Carlo valuation model for previous periods which are based, in part, upon inputs where there is little or no market data, requiring the Company to develop its own independent assumptions. (See Note 4). Stock-Based Compensation The Company accounts for stock-based compensation in accordance with the provisions of ASC 718, Compensation—Stock Compensation. Accordingly, compensation costs related to equity instruments granted are recognized over the requisite service periods of the awards on a straight-line basis at the grant-date fair value calculated using a Black-Scholes option-pricing model. Additionally, under the provisions of ASC 718, the Company is required to include an estimate of the number of awards that will be forfeited in calculating compensation costs. Any changes to the estimated forfeiture rates are accounted for prospectively. Stock-based compensation arrangements with non-employees are recognized at the grant-date fair value and then re-measured at each reporting period. Expense is recognized during the period the related services are rendered. Recent Accounting Pronouncements In April 2015, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update 2015-03 which simplifies the presentation of debt issuance costs by requiring that debt issuance costs related to a recognized debt liability be presented on the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts, rather than as a deferred charge. The recognition and measurement guidance for debt issuance costs are not affected by the amendment. The Company early-adopted the amendment effective January 1, 2015, which resulted in a change in the balance sheet presentation of net debt; in prior period disclosures the debt issuance costs related to the Company's debt liability were presented on the balance sheet as deferred charges within "Other prepaid and current assets". Upon adoption of the amended guidance, the debt issuance costs associated with the Company's debt liability are presented on the balance sheet as a direct deduction from the carrying amount of the debt liability. Within the December 31, 2015 and 2014, balance sheets, "Long-term debt, net of discount" includes less than $0.1 million and $0.1 million, respectively, of debt issuance costs. In November 2015, the FASB issued Accounting Standards Update 2015-17 which requires that deferred tax assets and liabilities, along with any related valuation allowances, be classified as noncurrent on the balance sheet, effective for annual periods, and for interim periods within those annual periods, beginning after December 15, 2016, for public companies, with early adoption permitted. The Company early-adopted the amendment effective December 31, 2015, classifying all deferred tax assets on the balance sheet as non-current. The Company has incurred operating losses since inception and has fully reserved its net deferred tax assets. Accordingly, the adoption of this amendment has no impact on the balance sheet presentation of deferred tax assets.

Debt

Debt	12 Months Ended
	Dec. 31, 2015
Debt
Debt	3. Debt Credit Facility In June 2014, the Company entered into a loan and security agreement (the "Credit Facility") with Oxford Finance LLC which provided for initial borrowings of $5.0 million under the term loan (the "Term A Loan") and additional borrowings of $15.0 million (the "Term B Loan") at the Company's option, for a maximum of $20.0 million. On June 30, 2014, the Company received proceeds of $5.0 million from the issuance of secured promissory notes under the Term A Loan. Upon achieving positive clinical development results in March 2015, the remaining $15.0 million under the Term B Loan became available to be drawn down, at the Company's sole discretion, until March 31, 2015. The Company did not elect to draw down the Term B Loan as of March 31, 2015. The secured promissory notes issued under the Credit Facility are due on July 1, 2018, and are collateralized by substantially all of the Company's personal property, other than its intellectual property. The Company is obligated to make monthly, interest-only payments on the Term A Loan until July 1, 2015, and, thereafter, to pay 36 consecutive, equal monthly installments of principal and interest from August 1, 2015, through July 1, 2018. The Term A Loan bears interest at an annual rate of 6.40%. In addition, a final payment equal to 8.0% of the Term A Loan is due upon the earlier of the maturity date or prepayment of the term loan. The Company is recognizing the final payment as interest expense using the effective interest method over the life of the Credit Facility. There are no financial covenants associated to the Credit Facility. However, so long as the Credit Facility is outstanding, there are negative covenants that limit or restrict the Company's activities, which include limitations on incurring indebtedness, granting liens, mergers or acquisitions, dispositions of assets, making certain investments, entering into certain transactions with affiliates, paying dividends or distributions, encumbering or pledging interest in its intellectual property and certain other business transactions. Additionally, the Credit Facility includes events of default, the occurrence and continuation of any of which provides the lenders the right to exercise remedies against the Company and the collateral securing the loans under the Credit Facility, which includes cash. These events of default include, among other things, non-payment of any amounts due under the Credit Facility, insolvency, the occurrence of a material adverse event, inaccuracy of representations and warranties, cross default to material indebtedness and a material judgment against the Company. Upon the occurrence of an event of default, all obligations under the Credit Facility shall accrue interest at a rate equal to the fixed annual rate plus five percentage points. In connection with the borrowing of the Term A Loan, the Company issued a warrant to purchase 8,230 shares of common stock at an exercise price of $15.19 (see Note 4). The warrant resulted in a debt discount of $0.1 million which is amortized into interest expense using the effective interest method over the life of the Term A Loan. In addition, the Company incurred debt issuance costs of $0.1 million in connection with the borrowing of the Term A Loan. The debt issuance costs were capitalized and included in long-term debt on the condensed balance sheet at the inception of the Term A Loan, and are amortized to interest expense using the effective interest method over the same term. As of December 31, 2015, the remaining unamortized discount and debt issuance costs associated with the debt were less than $0.1 million and less than $0.1 million, respectively. Estimated future principal payments due under the Credit Facility are as follows: Years Ending December 31, (in thousands) 2016 $ 2017 2018 ​ ​ ​ ​ ​ Total $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ During the years ended December 31, 2015 and 2014, the Company recognized $0.5 million and $0.3 million of interest expense and made cash interest payments of $0.3 million and $0.1 million related to the Credit Facility, respectively. Convertible Notes In January 2012, the Company issued $6.0 million of 10% convertible promissory notes to certain existing investors for cash. In September and November 2012, the Company issued an aggregate of $9.7 million of 10% convertible promissory notes that mature on September 4, 2013, for cash to certain existing investors. In connection with the September convertible note financing, the Company and the holders of the January 2012, convertible promissory notes agreed to extend the maturity date of the January 2012, notes to September 4, 2013. In February 2013, these convertible promissory notes, with an outstanding principal amount of $15.7 million and accrued interest of $0.9 million, were amended and then converted into 16,623,092 shares of Series A preferred stock, in accordance with their terms and at their conversion price of $1.00 per share, and following such conversion, the notes were cancelled. The holders of the September convertible promissory notes received the benefit of a deemed conversion price of the September convertible promissory notes that were below the estimated fair value of the Series A convertible preferred stock at the time of their issuance. The fair value of this beneficial conversion feature was estimated to be $0.3 million. The fair value of this beneficial conversion feature was recorded to debt discount and amortized to interest expense using the effective interest method over the term of the convertible promissory notes. As a result of the conversion of the convertible promissory notes into shares of Series A preferred stock in February 2013, the Company recorded the remaining accretion of the beneficial conversion feature of $0.2 million as interest expense during the year ended December 31, 2013. In connection with the issuance of the September and the November 2012, convertible promissory notes, the Company issued warrants to purchase shares of Series A preferred stock for an aggregate price less than $0.1 million. The estimated fair value of the warrants at issuance was $0.3 million. The proceeds from the sale of the preferred stock and warrants were allocated with $9.4 million to the convertible promissory notes and $0.3 million to warrants. This resulted in a discount on the convertible promissory notes which was amortized into interest expense using the effective interest method over the life of the convertible promissory notes (see Note 4). The company recorded $0.1 million of interest expense for the accretion of the discount during the year ended December 31, 2012. As a result of the conversion of the convertible promissory notes into shares of Series A preferred stock in February 2013, the Company recorded $0.2 million of interest expense for the accretion of this discount during the year ended December 31, 2013. In April 2008, the Company acquired all of the capital stock of Esperion from Pfizer in exchange for a non-subordinated convertible note in the original principal amount of $5.0 million. This convertible promissory note had a maturity date of April 28, 2018. The note bore interest at 8.931% annually, payable semiannually on June 30 and December 31 by adding such unpaid interest to the principal of the note, which would thereafter accrue interest. The Company accrued interest of $0.3 million and $0.6 million during the years ended December 31, 2013 and 2012, respectively. In May 2013, the Company entered into a stock purchase agreement with Pfizer Inc. and sold 6,750,000 shares of Series A-1 preferred stock at a price of $1.1560 per share, which was the fair value at the transaction date. The purchase price was paid through the cancellation of all outstanding indebtedness, including accrued interest, under the Pfizer convertible promissory note, which had an outstanding balance, including accrued interest, of $7.8 million as of May 29, 2013. The Series A-1 preferred stock issued in connection with this transaction was subsequently converted into 966,218 shares of common stock upon completion of the IPO on July 1, 2013.

