Stockholders’ Equity (Tables)

12 Months Ended

Dec. 31, 2015

Stockholders' Equity

Schedule of stock-based compensation expense recognized by award type

	Year Ended December 31,
	2014		2015
Options awards	$	3,299,870	$	4,933,916
Restricted stock awards		84,930		746,437
Total stock-based compensation expense	$	3,384,800	$	5,680,353

Schedule of total compensation cost recognized for all stock-based compensation awards in the statements of operations

	Year Ended December 31,
	2014		2015
Research and development	$	933,814	$	1,569,886
General and administrative		2,450,986		4,110,467
Total stock-based compensation expense	$	3,384,800	$	5,680,353

Summary of activity for all options

					Fair Value
		Weighted Average		Weighted Average	of		Aggregate
		Exercise Price		Contractual	Options		Intrinsic
	Shares	Per Share		Life (In Years)	Granted		Value
Outstanding—December 31, 2013	3,263,417	$	5.36	9.0			$	13,575,815
Granted	253,750		5.77		$	1,145,545
Exercised	(121,816)		1.49
Forfeited	(118,043)		5.56
Outstanding—December 31, 2014	3,277,308	$	5.53	8.5			$	—
Granted	785,900		5.15		$	2,944,407
Exercised	(24,261)		1.35
Forfeited	(18,164)		5.51
Outstanding—December 31, 2015	4,020,783	$	5.48	7.8			$	—
Vested and expected to vest—December 31, 2015	3,965,121	$	5.48	7.8			$	—
Exercisable at December 31, 2015	2,612,588	$	5.41	7.5			$	—

Schedule of weighted-average assumptions estimated at the date of grant using the Black Scholes option pricing model for per share weighted average fair value of the options granted to employees and non employees

	2014			2015			2015
	All grants			Executive grants			All others
Expected stock price volatility		90	%		75	%		91	%
Expected term of options		6.9	years		8	years		5.3	years
Risk‑free interest rate		2.19	%		1.73	%		1.33	%
Expected annual dividend yield		—	%		—	%		—	%
Weighted average grant date fair value	$	4.51		$	3.84		$	3.44

Schedule of expense is expected to be recognized

December 31, 2016	$	3,926,109
December 31, 2017		871,911
December 31, 2018		283,294
December 31, 2019		19,046
	$	5,100,360

Summary of restricted stock activity

		Weighted
		Average
		Grant Date
		Fair Value
	Shares	per Share
Unvested Balance—December 31, 2013	80,237
Granted	—	$	—
Vested	(59,279)
Forfeited	(9,193)
Unvested Balance—December 31, 2014	11,765
Granted	450,000	$	5.25
Vested	(11,765)
Forfeited	—
Unvested Balance—December 31, 2015	450,000

Schedule of warrants outstanding to purchase shares of the entity's common stock

		Number of
		Shares	Exercise
Warrant Series	Underlying Equity Security	Issuable	Price		Expiration Date
2007 Warrant	common stock	1,961	$	7.65	May 2017
2006 Warrants	common stock	21,786	$	7.65	March 2016-May 2016
2009/2010 Warrants	common stock	52,815	$	0.85	November 2019-March 2020