Actinium Pharmaceuticals Presents New Data for ATNM-400 at 2025 San Antonio Breast Cancer Symposium

Actinium Pharmaceuticals announced the presentation of new preclinical data for ATNM-400, its first-in-class Actinium-225 based antibody radioconjugate, at the 2025 San Antonio Breast Cancer Symposium. The poster, titled "Anti-Tumor Activity of ATNM-400, a First-in-Class Actinium-225 Antibody Radioconjugate, in Hormone-Positive, Triple-Negative, Tamoxifen-Resistant and Trastuzumab-Resistant Breast Cancer Models," showcases the following key findings: Potent Efficacy Across Breast Cancer Subtypes: ATNM-400 demonstrated significant tumor-growth inhibition in HR+ and TNBC in vivo models, with all treatment regimens well tolerated and no significant changes in body weight observed. Potent Activity in Standard-of-care Treatment-Resistant Breast Cancer Models: Trastuzumab-resistant BT474-Clone5 breast cancer cells or Tamoxifen-resistant MCF7-Tam1 breast cancer cells exhibited increased target expression, resulting in enhanced in vitro cytotoxicity with ATNM-400. Combining ATNM-400 with either trastuzumab or tamoxifen resulted in greater cytotoxicity versus monotherapy and produced in vivo tumor regression in the trastuzumab-resistant model. Mechanistic Evidence of Irreversible DNA Damage: Activation of phosphorylation of AKT was observed in trastuzumab resistant BT474-Clone5 breast cancer cells, as well as a significant increase in the total level of the ATNM-400 target antigen in the in vivo trastuzumab-resistant breast cancer model. ATNM-400 treatment of these trastuzumab-resistant breast cancer cells caused significant increase in phosphorylated H2AZ, consistent with alpha-particle-driven double-strand DNA damage. Favorable Biodistribution: Sustained tumor uptake in a breast cancer model through 144 hours and rapid clearance from normal organs supports a potentially differentiated safety profile. Pan-Tumor Potential: These results, together with previously published ATNM-400 data in prostate and lung cancer, reinforce the program's broad applicability across solid tumors.