Role of phase II drug metabolizing enzymes in cancer chemoprevention.

Chemical insults, such as environmental or occupational carcinogenic agents, play a major role in the pathogenesis of many cancers. Many carcinogens exert genotoxic and cytotoxic effects via bioactivation into electrophilic species, a process catalyzed primarily by phase I drug metabolizing enzymes, typically cytochrome P450s. These reactive intermediates can induce DNA and RNA damage, and formation of protein adducts. The reactive species are often detoxified by phase II drug metabolizing enzymes, such as glutathione S-transferases (GSTs), UDP-glucuronosyl transferases (UGTs), sulfotransferase (ST) and N-acetyltransferase (NAT). Phase II enzymes classically conjugate these hydrophobic intermediates to a water-soluble group, thus masking their reactive nature, and allowing subsequent excretion. Therefore, strategies that modulate the levels of phase II enzymes by either pharmacological or nutritional means can lead to enhanced elimination of reactive species. Agents that preferentially activate phase II over phase I enzymes can be beneficial as chemopreventives. Compounds, such as isothiocyanates and dithiolthiones have been shown to act as transcriptional activators of phase II enzymes. A consensus enhancer element, known as antioxidant response element (ARE), in the regulatory domains of many phase II genes and an ARE-binding transcription factor nuclear factor E2-related factor 2 (Nrf2) have been implicated in the action of many chemopreventive agents. In this review, we will discuss the mechanisms of regulation of phase II enzymes, including the signal transduction events elicited by chemopreventive agents. We will also summarize the data available for these agents in preclinical models of tumorigenesis. Some chemopreventive agents have progressed to various stages of clinical trials, e.g. biomarker studies in healthy volunteers or in susceptible populations. These clinical data will be reviewed. Finally, we will provide a commentary on implementation of discovery and development programs for novel chemopreventive agents that are based on rational drug design, with lead optimization towards a safe and efficacious regimen in man.