Stockholders' Equity (Deficit)

3 Months Ended

Mar. 31, 2025

Share-Based Payment Arrangement [Abstract]

Stockholders' Equity (Deficit)

10.Stockholders’ Equity (Deficit)

Our authorized capital stock consists of 520,000,000 shares, all with a par value of $0.0001 per share, of which 500,000,000 shares are designated as common stock and 20,000,000 shares are designated as preferred stock. There were no shares of preferred stock outstanding as of March 31, 2025 and December 31, 2024.

Equity Offerings

As part of our July 2019 underwritten public offering, we issued and sold pre-funded warrants to purchase 117,801 shares of common stock in an underwritten public offering pursuant to a shelf registration on Form S-3.

Each pre-funded warrant entitles the holder to purchase one share of common stock at an exercise price of $0.0025 per share and expires seven years from the date of issuance. These warrants were recorded as a component of stockholders’ equity (deficit) within additional paid-in capital. Per the terms of the warrant agreement, a holder of the outstanding warrants is not entitled to exercise any portion of any pre-funded warrant if, upon exercise of the warrant, the holder’s ownership (together with its affiliates) of our common stock or combined voting power of our securities beneficially owned by such holder (together with its affiliates) would exceed 9.99% after giving effect to the exercise (2019 Warrant Maximum Ownership Percentage). Upon at least 61 days’ prior notice to us by the holder, any holder may increase or decrease the 2019 Warrant Maximum Ownership Percentage to any other percentage not to exceed 19.99%. No July 2019 pre-funded warrants were exercised during the three months ended March 31, 2025, and as of March 31, 2025, pre-funded warrants to purchase 101,089 shares of our common stock from the July 2019 underwritten public offering were outstanding.

As part of the May 2020 underwritten public offering, we issued and sold pre-funded warrants to purchase 114,678 shares of common stock in an underwritten public offering pursuant to a shelf registration on Form S-3. Additionally, as part of the December 2020 underwritten public offering, we issued and sold pre-funded warrants to purchase 81,632 shares of common

stock in an underwritten public offering pursuant to a shelf registration on Form S-3. These warrants were recorded as a component of stockholders' equity (deficit) within additional paid-in capital.

The terms of the pre-funded warrants issued and sold as part of the 2020 public offerings were similar to those issued and sold in 2019. No May 2020 or December 2020 pre-funded warrants were exercised during the three months ended March 31, 2025. As of March 31, 2025, 38,735 and 55,387 of the pre-funded warrants to purchase shares of our common stock issued and sold as part of the May 2020 and December 2020 underwritten public offerings, respectively, were outstanding.

In January 2024, we issued and sold pre-funded warrants to purchase 1,090,907 shares of common stock at a price of $13.7475 per warrant in a registered direct offering pursuant to a shelf registration on Form S-3. The gross proceeds from this sale were $15.0 million, resulting in net proceeds of $14.8 million after deducting offering expenses payable by us.

Each of the January 2024 pre-funded warrants issued entitles the holder to purchase one share of common stock at an exercise price of $0.0025 per share, with no expiration date. These warrants were recorded as a component of stockholders’ equity (deficit) within additional paid-in capital. Per the terms of the warrant agreement, a holder of the outstanding warrants is not entitled to exercise any portion of any pre-funded warrant if, upon exercise of the warrant, the holder’s ownership (together with its affiliates) of our common stock or combined voting power of our securities beneficially owned by such holder (together with its affiliates) would exceed 9.99% after giving effect to the exercise (January 2024 Maximum Ownership Percentage). Upon at least 61 days’ prior notice to us by the holder, any holder may increase or decrease the January 2024 Maximum Ownership Percentage to any other percentage not to exceed 19.99%. No January 2024 pre-funded warrants were exercised during the three months ended March 31, 2025 and all 1,090,907 of the January 2024 pre-funded warrants remain outstanding as of March 31, 2025.

