Abstract 4557: Development of a melanocortin receptor 1 targeted probe for molecular imaging of melanoma
Authors: Tafreshi Narges K. , Barkey Natalie M. , McCardle Timothy W. , Enkemann Steven A. , Estrella Veronica C. , Chen Tingan , Vagner Josef , Gillies Robert J. , Morse David L. 
Published: August 2012

Abstract
Melanoma is curable by means of surgical excision if diagnosed in early stages, but once the metastatic stage is reached, prognosis is poor because the tumor is resistant to most cures. Therefore, a melanoma targeted probe that can deliver therapy to metastases at high dosages could increase the chances of treatment. Melanocortin 1 receptor (MC1R) is overexpressed in most human melanoma metastases, thus making it a promising target for imaging and therapy of melanomas. To confirm that MC1R is a specific melanoma marker, mRNA and protein expression was confirmed and quantified in tumor and normal unaffected patient tissue samples. MC1R mRNA expression was highly and generally expressed among melanoma samples surveyed. In contrast, MC1R expression was not elevated in other skin cancers, normal skin and organs involved in toxicity and clearance, i.e. heart, spleen, liver and kidney. To determine MC1R protein expression in patient samples, immunohistochemistry was performed on a melanoma tissue microarray containing 267 samples. None of the normal skin samples (n = 19) had staining with a pathology score of α4. Benign lesions (n = 65), samples of local invasion to regional lymph nodes (n = 35) and metastatic melanoma (n=40) had moderate to high staining in 15, 33 and 47% of the samples, respectively. We have previously reported the development of a peptidomimetic ligand with high specificity and affinity for MC1R. In this study, we have conjugated this ligand to an infrared dye to generate a MC1R specific optical probe (named ML-800). Our whole-cell binding assay using A375/MC1R human melanoma engineered cells was used to determine the high binding affinity of ML-800 (0.4± 0.1 nM Ki). The cellular uptake of the probe was studied in A375/MC1R cells by fluorescence microscopy both in vitro and in vivo using a dorsal skin-fold window-chamber mouse model. The in vivo tumor targeting of ML-800 was evaluated by intravenous injection of probe into nude mice bearing bilateral subcutaneous tumors of A375 with low number of MC1R receptors and A375/MC1R with high expression of MC1Rs. Fluorescence imaging showed that the agent has higher uptake values in A375/MC1R tumors than those in A375M tumors (P, 0.05), demonstrating differentiation of probe retention in tumors with different levels of expression. In conclusion, the imaging probe designed in this study demonstrates the potential for the development of agents that can deliver imaging contrast and therapy to melanoma metastases that express MC1R. Since radiopeptides have proven their usefulness for diagnostic imaging and radiotherapy, in the future, this ligand could be developed as a targeted delivery vehicle for non-invasive nuclear imaging to detect regional lymph node involvement, or delivery of radiotherapy. Furthermore, by attachment to the MC1R ligand with a cleavable linker, cytotoxins may also be targeted to tumor cells for receptor mediated endocytosis and intracellular release.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4557. doi:1538-7445.AM2012-4557