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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2020
Share-based Payment Arrangement [Abstract]  
Stock Option Valuation The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees, directors and non- employees:
Three Months Ended
March 31,
20202019
Risk-free interest rate1.0 %2.4 %
Expected term (in years)6.015.90
Expected volatility94.1 %90.6 %
Expected dividend yield0 %0 %
Summary of Stock Option Activity
The following table summarizes the Company’s stock option activity for the three months ended March 31, 2020:
Number of
Shares		Weighted
Average
Exercise
Price		Weighted
Average
Contractual
Term (Years)		Aggregate
Intrinsic
Value
Outstanding as of December 31, 20191,297,029  $17.05  8.4$1,286  
Granted206,775  10.22  
Exercised(13,006) 7.38  
Forfeited(22,593) 17.46  
Outstanding as of March 31, 20201,468,205  $16.17  8.4$958  
Exercisable as of March 31, 2020543,825  $21.17  7.1$771  
Vested and expected to vest as of March 31, 20201,270,027  $16.32  8.3$938  
Summary of Stock-Based Compensation Expense Classification Stock-based compensation expense was classified in the consolidated statements of operations as follows:
Three Months Ended
March 31,
20202019
Research and development expense$196  $84  
General and administrative expense417  178  
Total stock-based compensation$613  $262