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Stock Incentive Plans (Tables)
3 Months Ended
Mar. 31, 2021
Share-based Payment Arrangement [Abstract]  
Schedule of Stock-Based Compensation
Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):
 Three Months Ended March 31,
 20212020
Research and development$754 $623 
General and administrative1,026 786 
     Total stock-based compensation expense$1,780 $1,409 
Schedule of Weighted Average Assumptions Used 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:
Three Months Ended March 31,
 20212020
Risk-free interest rate0.44 %1.73 %
Dividend yield— — 
Expected term5.286.08
Expected volatility83.26 %87.96 %
Weighted-average fair value of common stock$3.13 $2.30 
Summary of Options Activity
The following table summarizes the stock option activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan:
   Weighted-average 
  remainingAggregate
 Number ofWeighted-averagecontractual termintrinsic value
 optionsexercise price ($)(in years)(in thousands)
Employees    
Outstanding at December 31, 20207,302,176 $3.98 8.43$4,456 
Granted2,628,000 $3.13   
Exercised(153,278)$1.47   
Forfeited(62,577)$5.19   
Outstanding at March 31, 20219,714,321 $3.78 8.65$15,665 
Vested at March 31, 20213,064,130 $5.46 7.63$4,313 
Vested and expected to vest at March 31, 20218,920,548 $3.88 8.59$14,231 
Non-employee consultants    
Outstanding at December 31, 2020473,073 $5.89 5.23$86 
Granted— $— 
Exercised— $— 
Forfeited— $— 
Outstanding at March 31, 2021473,073 $5.89 4.98$366 
Vested at March 31, 2021462,186 $5.82 4.92$366 
Vested and expected to vest at March 31, 2021473,073 $5.89 4.98$366 
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award
The following table summarizes the Company’s restricted stock units under the 2016 Plan and 2018 Inducement Incentive Award Plan:
 Number of sharesWeighted average
grant date
fair value ($)
Unvested at December 31, 2020
87,500 $6.03 
Granted468,550 2.99 
Vested(10,937)6.03 
Reserved for issuance98,750 2.99 
Forfeited(800)2.99 
Unvested at March 31, 2021
643,063 $3.35