Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Summary of Assumptions Used for Estimating the Fair Value of Stock Purchase Rights Granted Under 2018 Employee Stock Purchase Plan

The estimated fair value of stock purchase rights granted under the 2018 ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions:

 

 

 

Year ended December 31,

 

 

 

2021

 

 

2020

 

Expected dividend yield

 

 

—

%

 

 

—

%

Expected term

 

0.57 years

 

 

0.48 years

 

Risk-free interest rate

 

 

0.1

%

 

 

0.8

%

Expected volatility

 

 

88.0

%

 

 

81.0

%

 
Schedule of Weighted-Average Assumptions Used in Black-Scholes Option-Pricing Model to Estimate Fair Value of Employee Stock Options at Grant Date

The fair value of each stock option granted to an employee or a director was estimated as of the date of grant using the Black-Scholes model with the following weighted-average assumptions:

 

 

 

Year Ended December 31,

 

 

 

2021

 

 

2020

 

 

2019

 

Expected dividend yield

 

 

—

%

 

 

—

%

 

 

—

%

Expected term

 

6.01 years

 

 

6.01 years

 

 

6.01 years

 

Risk-free interest rate

 

 

1.1

%

 

 

1.1

%

 

 

2.3

%

Expected volatility

 

 

79.0

%

 

 

73.0

%

 

 

83.0

%
Schedule of Stock Options Activity

A summary of the 2015 Plan, 2018 Plan and 2021 Plan activity is as follows:

 

 

 

 

 

 

Options Outstanding

 

 

 

Number of
Shares
Available
for
Issuance

 

 

Number
of Shares

 

 

Weighted-
Average
Exercise Price

 

 

Weighted-
Average
Remaining
Contractual
Term (in years)

 

 

Aggregate
Intrinsic
Value
(in thousands)

 

Balance at December 31, 2020

 

 

3,108,412

 

 

 

4,166,441

 

 

$

8.13

 

 

 

8.15

 

 

$

2,092

 

Authorized

 

 

2,692,507

 

 

 

—

 

 

$

—

 

 

 

 

 

 

 

Granted

 

 

(3,195,124

)

 

 

2,314,524

 

 

$

11.87

 

 

 

 

 

 

 

Exercised

 

 

—

 

 

 

(692,038

)

 

$

4.86

 

 

 

 

 

 

 

Cancelled

 

 

853,392

 

 

 

(681,592

)

 

$

11.50

 

 

 

 

 

 

 

Balance at December 31, 2021

 

 

3,459,187

 

 

 

5,107,335

 

 

$

9.82

 

 

 

8.13

 

 

$

17,153

 

Vested and exercisable – December 31, 2021

 

 

 

 

 

2,253,957

 

 

$

8.70

 

 

 

7.21

 

 

$

9,901

 

Vested and expected to vest – December 31, 2021

 

 

 

 

 

4,741,858

 

 

$

9.74

 

 

 

8.06

 

 

$

16,267

 
Summary of Restricted Stock Unit Activity The following table summarizes our restricted stock unit activity during the year ended December 31, 2021:

 

 

 

Number of
Shares

 

 

Weighted Average Grant Date Fair Value

 

Outstanding, unvested as December 31, 2020

 

 

—

 

 

$

—

 

   Issued

 

 

880,600

 

 

$

5.29

 

   Vested

 

 

—

 

 

$

—

 

   Canceled/Forfeited

 

 

(171,800

)

 

$

5.29

 

Outstanding, unvested as December 31, 2021

 

 

708,800

 

 

$

5.29

 
Schedule of Stock Based Compensation Expense

Total stock-based compensation for all awards granted to employees, consultants and our 2018 ESPP, before taxes, is as follows (in thousands):

 

 

 

Year Ended December 31,

 

 

 

2021

 

 

2020

 

 

2019

 

Research and development expenses

 

$

6,626

 

 

$

4,453

 

 

$

3,437

 

General and administrative expenses

 

 

3,926

 

 

 

2,657

 

 

 

1,862

 

Total

 

$

10,552

 

 

$

7,110

 

 

$

5,299