In as little as six weeks, melanoma can turn life-threatening. Not only is melanoma a fast growing skin cancer, but its annual incidence rate is nearly 100,000 in the United States. Disease identified early can be cured by surgery, but metastatic melanoma is lethal. Despite the striking progress in treating metastatic melanoma using MAPK inhibitors and immune checkpoint inhibitors, improvements in objective response rate (<50%) and 5-yr survival (<30%) are critically needed. Melanocortin 1 receptor (MC1R), a G protein-coupled receptor, has been investigated as a potential target for drug delivery (e.g., peptide receptor radionuclide therapies, peptide-drug conjugates, etc.) to metastatic melanoma cells, as it is overexpressed in melanomas but largely absent in normal tissues. This study aims to further investigate the potential of MC1R as a target for drug delivery, by determining its expression in melanoma cancer cell lines, human melanoma biopsies, as well as normal tissue specimen. Bmax and MC1Rs per cell in five human melanoma cell lines (TXM13, TXM40, Sk-Mel-28, A375, and A2058) were calculated from saturation binding assays using synthetic MC1R ligand [125I]Nle-D-Phe-α-MSH (125I-NDP-α-MSH). An immunohistochemistry (IHC) staining protocol was developed using paraffin-embedded xenografts of TXM13, TXM40, Sk-Mel-28, A375, and A2058 human melanoma cells. IHC staining of MC1R was performed using human melanoma biopsy samples of stage III and IV metastatic melanoma obtained from University of Iowa Tissue Procurement Core Facility. MC1R-IHC was also performed on Caucasian North American normal tissue specimens including skin, muscle, pituitary gland, adrenal gland, cerebellum, and testis obtained from Amsbio. 125I-NDP-α-MSH binding assay results revealed positive staining in all melanoma cell lines, with the density of 2,762 receptors/cell in TXM13, 461 receptors/cell in Sk-Mel-28, 343 receptors/cell in TXM40, 164 receptors/cell in A375, and 93 receptors/cell in A2058. The IHC staining also demonstrated positive immunoreactivity against MC1R in stage III and IV human metastatic melanoma biopsy samples. Heterogenous expression of MC1R was found within the biopsy samples and among tumor biopsy samples, and the staining intensity fell within the range of the melanoma cancer cell line xenografts. Conversely, negative IHC staining of MC1R was found in normal tissue specimen, indicating that the expression of MC1R in normal tissues is largely absent. This study establishes an MC1R-IHC staining protocol optimized for clinical applications, and our data demonstrates high expression of MC1R in human melanoma biopsy samples as well as in melanoma cancer cell lines. The expression of MC1R in normal tissue specimens is largely absent. These results indicate that MC1R-targeted therapies, such as peptide receptor radionuclide therapies, are a promising alternative to current therapies for metastatic melanoma.

Citation Format: Brenna Marks, Mengshi Li, Edwin Sagastume, Michael Schultz, Frances Johnson. Assessing Melanocortin 1 receptor as a target for metastatic melanoma drug delivery [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1171.