Melanoma Imaging Using 18 F-Labeled α-Melanocyte-Stimulating Hormone Derivatives with Positron Emission Tomography.

Melanocortin 1 receptor (MC1R) is specifically expressed in the majority of melanomas, a leading cause of death related to skin cancers. Accurate staging and early detection is crucial in managing melanoma. Based on the α-melanocyte-stimulating hormone (αMSH) sequence, MC1R-targeted peptides have been studied for melanoma imaging, predominately for use with single-photon emission computed tomography, with few attempts made for positron emission tomography (PET). 18 F is a commonly used PET isotope due to readily available cyclotron production, pure positron emission, and a favorable half-life (109.8 min). In this study, we aim to design and evaluate αMSH derivatives that enable radiolabeling with 18 F for PET imaging of melanoma. We synthesized three imaging probes based on the structure of Nle4 -cyclo[Asp5 -His-d-Phe7 -Arg-Trp-Lys10 ]-NH2 (Nle-CycMSHhex ), with a Pip linker (CCZ01064), an Acp linker (CCZ01070), or an Aoc linker (CCZ01071). 18 F labeling was enabled by an ammoniomethyl-trifluoroborate (AmBF3 ) moiety. In vitro competition binding assays showed subnanomolar inhibition constant ( Ki ) values for all three peptides. The 18 F radiolabeling was performed via a one-step 18 F-19 F isotope exchange reaction that resulted in high radiochemical purity (>95%) and good molar activity (specific activity) ranging from 40.7 to 66.6 MBq/nmol. All three 18 F-labeled peptides produced excellent tumor visualization with PET imaging in C57BL/6J mice bearing B16-F10 tumors. The tumor uptake was 7.80 ± 1.77, 5.27 ± 2.38, and 5.46 ± 2.64% injected dose per gram of tissue (%ID/g) for [18 F]CCZ01064, [18 F]CCZ01070, and [18 F]CCZ01071 at 1 h post-injection (p.i.), respectively. Minimal background activity was observed except for kidneys at 4.99 ± 0.20, 4.42 ± 0.54, and 13.55 ± 2.84%ID/g, respectively. The best candidate [18 F]CCZ01064 was further evaluated at 2 h p.i., which showed increased tumor uptake at 11.96 ± 2.31%ID/g and further reduced normal tissue uptake. Moreover, a blocking study was performed for CCZ01064 at 1 h p.i., where tumor uptake was significantly reduced to 1.97 ± 0.60%ID/g, suggesting the tumor uptake was receptor mediated. In conclusion, [18 F]CCZ01064 showed high tumor uptake, low normal tissue uptake, and fast clearance and is therefore a suitable and promising candidate for PET imaging of melanoma.