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Stock Plans and Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Summary of Stock Option Activity
A summary of stock option activity under the 2010 Plan and inducement awards are summarized as follows:
Number of
Options		Weighted
Average
Exercise
Price per
Share		Weighted
Average
Remaining Contractual Life		Aggregate Intrinsic Value
(000's)
Outstanding, December 31, 2020
8,668,005 $2.71 7.98
Granted2,006,100 9.07 
Exercised(98,886)1.67 
Canceled(211,450)10.09 
Outstanding, December 31, 2021
10,363,769 $3.80 7.41$24,749 
Exercisable at December 31, 2021
6,127,217 $3.24 6.77$16,572 
Vested and unvested expected to vest10,083,019 $3.76 7.37$24,287 
The weighted average grant date fair values of stock options granted during the years ended December 31, 2021, 2020, and 2019 were $7.25, $0.84, and $0.85, respectively, and were calculated using the following estimated assumptions under the Black-Scholes option pricing model:
For the Year Ended
December 31,
202120202019
Expected term (years)5.55.55.5
Risk-free interest rate
0.4-1.4%
0.4-1.7%
1.5-2.6%
Expected volatility
107-111%
80-81%
76-79%
Expected dividend yieldNoneNoneNone
Nonvested Restricted Stock Shares Activity The following table presents a summary of outstanding RSAs under the 2010 Plan as of December 31, 2021:
Number of
Shares		Weighted
Average
Grant Date Fair Value
Unvested, December 31, 2020
20,624 $3.45 
Awarded— — 
Vested(10,312)3.45 
Forfeited— — 
Unvested, December 31, 2021
10,312 $3.45 
Summary of Compensation Expense Related to ESPP For the years ended December 31, 2021, 2020, and 2019, the Company recorded compensation expense related to its ESPP and calculated the fair value of shares expected to be purchased under the ESPP using the Black-Scholes models with the following assumptions:
For the Year Ended December 31,
202120202019
Expected term
6-24 months
6-24 months
6 - 24 months
Risk-free interest rate
0.1-0.5%
0.1-0.2%
1.5-2.1%
Volatility
51-154%
97-219%
92-97%
DividendsNoneNoneNone
Employee and Non-Employee Stock-Based Compensation Expense Allocation
For the years ended December 31, 2021, 2020, and 2019, the Company recorded employee and non-employee stock-based compensation expense to the following line items in its Costs and Expenses section of the Consolidated Statements of Operations and Comprehensive Loss: 
For the Year Ended December 31,
202120202019
Research and development expenses$1,844 $731 $575 
General and administrative expenses3,435 1,967 2,083 
Total stock-based compensation expense$5,279 $2,698 $2,658