Warrants

Warrants	12 Months Ended
	Dec. 31, 2015
Warrants
Warrants	4. Warrants In connection with the Credit Facility entered into in June 2014, the Company issued a warrant to purchase 8,230 shares of common stock at an exercise price of $15.19. The warrant will terminate on the earlier of June 30, 2019 and the closing of a merger or consolidation transaction in which the Company is not the surviving entity. The warrant was recorded at fair value of $0.1 million to additional-paid-in-capital in accordance with ASC 815-10 based upon the allocation of the debt proceeds. The Company estimated the fair value of the warrant using a Black-Scholes option-pricing model, which is based, in part, upon subjective assumptions including but not limited to stock price volatility, the expected life of the warrant, the risk-free interest rate and the fair value of the common stock underlying the warrant. The Company estimates the volatility of its stock based on public company peer group historical volatility that is in line with the expected remaining life of the warrant. The risk-free interest rate is based on the U.S. Treasury zero-coupon bond for a maturity similar to the expected remaining life of the warrant. The expected remaining life of the warrant is assumed to be equivalent to its remaining contractual term. In connection with its various convertible note financing transactions, the Company issued warrants to purchase shares of preferred stock which had provisions where the underlying issuance was contingently redeemable based on events outside the Company's control and were recorded as a liability in accordance with ASC 480-10. The warrants were classified as liabilities and were recorded on the Company's balance sheet at fair value on the date of issuance and marked-to-market on each subsequent reporting period, with the fair value changes recognized in the statement of operations. Subsequent to the pricing of the IPO, the Company estimated the fair values of the warrants at each reporting period using a Black-Scholes option-pricing model, which is based, in part, upon subjective assumptions including but not limited to stock price volatility, the expected life of the warrants, the risk-free interest rate and the fair value of the common stock underlying the warrants. The Company estimates the volatility of its stock based on public company peer group historical volatility that is in line with the expected remaining life of the warrants. The risk-free interest rate is based on the U.S. Treasury zero-coupon bond for a maturity similar to the expected remaining life of the warrants. The expected remaining life of the warrants is assumed to be equivalent to their remaining contractual term. Prior to the pricing of the IPO, a Monte Carlo valuation model was utilized to estimate the fair value of the warrants based on the probability and timing of future financings. The assumptions used in calculating the estimated fair market value at each reporting period prior to the closing of the Company's IPO represented the Company's best estimate but do, however, involve inherent uncertainties. The estimated fair value of the warrants was determined using the Monte Carlo valuation model which totaled $0.3 million and was comprised of $0.1 million and $0.2 million as of and for the September and November 2012 financings, respectively, and was recorded as a discount on the related convertible promissory notes and amortized as interest expense over the term of the convertible promissory notes. Inherent in the Monte Carlo valuation model are assumptions related to expected stock-price volatility, expected life and risk-free interest rate. The Company estimates the volatility of its stock based on public company peer group historical volatility that is in line with the expected remaining life of the warrants. The risk-free interest rate is based on the U.S. Treasury zero-coupon bond on the grant date for a maturity similar to the expected remaining life of the warrants. The expected life of the warrants is assumed to be equivalent to their remaining contractual term. The dividend rate is based on the historical rate, which the Company anticipates to remain at zero. The Monte Carlo model was used prior to the closing of the Company's IPO to appropriately value the potential future exercise price based on various exit scenarios. This required Level 3 inputs which are based on the Company's estimates of the probability and timing of potential future financings. Upon the closing of the Company's IPO, all warrants exercisable for 1,940,000 shares of Series A preferred stock, at an exercise price of $1.00 per share (unadjusted for stock splits), were automatically converted into warrants exercisable for 277,690 shares of common stock, at an exercise price of $6.99 per share. As a result, the Company concluded the warrants outstanding no longer met the criteria to be classified as liabilities and were reclassified to additional paid-in capital at fair value on the date of reclassification. During the years ended December 31, 2015, 2014 and 2013, the Company recognized a loss of $0, $0 and $2.6 million, respectively, relating to the change in the fair value of the warrant liability. During the year ended December 31, 2015, 29,330 warrants were net exercised for 25,445 shares of the Company's common stock. The remaining 248,360 warrants outstanding as of December 31, 2015, expire in February 2018. As of December 31, 2015, the Company had warrants outstanding that were exercisable for a total of 256,590 shares of common stock at a weighted-average exercise price of $7.25 per share.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies
Commitments and Contingencies	5. Commitments and Contingencies In February 2014, the Company entered into an operating lease agreement for its principal executive offices located in Ann Arbor, Michigan commencing in April 2014, with a term of 63 months. The Company's lease provides for fixed monthly rent for the term of the lease, with monthly rent increasing every 12 months subsequent to the first three months of the lease, and also provides for certain rent adjustments to be paid as determined by the landlord. In May 2014, the Company entered into the third amendment to the operating lease agreement for its laboratory facility in Plymouth, Michigan. The amendment provides in part that (i) the expiration date of the term of the lease is extended from April 2014 to April 2017, (ii) the rentable laboratory space is adjusted to 3,045 square feet, (iii) the Company's proportionate share of the landlord's expenses and taxes is adjusted to 7.40%, (iv) the Company may exercise its option to renew the lease for one term of three years through written notice to the landlord by February 2017, and (v) the annual base rent under the lease is decreased to $37,000, subject to increases and adjustments provided in the lease. In December 2015, the Company entered into a termination agreement for its laboratory facility. Pursuant to the termination, the Plymouth lease was terminated, effective retroactively on October 30, 2015 (the "Termination Date"), rather than April 30, 2017, as contemplated by the Plymouth lease, with the Company having no further rent obligations to the Landlord pursuant to the Plymouth lease after the Termination Date. In August 2015, the Company entered into an operating lease agreement to increase its office space and support its clinical development operations located in Ann Arbor, Michigan, commencing September 2015, with a term of 49 months. The Company's lease provides for fixed monthly rent for the term of the lease, with monthly rent increasing every 12 months subsequent to the first month of the lease. The total rent expense for the years ended December 31, 2015, 2014 and 2013, was approximately $0.2 million, $0.3 million and $0.3 million, respectively. The following table summarizes the Company's future minimum lease payments as of December 31, 2015: Total Less than 1 Year 1 - 3 Years 3 - 5 Years More than 5 Years (in thousands) Operating lease $ $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total $ $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2015
Property and Equipment
Property and Equipment	6. Property and Equipment Property and equipment consist of the following: December 31, 2015 2014 (in thousands) Lab equipment $ $ Computer equipment Software Furniture and fixtures Leasehold improvements Assets in Progress — ​ ​ ​ ​ ​ ​ ​ ​ Subtotal Less accumulated depreciation and amortization ​ ​ ​ ​ ​ ​ ​ ​ Property and equipment, net $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Depreciation expense was $0.2 million, $0.2 million and $0.1 million for the years ended December 31, 2015, 2014 and 2013, respectively.

Other Accrued Liabilities

Other Accrued Liabilities	12 Months Ended
	Dec. 31, 2015
Other Accrued Liabilities
Other Accrued Liabilities	7. Other Accrued Liabilities Other accrued liabilities consist of the following: December 31, 2015 2014 (in thousands) Accrued compensation $ $ Accrued professional fees Accrued franchise and property taxes Accrued interest Accrued other ​ ​ ​ ​ ​ ​ ​ ​ Total other accrued liabilities $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Investments

Investments	12 Months Ended
	Dec. 31, 2015
Investments
Investments	8. Investments The following table summarizes the Company's cash equivalents and investments: December 31, 2015 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Cash equivalents: Money market funds $ $ — $ — $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Short-term investments: Certificates of deposit — ) U.S treasury notes — ) U.S. government agency securities — ) Long-term investments: Certificates of deposit — ) U.S. treasury notes — ) U.S. government agency securities — ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total $ $ — $ ) $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2014 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Cash equivalents: Money market funds $ $ — $ — $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Short-term investments: Certificates of deposit — — U.S treasury notes — U.S. government agency securities — ) Long-term investments: Certificates of deposit — — U.S. treasury notes — ) U.S. government agency securities — ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total $ $ $ ) $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ At December 31, 2015, remaining contractual maturities of available-for-sale investments classified as current on the balance sheet were less than 12 months, and remaining contractual maturities of available-for-sale investments classified as long-term were less than two years. During the years ended December 31, 2015, 2014 and 2013, other income, net in the statements of operations includes interest income on available-for-sale investments of $1.5 million, $0.4 million and $0.1 million, and expense for the amortization of premiums and discounts on investments of $0.6 million, $0.2 million and less than $0.1 million, respectively. There were no unrealized gains or losses on investments reclassified from accumulated other comprehensive loss to other income, net in the statements of operations during the year ended December 31, 2015.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2015
Fair Value Measurements
Fair Value Measurements	9. Fair Value Measurements The Company follows accounting guidance that emphasizes that fair value is a market-based measurement, not an entity-specific measurement. Fair value is defined as "the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date." Fair value measurements are defined on a three level hierarchy: Level 1 inputs: Quoted prices for identical assets or liabilities in active markets; Level 2 inputs: Observable inputs other than Level 1 prices, such as quoted market prices for similar assets or liabilities or other inputs that are observable or can be corroborated by market data; and Level 3 inputs: Unobservable inputs that are supported by little or no market activity and require the reporting entity to develop assumptions that market participants would use when pricing the asset or liability. The following table presents the Company's financial assets and liabilities that have been measured at fair value on a recurring basis: Description Total Level 1 Level 2 Level 3 (in thousands) December 31, 2015 Assets: Money market funds $ $ $ — $ — Available-for-sale securities: Certificates of deposit — — U.S. treasury notes — — U.S. government agency securities — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total assets at fair value $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2014 Assets: Money market funds $ $ $ — $ — Available-for-sale securities: Certificates of deposit — — U.S. treasury notes — — U.S. government agency securities — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total assets at fair value $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ There were no transfers between Levels 1, 2 or 3 during the years ended December 31, 2015 or December 31, 2014. Fair Value Measurements on a Nonrecurring Basis In addition to items that are measured at fair value on a recurring basis, the Company also measures assets held for sale at the lower of its carrying amount or fair value on a nonrecurring basis. The Company recognized an impairment expense and other losses relating to assets held for sale during the years ended December 31, 2015, 2014, and 2013, of $0, $0, and less than $0.1 million, respectively, based on recent market sales data for similar equipment less the related costs to sell and recent purchase offers, which are Level 3 inputs. There are no assets held for sale as of December 31, 2015.

