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Equity Incentive Plans and Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Summary of Restricted Stock Activity

The following table summarizes restricted stock activity during the nine months ended September 30, 2020:

 

  

 

Number

of

Shares

 

 

Weighted-

Average

Grant Date

fair value

 

Outstanding and unvested, as of December 31, 2019

 

 

302,211

 

 

$

1.51

 

Issued

 

 

—

 

 

$

—

 

Vested

 

 

(130,257

)

 

$

1.24

 

Repurchases

 

 

(12,121

)

 

$

1.22

 

Outstanding and unvested, as of September 30, 2020

 

 

159,833

 

 

$

1.79

 

 

The following table summarizes restricted stock activity during the nine months ended September 30, 2019:

 

  

 

Number

of

Shares

 

 

Weighted-

Average

Grant Date

fair value

 

Outstanding and unvested, as of December 31, 2018

 

 

703,188

 

 

$

1.22

 

Issued

 

 

—

 

 

$

—

 

Vested

 

 

(335,610

)

 

$

1.01

 

Exercised

 

 

363

 

 

$

2.15

 

Repurchases

 

 

(7,051

)

 

$

2.01

 

Outstanding and unvested, as of September 30, 2019

 

 

360,890

 

 

$

1.43

 
Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for Stock Option Awards

The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for stock option awards with the following assumptions:

 

  

 

Three Months Ended September 30,

 

 

Nine Months Ended September 30,

 

 

 

2020

 

 

2019

 

 

2020

 

 

2019

 

Expected volatility

 

72.39% - 74.79%

 

 

81.10%

 

 

72.10% - 77.50%

 

 

78.50% - 82.55%

 

Risk-free interest rate

 

0.27% - 0.38%

 

 

1.43%

 

 

0.27% - 0.82%

 

 

1.43% - 2.59%

 

Expected dividend

 

 

—

 

 

 

—

 

 

 

—

 

 

 

—

 

Expected term (in years)

 

5.26 - 6.08

 

 

5.83 - 6.06

 

 

5.26 - 6.75

 

 

5.00 - 6.08

 

Underlying common stock fair value

 

21.54 - 24.64

 

 

 

7.08

 

 

6.22 - 24.64

 

 

5.15 - 7.08

 

 

 
Summary of Option Activity Under the 2015 Plan and 2020 Plan

A summary of option activity under the 2015 Plan and the 2020 Plan is as follows:

 

  

 

Number

of

Options

 

 

Weighted-

Average Exercise

Price per Share

 

 

Weighted-

Average

Remaining

Contractual

Term (in Years)

 

 

Aggregate

Intrinsic

Value

 

Outstanding as of December 31, 2019

 

 

1,337,501

 

 

$

3.22

 

 

 

9.18

 

 

$

5,157

 

Granted

 

 

1,567,198

 

 

$

9.26

 

 

 

 

 

 

 

 

 

Exercised

 

 

(82,454

)

 

$

2.49

 

 

 

 

 

 

 

 

 

Forfeited

 

 

(13,768

)

 

$

5.67

 

 

 

 

 

 

 

 

 

Outstanding as of September 30, 2020

 

 

2,808,477

 

 

$

6.60

 

 

 

9.05

 

 

$

45,033

 

Exercisable as of September 30, 2020

 

 

629,988

 

 

$

4.02

 

 

 

8.60

 

 

$

11,750

 

Vested and expected to vest as of September 30, 2020

 

 

2,808,477

 

 

$

6.60

 

 

 

9.05

 

 

$

45,033

 

 

  

 

Number

of

Options

 

 

Weighted-

Average Exercise

Price per Share

 

 

Weighted-

Average

Remaining

Contractual

Term (in Years)

 

 

Aggregate

Intrinsic

Value

 

Outstanding as of December 31, 2018

 

 

113,174

 

 

$

2.08

 

 

 

9.77

 

 

$

348

 

Granted

 

 

1,220,104

 

 

$

3.08

 

 

 

 

 

 

 

 

 

Exercised

 

 

(28,714

)

 

$

5.08

 

 

 

 

 

 

 

 

 

Forfeited

 

 

(4,458

)

 

$

2.08

 

 

 

 

 

 

 

 

 

Outstanding as of September 30, 2019

 

 

1,300,106

 

 

$

2.94

 

 

 

9.37

 

 

$

5,385

 

Exercisable as of September 30, 2019

 

 

118,835

 

 

$

2.15

 

 

 

9.30

 

 

$

585

 

Vested and expected to vest as of September 30, 2019

 

 

1,300,106

 

 

$

2.94

 

 

 

9.37

 

 

$

5,385

 
Summary of Components and Classification of Stock-based Compensation Expense for Stock-based Awards

The following table presents the components and classification of stock-based compensation expense for the Company’s stock-based awards for the three and nine months ended September 30, 2020 and 2019 (in thousands):

 

  

 

Three Months Ended

September 30,

 

 

Nine Months Ended

September 30,

 

 

 

2020

 

 

2019

 

 

2020

 

 

2019

 

Restricted stock awards and founders’ common stock awards

 

$

51

 

 

$

118

 

 

$

160

 

 

$

262

 

Stock options and ESPP

 

 

1,118

 

 

 

362

 

 

 

2,220

 

 

 

1,036

 

Total stock-based compensation expense

 

$

1,169

 

 

$

480

 

 

$

2,380

 

 

$

1,298

 

Research and development expenses

 

$

564

 

 

$

198

 

 

$

1,015

 

 

$

423

 

General and administrative expenses

 

$

605

 

 

$

282

 

 

$

1,365

 

 

$

875

 
Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for 2020 Employee Stock Purchase Plan The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for the 2020 ESPP with the following assumptions:

 

 

 

Three Months Ended

September 30,

 

 

Nine Months Ended

September 30,

 

 

 

2020

 

 

2019

 

 

2020

 

 

2019

 

Risk-free interest rate

 

 

0.11

%

 

 

—

 

 

 

0.11

%

 

 

—

 

Expected term of options (in years)

 

 

0.58

 

 

 

—

 

 

 

0.58

 

 

 

—

 

Expected stock price volatility

 

 

83.72

%

 

 

—

 

 

 

83.72

%

 

 

—

 

Expected dividends

 

 

—

 

 

 

—

 

 

 

—

 

 

 

—