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Stockholders' Equity
 9 Months Ended
Sep. 30, 2021
Equity [Abstract]  
Stockholders' Equity

10. Stockholders’ Equity

Public Offering Warrants

In June 2014, as a component of our public offering, we sold 600,000 pre-funded warrants to purchase shares of our common stock. The pre-funded warrants have an exercise price of $0.01 per share and expired on June 30, 2021. In April 2021, warrant holders exercised 195,574 warrants using the cashless exercise provision, which resulted in the issuance of 195,461 shares and no cash proceeds. As of September 30, 2021, no warrants from the June 2014 public offering remain outstanding.

Option Plan Activity

The following table summarizes the stock option activity for the nine months ended September 30, 2021:

 

 

 

Shares

(in thousands)

 

 

Weighted-

average

Exercise Price

 

 

Weighted-

average

Remaining

Contractual

Term (Years)

 

Outstanding at December 31, 2020

 

 

18,912

 

 

$

19.59

 

 

 

7.21

 

Granted

 

 

885

 

 

$

14.96

 

 

 

 

 

Exercised

 

 

(370

)

 

$

15.29

 

 

 

 

 

Expired and forfeited

 

 

(1,170

)

 

$

22.15

 

 

 

 

 

Outstanding at September 30, 2021

 

 

18,257

 

 

$

19.29

 

 

 

6.53

 

 

The following table summarizes the restricted stock unit activity for the nine months ended September 30, 2021:

 

 

 

Shares

(in thousands)

 

 

Weighted Average Grant Date Fair Value

 

Outstanding at December 31, 2020

 

 

603

 

 

$

15.84

 

Granted

 

 

193

 

 

$

14.73

 

Released

 

 

(115

)

 

$

15.99

 

Forfeited

 

 

(40

)

 

$

15.84

 

Outstanding at September 30, 2021

 

 

641

 

 

$

15.48

 

 

 

For the nine months ended September 30, 2021, 369,978 shares of common stock were issued pursuant to the exercise of stock options, resulting in net proceeds of $5.7 million.

Stock-based Compensation

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

September 30,

 

 

Nine Months Ended

September 30,

 

 

 

2021

 

 

2020

 

 

2021

 

 

2020

 

Research and development

 

$

4,521

 

 

$

4,612

 

 

$

14,130

 

 

$

14,580

 

General and administrative

 

 

2,836

 

 

 

3,277

 

 

 

8,414

 

 

 

10,012

 

Sales and marketing

 

 

3,879

 

 

 

3,206

 

 

 

11,419

 

 

 

9,591

 

Total stock-based compensation expense

 

$

11,236

 

 

$

11,095

 

 

$

33,963

 

 

$

34,183

 

 

 

As of September 30, 2021, there was $86.0 million of total unrecognized compensation cost related to non-vested, stock-based payment awards granted under all of our equity compensation plans and all non-plan option grants. Total unrecognized compensation cost will be adjusted for future changes in estimated forfeitures. We expect to recognize this compensation cost over a weighted-average period of 2.5 years.

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.

We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model with the following weighted-average assumptions:

 

 

Nine Months Ended

 

 

 

September 30,

 

 

 

2021

 

 

2020

 

Risk-free interest rate

 

 

1.0

%

 

 

0.9

%

Dividend yield

 

 

0.0

%

 

 

0.0

%

Volatility

 

 

65.8

%

 

 

67.3

%

Expected life (years)

 

 

6

 

 

 

6

 

 

We estimate 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. There were no new offering periods during the three months ended September 30, 2021 and 2020.