Abstract 3791: Milk thistle as an inhibitor of PXR-mediated CYP3A4 induction

Abstract

Abstract Introduction Many anticancer drugs, such as paclitaxel and erlotinib, are able to induce the expression of cytochrome P450 (CYP450) 3A4.1 Induction of intestinal and hepatic CYP3A4 can result in a decreased uptake and an increased metabolism of anticancer drugs. Consequently, systemic levels decrease, possibly resulting in a lower efficacy. The major mechanism behind this CYP3A4 induction is the activation of the pregnane X receptor (PXR)1 and therefore inhibition of PXR might prevent CYP3A4 induction. As 54-77% of the cancer patients use complementary and alternative medicine (CAM) including herbs and dietary supplements, these CAM are interesting to screen for their ability to inhibit PXR-mediated CYP3A4. For the current study, the following CAM were selected: α-carotene, Echinacea, garlic, Ginkgo biloba, P. ginseng, grape seed, green tea, milk thistle, saw palmetto, St. John's Wort, valerian, vitamin B6+12 and vitamin C.2,3 Aim The aim was to screen CAM for their ability to inhibit PXR-mediated CYP3A4 induction in LS180 cells, a human colon adenocarcinoma-derived cell line. Method The selected CAM were screened for possible inhibition of PXR-mediated CYP3A4 induction, using a CYP3A4 reporter gene assay and a real-time polymerase chain reaction (RT-PCR) assay. In both assays, LS180 cells were exposed to fresh medium containing 0.1% DMSO or model inducers of CYP3A4 (10 μM rifampicin, 20 μM erlotinib or 20 μM paclitaxel) with or without addition of known PXR inhibitors (10 μM ketoconazole and 10 μM A792611) or 50-100 μg/ml standardized CAM (table 1) for 24 hours. Data were analyzed by using the Student's unpaired t-test and were considered statistically significant when p< 0.05. Results Milk thistle was the most potent inhibitor of PXR-mediated CYP3A4 induction compared to the other CAM. The reporter gene assay demonstrated that 100 and 75 μg/ml milk thistle significantly inhibited PXR-mediated CYP3A4 induction by rifampicin by respectively 88% and 75%; by paclitaxel by respectively 90% and 81%; and by erlotinib by 82% and 49%. According to the RT-PCR assay 100 and 75 μg/ml milk thistle significantly inhibited PXR-mediated CYP3A4 induction by rifampicin by respectively 91% and 90%. Conclusion Milk thistle significantly inhibited PXR-mediated CYP3A4 induction at the transcriptional level, demonstrated by a reporter gene and RT-PCR assay. Thus, milk thistle is a suitable candidate for the inhibition of PXR-mediated CYP3A4 induction. Consequently, the concomitant use of milk thistle with anticancer drugs can possibly prevent the decrease in systemic levels of anticancer drugs resulting in a higher efficacy, but also a higher risk for toxicity.