Modulating transcriptional control of drug metabolism: A novel paradigm in drug therapeutics

893 Individual variation in drug metabolism is a major cause of treatment failures during cancer therapy. Because variations can lead to unpredictable and erratic drug side effects, there is a considerable need to modulate the pharmacodynamics of therapeutics to maximize efficacy and minimize side effects. Individual variations stem from differences in activity and abundance of drug metabolizing enzymes, including the cytochrome p450 family, whose expression is regulated at the level of transcription by orphan nuclear receptors. We have found that ketoconazole, a known inhibitor of p450 enzyme activity, represses the transcription of genes central to drug metabolism and transport by blocking the activation of nuclear receptors. Ketoconazole prevented the activation of endogenous as well as transiently transfected genes in cell lines and primary human and mouse hepatocytes. This inhibition was mediated by blocking the activation of nuclear receptors hPXR and CAR but not other nuclear receptors such as ERα or PPARγ. Ketoconazole disrupted the interaction between hPXR and co-activators in both mammalian two-hybrid assays and in co-activator dependent receptor ligand binding assays (CARLA). Furthermore, ketoconazole specifically inhibited PXR mediated gene transcription of cyp3a11, mdr1 and mrp2 in mice. Using LORR (Loss of Righting Reflex), we found that treatment of mice with ketoconazole inhibited the metabolism, and hence the functional consequences, of a CYP450 substrate-tribromoethanolamine. Finally, in wild type mice (n=6), we demonstrate that the pharmacokinetics of paclitaxel are normalized. Short term treatment with ketoconazole markedly reduces variation in treated population, and this effect coincides with decreased transcription of cyp3a11 and mrp2 in liver and mdr-1 in small bowel. These studies demonstrate for the first time that ketoconazole can inhibit the transcriptional activation of drug metabolism genes in vivo. Thus, ketoconazole has the potential to be used clinically to reduce individual variability in therapeutic responses.