Flexion Therapeutics (FLXN) 10-K 24 Mar 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Mar. 20, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	FLXN
Entity Registrant Name	Flexion Therapeutics Inc
Entity Central Index Key	1419600
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		21,456,419
Entity Public Float			$89,394,000

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$103,097,522	$16,188,254
Marketable securities	48,527,156	250,047
Prepaid expenses and other current assets	502,314	181,962
Total current assets	152,126,992	16,620,263
Property and equipment, net	1,109,391	375,239
Deferred offering costs	0	1,623,540
Other assets	12,375	28,875
Restricted cash	128,000	128,000
Total assets	153,376,758	18,775,917
Current liabilities:
Accounts payable	1,584,822	1,279,874
Accrued expenses and other current liabilities	3,213,704	2,256,680
Current portion of long-term debt	2,000,000	1,500,000
Total current liabilities	6,798,526	5,036,554
Long-term debt	1,593,333	3,546,667
Other long-term liabilities	43,008	90,373
Total liabilities	8,434,867	8,673,594
Commitments and contingencies
Preferred Stock, $0.001 par value; 10,000,000 and 0 shares authorized at December 31, 2014 and 2013; respectively, and 0 shares issued and outstanding at December 31, 2014 and 2013	0	0
Stockholders' equity (deficit):
Common stock, $0.001 par value; 100,000,000 and 94,000,000 shares authorized at December 31, 2014 and 2013, respectively; 21,440,058 and 794,090 shares issued and outstanding at December 31, 2014 and 2013, respectively	21,440	794
Additional paid-in capital	238,402,514	1,458,503
Accumulated other comprehensive income	-5,240	-28
Accumulated deficit	-93,476,823	-66,163,159
Total stockholders' equity (deficit)	144,941,891	-64,703,890
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	153,376,758	18,775,917
Convertible Preferred Stock [Member]
Current liabilities:
Convertible preferred stock (Series A and B), $0.001 par value; 0 and 73,780,250 shares authorized, 0 and 72,780,250 shares issued and outstanding as of December 31, 2014 and 2013; aggregate liquidation preference of $0 and $75,043,464 at December 31, 2014 and 2013		$74,806,213

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Preferred stock, par value	$0.00	$0.00
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$0.00	$0.00
Common stock, shares authorized	100,000,000	94,000,000
Common stock, shares issued	21,440,058	794,090
Common stock, shares outstanding	21,440,058	794,090
Convertible Preferred Stock [Member]
Preferred stock, par value	$0.00	$0.00
Preferred stock, Shares authorized	0	73,780,250
Preferred Stock, Shares issued	0	72,780,250
Preferred stock, shares outstanding	0	72,780,250
Preferred stock, aggregate liquidation preference	$0	$75,043,464

Consolidated_Statements_of_Ope

Consolidated Statements of Operations and Comprehensive Loss (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenue	$0	$0	$0
Operating expenses:
Research and development	17,923,348	11,060,912	11,065,137
General and administrative	9,063,926	6,704,297	3,946,505
Total operating expenses	26,987,274	17,765,209	15,011,642
Loss from operations	-26,987,274	-17,765,209	-15,011,642
Other income (expense):
Interest income	478,715	233,999	193,900
Interest expense	-401,370	-448,889
Other income (expense), net	-403,735	-206,625	-163,877
Total other income (expense)	-326,390	-421,515	30,023
Net loss	-27,313,664	-18,186,724	-14,981,619
Net loss attributable to common stockholders	-27,313,664	-18,186,724	-14,981,619
Net loss per share attributable to common stockholders, basic and diluted	($1.97)	($23.02)	($27.58)
Weighted average common shares outstanding, basic and diluted	13,893,961	790,038	543,301
Other comprehensive (loss) income:
Unrealized (losses) gains from available-for-sale securities, net of tax of $0	-5,212	-2,478	1,647
Total other comprehensive (loss) income	-5,212	-2,478	1,647
Comprehensive loss	($27,318,876)	($18,189,202)	($14,979,972)

Consolidated_Statements_of_Ope1

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Other comprehensive (loss) income, Tax	$0	$0	$0

Consolidated_Statements_of_Cha

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (USD $)	Total	Series A and B Convertible Preferred Stock [Member]	Series A and B Convertible Preferred Stock [Member]	Series A and B Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Deficit Accumulated During the Development Stage [Member]
			Series A Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]
Balance (in shares) at Dec. 31, 2011		41,950,000			530,240
Balance at Dec. 31, 2011	($32,681,542)	$41,835,747			$530	$311,941	$803	($32,994,816)
Issuance of Stock, net of issuance costs			13,081,988	19,888,478
Issuance of Stock, net of issuance costs (in shares)			13,093,464	17,736,786
Exercise of stock options	42,110				259	41,851
Exercise of stock options (in shares)	258,981				258,982
Stock-based compensation expense	96,278					96,278
Net loss	-14,981,619							-14,981,619
Other comprehensive income (loss)	1,647						1,647
Balance (in shares) at Dec. 31, 2012		72,780,250			789,222
Balance at Dec. 31, 2012	-47,523,126	74,806,213			789	450,070	2,450	-47,976,435
Exercise of stock options	12,271				5	12,266
Exercise of stock options (in shares)	4,868				4,868
Stock-based compensation expense	996,167					996,167
Net loss	-18,186,724							-18,186,724
Other comprehensive income (loss)	-2,478						-2,478
Balance (in shares) at Dec. 31, 2013		72,780,250			794,090
Balance at Dec. 31, 2013	-64,703,890	74,806,213			794	1,458,503	-28	-66,163,159
Conversion of Series A and Series B Convertible Preferred Stock	74,806,213	-74,806,213			8,952	74,797,261
Conversion of Series A and Series B Convertible Preferred Stock (in shares)		72,780,250			8,952,057
Conversion of Series A and Series B Convertible Preferred Stock (in shares)		-72,780,250			-8,952,057
Issuance of Stock, net of issuance costs	159,322,738				11,546	159,311,192
Issuance of Stock, net of issuance costs (in shares)					11,546,000
Exercise of stock options	304,496				141	304,355
Exercise of stock options (in shares)	141,141				141,141
Employee Stock Purchase Plan	80,631				7	80,624
Employee Stock Purchase Plan (in shares)					6,770
Stock-based compensation expense	2,450,579					2,450,579
Net loss	-27,313,664							-27,313,664
Other comprehensive income (loss)	-5,212						-5,212
Balance (in shares) at Dec. 31, 2014					21,440,058
Balance at Dec. 31, 2014	$144,941,891				$21,440	$238,402,514	($5,240)	($93,476,823)

Consolidated_Statements_of_Cha1

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2012
Series A Convertible Preferred Stock [Member]
Issuance of Convertible Preferred Stock, issuance costs	$11,476
Series B Convertible Preferred Stock [Member]
Issuance of Convertible Preferred Stock, issuance costs	$111,522

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities
Net loss	($27,313,664)	($18,186,724)	($14,981,619)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	120,341	79,808	43,233
Stock-based compensation expense	2,450,579	996,167	96,278
Amortization of premium (discount) on marketable securities	366,231	151,138	137,163
Loss on disposal of property and equipment	0	14,111
Other non-cash charges	16,500	63,167
Changes in operating assets and liabilities:
Prepaid expenses and other current and long-term assets	-320,352	343,896	-178,571
Accounts payable	518,032	475,821	-176,272
Accrued expenses and other current and long-term liabilities	1,017,412	-124,328	1,079,925
Net cash used in operating activities	-23,144,921	-16,186,944	-13,979,863
Cash flows from investing activities
Purchases of property and equipment	-802,481	-405,315	-34,914
Change in restricted cash		-98,000
Purchases of marketable securities	-79,383,553	-15,015,663	-28,465,790
Redemption of marketable securities	30,735,000	31,160,000	18,967,000
Net cash provided by (used in) investing activities	-49,451,034	15,641,022	-9,533,704
Cash flows from financing activities
Proceeds from borrowings under term loan		5,000,000
Payments of debt issuance costs		-40,715	-21,161
Payments on debt	-1,500,000
Payment of public offering costs	-1,517,484	-1,072,710
Proceeds from the issuance of common stock	162,137,580
Proceeds from the exercise of stock options	304,496	12,271	42,110
Proceeds from Employee Stock Purchase Plan	80,631
Net cash provided by financing activities	159,505,223	3,898,846	32,991,415
Net increase (decrease) in cash and cash equivalents	86,909,268	3,352,924	9,477,848
Cash and cash equivalents at beginning of period	16,188,254	12,835,330	3,357,482
Cash and cash equivalents at end of period	103,097,522	16,188,254	12,835,330
Supplemental disclosures of cash flow information:
Cash paid for interest	364,889	367,778
Supplemental disclosures of non-cash financing activities:
Debt issuance costs included in accounts payable and accrued expenses			36,318
Deferred initial public offering costs included in accounts payable or accrued expenses		550,830
Public offering costs included in accounts payable or accrued expenses	224,648
Conversion of convertible preferred stock into common stock	74,806,213
Series A Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of Series A Convertible Preferred Stock, net of issuance costs			13,081,988
Series B Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of Series A Convertible Preferred Stock, net of issuance costs			$19,888,478

Nature_of_the_Business

Nature of the Business	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Nature of the Business	1	Nature of the Business
	Flexion Therapeutics, Inc. (“Flexion” or the “Company”) was incorporated under the laws of the state of Delaware on November 5, 2007. Flexion is a specialty pharmaceutical company focused on the development and commercialization of novel, injectable pain therapies. The Company is targeting anti-inflammatory and analgesic therapies for the treatment of patients with musculoskeletal conditions, beginning with osteoarthritis, a type of degenerative arthritis (“OA”) and post-operative pain. Flexion’s broad and diversified portfolio of product candidates addresses the OA pain treatment spectrum, from moderate to severe pain, and provides the Company with multiple opportunities to achieve its goal of commercializing novel, patient-focused pain therapies.
	The Company is subject to risks and uncertainties common to early-stage companies in the biopharmaceutical industry, including, but not limited to, new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations, and ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance reporting capabilities. The Company’s product candidates are all in the development stage. There can be no assurance that development efforts, including clinical trials, will be successful. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales.
	The accompanying consolidated financial statements have been prepared on a basis which assumes that the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the normal course of business. The Company has incurred recurring losses and negative cash flows from operations. As of December 31, 2014 and 2013, the Company had cash, cash equivalents and marketable securities of $151,624,678 and $16,438,301, respectively. Management believes that current cash, cash equivalents and marketable securities on hand at December 31, 2014 should be sufficient to fund operations for at least the next twelve months. The future viability of the Company is dependent on its ability to raise additional capital to finance its operations and to fund increased research and development costs in order to seek approval for commercialization of its product candidates. The Company may not be able to obtain financing on acceptable terms, or at all. The Company’s failure to raise capital as and when needed would have a negative impact on its financial condition and its ability to pursue its business strategies. This capital is necessary for the Company to perform the research and development activities required to develop the Company’s product candidates in order to generate future revenue streams.