In September 2024, we issued and sold 758,900 shares of common stock at an offering price of $8.25 per share and pre-funded warrants to purchase 3,604,780 shares of common stock at an offering price of $8.2499 per warrant in a registered direct offering pursuant to a shelf registration on Form S-3. The gross proceeds from this sale were $36.0 million, resulting in net proceeds of $35.8 million after deducting offering expenses payable by us.

Each of the September 2024 pre-funded warrants issued entitles the holder to purchase one share of common stock at an exercise price of $0.0001 per share, with no expiration date. These warrants were recorded as a component of stockholders’ equity (deficit) within additional paid-in capital. Per the terms of the warrant agreement, a holder of the outstanding warrants is not entitled to exercise any portion of any pre-funded warrant if, upon exercise of the warrant, the holder’s ownership (together with its affiliates) of our common stock or combined voting power of our securities beneficially owned by such holder (together with its affiliates) would exceed, at the holder's election, 4.99%, 9.99% or 19.99% after giving effect to the exercise (the September 2024 Warrant Maximum Ownership Percentage). Upon at least 61 days’ prior notice to us by the holder, any holder may increase or decrease the September 2024 Warrant Maximum Ownership Percentage to any other percentage not to exceed 19.99%. No September 2024 pre-funded warrants were exercised during the three months ended March 31, 2025 and as of March 31, 2025, pre-funded warrants to purchase 3,596,146 shares of our common stock from the September 2024 direct offering were outstanding.

ATM Facilities

In the past three years, we have entered into two separate sales agreements with Cowen and Company, LLC (Cowen): in November 2021 (2021 ATM Facility) and in November 2023 (2023 ATM Facility). Each ATM facility provides or provided for the sale, in our sole discretion, of shares of our common stock having an aggregate offering price of up to $100.0 million, through Cowen, as our sales agent. We filed a registration statement on Form S-3 registering the offer and sale of these shares under the Securities Act (2023 Registration Statement). Upon the effectiveness of the 2023 Registration Statement, the 2021 ATM Facility was terminated, and no further sales can be made under the 2021 ATM Facility. The issuance and sale of these shares by us pursuant to the ATM facilities are deemed “at the market” offerings defined in Rule 415 under the Securities Act of 1933, as amended (Securities Act), and were registered under the Securities Act. Commissions of up to 3.0% are due on the gross sales proceeds of the common stock sold under each ATM facility.

During the three months ended March 31, 2024, we sold an aggregate of 492,855 shares of common stock under the 2023 ATM Facility, at an average price of $19.23 per share, for gross proceeds of $9.5 million and net proceeds of $9.3 million, after deducting commissions and other offering expenses payable by us.

During the three months ended March 31, 2025, no shares of common stock were sold under the 2023 ATM Facility.

As of March 31, 2025, we had $88.7 million of common stock remained available to be sold under the 2023 ATM Facility.

Equity Incentive Plans

In June 2024, we adopted the 2024 Equity Incentive Plan (2024 EIP), under which we may grant stock options, restricted stock awards (RSAs) and RSUs to employees, directors, consultants and other service providers. RSUs generally vest over two to four years. The 2014 Equity Incentive Plan, as amended (2014 EIP), expired March 31, 2024, after which no new awards can be granted from it. All awards granted prior to the 2014 EIP expiration continue to remain outstanding and governed in accordance with the rules set forth in the 2014 EIP and the terms of the associated grant notice. To the extent forfeited, cancelled or expired, certain awards granted under the 2014 EIP will become available for grant under the 2024 EIP.

RSUs generally vest over two to four years. We have granted performance-based RSUs to certain of our employees that provide for the issuance of common stock if specified Company performance criteria related to tab-cel regulatory milestones are achieved. The number of performance-based RSUs that ultimately vests depends upon if and which performance criteria are achieved, as well as the employee’s continuous service, as defined in the 2014 EIP and 2024 EIP, through the date of vesting. The fair value of performance-based RSUs is determined as the closing stock price on the date of grant.