Convertible Preferred Stock and

Convertible Preferred Stock and Stockholders' Equity	12 Months Ended
	Dec. 31, 2015
Convertible Preferred Stock and Stockholders' Equity
Convertible Preferred Stock and Stockholders' Equity	10. Convertible Preferred Stock and Stockholders' Equity In January 2012, the Company issued $6.0 million of 10% convertible promissory notes to certain existing investors for cash. In September and November 2012, the Company issued an aggregate of $9.7 million of 10% convertible promissory notes to certain existing investors for cash. In February 2013, these convertible promissory notes, with an outstanding principal of $15.7 million and accrued interest of $0.9 million, were amended and then converted into 16,623,092 shares of Series A preferred stock, in accordance with their terms and at their conversion price of $1.00 per share, and following such conversion, the notes were cancelled. Each share of Series A preferred stock issued in the financing was convertible into 0.143 shares of common stock upon the closing of the Company's IPO. On April 19, 2013, the Company issued and sold an aggregate of 17,000,000 shares of Series A preferred stock at a price of $1.00 per share for proceeds of $16.9 million, net of issuance costs of $0.1 million, to funds affiliated with Longitude Capital and certain existing investors. Each share of Series A preferred stock issued in the financing was convertible into 0.143 shares of common stock upon the closing of the Company's IPO. On May 29, 2013, the Company entered into a stock purchase agreement with Pfizer Inc. and issued and sold 6,750,000 shares of Series A-1 preferred stock at a price of $1.1560 per share. The purchase price was paid through the cancellation of all outstanding indebtedness, including accrued interest, under the Pfizer convertible promissory note, which had an aggregate balance, including accrued interest, of $7.8 million as of May 29, 2013. Each share of Series A-1 preferred stock issued in the agreement was convertible into 0.143 shares of common stock upon the closing of the Company's IPO. Upon the closing of the Company's IPO on July 1, 2013, all of the outstanding shares of convertible preferred stock were converted into 9,210,999 shares of common stock. As of December 31, 2015, the Company did not have any convertible preferred stock issued or outstanding.

Stock Compensation

Stock Compensation	12 Months Ended
	Dec. 31, 2015
Stock Compensation
Stock Compensation	11. Stock Compensation 2013 Stock Option and Incentive Plan In May 2015, the Company's stockholders approved the amended and restated 2013 Stock Option and Incentive Plan (as amended, the "2013 Plan") which, among other things, increased the number of shares of common stock reserved for issuance thereunder. The number of shares of common stock available for awards under the 2013 Plan was increased by 923,622 shares from 2,051,378 shares to 2,975,000 shares, plus (i) shares of common stock that are forfeited, cancelled, held back upon the exercise or settlement of an award to cover the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of common stock or otherwise terminated (other than by exercise) under the 2013 Plan and the Company's 2008 Incentive Stock Option and Restricted Stock Plan are added back to the shares of common stock available for issuance under the 2013 Plan, and (ii) on January 1, 2016, and each January 1, thereafter, the number of shares of common stock reserved and available for issuance under the 2013 Plan will be cumulatively increased by 2.5% of the number of shares of common stock outstanding on the immediately preceding December 31, or such lesser number of shares of common stock determined by the compensation committee. The 2013 Plan provides for the granting of stock options, stock appreciation rights, restricted stock awards, restricted stock units ("RSUs"), unrestricted stock awards, cash-based awards, performance share awards and dividend equivalent rights. The Company incurs stock-based compensation expense related to stock options and RSUs. The fair value of RSUs is determined by the closing market price of the Company's common stock on the date of grant. The fair value of stock options is calculated using a Black-Scholes option-pricing model. The Company accounts for stock-based compensation in accordance with the provisions of ASC 718, Compensation—Stock Compensation. Accordingly, compensation costs related to equity instruments granted are recognized over the requisite service periods of the awards on a straight-line basis at the grant-date fair value, taking into account estimated forfeitures. 2008 Stock Option and Restricted Stock Plan In April 2008, the Company adopted the 2008 Plan, administered by the Board of Directors or a committee appointed by the Board of Directors. The 2008 Plan provides for the granting of stock options and restricted stock to employees and nonemployees of the Company. Options granted under the 2008 Plan may either be incentive stock options, restricted stock awards or nonqualified stock options. Stock options and restricted stock grants may be granted to employees, directors and consultants. Stock awards under the 2008 Plan may be granted for up to ten years from the adoption of the 2008 Plan at prices no less than 100 percent of the fair value of the shares on the date of the grant as determined by (i) the closing price of the Company's common stock on any national exchange, (ii) the National Association of Securities Dealers Inc. Automated Quotation System ("NASDAQ"), if so authorized for quotation as a NASDAQ security, or (iii) by reasonable application of a reasonable valuation method. The valuation methods utilized by the Company are consistent with the AICPA Technical Practice Aid. Under the 2013 Plan and the 2008 Plan the vesting of options granted or restricted awards given will be determined individually with each option grant. Generally, 25 percent of the granted amount will vest upon the first anniversary of the option grant with the remainder vesting ratably on the first day of each calendar quarter for the following three years. Stock options have a 10 year life and expire if not exercised within that period, or if not exercised within 90 days of cessation of providing service to the Company. During the year ended December 31, 2015, the Company granted 25,000 RSUs to employees with a fair value for each outstanding RSU of $57.54. No RSUs were vested as of December 31, 2015. During the year ended December 31, 2015, equity compensation cost related to RSUs was $0.1 million. As of December 31, 2015, there was approximately $1.2 million of unrecognized compensation cost related to unvested RSUs, adjusted for forfeitures, which will be recognized over a weighted-average period of approximately 3.5 years. The following table summarizes the activity relating to the Company's options to purchase common stock for the year ended December 31, 2015: Number of Options Weighted-Average Exercise Price Per Share Weighted-Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2014 $ $ Granted $ Forfeited or expired (vested and unvested) ) $ Exercised ) $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2015 $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The following table summarizes information about the Company's stock option plan as of December 31, 2015: Number of Options Weighted-Average Exercise Price Per Share Weighted-Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Vested and expected to vest at December 31, 2015 $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Exercisable at December 31, 2015 $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The total intrinsic value of stock options exercised during the years ended December 31, 2015, 2014 and 2013, was $7.4 million, $1.4 million, and $0.2 million, respectively. The following table shows the weighted-average assumptions used to compute the stock-based compensation costs for the stock options granted to employees and non-employees during the period from December 31, 2015, to December 31, 2013, using the Black-Scholes option-pricing model: Year ended December 31, 2015 2014 2013 Risk-free interest rate % % % Dividend yield — — — Weighted-average expected life of options (years) Volatility % % % The risk-free interest rate assumption was based on the United States Treasury's rates for U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term of the award being valued. The assumed dividend yield was based on the Company's expectation of not paying dividends in the foreseeable future. The weighted-average expected life of the options was calculated using the simplified method as prescribed by the Securities and Exchange Commission Staff Accounting Bulletin No. 107 ("SAB No. 107"). This decision was based on the lack of relevant historical data due to the Company's limited historical experience. In addition, due to the Company's limited historical data, the estimated volatility also reflects the application of SAB No. 107, incorporating the historical volatility of comparable companies whose share prices are publicly available. The weighted-average grant-date fair values of stock options granted during the years ended December 31, 2015, 2014 and 2013, were $38.44, $10.15 and $7.14, respectively. During the years ended December 31, 2015, 2014 and 2013, the Company recognized stock-based compensation expense related to stock options of $12.6 million, $3.7 million and $1.2 million, respectively. As of December 31, 2015, there was approximately $38.5 million of unrecognized compensation cost related to unvested options, adjusted for forfeitures, which will be recognized over a weighted-average period of approximately 3.0 years.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2015
Employee Benefit Plan
Employee Benefit Plan	12. Employee Benefit Plan During 2008, the Company adopted the Esperion Therapeutics, Inc. 401(k) Plan (the "401(k) Plan"), which qualifies as a deferred salary arrangement under Section 401(k) of the Internal Revenue Code. Under the 401(k) Plan, participating employees may defer a portion of their pretax earnings. The Company may, at its sole discretion, contribute for the benefit of eligible employees. Company contributions to the 401(k) Plan during the years ended December 31, 2015, 2014 and 2013, were $0.1 million, $0 and $0, respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2015
Income Taxes
Income Taxes	13. Income Taxes There was no provision for income taxes for the years ended December 31, 2015, 2014 and 2013, because the Company has incurred operating losses since inception. At December 31, 2015, the Company concluded that it is not more likely than not that the Company will realize the benefit of its deferred tax assets due to its history of losses. Accordingly, a full valuation allowance has been applied against the net deferred tax assets. As of December 31, 2015, 2014 and 2013, the Company had deferred tax assets, before valuation allowance, of approximately $50.6 million, $34.2 million and $22.8 million, respectively. Realization of the deferred assets is dependent upon future taxable income, if any, the amount and timing of which are uncertain. Accordingly, the net deferred tax assets have been fully offset by a valuation allowance. As of December 31, 2015, 2014 and 2013, the Company had federal net operating loss carryforwards of approximately $137.4 million, $95.1 million and $62.3 million, respectively. The federal net operating loss carryforwards will expire at various dates beginning in 2028, if not utilized. As of December 31, 2015, 2014 and 2013, the Company had state net operating loss carryforwards of approximately $15.4 million, $16.6 million and $33.1 million, respectively. The state net operating loss carryforwards will expire at various dates beginning in 2022, if not utilized. The Company has $4.5 million of NOLs related to excess tax benefits generated upon the settlement of stock awards that increased net operating loss. The Company cannot record the benefit of these losses in the financial statements until the losses are utilized to reduce its income taxes payable at which time it will recognize the tax benefit in equity. A reconciliation of the U.S. statutory income tax rate to the Company's effective tax rate is as follows: December 31, 2015 2014 2013 Federal income tax (benefit) at statutory rate )% )% )% Change in tax rate % % % Permanent items % % % Other % % % Change in valuation allowance % % % ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Effective income tax rate % % % ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ If the Company experiences a greater than 50 percentage point aggregate change in ownership of certain significant stockholders over a three-year period, a Section 382 ownership change could be deemed to have occurred. If a section 382 change occurs, the Company's future utilization of the net operating loss carryforwards and credits as of the ownership change will be subject to an annual limitation under Section 382 of the Internal Revenue Code of 1986, as amended, and similar state provisions. Such an annual limitation may result in the expiration of net operating losses before utilization. The Company's reserves related to taxes are based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings or positions is more likely than not to be realized following resolution of any potential contingencies present related to the tax benefit. The Company recognized no material adjustment for unrecognized income tax benefits. Through December 31, 2015, the Company had no unrecognized tax benefits or related interest and penalties accrued. Significant components of the Company's deferred tax assets are summarized in the table below: December 31, 2015 2014 (in thousands) Deferred tax assets: Federal and state operating loss carryforwards $ $ Equity compensation Temporary differences ​ ​ ​ ​ ​ ​ ​ ​ Total deferred tax assets Valuation allowance ) ) ​ ​ ​ ​ ​ ​ ​ ​ Net deferred tax assets $ — $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Net Loss Per Common Share

