Atossa Therapeutics Receives FDA "Study May Proceed" Letter for (Z)-Endoxifen Investigational New Drug Application for Metastatic Breast Cancer

Atossa Therapeutics (Nasdaq: ATOS) announced on January 6, 2026 that the U.S. Food and Drug Administration issued a "Study May Proceed" letter for its investigational new drug application for (Z)-endoxifen in metastatic ER+/HER2- breast cancer.

Rhea-AI Summary

Atossa Therapeutics (Nasdaq: ATOS) announced on January 6, 2026 that the U.S. Food and Drug Administration issued a "Study May Proceed" letter for its investigational new drug application for (Z)-endoxifen in metastatic ER+/HER2- breast cancer.

The company presented this as a regulatory milestone to advance a clinical study testing (Z)-endoxifen's activity, including in tumors with resistance to other endocrine therapies and targeting the oncogenic signaling pathway PKCβ1. No clinical efficacy or safety results were reported in this announcement.

Positive

• FDA "Study May Proceed" letter received on Jan 6, 2026
• Targeting metastatic ER+/HER2- breast cancer, a clear patient population
• Program aims at tumors resistant to other endocrine therapies
• PKCβ1 pathway targeted, offering a distinct mechanism of action

Negative

• No clinical efficacy or safety results reported in the announcement
• Investigation remains pre-approval and investigational, outcome uncertain

Argus Jan 6 session

+7.00% close to close Open Argus

Details

News Market Reaction – ATOS

On Jan 6, the day this news came out, ATOS closed 7.00% above the previous close.

Data tracked by StockTitan Argus for the Jan 6 session.

Key Figures

Patients completing therapy: 95% MRI tumor volume change: -72% ctDNA clearance: 70% +5 more

Patients completing therapy

95%

Proportion completing ≥75% therapy in (Z)-endoxifen trial (SABCS 2025)

MRI tumor volume change

-72%

Median MRI tumor volume change in (Z)-endoxifen data (SABCS 2025)

ctDNA clearance

70%

Initially ctDNA-positive patients achieving clearance with (Z)-endoxifen

Ki-67 response rate

86%

EVANGELINE run-in patients with Week 4 Ki-67 ≤10%

Q3 2025 net loss

$8.7M

Net loss for Q3 2025

Operating expenses

$9.3M

Q3 2025 operating expenses; R&D $5.4M, G&A $3.9M

Cash & equivalents

$51.8M

Cash and cash equivalents at Sep 30, 2025

ATM facility

$100.0M

At-the-market facility capacity with no sales during 2025 to date

Historical Context

5 past events · Latest: Dec 04

5 events

1. Dec 04

   Regulatory strategy update

   24h Move

   +2.7%

   FDA Type C meeting clarified expedited (Z)-endoxifen development pathways.

2. Dec 09

   Patent issuance

   24h Move

   +3.5%

   New U.S. patent broadened protection for enteric oral (Z)-endoxifen.

3. Dec 11

   Pediatric designation

   24h Move

   +6.5%

   FDA Rare Pediatric Disease designation for DMD program with PRV potential.

4. Dec 15

   Clinical data update

   24h Move

   -13.4%

   Multiple positive (Z)-endoxifen updates at SABCS across several settings.

5. Dec 17

   R&D award

   24h Move

   -1.5%

   External recognition for (Z)-endoxifen precision endocrine therapy work.

24h Move is the share-price change in the day after each event; other market factors may also have contributed.

Key Terms

investigational new drug application, metastatic breast cancer, ER+/HER2-, protein kinase C beta 1 (PKCβ1)

4 terms

investigational new drug application regulatory

"which was the subject of a recent investigational new drug application for (Z)-endoxifen."

An investigational new drug application is a formal request made to regulatory authorities to begin testing a new medication in humans. It is a critical step in the drug development process, as approval indicates the drug has passed initial safety checks and can be studied further. For investors, this signals that a potential new treatment is progressing through its early testing stages, which can impact the company's future growth prospects.

metastatic breast cancer medical

"the Company's study in metastatic breast cancer which was the subject of a recent"

Metastatic breast cancer is breast cancer that has spread beyond the breast and nearby lymph nodes to other organs, such as bones, liver, lungs or brain. For investors it matters because these advanced-stage cases often require long-term, complex and costly treatments, drive demand for specialty drugs and diagnostics, and influence regulatory approvals, pricing negotiations and the long-term revenue potential of companies developing therapies aimed at slowing spread or improving quality of life. An everyday analogy: it’s like a weed that has taken root in multiple beds rather than just one garden patch, requiring broader and more sustained effort to manage.

ER+/HER2- medical

"expand the use of (Z)-endoxifen for metastatic ER+/HER2- Breast Cancer."

