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Stock Incentive Plans (Tables)
 12 Months Ended
Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Schedule of Weighted Average Assumptions Used The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:
 
Year Ended December 31,
 
2019
 
2018
 
2017
Risk-free interest rate
1.86
%
 
2.85
%
 
2.03
%
Dividend yield
—

 
—

 
—

Expected term
5.94

 
6.06

 
5.90

Expected volatility
87.66
%
 
85.17
%
 
84.52
%
Weighted-average fair value of common stock
$
2.01

 
$
8.45

 
$
15.32
Summary of Activity Under the 2008 Plan and the 2016 Plan
The following table summarizes the activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan:
 
 
 
 
 
Weighted‑average
 
 
 
 
 
 
 
remaining
 
Aggregate
 
Number of
 
Weighted-average
 
contractual term
 
intrinsic value
 
options
 
exercise price ($)
 
(in years)
 
(in thousands)
Employee awards
 
 
 
 
 
 
 
Outstanding at December 31, 2018
3,681,575

 
$
9.49

 
7.77
 
$
300

Granted
3,876,255

 
$
2.01

 
 
 
 
Exercised
(125,600
)
 
$
1.20

 
 
 
 
Forfeited
(1,108,634
)
 
$
10.38

 
 
 
 
Outstanding at December 31, 2019
6,323,596

 
$
4.91

 
8.71
 
$
1,716

 
 
 
 
 
 
 
 
Vested at December 31, 2019
1,485,228

 
$
10.02

 
6.73
 
$
90

Vested and expected to vest at December 31, 2019
5,843,053

 
$
5.10

 
8.65
 
$
1,500

 
 
 
 
 
 
 
 
Non‑employee awards
 
 
 
 
 
 
 
Outstanding at December 31, 2018
412,404

 
$
6.44

 
6.42
 
$
28

Granted
73,489

 
$
1.86

 
 
 
 
Exercised
—

 
$
—

 
 
 
 
Forfeited
(12,820
)
 
$
0.47

 
 
 
 
Outstanding at December 31, 2019
473,073

 
$
5.89

 
6.23
 
$
38

 
 
 
 
 
 
 
 
Vested at December 31, 2019
330,528

 
$
5.61

 
5.06
 
$
21

Vested and expected to vest at December 31, 2019
473,073

 
$
5.89

 
6.23
 
$
38
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award
The following table summarizes the status of the Company’s restricted stock units:
 
Number of shares
 
Weighted average fair value ($)
Unvested at December 31, 2018
175,000

 
$
6.03

Granted
100,000

 
1.97

Vested
93,750

 
1.65

Forfeited
—

 
—

Unvested at December 31, 2019
181,250

 
$
5.00
Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock
The Company recorded stock-based compensation expense related to stock option awards, restricted stock units and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):
 
Year Ended December 31,
 
2019

2018

2017
Research and development
$
2,079

 
$
2,453

 
$
1,779

General and administrative
3,082

 
3,261

 
2,302

     Total stock-based compensation expense
$
5,161

 
$
5,714

 
$
4,081
Non-employee stock options  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Schedule of Weighted Average Assumptions Used
The estimated grant date fair values of non-employee stock option awards granted under the 2016 Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:
 
Year Ended December 31,
 
2019
 
2018
 
2017
Risk-free interest rate
1.92
%
 
2.77
%
 
—
%
Dividend yield
—

 
—

 
—

Expected life (in years)
5.33

 
5.81

 
0.00

Expected volatility
88.60
%
 
85.86
%
 
—
%