Net Loss Per Common Share	12 Months Ended
	Dec. 31, 2015
Net Loss Per Common Share
Net Loss Per Common Share	14. Net Loss Per Common Share Basic net loss per share is calculated by dividing net loss by the weighted-average number of common shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is computed by dividing net loss by the weighted-average number of common share equivalents outstanding for the period determined using the treasury-stock method. For purposes of this calculation, convertible preferred stock, convertible debt, warrants for preferred stock and stock options are considered to be common stock equivalents and are only included in the calculation of diluted net loss per share when their effect is dilutive. Interest expense for convertible debt that is dilutive is added back to net income in the calculation of diluted net loss per share. The shares outstanding at the end of the respective periods presented below, after giving effect for the 1-for-6.986 reverse stock split, were excluded from the calculation of diluted net loss per share due to their anti-dilutive effect: December 31, 2015 2014 2013 Warrants for common stock Common shares under option Unvested restricted stock ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total potential dilutive shares ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2015
Selected Quarterly Financial Data (Unaudited)
Selected Quarterly Financial Data (Unaudited)	15. Selected Quarterly Financial Data (Unaudited) The following table summarizes the unaudited quarterly financial data for the last two years: 2015 March 31 June 30 September 30 December 31 (in thousands, except share and per share data) Operating expenses: Research and development $ $ $ $ General and administrative ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total operating expenses Loss from operations: ) ) ) ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Interest expense ) ) ) ) Other income, net ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss $ ) $ ) $ ) $ ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss per common share (basic and diluted) $ ) $ ) $ ) $ ) Weighted-average shares outstanding (basic and diluted) 2014 March 31 June 30 September 30 December 31 (in thousands, except share and per share data) Operating expenses: Research and development $ $ $ $ General and administrative ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total operating expenses Loss from operations: ) ) ) ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Interest expense — ) ) ) Other income, net ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss $ ) $ ) $ ) $ ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss per common share (basic and diluted) $ ) $ ) $ ) $ ) Weighted-average shares outstanding (basic and diluted)

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2015
Subsequent Events
Subsequent Events	16. Subsequent Events On January 12, 2016, a purported stockholder of the Company filed a putative class action lawsuit in the United States District Court for the Eastern District of Michigan, against the Company and Tim Mayleben, captioned Kevin L. Dougherty v. Esperion Therapeutics, Inc., et al. (No. 16-cv-10089). The lawsuit alleges that the Company and Mr. Mayleben violated Sections 10(b) and 20(a) of the Securities Exchange Act of 1934 and SEC Rule 10b-5 by allegedly failing to disclose in an August 17, 2015, public statement that the FDA would require a cardiovascular outcomes trial before approving the Company's lead product candidate. The lawsuit seeks, among other things, compensatory damages in connection with an allegedly inflated stock price between August 18, 2015, and September 28, 2015, as well as attorneys' fees and costs. In light of, among other things, the early stage of the litigation, the Company is unable to predict the outcome of this matter and is unable to make a meaningful estimate of the amount or range of loss, if any, that could result from an unfavorable outcome.

Summary of Significant Accoun25

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
Summary of Significant Accounting Policies
Use of Estimates	Use of Estimates The preparation of financial statements in accordance with generally accepted accounting principles in the United States requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, expenses and related disclosures. Actual results could differ from those estimates. Prior to the completion of the IPO on July 1, 2013, the Company utilized significant estimates and assumptions in determining the fair value of its common stock. The Company utilized valuation methodologies in accordance with the framework of the 2004 American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation , to estimate the fair value of its common stock. Each valuation methodology includes estimates and assumptions that require the Company's judgment. These estimates and assumptions include a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector, the prices at which the Company sold shares of its preferred stock, the superior rights and preferences of securities senior to its common stock at the time and the likelihood of achieving a liquidity event, such as an IPO or sale. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock at each valuation date.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company invests its excess cash in bank deposits, money market accounts, and short-term investments. The Company considers all highly liquid investments with an original maturity of 90 days or less at the time of purchase to be cash equivalents. Cash equivalents are reported at fair value.
Investments	Investments Investments are considered to be available-for-sale and are carried at fair value. Unrealized gains and losses, if any, are reported as a separate component of stockholders' equity. The cost of investments classified as available-for-sale are adjusted for the amortization of premiums and accretion of discounts to maturity and recorded in other income, net. Realized gains and losses, if any, are determined using the specific identification method and recorded in other income, net. Investments with original maturities beyond 90 days at the date of purchase and which mature at, or less than twelve months from, the balance sheet date are classified as current. Investments with a maturity beyond twelve months from the balance sheet date are classified as long-term.
Concentration of Credit Risk	Concentration of Credit Risk Cash, cash equivalents, and marketable securities consist of financial instruments that potentially subject the Company to concentrations of credit risk. The Company has established guidelines for investment of its excess cash and believes the guidelines maintain safety and liquidity through diversification of counterparties and maturities.
Segment Information	Segment Information The Company views its operations and manages its business in one operating segment, which is the business of researching, developing and commercializing therapies for the treatment of patients with elevated LDL-C.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company's cash, cash equivalents and investments are carried at fair value. Financial instruments, including other prepaid and current assets, accounts payable and accrued liabilities are carried at cost, which approximates fair value. Debt is carried at amortized cost, which approximates fair value.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are recorded at cost, less accumulated depreciation. Depreciation is provided using the straight-line method over the estimated useful lives of the respective assets, generally three to ten years. Leasehold improvements are amortized over the lesser of the lease term or the estimated useful lives of the related assets.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition are less than its carrying amount. The impairment loss, if recognized, would be based on the excess of the carrying value of the impaired asset over its respective fair value. Excluding impairment losses recorded on assets held for sale, no other impairment losses have been recorded through December 31, 2015.
Research and Development	Research and Development Research and development expenses consist of costs incurred to further the Company's research and development activities and include salaries and related benefits, costs associated with clinical activities, nonclinical activities, regulatory activities, manufacturing activities to support clinical activities, research-related overhead expenses and fees paid to external service providers that conduct certain research and development, clinical, and manufacturing activities on behalf of the Company. Research and development costs are expensed as incurred.
Accrued Clinical Development Costs	Accrued Clinical Development Costs Outside research costs are a component of research and development expense. These expenses include fees paid to clinical research organizations and other service providers that conduct certain clinical and product development activities on behalf of the Company. Depending upon the timing of payments to the service providers, the Company recognizes prepaid expenses or accrued expenses related to these costs. These accrued or prepaid expenses are based on management's estimates of the work performed under service agreements, milestones achieved and experience with similar contracts. The Company monitors each of these factors and adjusts estimates accordingly.
Income Taxes	Income Taxes The Company utilizes the liability method of accounting for income taxes as required by ASC 740, Income Taxes. Under this method, deferred tax assets and liabilities are determined based on differences between financial reporting and the tax basis of assets and liabilities and are measured using enacted tax rates and laws that will be in effect when the differences are expected to reverse. The Company has incurred operating losses since inception. Accordingly, it is not more likely than not that the Company will realize deferred tax assets and as such, it has recorded a full valuation allowance.
Warrants	Warrants The Company accounts for its warrants issued in connection with its various financing transactions based upon the characteristics and provisions of the instrument. Warrants classified as liabilities are recorded on the Company's balance sheet at their fair value on the date of issuance and are marked-to-market on each subsequent reporting period, with the fair value changes recognized in the statement of operations. Warrants classified as additional-paid-in-capital are recorded on the Company's balance sheet at their fair value on the date of issuance. The warrants are measured using the Black-Scholes option-pricing model subsequent to the pricing of the Company's IPO and a Monte Carlo valuation model for previous periods which are based, in part, upon inputs where there is little or no market data, requiring the Company to develop its own independent assumptions. (See Note 4).
Stock-Based Compensation	Stock-Based Compensation The Company accounts for stock-based compensation in accordance with the provisions of ASC 718, Compensation—Stock Compensation. Accordingly, compensation costs related to equity instruments granted are recognized over the requisite service periods of the awards on a straight-line basis at the grant-date fair value calculated using a Black-Scholes option-pricing model. Additionally, under the provisions of ASC 718, the Company is required to include an estimate of the number of awards that will be forfeited in calculating compensation costs. Any changes to the estimated forfeiture rates are accounted for prospectively. Stock-based compensation arrangements with non-employees are recognized at the grant-date fair value and then re-measured at each reporting period. Expense is recognized during the period the related services are rendered.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In April 2015, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update 2015-03 which simplifies the presentation of debt issuance costs by requiring that debt issuance costs related to a recognized debt liability be presented on the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts, rather than as a deferred charge. The recognition and measurement guidance for debt issuance costs are not affected by the amendment. The Company early-adopted the amendment effective January 1, 2015, which resulted in a change in the balance sheet presentation of net debt; in prior period disclosures the debt issuance costs related to the Company's debt liability were presented on the balance sheet as deferred charges within "Other prepaid and current assets". Upon adoption of the amended guidance, the debt issuance costs associated with the Company's debt liability are presented on the balance sheet as a direct deduction from the carrying amount of the debt liability. Within the December 31, 2015 and 2014, balance sheets, "Long-term debt, net of discount" includes less than $0.1 million and $0.1 million, respectively, of debt issuance costs. In November 2015, the FASB issued Accounting Standards Update 2015-17 which requires that deferred tax assets and liabilities, along with any related valuation allowances, be classified as noncurrent on the balance sheet, effective for annual periods, and for interim periods within those annual periods, beginning after December 15, 2016, for public companies, with early adoption permitted. The Company early-adopted the amendment effective December 31, 2015, classifying all deferred tax assets on the balance sheet as non-current. The Company has incurred operating losses since inception and has fully reserved its net deferred tax assets. Accordingly, the adoption of this amendment has no impact on the balance sheet presentation of deferred tax assets.

