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Figure 4. Elevated cellular drug metabolism in relation to pregnane X receptor or steroid and xenobiotic receptor. Induction of glucuronidation of SN-38 within colorectal cancer cells renders cells resistant to irinotecan-based therapy. PXR and SXR are well known as they are expressed mainly in mammalian livers and gastrointestinal tracts and are involved in the induction of various classes of drug-metabolizing enzymes and drug transporters to detoxify therapeutic drugs such as irinotecan and SN-38. PXR: Pregnane X receptor; SXR: steroid and xenobiotic receptor; RXR: retinoid X receptor.