Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2019

Stockholders' Equity

Schedule of share-based compensation expense recognized

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

	Year Ended December 31,
	2019		2018		2017
Research and development	$	825	$	1,077	$	1,954
General and administrative		2,407		3,290		4,433
Total stock-based compensation	$	3,232	$	4,367	$	6,387

Summary of stock option activity

	Options Outstanding
				Weighted
				Average
		Weighted		Remaining
		Average		Contractual
	Number of	Exercise		Term
	Shares	Price		(in years)
Balance, December 31, 2017	8,624,223	$	5.22	7.17
Granted	4,065,375		1.70
Exercised	(132,952)		0.63
Forfeited/Cancelled	(4,291,439)		(4.38)
Balance, December 31, 2018	8,265,207	$	3.99	6.99
Granted	1,033,000		1.10
Exercised	(41,968)		0.68
Forfeited/Cancelled	(1,687,935)		(4.94)
Balance, December 31, 2019	7,568,304	$	3.40	7.01
Vested or expected to vest at December 31, 2019	7,568,304	$	3.40	7.01
Exercisable at December 31, 2019	4,259,251	$	4.24	5.92

Schedule of weighted-average assumptions:

	Year Ended December 31,
	2019		2018		2017
Expected term of options (in years)	6.1		5.8		6.2
Risk-free interest rate	1.9	%	2.8	%	2.0	%
Expected volatility	98.9	%	71.7	%	75.6	%
Dividend yield	—	%	—	%	—	%

Schedule of changes in the status of non-vested RSU

		Weighted
		Average
	Number of	Grant Date
	Awards	Fair Value
Non-vested at December 31, 2017	—	$	—
Granted	1,255,000		0.65
Exercised	—		—
Forfeited/Cancelled	—		—
Non-vested at December 31, 2018	1,255,000	$	0.65
Granted	2,500,585		0.74
Vested	(572,500)		0.65
Forfeited	(237,500)		0.65
Non-vested at December 31, 2019	2,945,585	$	0.73

Schedule of shares available to be granted under equity incentive plans

At December 31, 2019, the Company has the following shares available to be granted:

		Inducement
	2013 Plan	Plan
Available at December 31, 2018	2,603,983	144,125
Authorized	3,459,343	—
Granted	(3,533,585)	—
Forfeited/Cancelled	1,864,560	60,875
Available at December 31, 2019	4,394,301	205,000

Schedule of shares of common stock reserved/available

At December 31, 2019, the Company has reserved the following shares of common stock for issuance:

Stock options outstanding under 2013 Plan	7,273,304
Restricted stock units outstanding under 2013 Plan	2,945,585
Shares reserved for future issuance under 2013 Plan	4,394,301
Stock options outstanding under Inducement Plan	295,000
Shares reserved for future issuance under Inducement Plan	205,000
Shares reserved for future issuance under 2013 Employee Stock Purchase Plan	225,806
Warrants outstanding	623,091
Total shares of common stock reserved for future issuance	15,962,087