In July 2016, the Company adopted the 2016 Stock Incentive Plan (the “2016 Plan”), which provided for the grant of restricted stock awards, restricted stock units, incentive stock options, non-statutory stock options, and other stock-based awards to the Company’s eligible employees, officers, directors, consultants, and advisors. As of the effective date of the 2019 Stock Incentive Plan (the “2019 Plan”), and as of September 30, 2021, no shares remained available for future issuance under the 2016 Plan. Any options or awards outstanding under the 2016 Plan remain outstanding and effective.

On July 2, 2019, the Company’s stockholders approved the 2019 Plan, which became effective on July 17, 2019. The 2019 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards to the Company’s officers, employees, directors, consultants and advisors. The number of shares initially reserved for issuance under the 2019 Plan was 2,017,142 shares, plus the shares of common stock remaining available for issuance under the 2016 Plan as of July 17, 2019. The number of shares reserved was annually increased on January 1, 2020 and will be increased on each January 1 thereafter through January 1, 2029 by the least of (i) 2,000,000 shares, (ii) 4% of the number of shares of the Company’s common stock outstanding on the first day of each such year or (iii) an amount determined by the Company’s board of directors. On January 1, 2021, the number of shares reserved for issuance under the 2019 Plan was increased by 1,122,696 shares. As of September 30, 2021, there were 579,016 shares available for future issuance under the 2019 Plan.

The shares of common stock underlying any awards that expire, terminate, or are otherwise surrendered, cancelled, forfeited or repurchased by the Company under the 2016 Plan or the 2019 Plan will be added back to the shares of common stock available for issuance under the 2019 Plan. As of July 17, 2019, no further awards will be made under the 2016 Plan.

The Company may repurchase unvested shares at the original purchase price if employees or non-employees are terminated or cease their employment or service relationship with the Company. Shares of common stock repurchased from employees and non-employees are shares held in the Company’s treasury (“Treasury Shares”). The board of directors may, at its discretion, authorize that the Treasury Shares be returned to the pool of authorized but unissued common stock.

The shares of common stock underlying restricted stock awards typically vest over a four-year period. The shares of common stock are recorded in stockholders’ equity as they vest.

The following table summarizes the Company’s restricted stock activity under the 2019 Plan and 2016 Plan during the nine months ended September 30, 2021:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2020		82,978		$	3.19
Granted		—			—
Vested		(60,043	)		3.15
Repurchased		(2,971	)		3.22
Unvested at September 30, 2021		19,964		$	3.32

Stock options granted by the Company typically vest over a four-year period and have a ten year contractual term. Shares issued upon the exercise of stock options are issued from the Company’s pool of authorized but unissued common stock. In addition to stock options granted under the 2019 Plan and 2016 Plan, the Company has granted stock options as material inducements to employment in accordance with Nasdaq Listing Rule 5635(c)(4), which were granted outside of the 2019 Plan and 2016 Plan. The following table summarizes the Company’s stock option activity during the nine months ended September 30, 2021:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2020		2,962,347		$	11.57		8.55	$	5,767,895
Granted		2,662,007			14.22
Exercised		(274,745	)		9.45
Cancelled		(386,740	)		12.48
Outstanding at September 30, 2021		4,962,869		$	13.04		8.72	$	75,619,778
Exercisable at September 30, 2021		1,234,740		$	11.48		7.74	$	20,653,203

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 as of the balance sheet date for those options that had exercise prices lower than the fair value of the Company’s common stock.

The weighted average grant date fair value of stock options granted in the three and nine months ended September 30, 2021 was $15.25 per share and $10.33 per share, respectively. The weighted average grant date fair value of stock options granted in the three and nine months ended September 30, 2020 was $4.84 per share and $10.41 per share, respectively. The total intrinsic value of stock options exercised during the three and nine months ended September 30, 2021 was $2.9 million and $3.0 million, respectively. The total intrinsic value of stock options exercised during the three and nine months ended September 30, 2020 was $0.1 million and $1.2 million, respectively. The fair value of stock options granted during the three and nine months ended September 30, 2021 and 2020 has been calculated on the date of grant using the following weighted average assumptions:

	Three Months Ended September 30, 2021			Three Months Ended September 30, 2020			Nine Months Ended September 30, 2021			Nine Months Ended September 30, 2020
Risk-free interest rate		1.0	%		0.3	%		0.9	%		1.4	%
Expected dividend yield		0.0	%		0.0	%		0.0	%		0.0	%
Expected term (years)		6.1			6.0			6.0			6.0
Expected stock price volatility		89.3	%		78.1	%		86.9	%		75.9	%

The following table summarizes the Company’s restricted stock activity outside of the 2019 Plan and 2016 Plan during the nine months ended September 30, 2021:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2020		42,858		$	2.93
Granted		—			—
Vested		(42,858	)		2.93
Repurchased		—			—
Unvested at September 30, 2021		—		$	—

The aggregate intrinsic value of all restricted stock awards that vested during the three months ended September 30, 2021 and 2020 was $0.3 million and $0.7 million, respectively. The aggregate intrinsic value of all restricted stock awards that vested during the nine months ended September 30, 2021 and 2020 was $1.2 million and $8.3 million, respectively.

The total compensation cost recognized in the statements of operations associated with all stock-based compensation awards granted by the Company is as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
General and administrative	$	1,688	$	997	$	4,534	$	2,950
Research and development		970		779		2,743		2,723
Total stock-based compensation expense	$	2,658	$	1,776	$	7,277	$	5,673

As of September 30, 2021, the Company had an aggregate of $33.2 million of unrecognized stock-based compensation expense, which is expected to be recognized over a weighted average period of 2.85 years.

On July 2, 2019, the Company’s stockholders approved the 2019 Employee Stock Purchase Plan (the “ESPP”), which became effective on July 17, 2019. A total of 252,142 shares of common stock were initially reserved for issuance under the ESPP. In addition, the number of shares of common stock reserved under the ESPP was annually increased on January 1, 2020, and will be increased on each January 1 thereafter through January 1, 2029, by the least of (i) 428,571 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the first day of each such year or (iii) an amount determined by the Company’s board of directors. On January 1, 2021, the number of shares reserved for issuance under the 2019 ESPP was increased by 280,674 shares. As of September 30, 2021, there were 729,900 shares available for future issuance under the ESPP.