ABSTRACT
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.
