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Stockholders' Equity Common Stock (Tables)
 12 Months Ended
Dec. 31, 2019
Equity [Abstract]  
Summary of Fair Value of Stock Option

The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis:

 

 

 

Year Ended December 31,

 

 

 

2019

 

 

2018

 

 

2017

 

Risk-free interest rate

 

 

2.64

%

 

 

2.39

%

 

 

2.20

%

Expected term (in years)

 

 

6.0

 

 

 

6.0

 

 

 

6.0

 

Expected volatility

 

 

88.4

%

 

 

76.0

%

 

 

80.9

%

Expected dividend yield

 

 

0

%

 

 

0

%

 

 

0

%
Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2019:

 

 

 

Number of

Shares

 

 

Weighted

Average

Exercise

Price

 

 

Weighted

Average

Remaining

Contractual

Term

 

 

Aggregate

Intrinsic

Value

 

 

 

 

 

 

 

 

 

 

 

(in years)

 

 

 

 

 

Outstanding as of December 31, 2018

 

 

7,561,719

 

 

$

12.26

 

 

 

7.23

 

 

$

4,958

 

Granted

 

 

2,456,850

 

 

 

5.79

 

 

 

 

 

 

 

 

 

Exercised

 

 

(90,125

)

 

 

1.61

 

 

 

 

 

 

 

 

 

Forfeited

 

 

(1,617,761

)

 

 

12.82

 

 

 

 

 

 

 

 

 

Outstanding as of December 31, 2019

 

 

8,310,683

 

 

$

10.36

 

 

 

7.01

 

 

$

3,427

 

Options vested and expected to vest as of December 31, 2019

 

 

8,310,683

 

 

$

10.36

 

 

 

7.01

 

 

$

3,427

 

Options exercisable as of December 31, 2019

 

 

4,815,001

 

 

$

12.47

 

 

 

5.78

 

 

$

3,296

 
Summary of Restricted Stock Activity The table below summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2019:

 

 

 

Number

of Shares

 

 

Weighted

Average Grant

Date Fair Value

 

Unvested restricted stock units as of December 31, 2018

 

 

226,900

 

 

$

9.64

 

Granted

 

 

20,000

 

 

$

2.69

 

Forfeited

 

 

(22,500

)

 

$

8.77

 

Vested

 

 

(94,400

)

 

$

9.46

 

Unvested restricted stock units as of December 31, 2019

 

 

130,000

 

 

$

8.86

 
Summary of Stock Based Compensation Expense

The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its consolidated statements of operations and comprehensive loss:

 

 

 

Year Ended December 31,

 

 

 

2019

 

 

2018

 

 

2017

 

Research and development expenses

 

$

4,613

 

 

$

8,388

 

 

$

8,115

 

General and administrative expenses

 

 

3,731

 

 

 

8,253

 

 

 

9,247

 

 

 

$

8,344

 

 

$

16,641

 

 

$

17,362