Financing_TransactionsCommon_S

Financing Transactions/Common Stock	12 Months Ended
	Dec. 31, 2014
Common Stock [Member]
Financing Transactions/Common Stock	12	Common Stock
	Upon inception of the Company on November 5, 2007, the Company authorized 10,000,000 shares of common stock and issued 109 shares to the founders. In 2009, the Company amended its Certificate of Incorporation and authorized an additional 69,000,000 shares of common stock, $0.001 par value, bringing the total number of shares of common stock authorized to 79,000,000. In 2012, the Company amended its Certificate of Incorporation and authorized an additional 15,000,000 shares of common stock, $0.001 par value, bringing the total number of shares of common stock authorized to 94,000,000. In 2014, the Company amended its Certificate of Incorporation and authorized an additional 6,000,000 shares of common stock, $0.001 par value, bringing the total number of shares of common stock authorized to 100,000,000.
	In December 2008, the Company issued 301,350 shares of common stock for proceeds of $20,750. Additionally 110,701 shares were issued to the founders of the Company, 73,800 shares were issued to a prospective preferred stock investor, and 116,849 shares were issued to non-employee consultants. The 110,701 shares issued to the founders and 116,849 of the shares issued to non-employee consultants were subject to restrictions that were satisfied over a four-year vesting period. In 2009, the Company issued 184,501 shares of common stock in connection with a licensing agreement.
	On February 18, 2014, the Company completed an initial public offering (“IPO”) of its common stock, which resulted in the sale of 5,750,000 shares of common stock at a price to the public of $13.00 per share, including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. In connection with the closing of the IPO, all of the Company’s outstanding redeemable convertible preferred stock automatically converted to common stock as of February 18, 2014, resulting in an additional 8,952,057 shares of common stock of the Company becoming outstanding.
	On December 17, 2014, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 5,796,000 shares of the Company’s common stock at a price to the public of $17.00 per share including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares.
	Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any, subject to the preferential dividend rights of the holders of the Series A and B Preferred Stock. As of December 31, 2014, no dividends have been declared.
IPO [Member]
Financing Transactions/Common Stock	2	Financing Transactions
	On February 18, 2014, the Company completed an initial public offering (“IPO”) of its common stock, which resulted in the sale of 5,750,000 shares of common stock at a price to the public of $13.00 per share, including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. The Company received net proceeds from the IPO of $67.2 million after deducting underwriting discounts, commissions, and offering costs paid by the Company. In preparation for the IPO, the Company’s Board of Directors and stockholders approved a 1-for-8.13 reverse stock split of the Company’s common stock and a proportional adjustment to the existing conversion ratios for each series of Convertible Preferred Stock, effective January 27, 2014. All share and per share amounts in the consolidated financial statements and notes thereto have been retroactively adjusted, where necessary, to give effect to this reverse stock split. In connection with the closing of the IPO, all of the Company’s outstanding redeemable convertible preferred stock automatically converted to common stock as of February 18, 2014, resulting in an additional 8,952,057 shares of common stock of the Company becoming outstanding.
	On December 17, 2014, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 5,796,000 shares of the Company’s common stock at a price to the public of $17.00 per share including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. The Company received net proceeds from the follow-on financing of $92.2 million after deducting underwriting discounts, commissions, and offering costs paid by the Company.
	The Company’s total issued common stock as of December 31, 2014 was 21,440,058 shares.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	3	Summary of Significant Accounting Policies
	Basis of Presentation
	The accompanying consolidated financial statements have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (the “SEC”) and Generally Accepted Accounting Principles (“GAAP”) for financial information, including the accounts of the Company and its wholly owned subsidiary after elimination of all significant intercompany accounts and transactions.
	Use of Estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and judgments that may affect the reported amounts of assets and liabilities, expenses and related disclosures. The Company bases estimates and judgments on historical experience and on various other factors that it believes to be reasonable under the circumstances. The most significant estimates in these consolidated financial statements include useful lives with respect to long-lived assets, such as property and equipment and leasehold improvements, accounting for stock-based compensation, and accrued expenses, including clinical research costs. The Company’s actual results may differ from these estimates under different assumptions or conditions. The Company evaluates its estimates on an ongoing basis. Changes in estimates are reflected in reported results in the period in which they become known by the Company’s management.
	Consolidation
	The accompanying consolidated financial statements include the Company and its wholly-owned subsidiary, Flexion Securities Corporation, Inc. The Company has eliminated all intercompany transactions for the year ended December 31, 2014, the year Flexion Securities Corporation, Inc. was established.
	Cash and Cash Equivalents
	The Company considers all highly liquid investments with a maturity of three months or less at the date of purchase to be cash equivalents. The Company currently invests available cash in money market funds of a major financial institution, corporate bonds, government obligations and commercial paper.
	Marketable Securities
	Marketable securities consist of investments with original maturities greater than ninety days and less than one year from the balance sheet date. Long-term investments consist of investments with maturities of greater than one year. The Company classifies all of its investments as available-for-sale securities. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Realized gains and losses are determined on a specific identification basis and are included in other income (loss). Amortization and accretion of discounts and premiums is recorded in other income.
	Restricted Cash
	The Company purchased a $30,000 certificate of deposit to collateralize a credit card account with a commercial bank that was classified as long-term restricted cash as of December 31, 2014 and 2013. In addition, the Company posted a letter of credit to the lessor of the Company’s Burlington facility in the amount of $98,000 as a security deposit pursuant to the lease agreement in the year ended December 31, 2014. That amount was classified as long-term restricted cash at December 31, 2014 and 2013.
	Property and Equipment
	Property and equipment are stated at cost less accumulated depreciation. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives:
			Estimated
			Useful Life
			(Years)
	Computers, software and office equipment			3
	Manufacturing equipment			7
	Furniture and fixtures			5
	Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Costs of major additions and betterments are capitalized and depreciated on a straight-line basis over their useful lives. Repairs and maintenance costs are expensed as incurred. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is credited or charged to income.
	Impairment of Long-Lived Assets
	The Company reviews its 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. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends, and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows.
	Debt Issuance Costs, net
	Debt issuance costs, net represent legal costs related to the Company’s Credit and Security Agreement (Note 9). These costs are recorded as debt issuance costs on the balance sheets at the time they are incurred and are being amortized to interest expense through the scheduled final principal payment date. As of December 31, 2014 and 2013, the carrying value of debt issuance costs was $16,500 and $16,500, respectively, reported in prepaid expenses and other current assets and $12,375 and $28,875, respectively, reported in other assets. In addition, $16,500 of debt issuance costs were amortized and recognized as interest expense in the statement of operations for the year ended December 31, 2014 and 2013. The Company did not recognize any interest expense related to debt issuance costs during the year ended December 31, 2012.
	Deferred Offering Costs
	The Company capitalizes certain legal, accounting and other third-party fees that are directly associated with in-process equity financings as other assets until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ deficit as a reduction of additional paid-in capital generated as a result of the offering. If the equity financing is no longer considered probable of being consummated, the deferred offering costs would be expensed immediately as a charge to operating expenses in the consolidated statement of operations. As of December 31, 2013, the Company recorded deferred offering costs of $1,623,540 in the accompanying balance sheet in contemplation of a 2014 equity financing. The Company completed its initial public offering in February 2014 and did not record any deferred offering costs as of December 31, 2014.
	Research and Development
	Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, depreciation, clinical trial and related clinical manufacturing costs, contract services and other related costs. Research and development costs are expensed to operations as the related obligation is incurred.
	Patent Costs
	All patent-related costs incurred in connection with filing and prosecuting patent applications are recorded as general and administrative expenses as incurred, as recoverability of such expenditures is uncertain.
	Accounting for Stock-Based Compensation
	The Company measures all stock options and other stock based-awards granted to employees at the fair value at the date of grant using the Black-Scholes option-pricing model. The fair value of the awards is recognized as expense, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. The straight-line method of expense recognition is applied to all awards with service-only conditions.
	For stock-based awards granted to non-employees, compensation expense is recognized over the period during which services are rendered by such non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is re-measured using the then current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model.
	The Company classifies stock-based compensation expense in the consolidated statements of operations in the same manner in which the award recipient’s payroll costs are classified, or in the case of a non-employee, in the same manner as the award recipient’s service costs are classified.
	The Company recognizes compensation expense only for the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for service-based awards. The impact of a forfeiture rate adjustment will be recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company’s estimate, the Company may be required to record adjustments to stock-based compensation expense in future periods.
	Concentration of Credit Risk and Significant Suppliers
	Financial instruments that potentially expose the Company to concentration of credit risk consist primarily of commercial paper and bonds. The Company generally invests its cash in money market funds, government and corporate bonds, and commercial paper at one financial institution. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships.
	The Company is completely dependent on third-party manufacturers and product suppliers for preclinical research activities. In particular, the Company relies and expects to continue to rely exclusively on one manufacturer and relies on the manufacturer to purchase from third-party suppliers the materials necessary to produce its product candidates for its clinical trials. These research programs would be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients.
	Comprehensive Loss
	Comprehensive income (loss) includes net loss as well as other changes in stockholders’ deficit that result from transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive income (loss) in all periods presented was unrealized gains (losses) on available-for-sale securities.
	Income Taxes
	The Company accounts for income taxes using the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred taxes are determined based on the differences between financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.
	The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.
	Fair Value Measurements
	Fair value is defined as the exchange price that would be received to sell an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants at the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:
	•  Level 1 —		Quoted market prices in active markets for identical assets or liabilities. Level 1 consists primarily of financial instruments whose value is based on quoted market prices, such as exchange-traded instruments and listed equities.
	•  Level 2 —		Observable inputs (other than Level 1 quoted prices) such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.
	•  Level 3 —		Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.
	The Company’s financial instruments consist of cash equivalents, marketable securities, restricted cash, accounts payable and accrued expenses, and its term loan (Note 9). The estimated fair value of the Company’s financial instruments approximates their carrying values.
	Net Loss Per Share
	The Company follows the two-class method when computing net loss per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based on their respective rights to receive dividends as if all income for the period had been distributed.
	The Company’s convertible preferred shares contractually entitle the holders of such shares to participate in dividends, but do not contractually require the holders of such shares to participate in the losses of the Company. Accordingly, in periods in which the Company reports a net loss or a net loss attributable to common stockholders resulting from preferred stock dividends, net losses are not allocated to participating securities. In periods of net loss, the Company does not increase its net loss attributable to common stockholders by accreting dividends on preferred stock, as the dividends are not cumulative under the terms of the preferred stock. The Company reported a net loss attributable to common stockholders for the year ended December 31, 2014, 2013 and 2012.
	Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities, including outstanding stock options and unvested restricted common stock. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock options and unvested restricted common stock. For periods in which the Company has reported net losses, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Potential common shares will always be anti-dilutive for periods in which the Company has reported a net loss. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders for the years ended December 31, 2014, 2013 and 2012.
	Segment Data
	The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. The Company is a specialty pharmaceutical company focused on the development and commercialization of novel, injectable pain therapies. No revenue has been generated since inception, and all assets are held in the United States.
	Recently Issued and Adopted Accounting Pronouncements
	In June 2014, the Financial Accounting Standard Board, or FASB, issued amended accounting guidance for development stage entities. The amendment eliminates certain financial reporting requirements for development stage entities, specifically, the presentation of inception-to-date information, the development stage entity label on the financial statements, the description of the activities in which the entity is engaged, and disclosure in the first year that the entity is no longer a development stage entity that it had been in prior years. In addition, the amendment clarifies guidance regarding entities that have not commenced planned principal operations. The amendment is effective for annual reporting periods beginning after December 15, 2014, and interim periods therein with early adoption permitted. The Company elected to early adopt this standard in the period ended June 30, 2014. Other than a change in presentation, the adoption of this guidance did not have an impact on the Company’s financial statements.
	In May 2014, the FASB issued guidance which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. This guidance will be effective prospectively for fiscal years, and interim periods within those years, beginning after December 15, 2016, and early adoption is not permitted. The Company is currently evaluating the potential impact that the adoption of this guidance and the related transition guidance may have on the Company’s financial statements.
	In August 2014, the FASB issued accounting guidance for the disclosure of uncertainties related to an entity’s ability to continue as a going concern. The new standard requires management to perform an assessment at interim and annual periods as to the entity’s ability to continue as a going concern and provides specific disclosure guidance. This guidance will be effective for fiscal years, and interim periods within those years, beginning after December 15, 2016 and early adoption is permitted. The Company is currently evaluating the potential impact that the adoption of this guidance may have on the Company’s financial statements.

