Targeting the Melanocortin 1 Receptor by Selective Small Analogs of α-Melanocortin for Melanoma Prevention

Cincinnati Va Medical Center Research, Cincinnati OH

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Abstract

Solar ultraviolet radiation (UV) is the main etiological factor for melanoma, the deadliest form of skin cancer, and the fifth most diagnosed cancer in veterans. The incidence of melanoma is higher in veterans than in civilians, particularly in individuals with light skin color and poor tanning ability. Over 65% of all veterans have this phenotype. Deployment of millions of soldiers in geographical areas such as Iraq and Afghanistan increases their risk for melanoma due to unavoidable daily excessive sun exposure without optimal protection. Given the limitations of advanced therapeutic options for melanoma, it is important to focus on developing new chemoprevention strategies to reduce its incidence. Our goal is to develop a melanoma prevention strategy based on targeting the melanocortin 1 receptor (MC1R), the product of a bona fide melanoma predisposition gene known to be a major regulator of human pigmentation and the response of melanocytes to solar UV. We were the first to report the seminal findings that activation of the MC1R, a membrane-bound Gs protein-coupled receptor expressed on melanocytes, by its agonist ?-melanocyte stimulating hormone (?-melanocortin; ?-MSH) reduces UV-induced DNA damage by enhancing DNA repair and antioxidant capacities, in addition to stimulating the synthesis of photoprotective eumelanin. Others have shown that expression of a loss-of- function variant of MC1R is associated with increased UV signature mutations and the common somatic V600E mutation in BRAF, that drive melanomagenesis. About 50% of all White Caucasians, including about 7 million veterans, are heterozygous carriers of a MC1R variant, a genotype associated with increased risk for melanoma. These individuals will particularly benefit from our prevention strategy that will enhance the activation of the MC1R. We have developed tetra- and tripeptide analogs of ?-MSH that mimic the effects of ?-MSH on human melanocytes, and are stable and lipophilic, and unique in their high selectivity for the MC1R. The cosmetic use of these peptides is covered by a published patent application. The tetrapeptides are 100 fold more potent, and the tripeptides are only 10 fold less potent than ?-MSH in activating MC1R signaling and its downstream effects in cultured human melanocytes. These peptides enhanced repair of UV-induced DNA photoproducts, and increased pigmentation of cultured human skin substitutes (CSS) without any UV exposure. These small peptides, unlike ?-MSH, should not cause immunosuppression, since they lack the COOH-terminal 11-13 amino acid residues (Lys-Pro-Val) of ?-MSH that are responsible for its anti- inflammatory and immunosuppressive effects. We propose that our tri- and tetrapeptide analogs of ?-MSH that are selective for the MC1R will be developed as topical agents to stimulate skin pigmentation and reduce UV-induced DNA damage, in order to maintain the genomic stability of melanocytes, and therefore reduce the risk for melanoma in veterans. Our Specific Aims are: I: To develop topical formulations of MC1R-selective tetrapeptide and tripeptidepeptide, demonstrate their permeation through the epidermis, quantify their concentrations in the skin layers, and validate their photoprotective effects on intact human skin or CSS in vitro when applied topically. II. To test the effects of the two peptides on pre-clinical animal models, including human CSS that will be grafted onto immunodeficient mice, and a miniature pig model with skin similar to that of humans. Endpoints will include measurement of DNA damage, changes in pigmentation, proliferation and apoptosis of keratinocytes and melanocytes. Blood samples will be obtained for preliminary pharmacokinetics testing and tissue samples will be examined for gross toxicological effects. The outcome of this project should advance us closer towards application for Investigational New Drug and conducting clinical trials.