In August 2015, the board of directors adopted the 2015 Equity Incentive Plan, as amended (the “2015 Plan”), which provides for the grant of incentive stock options, nonqualified stock options or other awards including stock appreciation rights and restricted stock awards to the Company’s employees, officers, directors, advisors, and consultants for the purchase of up to 1.5 million shares of the Company’s common stock. In July 2018 and January 2019, the 2015 Plan was amended to increase the number of shares reserved thereunder by 1.0 million and 0.4 million shares, respectively. In March 2020, the Company’s board of directors and stockholders voted to increase the number of shares reserved for issuance under the 2015 Plan by 1.4 million shares. In May 2020, the board of directors adopted the 2020 Stock Options and Incentive Plan (the “2020 Plan”). The 2015 plan was suspended and no further grants may be issued under the 2015 Plan.

The 2020 Plan provides for the grant of incentive stock options, nonqualified stock options or other awards including stock appreciation rights, restricted stock awards and restricted stock units to the Company’s employees, officers, directors, advisors, and consultants for the purchase of up to 4.2 million shares of the Company’s common stock. In addition, to the extent that awards outstanding under the 2020 Plan or the 2015 Plan are cancelled, forfeited or held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the registrant prior to vesting, satisfied without any issuance of stock, expire or are otherwise terminated (other than by exercise) subsequent to May 2020, the shares of common stock reserved for issuance pursuant to such awards will become available for issuance as shares of common stock under the 2020 Plan. The 2020 Plan provides that an additional number of shares will automatically be added to the shares authorized for issuance under the 2020 Plan on January 1 of each year beginning January 1, 2021. The number of shares added each January 1 will be equal to the lesser of: (i) 5% of the outstanding shares on the immediately preceding December 31 or (ii) such amount as determined by the administrator of the 2020 Plan, which is the compensation committee of the board of directors. As of December 31, 2020, 3.6 million shares remained available for issuance under the 2020 Plan.

Prior to the adoption of the 2020 Plan, options under the 2015 Plan could be granted for periods of up to 10 years and at prices no less than 100% of the estimated fair value of the shares on the date of grant as determined by the board of directors, provided, however, that the exercise price of an incentive stock option granted to a 10.0% shareholder could not be less than 110.0% of the estimated fair value of the shares on the date of grant and the option was not exercisable after the expiration of five years from the date of grant.

Options under the 2020 Plan may be granted for periods of up to 10 years and at prices no less than the market price of the Company’s common stock on the date of grant, provided, however, that the exercise price of an incentive stock option granted to a 10.0% shareholder shall not be less than 110.0% of the estimated fair value of the shares on the date of grant and the option is not exercisable after the expiration of five years from the date of grant.

The Company granted restricted stock awards under the 2015 Plan. The purchase price of the restricted common stock awards was the estimated fair value as determined by the board of directors at the issuance date. The shares vest from one to four years and vesting could be accelerated upon a change in control. A holder of an award may pay a total purchase price or a part of the purchase price for granted shares at any time during the vesting periods. Upon termination of employment, the Company has the right to repurchase any unvested restricted shares. The repurchase price for unvested shares of common stock will be the lower of (i) the fair market value on the date of repurchase or (ii) their original purchase price. During the vesting term, holders of restricted stock awards are deemed to be a common stock shareholder and have dividends and voting rights.

The Company accounted for restricted stock awards as early exercised options and recognized a liability in other liabilities when cash was received for the purchase of shares of restricted stock. As shares of restricted stock vested, the Company reclassified the liability to common stock and additional paid in capital. As of December 31, 2020 and 2019, the Company recorded a liability included in accrued expenses and other liabilities of $10,000 and $22,000, respectively.

There have been no restricted stock awards granted during the years ended December 31, 2020 and 2019.

The following table summarizes restricted stock activity during the year ended December 31, 2020:

	Number of Shares			Weighted- Average Grant Date fair value
Outstanding and unvested, as of December 31, 2019		302,211		$	1.51
Issued		—		$	—
Vested		(163,568	)	$	1.30
Repurchases		(12,121	)	$	1.22
Outstanding and unvested, as of December 31, 2020		126,522		$	1.85

Restricted stock awards of 4,195 shares with a weighted-average grant date fair value of $0.08 per share, were not purchased by the award holders as of December 31, 2020. As these shares of the restricted common stock awards were not issued, they are not included in the table above.

