ATNM News & Events

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.

Actinium Pharmaceuticals announced preclinical data for ATNM-400, a first-in-class Actinium-225 antibody radioconjugate, in hormone receptor positive, HER2 positive and triple-negative breast cancer that will be presented at the San Antonio Breast Cancer Symposium, SABCS, being held December10-14, 2025 in SanAntonio, Texas. The data demonstrates significant anti-tumor activity in breast cancer models resistant to standard-of-care therapies including endocrine therapy tamoxifen and HER2-targeted therapy trastuzumab as well as potent tumor growth inhibition in TNBC models. The data highlight ATNM-400's potential to address critical unmet needs in patients who have exhausted treatment options following endocrine therapy or HER2-targeted therapy failure. These data add to ATNM-400's robust preclinical data package that also encompasses metastatic castrate-resistant prostate cancer and non-small cell lung cancer.

Actinium Pharmaceuticals (ATNM) announced the presentation of the first ever preclinical data of ATNM-400 in non-small cell lung cancer, or NSCLC. ATNM-400 is Actinium's lead solid tumor program, which is also being studied in prostate cancer. ATNM-400 demonstrated superior efficacy with 3-5x greater tumor growth inhibition compared to standard-of-care therapies across EGFR-mutant NSCLC including AstraZeneca's (ASZN) Osimertinib and Dato-DXd and J&J's (JNJ) Amivantamab. ATNM-400 also demonstrated synergistic activity in combination with osimertinib with complete tumor regression in 100% of tumor-bearing animals. Data of external beam radiotherapy combined with osimertinib resulted in improved progression free survival compared to osimertinib alone, providing a strong clinical rationale for combining targeted alpha-therapy via ATNM-400 with EGFR targeting therapies. ATNM-400 produced superior tumor growth inhibition in animals bearing human lung cancer NCI-H1975. TGI with ATNM-400 was 3-5 times higher than what was achieved with the current standard of care EGFR-mutant NSCLC therapies. The combination of ATNM-400 and osimertinib produced cures in all tumor-bearing animals with complete tumor regression, outperforming either agent alone and suggesting synergistic activity.

Actinium Pharmaceuticals highlighted new preclinical data for ATNM-400, its novel, first-in-class antibody radioconjugate armed with the potent alpha-emitter Actinium-225, or Ac-225, at the 32nd Annual Prostate Cancer Foundation, or PCF, Scientific Retreat being held October 23 - 25, 2025 in Carlsbad, CA. ATNM-400, which targets a non-PSMA antigen associated with prostate cancer progression and treatment resistance, demonstrated superior tumor control and improved overall survival compared with the active agents in key standard-of-care therapies including enzalutamide, the active agent in Xtandi and 177Lu-PSMA-617, the active agent in Pluvicto, as well as 225Ac-PSMA-617 across multiple preclinical prostate cancer models. These data further reinforce ATNM-400's potential as a paradigm changing, PSMA-independent Ac-225 alpha radiotherapy, offering opportunities for use as a monotherapy, in combination regimens, and in patients relapsing after ARPI or PSMA-directed treatments. ATNM-400 data has now been presented at the American Association for Cancer Research, Society for Nuclear Medicine and Molecular Imaging and PCF annual conferences in 2025 demonstrating its growing potential in various treatment settings in prostate cancer. In addition, data highlighting its potential in non-small cell lung cancer will be presented at the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics on Saturday, October 25. Key Findings in Prostate Cancer Treatment Setting include: Potent and Durable Tumor Control, Increased Overall Survival and Synergistic Activity in Enzalutamide Resistant Prostate Cancer; ATNM-400 demonstrated superior and 5-times more durable anti-tumor efficacy extending to 100 days compared to approximately 20 days with enzalutamide alone. Given that ARPI therapies like enzalutamide are known to increase the ATNM-400 target antigen expression, combination treatment with ATNM-400 and enzalutamide was synergistic, resulting in complete tumor eradication in 40% of treated animals and significantly prolonged survival, whereas enzalutamide alone provided no durable disease control.