Actinium Pharmaceuticals, Inc. (NYSE American: ATNM) presented research findings at the Society of Nuclear Medicine and Molecular Imaging 2026 Annual Meeting on May 31, demonstrating how chelator-to-antibody ratio optimization can improve the targeting and pharmacokinetics of Actinium-225 radioconjugates.

The company's poster presentation showed that antibody-DOTA conjugates with chelator-to-antibody ratios of 1.7 or higher enabled robust Actinium-225 labeling, while ratios of 0.7 proved insufficient. The research found that antigen binding remained high at low ratios (91-98% at ratios 0.7-3.2) but decreased at higher ratios (79-85% at ratios 7-9).

In animal studies, both low-ratio (2.5) and high-ratio conjugates showed comparable tumor uptake and retention through 192 hours. However, the low-ratio conjugate significantly reduced liver and spleen uptake compared to higher ratios, potentially supporting a wider therapeutic window.

The conjugates maintained stability over seven days with radiochemical purity above 97%, according to the data presented. Chelator-to-antibody ratio determines how many radioactive payloads each tumor-targeting antibody carries in these radioconjugate therapies.

"Our CAR optimization data underscore the depth of our radiochemistry expertise and our focus on maximizing therapeutic index," said Sandesh Seth, Actinium's Chairman and CEO. "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."

Actinium develops targeted radiotherapies for cancer treatment, with a pipeline that includes programs for solid tumors and blood cancers. The company holds approximately 250 patents and patent applications related to its radiotherapy platform, according to the press release.