Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

Summary of Assumptions Used in Black-Scholes Option Pricing Model

The fair values of stock options granted during the three and six months ended June 31, 2019 and 2018 were estimated as of the dates of grant using the Black-Scholes option pricing model with the following assumptions:

	Three Months Ended June 30,		Six Months Ended June 30,
	2019	2018	2019	2018
Risk-free interest rate	1.95% - 2.36%	2.85% - 2.94%	1.95% - 2.62%	2.51% - 2.94%
Expected volatility	73.06% - 73.78%	85.7% - 92.8%	73.06%-75.19%	85.7% - 92.8%
Expected dividend yield	0%	0%	0%	0%
Expected life of employee options (in years)	6.25	6.25	6.25	6.25
Expected life of non-employee options    (in years)	—	—	—	—

Summary of Compensation Expense Relating to Stock Options

During the three and six months ended June 30, 2019 and 2018, the Company recognized compensation expense relating to stock options (excluding compensation expense related to i) the issuance of common stock relating to the consulting agreement for $99 in G&A expense for the three and six months ended June 30, 2019; ii) the vesting of executives’ restricted stock units for $604 in R&D expense and $590 in G&A expense for the three and six months ended June 30, 2019; and iii) compensation expense relating to the Board of Directors’ restricted stock units for $41 in G&A expense for the three and six months ended June 30, 2019), as follows:  

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	1,326	$	852	$	2,408		1,500
General and administrative		1,355		1,217		2,507		2,440
Total stock option expense	$	2,681	$	2,069	$	4,915	$	3,940

Summary of Stock Option Activity

A summary of stock option award activity related to employees, non-employee members of the Company’s Board of Directors and non-employee consultants as of and for the six months ended June 30, 2019 is presented below:

	Number of Shares			Weighted Average Exercise Price
Outstanding, December 31, 2018		4,004,422		$	13.34
Granted		1,198,000			16.77
Exercised		(395,997	)		10.63
Forfeited		(100,703	)		16.22
Outstanding, June 30, 2019		4,705,722		$	14.39
Options exercisable, June 30, 2019		2,034,916

Re-Measurement [Member]

Summary of Assumptions Used in Black-Scholes Option Pricing Model

Prior to January 1, 2019, the Company used the Black-Scholes option valuation model to re-measure the fair value of all outstanding options that had been granted to non-employee consultants during the vesting period of each tranche in accordance with ASC 505-50. On January 1, 2019, the Company used the Black-Scholes option valuation model to re-measure the fair value of all outstanding unvested options that had been granted to non-employee consultants in accordance with ASU 2018-07 (see Note 2, Other Accounting Pronouncements Recently Adopted). The range of assumptions used by the Company on January 1, 2019 and June 30, 2018 are as follows:  

	January 1,	June 30,
	2019	2018
Risk-free interest rate	2.59% - 2.62%	1.92% - 2.82%
Expected volatility	58.9% - 84.6%	60.4% - 96.2%
Expected dividend yield	0%	0%
Expected life of non-employee options (in years)	0.81 - 8.19	0.25 - 8.69