Stock-based Compensation (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2016

Dec. 31, 2015

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-based Compensation Expense

	Three Months Ended March 31,
	2016		2015
Research and development	$	303,624	$	41,576
General and administrative		1,230,264		41,196
Total share-based compensation expense	$	1,533,888	$	82,772

	Years Ended December 31,
	2015		2014
Research and development	$	929,633	$	163,019
General and administrative		3,534,062		441,957
Total share-based compensation expense	$	4,463,695	$	604,976

Summary of Stock Option Activity

	Options Outstanding			Weighted-Average Exercise Price
Outstanding at December 31, 2015		3,243,041		$	2.36
Granted		1,346,000			0.76
Exercised		—			—
Forfeited/cancelled/expired		(93,337	)		2.21
Outstanding and expected to vest at March 31, 2016		4,495,704		$	1.93
Vested and exercisable at March 31, 2016		179,437		$	3.66

	Options Outstanding			Weighted Average Exercise Price
Outstanding at December 31, 2013		152,017		$	1.19
Granted		90,876			8.47
Exercised		—			—
Forfeited/cancelled/expired		—			—
Outstanding and expected to vest at December 31, 2014		242,893		$	3.92
Granted		3,015,850			2.23
Exercised		(2,779	)		0.29
Forfeited/cancelled/expired		(12,923	)		7.42
Outstanding and expected to vest at December 31, 2015		3,243,041		$	2.36
Vested and exercisable at December 31, 2015		178,001		$	3.59

Summary of activity related to restricted stock grants

	Shares		Weighted Average Grant-Date Fair Value
Non-vested at December 31, 2015		2,300,850	$	2.28
Granted		—		—
Vested		—		—
Forfeited		—		—
Non-vested at March 31, 2016		2,300,850	$	2.28

	Shares		WeightedAverage Grant-Date Fair Value
Non-vested at December 31, 2014		—	$	—
Granted		2,300,850		2.28
Vested		—		—
Forfeited		—		—
Non-vested at December 31, 2015		2,300,850	$	2.28

Valuation Assumptions Used to Determine Stock-based Expense

	Years Ended December 31,
	2015			2014
Risk-free interest rate	0.9 to 1.8 %			0.1 to 2 %
Dividend yield		0	%		0	%
Expected volatility	81to87 %			84to100 %
Expected life of options, in years	5.5 and 6.0			5 and 6.25
Weighted average grant date fair value	$	1.56		$	4.73

Common Stock Reserved for Future Issuance

Common stock reserved for conversion of preferred stock and warrants	21,662,200
Common stock reserved for exercise of warrants	8,876,336
Common stock options outstanding	4,495,704
Authorized for future grant or issuance under the Stock Plan	1,704,426
Unvested restricted stock	2,300,850
Total	39,039,516

Common stock reserved for conversion of preferred stock and warrants	29,692,068
Common stock options outstanding	3,243,041
Authorized for future grant or issuance under the Stock Plan	2,970,012
Unvested restricted stock	2,300,850
Total	38,205,971