Radiofluorinated Rhenium Cyclized α-MSH Analogues for PET Imaging of Melanocortin Receptor 1

In order to accomplish in vivo molecular imaging of melanoma biomarker melanocortin 1 receptor (MC1R), several α-melanocyte-stimulating hormone (α-MSH) analogues have been labeled with N-succinimidyl-4-¹⁸F-fluorobenzoate (¹⁸)F-SFB) and studied as positron emission tomography (PET) probes in our recent studies. To further pursue a radiofluorinated α-MSH peptide with high clinical translation potential, we utilized 4-nitrophenyl 2-¹⁸F-fluoropropionate (¹⁸F-NFP) to radiofluorinate the transition metal rhenium cyclized α-MSH metallopeptides for PET imaging of MC1R positive malignant melanoma. Metallopeptides Ac-d,Lys-ReCCMSH(Arg¹¹) (two isomers, namely RMSH-1 and RMSH-2) were synthesized using conventional solid phase peptide synthesis chemistry and rhenium cyclization reaction. The two isomers were then conjugated with ¹⁹F-NFP or ¹⁸F-NFP. The resulting cold or radiofluorinated metallopeptides, (¹⁸/¹⁹)F-FP-RMSH-1 and (¹⁸/¹⁹)F-FP-RMSH-2, were further evaluated for their in vitro receptor binding affinities, in vivo biodistribution, and small-animal PET imaging properties. The binding affinities of ¹⁹F-FP-RMSH-1 and ¹⁹F-FP-RMSH-2 were determined to be within low nanomolar range. In vivo studies revealed that both F-labeled metallopeptides possessed good tumor uptake in the B16F10 murine model with high MC1R expression, while possessing much lower uptake in A375M human melanoma xenografts. Moreover, ¹⁸F-FP-RMSH-1 displayed more favorable in vivo performance in terms of higher tumor uptake and much lower accumulation in the kidney and liver, when compared to that of ¹⁸F-FP-RMSH-2 at 2 h postinjection (p.i.). ¹⁸F-FP-RMSH-1 also displayed lower liver and lung uptake when compared with that of the same peptide labeled with ¹⁸F-SFB (named as ¹⁸F-FB-RMSH-1). Small animal PET imaging of ¹⁸F-FP-RMSH-1 in mice bearing B16F10 tumors at 1 and 2 h showed good tumor imaging quality. As expected, much lower tumor uptake and poorer tumor/normal organ contrast were observed for A375M model compared to those of the B16F10 model. ¹⁸F-FP-RMSH-1 also exhibited higher tumor uptake and better tumor retention when compared with ¹⁸F-FB-RMSH-1. ¹⁸F-FP-RMSH-1 demonstrates significant advantages over ¹⁸F-FB-RMSH-1 and ¹⁸F-FP-RMSH-2. It is a promising PET probe for imaging MC1R positive melanoma and MC1R expression in vivo.