The Company and Basis of Pres26

The Company and Basis of Presentation (Tables)	12 Months Ended
	Dec. 31, 2015
Initial Public Offering
Summary of the Company's capitalization upon closing of its public offering	Total common stock issued as of June 30, 2013 Conversion of Series A preferred stock into common stock upon closing of IPO Conversion of Series A-1 preferred stock into common stock upon closing of IPO Sales of common stock through IPO ​ ​ ​ ​ ​ Common stock issued as of July 1, 2013 Issuance of common stock to underwriters due to exercise of over-allotment ​ ​ ​ ​ ​ Total common stock issued as of July 11, 2013 ​ ​ ​ ​ ​

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2015
Debt
Schedule of estimated future principal payments due	Years Ending December 31, (in thousands) 2016 $ 2017 2018 ​ ​ ​ ​ ​ Total $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies
Summary of future minimum lease payments	The following table summarizes the Company's future minimum lease payments as of December 31, 2015: Total Less than 1 Year 1 - 3 Years 3 - 5 Years More than 5 Years (in thousands) Operating lease $ $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total $ $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2015
Property and Equipment
Schedule of property and equipment	December 31, 2015 2014 (in thousands) Lab equipment $ $ Computer equipment Software Furniture and fixtures Leasehold improvements Assets in Progress — ​ ​ ​ ​ ​ ​ ​ ​ Subtotal Less accumulated depreciation and amortization ​ ​ ​ ​ ​ ​ ​ ​ Property and equipment, net $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Other Accrued Liabilities (Tabl

Other Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2015
Other Accrued Liabilities
Schedule of other accrued liabilities	December 31, 2015 2014 (in thousands) Accrued compensation $ $ Accrued professional fees Accrued franchise and property taxes Accrued interest Accrued other ​ ​ ​ ​ ​ ​ ​ ​ Total other accrued liabilities $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2015
Investments
Summary of the Company's cash equivalents and investments	December 31, 2015 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Cash equivalents: Money market funds $ $ — $ — $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Short-term investments: Certificates of deposit — ) U.S treasury notes — ) U.S. government agency securities — ) Long-term investments: Certificates of deposit — ) U.S. treasury notes — ) U.S. government agency securities — ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total $ $ — $ ) $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2014 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Cash equivalents: Money market funds $ $ — $ — $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Short-term investments: Certificates of deposit — — U.S treasury notes — U.S. government agency securities — ) Long-term investments: Certificates of deposit — — U.S. treasury notes — ) U.S. government agency securities — ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total $ $ $ ) $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2015
Fair Value Measurements
Schedule of the Company's financial assets and liabilities that have been measured at fair value on a recurring basis	Description Total Level 1 Level 2 Level 3 (in thousands) December 31, 2015 Assets: Money market funds $ $ $ — $ — Available-for-sale securities: Certificates of deposit — — U.S. treasury notes — — U.S. government agency securities — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total assets at fair value $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2014 Assets: Money market funds $ $ $ — $ — Available-for-sale securities: Certificates of deposit — — U.S. treasury notes — — U.S. government agency securities — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total assets at fair value $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Stock Compensation (Tables)

Stock Compensation (Tables)	12 Months Ended
	Dec. 31, 2015
Stock Compensation
Summary of activity relating to the Company's options to purchase common stock	Number of Options Weighted-Average Exercise Price Per Share Weighted-Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2014 $ $ Granted $ Forfeited or expired (vested and unvested) ) $ Exercised ) $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2015 $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Summary of information about the stock option plan	Number of Options Weighted-Average Exercise Price Per Share Weighted-Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Vested and expected to vest at December 31, 2015 $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Exercisable at December 31, 2015 $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of weighted-average assumptions used to compute the stock-based compensation costs for the stock options granted to employees and non-employees	Year ended December 31, 2015 2014 2013 Risk-free interest rate % % % Dividend yield — — — Weighted-average expected life of options (years) Volatility % % %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
Income Taxes
Schedule of reconciliation of the U.S. statutory income tax rate to the Company's effective tax rate	December 31, 2015 2014 2013 Federal income tax (benefit) at statutory rate )% )% )% Change in tax rate % % % Permanent items % % % Other % % % Change in valuation allowance % % % ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Effective income tax rate % % % ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of significant components of the Company's deferred tax assets	December 31, 2015 2014 (in thousands) Deferred tax assets: Federal and state operating loss carryforwards $ $ Equity compensation Temporary differences ​ ​ ​ ​ ​ ​ ​ ​ Total deferred tax assets Valuation allowance ) ) ​ ​ ​ ​ ​ ​ ​ ​ Net deferred tax assets $ — $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Net Loss Per Common Share (Tabl

Net Loss Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2015
Net Loss Per Common Share
Schedule of anti-dilutive securities excluded from the calculation of diluted net loss per share	December 31, 2015 2014 2013 Warrants for common stock Common shares under option Unvested restricted stock ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total potential dilutive shares ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Selected Quarterly Financial 36

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Selected Quarterly Financial Data (Unaudited)
Schedule of unaudited quarterly financial data	2015 March 31 June 30 September 30 December 31 (in thousands, except share and per share data) Operating expenses: Research and development $ $ $ $ General and administrative ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total operating expenses Loss from operations: ) ) ) ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Interest expense ) ) ) ) Other income, net ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss $ ) $ ) $ ) $ ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss per common share (basic and diluted) $ ) $ ) $ ) $ ) Weighted-average shares outstanding (basic and diluted) 2014 March 31 June 30 September 30 December 31 (in thousands, except share and per share data) Operating expenses: Research and development $ $ $ $ General and administrative ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total operating expenses Loss from operations: ) ) ) ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Interest expense — ) ) ) Other income, net ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss $ ) $ ) $ ) $ ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss per common share (basic and diluted) $ ) $ ) $ ) $ ) Weighted-average shares outstanding (basic and diluted)

