Atossa Therapeutics Announces Plans to Pursue Metastatic Breast Cancer Indication for (Z)-Endoxifen and Continued Engagement with FDA on Additional Indications (2025)

Atossa Therapeutics, Inc.(Nasdaq: ATOS)(“Atossa” or the “Company”), a clinical-stage biopharmaceuticalcompany dedicated to the prevention and treatment of breast cancer,today announced its strategic decision to pursue a metastaticbreast cancer indication for (Z)-endoxifen. Atossa believes thatpursuing a metastatic indication may offer a more efficientregulatory pathway to deliver (Z)-endoxifen to women in urgentneed, and simultaneously plans to work with the FDA to advanceadditional indications, such as breast cancer prevention andneoadjuvant therapy, that often require larger and longer clinicaltrials.

Addressing the High Unmet Need inMetastatic Breast Cancer

Metastatic breast cancer remains a significantarea of unmet medical need, with current treatment options oftenproviding limited durability of response and substantial sideeffects. (Z)-endoxifen, a potent and well-tolerated selectiveestrogen receptor modulator (SERM), has shown encouraging signalsin previous clinical trials, which we believe supports itspotential to fill this critical gap in treatment.

Promising Data from Phase 1 and Phase 2Trials

(Z)-endoxifen has shown encouraging results inearly-stage trials, which we believe reinforces its potential as atransformative therapy for breast cancer. Key findings from earlyclinical programs include:

• Significantly improvedprogression-free survival (PFS) in CDK4/6i-naïvepatients: (Z)-endoxifen more than doubled the medianPFS compared to tamoxifen (7.2 vs. 2.4 months), highlighting itspotency in an endocrine-sensitive population.

• Substantial activityobserved even after tamoxifen progression: Patientsin the crossover arm who progressed on tamoxifen and switched to(Z)-endoxifen experienced clinical benefit, including partialresponses and prolonged stable disease exceeding 2-3 years in somecases.

• Favorable safetyprofile: Despite its higher potency, (Z)-endoxifenhas not shown unexpected safety concerns beyond what is typicallyseen with tamoxifen and has been generally well-tolerated.
  

Dr. Steven Quay, Chairman and Chief ExecutiveOfficer of Atossa Therapeutics, stated: “Our decision to advance(Z)-endoxifen into a metastatic breast cancer indicationunderscores our unwavering commitment to developing a best-in-classtherapy for women facing this devastating disease. The encouragingclinical data that has been generated to date supports thepotential of (Z)-endoxifen to provide a meaningful benefit topatients who have exhausted other treatment options. By pursuingthis strategy, we believe we are not only addressing an urgentmedical need but also fortifying the path forward for expanding(Z)-endoxifen’s role across the full spectrum of breast cancerprevention and treatment. We look forward to providing futureupdates as we execute this plan.”

The Strategic Advantage of a MetastaticIndication

We believe that pursuing an initial approval inmetastatic breast cancer provides an efficient regulatory andclinical path to market, potentially allowing Atossa to morerapidly bring (Z)-endoxifen to patients who need it most. Thisapproach can also strengthen the foundation for the expansion of(Z)-endoxifen into earlier-stage disease settings, where(Z)-endoxifen has already shown significant promise in reducingtumor proliferation and preventing recurrence.

Advancing Additional Indications inPrevention and Early-Stage Disease

Beyond metastatic disease, Atossa remainscommitted to demonstrating the broad clinical utility of(Z)-endoxifen across the breast cancer treatment paradigm. Sincethe release of compelling data at the end of 2024, the Company isactively engaging with the FDA to further define a regulatory pathforward for additional indications, including in:

• Breast CancerPrevention: Prior studies have indicated that(Z)-endoxifen can significantly reduce breast tissue density andestrogen receptor activity, key risk factors for developing breastcancer.

• NeoadjuvantTherapy: Data from early-stage patients indicate thatthe 4-week Ki-67 ≤10% response rate remained consistently above 85percent across all dose levels, regardless of the presence ofovarian function. These findings suggest that (Z)-endoxifen mayeffectively reduce tumor proliferation, which may lead to improvedsurgical outcomes and long-term prognosis.
  

Atossa Therapeutics remains dedicated toaccelerating the development of (Z)-endoxifen with the goal ofhelping patients across all stages of breast cancer to benefit fromits therapeutic potential.

About Atossa Therapeutics

Atossa Therapeutics, Inc. (Nasdaq: ATOS) is aclinical-stage biopharmaceutical company dedicated to transformingbreast cancer treatment through innovative science andpatient-focused solutions. The company’s lead product candidate,(Z)-endoxifen, is a highly potent SERM designed for use across thebreast cancer spectrum, including prevention, neoadjuvant,adjuvant, and metastatic settings. Atossa is committed to advancingits robust clinical research programs to improve patient outcomeswhile creating sustainable value for shareholders. For moreinformation, visit atossatherapeutics.com.

FORWARD LOOKING STATEMENTS

This press release contains certain informationthat may constitute forward-looking statements within the meaningof the Private Securities Litigation Reform Act of 1995. We mayidentify these forward-looking statements by the use of words suchas “expect,” “potential,” “continue,” “may,” “will,” “should,”“could,” “would,” “seek,” “intend,” “plan,” “estimate,”“anticipate,” “believe,” “design,” “predict,” “future,” or othercomparable words. All statements made in this press release thatare not statements of historical fact, including statementsregarding 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 receiveregulatory approval, benefits of the Company’s strategy of pursuinga metastatic indication for (Z)-endoxifen, the expected design andenrollment 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 thispress release are subject to risks and uncertainties that may causeactual results, outcomes, or the timing of actual results oroutcomes, to differ materially from those projected or anticipated,including risks and uncertainties associated with: our ability toobtain patent coverage for our product candidates; macroeconomicconditions and increasing geopolitical instability; the expectedtiming of releasing data; any variation between interim orpreliminary and final clinical results or analysis; actions andinactions by the FDA and foreign regulatory bodies; the outcome ortiming of regulatory approvals needed by Atossa, including thoseneeded to continue our planned (Z)-endoxifen trials; our ability tosatisfy regulatory requirements; our ability to regain complianceor maintain compliance with the continued listing requirements ofthe Nasdaq Stock Market; our ability to successfully develop andcommercialize new therapeutics; the success, costs and timing ofour development activities, including our ability to successfullyinitiate 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 toperform adequately; our estimates on the size and characteristicsof our potential markets; our ability to successfully defendlitigation and other similar complaints and to establish andmaintain 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 ourtrials of (Z)-endoxifen in women with breast cancer, and whetherthe studies will meet their objectives; our expectations as tofuture financial performance, expense levels and capital sources,including our ability to raise capital; our ability to attract andretain key personnel; our anticipated working capital needs andexpectations around the sufficiency of our cash reserves; and otherrisks and uncertainties detailed from time to time in Atossa’sfilings with the Securities and Exchange Commission, includingwithout limitation its Annual Reports on Form 10-K and QuarterlyReports on 10-Q. Forward-looking statements are presented as of thedate of this press release. Except as required by law, we do notintend to update any forward-looking statements, whether as aresult of new information, future events or circumstances orotherwise.

Investor and Media Contact:

Michael Parks
VP, Investor and Public Relations
484-356-7105
michael.parks@atossainc.com