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Equity Incentive Plan (Tables)
 6 Months Ended
Jun. 30, 2024
Equity [Abstract]  
Summary of Stock Option Activity

The following table summarizes the stock option activity for the six months ended June 30, 2024:

 

 

 

Shares
(in thousands)

 

 

Weighted-
Average
Exercise Price

 

 

Weighted-
Average
Remaining
Contractual
Term (Years)

 

Outstanding at December 31, 2023

 

 

24,575

 

 

$

7.06

 

 

 

5.60

 

Granted

 

 

6,826

 

 

$

2.23

 

 

 

 

Exercised

 

 

(372

)

 

$

2.46

 

 

 

 

Expired and forfeited

 

 

(3,584

)

 

$

11.17

 

 

 

 

Outstanding at June 30, 2024

 

 

27,445

 

 

$

5.38

 

 

 

7.74

 

 
Summary of Fair Value of Option Grant on Grant Date and Employee Stock Purchase Plan Using Black-Scholes Option Pricing Model and Market Based Stock Option Grants Monte Carlo Simulation Model

We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model and for market-based stock option grants using the Monte Carlo simulation model. The following are the weighted-average assumptions:

 

 

 

For the Six Months Ended

 

 

 

June 30,

 

 

 

2024

 

 

2023

 

Risk-free interest rate

 

 

4.2

%

 

 

3.6

%

Dividend yield

 

 

0.0

%

 

 

0.0

%

Volatility

 

 

74.9

%

 

 

67.4

%

Expected life (years)

 

6 to 10

 

 

6 to 10

 

 

We estimated the fair value of each purchase right granted under our 1997 Employee Stock Purchase Plan, as amended, at the beginning of each new offering period using the Black-Scholes option pricing model. The following are the weighted average assumptions:

 

 

 

For the Six Months Ended

 

 

 

June 30,

 

 

 

2024

 

 

2023

 

Risk-free interest rate

 

 

5.4

%

 

 

5.1

%

Dividend yield

 

 

0.0

%

 

 

0.0

%

Volatility

 

 

83.6

%

 

 

97.1

%

Expected life (months)

 

 

6

 

 

 

6

 
Summary of Restricted Stock Unit Activity

The following table summarizes the restricted stock unit activity (“RSUs”) for the six months ended June 30, 2024:

 

 

 

Shares
(in thousands)

 

 

Weighted-Average Grant Date Fair Value

 

Outstanding at December 31, 2023

 

 

1,405

 

 

$

4.43

 

Granted

 

 

1,482

 

 

$

2.30

 

Released

 

 

(829

)

 

$

2.63

 

Expired and forfeited

 

 

(415

)

 

$

3.67

 

Outstanding at June 30, 2024

 

 

1,642

 

 

$

2.97

 

 

The fair value of RSUs is estimated based on the closing market price of our common stock on the date of the grant. RSUs generally vest quarterly over a four-year period.
Summary of Stock-Based Compensation Expense related to Stock-Based Payment Awards Granted

The following table summarizes stock-based compensation expense related to stock-based payment awards granted pursuant to all of our equity compensation arrangements (in thousands):

 

 

 

Three Months Ended
June 30,

 

 

Six Months Ended
June 30,

 

 

 

2024

 

 

2023

 

 

2024

 

 

2023

 

Research and development

 

$

553

 

 

$

2,694

 

 

$

1,254

 

 

$

4,430

 

General and administrative

 

 

2,060

 

 

 

6,925

 

 

 

3,938

 

 

 

10,457

 

Sales and marketing

 

 

1,957

 

 

 

4,281

 

 

 

2,753

 

 

 

6,960

 

Total stock-based compensation expense

 

$

4,570

 

 

$

13,900

 

 

$

7,945

 

 

$

21,847