Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2019

Summary of Assumptions Used in Black-Scholes Option Pricing Model

The fair values of stock options granted during the three and nine months ended September 30, 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		Nine Months Ended
	September 30,		September 30,
	2019	2018	2019	2018
Risk-free interest rate	1.62%	2.73% - 2.99%	1.62%  - 2.62%	2.51% - 2.99%
Expected volatility	71.8%	82.6% - 87.6%	71.8% - 75.2%	82.6% - 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 nine months ended September 30, 2019 and 2018, the Company recognized compensation expense relating to stock options as follows:

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2019		2018		2019		2018
Research and development	$	1,391	$	901	$	3,799		2,401
General and administrative		1,322		918		3,829		3,358
Total stock option expense	$	2,713	$	1,819	$	7,628	$	5,759

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 nine months ended September 30, 2019 is presented below:

		Weighted
	Number of	Average Exercise
	Shares	Price
Outstanding, December 31, 2018	4,004,422	$	13.34
Granted	1,218,000		16.88
Exercised	(546,265)		11.08
Forfeited	(100,703)		16.22
Outstanding, September 30, 2019	4,575,454	$	14.50
Options exercisable, September 30, 2019	2,126,935

Share-based Payment Arrangement, Nonemployee [Member] | Consultants [Member] | 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 September 30, 2018 are as follows:

	January 1,	September 30,
	2019	2018
Risk-free interest rate	2.59% - 2.62%	2.06% - 3.02%
Expected volatility	58.9% - 84.6%	77.4% - 80.9%
Expected dividend yield	0%	0%
Expected life of non-employee options (in years)	0.81 - 8.19	0.26 - 8.44