⁶⁴Cu-Labeled Bioorthogonal Probe for Targeted Positron Emission Tomography Imaging of Malignant Melanoma

Malignant melanoma is the most severe form of skin cancer, and its incidence is increasing worldwide. In melanoma, melanocytes overexpress melanin, which can act as a specific biomarker for early detection. The benzamide (BZA) structure is known to bind to melanin, and it can be used as a skeletal structure for radiopharmaceuticals for visualizing malignant melanoma. In this study, we synthesized a novel BZA derivative, including a bioorthogonal chelator, and labeled it with ⁶⁴Cu for biological evaluation. Cell uptake of the ((6-((2-(dimethylamino)ethyl)carbamoyl)pyridin-3-yl)methyl)carbamodithioic acid (MI-0201B2) pretargeted ⁶⁴Cu treatment group was threefold higher (1.73 ± 0.17%) in melanin-expressed B16F10 (mouse melanoma) cells than in negative control cells. MicroPET imaging of the B16F10-bearing C57BL/6 mice (n = 4) showed tumor uptake of the MI-0201B2 pretargeted ⁶⁴Cu injection group of 4.42 ± 0.52%ID/g at 1 h, with this increasing until 48 h (7.74 ± 1.10%ID/g). By contrast, when only ⁶⁴Cu and ⁶⁴Cu-MI-0201B2 were injected, the tumor uptakes were lower than the MI-0201B2 pretargeted ⁶⁴Cu injection group (6.49 ± 0.48%ID/g), with values of 3.94 ± 0.09 and 4.53 ± 0.09%ID/g at 24 h, respectively. Ex-vivo biodistribution studies revealed that the MI-0201B2 pretargeted ⁶⁴Cu injection group showed specific tumor uptake and retention for 48 h (4.92 ± 0.74%ID/g at 48 h). The novel chelator MI-0201B2 exhibited excellent melanin targeting activity, demonstrating its potential as a PET imaging agent for melanoma diagnosis utilizing a bioorthogonal strategy.