Melanoma-targeting properties of (99m)technetium-labeled cyclic alpha-melanocyte-stimulating hormone peptide analogues

Preliminary reports have demonstrated that (99m)technetium (Tc)-labeled cyc lic [Cys(3,4,10), D-Phe(7)]alpha -MSH3-13 (CCMSH) exhibits high tumor uptak e and retention values in a murine melanoma mouse model, In this report, th e tumor targeting mechanism of Tc-99m-CCMSH was studied and compared with f our other radiolabeled ol-melanocyte stimulating hormone (alpha -MSH) pepti de analogues: I-125-(Tyr(2))-[Nle(4), D-Phe(7)]alpha -MSH [I-125-(Tyr(2))-N DP]; Tc-99m-CGCG-NDP; Tc-99m-Gly(11)-CCMSH; and Tc-99m-Nle(11)-CCMSH. In vi tro receptor binding, Internalization, and cellular retention of radiolabel ed alpha -MSH analogues in B16/F1 murine cell line demonstrated that >70% o f the receptor-bound radiolabeled analogues were internalized together with the receptor, Ninety % of the internalized I-125-(Tyr(2))-NDP, whereas onl y 36% of internalized Tc-99m-CCMSH, was released from the cells into the me dium during a 4-h incubation at 37 degreesC, Two mouse models, C57 mice and severe combined immunodeficient (Scid) mice, inoculated s.c. with B16/F1 m urine and TXM-13 human melanoma cells were used for the in vivo studies. Tu mor uptake values of 11.32 and 2.39 [% injected dose (ID)/g] for Tc-99m-CCM SH at 4 h after injection, resulted In an uptake ratio of tumor:blood of 39 .0 and 11.5 in murine meianoma-C57 and human melanoma-Scid mouse models, re spectively, Two strategies for decreasing the nonspecific kidney uptake of Tc-99m-CCMSH, substitution of Lys(11) in CCMSH with Gly(11) or Nle(11), and lysine coinjection, were evaluated. The biodistribution data for the modif ied peptides showed that Lys(11) replacement dramatically decreased the kid ney uptake, whereas the tumor uptakes of Tc-99m-Nle(11)- acid Tc-99m-Gly(11 )-CCMSH were significantly lower than that of Tc-99m-CCMSH. Lysine coinject ion significantly decreased the kidney uptake (e.g,, from 14.6%, ID/g to 4. 5% ID/g at 4 h after injection in murine melanoma-C57 mice) without signifi cantly changing the value of tumor uptake of Tc-99m-CCMSH. In conclusion, t he compact cyclic structure of Tc-99m-CCMSH, its resistance to degradation, and its enhanced intracellular retention are the major contributing factor s to the superior in vivo tumor targeting properties of Tc-99m-CCMSH. Lys(1 1) residue in Tc-99m-CCMSH is critical to the tumor targeting in vivo, and lysine coinjection rather than lysine replacement can significantly decreas e the nonspecific renal radioactivity accumulation without impeding the hig h melanoma-targeting properties of Tc-99m-CCMSH. The metal-cyclized CCMSH m olecule displays excellent potential for the development of melanoma-specif ic diagnostic and therapeutic agents.