Fair_Value_of_Financial_Assets

Fair Value of Financial Assets	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets	4	Fair Value of Financial Assets
	The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2014 and 2013 and indicate the level of the fair value hierarchy utilized to determine such fair value:
			Fair Value Measurements as of December 31, 2014 Using:
			Level 1		Level 2		Level 3		Total
	Assets:
	Cash equivalents		$	—	$	101,687,995	$	—	$	101,687,995
	Marketable securities			—		48,527,156		—		48,527,156
			$	—	$	150,215,151	$	—	$	150,215,151
			Fair Value Measurements as of December 31, 2013 Using:
			Level 1		Level 2		Level 3		Total
	Assets:
	Cash equivalents		$	—	$	14,957,788	$	—	$	14,957,788
	Marketable securities			—		250,047		—		250,047
			$	—	$	15,207,835	$	—	$	15,207,835
	As of December 31, 2014 and 2013, the Company’s cash equivalents that are invested in money market funds are valued based on Level 2 inputs. The Company measures the fair value of marketable securities using Level 2 inputs and primarily relies on quoted prices in active markets for similar marketable securities. During the years ended December 31, 2014 and 2013, there were no transfers between Level 1, Level 2 and Level 3. Amortization and accretion of discounts and premiums are recorded in other income.
	As outlined in Note 9, the Company has a term loan with MidCap Financial SBIC, LP (“MidCap”). The term loan outstanding under the Company’s credit and security agreement with MidCap is reported at its carrying value in the accompanying balance sheet. The Company determined the fair value of the term loan using an income approach, utilizing a discounted cash flow analysis based on current market interest rates for debt issuances with similar remaining years to maturity, adjusted for credit risk. The term loan was valued using Level 2 inputs as of December 31, 2014. The result of the calculation yielded a fair value that approximates carrying value.

Marketable_Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	5	Marketable Securities
	As of December 31, 2014 and 2013, the fair value of available-for-sale marketable securities by type of security was as follows:
			December 31, 2014
			Amortized Cost		Gross Unrealized		Gross Unrealized			Fair Value
					Gains		Losses
	Commercial paper		$	8,991,820	$	7,570	$	—		$	8,999,390
	U.S. Government obligations			28,300,921		181		(5,101	)		28,296,001
	Corporate bonds			11,239,655		2		(7,892	)		11,231,765
			$	48,532,396	$	7,753	$	(12,993	)	$	48,527,156
			December 31, 2013
			Amortized Cost		Gross Unrealized		Gross Unrealized			Fair Value
					Gains		Losses
	Corporate bonds			250,075		—		(28	)		250,047
			$	250,075	$	—	$	(28	)	$	250,047
	At December 31, 2014 and 2013, marketable securities consisted of investments that mature within one year.

Prepaid_Expenses_and_Other_Cur

Prepaid Expenses and Other Current Assets and Other Assets	12 Months Ended
	Dec. 31, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets and Other Assets	6	Prepaid Expenses and Other Current Assets and Other Assets
	Prepaid expenses and other current assets and other assets consisted of the following as of December 31, 2014 and 2013:
			December 31,
			2014		2013
	Prepaid expenses		$	269,199	$	132,266
	Debt issuance costs			16,500		16,500
	Interest receivable on marketable securities			216,615		33,196
	Total prepaid expenses and other current assets		$	502,314	$	181,962
			December 31,
			2014		2013
	Debt issuance costs		$	12,375	$	28,875
	Total other assets		$	12,375	$	28,875
	In connection with entering into a credit and security agreement (Note 9), the Company incurred debt acquisition costs in the amount of $61,876. The Company capitalized these costs and is amortizing them as interest expense over the term of the term loan using the effective interest rate method. Total amortization expense for the debt issuance costs was $16,500 for each of the years ended December 31, 2014 and 2013.

Property_and_Equipment_Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	7	Property and Equipment, Net
	Property and equipment, net, as of December 31, 2014 and 2013 consisted of the following:
			December 31,
			2014			2013
	Computer and office equipment		$	229,980		$	257,346
	Manufacturing equipment			153,140			99,684
	Furniture and fixtures			181,366			149,183
	Software			77,454			—
	Leasehold improvements			134,573			130,626
	Construction—in Progress			601,317			—
				1,377,830			636,839
	Less: Accumulated depreciation			(268,439	)		(261,600	)
	Total property and equipment, net		$	1,109,391		$	375,239
	Depreciation expense for the years ended December 31, 2014, 2013 and 2012, was $120,341, $79,808, and $43,233, respectively. During the years ended December 31, 2014 and 2013, $ 113,502 and $16,695 of property and equipment was disposed of, resulting in a loss of $0 and $14,111, respectively.

Accrued_Expenses_and_Other_Cur

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2014
Payables and Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	8	Accrued Expenses and Other Current Liabilities
	Accrued expenses and other current liabilities at December 31, 2014 and 2013 consisted of the following:
			December 31,
			2014		2013
	Clinical research		$	1,035,510	$	163,521
	Contract manufacturing services			294,900		529,287
	Payroll and other employee-related expenses			1,172,978		792,165
	Preclinical services			119,500		5,685
	Consultant fees and expenses			26,900		21,718
	Professional services fees			439,874		642,052
	Interest expense			24,111		34,445
	Other			99,931		67,807
	Total accrued expenses and other current liabilities		$	3,213,704	$	2,256,680

Longterm_Debt

Long-term Debt	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Long-term Debt	9	Long-term Debt
	On January 3, 2013, the Company entered into a credit and security agreement with MidCap under which it immediately borrowed $5,000,000 as a term loan. The term loan accrues interest monthly at an interest rate of 8.0% per annum and has a term of 45 months. As the term loan has a 15-month interest-only period, the term loan principal balance, along with any accrued interest, is to be paid in 30 equal monthly installments beginning April 1, 2014 and ending September 1, 2016. In addition to these principal payments, the Company is required to make a payment of $175,000 to the lender on September 1, 2016, which is being accreted to the carrying value of the debt using the effective interest rate method. As of December 31, 2014, the carrying value of the term loan was $3,593,333 of which $2,000,000 was classified as the current portion of long-term debt on the balance sheet as of December 31, 2014. In connection with the credit and security agreement, the Company granted MidCap a security interest in all of the Company’s personal property now owned or hereafter acquired, excluding intellectual property but including the proceeds from the sale, if any, of intellectual property, and a negative pledge on intellectual property. The credit and security agreement also contains certain representations, warranties and non-financial covenants of the Company. As of December 31, 2014, the Company was compliant with all covenants

Preferred_and_Convertible_Pref

Preferred and Convertible Preferred Stock

12 Months Ended

Dec. 31, 2014

Preferred and Convertible Preferred Stock

11

Preferred Stock

On February 17, 2014, the Company filed an amended and restated Certificate of Incorporation (the “Restated Certificate”) in connection with the closing of the Company’s initial public offering. As of December 31 2014, under the Restated Certificate the Company is authorized to issue 10,000,000 shares of preferred stock with a par value of $0.001 per share.

Convertible Preferred Stock [Member]

Preferred and Convertible Preferred Stock

10

Convertible Preferred Stock

As of December 31, 2013, the Company’s Certificate of Incorporation, as amended, authorized the Company to issue 73,780,250 shares of preferred stock with a par value of $0.001 per share. The Company has issued Series A and Series B Convertible Preferred Stock (collectively, the “Convertible Preferred Stock”). Of the 73,780,250 authorized shares, 72,780,250 shares are issued; 55,043,464 of the issued shares are designated as Series A Convertible Preferred Stock (“Series A Preferred Stock”) and 18,736,786 are designated as Series B Convertible Preferred Stock (“Series B Preferred Stock”).

In December 2012, the Company issued 17,736,786 shares of Series B Preferred Stock at a purchase price of $1.1276 per share (“Series B Original Issue Price”). The Series B Preferred Stock issuance resulted in net proceeds of $19,888,478. Included in the Series B Preferred Stock issuance were contingently issuable warrants for the purchase of 218,160 shares of common stock at $0.01 per share. These warrants were contingently issuable if the Company did not initiate and enroll the first patient in a multi-dose Phase 2b clinical trial with planned enrollment of at least 100 patients for the Company’s product candidate identified as FX005 by February 15, 2014. Additionally, the obligation to issue the warrants would terminate on the earlier to occur of (i) the consummation of a registered initial public offering, which is defined as a firmly underwritten public offering pursuant to an effective registration statement under the Securities Act of 1933, as amended, covering the offer and sale of common stock for the account of the Company and (ii) the occurrence of a liquidation event or the consummation of a deemed liquidation event. The Company determined that the contingently issuable common stock warrants were equity in nature. As such, the Company allocated the Series B Preferred Stock proceeds to both the Series B Preferred Stock and the contingently issuable common stock warrants based on relative fair values at the issuance date. Given the low probability assessed by the Company of not satisfying the contingency and having to issue the warrants, the amount ascribed to the warrants was immaterial and the full amount of the proceeds was allocated to the Series B Preferred Stock.

On February 18, 2014, the Company completed an initial public offering (“IPO”) of its common stock. In preparation for the IPO, the Company’s Board of Directors and stockholders approved a 1-for-8.13 reverse stock split of the Company’s common stock and a proportional adjustment to the existing conversion ratios for each series of Convertible Preferred Stock. On January 27, 2014, the Company and holders of a majority of the Company’s Series A Convertible Preferred Stock and Series B Convertible Preferred Stock entered into a Conversion, Amendment and Waiver Agreement. This agreement amended the milestone date associated with the contingently issuable common stock warrants from February 15, 2014 to February 21, 2014. In connection with the closing of the IPO, all of the Company’s outstanding redeemable convertible preferred stock automatically converted to common stock as of February 18, 2014, resulting in an additional 8,952,057 shares of common stock of the Company becoming outstanding. As the Company’s IPO was completed prior to February 21, 2014 these warrants expired.

The following is a summary of the Company’s Convertible Preferred Stock as of December 31, 2013:

December 31, 2013

Preferred

Preferred

Carrying

Liquidation

Common Stock

Shares

Shares

Value

Preference

Issuable

Authorized

Issued and

Upon Conversion

Outstanding

Series A Preferred Stock

55,043,464

55,043,464

$

54,917,735

$

55,043,464

6,770,411

Series B Preferred Stock

18,736,786

17,736,786

19,888,478

20,000,000

2,181,646

73,780,250

72,780,250

$

74,806,213

$

75,043,464

8,952,057

The holders of the preferred stock had the following rights and preferences:

Voting Rights

The holders of preferred stock were entitled to vote, together with the holders of common stock, on all matters submitted to stockholders for a vote. Each preferred stockholder was entitled to the number of votes equal to the number of shares of common stock into which each preferred share is convertible at the time of such vote.

Dividends

The holders of both Series A and B Preferred Stock were entitled to receive, out of funds legally available, non-cumulative dividends at an annual rate of 8.0%, when and if declared by the board of directors. Holders of Series B Preferred Stock receive such dividends in preference to any dividend on Series A Preferred Stock or common stock. After payment of any amounts payable to holders of Series B Preferred Stock, the holders of Series A Preferred Stock were entitled to receive such dividends in preference to any dividend on common stock. No dividends have been declared or paid from inception to December 31, 2014.

Liquidation

Prior to the conversion of the Company’s Series A and Series B preferred stock on February 18, 2014, in connection with the closing of the Company’s IPO, the following liquidation preferences and conversion rates were in effect. The Company does not currently have any holders of securities with either liquidation preferences or conversion rights.

Upon any liquidation, dissolution, or winding up of the Company, whether voluntary or involuntary (each, a “Liquidation Event”), before any distribution or payment would have been made to the holders of any common stock, the Company would have made payment to the holders of preferred stock as follows: The holders of the Series B Preferred Stock, in preference to the holders of the Series A Preferred Stock and common stock, were entitled to be paid out of the assets of the Company legally available for distribution for each share of Series B Preferred Stock held by them, an amount per share equal to the applicable Series B Original Issue Price plus all declared and unpaid dividends on such shares of Series B Preferred Stock, if any (the “Series B Liquidation Preference”).

After the payment of the full liquidation preference of the Series B Preferred Stock, the holders of Series A Preferred Stock, in preference to holders of common stock, would have been entitled to be paid out of the assets of the Company legally available for distribution for each share of Series A Preferred Stock held by them, an amount per share of Series A Preferred Stock equal to the Series A Original Issue Price plus all declared and unpaid dividends on the Series A Preferred Stock, if any (the “Series A Liquidation Preference”).

After payment in full of the Series B Liquidation Preference and the Series A Liquidation Preference as set forth above, the remaining assets of the Company legally available for distribution, if any, would have been distributed ratably to the holders of the common stock.

