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Stock Incentive Plans (Tables)
 6 Months Ended
Jun. 30, 2020
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:
 
Three Months Ended June 30,
 
Six Months Ended June 30,
 
2020
 
2019
 
2020
 
2019
Risk-free interest rate
0.39
%
 
2.18
%
 
1.61
%
 
2.41
%
Dividend yield
—

 
—

 
—

 
—

Expected term
5.56

 
5.87

 
6.03

 
6.03

Expected volatility
89.70
%
 
88.71
%
 
88.12
%
 
87.56
%
Weighted-average fair value of common stock
$
2.92

 
$
2.48

 
$
2.36

 
$
2.42
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, 2019
6,323,596

 
$
4.91

 
8.71
 
$
1,716

Granted
1,878,309

 
$
2.36

 
 
 
 
Exercised
(42,628
)
 
$
2.42

 
 
 
 
Forfeited
(1,158,737
)
 
$
5.84

 
 
 
 
Outstanding at June 30, 2020
7,000,540

 
$
4.09

 
8.69
 
$
3,736

 
 
 
 
 
 
 
 
Vested at June 30, 2020
1,632,355

 
$
7.62

 
7.37
 
$
337

Vested and expected to vest at June 30, 2020
6,472,944

 
$
4.22

 
8.64
 
$
3,381

 
 
 
 
 
 
 
 
Non‑employee awards
 
 
 
 
 
 
 
Outstanding at December 31, 2019
473,073

 
$
5.89

 
6.23
 
$
38

Granted
—

 
$
—

 
 
 
 
Exercised
—

 
$
—

 
 
 
 
Forfeited
—

 
$
—

 
 
 
 
Outstanding at June 30, 2020
473,073

 
$
5.89

 
5.73
 
$
72

 
 
 
 
 
 
 
 
Vested at June 30, 2020
413,191

 
$
5.45

 
5.37
 
$
72

Vested and expected to vest at June 30, 2020
473,073

 
$
5.89

 
5.73
 
$
72
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
grant date
fair value ($)
Unvested at December 31, 2019
181,250

 
$
5.00

Granted
—

 
—

Vested
21,875

 
6.03

Forfeited
—

 
—

Unvested at June 30, 2020
159,375

 
$
5.05
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):
 
Three Months Ended June 30,
 
Six Months Ended June 30,
 
2020
 
2019
 
2020
 
2019
Research and development
$
618

 
$
445

 
$
1,241

 
$
964

General and administrative
863

 
806

 
1,649

 
1,467

     Total stock-based compensation expense
$
1,481

 
$
1,251

 
$
2,890

 
$
2,431
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:
 
Three Months Ended June 30,

Six Months Ended June 30,
 
2020

2019

2020

2019
Risk-free interest rate
—
%
 
2.07
%
 
—
%
 
2.10
%
Dividend yield
—

 
—

 
—

 
—

Expected life (in years)
0

 
5.49

 
0

 
5.50

Expected volatility
—
%
 
88.46
%
 
—
%
 
88.44
%
Weighted-average fair value of common stock
$
—

 
$
2.04

 
$
—

 
$
2.04