Actinium shares early ATNM-400 prostate cancer trial data at EANM

Actinium Pharmaceuticals, Inc. (NYSE American: ATNM) presented initial clinical results from a first-in-human Phase 1 study of ATNM-400, an investigational HER3-targeted alpha radiotherapy for metastatic castration-resistant prostate cancer (mCRPC), at the 39th Annual Congress of the European Association of Nuclear Medicine in Vienna, Austria on Oct. 19.

ATNM-400 pairs the monoclonal antibody barecetamab with Actinium-225, an alpha-emitting isotope with a 10-day half-life. The study uses a theranostic approach, employing Zirconium-89-labeled barecetamab PET imaging to assess tumor uptake and inform dosing.

The first cohort of three patients with previously treated mCRPC received ATNM-400 at 1.0 µCi/kg. All had prior androgen receptor pathway inhibitor therapy; two had received chemotherapy and one had also received lutetium-177 PSMA therapy. No dose-limiting toxicities, xerostomia, or Grade 3/4 hematologic adverse events were observed. No elevations in liver enzymes or creatinine, injection-site reactions, or infusion-related reactions were reported.

Serial PET imaging showed lesion uptake retained through Day 7, consistent with the half-life of Actinium-225. Following clearance of the first dose level, the study advanced to a second dose level of 1.5 µCi/kg. Planned dose levels include 2.25 and 3.0 µCi/kg.

The study uses a 3+3 dose-escalation design followed by dose expansion. Eligible patients must have received at least one androgen receptor pathway inhibitor and at least one taxane regimen. Prior lutetium-177 PSMA treatment is permitted. ATNM-400 is administered intravenously every six weeks for up to four cycles.

Preclinical data presented showed antitumor activity across PSMA-high, PSMA-low, and PSMA-negative prostate cancer models. The company said HER3 targeting provides a mechanism distinct from PSMA-directed therapies such as Novartis' Pluvicto.

According to the press release, the study is designed to establish a recommended Phase 2 dose and evaluate safety, dosimetry, pharmacokinetics, and antitumor activity including PSA responses and radiographic progression-free survival.

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