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Stock-Based Compensation (Tables)
3 Months Ended
Mar. 31, 2021
Stock-Based Compensation  
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block]

Three Months Ended

March 31, 

By Expense Category:

    

2021

    

2020

Research and development

$

251,000

$

275,141

General and administrative

 

494,000

 

603,822

Total stock-based compensation expense

$

745,000

$

878,963

Three Months Ended

March 31, 

By Type of Award:

    

2021

    

2020

Stock options

$

762,000

$

854,770

Restricted stock units

 

(17,000)

 

24,193

Total stock-based compensation expense

$

745,000

$

878,963

Share-based Payment Arrangement, Nonvested Award, Cost [Table Text Block]

As of March 31, 2021

Weighted

Average

Remaining

Recognition

    

Unamortized

    

Period

Expense

(Years)

Stock options

$

9,859,000

 

3.4

Restricted stock units

$

865,000

1.8

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

Three Months Ended

March 31, 

    

2021

    

2020

Expected dividend yield

Risk-free interest rate

 

0.62% - 1.67%

 

0.90% - 1.60%

Expected volatility

 

93% - 94%

 

87% - 90%

Expected life (years)

 

5.2 - 6.1

 

6.1

Share-based Payment Arrangement, Option, Activity [Table Text Block]

    

    

    

Weighted

    

Weighted

Average

Average

Contractual

Aggregate

Number of

Exercise

Term

Intrinsic

Shares

Price

(in years)

Value

Outstanding as of December 31, 2020

 

4,692,071

$

5.51

 

  

 

  

Granted

 

1,162,918

$

2.80

 

  

 

  

Exercised

 

(623)

$

2.33

 

  

 

  

Cancelled

 

(811,578)

$

7.46

 

  

 

  

Outstanding as of March 31, 2021

 

5,042,788

$

4.57

 

8.4

$

98,081

Exercisable as of March 31, 2021

 

1,042,063

$

8.21

 

3.9

$

11,020

Vested and expected to vest as of March 31, 2021

 

4,998,291

$

4.55

 

8.4

$

98,081

Schedule of Nonvested Restricted Stock Units Activity [Table Text Block]

    

    

    

Weighted

Average

Grant-Date

Number of

Fair Value

RSUs

(per RSU)

Nonvested as of December 31, 2020

 

88,131

$

4.68

Granted

 

334,015

 

2.97

Vested

 

(10,366)

 

3.31

Forfeited

 

(52,403)

 

5.59

Nonvested as of March 31, 2021

 

359,377

$

3.00