On June 1, 2026 Actinium Pharmaceuticals, Inc. (NYSE American: ATNM) (Actinium or the Company), a pioneer in the development of targeted radiotherapies, reported new radiochemistry data at the Society of Nuclear Medicine and Molecular Imaging (SNMMI) 2026 Annual Meeting taking place in Los Angeles, California.
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The poster presented a systematic evaluation of chelator-to-antibody ratio (CAR) optimization for Actinium-225 (225Ac)-labeled antibody radioconjugates, a critical but often underappreciated design parameter that directly influences radiolabeling efficiency, antigen binding, internalization, and biodistribution. The findings reinforce a proprietary radiochemistry capability that underpins the Company’s broader pipeline. Practically, CAR governs how many radioactive payloads each tumor-seeking antibody carries. Too few a payload causes a radioconjugate drug to under deliver, too many and the antibody loses its aim and leaks dose into healthy tissue. Therefore, identifying the optimal ratio is what separates a technically active therapy from one that can be dosed effectively and safely.
Sandesh Seth, Actinium’s Chairman and CEO, said, "Our CAR optimization data underscore the depth of our radiochemistry expertise and our focus on maximizing therapeutic index, which we believe is a key differentiator of our platform. Getting the chelator-to-antibody ratio right is fundamental to preserving the biological integrity of an antibody while enabling robust 225Ac labeling and favorable pharmacokinetics. These findings have direct implications for the design of next-generation radioconjugates, and reinforce a radiochemistry advantage that supports successful clinical translation across our pipeline."
Highlights from the SNMMI 2026 Poster Presentation
Poster Titled: Optimizing Chelator-to-Antibody Ratio Improves Tumor Targeting and Pharmacokinetics of 225Ac-Labeled Antibodies
Optimizing the chelator-to-antibody ratio (CAR) is a critical but often underappreciated design parameter in antibody-based radiopharmaceuticals. Actinium’s scientists conducted a systematic evaluation of CAR for 225Ac-labeled antibody radioconjugates, with findings directly applicable to the development of Actinium’s radiopharma pipeline:
Antibody-DOTA conjugates spanning CAR 0.7–9 were prepared. CAR ≥1.7 enabled robust 225Ac labeling, while CAR 0.7 was insufficient. This establishes the minimum loading needed to carry an effective radioactive dose
Antigen binding stayed high at low CAR (91–98% at CAR 0.7–3.2) but fell at high CAR (79–85% at CAR 7–9), and low-CAR conjugates internalized more, keeping more of the antibody functional. Median Fluorescence Intensity (MFI) is a proxy for binding or antigen affinity retention. Findings suggest overloading the antibody degrades its ability to find and enter cancer cells
In vivo, both conjugates showed comparable tumor uptake and sustained retention through 192 h, but the low-CAR (2.5) conjugate significantly reduced liver and spleen uptake, providing a basis for a wider therapeutic index
Both conjugates remained stable over 7 days (radiochemical purity above 97%), showing that CAR can be optimized for performance without compromising manufacturability or clinical supply. Safety and targeting gains can come with no manufacturing trade-off
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A wider therapeutic index could allow more dose to reach the tumor at a given level of safety, a proprietary radiochemistry framework Actinium is applying to de-risk its broader pipeline resulting in lower technical risk across the portfolio
(Press release, Actinium Pharmaceuticals, JUN 1, 2026, View Source [SID1234666322])