Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2021

Stock Option Activity

The following summarizes option activity under the 2017 Inducement Plan, 2015 Plan and 2012 Plan:

	Number of Options			Weighted- Average Exercise Price
Outstanding — December 31, 2020		3,327,330		$	42.07
Options granted		658,480			43.57
Options exercised		(76,338	)		19.82
Options canceled		(170,068	)		52.52
Outstanding — June 30, 2021		3,739,404		$	42.32

Valuation Assumptions for Stock Awards

The fair values of stock options granted to employees were calculated using the following assumptions:

	Three Months Ended June 30,		Six Months Ended June 30,
	2021	2020	2021	2020
Expected term (in years)	5.3-6.1	5.1-6.1	5.3-6.1	5.1-6.1
Volatility	68.1%-72.2%	71.0%-71.8%	68.1%-72.7%	69.6%-71.8%
Risk-free interest rate	1.0%-1.1%	0.4%	0.9%-1.1%	0.4%-1.8%
Dividend yield	—	—	—	—

Stock-based Compensation Expense Recognized

Total stock-based compensation recognized by function included in the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Research and development	$	4,906	$	3,430	$	10,216	$	8,780
Selling, general and administrative		15,064		13,141		30,134		24,158
Total stock-based compensation expense	$	19,970	$	16,571	$	40,350	$	32,938

Summary of Share-based Compensation Arrangements by Share-based Payment Award

The target prices and vesting tranches are set forth in the table below:

Stock Price Targets		Number of Units Allowed
	$ 109.20		82,940
	$ 145.60		145,145
	$ 182.00		186,615

Restricted Stock Units [Member]

Non-Vested Restricted Stock Activity

The following table summarizes activity of restricted stock units, or RSUs, granted to employees with service-based vesting under the 2017 Inducement Plan and 2015 Plan and related information:

	Number of RSUs			Weighted- Average Grant Date Fair Value
Non-vested units — December 31, 2020		2,210,356		$	57.80
RSUs granted		1,409,417			44.28
RSUs vested		(379,635	)		55.06
RSUs forfeited		(302,565	)		53.34
Non-vested units — June 30, 2021		2,937,573		$	52.13

Market Condition Awards Granted to Employees [Member]

Summary of Activity of Market-Condition Awards

The following table summarizes the Market-Condition RSU Awards granted and forfeited during the six months ended June 30, 2021:

	Number of RSUs			Weighted- Average Grant Date Fair Value		Weighted- Average Remaining Vesting Period (years)		Aggregate Intrinsic Value
Non-vested market-condition awards — December 31, 2020		414,700		$	49.95		1.21
Granted		42,400			13.80		2.23
Forfeited		(42,400	)		49.95		0.71
Non-vested market-condition awards — June 30, 2021		414,700		$	46.26		0.76	$	14,523

Valuation Assumptions of Market-Condition Awards

The following table summarizes the assumptions used to estimate the fair value of the Market-Condition RSU Awards as of the grant date:

Valuation date stock price	$	37.20
Expected term		3.1 years
Volatility		66.6	%
Risk-free interest rate		0.3	%
Dividend yield		—