HBZmacropa-NCS: A Bifunctional Chelator for Actinium-225 Targeted Alpha Therapy.

Actinium-225 (Ac) is one of the most promising radionuclides for targeted alpha therapy (TAT). With a half-life of 9.92 days and a decay chain that emits four high-energy α particles, Ac is well-suited for TAT when conjugated to macromolecular targeting vectors that exhibit extended in vivo circulation times. The implementation of Ac in these targeted constructs, however, requires a suitable chelator that can bind and retain this radionuclide in vivo. Previous work has demonstrated the suitability of a diaza-18-crown-6 macrocyclic chelator Hmacropa for this application. Building upon these prior efforts, in this study, two rigid variants of Hmacropa, which contain either one (HBZmacropa) or two (HBZmacropa) benzene rings within the macrocyclic core, were synthesized and investigated for their potential use for Ac TAT. The coordination chemistry of these ligands with La, used as a nonradioactive model for Ac, was carried out. Both NMR spectroscopic and X-ray crystallographic studies of the La complexes of these ligands revealed similar structural features to those found for the related complex of Hmacropa. Thermodynamic stability constants of the La complexes, however, were found to be 1 and 2 orders of magnitude lower than those of Hmacropa for HBZmacropa and HBZmacropa, respectively. The decrease in thermodynamic stability was rationalized via the use of density functional theory calculations. Ac radiolabeling and serum stability studies with HBZmacropa showed that this chelator compares favorably with Hmacropa. Based on these promising results, a bifunctional version of this chelator, HBZmacropa-NCS, was synthesized and conjugated to the antibody codrituzumab (GC33), which targets the liver cancer biomarker glypican-3 (GPC3). The resulting GC33-BZmacropa conjugate and an analogous GC33-macropa conjugate were evaluated for their Ac radiolabeling efficiencies, antigen-binding affinities, and in vivo biodistribution in HepG2 liver cancer tumor-bearing mice. Although both conjugates were comparably effective in their radiolabeling efficiencies, [Ac]Ac-GC33-BZmacropa showed slightly poorer serum stability and biodistribution than [Ac]Ac-GC33-macropa. Together, these results establish HBZmacropa-NCS as a new bifunctional chelator for the preparation of Ac radiopharmaceuticals.