Stock-based Compensation (Tables)
 12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Stock-based Compensation Expense Related to Stock Options The following table summarizes stock-based compensation expense related to stock options, restricted stock awards (RSAs) and RSUs granted to employees, non-employee directors and consultants, and the 2019 ESPP (as defined below) included in the consolidated statements of operations as follows (in thousands):
Years Ended December 31,
20212020
Research and development$1,357 $1,922 
Selling and marketing1,870 2,388 
General and administrative5,671 8,387 
Total$8,898 $12,697 
Summary of Stock Option Information
The following table summarizes share option activity for the year ended December 31, 2021:
OptionsWeighted Average Exercise PriceWeighted Average Remaining Contractual Term (in Years)Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 20208,935,801 $6.08 7.76$338 
     Granted3,208,525 $2.77 
     Exercised— $— 
     Cancelled(1,519,264)$4.30 
Outstanding as of December 31, 202110,625,062 $5.34 6.80$— 
Options vested and expected to vest as of December 31, 202110,625,062 $5.34 6.80$— 
Options exercisable as of December 31, 20217,384,269 $6.26 5.93$— 

The following table summarizes certain information regarding stock options for the years ended December 31, 2021 and 2020 (in thousands, except per share data):
20212020
Weighted average grant date fair value per share of options granted during the period$2.18 $3.20 
Fair value per share of options vested during the period$3.15 $3.51 
Cash received from options exercised during the period$— $83 
Intrinsic value of options exercised during the period$— $47 
Weighted-Average Assumptions Rate The fair value of noncash stock-based compensation for stock options granted to employees and non-employees was estimated on the date of grant using the Black-Scholes option pricing model based on the following weighted-average assumptions for options granted for the periods indicated.
Years Ended December 31,
20212020
Expected volatility101.1 %82.7 %
Risk-free interest rate0.7 %0.6 %
Expected dividend yield— %— %
Expected term (years)5.96.0
The fair value of shares to be issued to employees under the 2019 ESPP is estimated using a Black-Scholes option-pricing model at the grant date, which requires the use of subjective and complex assumptions, including (i) the expected stock price volatility, (ii) the calculation of the expected term of the award, (iii) the risk-free interest rate and (iv) the expected dividend yield. The following weighted average assumptions were used in the calculation of fair value of shares under the 2019 ESPP at the grant dates for the period indicated.
Year Ended December 31, 2021Year Ended December 31, 2020
Expected volatility83.9 %94.8 %
Risk-free interest rate0.1 %0.1 %
Expected dividend yield— %— %
Expected term (years)0.50.5
Summary of Restricted Stock Awards and Restricted Stock Units
The following table summarizes RSAs activity for the year ended December 31, 2021:
Shares (RSAs)Weighted Average Fair Value per Share
Unvested as of December 31, 202080,000 $4.78 
      Granted1,777,500 $3.25 
      Forfeited(1,073,833)$3.27 
      Released(783,667)$3.36 
Unvested as of December 31, 2021— $—