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Stock-based Compensation (Tables)
12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Summary of Stock Options Activity

The following table summarizes the stock options activity for the year ended December 31, 2021:

 

 

 

 

 

 

Options Outstanding

 

 

 

 

 

 

 

 

 

Number of
Shares
Available
for Grant

 

 

Number of
Shares
Underlying
Outstanding
Options

 

 

Weighted-
Average
Exercise
Price

 

 

Weighted-
Average
Remaining
Contractual
Term
(in years)

 

 

Aggregate
Intrinsic
Value
(in thousands)

 

Balances at December 31, 2020

 

 

2,606,546

 

 

 

3,194,113

 

 

$

11.05

 

 

 

8.46

 

 

$

97,105

 

Options authorized

 

 

1,334,099

 

 

 

 

 

 

 

 

 

 

 

 

 

Options granted

 

 

(1,566,150

)

 

 

1,566,150

 

 

 

34.83

 

 

 

 

 

 

 

Options exercised

 

 

 

 

 

(563,387

)

 

 

6.26

 

 

 

 

 

 

 

Options expired

 

 

4,324

 

 

 

(4,324

)

 

 

36.33

 

 

 

 

 

 

 

Options forfeited

 

 

311,278

 

 

 

(311,278

)

 

 

29.17

 

 

 

 

 

 

 

Balances at December 31, 2021

 

 

2,690,097

 

 

 

3,881,274

 

 

$

19.86

 

 

$

7.97

 

 

$

24,666

 

Shares exercisable, December 31,
   2021

 

 

 

 

 

1,592,139

 

 

$

12.75

 

 

$

7.07

 

 

$

17,280

 

Shares vested and expected to
   vest, December 31, 2021

 

 

 

 

 

3,881,274

 

 

$

19.86

 

 

$

7.97

 

 

$

24,666

 

 

Summary of Stock Compensation Expense for Employees and Nonemployees by Function

The following table is a summary of stock compensation expense for employees and nonemployees by function (in thousands):

 

 

 

Year Ended December 31,

 

 

 

 

2021

 

 

2020

 

 

Research and development

 

$

6,847

 

 

$

2,670

 

 

General and administrative

 

 

6,952

 

 

 

2,314

 

 

Total stock-based compensation expense

 

$

13,799

 

 

$

4,984

 

 

Schedule of Assumptions Used in Black-Scholes Valuation Model to Estimate Fair Value of Stock Options

The Company estimates the fair value of employee and nonemployee stock options using the Black-Scholes option pricing model. The fair value of employee and nonemployee options is recognized on a straight-line basis over the requisite service period of the awards. The fair value of the Company's stock options was estimated using the following assumptions for the years ended December 31, 2021 and 2020.
 

 

 

Year Ended December 31, 2021

 

Year Ended December 31, 2020

 

 

Employee

 

 

Nonemployee

 

Employee

 

 

Nonemployee

Expected term

 

5.8 – 6.3 years

 

 

6.3 years

 

5.5 6.3 years

 

 

6.0 years

Expected volatility

 

80.0% – 84.3%

 

 

83.5%

 

82.1% – 83.8%

 

 

82.1% – 83.1%

Risk-free interest rate

 

0.6% – 1.3%

 

 

0.6%

 

0.4% – 0.7%

 

 

0.4% – 0.7%

Expected dividend yield

 

0%

 

 

0%

 

0%

 

 

0%