Note 5 - Equity 

In August 2020, the Company entered into the Capital on Demand™ Sales Agreement with JonesTrading Institutional Services LLC, “JonesTrading”, pursuant to which the Company may sell, from time to time, through or to JonesTrading, up to an aggregate of $200 million of its common stock. On June 28, 2022, the Company entered into an Amended and Restated Capital on Demand™ Sales Agreement (the “A&R Sales Agreement”) with JonesTrading and B. Riley Securities, Inc. (“B. Riley”). The A&R Sales Agreement modifies the original Capital on Demand™ Sales Agreement to include B. Riley Securities as an additional sales agent thereunder. Shares of common stock were offered pursuant to a shelf registration statement on Form S-3 (File No. 333-242322) filed with the SEC on August 7, 2020 (the “Prior Shelf Registration Statement”). On August 11, 2023, the Company filed a registration statement on Form S-3 (File No. 333-273911), which registration statement was amended on February 2, 2024, and declared effective on February 5, 2024, to replace the Prior Shelf Registration Statement, including a base prospectus which covers the offering, issuance and sale of up to $500 million of common stock, preferred stock, warrants, units and/or subscription rights; and a sales agreement prospectus covering the offering, issuance and sale of up to a maximum aggregate offering price of $200 million of common stock that may be issued and sold under the A&R Sales Agreement.

The Company did not sell any shares of common stock during the three months ended March 31, 2026 and 2025, respectively.

Stock Options

The following is a summary of stock option activity for the three months ended March 31, 2026:

During the three months ended March 31, 2026, the Company granted newly hired employees options to purchase 30 thousand shares of common stock with an exercise price ranging from $1.08 to $1.28 per share, a term of 10 years, and a vesting period of 4 years. The stock options had an aggregated fair value of $26 thousand that was calculated using the Black-Scholes option-pricing model. Variables used in the Black-Scholes option-pricing model include: (1) discount rate range from 3.7% to 4.0%, (2) expected life of 6 years, (3) expected volatility range from 86.8% to 87.3%, and (4) zero expected dividends. During the three months ended March 31, 2025, the Company granted options to purchase 25 thousand shares.

On March 31, 2025, the Board of Directors approved of the cancellation of stock options to purchase an aggregate of 4.9 million shares of common stock held by certain current employees and directors that were initially granted under the Amended and Restated 2013 Stock Plan and the 2019 Stock Plan. Such cancellations were subject to the consent of the applicable holders of the stock options, which the Company received. The cancellation of these stock options resulted in the recording of $8.7 million in stock compensation expense for the three months ended March 31, 2025.

The fair values of all options issued and outstanding are being amortized over their respective vesting periods. The unrecognized compensation expense at March 31, 2026 was $0.1 million related to unvested stock options, which is expected to be expensed over a weighted average of 2.9 years.

Warrants

Following is a summary of warrant activity for the three months ended March 31, 2026: