Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Summary of Assumptions Used in Black-Scholes Option Pricing Model

The fair values of stock options granted during the three months ended March 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 March 31,
	2019	2018
Risk-free interest rate	2.54% - 2.62%	2.51% - 2.71%
Expected volatility	74.29% - 75.19%	85.9%
Expected dividend yield	0%	0%
Expected life of employee options (in years)	6.25	6.25
Expected life of non-employee options    (in years)	—	—

Summary of Compensation Expense Relating to Stock Options

During the three months ended March 31, 2019 and 2018, the Company recognized compensation expense in the accompanying Condensed Statements of Comprehensive Loss relating to stock options, as follows:  

	Three Months Ended March 31,
	2019		2018
Research and development	$	1,082	$	649
General and administrative		1,152		1,222
Total stock option expense	$	2,234	$	1,871

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 three months ended March 31, 2019 is presented below:

	Number of Shares			Weighted Average Exercise Price
Outstanding, December 31, 2018		4,004,422		$	13.34
Granted		957,000			16.06
Exercised		(17,291	)		13.52
Forfeited		(18,490	)		16.55
Outstanding, March 31, 2019		4,925,641		$	13.86
Options exercisable, March 31, 2019		2,134,424

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 March 31, 2018 are as follows:  

	January 1,	March 31,
	2019	2018
Risk-free interest rate	2.59% - 2.62%	1.82% - 2.70%
Expected volatility	58.9% - 84.6%	78.2% - 101.0%
Expected dividend yield	0%	0%
Expected life of non-employee options (in years)	0.81 - 8.19	0.50 - 8.94