ER+/HER2- describes a type of breast cancer where the cancer cells grow in response to the hormone estrogen (ER positive) but do not have excess levels of the HER2 protein (HER2 negative). This classification helps determine the most effective treatments and can influence prognosis. For investors, understanding such medical details can signal potential shifts in healthcare demand and the development of targeted therapies.

protein kinase C beta 1 (PKCβ1) medical

"and in the oncogenic signaling pathway, protein kinase C beta 1 (PKCβ1), may offer"

Protein kinase C beta 1 (PKCβ1) is a specific form of an enzyme that acts like a cellular switch or traffic controller, changing the activity of other proteins to influence cell growth, survival and communication. Investors care because PKCβ1 can be a drug target or biomarker: therapies that modify its activity may alter the course of diseases, affect clinical trial results, regulatory decisions and ultimately the commercial value of companies developing related treatments.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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SEATTLE, Jan. 6, 2026 /PRNewswire/ -- Atossa Therapeutics, Inc. (Nasdaq: ATOS), a clinical-stage biopharmaceutical company developing innovative medicines for unmet medical needs, today announced that the U.S. Food and Drug Administration ("FDA") issued a "Study May Proceed" letter for the Company's study in metastatic breast cancer which was the subject of a recent investigational new drug application for (Z)-endoxifen.

"This letter marks an important regulatory milestone for Atossa and to potentially expand the use of (Z)-endoxifen for metastatic ER+/HER2- Breast Cancer. We believe its activity, even in tumors that have developed resistance to other endocrine therapies and in the oncogenic signaling pathway, protein kinase C beta 1 (PKCβ1), may offer a new tool in treating this disease.," said Dr. Steven Quay, M.D., Ph.D., Atossa Therapeutics President and Chief Executive Officer. "We appreciate the FDA's review and look forward to advancing this clinical investigation."

About (Z)-Endoxifen

(Z)-Endoxifen is a potent Selective Estrogen Receptor Modulator/Degrader (SERM/D) with demonstrated activity across multiple mechanisms of interest. Atossa is evaluating its potential applications in oncology and rare diseases. The Company's proprietary oral formulation has shown a favorable safety profile and pharmacology distinct from tamoxifen, including ER-targeted effects and PKC inhibition. Atossa's (Z)-Endoxifen is not approved for any indication.

Atossa's (Z)-Endoxifen program is supported by a growing global intellectual property portfolio, including multiple recently issued U.S. patents and numerous pending applications worldwide.

About Atossa Therapeutics

Atossa Therapeutics, Inc. (Nasdaq: ATOS) is a clinical-stage biopharmaceutical company developing innovative medicines in oncology and other areas of significant unmet need. The Company's lead product candidate, (Z)-Endoxifen, is currently in development across several clinical settings. More information is available at https://atossatherapeutics.com.

Forward-Looking Statements

This press release contains certain "forward-looking statements" within the meaning of applicable securities laws, including but not limited to, our expectations regarding the Company's development and regulatory strategy and related milestones, the potential indications that the Company may pursue for (Z)-Endoxifen, the potential for (Z)-Endoxifen to receive regulatory approval and the timing thereof, and the potential market and growth opportunities for the Company. Words such as "expect," "potential," "continue," "may," "will," "should," "could," "would," "seek," "intend," "plan," "estimate," "anticipate," "believe," "design," "predict," "future," or other similar expressions or statements regarding intent, belief or current expectations, 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, without limitation, risks and uncertainties associated with: our ability to successfully execute our strategy to shorten our clinical development timelines and pursue a metastatic breast cancer indication or other indications for our lead program, (Z)-Endoxifen; expected timing, completion and results of our preclinical studies, clinical trials and research and development programs; the unpredictable relationship between preclinical study results and clinical study results; the timing or likelihood of regulatory filings and approvals; the outcome or timing of necessary regulatory approvals; our ability to regain and maintain compliance with Nasdaq listing requirements; our ability to establish and maintain intellectual property rights covering our products; the impact of general macroeconomic conditions on our business; our ability to raise capital; and other risks and uncertainties detailed from time to time in Atossa's filings with the SEC, including, without limitation, its Annual Reports on Form 10-K and Quarterly Reports on Form 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.

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SOURCE Atossa Therapeutics Inc

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

What did Atossa (ATOS) announce on January 6, 2026 about (Z)-endoxifen?

Atossa announced the FDA issued a "Study May Proceed" letter for an IND study of (Z)-endoxifen in metastatic ER+/HER2- breast cancer.

What does the FDA "Study May Proceed" letter mean for ATOS stock (ATOS)?

It authorizes the company to move forward with the planned clinical study, representing a regulatory milestone but not an approval of safety or efficacy.

Which patient population will Atossa's (Z)-endoxifen study target for ATOS?

The study targets patients with metastatic ER+/HER2- breast cancer, including tumors that developed resistance to other endocrine therapies.

Does the announcement include clinical trial results for (Z)-endoxifen from Atossa (ATOS)?

No; the announcement did not report any clinical efficacy or safety results.

What biological pathway does Atossa say (Z)-endoxifen may affect in the ATOS study?

The company highlighted potential activity involving the oncogenic signaling pathway PKCβ1.