Novel ⁶⁴Cu-Labeled NOTA-Conjugated Lactam-Cyclized Alpha-Melanocyte-Stimulating Hormone Peptides with Enhanced Tumor to Kidney Uptake Ratios

The aim of this study was to evaluate the effect of linker on tumor targeting and biodistribution of ⁶⁴Cu-NOTA-PEG₂Nle-CycMSH_hex {⁶⁴Cu-1,4,7-triazacyclononane-1,4,7-triyl-triacetic acid-polyethylene glycol-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH₂} and ⁶⁴Cu-NOTA-AocNle-CycMSH_hex {⁶⁴Cu-NOTA-8-aminooctanoic acid-Nle-CycMSH_hex} on melanoma-bearing mice. NOTA-PEG₂Nle-CycMSH_hex and NOTA-AocNle-CycMSH_hex were synthesized and purified by HPLC. The melanocortin-1 (MC1) receptor binding affinities of the peptides were examined on B16/F10 melanoma cells. The biodistributions of ⁶⁴Cu-NOTA-PEG₂Nle-CycMSH_hex and ⁶⁴Cu-NOTA-AocNle-CycMSH_hex were determined on B16/F10 melanoma-bearing C57 mice. The melanoma imaging property of ⁶⁴Cu-NOTA-PEG₂Nle-CycMSH_hex was further examined on B16/F10 melanoma-bearing C57 mice because of its higher melanoma uptake than ⁶⁴Cu-NOTA-AocNle-CycMSH_hex. The IC₅₀ values of NOTA-PEG₂Nle-CycMSH_hex and NOTA-AocNle-CycMSH_hex were 1.24 ± 0.07 and 2.75 ± 0.48 nM on B10/F10 melanoma cells. ⁶⁴Cu-NOTA-PEG₂Nle-CycMSH_hex and ⁶⁴Cu-NOTA-AocNle-CycMSH_hex were readily prepared with more than 90% radiolabeling yields and showed MC1R-specific binding on B16/F10 cells. ⁶⁴Cu-NOTA-PEG₂Nle-CycMSH_hex exhibited higher tumor uptake than ⁶⁴Cu-NOTA-AocNle-CycMSH_hex at 0.5, 2, 4, and 24 h post-injection. The tumor uptake of ⁶⁴Cu-NOTA-PEG₂Nle-CycMSH_hex was 16.23 ± 0.42, 19.59 ± 1.48, 12.83 ± 1.69, and 8.78 ± 2.29% ID/g at 0.5, 2, 4, and 24 h post-injection, respectively. Normal organ uptake of ⁶⁴Cu-NOTA-PEG₂Nle-CycMSH_hex was lower than 2% ID/g at 2 h post-injection except for kidney uptake. The renal uptake of ⁶⁴Cu-NOTA-PEG₂Nle-CycMSH_hex was 3.66 ± 0.52, 3.27 ± 0.52, and 1.47 ± 0.56 ID/g at 2, 4, and 24 h post-injection, respectively. ⁶⁴Cu-NOTA-PEG₂Nle-CycMSH_hex showed high tumor to normal organ uptake ratios after 2 h post-injection. The B16/F10 melanoma lesions could be clearly visualized by positron emission tomography (PET) using ⁶⁴Cu-NOTA-PEG₂Nle-CycMSH_hex as an imaging probe at 2 h post-injection. High tumor uptake and low kidney uptake of ⁶⁴Cu-NOTA-PEG₂Nle-CycMSH_hex underscored its potential as an MC1R-targeted theranostic peptide for melanoma imaging and therapy.