Actinium's HER3-Targeted Radiotherapy ATNM-400 to Feature at ESMO 2026; Shares Fall 3.78%

Actinium Pharmaceuticals (ATNM) said its first-in-human Phase 1 study of ATNM-400, an investigational HER3-targeted radiotherapy, will be presented at the ESMO 2026 Congress in Madrid from October 23-27. The presentation is listed as an ePoster under "Trial in Progress." The multicenter, open-label study evaluates ATNM-400 in patients with metastatic castration-resistant prostate cancer (mCRPC), including those with and without prior PSMA-directed therapy. The therapy pairs a HER3-targeting antibody with the alpha-emitting isotope Actinium-225, an approach distinct from PSMA-targeted treatments. The trial assesses safety, tolerability, dosimetry, pharmacokinetics and antitumor activity to determine a recommended Phase 2 dose. Patients undergo Zirconium-89-labeled imaging to assess tumor uptake before treatment. ATNM traded at $1.54, down 3.78%.