Complexes of Di- and Triazacrown Ethers with Heavy Metal Ions in Water Solution

H2BZmacropa-NCS: A Bifunctional Chelator for Actinium-225 Targeted Alpha Therapy

Actinium-225 (225Ac) 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, 225Ac is well-suited for TAT when conjugated to macromolecular targeting vectors that exhibit extended in vivo circulation times. The implementation of 225Ac 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 H2macropa for this application. Building upon these prior efforts, in this study, two rigid variants of H2macropa, which contain either one (H2BZmacropa) or two (H2BZ2macropa) benzene rings within the macrocyclic core, were synthesized and investigated for their potential use for 225Ac TAT. The coordination chemistry of these ligands with La3+, used as a nonradioactive model for Ac3+, was carried out. Both NMR spectroscopic and X-ray crystallographic studies of the La3+ complexes of these ligands revealed similar structural features to those found for the related complex of H2macropa. Thermodynamic stability constants of the La3+ complexes, however, were found to be 1 and 2 orders of magnitude lower than those of H2macropa for H2BZmacropa and H2BZ2macropa, respectively. The decrease in thermodynamic stability was rationalized via the use of density functional theory calculations. 225Ac radiolabeling and serum stability studies with H2BZmacropa showed that this chelator compares favorably with H2macropa. Based on these promising results, a bifunctional version of this chelator, H2BZmacropa-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 225Ac 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, [225Ac]Ac-GC33-BZmacropa showed slightly poorer serum stability and biodistribution than [225Ac]Ac-GC33-macropa. Together, these results establish H2BZmacropa-NCS as a new bifunctional chelator for the preparation of 225Ac radiopharmaceuticals.

Complexation of Bi3+, Ac3+, Y3+, Lu3+, La3+ and Eu3+ with benzo-diaza-crown ether with carboxylic pendant arms

Polyaminopolycarboxylates are attractive ligands for binding cationic radionuclides for synthesis of radiopharmaceuticals with target delivery to tumor cells. Nowadays beta emitting Y−90 and Lu−177 are used as therapeutic agents, while Ac−225 and Bi−213 are considered as perspective for alpha therapy. In the present study new data on complexation of Y3+, Lu3+, Ac3+ and Bi3+ with 2,2’-(15-formyl-2,3,5,6,8,9,11,12-octahydrobenzo [b][1,4,10,7,13]trioxadiazacyclopentadecine-4,10- diyl)diacetic acid are presented. For ligand and complexes characterization potentiometric titration, solvent extraction, chromatography and solubility techniques were applied. The highest values of stability constants within the range of log K = 5.8 – 7.5 were found for Ac3+ and REE. Fast complex formation is established which is beneficial for practical applications in radiopharmaceutical synthesis.