Stock options are granted at prices no less than 100% of the estimated fair value of the shares on the date of grant as determined by the board of directors, provided, however, that the exercise price of an option granted to a 10% shareholder cannot be less than 110% of the estimated fair value of the shares on the date of grant. Options granted generally vest over three to four years and expire in seven to ten years. We have granted performance-based stock options to certain of our employees that provide for the issuance of a right to purchase a share of common stock if specified Company performance criteria related to tab-cel regulatory milestones are achieved. The vesting of performance-based stock options depends upon if and when the performance criteria are achieved, as well as the employee’s continuous service as defined in the 2014 EIP and 2024 EIP, through the date of vesting.

As of March 31, 2025, a total of 462,897 shares of common stock were reserved for issuance under the 2014 EIP, of which all were subject to outstanding options and RSUs, including performance-based awards.

As of March 31, 2025, a total of 415,093 shares of common stock were reserved for issuance under the 2024 EIP, of which 36,093 shares were available for future grant and 379,000 shares were subject to outstanding options and RSUs.

In February 2018, we adopted the 2018 Inducement Plan (Inducement Plan), under which we may grant options, stock appreciation rights, RSAs and RSUs to new employees. In November 2020, September 2021 and June 2022 we amended the Inducement Plan to reserve an additional 60,000 shares of the Company’s common stock for issuance under the Inducement Plan in each case.

As of March 31, 2025, 168,813 shares of common stock were reserved for issuance under the Inducement Plan, of which 126,007 shares were available for future grant and 42,806 shares were subject to outstanding options and RSUs.

In May 2014, we adopted the 2014 Employee Stock Purchase Plan (2014 ESPP), which permits eligible employees to purchase common stock at a discount through payroll deductions during defined offering periods. Following stockholder approval, in June 2024, we amended the 2014 ESPP to increase the number of shares of our common stock available for issuance under the 2014 ESPP by 40,000 shares.

Restricted Stock Units

The following is a summary of RSU activity under our 2014 EIP, 2024 EIP and Inducement Plan:

	RSUs
	Shares			Weighted Average Grant Date Fair Value
Balance as of December 31, 2024		414,470		$	39.09
Granted		339,100		$	7.38
Forfeited		(10,042	)	$	87.96
Vested		(65,580	)	$	50.66
Balance as of March 31, 2025		677,948		$	21.39

As of March 31, 2025, there was $8.6 million of unrecognized stock-based compensation expense related to RSUs that is expected to be recognized over a weighted average period of 1.6 years. This excludes unrecognized stock-based compensation expense for performance-based RSUs that were deemed not probable of vesting in accordance with U.S. GAAP.

Stock Options

The following is a summary of stock option activity under our 2014 EIP, 2024 EIP and Inducement Plan:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Balance as of December 31, 2024		234,143		$	217.91		7.0	$	27
Granted		—					—		—
Exercised		—			—		—		—
Forfeited or expired		(27,388	)		257.66		—		—
Balance as of March 31, 2025		206,755		$	212.64		5.9	$	—

Aggregate intrinsic value represents the difference between the closing stock price of our common stock on March 31, 2025 and the exercise price of outstanding, in-the-money options. As of March 31, 2025, there was $2.9 million of unrecognized stock-based compensation expense related to stock options that is expected to be recognized over a weighted average period of 0.9 years. This excludes unrecognized stock-based compensation expense for performance-based stock options that were deemed not probable of vesting in accordance with U.S. GAAP.

Reserved Shares

The following shares of common stock were reserved for future issuance under our equity incentive plans as of March 31, 2025:

	Total Shares Reserved
2014 Equity Incentive Plan		462,897
2018 Inducement Plan		168,813
2024 Equity Incentive Plan		415,093
2014 Employee Stock Purchase Plan		23,731
Total reserved shares of common stock		1,070,534

Stock-based Compensation Expense

The following is a summary of stock-based compensation expense for the periods presented:

	Three Months Ended March 31,
	2025		2024
	(in thousands)
Research and development	$	1,598	$	4,713
General and administrative		2,611		3,684
Total stock-based compensation expense	$	4,209	$	8,397