Stock Incentive Plans
 12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Stock Incentive Plans Stock Incentive Plans
The Company maintains the 2008 Stock Incentive Plan, or the 2008 Plan, for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non-qualified stock option and restricted stock awards as determined by the Board.
In June 2016, the Company’s stockholders approved the 2016 Incentive Award Plan, or the 2016 Plan, which authorized 1,210,256 shares of common stock for future issuance under the 2016 Plan and the Company ceased granting awards under the 2008 Plan. Upon the effective date of the 2016 Plan, awards issued under the 2008 Plan remain subject to the terms of the 2008 Plan. Awards granted under the 2008 Plan that expire, lapse or terminate become available under the 2016 Plan as shares available for future grants.
Additionally, pursuant to the terms of the 2016 Plan, the Board is authorized to grant awards with respect to common stock, and may delegate to a committee of one or more members of the Board or executive officers of the Company the authority to grant options and restricted stock units. On December 9, 2020, the Board established a Stock Option Committee authorized to grant awards to certain employees and consultants subject to conditions and limitations within the 2016 Plan. In January 2021 and 2020, the number of shares of common stock that may be issued under the 2016 Plan was increased by 4,322,850 and 3,453,022 shares, respectively. As of December 31, 2021, 1,925,537 shares remain available for future issuance under the 2016 Plan.
In September 2018, the Company’s 2018 Employment Inducement Incentive Award Plan, or the 2018 Inducement Incentive Award Plan was adopted by the Board without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules, which authorized 1,175,000 shares of its common stock for issuance. In March 2019, the Board approved the amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 2,000,000 shares of the Company’s common stock for issuance thereunder. As of December 31, 2021, there are 1,591,661 shares available for future grant under the 2018 Inducement Incentive Award Plan.
Stock-Based Compensation Expense
Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows (in thousands):
 Year Ended December 31,
 202120202019
Research and development$3,204 $2,271 $2,079 
General and administrative4,516 3,151 3,082 
Total stock-based compensation expense$7,720 $5,422 $5,161 

Stock Options
The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:
Year Ended December 31,
 202120202019
Risk-free interest rate0.79 %1.11 %1.86 %
Dividend yield— — — 
Expected term6.036.045.94
Expected volatility95.04 %91.00 %87.66 %
Weighted-average fair value of common stock$3.58 $2.49 $2.01 

The weighted average grant date fair value of stock options granted to employees during the years ended December 31, 2021, 2020 and 2019 was $2.73, $1.86, and $1.47 respectively.
As of December 31, 2021 and 2020, total unrecognized compensation expense related to unvested employee stock options was $11.5 million and $8.0 million, respectively, which is expected to be recognized over a weighted average period of 2.7 years and 2.3 years, respectively.

The following table summarizes the stock option activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan:
   Weighted-average 
  remainingAggregate
 Number ofWeighted-averagecontractual termintrinsic value
 optionsexercise price ($)(in years)(in thousands)
Employees    
Outstanding at December 31, 20207,302,176 $3.98 8.43$4,456 
Granted4,625,611 $3.58   
Exercised(398,754)$1.72   
Forfeited(912,233)$2.84   
Outstanding at December 31, 202110,616,800 $3.99 8.19$4,982 
Vested at December 31, 20214,459,309 $4.72 7.23$3,149 
Vested and expected to vest at December 31, 20219,917,248 $4.03 8.13$4,796 
Non-employee consultants    
Outstanding at December 31, 2020473,073 $5.89 5.23$86 
Exercised(50,000)$2.04 
Outstanding at December 31, 2021423,073 $6.34 3.85$42 
Vested at December 31, 2021423,073 $6.34 3.85$42 
Vested and expected to vest at December 31, 2021423,073 $6.34 3.85$42 
Restricted Stock Units
During the year ended December 31, 2021, the Company granted 407,700 restricted stock awards with a weighted average fair value of $3.11 per share based on the closing price of the Company’s common stock on the date of grant to employees under the 2016 Plan, which will vest over a four year term. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% for restricted stock awards to employees based on historical attrition trends. In addition, the Company awarded 197,500 restricted stock units to executives under the 2016 Plan. These restricted stock units vested in two equal installments on the dates applicable performance conditions were achieved during the year ended December 31, 2021.
Unrecognized compensation expense for all restricted stock units was $0.9 million as of December 31, 2021, which is expected to be recognized over a weighted average period of 2.7 years.
The following table summarizes the Company’s restricted stock units under the 2016 Plan and 2018 Inducement Incentive Award Plan:
 Number of sharesWeighted average
grant date
fair value ($)
Unvested at December 31, 2020
87,500 $6.03 
Granted605,200 2.10 
Vested(201,250)1.31 
Forfeited(97,000)1.76 
Unvested at December 31, 2021
394,450 $3.45 

Employee Stock Purchase Plan
In June 2016, the Company approved the 2016 Employee Stock Purchase Plan, or the ESPP, which authorized 173,076 shares of common stock for future issuance under the ESPP to participating employees. In January 2021 and 2020, the number of shares of common stock authorized for issuance under the ESPP was increased by 1,080,711 shares and 863,254 shares, respectively. During the year ended December 31, 2021, the Company issued 58,794 shares of common stock under the ESPP. As of December 31, 2021, 2,522,366 shares remain available for future issuance under the ESPP.
For each of the years ended December 31, 2021 and 2020, the Company recognized $0.1 million of stock-based compensation expense under the ESPP, respectively.