Development of α-melanocortin analogs for melanoma prevention and targeting

Melanocortins, particularly α-Melanocortin (α-Melanocyte stimulating hormone, α-MSH), were first identified as the physiological regulators of pigmentation in many vertebrate species. Their role in regulating human pigmentation was unequivocally demonstrated in the 1990s, with the cloning of the human melanocortin 1 receptor (MC1R) gene from human melanocytes and the demonstration that functional MC1R is expressed by these cells. α-Melanocyte stimulating hormone is a tridecapeptide, with the core sequence His(6)-Phe(7)-Arg(8)-Trp(9) shared with β- and γ-MSH and identified as essential for receptor activation and stimulation of pigmentation. The small size of α-MSH makes it an attractive molecule for drug design. There has been longstanding interest in the development of melanocortin analogs that target the MC1R expressed on normal melanocytes and melanoma cells. The aim has been to develop MC1R agonists that stimulate melanogenesis and confer photoprotection to human melanocytes and thus prevent skin cancer formation. Recent findings that the physiological α-MSH not only stimulates melanogenesis, but also reduces the extent of DNA damage caused by exposure to solar ultraviolet radiation have further rejuvenated the interest in developing synthetic MC1R agonists for skin cancer prevention. α-Melanocortin analogs have also been developed for imaging of melanoma tumors, localization of residual metastasis and specific delivery of radionuclides to eradicate melanoma tumors, sparing normal tissues. The main challenge is to develop specific MC1R agonists that will target melanocytes for skin cancer prevention, or for localization and treatment of metastatic melanoma.