Introduction of a Polyethylene Glycol Linker Improves Uptake of [sup.67]Cu-NOTA-Conjugated Lactam-Cyclized Alpha-Melanocyte-Stimulating Hormone Peptide in Melanoma

There is a need to develop new theranostic approaches for malignant melanoma. Only 35% of patients with metastatic melanoma reach the milestone of 5-year survival, despite the success of new immunotherapy. We have developed a new class of peptides to target melanocortin-1 receptors (MC1Rs) that display elevated levels in human melanoma. In this study, we examined the melanoma targeting and biodistribution properties of two [sup.67]Cu-tagged peptides in tumor-bearing mice. We found that one of the peptides, namely [sup.67]Cu-NOTA-PEG[sub.2]Nle-CycMSH[sub.hex], exhibited favorable melanoma targeting and biodistribution properties that underscored its potential as an MC1R-targeted therapeutic peptide for melanoma treatment in the future. The aim of this study was to evaluate the effect of linker on tumor targeting and biodistribution of [sup.67]Cu-NOTA-PEG[sub.2]Nle-CycMSH[sub.hex] {[sup.67]Cu-1,4,7-triazacyclononane-1,4,7-triyl-triacetic acid-polyethylene glycol-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH[sub.2]} and [sup.67]Cu-NOTA-GGNle-CycMSH[sub.hex] {[sup.67]Cu-NOTA-GlyGlyNle-CycMSH[sub.hex]} on melanoma-bearing mice. NOTA-PEG[sub.2]Nle-CycMSH[sub.hex] and NOTA-GGNle-CycMSH[sub.hex] were synthesized and purified by HPLC. The biodistribution of [sup.67]Cu-NOTA-PEG[sub.2]Nle-CycMSH[sub.hex] and [sup.67]Cu-NOTA-GGNle-CycMSH[sub.hex] was determined in B16/F10 melanoma-bearing C57 mice. The melanoma imaging property of [sup.67]Cu-NOTA-PEG[sub.2]Nle-CycMSH[sub.hex] was further examined in B16/F10 melanoma-bearing C57 mice. [sup.67]Cu-NOTA-PEG[sub.2]Nle-CycMSH[sub.hex] exhibited higher tumor uptake than [sup.67]Cu-NOTA-GGNle-CycMSH[sub.hex] at 2, 4, and 24 h post-injection. The tumor uptake of [sup.67]Cu-NOTA-PEG[sub.2]Nle-CycMSH[sub.hex] was 27.97 ± 1.98, 24.10 ± 1.83, and 9.13 ± 1.66% ID/g at 2, 4, and 24 h post-injection, respectively. Normal organ uptake of [sup.67]Cu-NOTA-PEG[sub.2]Nle-CycMSH[sub.hex] was lower than 2.6% ID/g at 4 h post-injection, except for kidney uptake. The renal uptake of [sup.67]Cu-NOTA-PEG[sub.2]Nle-CycMSH[sub.hex] was 6.43 ± 1.31, 2.60 ± 0.79, and 0.90 ± 0.18% ID/g at 2, 4, and 24 h post-injection, respectively. [sup.67]Cu-NOTA-PEG[sub.2]Nle-CycMSH[sub.hex] showed high tumor to normal organ uptake ratios after 2 h post-injection. The B16/F10 melanoma lesions could be clearly visualized by single photon emission computed tomography (SPECT) using [sup.67]Cu-NOTA-PEG[sub.2]Nle-CycMSH[sub.hex] as an imaging probe at 4 h post-injection. The f