12. Equity Incentive Plans, Equity Inducement Plans, Employee Stock Purchase Plan and Stock-Based Compensation

2004 Equity Incentive Plan, 2018 Equity Incentive Plan, 2021 Equity Inducement Plan, and Amended and Restated 2021 Equity Inducement Plan

In September 2018, the Company adopted the 2018 Equity Incentive Plan (“2018 Plan”), which became effective on September 25, 2018. As a result, the Company will not grant any additional awards under the 2004 Equity Incentive Plan (“2004 Plan”). The terms of the 2004 Plan and applicable award agreements will continue to govern any outstanding awards thereunder. In addition to the shares of common stock reserved for future issuance under the 2004 Plan that were added to the 2018 Plan upon its effective date, the Company initially reserved 2,300,000 shares of common stock for issuance under the 2018 Plan. In addition, the number of shares of common stock reserved for issuance under the 2018 Plan will automatically increase on the first day of January for a period of up to ten years, commencing on January 1, 2019, in an amount equal to 5% of the total number of shares of the Company’s capital stock outstanding on the last day of the preceding year (rounded to the nearest whole share), or a lesser number of shares determined by the Company’s board of directors. As a result, common stock reserved for issuance under the 2018 Plan was increased by 2,874,977 shares on January 1, 2023.

In August 2021, the Company adopted the 2021 Equity Inducement Plan (“2021 Plan”), which became effective on August 4, 2021. Upon its effective date, the Company initially reserved 750,000 shares of common stock for issuance pursuant to non-qualified stock options and restricted stock units (“RSUs”) under the 2021 Plan. In accordance with Rule 5635(c)(4) of the Nasdaq listing rules, equity awards under the 2021 Plan may only be made to an employee if he or she is granted such equity awards in connection with his or her commencement of employment with the Company and such grant is an inducement material to his or her entering into employment with the Company or such subsidiary. In addition, awards under the 2021 Plan may only be made to employees who have not previously been an employee or member of the Board (or any parent or subsidiary of the Company) or following a bona fide period of non-employment of the employee by the Company (or a parent or subsidiary of the Company). At all times the Company will reserve and keep available a sufficient number of shares as will be required to satisfy the requirements of all outstanding awards granted under the 2021 Plan.

In August 2022, the Company amended and restated the 2021 Plan (the “Amended and Restated 2021 Plan”) and reserved an additional 750,000 shares of common stock available for issuance under the Amended and Restated 2021 Plan to be granted by the Company to certain employees as a material inducement to their acceptance of employment with the Company in accordance with Nasdaq Listing Rule 5635(c)(4).

Additionally, in February 2023, the Company amended and restated the Amended and Restated 2021 Plan and reserved an additional 500,000 shares of common stock available for issuance under the Amended and Restated 2021 Plan to be granted by the Company to certain employees as a material inducement to their acceptance of employment with the Company in accordance with Nasdaq Listing Rule 5635(c)(4). The total number of shares reserved for issuance pursuant to the Amended and Restated 2021 Plan is 2,000,000 shares.

As of December 31, 2023, the Company had 1,777,919 shares available for grant under the 2018 Plan and the 2021 Plan.

The following table summarizes option activity under the Company’s 2004 Plan, 2018 Plan and 2021 Plan:


	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contract Term (Years)		Aggregate Intrinsic Value (in thousands)
Stock options outstanding at December 31, 2022		7,310,611		$	12.68		6.66	$	2,187
Granted		1,611,500			5.35
Exercised		(53,060	)		5.92
Canceled and forfeited		(964,019	)		12.63
Stock options outstanding at December 31, 2023		7,905,032		$	11.24		6.02	$	180
Stock options exercisable at December 31, 2023		5,717,050		$	12.42		5.14	$	14

The aggregate intrinsic value in the table above represents the total intrinsic value (the difference between our closing stock price on the last trading day of fiscal 2023 and the exercise prices, multiplied by the number of in-the-money stock options) that would have been received by the stock option holders had all stock option holders exercised their stock options on December 31, 2023. For the years ended December 31, 2023, 2022 and 2021, the aggregate intrinsic value of stock options exercised was $30,000, $0.1 million and $2.8 million, respectively, determined at the date of the option exercise.

For determining stock-based compensation expense, the fair-value-based measurement of each employee stock option was estimated as of the date of grant using the Black-Scholes option pricing model with assumptions as follows:


	Year Ended December 31,
	2023		2022		2021
Expected term (in years)	5.3-6.1		5.3-6.1		5.3-6.1
Expected volatility	80.7%-84.1%		81.8%-83.5%		80.9%-84.9%
Risk-free interest rate	3.6%-4.7%		1.7%-4.2%		0.6%-1.3%
Expected dividend		-		-		-

Expected Term—The expected term represents the period that the stock-based awards are expected to be outstanding. The Company used the “simplified” method to determine the expected term of options granted, which calculates the expected term as the average of the weighted-average vesting term and the contractual term of the option.

