Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of weighted-average assumptions estimated at the date of grant using the Black-Scholes option pricing model

	2018				2017				2016
Expected stock price volatility	106.64	-	113.19	%	74.45	-	83.29	%	74.69	-	87.67	%
Expected term of options	5.2	-	6.1	years	5.3	-	6.1	years	5.2	-	6.1	years
Risk‑free interest rate	2.32	-	3.1	%	1.78	-	2.18	%	1.07	-	1.86	%
Expected annual dividend yield			0	%			0	%			0	%

Schedule of unrecognized compensation expense expected to be recognized in future years

That expense is expected to be recognized over the next four years as follows, in thousands:

2019	$	3,413
2020		3,144
2021		1,161
2022		372
	$	8,090

Stock options

Schedule of total compensation cost recognized in the statement of operations

Total compensation cost recognized for all stock option awards in the statements of operations is as follows (in thousands):

	Year Ended December 31,
	2018		2017		2016
Research and development	$	1,712	$	724	$	980
General and administrative		2,995		1,697		1,975
Total	$	4,707	$	2,421	$	2,955

Summary of activity for all options

A summary of activity for the years ended December 31, 2018, 2017 and 2016 is presented below (in thousands, except share and per share amounts):

		Weighted‑
		Average		Aggregate
		Exercise Price		Intrinsic
	Shares	Per Share		Value
Outstanding—December 31, 2015	1,799,226	$	7.74
Granted	603,975		2.42
Exercised	(118,365)		1.04
Forfeited	(45,792)		12.08
Outstanding—December 31, 2016	2,239,044		6.57
Granted	1,884,800		4.02
Exercised	(98,402)		1.46
Forfeited	(141,358)		7.09
Expired	(129,764)		11.75
Outstanding—December 31, 2017	3,754,320		5.22
Granted	1,372,000		6.60
Exercised	(12,308)		1.18
Forfeited	(162,603)		5.00
Outstanding—December 31, 2018	4,951,409	$	5.62	$	2,429
Exercisable—December 31, 2018	2,894,726	$	5.69	$	1,836
Exercisable and expected to vest—December 31, 2018	4,951,409	$	5.62	$	2,429

Restricted stock

Schedule of total compensation cost recognized in the statement of operations

Total compensation cost recognized for all restricted stock awards in the statements of operations for the year ended December 31, 2018 is as follows (in thousands):

	Year Ended
	December 31,
	2018		2017
Research and development	$	19	$	125
General and administrative		62		334
Total	$	81	$	459

Summary of activity for all restricted stock

A summary of activity for the years ended December 31, 2018, 2017 and 2016 is presented below (in thousands, except share and per share amounts):

		Weighted‑average
		Grant Date
	Shares	Fair Value per Share
Outstanding—December 31, 2015	—	$	—
Granted	196,275		1.51
Forfeited	(375)		1.50
Outstanding—December 31, 2016	195,900		1.51
Granted	245,200		1.26
Vested	(179,700)		1.12
Forfeited	(21,600)		0.17
Outstanding—December 31, 2017	239,800		1.21
Vested	(120,600)		1.21
Forfeited	(14,000)		1.21
Outstanding—December 31, 2018	105,200	$	1.21
Expected to vest—December 31, 2018	105,200	$	1.21