Actinium Pharmaceuticals to Present Preclinical Efficacy of ATNM-400 at SABCS 2026

Actinium Pharmaceuticals, Inc. (NYSE American: ATNM) has revealed that an abstract detailing the preclinical efficacy of ATNM-400 against therapy-resistant breast cancer subtypes will be presented as a poster at the 2026 San Antonio Breast Cancer Symposium (SABCS) scheduled for December 8–11, 2026, in San Antonio, Texas.

ATNM-400 is described as a first-in-class antibody radioconjugate that targets HER3, an antigen involved in tumor survival and treatment resistance. Combining a monoclonal antibody's targeting specificity with alpha-particle radiation, ATNM-400 aims to deliver a potent cytotoxic payload to tumors.

The findings relevant to breast cancer enhance previously reported results indicating antitumor activity in preclinical models of non-small cell lung cancer (NSCLC) characterized by EGFR, KRAS, and ALK mutations. The results also demonstrated improved responses to standard therapies in colorectal cancer (CRC) models and sustained activity in ovarian cancer models, even in treatment-resistant situations. Collectively, these findings suggest the broad therapeutic potential of ATNM-400, warranting further exploration as a standalone or combined therapy across various solid tumor types, while complementing ongoing clinical studies in prostate cancer.

Sandesh Seth, Actinium's Chairman and CEO, expressed enthusiasm for showcasing the efficacy of ATNM-400 in therapy-resistant breast cancer models. He emphasized that the findings build on prior data regarding NSCLC, colorectal, and ovarian cancer, reinforcing the company's strategy to develop ATNM-400 as a pan-cancer radiotherapy.

Presentation Details at SABCS 2026

• 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, 5:00–6:30 p.m. CST
• Location: Exhibit Hall

ATNM-400 combines the HER3-targeting monoclonal antibody barecetamab with Actinium-225 (²²⁵Ac), capitalizing on the unique localization and delivery of high-energy alpha radiation. The theranostic design combines imaging capabilities with therapeutic efficacy, providing noninvasive visualization of antibody distribution during development.

Distinct from PSMA-targeted radiotherapies, ATNM-400 offers a unique approach by targeting HER3, relevant for patients with varying treatment responses, including those with low or negative PSMA expression. The preclinical data advocate for the investigation of ATNM-400's potential to serve as a transformative therapy against resistance in solid tumors, including ongoing research in prostate cancer candidates.

Actinium Pharmaceuticals is committed to advancing targeted radiotherapies to enhance cancer treatment outcomes through innovative approaches, leveraging their expertise in tumor biology, translational medicine, and radiochemistry.