For the years ended December 31, 2024, 2023 and 2022, the Company recorded stock-based compensation expense as follows (in thousands):


	2024		2023		2022
Research and development	$	1,779	$	1,481	$	1,729
Selling, general and administrative		20,472		12,769		6,178
Total stock-based compensation	$	22,251	$	14,250	$	7,907

The Company may issue stock options, restricted stock, stock appreciation rights and restricted stock units (collectively, the Awards) to employees, directors, and consultants of the Company under the 2014 and 2018 Stock Incentive Plans (the 2014 Plan and the 2018 Plan or collectively, the Plans). At December 31, 2024, no shares remain available for future issuance under the 2014 Plan. Under the 2018 Plan, 21,000,000 shares are reserved for issuance and as of December 31, 2024, 2,254,102 shares remain available for future issuance.

The Company has granted stock options to employees, officers, directors, and consultants generally at exercise prices equal to the market price of the common stock at grant date. Option awards generally vest over a period of 1 to 3 years of continuous service and have contractual terms of 7 years. Certain awards provide for accelerated vesting if there is a change in control. The Company issues new shares as shares are required to be delivered upon exercise of outstanding stock options.

During the years ended December 31, 2024, 2023 and 2022, options to purchase 3,429,184, 1,651,345 and 3,172,342 shares, respectively, of the Company’s common stock were exercised with gross proceeds to the Company of approximately $13.5 million, $2.8 million and $9.6 million, respectively. During the years ended December 31, 2024, 2023 and 2022, no options to purchase shares of the Company’s common stock were exercised on a “cashless” basis.

During the years ended December 31, 2024, 2023 and 2022 the Company recorded non-cash stock-based compensation expense related to stock options totaling approximately $16.8 million, $11.1 million and $6.3 million, respectively.

During the years ended December 31, 2024, 2023 and 2022, the Company granted seven-year options to purchase an aggregate of 2,476,946, 3,598,535 and 1,386,500 shares, respectively, of the Company’s common stock to certain of the Company’s officers, employees, directors, and consultants.

Stock option activity under the Company’s Plans for the year ended December 31, 2024 is summarized as follows:


	Number of Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at beginning of year		14,177,488		$	7.73
Granted		2,476,946			18.95
Exercised or released		(3,429,184	)		3.94
Forfeited or cancelled		(137,337	)		15.29
Expired		(11,124	)		16.47
Outstanding at end of year		13,076,789		$	10.76		4.26	$	132,570
Exercisable at end of year		7,353,893		$	6.53		2.93	$	105,449

Other information pertaining to stock option activity during the years ended December 31, 2024, 2023 and 2022 was as follows:


	2024		2023		2022
Weighted–average fair value of granted stock options	$	9.53	$	8.66	$	8.52
Total fair value of vested stock options (in thousands)	$	12,860	$	8,278	$	6,096
Total intrinsic value of exercised stock options (in thousands)	$	53,230	$	22,265	$	31,881

As of December 31, 2024, there was approximately $41.7 million of unrecognized compensation expense related to non-vested stock option awards granted under the Plans. That cost is expected to be recognized over a weighted average period of approximately 2.72 years.

The Company utilizes the Black-Scholes option-pricing model to determine the fair value of stock options on the date of grant. This model derives the fair value of stock options based on certain assumptions related to the expected stock price volatility, expected option life, risk-free interest rate and dividend yield. Expected volatility is based on reviews of historical volatility of the Company’s common stock. The Company estimates the expected option life for options granted to employees and directors based upon the simplified method. Under this method, the expected life is presumed to be the mid-point between the vesting date and the end of the contractual term. The Company will continue to use the simplified method until it has sufficient historical exercise data to estimate the expected life of the options. The risk-free interest rate assumption is based upon the U.S. Treasury yield curve appropriate for the estimated life of the stock option awards. The expected dividend rate is zero. Forfeitures are recognized as a reduction of stock-based compensation expense as they occur.

Assumptions used during the years ended December 31, 2024, 2023 and 2022 were as follows:


	2024	2023	2022
Risk free interest rate	3.70% to 4.70%	3.55% to 4.92%	1.27% to 4.07%
Expected term	4.5 to 5.0 years	4.5 to 5.2 years	4.5 years
Expected volatility	54.1% to 61.5%	68.0% to 71.0%	68.4% to 69.5%
Expected dividend yield	—%	—%	—%
Expected forfeiture rate	—%	—%	—%

Under the 2018 Plan, participants may be granted restricted stock units, each of which represents a conditional right to receive shares of common stock in the future. The restricted stock units granted under this plan generally vest ratably over a three-year period. Upon vesting, the restricted stock units will convert into an equivalent number of shares of common stock. The amount of expense relating to the restricted stock units is based on the closing market price of the Company’s common stock on the date of grant and is amortized on a straight-line basis over the requisite service period. Restricted stock unit activity for the year ended December 31, 2024 was as follows:


	Number of Restricted Stock Units			Weighted Average Grant Date Fair Value
Nonvested balance at beginning of year		854,459		$	12.48
Granted		228,029			20.45
Vested		(364,861	)		11.19
Forfeited		(8,667	)		18.59
Nonvested balance at end of year		708,960		$	15.63

During the years ended December 31, 2024, 2023 and 2022, the Company recorded non-cash stock-based compensation expense related to restricted stock units totaling $5.5 million, $3.2 million and $1.6 million, respectively.