In May 2017, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the 2016 ESPP). The 2016 ESPP is structured as a qualified employee stock purchase plan under Section 423 of the Internal Revenue Code of 1986, as amended, and is not subject to the provisions of the Employee Retirement Income Security Act of 1974. The Company reserved 800,000 shares of common stock for issuance under the 2016 ESPP. The 2016 ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 10% of their eligible compensation, subject to any plan limitations. The 2016 ESPP provides for six-month offering periods ending on May 31 and November 30 of each year. At the end of each offering period, employees are able to purchase shares at 85% of the fair market value of the Company’s common stock on the last day of the offering period. During the six months ended June 30, 2026 and 2025, 33,092 and 56,880 shares of common stock were purchased under the 2016 ESPP, respectively.

In October 2013, the Company implemented the 2013 Equity Incentive Plan (2013 Plan). In May 2023, the 2013 Plan was terminated, and no further awards may be issued under the plan. If an option granted under the 2013 Plan expires or terminates for any reason without having been fully exercised, if any shares of restricted stock are forfeited, or if any award terminates, expires or is settled without all or a portion of the shares of common stock covered by the award being issued, such shares will become available for issuance under the 2023 Equity Incentive Plan (2023 Plan).

The 2023 Plan was effective as of stockholder approval in May 2023. The 2023 Plan provides for grants of stock options and other stock-based awards, as well as cash-based performance awards. The 2023 Plan originally authorized the issuance of up to an aggregate of 4,850,000 shares of common stock. The Board of Directors and stockholders of the Company subsequently approved amendments to the 2023 Plan to increase the number of shares of common stock available to a total of

9,350,000 shares. If an option expires or terminates for any reason without having been fully exercised, if any shares of restricted stock are forfeited, or if any award terminates, expires or is settled without all or a portion of the shares of common stock covered by the award being issued, such shares are available for the grant of additional awards. However, any shares that are withheld (or delivered) to pay withholding taxes or to pay the exercise price of an option are not available for the grant of additional awards.


	Three Months Ended June 30,				Six Months Ended June 30,
	2026		2025		2026		2025
Research and development	$	753	$	2,237	$	2,170	$	5,320
General and administrative	1,221		1,453		2,450		2,756
Total stock-based compensation expense	$	1,974	$	3,690	$	4,620	$	8,076

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table for options issued during the period indicated:


	Six Months Ended June 30,
	2026	2025
Expected dividend yield	0%	0%
Expected volatility	112.6% - 114.0%	110.7% - 115.6%
Risk-free interest rate	3.7% - 4.4%	3.9% - 4.5%
Expected term	6.22 years	6.11 years


	Shares	Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2025	14,106,148	$	11.72		$	125
Granted	2,975,160	1.91		—	$	—
Exercised	(23,032)	2.92		—	$	24
Forfeited	(1,251,731)	4.28		—	$	—
Expired	(1,046,363)	15.69		—	$	—
Outstanding, June 30, 2026	14,760,182	$	10.10	6.4	$	13,003
As of June 30, 2026:
Exercisable	10,006,891	$	13.26	5.1	$	2,327
Vested and expected to vest	14,359,528	$	10.28	6.3	$	12,146

As of June 30, 2026, the total unrecognized compensation expense related to unvested stock options, net of related forfeiture estimates, was approximately $9.6 million, which the Company expects to recognize over a weighted-average period of approximately 1.2 years.

The following table summarizes additional information on stock options (in thousands, except per share amounts):


	Six Months Ended June 30,
		2026		2025
Weighted-average fair value per share of stock options granted		$	1.60	$	2.03
Total intrinsic value of stock options exercised		$	24	$	—
Total cash received for stock options exercised		$	67	$	—
Total grant date fair value of stock options vested		$	4,285	$	9,494

Restricted stock units (RSUs) are valued based on the closing price of the Company’s common stock on the date of the grant. The fair value of RSUs is recognized and amortized on a straight-line basis over the requisite service period of the award.

At June 30, 2026, there was $2.1 million of total unrecognized compensation cost related to unvested RSUs, which the Company expects to recognize over a remaining weighted-average period of approximately 1.2 years.