Synthesis and Evaluation of a Macrocyclic Actinium-225 Chelator, Quality Control and In Vivo Evaluation of 225 Ac-crown-αMSH Peptide.

Targeted alpha-therapy (TAT) has great potential for treating a broad range of late-stage cancers by delivering a focused and lethal radiation dose to tumors. Actinium-225 (225 Ac) is an emerging alpha emitter suitable for TAT; however, the availability of chelators for Ac remains limited to a small number of examples (DOTA and macropa). Herein, we report a new Ac macrocyclic chelator named 'crown', which binds quantitatively and rapidly (<10 min) to Ac at ambient temperature. We synthesized 225 Ac-crown-αMSH, a peptide targeting the melanocortin 1 receptor (MC1R), specifically expressed in primary and metastatic melanoma. Biodistribution of 225 Ac-crown-αMSH showed favorable tumor-to-background ratios at 2 h post injection in a preclinical model. In addition, we demonstrated dramatically different biodistrubution patterns of 225 Ac-crown-αMSH when subjected to different latency times before injection. A combined quality control methodology involving HPLC, gamma spectroscopy and radioTLC is recommended.