Shares of Series A and B Preferred Stock would not have been entitled to be converted into shares of common stock in order to participate in any distribution, or series of distributions, as shares of common stock, without first foregoing participation in the distribution, or series of distributions, as shares of Series A and B Preferred Stock.

Conversion

Each share of Series A and B Preferred Stock was convertible at any time at the option of the stockholder into fully paid and non-assessable shares of common stock. The number of shares of common stock to which a holder of any series of Series A and B Preferred Stock was entitled upon conversion was the product obtained by multiplying the Conversion Rate then in effect for such series of preferred stock by the number of shares of such series of preferred stock being converted.

The conversion rate in effect at any time for conversion of the preferred stock (the “Conversion Rate”) was (i) for the Series A Preferred Stock, the quotient obtained by dividing the Series A Original Issue Price by the Conversion Price of the Series A Preferred Stock and (ii) for the Series B Preferred Stock, the quotient obtained by dividing the Series B Original Issue Price by the Conversion Price of the Series B Preferred Stock.

The Conversion Price of Series A Preferred Stock was $8.13 as of December 31, 2013, and the Conversion Price of Series B Preferred Stock was $9.166575 as of December 31, 2013. As of December 31, 2013, all outstanding shares of Series A Preferred Stock and Series B Preferred Stock were convertible into common stock on a 0.123001-for-1 basis. The Conversion Price of each series of preferred stock was subject to adjustment for stock splits, stock dividends and other recapitalizations in accordance with the Company’s Certificate of Incorporation, as amended.

In addition to the above optional conversion feature, the Series A and B Preferred Stock included a mandatory conversion feature whereby upon a qualifying initial public offering, all outstanding shares of Series A and B Preferred Stock would automatically be converted into shares of common stock at the then effective conversion rate. A qualifying initial public offering is an initial public offering in which (i) the per share price is at least $27.48 (as adjusted for any stock splits, dividends, combinations, recapitalizations and the like after the filing date), and (ii) the gross cash proceeds to the Company (before underwriting discounts, commissions and fees) are at least $40,000,000. All shares that are required to be surrendered per the provisions above were deemed to have been retired and canceled and may not be reissued as shares of preferred stock.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	13	Commitments and Contingencies
	Operating Leases
	In May 2013, the Company entered into a lease for office space in Burlington, Massachusetts. The lease is for a 42-month term with minimum monthly lease payments beginning at $17,588 per month and escalating over the lease term. The Company provided a letter of credit to the lessor in the amount of $98,000 as a security deposit pursuant to the lease agreement to secure its obligations under the lease. During January 2015, this letter of credit was reduced to $74,000 pursuant to the original lease agreement.
	The Company incurred rent expense of $242,946, $227,381, and $123,023 for the years ended December 31, 2014, 2013 and 2012, respectively .
	Future minimum lease payments under operating leases as of December 31, 2014, together with the new Burlington lease, were as follows:
	Year Ending December 31,
	2015			289,972
	2016			243,532
	Total		$	533,504
	AstraZeneca License Agreements
	On September 3, 2010, the Company entered into an exclusive license agreement with AstraZeneca for FX007. The agreement grants the Company an exclusive, royalty-bearing right and license to the patent rights detailed in the agreement. Per the terms of the license agreement, the Company was required to pay a nonrefundable fee of $1,000,000: $500,000 within thirty days of the execution of the agreement and $500,000 six months after the execution of the agreement. The Company recorded $1,000,000 as research and development expense during 2010. The agreement includes terms for potential future milestone payments including up to an aggregate of $21 million upon the achievement of certain regulatory and development milestones for a first licensed product for OA indications, or up to an aggregate of $15 million upon the achievement of certain regulatory and development milestones for a first-licensed product for non-OA indications. Upon commercialization of a product that results from the technology licensed under the agreement, the Company will owe AstraZeneca tiered royalty payments on net sales based on a percentage ranging from low single digits to low double digits, depending on the volume of sales of the applicable product, as well as up to $75 million in additional payments based on the achievement of certain sales milestones. There were no payments made or expenses recorded under this agreement in 2014, 2013 or 2012.
	On June 12, 2009, the Company entered into an exclusive license agreement with AstraZeneca for FX005. The agreement grants the Company an exclusive, royalty-bearing right and license to the patent rights detailed in the agreement. Per the terms of the license agreement, the Company paid a nonrefundable fee of $1,000,000 upon execution of the agreement. The Company recorded the $1,000,000 payment as research and development expense during 2009. The agreement includes terms for potential future milestone payments including up to an aggregate of $17 million upon the achievement of certain regulatory and development milestones for a first licensed product for OA indications, or up to an aggregate of $11 million upon the achievement of certain regulatory and development milestones for a first-licensed product for non-OA indications. Upon commercialization of a product that results from the technology licensed under the agreement, the Company will owe AstraZeneca tiered royalty payments on net sales based on a percentage ranging from low to high single digits, depending on the volume of sales of the applicable product, as well as up to $45 million in additional payments based on the achievement of certain sales milestones. There were no payments made or expenses recorded under this agreement in 2014, 2013 or 2012.

StockBased_Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	14	Stock-Based Compensation
	2013 Equity Incentive Plan
	On January 27, 2014, the Company’s stockholders approved the 2013 Equity Incentive Plan (the “2013 Plan”), which became effective on February 11, 2014, the date of execution of the underwriting agreement pursuant to which the Company’s common stock is priced for its initial public offering. Prior to the effective date of the 2013 Plan, the Company granted stock-based awards pursuant to the 2009 Stock Incentive Plan (the “2009 Plan), which had similar features to the 2013 Plan. The 2013 Plan provides for the grant of incentive stock options (“ISOs”), non-statutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance-based stock awards, and other forms of equity compensation. Initially, the maximum number of shares of the Company’s common stock that may be issued pursuant to stock awards under the 2013 Plan is 2,337,616, which is the sum of (i) 1,230,012 shares, plus (ii) the number of shares remaining available for grant under the 2009 Plan, plus (iii) any shares subject to outstanding stock options or other stock awards that would have otherwise returned to the 2009 Plan (such as upon the expiration or termination of a stock award prior to vesting). Additionally, the number of shares of common stock reserved for issuance under the 2013 Plan will automatically increase on January 1 of each year, beginning on January 1, 2015 and continuing through and including January 1, 2023, by 4% of the total number of shares of the Company’s capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the board of directors. The maximum number of shares that may be issued upon the exercise of ISOs under the 2013 Plan is 4,684,989 shares. As of December 31, 2014, there were 750,791 options outstanding under the 2009 Plan.
	The Company currently grants stock-based awards pursuant to the 2013 Plan. As of December 31, 2014, 906,077 shares were available for future issuance under the 2013 Plan. Stock option vesting typically occurs over four years for employees and directors and is at the discretion of the board of directors. Options granted have a maximum term of up to 10 years.
	Stock Options
	During the years ended December 31, 2014, 2013 and 2012, the Company granted stock options for the purchase of 727,575, 201,721, 403,382, shares of common stock, respectively, to certain employees, non-employee consultants and directors. The vesting conditions for most of these awards are time-based, and the awards typically vest 25% after one year and monthly thereafter for the next 36 months, except for annual option grants to non-employee directors of the Company whose initial grants vest 25% after one year and monthly thereafter for the next 24 months and whose annual grants vest in equal monthly installments during the 12-month period following the grant date, pursuant to the Company’s Non-Employee Director Compensation Policy. Awards typically expire after 10 years.
	Of the stock options granted in 2012 for the purchase of 403,382 shares of common stock options for the purchase of 264,944 shares were granted with performance-based vesting conditions to certain executives. The options were to vest in the event of a corporate transaction with the amount to vest contingent upon the transaction. The grant date fair value of these options was $236,940. In September 2012, performance-based options for the purchase of 18,450 shares of common stock were forfeited. No expense was recognized related to these options for the year ended December 31, 2012 as the performance conditions were not considered probable of achievement at December 31, 2012. On July 16, 2013, in connection with the Company’s proposed initial public offering, the board of directors exercised its election to change the vesting conditions of these stock options from performance-based vesting to time-based vesting. As a result, these stock options now vest over a four-year period commencing effective August 29, 2012. The change in the vesting conditions was accounted for as a modification of these stock options. The modification resulted in an aggregate increase in the fair value of the options of $2,185,729, of which $481,729 was recorded as stock-based compensation expense on the modification date, July 16, 2013, and $1,704,000 was unrecognized stock-based compensation expense, which is expected to be recognized over the remaining vesting terms of the options through August 2016.
	Stock Option Valuation
	The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of its publicly-traded peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The relevant data used to determine the value of the stock option grants for the years ended December 31, 2014, 2013 and 2012 is as follows:
			December 31,
			2014			2013			2012
	Risk-free interest rates			1.54-2.04	%		1	%		0.93	%
	Expected dividend yield			0	%		0	%		0	%
	Expected term (in years)			6			6			6.1
	Expected volatility			61.9-68.0	%		71	%		71	%
	The following table summarizes stock option activity for the years ended December 31, 2014 and 2013:
			Shares			Weighted
			Issuable			Average
			Under			Exercise
			Options			Price
	Outstanding as of December 31, 2013			834,983		$	2.99
	Granted			727,575			16.81
	Exercised			(141,141	)		2.16
	Canceled or forfeited			(132,335	)		9.06
	Outstanding as of December 31, 2014			1,289,082		$	10.26
	Options vested and expected to vest at December 31, 2014			1,100,564
	Options exercisable at December 31, 2014			450,109
	The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock for those options that had exercise prices lower than the fair value of the Company’s common stock. A total of 141,141, 4,868 and 258,891 options were exercised during the years ended December 31, 2014, 2013 and 2012, respectively. The aggregate intrinsic value of stock options exercised was $1,476,861, $45,125 and $610,600 for the years ended December 31, 2014, 2013 and 2012, respectively.
	At December 31, 2014, 2013 and 2012 the Company had options for the purchase of 1,289,082, 834,983, and 662,730 shares of common stock outstanding, with a weighted average remaining contractual term of 8.1, 8.5 and 7.95 years, respectively, and with a weighted average exercise price of $10.26, $2.99 and $1.87 per share, respectively. At December 31, 2014, 2013 and 2012 there were options for the purchase of 450,109, 386,334 and 198,439 shares of common stock exercisable under these stock option awards, respectively.
	The weighted average grant date fair value of options granted during the years ended December 31, 2014, 2013 and 2012 was $9.99, $6.99 and $1.63, respectively.
	Restricted Common Stock
	The Company’s 2009 and 2013 Plans provide for the award of restricted stock. The Company has granted restricted common stock with time-based vesting conditions. Unvested shares of restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting.
	During the years ended December 31, 2014, 2013, and 2012 the Company did not issue any shares of restricted common stock.
	As of December 31, 2014 and 2013, there were no shares related to restricted stock awards that were unvested and subject to repurchase.
	The aggregate intrinsic value of restricted stock awards is calculated as the difference between the grant date fair value of the restricted stock awards and the fair value of the Company’s common stock at December 31, 2014, 2013, and 2012, respectively. For the years ended December 31, 2014, 2013 and 2012, the aggregate intrinsic value of vested restricted stock awards was $0, $0, and $1,422,711, respectively, and for restricted stock awards expected to vest was $0, $0 and $1,741, respectively. The weighted average remaining contractual term for restricted stock awards as of December 31, 2014, 2013 and 2012 was 0 years. The fair value of restricted stock awards that vested during the years ended December 31, 2014, 2013 and 2012 was $0, $0, and $2.52, respectively, per share.
	Stock-based Compensation
	The Company recorded stock-based compensation expense related to stock options and restricted stock for the years ended December 31, 2014, 2013 and 2012 as follows:
			Year Ended December 31,
			2014			2013			2012
	Research and development		$	684,511		$	347,470		$	36,258
	General and administrative			1,766,068			648,697			60,020
			$	2,450,579		$	996,167		$	96,278
	As of December 31, 2014, unrecognized stock-based compensation expense for stock options outstanding was $7,835,365, which is expected to be recognized over a weighted average period of 2.9 years.
	Employee Stock Purchase Plan
	On January 27, 2014, the Company’s stockholders approved the Employee Stock Purchase Plan. A total of 209,102 shares of common stock were reserved for issuance under this plan. The Employee Stock Purchase Plan became effective on February 11, 2014, the date of execution of the underwriting agreement pursuant to which the Company’s common stock is priced for its initial public offering. During the year ended December 31, 2014, 6,770 shares were purchased by employees under the plan.