The Company and Basis of Pres37

The Company and Basis of Presentation (Details)-10K $ / shares in Units, $ in Millions	Mar. 24, 2015USD ($)$ / sharesshares	Oct. 21, 2014USD ($)$ / sharesshares	Jul. 11, 2013USD ($)$ / sharesshares	Jul. 11, 2013$ / sharesshares	Jul. 01, 2013USD ($)$ / sharesshares	Jun. 11, 2013	Dec. 31, 2015$ / sharesshares	Dec. 31, 2013$ / sharesshares	Dec. 31, 2014$ / shares	Jun. 30, 2014shares
Reverse Stock Split
Reverse stock split ratio of outstanding common stock						0.1431	0.1431
Par value of common stock (in dollars per share) | $ / shares							$ 0.001	$ 0.001	$ 0.001
Common stock | Warrant
Initial Public Offering
Warrants exercisable into common stock (in shares)					277,690		25,445			8,230
Series A preferred stock
Initial Public Offering
Number of shares of convertible preferred stock converted into common stock								(57,598,092)
Series A-1 preferred stock
Initial Public Offering
Number of shares of convertible preferred stock converted into common stock								(6,750,000)
Initial Public Offering
Initial Public Offering
Original issue price (in dollars per share) | $ / shares	$ 100	$ 20			$ 14
Net proceeds from the IPO, including proceeds from the exercise of the underwriters' over-allotment option | $	$ 190	$ 91.6	$ 72.2
Initial Public Offering | Common stock
Initial Public Offering
Common stock sold (in shares)	2,012,500	4,887,500			5,000,000
Number of shares of convertible preferred stock converted into common stock					9,210,999
Initial Public Offering | Common stock | Additional paid-in capital
Initial Public Offering
Reclassification of convertible preferred stock warrant liability to additional paid-in capital | $					$ 2.9
Initial Public Offering | Series A preferred stock
Initial Public Offering
Number of shares of convertible preferred stock converted into common stock					8,244,781
Initial Public Offering | Series A-1 preferred stock
Initial Public Offering
Number of shares of convertible preferred stock converted into common stock					966,218
Over-allotment option
Initial Public Offering
Common stock sold (in shares)			750,000	750,000
Original issue price (in dollars per share) | $ / shares			$ 14	$ 14
Over-allotment option | Common stock
Initial Public Offering
Common stock sold (in shares)	262,500	637,500

The Company and Basis of Pres38

The Company and Basis of Presentation (Details 2) - shares	Mar. 24, 2015	Oct. 21, 2014	Jul. 11, 2013	Jul. 11, 2013	Jul. 01, 2013	Dec. 31, 2015	Dec. 31, 2013
Movement in Company's capitalization
Balance (in shares)				396,414	396,414	20,352,876
Balance (in shares)			15,357,413	15,357,413	14,607,413	22,518,907
Series A preferred stock
Movement in Company's capitalization
Conversion of preferred stock into common stock upon closing of IPO (in shares)							(57,598,092)
Series A-1 preferred stock
Movement in Company's capitalization
Conversion of preferred stock into common stock upon closing of IPO (in shares)							(6,750,000)
Initial Public Offering | Series A preferred stock
Movement in Company's capitalization
Conversion of preferred stock into common stock upon closing of IPO (in shares)					8,244,781
Initial Public Offering | Series A-1 preferred stock
Movement in Company's capitalization
Conversion of preferred stock into common stock upon closing of IPO (in shares)					966,218
Initial Public Offering | Common stock
Movement in Company's capitalization
Conversion of preferred stock into common stock upon closing of IPO (in shares)					9,210,999
Common stock sold (in shares)	2,012,500	4,887,500			5,000,000
Over-allotment option
Movement in Company's capitalization
Common stock sold (in shares)			750,000	750,000
Over-allotment option | Common stock
Movement in Company's capitalization
Common stock sold (in shares)	262,500	637,500

Summary of Significant Accoun39

Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2015USD ($)item	Dec. 31, 2014USD ($)
Segment Information
Number of operating segments | item	1
Debt
Debt issuance costs		$ 100,000
Impairment of Long-Lived Assets
Other impairment losses	$ 0
Maximum
Debt
Debt issuance costs	$ 100,000
Property and equipment | Minimum
Property and equipment, net
Estimated useful lives	3 years
Property and equipment | Maximum
Property and equipment, net
Estimated useful lives	10 years

Debt (Details)

Debt (Details) - USD ($)

Jul. 01, 2013

Feb. 28, 2013

Jun. 30, 2014

Feb. 28, 2013

Dec. 31, 2012

Dec. 31, 2015

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

May. 29, 2013

Nov. 30, 2012

Jan. 31, 2012

Apr. 28, 2008

Debt

Debt issuance costs

$ 100,000

Estimated future principal payments due under the Credit Facility

Payments of long term debt

$ 638,000

Series A preferred stock

Estimated future principal payments due under the Credit Facility

Conversion of convertible promissory notes into Series A preferred stock (in shares)

16,623,092

Preferred stock, shares issued

23,975,000

23,975,000

Number of shares of preferred stock converted into shares of common stock

(57,598,092)

Series A-1 preferred stock

Estimated future principal payments due under the Credit Facility

Conversion of convertible promissory notes into Series A preferred stock (in shares)

6,750,000

Number of shares of preferred stock converted into shares of common stock

(6,750,000)

Warrant

Debt

Exercise price (in dollars per share)

$ 7.25

Series A preferred stock warrant

Debt

Warrants exercisable into stock (in shares)

1,940,000

Exercise price (in dollars per share)

$ 1

10% Convertible promissory notes | Series A preferred stock

Estimated future principal payments due under the Credit Facility

Aggregate principal amount

$ 15,700,000

$ 15,700,000

Accrued interest

$ 900,000

$ 900,000

Conversion of convertible promissory notes into Series A preferred stock (in shares)

16,623,092

16,623,092

Conversion price (in dollars per share)

$ 1

$ 1

Short term convertible notes issued January 2012

Debt

Interest rate (as a percent)

10.00%

Estimated future principal payments due under the Credit Facility

Aggregate principal amount

$ 6,000,000

Short term convertible notes issued September and November 2012

Debt

Interest rate (as a percent)

10.00%

Estimated future principal payments due under the Credit Facility

Aggregate principal amount

$ 9,700,000

Short term borrowings with related parties, net of debt discount

$ 9,400,000

Short term convertible notes issued September and November 2012 | Series A preferred stock warrant | Series A preferred stock | Maximum

Estimated future principal payments due under the Credit Facility

Aggregate purchase price of warrants

$ 100,000

Short term convertible notes issued September 2012

Estimated future principal payments due under the Credit Facility

Fair value of beneficial conversion feature

$ 300,000

Short term convertible notes issued September 2012 | Interest Expense.

Estimated future principal payments due under the Credit Facility

Amortization of debt discount and beneficial conversion

$ 200,000

$ 100,000

Pfizer note

Debt

Interest rate (as a percent)

8.931%

Estimated future principal payments due under the Credit Facility

Aggregate principal amount

$ 5,000,000

Outstanding principal amount

$ 7,800,000

Accrued interest during period

$ 300,000

$ 600,000

Pfizer note | Series A-1 preferred stock

Estimated future principal payments due under the Credit Facility

Preferred stock, shares issued

6,750,000

Number of shares of preferred stock converted into shares of common stock

966,218

Credit Facility

Debt

Maximum borrowing capacity

$ 20,000,000

Percentage of final payment equal to amount drawn under the Credit Facility

8.00%

Estimated future principal payments due under the Credit Facility

2,016

$ 1,604,000

2,017

1,709,000

2,018

1,049,000

Total

4,362,000

Interest expense

500,000

$ 300,000

Interest payments

300,000

$ 100,000

Term A Loan

Debt

Maximum borrowing capacity

$ 5,000,000

Period of consecutive equal monthly installments of principal and interest

36 months

Interest rate (as a percent)

6.40%

Term A Loan | Warrant

Debt

Warrants exercisable into stock (in shares)

8,230

Exercise price (in dollars per share)

$ 15.19

Unamortized discount

$ 100,000

Term A Loan | Secured promissory notes

Debt

Proceeds from issuance of debt

$ 5,000,000

Term B Loan

Debt

Maximum borrowing capacity

$ 15,000,000

Remaining borrowing capacity

$ 15,000,000

Warrants (Details)

Warrants (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2013	Jun. 30, 2014	Jul. 01, 2013	Nov. 30, 2012	Sep. 30, 2012
Warrants
(Loss)/gain recognized for change in the fair value of the warrant liability		$ (2,587)
Warrant
Warrants
Exercise price (in dollars per share)	$ 7.25
Warrants exercised	29,330
Number of shares of common stock to be purchased against outstanding warrants	256,590
Warrant | Total | Short term convertible notes issued September 2012
Warrants
Estimated fair value						$ 100
Warrant | Total | Short term convertible notes issued November 2012
Warrants
Estimated fair value					$ 200
Series A preferred stock warrant
Warrants
Warrants exercisable into stock (in shares)				1,940,000
Exercise price (in dollars per share)				$ 1
Number of shares of common stock to be purchased against outstanding warrants	248,360
Series A preferred stock warrant | Total | Short term convertible notes issued September and November 2012
Warrants
Estimated fair value					$ 300
Common stock
Warrants
Exercise price (in dollars per share)			$ 15.19
Common stock | Warrant
Warrants
Warrants exercisable into stock (in shares)	25,445		8,230	277,690
Fair value of warrants			$ 100
Share price of common stock (in dollars per share)				$ 6.99

