Caffeic acid phenethyl ester prodrug as an anti-melanoma agent

Tyrosinase and glutathione S-transferase (GST) are over-expressed in melanoma cancer. Tyrosinase is involved in melanogenesis, whereas GST plays a major role in detoxification of reactive anti-cancer agents. In this study, we investigated Caffeic Acid Phenethyl Ester (CAPE) as a substrate for tyrosinase to form reactive toxic metabolites in melanoma cells, and as a selective melanoma GST inhibitor due to CAPE's bioactivation by tyrosinase. Using this approach, CAPE prodrug was investigated as an anti-melanoma agent. CAPE (ICso, 15 μM) demonstrated selective toxicity towards melanocytic melanoma cells, which express functional tyrosinase, as compared with amelanotic melanoma and non-melanoma cells, which do not express functional tyrosinase. Tyrosinase dependent formation of CAPE-quinone in melanoma cells depleted intracellular GSH, increased ROS, increased apoptosis, and induced mitochondrial toxicity in melanoma cells. The effect of CAPE was prevented in human melanoma SKMEL- 28 cells treated with shRNA plasmid to down regulate tyrosinase. LC/MS/MS revealed that CAPE-glutathione conjugate is a major metabolite formed from CAPE bioactivation by tyrosinase. CAPE-glutathione conjugate and CAPE-quinone were shown to be potent inhibitors of GST compared to CAPE alone. CAPE was further investigated for their in vivo efficacy and toxicity in a skin melanoma tumor model in mice. CAPE (10 mg/Kg, ip) showed 40% tumor growth inhibition in melanoma animal model with no liver and kidney toxicity. Our data suggests that tyrosinase and GST are selective targets for CAPE in melanoma cells.