Introduction of an 8-Aminooctanoic Acid Linker Enhances Uptake of Tc-99m-Labeled Lactam Bridge-Cyclized alpha-MSH Peptide in Melanoma

The purpose of this study was to examine the effects of amino acid, hydrocarbon, and polyethylene glycol (PEG) linkers on the melanoma targeting and imaging properties of Tc-99m-labeled lactam bridge-cyclized HYNIC-linker-Nle-CycMSH(hex) (hydrazinonicotinamide-linker-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH2) peptides. Methods: Four novel peptides (HYNIC-GGGNle-CycMSH(hex), HYNIC-GSGNle-CycMSH(hex), HYNIC-PEG(2)Nle-CycMSH(hex), and HYNIC-AocNle-CycMSH(hex)) were designed and synthesized. The melanocortin-1 receptor binding affinities of the peptides were determined in B16/F1 melanoma cells. The biodistribution of Tc-99m(ethylenediaminediacetic acid [EDDA])-HYNIC-GGGNle-CycMSH(hex), Tc-99m(EDDA)-HYNIC-GSGNle-CycMSH(hex), Tc-99m(EDDA)-HYNIC-PEG(2)Nle-CycMSH(hex), and Tc-99m(EDDA)-HYNIC-AocNle-CycMSH(hex) were determined in B16/F1 melanoma-bearing C57 mice at 2 h after injection to select a lead peptide for further evaluation. The melanoma targeting and imaging properties of Tc-99m(EDDA)-HYNIC-AocNle-CycMSH(hex) were further examined because of its high melanoma uptake. Results: The inhibitory concentrations of 50% (IC50) for HYNIC-GGGNle-CycMSH(hex), HYNIC-GSGNle-CycMSH(hex), HYNIC-PEG(2)Nle-CycMSH(hex), and HYNIC-AocNle-CycMSH(hex) were 0.7 +/- 0.1, 0.8 +/- 0.09, 0.4 +/- 0.08, and 0.3 +/- 0.06 nM, respectively, in B16/F1 melanoma cells. Among these four Tc-99m-labeled peptides, Tc-99m(EDDA)-HYNIC-AocNle-CycMSH(hex) displayed the highest melanoma uptake (22.3 +/- 1.72 percentage injected dose/g) at 2 h after injection. Tc-99m(EDDA)-HYNIC-AocNle-CycMSH(hex) exhibited high tumor-to-normal-organ uptake ratios except for the kidneys. The tumor-to-kidney uptake ratios of Tc-99m (EDDA)-HYNIC-AocNle-CycMSH(hex) were 3.29, 3.63, and 6.78 at 2, 4, and 24 h, respectively, after injection. The melanoma lesions were clearly visualized by SPECT/CT using Tc-99m(EDDA)-HYNIC-AocNle-CycMSH(hex) as an imaging probe at 2 h after injection. Conclusion: High melanoma uptake and fast urinary clearance of Tc-99m (EDDA)-HYNIC-AocNle-CycMSH(hex) highlighted its potential for metastatic melanoma detection in the future.