[Development of targeted chemoradiotherapy for malignant melanoma by exploitation of metabolic pathway]

Malignant melanoma cell possesses a unique metabolic pathway which consists of conversion of tyrosine, an essential amino acid, to dopa and subsequently to dopaquinone in the presence of tyrosinase to form melanin, the end product of the metabolic pathway. This tyrosinase-mediated melanin biosynthesis occurs within normal melanocyte and its transformed cell, malignant melanoma cell. It is in general highly elevated in malignant melanoma cells compared to normal melanocytes. The objective of our research is to develop targeted therapeutic approach for malignant melanoma by utilizing this unique metabolic pathway in the presence of tyrosinase. Specifically we have synthesized sulpher homologue of tyrosine, cysteinylphenol and its amine derivative, cysteaminylphenol (CAP) and subsequently its N-acetyl and N-propionyl derivatives (N-acetyl and N-propionyl-CAP). These synthetic compounds are good tyrosinase substrates and possess high lipophilicity and penetration through the plasma membrane into melanoma cells. Our in vivo and in vitro studies using these synthetic compounds revealed following findings: (1) CAP and its derivatives possess selective cytotoxicity to human neoplastic cells, in particular tyrosinase-positive melanoma cells; (2) N-Acetyl-CAP and N-propionyl-CAP possess both cytostatic and cytocidal effect; (3) These synthetic compounds provide irreversible DNA damage to melanoma cells with high tyrosinase activity; (4) However, there is no irreversible DNA damage to non-pigmented, tyrosinase negative cells; (5) Pharmacological effect of CAP appears to be related to oxidative stress; (6) Radio-labelled CAP derivatives showed selective incorporation into melanin-forming melanoma cells; (7) This selective cytotoxicity can occur in non-melanin forming cells after transfection of human tyrosinase cDNA, resulting cytocidal effect. All these findings clearly indicate that our synthetic compounds which are good substrates of human tyrosinase can provide basis for the development of targeted chemotherapy and/or chemoradiotherapy. In addition the transfection of human tyrosinase cDNA will provide the rational approach for developing the targeted gene therapy to non-melanoma cells.