Abstract 517: an MC1R Targeted 225ac Radiopharmaceutical Agent for Treatment of Uveal Melanoma

Abstract Prognosis in metastatic uveal melanoma is poor with median survival being less than one year. There is no approved therapy for metastatic disease. Hence, new targeted therapies are needed. The melanocortin 1 receptor (MC1R) is expressed in 94% of uveal melanomas but is not expressed in normal tissues of concern for toxicity. We have developed a MC1R specific ligand (MC1RL), conjugated imaging contrast agents to it, demonstrated high selectivity for MC1R expressing tumors in mice, and demonstrated rapid systemic clearance, without retention in tissues of concern for toxicity. The aim of this study is to use MC1RL as a targeting scaffold for development of a radiopharmaceutical by conjugation of 225Ac chelate. 225Ac is a therapeutic alpha emitting radionuclide. A targeted approach to deliver 225Ac to uveal melanoma without causing toxicity in normal tissues would provide clinicians with a novel and powerful therapeutic option for treatment of metastatic disease. Here, we conjugated 1,4,7,10-tetraazacyclo-dodecane-1,4,7,10-tetraacetic acid (DOTA) to the MC1R specific scaffold (DOTA-MC1RL) and chelated the nonradioactive surrogate 139La (substitute for 225Ac) and demonstrated high binding affinity to MC1R (0.2 nM Ki). We anticipate that the radioactive conjugates will have comparable affinities. We also synthesized 225Ac-DOTA-MC1RL and showed a radiochemical yield greater than 95% and high radiochemical purity (≥99.8%) as determined by radio-TLC, radio-HPLC, and gamma-counter quantification. Moreover, 225Ac -DOTA-MC1RL showed excellent in vitro stability, i.e. 90% after 10 days in human serum at 37ºC. A maximum tolerated dosage (MTD) study was performed by administration of 0, 9, 18, 28, 37, 56, 74 and 148 kBq of 225Ac-DOTA-MC1RL. Animals were followed for 120 days and there were no signs of altered behavior among the groups. The group that received the highest dosage had a slightly but significantly lower increase in body weight over the course of the study, suggesting that 225Ac-DOTA-MC1RL is tolerated extremely well. Blood work and organ pathology did not show any significant deleterious effects even at the highest dose. In vitro cytotoxicity assays showed significant cell death in uveal and cutaneous cell lines in an MC1R dependent manner. Biodistribution studies showed tumor selectivity and a combination of renal and hepatic clearance with minimal retention in other normal tissues. In vivo efficacy studies in SCID mice bearing MC1R expressing human A375 subcutaneous xenograft tumors showed complete loss of tumor within one week of intravenous administration of 225Ac-DOTA-MC1RL relative to controls that had continued tumor growth. In conclusion, we have demonstrated radiosynthesis of 225Ac-DOTA-MC1RLwith high yield, purity and stability. In vitro studies demonstrated MC1R dependent cytotoxicity in uveal melanoma cells. In vivo studies demonstrated favorable biodistribution and significant antitumor efficacy. Citation Format: Narges K. Tafreshi, Michael L. Doligalski, Darpan N. Pandya, Hyun Joo Kil, Mikalai Budzevich, Thaddeus J. Wadas, Mark L. McLaughlin, David Morse. An MC1R targeted 225Ac radiopharmaceutical agent for treatment of uveal melanoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 517.