Net_Loss_Per_Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Net Loss Per Share	15. Net Loss Per Share
	Basic and diluted net loss per share attributable to common stockholders was calculated as follows for the years ended December 31, 2014, 2013 and 2012:
		Year Ended December 31,
		2014			2013			2012
	Numerator:
	Net loss	$	(27,313,664	)	$	(18,186,724	)	$	(14,981,619	)
	Accretion of dividends on convertible preferred stock		—			—			—
	Net income attributable to participating securities		—			—			—
	Net loss attributable to common stockholders:	$	(27,313,664	)	$	(18,186,724	)	$	(14,981,619	)
	Denominator:
	Weighted average common shares outstanding, basic and diluted		13,893,961			790,038			543,301
	Net loss per share attributable to common stockholders, basic and diluted	$	(1.97	)	$	(23.02	)	$	(27.58	)
	Stock options for the purchase of 1,185,253, 648,591, and 580,419 weighted average shares of common stock were excluded from the computation of diluted net loss per share attributable to common stockholders for the years ended December 31, 2014, 2013 and 2012, respectively, because those options had an anti-dilutive impact due to the net loss attributable to common stockholders incurred for those periods. In addition, 0, 0, and 36 weighted average shares of unvested restricted common stock were excluded from the computation of basic and diluted net loss per share attributable to common stockholders for the years ended December 31, 2014, 2013 and 2012, respectively.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	16	Income Taxes
	The Company has generated losses since inception. The Company has recorded no income tax benefits for those losses during the years ended December 31, 2014 and 2013, respectively, due to its uncertainty of realizing a benefit from those losses.
	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows:
			Year Ended December 31,
			2014			2013
	Federal statutory income tax rate			34	%		34	%
	State taxes, net of federal benefit			4.7			4.8
	Federal and state research and development tax credits			3.5			3.7
	Change in deferred tax asset valuation allowance			(37.8	)		(38.1	)
	Other			(4.4	)		(4.4	)
	Effective income tax rate			—	%		—	%
	The Company’s net deferred tax assets consisted of the following:
			December 31,
			2014			2013
	Net operating loss carryforwards		$	17,728,075		$	13,677,136
	Research and development tax credit carryforwards			3,185,552			2,333,715
	Capitalized research and development expenses, net			12,780,675			7,397,849
	Accruals and other temporary differences			131,571			84,716
	Total deferred tax assets			33,825,873			23,493,416
	Valuation allowance			(33,825,873	)		(23,493,416	)
	Net deferred tax asset		$	—		$	—
	As of December 31, 2014, the Company had federal and state net operating loss carryforwards of approximately $45,692,802 and $41,525,046, respectively, which begin to expire in 2029 for federal purposes and in 2030 for state purposes. In addition, the Company had federal and state research and development tax credit carryforwards of approximately $2,138,292 and $1,586,758, respectively, available to reduce future tax liabilities, which begin to expire in 2029 for federal purposes and 2025 for state purposes. Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which are comprised principally of net operating loss carryforwards and capitalized research and development expenses. Management has considered the Company’s history of cumulative net losses incurred since inception, as well as its lack of commercialization of any products or generation of any revenue from product sales since inception, and determined that it is more likely than not that the Company will not realize the benefits of its deferred tax assets. As a result, a full valuation allowance has been established at December 31, 2014 and 2013.
	Section 382 of the Internal Revenue Code of 1986, as amended (“Section 382”), contains rules that limit the ability of a company that undergoes an ownership change to utilize its net operating losses (“NOLs”) and tax credits existing as of the date of such ownership change. Under the rules, such an ownership change is generally any change in ownership of more than 50% of a company’s stock within a rolling three-year period. The rules generally operate by focusing on changes in ownership among stockholders considered by the rules as owning, directly or indirectly, 5% or more of the stock of a company and any change in ownership arising from new issuances of stock by the company. During the quarter ended June 30, 2014, the Company completed a Section 382 study through February 11, 2014. The results of this study showed that as of February 11, 2014, one historical ownership change within the meaning of Section 382 had occurred in 2009. As a result of this Section 382 limitation, approximately $0.3 million of NOLs will expire unutilized. In addition, the Company recently completed another Section 382 study through December 31, 2014. The results of this study showed that the Company experienced an ownership change in 2014 as part of the follow-on offering, however, none of the NOLs will expire due to the Section 382 limitation associated with the ownership change, assuming sufficient future taxable income and no future limitations. Subsequent ownership changes as defined by Section 382 may potentially limit the amount of net operating loss carryforwards that could be utilized annually to offset future taxable income. The Company has generated losses since inception and therefore has recorded no income tax benefits for those losses due to its uncertainty of realizing a benefit from those losses.
	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2014 and 2013 were as follows:
			Year Ended December 31,
			2014			2013
	Valuation allowance as of beginning of year		$	(23,493,416	)	$	(16,548,999	)
	Decreases recorded as benefit to income tax provision			2,020,565			1,726,638
	Increases recorded to income tax provision			(12,353,022	)		(8,671,055	)
	Valuation allowance as of end of year		$	(33,825,873	)	$	(23,493,416	)
	In each reporting period, the Company considers whether a tax position of the Company is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more likely than not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than fifty percent likelihood of being realized upon the ultimate settlement with the relevant taxing authority. No liabilities for unrecognized tax benefits were recorded as of December 31, 2014 and 2013.
	The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. The Company’s tax years are still open under statute from 2011 to the present. Earlier years may be examined to the extent that tax credit or net operating loss carryforwards are used in future periods. The resolution of tax matters is not expected to have a material effect on the Company’s consolidated financial statements.

Quarterly_Financial_Data_unaud

Quarterly Financial Data (unaudited)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (unaudited)	17	Quarterly Financial Data (unaudited)
	The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information.
			Three Months Ended
			March 31,			June 30,			September 30,			December 31,
			2014			2014			2014			2014
	Operating expenses		$	6,434,760		$	5,848,825		$	6,962,319		$	7,741,370
	Net loss			(6,542,244	)		(5,930,602	)		(7,035,607	)		(7,805,211	)
	Net loss per common share—basic and diluted		$	(0.86	)	$	(0.38	)	$	(0.45	)	$	(0.47	)
	Weighted average common shares—basic and diluted			7,632,786			15,619,151			15,624,963			16,523,385
			Three Months Ended
			March 31,			June 30,			September 30,			December 31,
			2013			2013			2013			2013
	Operating expenses		$	4,707,639		$	4,490,078		$	4,990,196		$	3,577,296
	Net loss			(4,798,087	)		(4,597,774	)		(5,100,378	)		(3,690,485	)
	Net loss per common share—basic and diluted		$	(6.08	)	$	(5.83	)	$	(6.46	)	$	(4.66	)
	Weighted average common shares—basic and diluted			789,222			789,222			789,222			792,432

