Synthesis and Characterization of a Melanoma-Targeted Fluorescence Imaging Probe by Conjugation of a Melanocortin 1 Receptor (MC1R) Specific Ligand

The incidence of malignant melanoma is rising more rapidly than that of any other cancer in the United States. The melanocortin 1 receptor (MC1R) is overexpressed in most human melanoma metastases, thus making it a promising target for imaging and therapy of melanomas. We have previously reported the development of a peptidomimetic ligand with high specificity and affinity for MC1R. Here, we have conjugated near-infrared fluorescent dyes to the C-terminus of this ligand via lysine-mercaptopropionic acid linkers to generate MC1R specific optical probes (MC1RL-800, 0.4 nM <i>K</i>_i; and MC1RL-Cy5, 0.3 nM <i>K</i>_i). Internalization of the imaging probe was studied in vitro by fluorescence microscopy using engineered A375/MC1R cells and B16F10 cells with endogenous MC1R expression. The in vivo tumor targeting of MC1RL-800 was evaluated by intravenous injection of probe into nude mice bearing bilateral subcutaneous A375 xenograft tumors with low MC1R expression and engineered A375/MC1R tumors with high receptor expression. Melanotic B16F10 xenografts were also studied. Fluorescence imaging showed that the agent has higher uptake values in tumors with high expression compared to low (<i>p</i> < 0.05), demonstrating the effect of expression levels on image contrast-to-noise. In addition, tumor uptake was significantly blocked by coinjection of excess NDP-α-MSH peptide (<i>p</i> < 0.05). In conclusion, the MC1R-specific imaging probe developed in this study displays excellent potential for the intraoperative detection of regional node involvement and for margin detection during melanoma metastasis resection