A Novel DOTA- -Melanocyte–Stimulating Hormone Analog for Metastatic Melanoma Diagnosis

Scintigraphic imaging of metastatic melanoma lesions requires highly tumor-specific radiolabeled compounds. Because both melanotic and amelanotic melanomas overexpress receptors for -melanocyte–stimulating hormone ( -MSH; receptor name: melanocortin type 1 receptor, or MC1R), radiolabeled -MSH analogs are potential candidates for melanoma diagnosis. The aim of this study was to develop a melanoma-selective radiolabeled -MSH analog suitable for melanoma diagnosis. Methods: The very potent -MSH analog [Nle4, D-Phe7]-MSH (NDP-MSH) and a newly designed -MSH octapeptide analog, [ Ala3, Nle4, Asp5, D-Phe7, Lys10]-MSH3–10 (MSHoct), were conjugated to the metal chelator 1,4,7,10-tetraazacyclododecane1,4,7,10-tetraacetic acid (DOTA) to enable radiometal incorporation. The resulting DOTA conjugates were evaluated in vitro for their MC1R-binding affinity and melanogenic activity in isolated mouse B16F1 cells and in vivo for their biodistribution in mouse models of primary and metastatic melanoma after labeling with 111In. Results: DOTA-MSHoct was shown to bind with high affinity (inhibitory concentration of 50% [IC50] 9.21 nmol/L) to the MC1R, although with lower potency than does DOTA-NDP-MSH (IC50 0.25 nmol/L). In B16F1 melanomabearing mice, both 111In-DOTA-NDP-MSH and 111In-DOTAMSHoct exhibited high MC1R-mediated uptake by melanoma, which differed by a factor of only 1.5 at 4 h after injection. The main route of excretion for both radioconjugates was the kidneys, whereby In-DOTA-MSHoct led to somewhat higher kidney values than did 111In-DOTA-NDP-MSH. In contrast, the latter was much more poorly cleared from other nonmalignant tissues, including bone, the most radiosensitive organ. Therefore, In-DOTA-MSHoct displayed higher uptake ratios of tumor to nontarget tissue (e.g., tumor-to-bone ratio 4 h after injection was 4.9 for 111In-DOTA-NDP-MSH and 53.9 for 111In-DOTAMSHoct). Lung and liver melanoma metastases could easily be visualized on tissue section autoradiographs after injection of In-DOTA-MSHoct. Radio–reversed-phase high-performance liquid chromatography analysis of urine samples revealed that most In-DOTA-MSHoct is excreted intact 4 h after injection, indicating good in vivo stability. Conclusion: 111In-DOTAMSHoct exhibits more favorable overall performance than does 111In-DOTA-NDP-MSH in murine models of primary and metastatic melanoma, making it a promising melanoma imaging agent.