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Events	18	Subsequent Events
	There were no subsequent events that require disclosure in the financial statements for the period ended December 31, 2014.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation
	The accompanying consolidated financial statements have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (the “SEC”) and Generally Accepted Accounting Principles (“GAAP”) for financial information, including the accounts of the Company and its wholly owned subsidiary after elimination of all significant intercompany accounts and transactions.
Use of Estimates	Use of Estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and judgments that may affect the reported amounts of assets and liabilities, expenses and related disclosures. The Company bases estimates and judgments on historical experience and on various other factors that it believes to be reasonable under the circumstances. The most significant estimates in these consolidated financial statements include useful lives with respect to long-lived assets, such as property and equipment and leasehold improvements, accounting for stock-based compensation, and accrued expenses, including clinical research costs. The Company’s actual results may differ from these estimates under different assumptions or conditions. The Company evaluates its estimates on an ongoing basis. Changes in estimates are reflected in reported results in the period in which they become known by the Company’s management.
Consolidation	Consolidation
	The accompanying consolidated financial statements include the Company and its wholly-owned subsidiary, Flexion Securities Corporation, Inc. The Company has eliminated all intercompany transactions for the year ended December 31, 2014, the year Flexion Securities Corporation, Inc. was established.
Cash and Cash Equivalents	Cash and Cash Equivalents
	The Company considers all highly liquid investments with a maturity of three months or less at the date of purchase to be cash equivalents. The Company currently invests available cash in money market funds of a major financial institution, corporate bonds, government obligations and commercial paper.
Marketable Securities	Marketable Securities
	Marketable securities consist of investments with original maturities greater than ninety days and less than one year from the balance sheet date. Long-term investments consist of investments with maturities of greater than one year. The Company classifies all of its investments as available-for-sale securities. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Realized gains and losses are determined on a specific identification basis and are included in other income (loss). Amortization and accretion of discounts and premiums is recorded in other income.
Restricted Cash	Restricted Cash
	The Company purchased a $30,000 certificate of deposit to collateralize a credit card account with a commercial bank that was classified as long-term restricted cash as of December 31, 2014 and 2013. In addition, the Company posted a letter of credit to the lessor of the Company’s Burlington facility in the amount of $98,000 as a security deposit pursuant to the lease agreement in the year ended December 31, 2014. That amount was classified as long-term restricted cash at December 31, 2014 and 2013.
Property and Equipment	Property and Equipment
	Property and equipment are stated at cost less accumulated depreciation. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives:
		Estimated
		Useful Life
		(Years)
	Computers, software and office equipment		3
	Manufacturing equipment		7
	Furniture and fixtures		5
	Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Costs of major additions and betterments are capitalized and depreciated on a straight-line basis over their useful lives. Repairs and maintenance costs are expensed as incurred. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is credited or charged to income.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets
	The Company reviews its 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. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends, and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows.
Debt Issuance Costs, net	Debt Issuance Costs, net
	Debt issuance costs, net represent legal costs related to the Company’s Credit and Security Agreement (Note 9). These costs are recorded as debt issuance costs on the balance sheets at the time they are incurred and are being amortized to interest expense through the scheduled final principal payment date. As of December 31, 2014 and 2013, the carrying value of debt issuance costs was $16,500 and $16,500, respectively, reported in prepaid expenses and other current assets and $12,375 and $28,875, respectively, reported in other assets. In addition, $16,500 of debt issuance costs were amortized and recognized as interest expense in the statement of operations for the year ended December 31, 2014 and 2013. The Company did not recognize any interest expense related to debt issuance costs during the year ended December 31, 2012.
Deferred Offering Costs	Deferred Offering Costs
	The Company capitalizes certain legal, accounting and other third-party fees that are directly associated with in-process equity financings as other assets until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ deficit as a reduction of additional paid-in capital generated as a result of the offering. If the equity financing is no longer considered probable of being consummated, the deferred offering costs would be expensed immediately as a charge to operating expenses in the consolidated statement of operations. As of December 31, 2013, the Company recorded deferred offering costs of $1,623,540 in the accompanying balance sheet in contemplation of a 2014 equity financing. The Company completed its initial public offering in February 2014 and did not record any deferred offering costs as of December 31, 2014.
Research and Development	Research and Development
	Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, depreciation, clinical trial and related clinical manufacturing costs, contract services and other related costs. Research and development costs are expensed to operations as the related obligation is incurred.
Patent Costs	Patent Costs
	All patent-related costs incurred in connection with filing and prosecuting patent applications are recorded as general and administrative expenses as incurred, as recoverability of such expenditures is uncertain.
Accounting for Stock-Based Compensation	Accounting for Stock-Based Compensation
	The Company measures all stock options and other stock based-awards granted to employees at the fair value at the date of grant using the Black-Scholes option-pricing model. The fair value of the awards is recognized as expense, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. The straight-line method of expense recognition is applied to all awards with service-only conditions.
	For stock-based awards granted to non-employees, compensation expense is recognized over the period during which services are rendered by such non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is re-measured using the then current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model.
	The Company classifies stock-based compensation expense in the consolidated statements of operations in the same manner in which the award recipient’s payroll costs are classified, or in the case of a non-employee, in the same manner as the award recipient’s service costs are classified.
	The Company recognizes compensation expense only for the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for service-based awards. The impact of a forfeiture rate adjustment will be recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company’s estimate, the Company may be required to record adjustments to stock-based compensation expense in future periods.
Concentration of Credit Risk and Significant Suppliers	Concentration of Credit Risk and Significant Suppliers
	Financial instruments that potentially expose the Company to concentration of credit risk consist primarily of commercial paper and bonds. The Company generally invests its cash in money market funds, government and corporate bonds, and commercial paper at one financial institution. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships.
	The Company is completely dependent on third-party manufacturers and product suppliers for preclinical research activities. In particular, the Company relies and expects to continue to rely exclusively on one manufacturer and relies on the manufacturer to purchase from third-party suppliers the materials necessary to produce its product candidates for its clinical trials. These research programs would be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients.
Comprehensive Loss	Comprehensive Loss
	Comprehensive income (loss) includes net loss as well as other changes in stockholders’ deficit that result from transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive income (loss) in all periods presented was unrealized gains (losses) on available-for-sale securities.
Income Taxes	Income Taxes
	The Company accounts for income taxes using the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred taxes are determined based on the differences between financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.
	The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.
Fair Value Measurements	Fair Value Measurements
	Fair value is defined as the exchange price that would be received to sell an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants at the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:
	•  Level 1 —	Quoted market prices in active markets for identical assets or liabilities. Level 1 consists primarily of financial instruments whose value is based on quoted market prices, such as exchange-traded instruments and listed equities.
	•  Level 2 —	Observable inputs (other than Level 1 quoted prices) such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.
	•  Level 3 —	Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.
	The Company’s financial instruments consist of cash equivalents, marketable securities, restricted cash, accounts payable and accrued expenses, and its term loan (Note 9). The estimated fair value of the Company’s financial instruments approximates their carrying values.
Net Loss Per Share	Net Loss Per Share
	The Company follows the two-class method when computing net loss per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based on their respective rights to receive dividends as if all income for the period had been distributed.
	The Company’s convertible preferred shares contractually entitle the holders of such shares to participate in dividends, but do not contractually require the holders of such shares to participate in the losses of the Company. Accordingly, in periods in which the Company reports a net loss or a net loss attributable to common stockholders resulting from preferred stock dividends, net losses are not allocated to participating securities. In periods of net loss, the Company does not increase its net loss attributable to common stockholders by accreting dividends on preferred stock, as the dividends are not cumulative under the terms of the preferred stock. The Company reported a net loss attributable to common stockholders for the year ended December 31, 2014, 2013 and 2012.
	Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities, including outstanding stock options and unvested restricted common stock. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock options and unvested restricted common stock. For periods in which the Company has reported net losses, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Potential common shares will always be anti-dilutive for periods in which the Company has reported a net loss. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders for the years ended December 31, 2014, 2013 and 2012.
Segment Data	Segment Data
	The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. The Company is a specialty pharmaceutical company focused on the development and commercialization of novel, injectable pain therapies. No revenue has been generated since inception, and all assets are held in the United States.
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted Accounting Pronouncements
	In June 2014, the Financial Accounting Standard Board, or FASB, issued amended accounting guidance for development stage entities. The amendment eliminates certain financial reporting requirements for development stage entities, specifically, the presentation of inception-to-date information, the development stage entity label on the financial statements, the description of the activities in which the entity is engaged, and disclosure in the first year that the entity is no longer a development stage entity that it had been in prior years. In addition, the amendment clarifies guidance regarding entities that have not commenced planned principal operations. The amendment is effective for annual reporting periods beginning after December 15, 2014, and interim periods therein with early adoption permitted. The Company elected to early adopt this standard in the period ended June 30, 2014. Other than a change in presentation, the adoption of this guidance did not have an impact on the Company’s financial statements.
	In May 2014, the FASB issued guidance which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. This guidance will be effective prospectively for fiscal years, and interim periods within those years, beginning after December 15, 2016, and early adoption is not permitted. The Company is currently evaluating the potential impact that the adoption of this guidance and the related transition guidance may have on the Company’s financial statements.
	In August 2014, the FASB issued accounting guidance for the disclosure of uncertainties related to an entity’s ability to continue as a going concern. The new standard requires management to perform an assessment at interim and annual periods as to the entity’s ability to continue as a going concern and provides specific disclosure guidance. This guidance will be effective for fiscal years, and interim periods within those years, beginning after December 15, 2016 and early adoption is permitted. The Company is currently evaluating the potential impact that the adoption of this guidance may have on the Company’s financial statements.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Property Plant and Equipment Estimated Useful Lives	Property and Equipment
	Property and equipment are stated at cost less accumulated depreciation. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives:
		Estimated
		Useful Life
		(Years)
	Computers, software and office equipment		3
	Manufacturing equipment		7
	Furniture and fixtures		5

Fair_Value_of_Financial_Assets1

Fair Value of Financial Assets (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2014 and 2013 and indicate the level of the fair value hierarchy utilized to determine such fair value:
		Fair Value Measurements as of December 31, 2014 Using:
		Level 1		Level 2		Level 3		Total
	Assets:
	Cash equivalents	$	—	$	101,687,995	$	—	$	101,687,995
	Marketable securities		—		48,527,156		—		48,527,156
		$	—	$	150,215,151	$	—	$	150,215,151
		Fair Value Measurements as of December 31, 2013 Using:
		Level 1		Level 2		Level 3		Total
	Assets:
	Cash equivalents	$	—	$	14,957,788	$	—	$	14,957,788
	Marketable securities		—		250,047		—		250,047
		$	—	$	15,207,835	$	—	$	15,207,835

Marketable_Securities_Tables

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security	As of December 31, 2014 and 2013, the fair value of available-for-sale marketable securities by type of security was as follows:
		December 31, 2014
		Amortized Cost		Gross Unrealized		Gross Unrealized			Fair Value
				Gains		Losses
	Commercial paper	$	8,991,820	$	7,570	$	—		$	8,999,390
	U.S. Government obligations		28,300,921		181		(5,101	)		28,296,001
	Corporate bonds		11,239,655		2		(7,892	)		11,231,765
		$	48,532,396	$	7,753	$	(12,993	)	$	48,527,156
		December 31, 2013
		Amortized Cost		Gross Unrealized		Gross Unrealized			Fair Value
				Gains		Losses
	Corporate bonds		250,075		—		(28	)		250,047
		$	250,075	$	—	$	(28	)	$	250,047

Prepaid_Expenses_and_Other_Cur1

Prepaid Expenses and Other Current Assets and Other Assets (Tables)	12 Months Ended
	Dec. 31, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets and Other Assets	Prepaid expenses and other current assets and other assets consisted of the following as of December 31, 2014 and 2013:
		December 31,
		2014		2013
	Prepaid expenses	$	269,199	$	132,266
	Debt issuance costs		16,500		16,500
	Interest receivable on marketable securities		216,615		33,196
	Total prepaid expenses and other current assets	$	502,314	$	181,962
		December 31,
		2014		2013
	Debt issuance costs	$	12,375	$	28,875
	Total other assets	$	12,375	$	28,875

Property_and_Equipment_Net_Tab

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Components of Property and Equipment, Net	Property and equipment, net, as of December 31, 2014 and 2013 consisted of the following:
		December 31,
		2014			2013
	Computer and office equipment	$	229,980		$	257,346
	Manufacturing equipment		153,140			99,684
	Furniture and fixtures		181,366			149,183
	Software		77,454			—
	Leasehold improvements		134,573			130,626
	Construction—in Progress		601,317			—
			1,377,830			636,839
	Less: Accumulated depreciation		(268,439	)		(261,600	)
	Total property and equipment, net	$	1,109,391		$	375,239

Accrued_Expenses_and_Other_Cur1

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2014
Payables and Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities at December 31, 2014 and 2013 consisted of the following:
		December 31,
		2014		2013
	Clinical research	$	1,035,510	$	163,521
	Contract manufacturing services		294,900		529,287
	Payroll and other employee-related expenses		1,172,978		792,165
	Preclinical services		119,500		5,685
	Consultant fees and expenses		26,900		21,718
	Professional services fees		439,874		642,052
	Interest expense		24,111		34,445
	Other		99,931		67,807
	Total accrued expenses and other current liabilities	$	3,213,704	$	2,256,680

Preferred_and_Convertible_Pref1

Preferred and Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Summary of Convertible Preferred Stock	The following is a summary of the Company’s Convertible Preferred Stock as of December 31, 2013:
		December 31, 2013
		Preferred		Preferred		Carrying		Liquidation		Common Stock
		Shares		Shares		Value		Preference		Issuable
		Authorized		Issued and						Upon Conversion
				Outstanding
	Series A Preferred Stock		55,043,464		55,043,464	$	54,917,735	$	55,043,464		6,770,411
	Series B Preferred Stock		18,736,786		17,736,786		19,888,478		20,000,000		2,181,646
			73,780,250		72,780,250	$	74,806,213	$	75,043,464		8,952,057

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Future Minimum Lease Payments under Operating Leases	Future minimum lease payments under operating leases as of December 31, 2014, together with the new Burlington lease, were as follows:
	Year Ending December 31,
	2015		289,972
	2016		243,532
	Total	$	533,504

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants	The relevant data used to determine the value of the stock option grants for the years ended December 31, 2014, 2013 and 2012 is as follows:
		December 31,
		2014			2013			2012
	Risk-free interest rates		1.54-2.04	%		1	%		0.93	%
	Expected dividend yield		0	%		0	%		0	%
	Expected term (in years)		6			6			6.1
	Expected volatility		61.9-68.0	%		71	%		71	%
Summary of Stock Option Activity	The following table summarizes stock option activity for the years ended December 31, 2014 and 2013:
		Shares			Weighted
		Issuable			Average
		Under			Exercise
		Options			Price
	Outstanding as of December 31, 2013		834,983		$	2.99
	Granted		727,575			16.81
	Exercised		(141,141	)		2.16
	Canceled or forfeited		(132,335	)		9.06
	Outstanding as of December 31, 2014		1,289,082		$	10.26
	Options vested and expected to vest at December 31, 2014		1,100,564
	Options exercisable at December 31, 2014		450,109
Stock-Based Compensation Expense Related to Stock Options	The Company recorded stock-based compensation expense related to stock options and restricted stock for the years ended December 31, 2014, 2013 and 2012 as follows:
		Year Ended December 31,
		2014			2013			2012
	Research and development	$	684,511		$	347,470		$	36,258
	General and administrative		1,766,068			648,697			60,020
		$	2,450,579		$	996,167		$	96,278

Net_Loss_Per_Share_Tables

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows for the years ended December 31, 2014, 2013 and 2012:
		Year Ended December 31,
		2014			2013			2012
	Numerator:
	Net loss	$	(27,313,664	)	$	(18,186,724	)	$	(14,981,619	)
	Accretion of dividends on convertible preferred stock		—			—			—
	Net income attributable to participating securities		—			—			—
	Net loss attributable to common stockholders:	$	(27,313,664	)	$	(18,186,724	)	$	(14,981,619	)
	Denominator:
	Weighted average common shares outstanding, basic and diluted		13,893,961			790,038			543,301
	Net loss per share attributable to common stockholders, basic and diluted	$	(1.97	)	$	(23.02	)	$	(27.58	)

