Share-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2020

Share-based Payment Arrangement [Abstract]

Summary of Share-Based Compensation Expense

The following table summarizes share-based compensation expense for the periods presented (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Research and development	$	2,088	$	1,955	$	6,163	$	6,154
General and administrative	3,495		3,861		10,643		12,144
Total share-based compensation expense	$	5,583	$	5,816	$	16,806	$	18,298

Fair Value of Options Granted

The fair value of the options granted to employees and non-employee directors during the three and nine months ended September 30, 2020, and 2019 was estimated as of the grant date using the Black-Scholes option-pricing model assuming the weighted-average assumptions listed in the following table:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2020	2019	2020	2019
Expected volatility	79.6%	81.0%	81.1%	81.4%
Risk-free interest rate	0.3%	1.8%	1.0%	2.3%
Expected dividend yield	—%	—%	—%	—%
Expected life (in years)	6.0	6.0	6.0	6.0
Weighted average grant date fair value	$8.56	$7.09	$8.22	$8.61

Summary of Company's Share Option Activity

The following table summarizes the Company’s stock option activity during the nine months ended September 30, 2020:

	Options	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2019	7,008,403	$	23.34	7.24	$	11,901
Granted	1,865,950	11.94
Exercised	(22,852)	9.42
Forfeited	(34,935)	13.18
Expired	(21,336)	33.61
Outstanding at September 30, 2020	8,795,230	$	20.97	7.37	$	1,994
Vested and expected to vest at September 30, 2020	8,378,260	$	21.34	7.07	$	1,986
Vested at September 30, 2020	4,859,859	$	25.93	6.26	$	1,918