Actinium Highlights Broad Anti-Tumor Activity in Hard-to-Treat Breast Cancer Models

Actinium Pharmaceuticals (NYSE American: ATNM) presented new preclinical data at the 2025 San Antonio Breast Cancer Symposium that reinforce the therapeutic potential of ATNM-400, its first-in-class Actinium-225–based targeted radiotherapy, across multiple breast cancer subtypes.

The data demonstrated robust anti-tumor activity in hormone receptor–positive, triple-negative, and treatment-resistant breast cancer models, including tumors resistant to tamoxifen and HER2-directed therapy. The findings support continued development of ATNM-400 as both a monotherapy and a combination candidate in settings with significant unmet need.

Target Biology and Mechanism Support Activity in Resistant Disease

ATNM-400 targets a validated cancer antigen that is overexpressed in breast cancer and further upregulated in tumors that have developed resistance to endocrine and HER2-targeted therapies. In preclinical models, delivery of the Actinium-225 alpha-emitter resulted in irreparable double-strand DNA damage, consistent with the high linear energy transfer associated with alpha-particle radiation.

This mechanism enables potent, localized tumor cell killing and may offer advantages over payloads that rely on DNA repair–dependent cytotoxicity.

Efficacy Across Molecular Subtypes and Prior Treatment Resistance

In vivo studies showed significant tumor growth inhibition across hormone receptor–positive and triple-negative breast cancer models. Notably, ATNM-400 demonstrated enhanced cytotoxicity in tamoxifen- and trastuzumab-resistant cell lines, where antigen expression was increased relative to treatment-sensitive counterparts.

Combination studies further suggested additive or synergistic activity when ATNM-400 was paired with standard-of-care therapies, resulting in improved tumor regression in resistant disease models.

Differentiated Biodistribution and Tolerability Profile

Preclinical biodistribution analyses showed sustained tumor uptake of ATNM-400 with rapid clearance from non-target tissues. This profile supports the potential for effective tumor targeting while minimizing off-target exposure, including pulmonary toxicity that has limited the use of some antibody-drug conjugates.

These findings contribute to a differentiated safety and tolerability profile for ATNM-400 within the targeted radiotherapy landscape.

Extension Beyond Breast Cancer

The SABCS data build on previously reported ATNM-400 activity in prostate cancer and non-small cell lung cancer models. Together, the results support Actinium’s strategy to position ATNM-400 as a multi-indication, pan-tumor targeted radiotherapy addressing solid tumors characterized by treatment resistance and limited therapeutic options.

Development Path Forward

Actinium continues to advance ATNM-400 toward clinical readiness as part of its next-generation radiotherapeutics pipeline. The breadth of activity observed across breast cancer subtypes and resistant disease strengthens the rationale for further development and potential clinical translation.