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows:
		Year Ended December 31,
		2014			2013
	Federal statutory income tax rate		34	%		34	%
	State taxes, net of federal benefit		4.7			4.8
	Federal and state research and development tax credits		3.5			3.7
	Change in deferred tax asset valuation allowance		(37.8	)		(38.1	)
	Other		(4.4	)		(4.4	)
	Effective income tax rate		—	%		—	%
Net Deferred Tax Assets	The Company’s net deferred tax assets consisted of the following:
		December 31,
		2014			2013
	Net operating loss carryforwards	$	17,728,075		$	13,677,136
	Research and development tax credit carryforwards		3,185,552			2,333,715
	Capitalized research and development expenses, net		12,780,675			7,397,849
	Accruals and other temporary differences		131,571			84,716
	Total deferred tax assets		33,825,873			23,493,416
	Valuation allowance		(33,825,873	)		(23,493,416	)
	Net deferred tax asset	$	—		$	—
Changes in Valuation Allowance for Deferred Tax Assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2014 and 2013 were as follows:
		Year Ended December 31,
		2014			2013
	Valuation allowance as of beginning of year	$	(23,493,416	)	$	(16,548,999	)
	Decreases recorded as benefit to income tax provision		2,020,565			1,726,638
	Increases recorded to income tax provision		(12,353,022	)		(8,671,055	)
	Valuation allowance as of end of year	$	(33,825,873	)	$	(23,493,416	)

Quarterly_Financial_Data_unaud1

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Recurring Adjustments Necessary for Fair Statement	The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information.
		Three Months Ended
		March 31,			June 30,			September 30,			December 31,
		2014			2014			2014			2014
	Operating expenses	$	6,434,760		$	5,848,825		$	6,962,319		$	7,741,370
	Net loss		(6,542,244	)		(5,930,602	)		(7,035,607	)		(7,805,211	)
	Net loss per common share—basic and diluted	$	(0.86	)	$	(0.38	)	$	(0.45	)	$	(0.47	)
	Weighted average common shares—basic and diluted		7,632,786			15,619,151			15,624,963			16,523,385
		Three Months Ended
		March 31,			June 30,			September 30,			December 31,
		2013			2013			2013			2013
	Operating expenses	$	4,707,639		$	4,490,078		$	4,990,196		$	3,577,296
	Net loss		(4,798,087	)		(4,597,774	)		(5,100,378	)		(3,690,485	)
	Net loss per common share—basic and diluted	$	(6.08	)	$	(5.83	)	$	(6.46	)	$	(4.66	)
	Weighted average common shares—basic and diluted		789,222			789,222			789,222			792,432

Nature_of_the_Business_Additio

Nature of the Business - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Incorporation date	5-Nov-07
Cash, cash equivalents and marketable securities	$151,624,678	$16,438,301

Financing_Transactions_Additio

Financing Transactions - Additional Information (Detail) (USD $)	0 Months Ended		12 Months Ended	0 Months Ended	1 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 17, 2014	Feb. 18, 2014	Dec. 31, 2014	Nov. 05, 2007	Dec. 31, 2008	Dec. 31, 2013
Class of Stock [Line Items]
Sale of common stock price per share	$17	$13
Net proceeds from initial public offering	$92.20	$67.20
Reverse stock split conversion ratio			1-for-8.13 reverse stock split
Common stock, shares outstanding			21,440,058			794,090
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued			11,546,000	109	301,350
Reverse stock split conversion ratio			0.123
Common stock issued upon conversion of redeemable preferred stock			8,952,057
IPO [Member]
Class of Stock [Line Items]
Common stock, shares issued	5,796,000	5,750,000
Reverse stock split conversion ratio			1-for-8.13 reverse stock split
Common stock, shares outstanding			21,440,058
IPO [Member] | Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Common stock issued upon conversion of redeemable preferred stock		8,952,057
IPO [Member] | Common Stock [Member]
Class of Stock [Line Items]
Reverse stock split conversion ratio			0.123

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash	$128,000	$128,000
Carrying value of debt issuance costs reported in prepaid expenses and other current assets	16,500	16,500
Carrying value of debt issuance costs reported in other assets	12,375	28,875
Deferred offering costs	0	1,623,540
Mid Cap [Member] | Secured Debt [Member]
Summary Of Significant Accounting Policies [Line Items]
Carrying value of debt issuance costs reported in prepaid expenses and other current assets	16,500	16,500
Carrying value of debt issuance costs reported in other assets	12,375	28,875
Amortization of debt issuance costs recognized as interest expense	16,500	16,500	0
Certificates of Deposit [Member]
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash	30,000	30,000
Letter of Credit [Member]
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash	$98,000	$98,000

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Property Plant and Equipment Estimated Useful Lives (Detail)	12 Months Ended
	Dec. 31, 2014
Computers, Software and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	3 years
Manufacturing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	7 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	5 years

Fair_Value_of_Financial_Assets2

Fair Value of Financial Assets - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$48,527,156	$250,047
Fair Value Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	101,687,995	14,957,788
Marketable securities	48,527,156	250,047
Assets, Total	150,215,151	15,207,835
Level 2 [Member] | Fair Value Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	101,687,995	14,957,788
Marketable securities	48,527,156	250,047
Assets, Total	$150,215,151	$15,207,835

Fair_Value_of_Financial_Assets3

Fair Value of Financial Assets - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]
Transfers between Level 1, Level 2 and Level 3	$0	$0

Marketable_Securities_Summary_

Marketable Securities - Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$48,532,396	$250,075
Gross Unrealized Gains	7,753
Gross Unrealized Losses	-12,993	-28
Fair Value	48,527,156	250,047
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,991,820
Gross Unrealized Gains	7,570
Fair Value	8,999,390
U S Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,300,921
Gross Unrealized Gains	181
Gross Unrealized Losses	-5,101
Fair Value	28,296,001
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,239,655	250,075
Gross Unrealized Gains	2
Gross Unrealized Losses	-7,892	-28
Fair Value	$11,231,765	$250,047

Marketable_Securities_Addition

Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Maturity period of investment	1 year	1 year

Prepaid_Expenses_and_Other_Cur2

Prepaid Expenses and Other Current Assets and Other Assets - Prepaid Expenses and Other Current Assets and Other Assets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$269,199	$132,266
Debt issuance costs	16,500	16,500
Interest receivable on marketable securities	216,615	33,196
Total prepaid expenses and other current assets	502,314	181,962
Debt issuance costs	12,375	28,875
Total other assets	$12,375	$28,875

Prepaid_Expenses_and_Other_Cur3

Prepaid Expenses and Other Current Assets and Other Assets - Additional Information (Detail) (Mid Cap [Member], Secured Debt [Member], USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Mid Cap [Member] | Secured Debt [Member]
Prepaid Expenses Other Current Assets And Other Assets [Line Items]
Capitalized debt acquisition costs	$61,876
Amortization expense for debt issuance costs	$16,500	$16,500	$0

Property_and_Equipment_Net_Com

Property and Equipment, Net - Components of Property and Equipment, Net (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property and equipment gross	$1,377,830	$636,839
Less: Accumulated depreciation	-268,439	-261,600
Total property and equipment, net	1,109,391	375,239
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	229,980	257,346
Manufacturing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	153,140	99,684
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	181,366	149,183
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	134,573	130,626
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	601,317
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	$77,454

Property_and_Equipment_Net_Add

Property and Equipment, Net - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Abstract]
Depreciation	$120,341	$79,808	$43,233
Property and equipment disposals	113,502	16,695
Gain (Loss) on sale of property and equipment	$0	($14,111)

Accrued_Expenses_and_Other_Cur2

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Payables and Accruals [Abstract]
Clinical research	$1,035,510	$163,521
Contract manufacturing services	294,900	529,287
Payroll and other employee-related expenses	1,172,978	792,165
Preclinical services	119,500	5,685
Consultant fees and expenses	26,900	21,718
Professional services fees	439,874	642,052
Interest expense	24,111	34,445
Other	99,931	67,807
Total accrued expenses and other current liabilities	$3,213,704	$2,256,680

LongTerm_Debt_Additional_Infor

Long-Term Debt - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Jan. 03, 2013
Debt Instrument [Line Items]
Current portion of long-term debt	$2,000,000	$1,500,000
Mid Cap [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Term loan, borrowings			5,000,000
Interest on term loan			8.00%
Term loan, Interest payment period	45 months
Term loan, payment description	15-month interest-only period, the term loan principal balance, along with any accrued interest, is to be paid in 30 equal monthly installments
Term loan, first periodic payment date	1-Apr-14
Term loan, last periodic payment date	1-Sep-16
Additional principal payments made to lender	175,000
Carrying value of term loan	3,593,333
Current portion of long-term debt	$2,000,000
Mid Cap [Member] | Secured Debt [Member] | Interest-Only-Strip [Member]
Debt Instrument [Line Items]
Term loan, Interest payment period	15 months
Mid Cap [Member] | Secured Debt [Member] | Principal-Only-Strip [Member]
Debt Instrument [Line Items]
Term loan, Interest payment period	30 months

Convertible_Preferred_Stock_Ad

Convertible Preferred Stock - Additional information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2013	Feb. 18, 2014
Temporary Equity [Line Items]
Reverse stock split conversion ratio	1-for-8.13 reverse stock split
Dividend at annual rate	8.00%
Dividends declared or paid	$0
Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock, Shares authorized			73,780,250
Preferred stock, par value			$0.00
Preferred Stock, Shares issued			72,780,250
Preferred stock conversion basis description	0.123001-for-1 basis
Common Stock [Member]
Temporary Equity [Line Items]
Common stock issued upon conversion of redeemable preferred stock	8,952,057
Reverse stock split conversion ratio	0.123
IPO [Member]
Temporary Equity [Line Items]
Reverse stock split conversion ratio	1-for-8.13 reverse stock split
IPO [Member] | Qualifying Initial Public Offering That Triggers Mandatory Conversion [Member] | Minimum [Member]
Temporary Equity [Line Items]
Initial public offering , per share price	$27.48
Gross cash proceed from initial public offering	40,000,000
IPO [Member] | Common Stock [Member]
Temporary Equity [Line Items]
Reverse stock split conversion ratio	0.123
Series A Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Net proceed from issuance of convertible preferred stock		13,081,988
Series A Convertible Preferred Stock [Member] | Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock, Shares authorized			55,043,464
Preferred Stock, Shares issued			55,043,464
Conversion price of preferred stock			$8.13
Series B Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Net proceed from issuance of convertible preferred stock		19,888,478
Series B Convertible Preferred Stock [Member] | Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock, Shares authorized			18,736,786
Preferred Stock, Shares issued			17,736,786
Preferred stock issued during period		17,736,786
Initial public offering , per share price		$1.13
Net proceed from issuance of convertible preferred stock		$19,888,478
Warrants issuable		218,160
Warrants issuable, per share		$0.01
Conversion price of preferred stock			$9.17
Redeemable Convertible Preferred Stock [Member] | IPO [Member]
Temporary Equity [Line Items]
Common stock issued upon conversion of redeemable preferred stock				8,952,057

Convertible_Preferred_Stock_Su

Convertible Preferred Stock - Summary of Convertible Preferred Stock (Detail) (Convertible Preferred Stock [Member], USD $)	12 Months Ended
	Dec. 31, 2013
Temporary Equity [Line Items]
Preferred Shares Authorized	73,780,250
Preferred Shares Issued	72,780,250
Preferred Shares Outstanding	72,780,250
Carrying Value	$74,806,213
Liquidation Preference	75,043,464
Common Stock Issuable Upon Conversion	8,952,057
Series A Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	55,043,464
Preferred Shares Issued	55,043,464
Preferred Shares Outstanding	55,043,464
Carrying Value	54,917,735
Liquidation Preference	55,043,464
Common Stock Issuable Upon Conversion	6,770,411
Series B Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized	18,736,786
Preferred Shares Issued	17,736,786
Preferred Shares Outstanding	17,736,786
Carrying Value	19,888,478
Liquidation Preference	$20,000,000
Common Stock Issuable Upon Conversion	2,181,646

Preferred_Stock_Additional_Inf

Preferred Stock - Additional Information (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Temporary Equity Disclosure [Abstract]
Preferred stock, shares authorized	10,000,000	0
Preferred stock, par value	$0.00	$0.00

