Towards the stable chelation of radium for biomedical applications with an 18-membered macrocyclic ligand

Targeted alpha therapy is an emerging strategy for the treatment of disseminated cancer. [Ra-223]RaCl2 is the only clinically approved alpha particle-emitting drug, and it is used to treat castrate-resistant prostate cancer bone metastases, to which [Ra-223]Ra2+ localizes. To specifically direct [Ra-223]Ra2+ to non-osseous disease sites, chelation and conjugation to a cancer-targeting moiety is necessary. Although previous efforts to stably chelate [Ra-223]Ra2+ for this purpose have had limited success, here we report a biologically stable radiocomplex with the 18-membered macrocyclic chelator macropa. Quantitative labeling of macropa with [Ra-223]Ra2+ was accomplished within 5 min at room temperature with a radiolabeling efficiency of >95%, representing a significant advancement over conventional chelators such as DOTA and EDTA, which were unable to completely complex [Ra-223]Ra2+ under these conditions. [Ra-223][Ra(macropa)] was highly stable in human serum and exhibited dramatically reduced bone and spleen uptake in mice in comparison to bone-targeted [Ra-223]RaCl2, signifying that [Ra-223][Ra(macropa)] remains intact in vivo. Upon conjugation of macropa to a single amino acid beta-alanine as well as to the prostate-specific membrane antigen-targeting peptide DUPA, both constructs retained high affinity for Ra-223, complexing >95% of Ra2+ in solution. Furthermore, [Ra-223][Ra(macropa-beta-alanine)] was rapidly cleared from mice and showed low Ra-223 bone absorption, indicating that this conjugate is stable under biological conditions. Unexpectedly, this stability was lost upon conjugation of macropa to DUPA, which suggests a role of targeting vectors in complex stability in vivo for this system. Nonetheless, our successful demonstration of efficient radiolabeling of the beta-alanine conjugate with Ra-223 and its subsequent stability in vivo establishes for the first time the possibility of delivering [Ra-223]Ra2+ to metastases outside of the bone using functionalized chelators, marking a significant expansion of the therapeutic utility of this radiometal in the clinic.