On Tuesday, Actinium Pharmaceuticals Inc. (NYSE:ATNM) announced encouraging initial clinical results from its first-in-human Phase 1 study of ATNM-400 for metastatic castration-resistant prostate cancer (mCRPC).
The company presented its findings at the Annual Congress of the European Association of Nuclear Medicine, demonstrating a favorable early safety profile and sustained tumor retention.
Following the announcement, the stock jumped nearly 9%. Trading volume reached 670,435 shares, more than 4.4 times the 100-day average of 150,386 shares, signaling unusually heavy trading activity.
Initial Safety Profile And Dose Escalation
The ongoing first-in-human trial employs a 3+3 dose-escalation framework in patients with progressive mCRPC who previously received at least one androgen receptor pathway inhibitor and at least one taxane regimen.
Investigators cleared the initial dose level of 1.0 µCi/kg across three heavily pretreated patients, all of whom received at least two cycles of ATNM-400, including two patients who completed three cycles. One patient had also previously undergone ¹⁷⁷Lu-PSMA therapy.
During the dose-limiting toxicity evaluation window and subsequent monitoring, researchers identified no dose-limiting toxicities, xerostomia, or Grade 3 or 4 hematologic adverse events.
Clinicians also observed no elevations in liver enzymes or creatinine, and no infusion-related or injection-site reactions.
Following the clearance of the initial cohort, dose escalation advanced to 1.5 µCi/kg, with additional planned increments at 2.25 and 3.0 µCi/kg.
Targeted Radiotherapy Mechanism And Theranostics
ATNM-400 represents a first-in-class, HER3-directed alpha radiotherapy that conjugates the monoclonal antibody barecetamab with Actinium-225 (²²⁵Ac).
Because HER3 promotes disease progression and resistance to androgen receptor pathway inhibitors, the agent offers a biology-driven treatment mechanism beyond PSMA.
Preclinical tests showed robust antitumor activity across PSMA-high, PSMA-low, and PSMA-negative models, as well as models exhibiting low HER3 expression.
The clinical trial uses a theranostic approach, conducting HER3-targeted PET scans using Zirconium-89-labeled barecetamab prior to intravenous ATNM-400 infusions given every six weeks.
Serial imaging verified that tumors retained the agent through Day 7, closely aligning with the 10-day half-life of ²²⁵Ac.
The study aims to determine the recommended Phase 2 dose while evaluating PSA responses, overall survival, and radiographic progression-free survival.
ATNM Price Action: Actinium Pharmaceuticals shares were up 8.85% at $1.43 at the time of publication on Tuesday, according to Benzinga Pro data.
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