Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Stockholders' Equity Note [Abstract]

Schedule of share-based compensation expense recognized

The estimated grant‑date fair value of the Company’s share‑based awards is amortized ratably over the awards’ service periods. Share‑based compensation expense recognized was as follows (in thousands):

	Year Ended December 31,
	2017		2016		2015
Research and development	$	1,954	$	3,511	$	1,460
General and administrative	4,433		2,392		1,967
Total stock-based compensation	$	6,387	$	5,903	$	3,427

Summary of stock option activity

A summary of stock option activity and related information through December 31, 2017 follows:

	Options Outstanding
	Number of Shares		Weighted Average Exercise Price			Weighted Average Remaining Contractual Term (in years)
Balance, December 31, 2015	4,630,073		$	4.98		7.87
Granted	2,067,500		8.43
Exercised	(149,622	)	1.71
Forfeitures	(177,373	)	(7.47		)
Balance, December 31, 2016	6,370,578		$	6.10		7.60
Granted	4,187,344		3.96
Exercised	(293,809	)	1.23
Forfeited/canceled	(1,639,890	)	(6.18		)
Balance, December 31, 2017	8,624,223		$	5.22		7.17
Vested or expected to vest at December 31, 2017	8,624,223		$	5.22		7.17
Exercisable at December 31, 2017	3,782,652		$	5.18		5.04

Schedule of weighted-average assumptions:

The per-share weighted-average grant date fair value of the options granted to employees and directors during the year ended December 31, 2017, 2016 and 2015 was estimated at $2.68, $5.26 and $4.49 per share, respectively, on the date of grant using the Black-Scholes option-pricing model with the following weighted-average assumptions:

	Year Ended
	December 31,
	2017		2016		2015
Expected term of options (in years)	6.2		6.2		6.2
Risk-free interest rate	2.0	%	1.5	%	1.7	%
Expected volatility	75.6	%	68.6	%	68.5	%
Dividend yield	—	%	—	%	—	%

Schedule of shares of common stock reserved/available

At December 31, 2017, the Company has reserved the following shares of common stock for issuance:

Stock options outstanding under 2013 Plan	8,417,223
Shares available for future grant under 2013 Plan	991,613
Stock options outstanding under Inducement Plan	207,000
Shares available for future grant under Inducement Plan	293,000
Employee stock purchase plan	225,806
Warrants outstanding	123,091
Total shares of common stock reserved for future issuance	10,257,733

At December 31, 2017, the Company has the following shares available to be granted:

	2013 Plan		Inducement Plan
Available at December 31, 2016	1,101,331		500,000
Authorized	2,230,736		—
Granted	(3,966,844	)	(220,500	)
Forfeited/canceled	1,626,390		13,500
Available at December 31, 2017	991,613		293,000