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Stock Based Compensation (Tables)
6 Months Ended
Jun. 30, 2022
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Summary of Option Activity Under Company's Equity Incentive Plans

The following table summarizes stock option activity under the Company’s 2018 Plan and related information:

 

 

 

Number of Shares

Underlying

Outstanding

Options

 

 

Weighted-

Average Exercise

Price

 

 

Weighted-

Average

Remaining

Contractual Term

(Years)

 

Outstanding — December 31, 2021

 

 

2,603,630

 

 

$

7.70

 

 

 

7.46

 

Options granted

 

 

1,314,200

 

 

$

0.72

 

 

 

 

 

Options forfeited

 

 

(1,173,227

)

 

$

4.19

 

 

 

 

 

Options expired

 

 

(47,192

)

 

$

12.84

 

 

 

 

 

Outstanding — June 30, 2022

 

 

2,697,411

 

 

$

5.64

 

 

 

7.85

 

Exercisable — June 30, 2022

 

 

1,375,625

 

 

$

9.01

 

 

 

 

 

Summary of Weighted Average Valuation Assumptions Used to Estimate Fair Value of Employee Stock Options The fair value of employee stock options was estimated using the following weighted-average assumptions:

 

 

Three Months Ended June 30,

 

 

Six Months Ended June 30,

 

 

 

2022

 

 

2021

 

 

2022

 

 

2021

 

Employee Stock Options:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Risk-free rate

 

 

2.93

%

 

 

0.96

%

 

 

2.22

%

 

 

0.74

%

Expected term (in years)

 

 

6.0

 

 

 

5.7

 

 

 

6.0

 

 

 

6.0

 

Dividend yield

 

 

 

 

 

 

 

 

Volatility

 

 

91.82

%

 

 

92.79

%

 

 

91.63

%

 

 

93.64

%

Weighted-average fair value of stock options granted

 

$

0.87

 

 

$

3.37

 

 

$

0.55

 

 

$

4.45

 

Summary of Stock-Based Compensation Expense Recognized

Total stock-based compensation expense recognized was as follows (in thousands):

 

 

 

Three Months Ended June 30,

 

 

Six Months Ended June 30,

 

 

 

2022

 

 

2021

 

 

2022

 

 

2021

 

Research and development

 

$

83

 

 

$

394

 

 

$

211

 

 

$

763

 

General and administrative(1)

 

 

263

 

 

 

589

 

 

 

650

 

 

 

1,246

 

Total stock-based compensation expense

 

$

346

 

 

$

983

 

 

$

861

 

 

$

2,009

 

 

 

(1)

Included in general and administrative stock-based compensation for the six months ended June 30, 2022 is stock-based compensation expense related to 32,684 shares of common stock issued to certain board members in lieu of their cash compensation. No shares of common stock were issued to board members for the three months ended June 30, 2022.