- Poster presentation to highlight preclinical efficacy of ATNM-400 across therapy-resistant breast cancer subtypes

- ATNM-400 is a first-in-class targeted alpha therapy directed against HER3, a driver of tumor biology and treatment resistance in multiple solid tumors

- Breast cancer findings reinforce ATNM-400's pan-tumor potential, building on previously reported preclinical results in non-small cell lung, colorectal and ovarian cancers

NEW YORK, Nov. 2, 2026 /PRNewswire/ -- Actinium Pharmaceuticals, Inc. (NYSE American: ATNM) (Actinium or the Company), a pioneer in differentiated targeted radiotherapies with the potential to transform cancer treatment, today announced that an abstract highlighting preclinical efficacy of ATNM-400 across therapy-resistant breast cancer subtypes has been accepted for poster presentation at the 2026 San Antonio Breast Cancer Symposium (SABCS), taking place December 8â11, 2026, in San Antonio, Texas.

The presentation will feature ATNM-400, the Company's investigational, first-in-class Actinium-225 (²²âµAc) antibody radioconjugate targeting HER3, an antigen implicated in tumor survival and treatment resistance. ATNM-400 combines the targeting specificity of a monoclonal antibody with potent alpha-particle radiation to deliver a tumor-directed cytotoxic payload.

The breast cancer findings build on previously reported preclinical results demonstrating antitumor activity in non-small cell lung cancer (NSCLC) models across EGFR, KRAS and ALK alterations, enhanced responses with standard-of-care therapies in colorectal cancer (CRC) models, and durable activity in ovarian cancer models, including treatment-resistant disease. Together, these findings reinforce ATNM-400's pan-tumor potential and support further evaluation as a standalone therapy and in combination with established treatments across multiple solid tumor indications, complementing its ongoing clinical development in prostate cancer.

"We are excited to showcase ATNM-400's activity in therapy-resistant breast cancer models at SABCS," said Sandesh Seth, Actinium's Chairman and CEO. "Building on our previously reported NSCLC, colorectal and ovarian cancer results, these findings strengthen our vision for developing ATNM-400 as a pan-cancer radiotherapy. HER3 is implicated in resistance to standard-of-care therapies across several solid tumors and treatment settings, which implies  ATNM-400's potential extends well beyond a single oncology setting. Together with our ongoing first-in-human prostate cancer program, this growing body of evidence supports our strategy to pursue ATNM-400's broad potential and address treatment-resistant disease across large patient populations." 

ATNM-400 SABCS 2026 Presentation Details

Abstract Title: ATNM-400 a first-in-class targeted alpha therapy targeting a driver of tumor biology and treatment resistance demonstrates efficacy across therapy-resistant breast cancer subtypes

Abstract Number: 1626

Presentation Number: P2-12-17

Presentation Type: Poster presentation

Presentation Date & Time: Wednesday, December 9, 2026, 5:00â6:30 p.m. CST

Location: Exhibit Hall

About ATNM-400 and HER3

ATNM-400 is an investigational antibody radioconjugate that pairs the HER3-targeting monoclonal antibody barecetamab with the alpha-emitting isotope Actinium-225. Its antibody-based design combines antigen-specific targeting with the potential for sustained tumor localization, supporting prolonged delivery of high-energy alpha radiation. Tumor uptake and retention observed through Day 7 with the â¸â¹Zr-labeled imaging counterpart support this development rationale alongside the approximately 10-day half-life of ²²âµAc. The theranostic approach enables noninvasive visualization of antibody distribution to inform therapeutic development.

