Atossa Therapeutics Announces Positive FDA Feedback, Advances Toward IND for (Z)-Endoxifen Clinical Program in ER+/HER2- Metastatic Breast Cancer

SEATTLE, July 29, 2025 /PRNewswire/ -- Atossa Therapeutics, Inc. (NASDAQ: ATOS), a clinical-stage biopharmaceutical company developing innovative medicines in oncology, today announced positive written feedback from the U.S. Food and Drug Administration (FDA) regarding the company's proposed dose optimization trial of (Z)-endoxifen for the treatment of estrogen receptor positive (ER+), human epidermal growth factor receptor 2 negative (HER2-) metastatic breast cancer.

With the FDA's feedback, Atossa is energized and moving quickly to advance (Z)-endoxifen in metastatic breast cancer.

The FDA has provided highly constructive responses ahead of the scheduled pre-Investigational New Drug (IND) meeting, affirming key elements of Atossa's clinical development plan, negating the need for a virtual meeting, and paving the way for a potential IND submission targeted for the fourth quarter of 2025.

"These FDA responses mark a significant milestone for the Company and are supportive of our comprehensive approach to developing (Z)-endoxifen for metastatic breast cancer," stated Dr. Steven Quay, Atossa's Chief Executive Officer and Chairman of the Board. "The detailed feedback received significantly advances our goal of submitting an IND by year-end. Importantly, the FDA's support of our dose optimization strategy and general agreement with our nonclinical data package leave us confident in our scientific rationale and overall regulatory approach."

Key Highlights:

• Dose Optimization Strategy Affirmed: FDA agreed that existing clinical and nonclinical data are sufficient to initiate Part A (monotherapy) of the proposed dose optimization study and provided clear guidance on randomization cohort sizes and study design enhancements.
  
  
• Combination Study Support: The Agency agreed with the scientific rationale for combining (Z)-endoxifen with approved breast cancer standard-of-care therapies, such as some CDK4/6 inhibitors, PI3K inhibitors, mTOR inhibitors, and capecitabine. While not all combinations will be investigated in this study, FDA guidance on the matter is helpful in narrowing the strategic approach for the potential IND.
  
  
• Nonclinical Data Package Deemed Adequate: FDA indicated the existing nonclinical safety data package is adequate to proceed without additional general toxicity or neurotoxicity studies.
  
  
• Agreement on Cardiac Safety Assessments for Monotherapy: FDA confirmed Atossa's cardiac safety assessment plan, including serial electrocardiograms (ECGs) and QT interval monitoring, is sufficient for the monotherapy portion of the trial.
  
  
• Next steps: In the coming weeks, Atossa will announce plans for the target patient population, combination backbone, and overall trial design for the upcoming dose-ranging study. The Agency acknowledged the Company's plan to file an IND in 2025 and encouraged Atossa to incorporate specific safety and eligibility refinements in the final protocol, which the Company has accepted.

Dr. Quay continued, "With the FDA's feedback now in hand, Atossa is energized and moving quickly. We believe the company is well positioned to maintain strategic momentum and meet regulatory milestones, bringing us closer to delivering (Z)-endoxifen to patients in need and driving shareholder value. Further updates will be provided as the trial design is finalized, and next steps are implemented."

Atossa's progress reflects the strong strategic momentum promised in the March 11, 2025 announcement of a metastatic focus; momentum that aligns well with FDA expectations and regulatory standards, including fulfilling Project Optimus requirements, a crucial next step in advancing (Z)-endoxifen toward potential approval.

The FDA's Project Optimus initiative emphasizes data-driven dose exploration to maximize benefit and minimize toxicity. In line with this guidance, Atossa will explore multiple dose levels in its upcoming clinical study to define the optimal dose for combination therapy, while seeking to maintain a balance between efficacy and patient safety.

About (Z)-Endoxifen

(Z)-endoxifen is a highly potent Selective Estrogen Receptor Modulator (SERM) with demonstrated ability to inhibit—and potentially degrade—estrogen receptors. It has shown activity even in tumors that have developed resistance to other endocrine therapies. Beyond its anti-estrogenic properties, (Z)-endoxifen also targets protein kinase C beta 1 (PKCβ1), an oncogenic signaling protein, at clinically achievable blood levels. Importantly, (Z)-endoxifen seems to deliver comparable or superior bone-protective effects relative to tamoxifen.

Atossa is developing a proprietary oral formulation of (Z)-endoxifen that is enteric-coated to bypass stomach acid, which would otherwise convert the active (Z)-isomer to its inactive (E)-form. This innovation ensures optimal bioavailability and therapeutic integrity. Clinical studies have shown Atossa's (Z)-endoxifen to be well tolerated in both healthy women and those with breast cancer. In over 700 subjects (healthy volunteers and breast cancer patients), doses up to 360 mg/day have been administered with no maximum tolerated dose (MTD) identified, supporting continued dose-ranging exploration.

Atossa is prioritizing the development of (Z)-endoxifen for the treatment of metastatic breast cancer, where novel therapeutic options are urgently needed. The compound is currently being evaluated in three Phase 2 trials: one in women with ductal carcinoma in situ (DCIS) and two in women with ER+/HER2- breast cancer. Atossa's (Z)-endoxifen program is supported by a growing global intellectual property portfolio, including three recently issued U.S. patents and numerous pending applications worldwide.

About Atossa Therapeutics

Atossa Therapeutics, Inc. (Nasdaq: ATOS) is a clinical-stage biopharmaceutical company dedicated to transforming breast cancer treatment through innovative science and patient-focused solutions. The company's lead product candidate, (Z)-endoxifen, is a highly potent SERM designed for use across the breast cancer spectrum, including prevention, neoadjuvant, adjuvant, and metastatic settings. Atossa is committed to advancing its robust clinical research programs to improve patient outcomes while creating sustainable value for shareholders. For more information, visit 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 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 and related timing, 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, 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; the expected timing of releasing data; 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.

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SOURCE Atossa Therapeutics Inc