Introduction of an 8-aminooctanoic acid linker enhances uptake of ^99mTc-labeled lactam bridge-cyclized α-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 ^99mTc-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-PEG₂Nle-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 ^99mTc(ethylenediaminediacetic acid [EDDA])-HYNIC-GGGNle-CycMSH_hex, ^99mTc(EDDA)-HYNIC-GSGNle-CycMSH_hex, ^99mTc(EDDA)-HYNIC-PEG₂Nle-CycMSH_hex, and ^99mTc(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 ^99mTc(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-PEG₂Nle-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 ^99mTc-labeled peptides, ^99mTc(EDDA)-HYNIC-AocNle-CycMSH_hex displayed the highest melanoma uptake (22.3 ± 1.72 percentage injected dose/g) at 2 h after injection. ^99mTc(EDDA)-HYNIC-AocNle-CycMSH_hex exhibited high tumor-to-normal-organ uptake ratios except for the kidneys. The tumor-to-kidney uptake ratios of ^99mTc (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 ^99mTc(EDDA)-HYNIC-AocNle-CycMSH_hex as an imaging probe at 2 h after injection. Conclusion: High melanoma uptake and fast urinary clearance of ^99mTc (EDDA)-HYNIC-AocNle-CycMSH_hex highlighted its potential for metastatic melanoma detection in the future.