Stock Based Compensation (Tables)

9 Months Ended

Sep. 30, 2019

Stock Based Compensation [Abstract]

Weighted-Average Assumptions for Stock Options

The weighted average assumptions that the Company used in the Black-Scholes pricing model to determine the fair value of the stock options granted to employees, non-employees and directors for the nine months ended September 30, 2019 and the stock options granted to employees, non-employees and directors for the nine months ended September 30, 2018 were as follows:

	Nine Months Ended September 30,
	2019			2018
Risk-free interest rate		2.40	%		2.61	%
Expected term (in years)		5.79			5.78
Expected volatility		75.13	%		88.30	%
Expected dividend yield		0.00	%		0.00	%
Exercise price	$	14.42		$	17.76
Fair value of common stock	$	14.42		$	17.76

Stock Option Activity

The following table summarizes stock option activity for the nine months ended September 30, 2019 under the Second Amended and Restated 2014 Stock Option and Incentive Plan:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2018		8,615,997		$	4.48		7.51	$	94,474
Granted		1,397,616			14.42		9.58
Exercised		(106,294	)		1.72				1,110
Cancelled		(298,782	)		7.49
Outstanding as of September 30, 2019		9,608,537		$	5.85		7.30	$	88,855
Options vested and exercisable as of September 30, 2019		7,143,825		$	2.87		6.72	$	84,535
Options unvested as of September 30, 2019		2,464,712		$	14.48		9.06

Stock-Based Compensation Expense

The total fair value of options vested during the nine months ended September 30, 2019 and 2018 was $49.4 million and $49.5 million, respectively.

Stock-Based Compensation

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	893	$	2,223	$	4,711	$	5,075
General and administrative		1,801		389		5,301		5,705
Total stock based compensation expense	$	2,694	$	2,612	$	10,012	$	10,780