Actinium Pharmaceuticals (NYSE American: ATNM) presented preclinical results for its investigational drug ATNM-400 at the 39th Annual Congress of the European Association of Nuclear Medicine (EANM), held Oct. 17–21, 2026, in Vienna, Austria. The data covered non-small cell lung cancer (NSCLC), colorectal cancer, and ovarian cancer models.

ATNM-400 is an antibody radioconjugate that pairs a HER3-targeting monoclonal antibody with the alpha-emitting isotope Actinium-225. The compound is currently in a Phase 1 clinical study in metastatic castration-resistant prostate cancer.

In NSCLC, the drug showed activity across EGFR-mutant, KRAS G12C-mutant, and ALK-rearranged models. In the KRAS G12C-mutant model, ATNM-400 monotherapy achieved 81% tumor growth inhibition (TGI), compared with 38% for adagrasib. Combining the two produced 105% TGI, consistent with tumor regression. In the ALK-rearranged model, ATNM-400 plus crizotinib achieved 97% TGI versus 55% for crizotinib alone.

In colorectal cancer, adding ATNM-400 to oxaliplatin and bevacizumab increased TGI to 72% from 25% in a model with moderate HER3 expression. In a high-expression model, ATNM-400 plus bevacizumab achieved 88% TGI, comparable to oxaliplatin plus bevacizumab at 85%.

In ovarian cancer, ATNM-400 monotherapy achieved 93% TGI in the OVCAR-5 model, compared with 54% for bevacizumab. Following bevacizumab failure, ATNM-400 retained activity with 50% TGI and prolonged survival.

The studies reported generally favorable tolerability across evaluated doses, with minimal changes in blood, liver, and kidney markers in ovarian cancer studies. A slight ALT elevation was observed with ATNM-400 plus bevacizumab in one colorectal model.

The findings were presented in one oral presentation and two e-posters. Actinium stated the results expand preclinical evidence beyond previously reported prostate and breast cancer data.