A 99mTc(CO)3-labeled pyrazolyl–α-melanocyte-stimulating hormone analog conjugate for melanoma targeting

Melanoma primary tumors can be, in most cases, removed surgically, whereas there is no satisfactory treatment for metastatic melanoma, being almost always lethal at this stage. Therefore, early detection of primary melanoma tumors is essential. The finding that melanocortin-1 receptor (MC1R) is overexpressed in isolated melanoma cells and melanoma tissues led to the radiolabeling of several α-melanocyte-stimulating hormone (α-MSH) analogs for early detection and treatment of melanoma. We have coupled the α-MSH analog Ac-Nle-Asp-His-d-Phe-Arg-Trp-Gly-Lys-NH2, through the ε-amino group of Lys11, to a pyrazolyl-containing chelator (pz). The resulting pz–α-MSH analog reacted with the fac-[99mTc(CO)3]+ moiety, giving [Ac-Nle4,Asp5,d-Phe7,Lys11(pz-99mTc(CO)3)]α-MSH4–11 in high yield, high specific activity and high radiochemical purity. This radioconjugate, which presents remarkable stability in vitro, exhibited time- and temperature-dependent internalization (4 h at 37°C; 56.7% maximum internalization) and high cellular retention (only 38% was released from the cell after 5 h) in murine melanoma B16F1 cells. A significant tumor uptake [4.2±0.9%ID/g, at 4 h postinjection (p.i.)] was also obtained in melanoma-bearing C57BL6 mice. The in vivo affinity and specificity of the radioconjugate to MC1R were demonstrated by receptor-blocking studies with the potent NDP-MSH agonist (63.5% reduction in tumor uptake at 4 h p.i.).