ATNM-400 is differentiated from PSMA-targeted radiotherapies by its focus on HER3, an antigen implicated in tumor survival and treatment resistance. PSMA-directed approaches include Novartis' Pluvicto and a broad pipeline of investigational therapies, including Telix Pharmaceuticals' TLX591-Tx, Clarity Pharmaceuticals' â¶â·Cu-SAR-bisPSMA, Convergent Therapeutics' CONV01-α, AstraZeneca's AZD2265 (formerly FPI-2265), Curium's lutetium-177 zadavotide guraxetan (¹â·â·Lu-PSMA-I&T), ARTBIO's AB001 and AdvanCell's ADVC001. These programs span small-molecule and antibody-based therapies using beta- or alpha-emitting isotopes. ATNM-400 combines HER3 targeting with an Actinium-225 antibody radioconjugate, offering a distinct biological approach with the potential to expand radiotherapy across the broad mCRPC population, including patients with PSMA-low or PSMA-negative disease and those who do not respond to or progress following PSMA-directed treatment. Its HER3-directed mechanism also supports investigation beyond prostate cancer in other HER3-expressing solid tumors.

HER3, also known as ERBB3, is a member of the epidermal growth factor receptor family involved in signaling pathways that support tumor growth, survival and resistance to treatment. HER3-directed antibody-drug conjugates in clinical development include BioNTech and MediLink's BNT326/YL202, Bristol Myers Squibb and SystImmune's EGFR Ã HER3 bispecific ADC izalontamab brengitecan (iza-bren), and Daiichi Sankyo and Merck's patritumab deruxtecan (HER3-DXd). These programs underscore the clinical development interest in HER3 as an oncology target. ATNM-400 applies a distinct payload strategy, using alpha radiation in place of a conventional cytotoxic drug. Its preclinical activity in prostate, non-small cell lung and breast cancer models supports investigation as a potential pan-cancer radiotherapy, as a monotherapy and in rational combinations.

About Actinium Pharmaceuticals, Inc.

Actinium is a pioneer in targeted radiotherapies designed to improve outcomes for patients with cancer. The company employs a biology-driven approach to develop differentiated radiopharmaceuticals for solid tumors and hematologic malignancies. Its mission is to transform cancer treatment through innovative radioconjugates that maximize therapeutic efficacy while minimizing toxicity to healthy tissue by combining expertise in tumor biology, translational medicine, and radiochemistry. Since inception, Actinium has focused on developing innovative radiotherapies. Its pipeline reflects this strategy across three areas: (1) solid tumor therapeutics including ATNM-400 and Actimab-A with pan-tumor potential; (2) Actimab-A as a therapeutic backbone for acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) in collaboration with the National Cancer Institute (NCI); and (3) targeted conditioning agents including Iomab-B for bone marrow transplant and Iomab-ACT for cell and gene therapy conditioning. ATNM-400 targets HER3, an antigen distinct from PSMA, and is in a first-in-human Phase 1 study in metastatic castration-resistant prostate cancer (mCRPC), with preclinical activity across mCRPC, NSCLC, breast, colorectal and ovarian cancers. Actimab-A has shown improved survival in relapsed/refractory AML with CLAG-M and is advancing toward a Phase 2/3 trial, with additional development ongoing through a CRADA with the NCI. Actinium is also advancing preclinical solid tumor programs and holds ~250 patents and patent applications, including intellectual property related to cyclotron-based production of Ac-225. For more information, please visit www.actiniumpharma.com.

Forward-Looking Statements

This press release may contain projections or other "forward-looking statements" within the meaning of the "safe-harbor" provisions of the private securities litigation reform act of 1995 regarding future events or the future financial performance of the Company which the Company undertakes no obligation to update. These statements are based on management's current expectations and are subject to risks and uncertainties that may cause actual results to differ materially from the anticipated or estimated future results, including the risks and uncertainties associated with preliminary study results varying from final results, estimates of potential markets for drugs under development, clinical trials, actions by the FDA and other governmental agencies, regulatory clearances, responses to regulatory matters, the market demand for and acceptance of Actinium's products and services, performance of clinical research organizations and other risks detailed from time to time in Actinium's filings with the Securities and Exchange Commission (the "SEC"), including without limitation its most recent annual report on form 10-K, subsequent quarterly reports on Forms 10-Q and Forms 8-K, each as amended and supplemented from time to time.

Investor Contact

Peter (PJ) Kelleher

LifeSci Advisors

[email protected]

(617) 430-7579

SOURCE Actinium Pharmaceuticals, Inc.

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