The Company adopted the 2016 Equity Incentive Plan (“2016 Plan”) in February 2016 and amended it in June 2017 and June 2018. Upon adoption of the 2016 Plan, no further grants were made under the 2005 Equity Incentive Plan (“2005 Plan”).

In April 2020, the Company’s Board of Directors adopted the Company’s 2020 Incentive Award Plan (“2020 Plan”, and together with the 2016 Plan and 2005 Plan, the “Plans”), and upon effectiveness of the 2020 Plan, the Company ceased granting equity-based awards under the 2016 Plan. The 2020 Plan provides for grant of incentive stock options and nonqualified stock options, stock appreciation rights, restricted stock, dividend equivalents, restricted stock units, performance awards and other share and cash-based awards to employees and consultants and members of the Board of Directors of the Company and its subsidiaries.

The initial number of shares of the Company’s common stock that may be issued under the 2020 Plan is 2,100,000 shares plus the number of shares of the Company’s common stock underlying outstanding awards under the 2005 Plan and 2016 Plan as of the effective date of the 2020 Plan that expire, lapse or are terminated, exchanged for cash, surrendered, repurchased, canceled or forfeited following the effective date of the 2020 Plan. The number of shares available under the 2020 Plan will automatically increase on January 1st of each year from 2021 to 2030 by the lesser of (i) 4% of the number of shares of common stock outstanding on the final day of the immediately preceding calendar year and (ii) a smaller number of shares determined by the Company’s Board of Directors. However, no more than 8,800,000 shares may be issued under the 2020 Plan pursuant to the exercise of incentive stock options. On January 1, 2020, the shares available for grant under the 2020 Plan was automatically increased by 517,295. As of March 31, 2021, the Company had 1,612,952 shares available for issuance under the 2020 Plan.

All stock option grants are nonqualified stock options except option grants to employees and officers intended to qualify as incentive stock options under the Internal Revenue Code of 1986, as amended. Stock options may not be granted at less than the fair market value of the Company’s common stock on the date of grant. Vesting periods of awards are determined by the Board of Directors or its compensation committee. Vesting periods of awards granted to date range from vesting upon grant to vesting over a four-year period. Vesting conditions are generally based on continued service. Additionally, the Company has granted certain awards which vest upon the achievement of certain financing and revenue milestones. Stock options granted under the Plans expire no more than 10 years from the date of grant.

Stock-based compensation expense included in the Company’s condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):

	Three Months Ended March 31,
	2021		2020
Research and development	$	142	$	40
General and administrative		457		94
Total	$	599	$	134

The Company did not record any stock-based compensation associated with milestone-based awards in the three months ended March 31, 2021 and 2020.

The fair value of each stock option granted to employees, directors and non-employees was estimated on the date of grant using the Black-Scholes option-pricing model, with the following weighted-average assumptions:

	Three Months Ended March 31,
	2021			2020
Risk-free interest rate		0.6	%		1.5	%
Expected dividend yield		—	%		—	%
Expected term (in years)		6.0			6.0
Expected volatility		85.4	%		78.0	%

A summary of the stock option activity under the Plans for the three months ended March 31, 2021 was as follows:

	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding December 31, 2020		1,428,886		$	10.41		7.7	$	5,165
Granted		365,700			10.75
Exercised		(30,391	)		8.63
Cancelled		(1,590	)		11.70
Outstanding at March 31, 2021		1,762,605		$	10.51		8.1	$	5,540
Exercisable at March 31, 2021		665,936		$	9.39		6.3	$	3,349
Vested and expected to vest at March 31, 2021		1,762,605		$	10.51		8.1	$	5,540

The weighted-average fair value of options granted to employees, directors and non-employees during the three months ended March 31, 2021 and 2020 was $7.65 and $5.84, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the three months ended March 31, 2021 and 2020 was approximately $0.2 million and $0, respectively.

As of March 31, 2021, total unrecognized stock-based compensation expense relating to unvested stock options was approximately $8.4 million. This amount is expected to be recognized over a weighted-average period of 3.2 years. Additionally, as of March 31, 2021, there was approximately $36,000 of unrecognized stock-based compensation related to a stock option award related to the achievement of a revenue-based milestone. As the Company believes the achievement of the revenue-based milestone is currently not probable, it has not recorded any stock-based compensation related to this award. The Company will continue to assess the probability of achieving the revenue-based milestone at each reporting period.

In April 2020, the Company’s Board of Directors adopted the Company’s 2020 Employee Stock Purchase Plan (“2020 ESPP”). The 2020 ESPP is structured as a qualified employee stock purchase plan under Section 423 of the Internal Revenue Code of 1986, as amended, and is not subject to the provisions of the Employee Retirement Income Security Act of 1974. The Company initially reserved 150,000 shares of common stock for issuance under the 2020 ESPP. In addition, the number of shares available for issuance under the 2020 ESPP will be annually increased on January 1st of each year from 2021 to 2030 by the lesser of (i) 0.5% of the number of shares of common stock outstanding on the final day of the immediately preceding calendar year and (ii) such smaller number of shares as is determined by the Company’s Board of Directors, provided that no more than 987,500 shares of common stock may be issued under the 2020 ESPP. On January 1, 2020, the shares available for grant under the 2020 ESPP was automatically increased by 64,661. The 2020 ESPP permits eligible participants to purchase common stock through payroll deductions of up to a specified percentage of their eligible compensation. On the first trading day of each offering period, each participant will automatically be granted an option to purchase shares of the Company’s common stock. The option will expire at the end of the applicable offering period, and will be exercised at that time to the extent of the payroll deductions accumulated during the offering period, subject to the limits set forth in the 2020 ESPP. The purchase price of the shares, in the absence of a contrary designation, will be 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the purchase date. As of March 31, 2021, no shares have been issued under the 2020 ESPP.