On June 3, 2026 Actinium Pharmaceuticals, Inc. (NYSE American: ATNM) (Actinium or the Company), a pioneer in the development of targeted radiotherapies, on June 2, 2026, reported new data on ATNM-400 in prostate cancer at the Society of Nuclear Medicine and Molecular Imaging (SNMMI) 2026 Annual Meeting taking place in Los Angeles, California.

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Androgen receptor pathway inhibitors (ARPIs) such as enzalutamide (Xtandi, Astellas/Pfizer), apalutamide (Erleada, Johnson & Johnson), and darolutamide (Nubeqa, Bayer) are foundational to advanced prostate cancer care, but virtually all patients eventually develop resistance and progress -with up to 50,0001 patients per year exhausting ARPI therapy- creating a large and recurring unmet need. ATNM-400, Actinium’s first-in-class Actinium-225 antibody radioconjugate, targets a non-PSMA antigen linked to aggressive prostate cancer biology and delivers a high-energy alpha-particle payload that kills tumor cells through a mechanism independent of PSMA expression and androgen receptor signaling. In preclinical head-to-head studies, this PSMA-independent mechanism allowed ATNM-400 to match or exceed PSMA-targeted radioligand therapies, including 177Lu-PSMA-617 (active ingredient of Pluvicto, Novartis) and 225Ac-PSMA-617, across PSMA-high, PSMA-low, and PSMA-negative models.

The new data presented at SNMMI 2026 address three key questions: whether ATNM-400 can overcome ARPI resistance in the mCRPC setting, whether it works as both a single agent and in combination, and how its dosing and safety profile support translation to patients. The data answer each: ATNM-400 remained potent against cells and tumors resistant to all three approved ARPIs, achieved 94% tumor growth inhibition as a single agent and durable complete responses in combination, and maintained a consistent, clean safety profile across both single-bolus and repeat-dose regimens.

Prostate cancer is one of oncology’s largest commercial opportunities: androgen receptor pathway inhibitors generate more than $12 billion in annual sales in 2025 and Pluvicto (177Lu-PSMA-617) reached $2.0 billion in 2025, yet up to 50,0001 patients exhaust ARPI therapy each year and roughly 30% of mCRPC patients are ineligible for PSMA-directed therapy. Because ATNM-400 acts independently of both androgen receptor signaling and PSMA, it is positioned to serve the full mCRPC continuum, from patients who have failed ARPIs to the PSMA-low and PSMA-negative populations with no approved targeted radiotherapy today. Actinium estimates this represents a combined opportunity of more than 100,000 patients annually, addressable as a monotherapy or in combination with existing standards of care.

Sandesh Seth, Actinium’s Chairman and CEO, said, "ARPI resistance is a central problem in advanced prostate cancer as once these drugs stop working there are few good options left. The new data are compelling as ATNM-400 stayed potent in cells and tumors resistant to all three approved ARPIs, outperformed apalutamide and darolutamide on its own similar to previous data with enzalutamide, and produced durable complete responses when combined with them. Equally important for development, ATNM-400 worked across single-dose and repeat-dose regimens with a consistent, clean safety profile, giving us real flexibility as we move toward the clinic. We believe these findings position ATNM-400 as a differentiated option for patients who have run out of ARPI choices. The same resistance-targeting biology underlies the compelling activity we have reported in non-small cell lung cancer and breast cancer, positioning ATNM-400 as a potential pan-tumor backbone for large, resistance-driven patient populations, alone or in combination. We look forward to sharing additional data across these programs in the coming months as we advance ATNM-400 toward the clinic."

Highlights from the SNMMI 2026 Poster Presentation

Poster Titled: ATNM-400: A First-in-Class Non-PSMA Actinium-225 Antibody Radioconjugate Demonstrates Superior Efficacy to PSMA-617 Radioligands and ARPIs With Favorable Safety Profile in Prostate Cancer Models

New data presented at SNMMI 2026 highlight several findings central to ATNM-400’s development in prostate cancer:

In a low-PSMA, moderate-target tumor model (22Rv1), a single bolus dose of ATNM-400 (40 or 30 µCi/kg) delivered superior tumor control versus fractionated dosing, and a repeat 30 µCi/kg regimen sustained tumor control and survival through day 60 with all regimens outperforming 177Lu-PSMA-617 and showing minimal deviation from vehicle in blood, liver, and kidney safety markers. Flexible, well-tolerated dosing gives clinicians multiple ways to balance efficacy and safety and points to a wide therapeutic window.

ATNM-400 produced potent, dose-dependent killing of prostate cancer cells that are resistant to enzalutamide, apalutamide, and darolutamide, driving cell viability toward zero after each ARPI had failed, while the ARPIs alone left the resistant cells largely intact. Demonstrating activity specifically after ARPI failure addresses the most common point of progression in advanced prostate cancer and supports ATNM-400 as a treatment option for patients with few alternatives.

As monotherapy in an ARPI-resistant tumor model (22Rv1), ATNM-400 achieved 94% tumor growth inhibition versus just 32% for apalutamide and 5% for darolutamide, roughly three- to eighteen-fold greater tumor control than the ARPIs themselves. Working on its own in tumors that no longer respond to standard ARPIs positions ATNM-400 as a potential stand-alone therapy after resistance, not only an add-on.

In combination with either apalutamide or darolutamide in the same ARPI-resistant model, ATNM-400 delivered durable tumor control at 107% tumor growth inhibition, with complete responses in at least 57% of mice (4/7 and 5/7). Turning resistant tumors into complete responses by pairing ATNM-400 with the very drugs that had failed supports a combination strategy that could extend the commercial and clinical value of approved ARPIs.

In a high-PSMA, high-target tumor model (C4-2), ATNM-400 matched or exceeded both PSMA-targeted radioligands-comparable to 225Ac-PSMA-617 and superior to 177Lu-PSMA-617-while being given at roughly one-thousandth the administered radioactivity of 177Lu-PSMA-617 (40 µCi/kg versus 40 mCi/kg), with durable survival and favorable tolerability. Because ATNM-400 acts independently of PSMA, this activity carries across PSMA-high, PSMA-low, and PSMA-negative models, reaching the PSMA-variable patients that PSMA-directed radioligand therapy serve least well. Moreover as the ATNM-400 target is not expressed in the salivary glands xerostomia, a limitation of PSMA directed therapies and especially PSMA-Ac-225 based agents, is not expected.

(Press release, Actinium Pharmaceuticals, JUN 3, 2026, View Source [SID1234666411])