Novel 64 Cu-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 ⁶⁴ Cu-NOTA-PEG ₂ Nle-CycMSH _hex { ⁶⁴ Cu-1,4,7-triazacyclononane-1,4,7-triyl-triacetic acid-polyethylene glycol-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH ₂ } and ⁶⁴ Cu-NOTA-AocNle-CycMSH _hex { ⁶⁴ Cu-NOTA-8-aminooctanoic acid-Nle-CycMSH _hex } on melanoma-bearing mice. NOTA-PEG ₂ Nle-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 ⁶⁴ Cu-NOTA-PEG ₂ Nle-CycMSH _hex and ⁶⁴ Cu-NOTA-AocNle-CycMSH _hex were determined on B16/F10 melanoma-bearing C57 mice. The melanoma imaging property of ⁶⁴ Cu-NOTA-PEG ₂ Nle-CycMSH _hex was further examined on B16/F10 melanoma-bearing C57 mice because of its higher melanoma uptake than ⁶⁴ Cu-NOTA-AocNle-CycMSH _hex . The IC ₅₀ values of NOTA-PEG ₂ Nle-CycMSH _hex and NOTA-AocNle-CycMSH _hex were 1.24 ± 0.07 and 2.75 ± 0.48 nM on B10/F10 melanoma cells. ⁶⁴ Cu-NOTA-PEG ₂ Nle-CycMSH _hex and ⁶⁴ Cu-NOTA-AocNle-CycMSH _hex were readily prepared with more than 90% radiolabeling yields and showed MC1R-specific binding on B16/F10 cells. ⁶⁴ Cu-NOTA-PEG ₂ Nle-CycMSH _hex exhibited higher tumor uptake than ⁶⁴ Cu-NOTA-AocNle-CycMSH _hex at 0.5, 2, 4, and 24 h post-injection. The tumor uptake of ⁶⁴ Cu-NOTA-PEG ₂ Nle-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 ⁶⁴ Cu-NOTA-PEG ₂ Nle-CycMSH _hex was lower than 2% ID/g at 2 h post-injection except for kidney uptake. The renal uptake of ⁶⁴ Cu-NOTA-PEG ₂ Nle-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. ⁶⁴ Cu-NOTA-PEG ₂ Nle-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 ⁶⁴ Cu-NOTA-PEG ₂ Nle-CycMSH _hex as an imaging probe at 2 h post-injection. High tumor uptake and low kidney uptake of ⁶⁴ Cu-NOTA-PEG ₂ Nle-CycMSH _hex underscored its potential as an MC1R-targeted theranostic peptide for melanoma imaging and therapy.