Localization of CaSR Antagonists in CaSR-expressing Medullary Thyroid Cancer

Objective: Image-based localization of medullary thyroid cancer (MTC) and parathyroid glands would improve the surgical outcomes of these diseases. MTC and parathyroid glands express high levels of calcium-sensing receptor (CaSR). The aim of this study was to prove the concept that CaSR antagonists specifically localize to CaSR-expressing tumors in vivo. Design: We synthesized two isomers of a known CaSR calcilytic, Calhex 231, and four new analogs, which have a favorable structure for labeling. Their antagonistic activity was determined using immunoblots demonstrating decreased ERK1/2 phosphorylation after calcium stimulation in human embryonic kidney cells overexpressing CaSR. Compound 9 was further radiolabeled with I-125 and evaluated in nude mice with and without heterotransplanted xenografts of MTC cell lines, TT and MZ-CRC-1, that do and do not express CaSR, respectively. Results: Two newly synthesized compounds, 9 and 11, exhibited better antagonistic activity than Calhex 231. The half-life of I-125-compound 9 in nude mice without xenografts was 9.9 hours. A biodistribution study in nude mice bearing both tumors demonstrated that the uptake of radioactivity in TT tumors was higher than in MZ-CRC-1 tumors at 24 hours: 0.39 +/- 0.24 vs 0.18 +/- 0.12 percentage of injected dose per gram of tissue (%ID/g) (P =.002), with a ratio of 2.25 +/- 0.62. Tumor-to-background ratios for TT tumors, but not MZ-CRC-1 tumors, increased with time. Tumorto-blood values increased from 2.02 +/- 0.52 at 1 hour to 3.29 +/- 0.98 at 24 hour (P =.015) for TT tumors, and 1.7 +/- 0.56 at 1 hour to 1.48 +/- 0.33 at 24 hour (P =.36) for MZ-CRC-1 tumors. Conclusions: OurnewCaSRantagonists specifically inhibitCaSRfunction in vitro, preferentially localize to CaSR-expressing tumors in vivo, and therefore have the potential to serve as scaffolds for further development as imaging pharmaceuticals.