Atossa Therapeutics Selects PSI as Contract Research Organization for Pivotal Dose-Ranging Study of (Z)-Endoxifen in Metastatic Breast Cancer

Monotherapy Phase 2 study designed with FDA input; IND expected in Q4 2025

Topline results anticipated in 2026

 Positions Atossa for transition into registrational Phase 3 development

SEATTLE, Aug. 20, 2025 /PRNewswire/ -- Atossa Therapeutics, Inc. (Nasdaq: ATOS) ("Atossa" or the "Company"), a clinical-stage biopharmaceutical company developing innovative medicines for breast cancer, announced today it has selected PSI, a leading global contract research organization (CRO), to operationalize and manage its planned (Z)-endoxifen monotherapy dose-ranging study in women with metastatic breast cancer (mBC). The study was designed following guidance from the U.S. Food and Drug Administration (FDA) and is intended to directly inform a subsequent Phase 3 trial.

The global Phase 2, multi-center dose-ranging study is designed to evaluate (Z)-endoxifen monotherapy for safety, pharmacokinetics/pharmacodynamics, and preliminary anti-tumor activity. Patient enrollment is expected to follow the Investigational New Drug (IND) filing in Q4 2025, with topline data anticipated in 2026.

"This is a defining milestone for Atossa," said Dr. Steven Quay, Chairman and Chief Executive Officer of Atossa. "With PSI as our partner, we are advancing (Z)-endoxifen into its final steps before Phase 3. Unlike existing endocrine therapies, (Z)-endoxifen has demonstrated clinical activity even in tumors resistant to aromatase inhibitors and fulvestrant. If successful, this program could reshape the standard of care in breast cancer treatment, including in the metastatic setting, and deliver hope to patients who currently face limited options."

PSI is widely recognized for excellence in global oncology trial execution with the ability to move swiftly from Phase 2 into Phase 3 trials. In 2024, 93 percent of PSI-managed studies enrolled on time or ahead of schedule, leveraging its proprietary machine-learning feasibility platform, VISIONAL™. Atossa selected PSI, following a highly competitive process, for its proven ability to:

• Deliver regulatory-grade data through rigorous quality and compliance systems.
  
  
• Consistently achieve on-time enrollment across diverse geographies.
  
  
• Seamlessly scale programs into Phase 3 and pivotal studies, often reducing operational risk and accelerating timelines.

"We are excited to partner with Atossa on this pivotal study of (Z)-endoxifen. Based on our assessment, the program is designed to transition seamlessly into Phase 3, and we are allocating significant global resources to accelerate its execution. We believe this is a high-priority program with the potential to reshape the standard of care in metastatic breast cancer." — said Nick Sinackevich, PSI's President.

Market Opportunity

Each year, approximately 5.6 percent of newly diagnosed invasive breast cancers in women in the U.S., or about 13,000 women annually, are diagnosed at the metastatic stage. In total, there are an estimated 170,000 women living with metastatic breast cancer in the United States. Current therapies often fail due to resistance to existing therapies, leaving patients with limited options. Atossa believes (Z)-endoxifen has the potential to become a first-in-class therapy in this multi-billion-dollar market.

Pipeline Momentum

In addition to the upcoming metastatic dose-ranging trial, Atossa is advancing additional Phase 2 studies in the ER+/HER2- neoadjuvant breast cancer setting (i.e., before definitive surgery), including studies of monotherapy of (Z)-endoxifen in early stage breast cancer; combination therapy with abemaciclib in women with early stage breast cancer at high risk of recurrence; and monotherapy in women with a biopsy-proven diagnosis of ductal carcinoma in situ (DCIS). Multiple clinical readouts are anticipated in the coming months.

About (Z)-Endoxifen

(Z)-endoxifen is a highly potent Selective Estrogen Receptor Modulator/ Degrader (SERM/D) with dual mechanisms of action:

• Estrogen receptor inhibition and degradation, including in tumors resistant to other endocrine therapies.
  
  
• Direct inhibition of protein kinase C beta 1 (PKCβ1), an oncogenic signaling protein, at clinically achievable blood levels.

