Actinium Pharmaceuticals Inc. (NYSE:ATNM) gave investors something to cheer about on Tuesday: early clinical data from its first-in-human Phase 1 study of ATNM-400 in metastatic castration-resistant prostate cancer (mCRPC). The company presented the findings at the Annual Congress of the European Association of Nuclear Medicine, and the market liked what it heard. Shares jumped nearly 9%, with trading volume reaching 670,435 shares, more than 4.4 times the 100-day average of 150,386 shares. That is the kind of volume that says people were paying attention.

Safety First, And So Far So Good

The ongoing trial uses a 3+3 dose-escalation design in patients with progressive mCRPC who have already received at least one androgen receptor pathway inhibitor and at least one taxane regimen. In other words, these are heavily pretreated patients with limited options.

Investigators cleared the initial dose level of 1.0 µCi/kg across three of those patients. All three received at least two cycles of ATNM-400, and two completed three cycles. One patient had also previously undergone ¹⁷⁷Lu-PSMA therapy.

During the dose-limiting toxicity evaluation window and subsequent monitoring, researchers found no dose-limiting toxicities, no xerostomia, and no Grade 3 or 4 hematologic adverse events. They also observed no elevations in liver enzymes or creatinine, and no infusion-related or injection-site reactions. For a first-in-human radiotherapy trial in a tough patient population, that is a clean early report card.

With the initial cohort cleared, dose escalation has advanced to 1.5 µCi/kg, with additional planned increments at 2.25 and 3.0 µCi/kg.

A Different Target Than PSMA

ATNM-400 is a first-in-class, HER3-directed alpha radiotherapy that conjugates the monoclonal antibody barecetamab with Actinium-225 (²²⁵Ac). HER3 is known to promote disease progression and resistance to androgen receptor pathway inhibitors, so targeting it offers a biology-driven mechanism beyond PSMA.

Preclinical tests showed robust antitumor activity across PSMA-high, PSMA-low, and PSMA-negative models, as well as models with low HER3 expression. That breadth matters because not every prostate cancer patient has high PSMA expression, which can limit PSMA-targeted therapies.

The trial also uses a theranostic approach, conducting HER3-targeted PET scans with Zirconium-89-labeled barecetamab before 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. That retention is a key piece of the puzzle for alpha-emitting radiotherapies.

The study aims to determine the recommended Phase 2 dose while evaluating PSA responses, overall survival, and radiographic progression-free survival.

Price Action: Actinium Pharmaceuticals shares were up 8.85% at $1.43 at the time of publication on Tuesday, according to market data.