Expected Volatility—Since the Company has limited information available on the volatility of its common stock due to its short trading history, the expected volatility was estimated based on the average historical volatilities of common stock of comparable publicly traded entities over a period equal to the expected term of the stock option grants. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available.

Risk-Free Interest Rate—The risk-free interest rate is based on the U.S. Treasury yield in effect at the time of grant for zero-coupon U.S. Treasury notes with maturities approximately equal to the expected term of the options.

Expected Dividend—The Company has never paid dividends on its common stock. Therefore, the Company used an expected dividend of zero.

Using the Black-Scholes option-valuation model, the weighted-average estimated grant-date fair value of employee stock options granted during the years ended December 31, 2023, 2022 and 2021 was $3.84, $5.03 and $14.24 per share, respectively.

Restricted stock units (“RSUs”) are share awards that entitle the holder to receive freely tradable shares of the Company’s common stock upon vesting. The RSUs cannot be transferred and the awards are subject to forfeiture if the holder’s employment terminates prior to the release of the vesting restrictions. The RSUs generally vest over a four-year period provided the employee remains continuously employed with the Company. The fair value of the RSUs is equal to the closing price of the Company’s common stock on the grant date.

A summary of the status and activity of non-vested RSUs for the year ended December 31, 2023 is as follows:


	Number of Shares			Weighted Average Grant-Date Fair Value
Non-vested December 31, 2022		3,958,478		$	11.70
Granted		2,823,725			5.41
Released		(1,155,644	)		12.31
Canceled		(381,686	)		9.92
Non-vested December 31, 2023		5,244,873		$	8.31

In September 2018, the Company adopted the 2018 Employee Stock Purchase Plan (“ESPP”), which became effective on September 26, 2018, in order to enable eligible employees to purchase shares of the Company’s common stock. The Company initially reserved 230,000 shares of common stock for sale under the ESPP. The aggregate number of shares reserved for sale under the ESPP will automatically increase on the first day of January for a period of up to ten years, commencing on January 1, 2019, in an amount equal to 1% of the total number of shares of the Company’s capital stock outstanding on the last day of the preceding year (rounded to the nearest whole share), or a lesser number of shares determined by the Company’s board of directors. As a result, common stock reserved for issuance under the ESPP was increased by 574,995 shares on January 1, 2023. The aggregate number of shares issued over the term of the Company’s ESPP, subject to stock-splits, recapitalizations or similar events, may not exceed 2,300,000 shares of the Company’s common stock.

The fair value of the ESPP shares is estimated using the Black-Scholes option pricing model. For the years ended December 31, 2023, 2022 and 2021, the fair value of ESPP shares was estimated using the following assumptions:


	Year Ended December 31,
	2023		2022		2021
Expected term (in years)	0.5		0.5		0.5
Expected volatility	79.6-89.9%		65.9-88.1%		65.9-111.4%
Risk-free interest rate	3.8%-5.5%		0.1%-3.8%			0.1	%
Expected dividend		-		-		-

During the years ended December 31, 2023, 2022 and 2021, 526,079, 270,516, and 145,809 shares, respectively, had been purchased. As of December 31, 2023, 914,911 shares were available for future issuance under the ESPP.

The Company believes that the fair value of the stock options, RSUs and ESPP shares is more reliably measurable than the fair value of services received.

Total stock-based compensation expense recognized was as follows:


	Year Ended December 31,
	2023		2022		2021
	(in thousands)
Research and development expense:
Stock options	$	2,183	$	2,287	$	2,208
Restricted stock units		8,885		7,227		4,280
ESPP		764		592		638
Subtotal		11,832		10,106		7,126
General and administrative expense:
Stock options		6,066		10,261		11,045
Restricted stock units		6,796		5,781		4,920
ESPP		214		156		150
Subtotal		13,076		16,198		16,115
Total	$	24,908	$	26,304	$	23,241

As of December 31, 2023, unrecognized stock-based compensation expense related to the unvested stock options and RSUs granted was $11.0 million and $30.9 million, respectively. The remaining unrecognized compensation cost is expected to be recognized over a weighted-average period of 2.0 years and 2.3 years, respectively. As of December 31, 2023, there is $0.3 million of unrecognized stock-based compensation expense related to the ESPP.