Preclinical and clinical studies suggest (Z)-endoxifen may deliver superior bone-protective effects relative to tamoxifen, while maintaining a favorable safety profile. Across more than 700 subjects dosed up to 360 mg/day, no maximum tolerated dose (MTD) has been identified, underscoring its potential for broad dose exploration. Clinical studies with tamoxifen demonstrate an inverse correlation between endoxifen blood concentration and hot flash severity score following adjustment for age, BMI, and menopausal status, suggesting it may deliver a better tolerance profile.

Atossa's proprietary oral, enteric-coated formulation is designed to preserve the active (Z)-isomer and promote optimal bioavailability, overcoming limitations of prior formulations.

About Atossa Therapeutics

Atossa Therapeutics, Inc. (Nasdaq: ATOS) is a clinical-stage biopharmaceutical company dedicated to transforming breast cancer treatment and prevention through innovative science and patient-focused solutions. The Company's lead product candidate, (Z)-endoxifen, is being developed as a therapy across the breast cancer spectrum, including prevention and treatment settings. For more information, please visit www.atossatherapeutics.com.

Forward-Looking Statements

This press release contains certain information that may constitute forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. We may identify these forward-looking statements by the use of words such as "expect," "potential," "continue," "may," "will," "should," "could," "would," "seek," "intend," "plan," "estimate," "anticipate," "believe," "design," "predict," "future," or other comparable words. All statements made in this press release that are not statements of historical fact, including statements regarding the Company's development strategy and related milestones, data related to the (Z)-endoxifen program, the safety, tolerability and efficacy of (Z)-endoxifen, the potential of (Z)-endoxifen as a breast cancer prevention and treatment agent, the potential indications that the Company may pursue for (Z)-endoxifen, the potential for (Z)-endoxifen to receive regulatory approval, including the potential IND submission, Phase 3 trial and related timing, the expected timing of releasing data; benefits of the Company's strategy of pursuing a metastatic indication for (Z)-endoxifen, the expected design and enrollment of trials and timing of data and related publications, the Company's progress across its pipeline, the strength of the Company's patent portfolio and expectations regarding related litigation, and the potential market and growth opportunities for the Company, are forward-looking statements. Forward-looking statements in this press release are subject to risks and uncertainties that may cause actual results, outcomes, or the timing of actual results or outcomes, to differ materially from those projected or anticipated, including risks and uncertainties associated with: our ability to obtain patent coverage for our product candidates; macroeconomic conditions and increasing geopolitical instability; any variation between interim or preliminary and final clinical results or analysis; actions and inactions by the FDA and foreign regulatory bodies; the outcome or timing of regulatory approvals needed by Atossa, including those needed to continue our planned (Z)-endoxifen trials; our ability to satisfy regulatory requirements; our ability to regain compliance or maintain compliance with the continued listing requirements of the Nasdaq Stock Market; our ability to successfully develop and commercialize new therapeutics; the success, costs and timing of our development activities, including our ability to successfully initiate or complete our clinical trials, including our (Z)-endoxifen trials; our anticipated rate of patient enrollment; our ability to contract with third-parties and their ability to perform adequately; our estimates on the size and characteristics of our potential markets; our ability to successfully defend litigation and other similar complaints and to establish and maintain intellectual property rights covering our products; whether we can successfully complete our clinical trial of oral (Z)-endoxifen in women with mammographic breast density and our trials of (Z)-endoxifen in women with breast cancer, and whether the studies will meet their objectives; our expectations as to future financial performance, expense levels and capital sources, including our ability to raise capital; our ability to attract and retain key personnel; our anticipated working capital needs and expectations around the sufficiency of our cash reserves; and other risks and uncertainties detailed from time to time in Atossa's filings with the Securities and Exchange Commission, including without limitation its Annual Reports on Form 10-K and Quarterly Reports on 10-Q. Forward-looking statements are presented as of the date of this press release. Except as required by law, we do not intend to update any forward-looking statements, whether as a result of new information, future events or circumstances or otherwise.

SOURCE Atossa Therapeutics Inc