H(2)BZmacropa-NCS: A Bifunctional Chelator for Actinium-225 Targeted Alpha Therapy

Actinium-225 ((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, (225)Ac is well-suited for TAT when conjugated to macromolecular targeting vectors that exhibit extended in vivo circulation times. The implementation of (225)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 H(2)macropa for this application. Building upon these prior efforts, in this study, two rigid variants of H(2)macropa, which contain either one (H(2)BZmacropa) or two (H(2)BZ(2)macropa) benzene rings within the macrocyclic core, were synthesized and investigated for their potential use for (225)Ac TAT. The coordination chemistry of these ligands with La(3+), used as a non-radioactive model for Ac(3+), was carried out. Both NMR spectroscopic and X-ray crystallographic studies of the La(3+) complexes of these ligands revealed similar structural features as those found for the related complex of H(2)macropa. Thermodynamic stability constants of the La(3+) complexes, however, were found to be one and two orders of magnitude lower than those of H(2)macropa for H(2)BZmacropa and H(2)BZ(2)macropa, respectively. The decrease in thermodynamic stability was rationalized via the use of density functional theory calculations. (225)Ac radiolabeling and serum stability studies with H(2)BZmacropa showed that this chelator compares favorably with H(2)macropa. Based on these promising results, a bifunctional version of this chelator, H(2)BZmacropa-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 were evaluated for their (225)Ac radiolabeling efficiencies, antigen-binding affinity, and in vivo biodistribution in HepG2 liver cancer tumor-bearing mice. Although both conjugates were comparably effective in their radiolabeling efficiencies, [(225)Ac]Ac-GC33-BZmacropa showed slightly poorer serum stability and biodistribution than [(225)Ac]Ac-GC33-macropa. Together, these results establish H(2)BZmacropa-NCS as a new bifunctional chelator for the preparation of (225)Ac radiopharmaceuticals.