Actinium Jumps 21% as ATNM-400 Makes Its First-in-Human Debut at EANM 2026

Actinium Pharmaceuticals, Inc. (NYSE: ATNM) shares rose about 21% to roughly $1.37 by mid-day Monday, up from Friday’s $1.13 close. The rally came as Actinium presented the first clinical data for ATNM-400 at the 39th Annual Congress of the European Association of Nuclear Medicine (EANM) in Vienna. ATNM-400 is its alpha-emitting radiotherapy for metastatic castration-resistant prostate cancer (mCRPC). The drug targets HER3, not PSMA, which sets it apart from most prostate cancer radiotherapies.

Key Points

• Actinium is presenting early data from its first-in-human Phase 1 dose-escalation study of ATNM-400 in mCRPC at EANM 2026 today.
• ATNM-400 is a first-in-class Actinium-225 antibody radioconjugate that targets HER3, an antigen distinct from PSMA.
• A second EANM presentation covers preclinical ATNM-400 results in KRAS- and ALK-driven non-small cell lung cancer.
• ATNM led today’s PRISM Pharmaceuticals Index movers, though it remains a volatile microcap with a market value near $43 million.

Actinium (NYSE: ATNM) Takes ATNM-400 Into the Clinic

The oral presentation (OP-525) covers the opening stage of the Phase 1 study. Actinium is using a Zirconium-89-labeled version of ATNM-400 as an imaging agent. With it, the company can measure safety, biodistribution and dosimetry. Serial PET scans then track how the drug reaches tumors and how it clears from the body. Actinium will use these dosimetry estimates to set doses for the therapeutic Actinium-225 version of the drug.

The step is important because it confirms where the antibody goes before the full radioactive payload is given. Actinium’s press release did not include patient numbers, safety findings or efficacy figures. Investors will need the full presentation to judge the early results.

A Differentiated Approach Beyond PSMA

Most of the prostate cancer radiopharmaceutical market targets PSMA. ATNM-400 targets HER3, which Actinium (ATNM) links to tumor survival and treatment resistance. That gives the company a possible option for patients whose disease stops responding to PSMA-based therapies.

PMV has followed the program since its preclinical stage. In 2025, Actinium reported data suggesting ATNM-400 may outperform blockbuster prostate cancer drugs. It later showed efficacy in resistant prostate cancer models. Today’s presentation is the program’s first move from animal models to patients.

Lung Cancer Data Supports a Pan-Tumor Strategy

At a second EANM presentation (EPS-149), Actinium is sharing preclinical results for ATNM-400 in KRAS- and ALK-driven non-small cell lung cancer models. The studies tested the drug alone and with KRAS or ALK inhibitors. This work builds on earlier EGFR-mutant research that Actinium showcased last year. It also adds to promising results in hard-to-treat breast cancer models. Together, these results support Actinium’s plan to develop ATNM-400 across many tumor types, regardless of mutation.

Radiopharma Dealmaking Raises the Stakes

The data arrives while radiopharmaceutical companies are making big deals. Last month, Telix agreed to acquire ITM for US$1.65 billion upfront. Actinium is also preparing to partner its own programs. Last week it hired industry veteran Sean Maenpaa to lead business development, with responsibility for licensing and strategic transactions across its targeted radiotherapy pipeline.

What’s Next for Actinium (NYSE: ATNM)

The Phase 1 study is ongoing. Imaging and dosimetry results will guide dosing of the therapeutic Actinium-225 version of the drug. Investors will watch for early safety and tumor-response data, along with any partnership news from Maenpaa’s new team. Follow ATNM and its peers on the PRISM Pharmaceuticals Index, and see today’s full PRISM Mid-Day Movers.