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 preclinical data on ATNM-400 in non-small cell lung cancer (NSCLC) at the Society of Nuclear Medicine and Molecular Imaging (SNMMI) 2026 Annual Meeting in Los Angeles, California.
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The new KRAS-mutant data, together with a growing body of EGFR-mutant data, demonstrate ATNM-400’s activity across the two major mutation driver classes in NSCLC and support a distinct strategic opportunity. ATNM-400 can be developed as a potential mutation-agnostic backbone for the broader NSCLC market, alone or in combination with standard-of-care therapies, rather than as another mutation-specific drug for a narrow subset.
NSCLC accounts for roughly 85% of the more than two million lung cancer cases diagnosed globally each year, a market more than twice the size of prostate cancer. It is also highly heterogeneous: no single mutation dominates, so treatments are fragmented across mutation-specific therapies such as EGFR, KRAS, BRAF, ALK and others, each marketed by different companies, each addressing only a molecular subset, and each ultimately limited by acquired resistance. ATNM-400, Actinium’s first-in-class Actinium-225 (Ac-225) antibody radioconjugate, is designed to break out of that single-mutation paradigm. Rather than blocking a specific mutant protein, it delivers a high-linear-energy-transfer alpha-particle payload that induces dense, irreversible double-strand DNA breaks and tumor-cell death independent of a tumor’s driver mutation or signaling pathway, the mechanistic basis of its mutation-agnostic activity.
ATNM-400’s target antigen, from previously published immunohistochemistry studies, is present in approximately 98% of NSCLC tumors and highly expressed in approximately 70% of those tumors. It is conserved across EGFR-, KRAS-, and other driver-defined subgroups, and further increased in tumors that have become resistant to EGFR, KRAS, and immune-checkpoint therapies. In new KRAS-mutant studies presented at SNMMI, sotorasib (active ingredient in LUMAKRAS/Amgen) and adagrasib (active ingredient in KRAZATI/BMS) increased ATNM-400’s target up to 3.5- and 3.8-fold, respectively, and adding ATNM-400 deepened tumor-cell killing beyond either inhibitor alone. These results demonstrate the same target-expression increasing, synergy-enabling biology shown previously with the EGFR inhibitor osimertinib, which produced tumor growth inhibition of 107% when combined with ATNM-400.
These combination benefits also broaden ATNM-400’s commercial opportunity. The franchises it could enhance are substantial; the KRAS inhibitor class in NSCLC is projected to exceed $5 billion in peak sales, and the EGFR-mutant segment has peak sales estimates over $15 billion, led by osimertinib (active ingredient in TAGRISSO/AZ), which generated $7.3 billion in 2025. This positions ATNM-400 to participate in these established markets by enhancing the standard of care, while also reaching the broader NSCLC population beyond any single mutation.
Sandesh Seth, Actinium’s Chairman and CEO, said, "Today’s data further strengthen our conviction that ATNM-400 represents a fundamentally different approach to treating NSCLC solid tumors. While most targeted therapies are designed for a single mutation-defined patient population, ATNM-400 combines the mutation-independent cell-killing power of Actinium-225 with a target that is broadly expressed across tumors and becomes even more abundant as resistance emerges. The consistency of activity we have now demonstrated across both EGFR- and KRAS-driven NSCLC supports our vision for ATNM-400 as a potential mutation-agnostic backbone therapy that can be used alone or in combination across large patient populations. We believe this opportunity could extend beyond lung cancer and may position ATNM-400 as a foundational therapy across multiple solid tumor indications."
Highlights from the SNMMI 2026 Poster Presentation
Poster Titled: ATNM-400: A First-in-Class Actinium-225 Antibody Radioconjugate Demonstrating Durable, Mutation-Agnostic Anti-Tumor Activity in Non-Small Cell Lung Cancer Models
New data demonstrated ATNM-400’s potential as a mutation-agnostic antibody radioconjugate therapy for NSCLC:
Across a panel of four lung cancer cell lines spanning EGFR- and KRAS-driver mutations, ATNM-400 bound and was internalized specifically by the three target-positive lines (NCI-H1975, NCI-H358 and Calu-3) but not by the target-negative line (A549), confirming that its activity is driven by target expression rather than by any particular mutation.
In a KRAS G12C model (NCI-H358), PET imaging of radiolabeled ATNM-400 showed the drug concentrating in the tumor with low uptake in healthy tissue, visually confirming that it reaches its target in a living animal and delivers its alpha payload where intended while largely sparing normal organs.
In a KRAS G13D model (Calu-3), a single dose of ATNM-400 drove tumor regression (124% and 135% inhibition at two dose levels), indicating marked antitumor potencywith full body-weight recovery. Deep responses from one dose with a clean tolerability profile point to a wide therapeutic window and dosing flexibility, favorable attributes for clinical translation.
Treatment with both approved KRAS inhibitors (sotorasib and adagrasib) raised ATNM-400’s target expression dose-dependently—up to roughly 3.5- to 3.8-fold—reaching statistical significance at every dose (p < 0.0001). The effect occurred with both agents, indicating a class-wide consequence of KRAS G12C inhibition that builds a rationale for combining ATNM-400 with these therapies.
Post both KRAS drugs ATMN400 expression increases
ATNM-400 plus sotorasib or adagrasib decreased cancer-cell viability beyond either inhibitor alone. The data demonstrate that ATNM-400 has development potential not only as a monotherapy but also in combination with these leading KRAS therapies and possibly the entire KRAS-mutant class.
In an EGFR-mutant model (NCI-H1975), ATNM-400 monotherapy achieved 75% tumor growth inhibition versus 40% for osimertinib; combined with osimertinib it reached 107% inhibition with complete cures in 100% of mice. Osimertinib raised target expression, providing a clear rationale for the combination.