Common_Stock_Additional_inform

Common Stock - Additional information (Detail) (USD $)	1 Months Ended	12 Months Ended			0 Months Ended
	Dec. 31, 2008	Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2009	Dec. 17, 2014	Feb. 18, 2014	Nov. 05, 2007	Dec. 31, 2013
		vote_per_share
Class of Stock [Line Items]
Common stock, share authorized		100,000,000	94,000,000	79,000,000			10,000,000	94,000,000
Additional common stock share authorized		6,000,000	15,000,000	69,000,000
Common stock, par value		$0.00	$0.00	$0.00				$0.00
Proceeds from the issuance of common stock	$20,750	$162,137,580
Sale of common stock price per share					17	13
Number of vote per common stock		1
Dividend declared		$0
IPO [Member]
Class of Stock [Line Items]
Common stock, shares issued					5,796,000	5,750,000
Redeemable Convertible Preferred Stock [Member] | IPO [Member]
Class of Stock [Line Items]
Common stock issued upon conversion of redeemable preferred stock						8,952,057
Restricted Stock [Member]
Class of Stock [Line Items]
Vesting period		4 years
Restricted Stock [Member] | Non-employee consultants [Member]
Class of Stock [Line Items]
Common stock, shares issued	116,849
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued	301,350	11,546,000					109
Issuance of common stock for licensing agreement				184,501
Common stock issued upon conversion of redeemable preferred stock		8,952,057
Common Stock [Member] | Prospective preferred stock investor [Member]
Class of Stock [Line Items]
Common stock, shares issued	73,800
Common Stock [Member] | Restricted Stock [Member] | Founders of the Company [Member]
Class of Stock [Line Items]
Common stock, shares issued	110,701

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional information (Detail) (USD $)	1 Months Ended	12 Months Ended					1 Months Ended
	31-May-13	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2010	Dec. 31, 2009	Jan. 31, 2015	Sep. 03, 2010	Jun. 12, 2009
Commitment And Contingencies [Line Items]
Minimum monthly lease payments	$17,588
Operating lease term	42 months
Rent expense		242,946	227,381	123,023
Research and development expense		17,923,348	11,060,912	11,065,137
AstraZeneca [Member]
Commitment And Contingencies [Line Items]
License agreement, nonrefundable fee required								1,000,000
Research and development expense					1,000,000	1,000,000
License agreement expense		0	0	0
AstraZeneca [Member] | Within Thirty Days of the Execution of the Agreement [Member]
Commitment And Contingencies [Line Items]
License agreement, nonrefundable fee required								500,000
AstraZeneca [Member] | Six Months After the Execution of the Agreement [Member]
Commitment And Contingencies [Line Items]
License agreement, nonrefundable fee required								500,000
AstraZeneca [Member] | Achievement of Certain Sales Milestones [Member]
Commitment And Contingencies [Line Items]
Milestone payment								75,000,000	45,000,000
AstraZeneca [Member] | OA Indications [Member]
Commitment And Contingencies [Line Items]
Milestone payment								21,000,000	17,000,000
AstraZeneca [Member] | Non - OA Indications [Member]
Commitment And Contingencies [Line Items]
Milestone payment								15,000,000	11,000,000
Letter of Credit [Member]
Commitment And Contingencies [Line Items]
Security deposit paid to lessor	98,000
Letter of Credit [Member] | Subsequent Event [Member]
Commitment And Contingencies [Line Items]
Reduction in letter of credit							$74,000

Commitments_and_Contingencies_2

Commitments and Contingencies - Future Minimum Lease Payments under Operating Leases (Detail) (USD $)	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
2015	$289,972
2016	243,532
Total	$533,504

StockBased_Compensation_Additi

Stock-Based Compensation - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended		1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 27, 2014	Jul. 16, 2013	Sep. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding	1,289,082	834,983	662,730
Stock options granted	727,575
Stock-based compensation expense	$2,450,579	$996,167	$96,278
Unrecognized stock-based compensation expense	7,835,365
Number of options exercised	141,141	4,868	258,981
Aggregate intrinsic value of stock options exercised	1,476,861	45,125	610,600
Weighted average remaining contractual term	8 years 1 month 6 days	8 years 6 months	7 years 11 months 12 days
Weighted average exercise price	$10.26	$2.99	$1.87
Options exercisable, shares	450,109	386,334	198,439
Weighted average grant date fair value of options granted	$9.99	$6.99	$1.63
Stock options recognized over weighted average period	2 years 10 months 24 days
2013 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Description of aggregate number of shares common stock that may be issued	Initially, the maximum number of shares of the Companybs common stock that may be issued pursuant to stock awards under the 2013 Plan is 2,337,616, which is the sum of (i) 1,230,012 shares, plus (ii) the number of shares remaining available for grant under the 2009 Plan, plus (iii) any shares subject to outstanding stock options or other stock awards that would have otherwise returned to the 2009 Plan (such as upon the expiration or termination of a stock award prior to vesting).
Share based compensation, shares issued in period	1,230,012
Percentage of outstanding shares of common stock	4.00%
2009 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding	750,791
2013 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future issuance	906,077
Employees Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future issuance				209,102
Number of shares purchased by employees				6,770
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period	4 years
Restricted stock, issued	0	0
Aggregate intrinsic value of vested restricted stock awards	0	0	1,422,711
Aggregate intrinsic value of vested restricted stock awards expected to vest	0	0	1,741
Weighted average remaining contractual term for restricted stock awards	0 years	0 years	0 years
Fair value of restricted stock awards that vested	$0	$0	$2.52
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period	4 years
Share based award, expiration period	10 years
Stock options granted	727,575	201,721	403,382
Percentage of share based award, vested	25.00%
Awards vesting right	The vesting conditions for most of these awards are time-based, and the awards typically vest 25% after one year and monthly thereafter for the next 36 months,
Employee Stock Option [Member] | Non Employee Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards vesting right	Annual option grants to non-employee directors of the Company whose initial grants vest 25% after one year and monthly thereafter for the next 24 months
Performance-Based Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period	4 years
Stock options granted			264,944
Grant date fair value of options			236,940
Stock options forfeited						18,450
Stock-based compensation expense			0		481,729
Aggregate increase in fair value of options					2,185,729
Unrecognized stock-based compensation expense					$1,704,000
Stock-based compensation, modification date	16-Jul-13
Maximum [Member] | 2013 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, authorized	2,337,616
Number of options exercised	4,684,989
Maximum [Member] | Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based award, expiration period	10 years

StockBased_Compensation_Schedu

Stock-Based Compensation - Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Risk-free interest rates		1.00%	0.93%
Risk-free interest rates	1.54%
Risk-free interest rates	2.04%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	6 years	6 years	6 years 1 month 6 days
Expected volatility		71.00%	71.00%
Expected volatility	61.90%
Expected volatility	68.00%

StockBased_Compensation_Summar

Stock-Based Compensation - Summary of Stock Option Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Shares Issuable Under Options, Beginning balance	834,983	662,730
Shares Issuable Under Options, Granted	727,575
Shares Issuable Under Options, Exercised	-141,141	-4,868	-258,981
Shares Issuable Under Options, Canceled or forfeited	-132,335
Shares Issuable Under Options, Ending balance	1,289,082	834,983	662,730
Shares Issuable Under Options, Options vested and expected to vest at December 31, 2014	1,100,564
Shares Issuable Under Options, Options exercisable December 31, 2014	450,109	386,334	198,439
Weighted Average Exercise Price, Beginning balance	$2.99	$1.87
Weighted Average Exercise Price, Granted	$16.81
Weighted Average Exercise Price, Exercised	$2.16
Weighted Average Exercise Price, Canceled or forfeited	$9.06
Weighted Average Exercise Price, Ending balance	$10.26	$2.99	$1.87

StockBased_Compensation_StockB

Stock-Based Compensation - Stock-Based Compensation Expense Related to Stock Options (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$2,450,579	$996,167	$96,278
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	684,511	347,470	36,258
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$1,766,068	$648,697	$60,020

Net_Loss_Per_Share_Schedule_of

Net Loss Per Share - Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net loss	($7,805,211)	($7,035,607)	($5,930,602)	($6,542,244)	($3,690,485)	($5,100,378)	($4,597,774)	($4,798,087)	($27,313,664)	($18,186,724)	($14,981,619)
Accretion of dividends on convertible preferred stock
Net income attributable to participating securities
Net loss attributable to common stockholders:									($27,313,664)	($18,186,724)	($14,981,619)
Denominator:
Weighted average common shares outstanding, basic and diluted	16,523,385	15,624,963	15,619,151	7,632,786	792,432	789,222	789,222	789,222	13,893,961	790,038	543,301
Net loss per share attributable to common stockholders, basic and diluted	($0.47)	($0.45)	($0.38)	($0.86)	($4.66)	($6.46)	($5.83)	($6.08)	($1.97)	($23.02)	($27.58)

Net_Loss_Per_Share_Additional_

Net Loss Per Share - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share attributable to common stockholders	185,253	648,591	580,419
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share attributable to common stockholders	0	0	36

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Operating Loss Carryforwards [Line Items]
Income tax benefits	$0	$0
Limit of utilization of net operating losses (NOLs) and tax credits description	Under the rules, such an ownership change is generally any change in ownership of more than 50% of a company's stock within a rolling three-year period. The rules generally operate by focusing on changes in ownership among stockholders considered by the rules as owning, directly or indirectly, 5% or more of the stock of a company and any change in ownership arising from new issuances of stock by the company.
Net operating losses (NOLs) expire unutilized	300,000
Liabilities for unrecognized tax benefits	0	0
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	45,692,802
Operating loss carryforwards expiration year	2029
Federal [Member] | Research and Development Tax Credit Carryforwards [Member]
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	2,138,292
Tax credit expiration year	2029
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	41,525,046
Operating loss carryforwards expiration year	2030
State and Local Jurisdiction [Member] | Research and Development Tax Credit Carryforwards [Member]
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$1,586,758
Tax credit expiration year	2025

Income_Taxes_Reconciliation_of

Income Taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	34.00%	34.00%
State taxes, net of federal benefit	4.70%	4.80%
Federal and state research and development tax credits	3.50%	3.70%
Change in deferred tax asset valuation allowance	-37.80%	-38.10%
Other	-4.40%	-4.40%
Effective income tax rate	0.00%	0.00%

Income_Taxes_Net_Deferred_Tax_

Income Taxes - Net Deferred Tax Assets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$17,728,075	$13,677,136
Research and development tax credit carryforwards	3,185,552	2,333,715
Capitalized research and development expenses, net	12,780,675	7,397,849
Accruals and other temporary differences	131,571	84,716
Total deferred tax assets	33,825,873	23,493,416
Valuation allowance	-33,825,873	-23,493,416	-16,548,999
Net deferred tax asset	$0	$0

Income_Taxes_Changes_in_Valuat

Income Taxes - Changes in Valuation Allowance for Deferred Tax Assets (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes Disclosure [Line Items]
Valuation allowance as of beginning of year			($16,548,999)
Valuation allowance as of end of year	-33,825,873	-23,493,416	-16,548,999
Decreases Recorded as Benefit to Income Tax Provision [Member]
Income Taxes Disclosure [Line Items]
Decreases (increase) recorded as benefit to income tax provision	2,020,565	1,726,638
Increases Recorded to Income Tax Provision [Member]
Income Taxes Disclosure [Line Items]
Decreases (increase) recorded as benefit to income tax provision	($12,353,022)	($8,671,055)

Quarterly_Financial_Data_unaud2

Quarterly Financial Data (unaudited) - Recurring Adjustments Necessary for Fair Statement (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$7,741,370	$6,962,319	$5,848,825	$6,434,760	$3,577,296	$4,990,196	$4,490,078	$4,707,639	$26,987,274	$17,765,209	$15,011,642
Net loss	($7,805,211)	($7,035,607)	($5,930,602)	($6,542,244)	($3,690,485)	($5,100,378)	($4,597,774)	($4,798,087)	($27,313,664)	($18,186,724)	($14,981,619)
Net loss per common share - basic and diluted	($0.47)	($0.45)	($0.38)	($0.86)	($4.66)	($6.46)	($5.83)	($6.08)	($1.97)	($23.02)	($27.58)
Weighted average common shares - basic and diluted	16,523,385	15,624,963	15,619,151	7,632,786	792,432	789,222	789,222	789,222	13,893,961	790,038	543,301