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Stock-based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

Summary of stock-based compensation

​

​	​	​	​	​	​	​	​	​	​	​	​
​	Three months ended June 30,					​	Six months ended June 30,
​	2019		​	2018		​	2019		​	2018
Research and development expenses	$	514		$	380	​	$	1,081		$	703
Selling, general and administrative expenses		3,648	​		3,146	​		7,344	​		5,551
Total stock-based compensation expense	$	4,162	​	$	3,526	​	$	8,425	​	$	6,254

Summary of restricted stock units activity

​

​	​	​	​	​	​
​	​	​	​	Weighted-Average
​	​	Shares	​	Grant Date Fair Value
Outstanding at December 31, 2018	​	514,603	​	$	20.67
Granted	​	590,205	​	​	15.78
Vested	​	(127,787)	​	​	21.54
Forfeited	​	(84,784)	​	​	18.26
Outstanding at June 30, 2019	​	892,237	​	$	17.54

Summary of stock option activity

​

​	​	​	​	​	​	​	​	​	​	​
​		​		​	​		Weighted-		​	​
​	​	​		​	Weighted-		Average	​	​	​
​	​	​		​	Average		Remaining	​	​	Aggregate
​	​	​	​	​	Exercise Price		Contractual	​	​	Intrinsic
​		Shares		​	per Share		Term (in years)		​	Value
Outstanding at December 31, 2018		3,585,856	​	$	16.20		8.0	​	$	11,170
Granted		897,312	​		15.67	​	​	​	​	​
Exercised		(26,911)	​	​	11.30	​	​	​	​	​
Cancelled		(266,141)	​		17.61	​	​	​	​	​
Outstanding at June 30, 2019		4,190,116	​	$	16.03		7.7	​	$	5,248
Exercisable at June 30, 2019		2,113,346	​	$	14.45		6.6	​	$	4,285

Schedule of fair value assumption using Black-Scholes option-pricing model

​

​	​	​	​	​	​
​	Six months ended June 30,
​	2019		​	2018
Risk-free interest rate	2.6	%	​	2.6	%
Volatility	63.3	%	​	65.0	%
Expected term (years)	6.06	​	​	6.11	​
Expected dividend yield	—	%	​	—	%

Performance share units

Summary of stock option activity

​

​	​	​	​	​	​
​	​	​	​	Weighted-Average
​	​	Shares	​	Grant Date Fair Value
Outstanding at December 31, 2018	​	—	​	$	—
Granted	​	99,400	​	​	15.90
Outstanding at June 30, 2019	​	99,400	​	$	15.90

X

- Definition

Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement.

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Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value.

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