Evaluation of [Tc-99m]Tc-tricarbonyl labeled alpha-melanocyte stimulating hormone derived peptide conjugates for melanoma imaging

The melanocortin 1 receptor (MC1R), since overexpressed in melanoma cells, is an attractive target for imaging or treatment of this type of malignancies. The α-melanocyte stimulating hormone (α-MSH) derived peptide NAP-NS1 (Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH2) binds with high affinity [1] and was engineered in three different ways for labeling with [Tc-99m]Tc-tricarbonyl. ɛ-Ahx-β-Ala-NAP-NS1, ethylenglycol (EG)-based linker NAP-NS1 and NAP-NS1 without linker were conjugated with bis(pyridin-2-ylmethyl)amine (DPA) obtaining yields of 42%, 38% and 45%, respectively. For standard purpose non-radioactive Re-DPA-peptides were prepared [2] with yields > 55%. The labeling with a one-pot method was faster than with a two steps method, and even receiving slightly higher radiochemical yields. High stability as well as negligible transschelation was observed for the [Tc-99m]Tc-tricarbonyl-peptide conjugates. The ethylenglycol based linker increased the hydrophilicity. Binding results, first of competition assays with the non-radioactive α-MSH conjugates, and second of saturation assays with the [Tc-99m]Tc-tricarbonyl-DPA peptides resulted in high affinity (low nanomolar range) where the labeled peptides showed a marginal loss of affinity; that was relevant for murine B16F10, as well as for human MeWo and TXM13 cells. The number of binding sites was considerably higher in murine melanoma cells. The three [Tc-99m]Tc-tricarbonyl-peptide conjugates exhibited similar in vitro properties. Preliminary in vivo studies were performed with [Tc-99m]Tc-tricarbonyl-DPA-ɛ-Ahx-β-Ala-NAP-NS1 in rats and showed good metabolic stability in blood and both a renal and hepatobiliary excretion. Hence, it displayed prospective radiochemical and radiopharmacological properties, suggesting a promising candidate for further investigation in a melanoma xenograft model.

[1] Gao et al. J. Amino Acids 2016; 48: 833-847.
[2] Alberto et al. J. Chem. Soc. Dalton 1994; 19: 2815-2820.