Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

Share-based Payment Arrangement [Abstract]

Schedule of stock-based compensation expense

Stock-based compensation expense is included in the unaudited condensed consolidated statements of operations as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	403	$	567	$	791	$	1,003
Selling, general and administrative	1,585		1,890		3,260		3,434
Total	$	1,988	$	2,457	$	4,051	$	4,437

Schedule of stock-based compensation expense by security types

The following table presents total stock-based compensation expense by security type included in the unaudited condensed consolidated statements of operations for the three and six months ended June 30, 2019 and 2018 (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Stock options	$	581	$	535	$	1,135	$	1,015
RSUs and RSAs	386		436		847		878
PSUs	316		426		707		844
PBOs	705		1,060		1,362		1,700
Total	$	1,988	$	2,457	$	4,051	$	4,437

Valuation assumptions

The weighted-average assumptions used to estimate the fair value of employee stock options and PBOs granted were as follows:

	Three Months Ended June 30,						Six Months Ended June 30,
	2019			2018			2019			2018
				(1)
Expected term (in years)	5.6			—			5.6			5.6
Volatility	55		%	—		%	56		%	60		%
Risk-free interest rate	2.28		%	—		%	2.48		%	2.70		%
Dividend yield	—		%	—		%	—		%	—		%
Weighted-average estimated fair value of stock options granted	$	10.02		$	—		$	11.40		$	5.02

⁽¹⁾ The Company did not grant employee stock options or PBOs in the three months ended June 30, 2018.