The aggregate fair value of restricted stock awards vested during the years ended December 31, 2020 and 2019 was $0.2 million and $0.4 million, respectively. Total intrinsic value of outstanding unvested restricted stock awards as of December 31, 2020 and 2019 was $2.9 million and $2.1 million, respectively.

Stock options issued under either the 2015 Plan or the 2020 Plan generally vest over a four-year period and expire ten years from the date of grant. Certain options provide for accelerated vesting if there is a change in control, as defined in the respective plans.

The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for stock option awards with the following assumptions:

	Year Ended December 31,
	2020		2019
Expected volatility	72.10% - 77.50%		74.80% - 82.53%
Risk-free interest rate	0.27% - 0.82%		1.43% - 2.59%
Expected dividend		—		—
Expected term (in years)	5.26 - 6.75		5.00 - 6.08
Underlying common stock fair value	6.22 - 28.79		5.15 - 7.08

A summary of option activity under the 2015 Plan and the 2020 Plan is as follows:

	Number of Options			Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (in Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2019		1,337,501		$	3.22		9.18	$	5,157
Granted		1,830,738		$	11.43
Exercised		(136,223	)	$	3.25
Forfeited		(38,161	)	$	8.13
Outstanding as of December 31, 2020		2,993,855		$	8.18		8.84	$	43,890
Exercisable as of December 31, 2020		736,601		$	4.19		8.22	$	13,664
Vested and expected to vest as of December 31, 2020		2,993,855		$	8.18		8.84	$	43,890

Aggregate intrinsic value represents the difference between the fair value of the underlying common stock and the exercise price as of December 31, 2020 and 2019. The weighted-average grant date fair value of options granted during years ended December 31, 2020 and 2019 was $7.27 per share and $4.37 per share, respectively.

As of December 31, 2020, there was $0.2 million of unrecognized compensation costs that is expected to be recognized over the weighted-average periods of 1.22 years related to restricted stock awards. As of December 31, 2020, there was $14.3 million of unrecognized compensation costs that is expected to be recognized over the weighted-average periods of 2.72 years related to stock options.

In June 2020, the Company adopted the 2020 ESPP. The Company reserved 700,000 shares of common stock for future issuance under the plan. The 2020 ESPP provides that the number of shares reserved and available for issuance will automatically increase on January 1 of each calendar year, beginning January 1, 2021, by the least of (i) 1.0% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, (ii) 700,000 shares or (iii) such lesser amount as determined by the administrator of the 2020 ESPP, which is the compensation committee of the board of directors.

Under the 2020 ESPP, eligible employees may purchase shares of our common stock through payroll deductions that cannot exceed 15% of each employee’s salary. The 2020 ESPP provides for a six-month offering period. At the end of the purchase period, eligible employees are permitted to purchase shares of common stock at the lower of 85% of the fair market value at the beginning of the offering period or 85% of the fair market value at the end of the purchase period, subject to tax limitations on the total value of the purchase. The 2020 ESPP is considered a compensatory plan, and the Company recorded $0.5 million in stock-based compensation expense for year ended December 31, 2020. There was no stock-based compensation expense attributed to the 2020 ESPP for the year ended December 31, 2019 as the 2020 ESPP was adopted in 2020. As of December 31, 2020, no shares of common stock were issued under the 2020 ESPP. The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for the 2020 ESPP with the following assumptions:

	Year Ended December 31,
	2020			2019
Risk-free interest rate		0.11	%		—
Expected term of options (in years)		0.58			—
Expected stock price volatility		83.72	%		—
Expected dividends		—			—

The following table presents the classification of stock-based compensation expense for the years ended December 31, 2020 and 2019 (in thousands):

	Year Ended December 31,
	2020		2019
Research and development expenses	$	1,719	$	584
General and administrative expenses		2,176		1,245
Total stock-based compensation expense	$	3,895	$	1,829