In 1997, our stockholders approved our Employee Stock Purchase Plan (“ESPP”) at which time a maximum of 10,000 shares of common stock were available for issuance. In December 2007, May 2009, June 2011, May 2014, May 2015, June 2016, June 2017, June 2019, June 2021, May 2022, June 2023 and June 2024, our stockholders authorized increases in the number of shares reserved for issuance under the ESPP by 5,000, 10,000, 25,000, 25,000, 100,000, 100,000, 200,000, 300,000, 200,000, 850,000, 400,000 and 1,200,000 shares, respectively, for a total of 3,425,000 shares reserved at December 31, 2024. Under the terms of the ESPP, employees can elect to have up to a maximum of 10% of their base earnings withheld to purchase shares of our common stock. The purchase price of the stock is 85% of the lower of the closing prices for our common stock on either: (i) the first trading day in the enrollment period, as defined in the ESPP, in which the purchase is made, or (ii) the purchase date. The length of the enrollment period is 6 months. Enrollment dates are the first business day of May and November. Under the ESPP, we issued 639,681 and 717,046 shares in 2024 and 2023, respectively. The weighted-average exercise price per share of the purchase rights exercised during 2024 and 2023 was $1.00 and $1.26, respectively. As of December 31, 2024, 2,537,126 shares of common stock have been issued under the ESPP and 920,953 shares of common stock are available for future issuance.

We currently have one equity incentive plan from which we can grant options, restricted stock awards, restricted stock units ("RSUs"), and stock appreciation rights to employees, officers, directors and consultants. In December 2007, the stockholders approved our 2007 Amended and Restated Equity Incentive Plan (“2007 Plan”) at which time a maximum of 150,000 shares of common stock were available for grant. In May 2010, June 2011, May 2014, May 2015, June 2016, June 2017, June 2019, June 2021, May 2022, June 2023 and June 2024, our stockholders approved amendments to our 2007 Plan to increase the maximum number of shares of common stock available for grant by 100,000, 4,500,000, 1,750,000, 4,300,000, 3,000,000, 5,000,000, 7,000,000, 2,000,000, 2,900,000, 8,490,000 and 7,500,000 shares of common stock, respectively, resulting in an aggregate of 46,690,000 shares of common stock authorized for issuance as of December 31, 2024. At December 31, 2024, there were 12,125,013 shares available for future grant under the 2007 Plan. Any shares that are subject to awards granted pursuant to the 2007 Plan that expire, are cancelled or are forfeited are available for future grant under the 2007 Plan.

In 2024, we granted options and RSUs to certain employees outside of the 2007 Plan as inducement equity awards. All such options to purchase our common stock were granted with an exercise price that equals fair market value of the underlying common stock on the grant dates and expire no later than 10 years from the date of grant. The options are exercisable in accordance with vesting schedules that generally provide for them to be fully vested and exercisable four years after the date of grant. All such RSUs vest ratably over four years. All stock options and RSU granted as inducement equity awards were approved by our Board of Directors and have been registered on Form S-8 with the SEC.

The following table summarizes the stock option activity for the year ended December 31, 2024:


	Outstanding Options
				Weighted-
				Average
	Number of			Exercise
	Shares			Price
Balance at December 31, 2023		24,575		$	7.06
Granted		8,158		$	2.20
Exercised		(485	)	$	2.47
Cancelled		(6,166	)	$	12.02
Balance at December 31, 2024		26,082		$	4.46

The following table summarizes the RSU activity for the year ended December 31, 2024:


	Outstanding RSUs
				Weighted-
				Average
	Number of			Grant Date
	Shares			Fair Value
Balance at December 31, 2023		1,405		$	4.43
Granted		2,175		$	1.65
Released		(1,116	)	$	2.49
Forfeited		(483	)	$	3.69
Balance at December 31, 2024		1,981		$	2.35

As of December 31, 2024, options outstanding have a weighted-average remaining contractual term of 7.8 years and options exercisable have a weighted-average remaining contractual term of 6.4 years.

Total intrinsic value of options outstanding, which is the difference between the exercise price and the closing price of our common stock on the last trading day of the calendar year, was $1.5 million at December 31, 2024 and $2.8 million at December 31, 2023. The were no options outstanding and in-the-money at December 31, 2022.

The total intrinsic value of stock option exercises, which is the difference between the exercise price and closing price of our common stock on the date of exercise, during the years ended December 31, 2024 and 2023 was $385,000 and $10,000, respectively. There were no option exercises during the year ended December 31, 2022.

