Equity-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2018

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

Equity-Based Compensation Expense Classification in Statement of Operations

Equity-based compensation expense is classified in the statements of operations as follows:



	Three Months Ended March 31,
	2018		2017
Research and development expense	$	96	$	44
General and administrative expense	269		336
	$	365	$	380

Assumptions Used to Determine Fair Value of Common Stock Option Grants

The fair value of the underlying common stock and the exercise price for the common stock options granted during the three months ended March 31, 2018 and 2017 are summarized in the table below:



Common Stock Options Granted During Period Ended:	Fair Value of Underlying Common Stock	Exercise Price of Common Stock Option
Three months ended March 31, 2018	$3.00	$3.00
Three months ended March 31, 2017	$4.21	$4.21

In addition to an assumption on the expected term of the option grants as discussed below, application of the Black-Scholes model requires additional inputs for which we have assumed the values described in the table below:



	Three Months Ended March 31,
	2018		2017
Expected term	5.9 to 6.0 years		5.3 to 6.5 years
Risk-free interest rate	2.72%		1.97% to 2.17%; weighted avg. 2.07%
Expected volatility	82.4%		80.8% to 83.1%; weighted avg. 81.0%
Forfeiture rate	5	%	5	%
Expected dividend yield	—	%	—	%

Common Stock Option Activity

A summary of common stock option activity as of the periods indicated is as follows:



	Options		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (thousands)
Outstanding - December 31, 2017	3,672,620		$	1.79	7.5	$	5,221
Granted	674,279		3.00
Exercised	(8,532	)	0.86
Forfeited	(2,638	)	1.03
Outstanding - March 31, 2018	4,335,729		$	1.98	7.5	$	5,221
Exercisable - March 31, 2018	2,597,078		$	1.51	7.0	$	4,245
Vested and Expected to Vest - March 31, 2018	3,916,640		$	2.11	7.5	$	5,190