Amendment to Current Report (Form 8-K/A)

Athira Pharma Announces Exclusive License to Lasofoxifene Phase 3 Development Program for Metastatic Breast Cancer Candidate and a Financing for up to $236 Million

Phase 3 Development Program of Novel Selective Estrogen Receptor Modulator (SERM) Represents Potential Multi-Billion Dollar Opportunity as Treatment Option for Patients with ESR1-Mutations

Financing Co-Led by Commodore Capital, Perceptive Advisors and TCGX Supports Development of Lasofoxifene through Phase 3 Clinical Topline Data Readout and Regulatory Milestones

Conference Call Today at 8:30 am Eastern Time

BOTHELL, Wash., December 18, 2025 - Athira Pharma, Inc. (NASDAQ: ATHA), a clinical-stage biopharmaceutical company dedicated to the development of novel therapeutics for high unmet medical needs, today announced that it has entered into an agreement to acquire the rights for the development and commercialization of lasofoxifene, a promising clinical asset in a potentially registrational Phase 3 trial. The ongoing Phase 3 ELAINE-3 clinical trial (NCT05696626) is greater than 50% enrolled with data expected in mid-2027.

Athira has acquired an exclusive global license (excluding Asia and certain countries in the Middle East) from Sermonix Pharmaceuticals, Inc. for rights to develop and commercialize lasofoxifene, a selective estrogen receptor modulator (SERM) for the potential treatment of metastatic breast cancer.

In conjunction with this transaction, Athira also announced an upfront financing of $90 million in private placement financing of common stock and warrants, with the warrants providing, if exercised, up to an additional $146 million to support development of the new program through key clinical and regulatory milestones. The financing was co-led by Commodore Capital, Perceptive Advisors, and TCGX, with participation from ADAR1, Blackstone Multi-Asset Investing, Kalehua Capital, Ligand Pharmaceuticals, New Enterprise Associates (NEA), Spruce Street Capital, and 9vc. The Company anticipates the upfront financing will support lasofoxifene development through its topline data readout and key regulatory milestones, with sufficient capital for runway into 2028.

"Today marks a defining moment for our company. This agreement for the rights to the Phase 3 lasofoxifene program for metastatic breast cancer is a significant step in building a pipeline with the potential to change lives and create enduring value," said Mark Litton, Ph.D., President and Chief Executive Officer of Athira. "This program provides a near-term opportunity to generate pivotal data necessary for the approval of lasofoxifene and to establish it as the new standard of care to treat ESR1-mutant breast cancer in patients who have progressed on aromatase inhibitors and prior CDK4/6 inhibitors. Supported by compelling Phase 2 clinical data and backed by blue-chip investors, we have a clear development strategy and are committed to advancing therapies that matter to patients while delivering value for our shareholders."

"In the ELAINE-2 clinical trial, lasofoxifene demonstrated the potential to provide meaningful combination efficacy with 13 months of progression-free survival in heavily pre-treated second- and third-line ESR1-mutated metastatic breast cancer patients. We believe lasofoxifene has the potential to be the preferred endocrine therapy for metastatic breast cancer patients given its tissue-selective SERM profile may allow for the preservation of estrogen function in non-breast tissue, which provides tolerability and potential

bone protection and quality of life benefits," noted David Portman, M.D., Chief Executive Officer of Sermonix. "With the Phase 3 trial now more than 50% enrolled, we look forward to delivering pivotal data in mid-2027 and advancing toward a regulatory submission."

"We are excited to announce this financing to help accelerate the development of lasofoxifene," said Cariad Chester, Managing Partner at TCGX. "With its differentiated profile, lasofoxifene has the potential to become the endocrine therapy of choice for the approximately 40% of breast cancer patients who develop ESR1 mutations and have progressed on aromatase inhibitors and prior CDK4/6 inhibitors. The accomplished team at Athira is committed to efficiently executing the ongoing pivotal trial of lasofoxifene, and we believe, lasofoxifene, may become the treatment of choice for oncologists and patients who are battling this challenging disease."

"The scientific and clinical data supporting lasofoxifene are compelling, and we are confident in Athira's leadership to drive the Company's next chapter with clarity, urgency, and excellence. We're proud to support this evolution and excited by the opportunity to deliver meaningful impact for patients and shareholders alike," stated Joseph Edelman, Founder and CEO of Perceptive Advisors.

Details of the Transaction

Sermonix License Agreement

In connection with the entry into the Sermonix license agreement, Athira will issue to Sermonix, as partial consideration, a pre-funded warrant to purchase approximately 5.5 million shares of common stock, with an exercise price of $0.001 per share. Athira will also be obligated to make certain payments to Sermonix of up to $100.0 million if Athira achieves certain commercialization or annual net sales milestones with respect to licensed products with respect to lasofoxifene, including royalty payments on the net sales of any licensed products in any licensed territory, ranging from sub-single digit to low-single digit royalties depending on pre-specified net sales.

Financing

On December 18, 2025, Athira entered into a securities purchase agreement with certain investors, pursuant to which Athira has agreed to issue and sell approximately 5.4 million shares of common stock, pre-funded warrants to purchase approximately 8.8 million shares of common stock and accompanying warrants to purchase approximately 23.0 million shares of common stock and/or pre-funded warrants (representing 162.5% of the aggregate shares and shares underlying the pre-funded warrants), with an exercise price of $6.35 per share (the "Series A common warrants"), and accompanying warrants to purchase approximately 21.3 million shares of common stock and/or pre-funded warrants (representing 150% of the aggregate shares and shares underlying the pre-funded warrants), with an exercise price of $7.62 per share (the "Series B common warrants") (the "Private Placement"). The common stock, including the accompanying Series A common warrants and Series B common warrants, will be sold at a price of $6.35 per share, and the pre-funded warrants, including the accompanying Series A common warrants and Series B common warrants, will be sold at the price per underlying pre-funded warrant share of the common stock less the exercise price of $0.001 per share.

The Private Placement is expected to close on or about December 23, 2025, subject to the satisfaction of customary closing conditions. Additional details regarding the Private Placement and the other transactions discussed herein will be included in a Current Report on Form 8-K to be filed by Athira with the Securities and Exchange Commission ("SEC").

Athira intends to use the net proceeds from the Private Placement to fund the development of lasofoxifene for the potential treatment of treatment-resistant metastatic breast cancer, and other clinical assets in its pipeline, such as ATH-1105 as a treatment for ALS, and for working capital and general corporate purposes.

Cantor is acting as exclusive financial advisor to Athira in connection with the license transaction and sole placement agent in connection with the Private Placement.

The securities being sold in the private placements discussed herein have not been registered under the Securities Act of 1933, as amended (the "Securities Act"), or state securities laws and may not be offered or sold in the United States absent registration with the SEC or an applicable exemption from applicable registration requirements. Athira has agreed to file registration statements with the SEC covering the resale of the shares of common stock issuable in connection with the private placements and upon exercise of the pre-funded warrants and warrants.