As of December 31, 2024 and 2023, the total intrinsic value of options outstanding and exercisable was $385,000 and $137,000, respectively. As of December 31, 2022, there were no options outstanding and exercisable that were in-the-money.


	Years Ended December 31,
	2024				2023				2022
			Weighted-				Weighted-				Weighted-
			Average				Average				Average
			Exercise				Exercise				Exercise
	Options		Price		Options		Price		Options		Price
Exercisable at end of year		9,589	$	8.88		11,390	$	12.87		13,650	$	17.86
Options vested or expected to vest		26,082	$	4.46		24,575	$	7.06		20,184	$	14.82

Exercise prices and weighted-average remaining contractual lives for the options outstanding as of December 31, 2024 were:


									Weighted-
			Weighted-						Average
			Average		Weighted-				Exercise
			Remaining		Average				Price of
Outstanding		Range of	Contractual		Exercise		Options		Options
Options		Exercise Prices	Life (in years)		Price		Exercisable		Exercisable
	5,241	$0.97 - $1.51		8.42	$	1.24		1,457	$	1.27
	13,321	$1.52 - $2.28		8.65		1.90		2,683		1.93
	3,826	$2.34 - $3.51		8.03		2.87		1,862		3.00
	87	$3.66 - $5.49		8.62		3.94		35		4.14
	20	$5.50 - $8.25		7.24		5.87		13		5.86
	535	$9.66 - $14.49		4.98		12.01		498		12.00
	1,394	$14.87 - $22.305		3.92		16.37		1,390		16.37
	1,598	$23.00 - $34.50		2.92		25.96		1,591		25.94
	60	$38.20 - $57.30		3.65		38.38		60		38.38
	26,082			7.82	$	4.46		9,589	$	8.88

On December 31, 2024, we had reserved 28,062,017 shares of common stock for future issuance upon exercise of outstanding options and vesting of outstanding restricted stock units granted under the 2007 Plan, as well as the inducement award grants.

The following summarizes stock-based compensation expense related to stock-based payment awards pursuant to our equity compensation arrangements (in thousands):


	December 31,
	2024		2023		2022
Research and development	$	1,856	$	6,505	$	10,025
General and administrative		7,138		16,846		19,552
Sales and marketing		3,968		9,503		13,403
Total stock-based compensation expense	$	12,962	$	32,854	$	42,980

As of December 31, 2024, there was $19.8 million of total unrecognized compensation cost related to non-vested, stock-based payment awards granted under all of our equity compensation plans and all non-plan option grants. Total unrecognized compensation cost will be adjusted for future changes in estimated forfeitures. We expect to recognize this compensation cost over a weighted-average period of 2.4 years.

The fair value of RSUs is estimated based on the closing market price of our common stock on the date of the grant. RSUs generally vest quarterly over a four-year period.

We estimated the fair value of each option grant and ESPP purchase right on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:

Options:


	December 31,
	2024			2023			2022
Risk-free interest rate		4.5	%		3.7	%		3.3	%
Dividend yield		—	%		—	%		—	%
Volatility		80.1	%		68.1	%		62.0	%
Expected life (years)		6		6 to 10				6

ESPP:


	December 31,
	2024			2023			2022
Risk-free interest rate		4.9	%		5.3	%		3.2	%
Dividend yield		—	%		—	%		—	%
Volatility		95.3	%		99.6	%		83.8	%
Expected life (months)		6			6			6

The weighted-average fair value of options granted was $1.40 and $1.13 for the years ended December 31, 2024 and 2023, respectively.

The weighted-average fair value of shares purchased through the ESPP was $1.53 and $0.64 for the years ended December 31, 2024 and 2023, respectively.

The risk-free interest rate assumption is based on observed interest rates on U.S. Treasury debt securities with maturities close to the expected term of our employee and director stock options and ESPP purchases.

The dividend yield assumption is based on our history and expectation of dividend payouts. We have never paid dividends on our common stock, and we do not anticipate paying dividends in the foreseeable future.

The expected life of employee and director stock options represents the average of the contractual term of the options and the weighted-average vesting period, as permitted under the simplified method. We have elected to use the simplified method, as we do not have enough historical exercise experience to provide a reasonable basis on which to estimate the expected term. The expected life for the ESPP purchase rights is six months, which represents the length of each purchase period.