Commitments and Contingencies42

Commitments and Contingencies (Details)	1 Months Ended			12 Months Ended
	Aug. 31, 2015	May. 31, 2014USD ($)ft²item	Feb. 28, 2014	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Commitments and Contingencies
Term period of lease	49 months		63 months
Operating lease period for monthly rent increasing in fixed monthly rent	12 months		12 months
Operating lease period after which monthly rent increase			3 months
Operating leases
Total				$ 705,000
Less than 1 Year				185,000
1-3 Years				387,000
3-5 Years				133,000
Total
Total				705,000
Less than 1 Year				185,000
1-3 Years				387,000
3-5 Years				133,000
Rent expense				$ 200,000	$ 300,000	$ 300,000
Third amendment to the operating lease agreement for laboratory facility in Plymouth, Michigan
Commitments and contingencies
Adjusted rentable laboratory space (in square feet) | ft²		3,045
Adjusted proportionate share of landlord's expenses and taxes (as a percent)		7.40%
Number of terms which Company may exercise to renew lease | item		1
Lease renewal term		3 years
Annual base rent under new lease agreement		$ 37,000

Property and Equipment (Detail)

Property and Equipment (Detail) -10K - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property and Equipment
Property and equipment, gross	$ 1,162	$ 1,183
Less accumulated depreciation and amortization	355	403
Property and equipment, net	807	780
Depreciation expense	236	160	$ 71
Lab equipment
Property and Equipment
Property and equipment, gross	232	519
Computer equipment
Property and Equipment
Property and equipment, gross	130	110
Software
Property and Equipment
Property and equipment, gross	73	74
Furniture and fixtures
Property and Equipment
Property and equipment, gross	568	320
Leasehold improvements
Property and Equipment
Property and equipment, gross	$ 159	158
Assets in Progress
Property and Equipment
Property and equipment, gross		$ 2

Other Accrued Liabilities (Deta

Other Accrued Liabilities (Detail)-10k - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Other Accrued Liabilities
Accrued compensation	$ 571	$ 313
Accrued professional fees	140	167
Accrued franchise and property taxes	37	107
Accrued interest	250	104
Accrued other	125	144
Total other accrued liabilities	$ 1,123	$ 835

Investments (Details)

Investments (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash equivalents and investments
Gross Unrealized Gains					$ 4,000					$ 4,000
Gross Unrealized Losses	$ (482,000)				(63,000)				$ (482,000)	(63,000)
Total, Amortized Cost	247,483,000				56,960,000				247,483,000	56,960,000
Total, Estimated Fair Value	247,001,000				56,901,000				247,001,000	56,901,000
Amortization of premiums and discounts on investments									(647,000)	(202,000)	$ (47,000)
Other income (expense), net	233,000	$ 248,000	$ 202,000	$ 93,000	57,000	$ 29,000	$ 17,000	$ 16,000	776,000	119,000	194,000
Interest Income
Cash equivalents and investments
Interest income on available for sale									1,500,000	400,000	100,000
Other Expense
Cash equivalents and investments
Amortization of premiums and discounts on investments									600,000	200,000
Other Expense | Maximum
Cash equivalents and investments
Amortization of premiums and discounts on investments											$ 100,000
Reclassification out of accumulated other comprehensive income (loss)
Cash equivalents and investments
Other income (expense), net									0
Short-term investments. | U.S. treasury notes
Cash equivalents and investments
Investments, Amortized Cost	12,620,000				9,020,000				12,620,000	9,020,000
Gross Unrealized Gains					4,000					4,000
Gross Unrealized Losses	(14,000)								(14,000)
Investments, Estimated Fair Value	12,606,000				9,024,000				12,606,000	9,024,000
Short-term investments. | U.S. government agency securities
Cash equivalents and investments
Investments, Amortized Cost	102,683,000				8,853,000				102,683,000	8,853,000
Gross Unrealized Losses	(110,000)				(8,000)				(110,000)	(8,000)
Investments, Estimated Fair Value	102,573,000				8,845,000				102,573,000	8,845,000
Long-term investments | U.S. treasury notes
Cash equivalents and investments
Investments, Amortized Cost	22,553,000				2,494,000				22,553,000	2,494,000
Gross Unrealized Losses	(105,000)				(5,000)				(105,000)	(5,000)
Investments, Estimated Fair Value	22,448,000				2,489,000				22,448,000	2,489,000
Long-term investments | U.S. government agency securities
Cash equivalents and investments
Investments, Amortized Cost	45,793,000				31,454,000				45,793,000	31,454,000
Gross Unrealized Losses	(183,000)				(50,000)				(183,000)	(50,000)
Investments, Estimated Fair Value	45,610,000				31,404,000				45,610,000	31,404,000
Money market funds
Cash equivalents and investments
Cash equivalents, Estimated Fair Value	31,761,000				357,000				31,761,000	357,000
Certificates of deposit | Short-term investments.
Cash equivalents and investments
Investments, Amortized Cost	19,774,000				2,934,000				19,774,000	2,934,000
Gross Unrealized Losses	(28,000)								(28,000)
Investments, Estimated Fair Value	19,746,000				2,934,000				19,746,000	2,934,000
Certificates of deposit | Long-term investments
Cash equivalents and investments
Investments, Amortized Cost	12,299,000				1,848,000				12,299,000	1,848,000
Gross Unrealized Losses	(42,000)								(42,000)
Investments, Estimated Fair Value	$ 12,257,000				$ 1,848,000				$ 12,257,000	$ 1,848,000

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($)	24 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Assets:
Total, Estimated Fair Value	$ 247,001,000	$ 56,901,000
Transfer of assets between levels	0
Transfer of liabilities between levels	0
U.S. treasury notes
Assets:
Available for sale securities		11,513,000
U.S. government agency securities
Assets:
Available for sale securities		40,249,000
Certificates of deposit
Assets:
Available for sale securities		4,782,000
Level 1 | U.S. treasury notes
Assets:
Available for sale securities		11,513,000
Level 1 | Certificates of deposit
Assets:
Available for sale securities		4,782,000
Level 2
Assets:
Total, Estimated Fair Value		40,249,000
Level 2 | U.S. government agency securities
Assets:
Available for sale securities		40,249,000
Recurring fair value measurement
Assets:
Money market fund	31,761,000	357,000
Total, Estimated Fair Value	247,001,000	56,901,000
Recurring fair value measurement | U.S. treasury notes
Assets:
Available for sale securities	35,054,000
Recurring fair value measurement | U.S. government agency securities
Assets:
Available for sale securities	148,183,000
Recurring fair value measurement | Certificates of deposit
Assets:
Available for sale securities	32,003,000
Recurring fair value measurement | Level 1
Assets:
Money market fund	31,761,000	357,000
Total, Estimated Fair Value	98,818,000	$ 16,652,000
Recurring fair value measurement | Level 1 | U.S. treasury notes
Assets:
Available for sale securities	35,054,000
Recurring fair value measurement | Level 1 | Certificates of deposit
Assets:
Available for sale securities	32,003,000
Recurring fair value measurement | Level 2
Assets:
Total, Estimated Fair Value	148,183,000
Recurring fair value measurement | Level 2 | U.S. government agency securities
Assets:
Available for sale securities	$ 148,183,000

Fair Value Measurements (Deta47

Fair Value Measurements (Details 2) - Nonrecurring fair value measurement - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value Measurements on a Nonrecurring Basis
Impairment loss related to the assets held for sale	$ 0	$ 0
Maximum
Fair Value Measurements on a Nonrecurring Basis
Impairment loss related to the assets held for sale			$ 0.1

Convertible Preferred Stock a48

Convertible Preferred Stock and Stockholders' Equity (Details) $ / shares in Units, $ in Thousands	Jul. 01, 2013$ / sharesshares	Apr. 19, 2013USD ($)$ / sharesshares	Feb. 28, 2013USD ($)$ / sharesshares	Feb. 28, 2013USD ($)$ / sharesshares	Dec. 31, 2013USD ($)shares	May. 29, 2013USD ($)$ / sharesshares	Nov. 30, 2012USD ($)	Jan. 31, 2012USD ($)	Apr. 28, 2008USD ($)
Convertible preferred stock and stockholders' deficit
Proceeds from issuance of preferred stock, net of issuance costs | $					$ 16,824
Pfizer note
Convertible preferred stock and stockholders' deficit
Interest rate (as a percent)									8.931%
Aggregate principal amount | $									$ 5,000
Outstanding principal amount | $						$ 7,800
Short term convertible notes issued January 2012
Convertible preferred stock and stockholders' deficit
Interest rate (as a percent)								10.00%
Aggregate principal amount | $								$ 6,000
Short term convertible notes issued September and November 2012
Convertible preferred stock and stockholders' deficit
Interest rate (as a percent)							10.00%
Aggregate principal amount | $							$ 9,700
Common stock | Warrant
Convertible preferred stock and stockholders' deficit
Share price of preferred stock (in dollars per share) | $ / shares	$ 6.99
Series A preferred stock
Convertible preferred stock and stockholders' deficit
Conversion of convertible promissory notes into Series A preferred stock (in shares)					16,623,092
Preferred stock, shares authorized		17,000,000
Share price of preferred stock (in dollars per share) | $ / shares		$ 1
Proceeds from issuance of preferred stock, net of issuance costs | $		$ 16,900
Stock issuance costs | $		$ 100			$ (120)
Conversion rate		0.143
Number of shares of convertible preferred stock converted into common stock					(57,598,092)
Series A preferred stock | 10% Convertible promissory notes
Convertible preferred stock and stockholders' deficit
Aggregate principal amount | $			$ 15,700	$ 15,700
Accrued interest | $			$ 900	$ 900
Conversion of convertible promissory notes into Series A preferred stock (in shares)			16,623,092	16,623,092
Conversion price (in dollars per share) | $ / shares			$ 1	$ 1
Conversion rate			0.143	0.143
Series A-1 preferred stock
Convertible preferred stock and stockholders' deficit
Conversion of convertible promissory notes into Series A preferred stock (in shares)					6,750,000
Stock issuance costs | $					$ (53)
Number of shares of convertible preferred stock converted into common stock					(6,750,000)
Series A-1 preferred stock | Pfizer note
Convertible preferred stock and stockholders' deficit
Preferred stock, shares authorized						6,750,000
Share price of preferred stock (in dollars per share) | $ / shares						$ 1.1560
Conversion rate						0.143
Number of shares of convertible preferred stock converted into common stock	966,218
Initial Public Offering | Common stock
Convertible preferred stock and stockholders' deficit
Number of shares of convertible preferred stock converted into common stock	9,210,999
Initial Public Offering | Series A preferred stock
Convertible preferred stock and stockholders' deficit
Number of shares of convertible preferred stock converted into common stock	8,244,781
Initial Public Offering | Series A-1 preferred stock
Convertible preferred stock and stockholders' deficit
Number of shares of convertible preferred stock converted into common stock	966,218

Stock Compensation (Details)

Stock Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	May. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stock compensation
Stock-based compensation expense		$ 12,600	$ 3,700	$ 1,200
Weighted-average period over which remaining unrecognized compensation cost will be recognized		3 years
Number of Options
Outstanding at the beginning of period (in shares)		1,729,586
Granted (in shares)		1,190,100
Forfeited or expired (vested and unvested) (in shares)		(128,738)
Exercised (in shares)		(128,086)
Outstanding at the end of the period (in shares)		2,662,862	1,729,586
Weighted-Average Exercise Price Per Share
Outstanding at the beginning of period (in dollars per share)		$ 11.44
Granted (in dollars per share)		60.33
Forfeited or expired (vested and unvested) (in dollars per share)		31.80
Exercised (in dollars per share)		9.19
Outstanding at the end of the period (in dollars per share)		$ 32.42	$ 11.44
Weighted-Average Remaining Contractual Term (Years)
Outstanding at the end of the period		8 years 4 months 10 days	8 years 5 months 5 days
Aggregate Intrinsic Value
Outstanding at the end of the period (in dollars)		$ 16,433	$ 50,155
Information about the stock option plan
Number of Options, vested and expected to vest (in shares)		2,604,616
Number of Options, exercisable (in shares)		1,074,320
Weighted-Average Price Per Share, vested and expected to vest (in dollars per share)		$ 32.10
Weighted-Average Price Per Share, exercisable (in dollars per share)		$ 13.06
Weighted-Average Remaining Contractual Term, vested and expected to vest		8 years 4 months 6 days
Weighted-Average Remaining Contractual Term, exercisable		7 years 6 months 4 days
Aggregate Intrinsic Value, vested and expected to vest (in dollars)		$ 16,304
Aggregate Intrinsic Value, exercisable (in dollars)		12,760
Total intrinsic value of stock options exercised		7,400	$ 1,400	$ 200
Additional disclosures
Unrecognized compensation cost (in dollars)		$ 38,500
Weighted-average grant-date fair value (in dollars per share)		$ 38.44	$ 10.15	$ 7.14
Assumptions used to compute the share-based compensation costs for stock options granted to employees and non-employees
Risk-free interest rate (as a percent)		1.65%	1.81%	1.45%
Weighted-average expected life of options (years)		6 years 1 month 10 days	6 years 3 months 26 days	6 years 3 months 4 days
Volatility (as a percent)		70.00%	75.00%	74.00%
RSUs
Stock compensation
Vested (in shares)		0
2013 Stock Option and Incentive Plan
Stock compensation
Increase in the number of shares of stock reserved and available for issuance	923,622
Percentage of increase in the number of shares reserved and available for issuance	2.50%
2013 Stock Option and Incentive Plan | RSUs
Stock compensation
Granted (in shares)		25,000
Fair value for each outstanding (in dollar per share)		$ 57.54
Stock-based compensation expense		$ 100
Unrecognized compensation cost		$ 1,200
Weighted-average period over which remaining unrecognized compensation cost will be recognized		3 years 6 months
2013 Stock Option and Incentive Plan | Minimum
Stock compensation
Shares reserved and approved for issuance	2,051,378
2013 Stock Option and Incentive Plan | Maximum
Stock compensation
Shares reserved and approved for issuance	2,975,000
2008 Incentive Stock Option and Restricted Stock Plan
Stock compensation
Expiration period		10 years
Period from cessation of employment within which options expire if not exercised		90 days
Grant period of stock awards		10 years
Purchase price expressed as a percentage of fair value of shares on the date of grant		100.00%
Vesting percentage on the first anniversary of the option grant		25.00%
Period for which remainder of grant amount will vest on the first day of each calendar quarter		3 years

Employee Benefit Plan (Details)

Employee Benefit Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee Benefit Plan
Contributions to the 401(k) Plan	$ 0.1	$ 0	$ 0

Income Taxes (Details)

Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes
Provision for income taxes	$ 0	$ 0	$ 0
Deferred tax assets, before valuation allowance	50,576,000	34,208,000	22,800,000
Net operating loss carryforwards
NOLs related to excess tax benefits	4,500,000
Federal
Net operating loss carryforwards
Net operating loss carryforwards	137,400,000	95,100,000	62,300,000
State
Net operating loss carryforwards
Net operating loss carryforwards	$ 15,400,000	$ 16,600,000	$ 33,100,000

Income Taxes (Detail 2)

Income Taxes (Detail 2) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of the U.S. statutory income tax rate to the Company's effective tax rate
Federal income tax (benefit) at statutory rate	(34.00%)	(34.00%)	(34.00%)
Change in tax rate	0.30%	2.10%	(0.00%)
Permanent items	1.30%	1.00%	4.90%
Other	(0.00%)	0.10%	(0.00%)
Change in valuation allowance	32.40%	30.80%	29.10%
Effective income tax rate	0.00%	0.00%	0.00%
Unrecognized tax benefits related to interest and penalties accrued	$ 0

Income Taxes (Detail 3)

Income Taxes (Detail 3) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Federal and state operating loss carryforwards	$ 45,848	$ 33,099
Equity compensation	4,387	971
Temporary differences	341	138
Total deferred tax assets	50,576	34,208	$ 22,800
Valuation allowance	$ (50,576)	$ (34,208)

Net Loss Per Common Share (Deta

Net Loss Per Common Share (Details)	Jun. 11, 2013	Dec. 31, 2015shares	Dec. 31, 2014shares	Dec. 31, 2013shares
Net Loss Per Common Share
Reverse stock split ratio of shares outstanding	0.1431	0.1431
Net Loss Per Common Share
Total potential dilutive shares		2,946,851	2,025,057	1,695,494
Warrant
Net Loss Per Common Share
Total potential dilutive shares		256,590	285,920	277,690
Common shares under option
Net Loss Per Common Share
Total potential dilutive shares		2,662,862	1,729,586	1,401,101
Unvested restricted stock
Net Loss Per Common Share
Total potential dilutive shares		27,399	9,551	16,703

Selected Quarterly Financial 55

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Expenses [Abstract]
Research and development	$ 7,956	$ 7,247	$ 7,209	$ 7,390	$ 6,200	$ 7,174	$ 6,528	$ 5,400	$ 29,802	$ 25,302	$ 16,014
General and administrative	5,278	5,672	5,253	4,035	3,180	2,526	2,726	2,490	20,238	10,922	6,745
Total operating expenses	13,234	12,919	12,462	11,425	9,380	9,700	9,254	7,890	50,040	36,224	22,759
Loss from operations	(13,234)	(12,919)	(12,462)	(11,425)	(9,380)	(9,700)	(9,254)	(7,890)	(50,040)	(36,224)	(22,759)
Interest expense.	(121)	(130)	(135)	(134)	(134)	(135)	(1)		(520)	(270)	(936)
Change in fair value of warrant liability											(2,587)
Other income (expense), net	233	248	202	93	57	29	17	16	776	119	194
Net loss	$ (13,122)	$ (12,801)	$ (12,395)	$ (11,466)	$ (9,457)	$ (9,806)	$ (9,238)	$ (7,874)	$ (49,784)	$ (36,375)	$ (26,088)
Net loss per common share (basic and diluted) (in dollars per share)	$ (0.58)	$ (0.57)	$ (0.55)	$ (0.56)	$ (0.49)	$ (0.64)	$ (0.60)	$ (0.51)	$ (2.26)	$ (2.22)	$ (3.31)
Weighted-average shares outstanding (basic and diluted) (in shares)	22,515,136	22,494,075	22,465,175	20,589,293	19,276,639	15,432,641	15,399,018	15,369,055	22,019,818	16,374,102	7,885,921