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Stock-Based Awards
 3 Months Ended
Mar. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Stock-Based Awards Stock-Based Awards
2014 Stock Incentive Plan
The Company’s 2014 Stock Incentive Plan (the “2014 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock awards, unrestricted stock awards or restricted stock units to employees, directors and consultants of the Company. The 2014 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of the stock options could not be less than 100% of the fair market value of a share of the Company’s common stock on the date of grant and the term of the stock options could not be greater than ten years.
As of December 31, 2018, all remaining shares available under the 2014 Plan were transferred to the Company’s 2018 Stock Option and Incentive Plan (the “2018 Plan”).
2018 Stock Option and Incentive Plan
In April 2018, the Company’s 2018 Plan was approved by its stockholders and became effective. The 2018 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards, cash-based awards and dividend equivalent rights to the Company’s officers, employees, non-employee directors and other key persons (including consultants). The number of shares initially reserved for issuance under the 2018 Plan was 1,545,454, plus the shares of common stock remaining available for issuance under the 2014 Plan, the reserved shares shall be cumulatively increased each January 1 by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s board of directors or compensation committee of the board of directors. The shares of common stock underlying any awards that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of stock, expire or are otherwise terminated (other than by exercise) under the 2018 Plan and the 2014 Plan will be added back to the shares of common stock available for issuance under the 2018 Plan.
As of March 31, 2023, 271,786 shares were available for future issuance under the 2018 Plan.
Stock options granted under the 2014 Plan and 2018 Plan to employees generally vest over three years and expire after ten years.
Stock Options
The following table summarizes the Company’s stock option activity since December 31, 2022:
 
 Number of
Shares		Weighted
Average
Exercise
Price		Weighted
Average
Remaining
Contractual
Term		Aggregate
Intrinsic
Value
   (in years) 
Outstanding as of December 31, 20228,233,330 $5.74 6.68$159 
Granted3,058,400 0.69 
Exercised— — 
Forfeited(100,599)4.76 
Outstanding as of March 31, 202311,191,131 $4.37 7.40$145 
Options exercisable at March 31, 20235,926,959 $5.90 5.74$118 
Vested and expected to vest at March 31, 202311,191,131 $4.37 7.40$145 
 
The weighted average grant-date fair value per share of stock options granted during the three months ended March 31, 2023 and year ended December 31, 2022 was $0.50 and $2.28, respectively.
As of March 31, 2023 and December 31, 2022, there were outstanding stock options held by non-employees for the purchase of 260,570 shares of common stock with service-based vesting conditions.
2018 Employee Stock Purchase Plan
In April 2018, the Company’s 2018 Employee Stock Purchase Plan (the “ESPP”) was approved by its stockholders and became effective. A total of 256,818 shares of common stock were initially reserved for issuance under this plan. In addition, the number of shares of common stock that may be issued under the ESPP automatically increased on January 1, 2019, and shall increase each January 1 thereafter through January 1, 2028, by the lesser of (i) 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 and (ii) such lesser number of shares as determined by the administrator of the Company’s ESPP. As of March 31, 2023, a total of 1,873,627 shares of common stock were reserved for issuance under this plan.  
For the three months ended March 31, 2023, the Company issued 137,917 shares of common stock under the ESPP. For the three months ended March 31, 2022, the Company issued 51,329 shares of common stock under the ESPP.
2021 Inducement Plan
In December 2021, the Company adopted the Company’s 2021 Inducement Plan (the “Inducement Plan”) pursuant to which the Company reserved 600,000 shares of common stock to be used exclusively for grants of equity-based awards to individuals who were not previously employees or directors of the Company, as an inducement material to the individual’s entry into employment with the Company within the meaning of Rule 5635(c)(4) of the Marketplace Rules of the Nasdaq Stock Market, Inc. The Inducement Plan provides for the grant of equity-based awards in the form of nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, unrestricted stock awards, and dividend equivalent rights. The Inducement Plan was adopted by the Company without stockholder approval pursuant to Rule 5635(c)(4) of the Marketplace Rules of the Nasdaq Stock Market, Inc.
The following table summarizes the Company’s stock option under the Inducement Plan activity since December 31, 2022:

Number of
Shares		Weighted
Average
Exercise
Price		Weighted
Average
Remaining
Contractual
Term		Aggregate
Intrinsic
Value
(in years)
Outstanding as of December 31, 2022210,400 $2.61 9.36$— 
Granted— — 
Exercised— — 
Forfeited— — 
Outstanding as of March 31, 2023210,400 $2.61 9.11$— 
Options exercisable at March 31, 202326,541 $3.64 8.92$— 
Vested and expected to vest at March 31, 2023210,400 $2.61 9.11$— 
The Company did not grant stock options under the Inducement Plan during the three months ended March 31, 2023. The weighted average grant-date fair value per share of stock options granted during the three months ended March 31, 2022 was $2.41. As of March 31, 2023, 389,600 shares were available for future issuance under the Inducement Plan.
Restricted Stock Units
The Company has granted restricted stock units (“RSUs”) with service-based vesting conditions. RSUs represent the right to receive shares of common stock upon meeting specified vesting requirements. Unvested shares of restricted common stock units may not be sold or transferred by the holder. These restrictions lapse according to the service-based vesting conditions of each award. In 2022, the Company granted 732,000 RSUs, of which 40% vested in August 2022 and 60% will vest in August 2023, as long as the applicable individual remains an employee of the Company at such time.
The table below summarizes the Company’s RSU activity since December 31, 2022:
 
 Number of
Shares		Weighted
Average
Grant-Date
Fair Value
Unvested restricted stock units as of December 31, 2022385,980 $3.64 
Granted— — 
Vested— — 
Forfeited— — 
Unvested restricted stock units as of March 31, 2023385,980 $3.64 
The expense related to RSUs granted to employees was $346 and $209 for the three months ended March 31, 2023 and 2022, respectively.
Stock-Based Compensation
The Company recorded stock-based compensation expense related to stock options, ESPP and restricted stock unit awards in the following expense categories of its condensed consolidated statements of operations and comprehensive income (loss):
 
 Three months ended March 31,
 20232022
Research and development expenses$646 $584 
General and administrative expenses1,000 1,281 
 $1,646 $1,865 
 
As of March 31, 2023, the Company had an aggregate of $10,059 of unrecognized stock-based compensation cost, which is expected to be recognized over a weighted average period of 1.35 years