Drug induced toxicity is caused either by parent compounds

Drug-induced toxicity is caused either by parent compounds or by their reactive metabolites that are generated through biotransformation primarily in the liver (Srivastava et al., 2010). The cytochrome P450 (CYP) superfamily members contribute to approximately 80% of this process (Evans and Relling, 1999). Previous work has indicated that CYP TRAM-34 mg mediated TP metabolism could have an impact on its toxicity (Li et al., 2008). In vitro studies demonstrated that two isoenzymes, CYP3A4 and CYP2C19, were involved in the conversion of TP into its mono-hydroxylated metabolites (Li et al., 2008). Further investigation showed that high active hepatic CYP3A levels induced by dexamethasone treatment significantly increased the level of the mono-hydroxylated metabolite of TP and decreased its hepatotoxicity in rats (Ye et al., 2010). In vivo studies demonstrated that inactivation of hepatic CYP enzymes abolished the metabolism of TP in the liver, which subsequently resulted in an increase in bioavailability and toxicity of TP in rats (Xue et al., 2011). It is well accepted that among the three major mechanisms for CYP involvement in drug–drug interactions, induction, inhibition and possibly stimulation, inhibition appears to be the most important in terms of known clinical problems (Guengerich, 1997). Therefore, investigations on the effects of CYP inhibition or inactivation in TP-induced toxicity, which to date remain unclear, could provide important information for the safe use of TP in the clinic. Recently, a sandwich-cultured hepatocyte model has been proved to be a valuable in vitro system that maintains specific hepatic cytomorphology and function relevant to drug metabolism, disposition and toxicity, and thus, closely resembles the in vivo setting. This hepatocyte model was recognized as a reliable toxicological method to determine mechanisms of drug-induced toxicity, identify biomarkers and predict chemical toxicity. Therefore, based on the previous in vivo findings, we hypothesized that the relationship between TP and CYP3A underlying the mechanism of TP-induced liver injury may be very complicated. Therefore, the aim of this study was initially to evaluate the inhibitory effects of TP on rat CYP3A in vitro. The study was further focused on the effect of CYP3A modulators on metabolism and toxicity of TP. The time-dependent CYP3A inhibition by TP was investigated using an IC50 shift assay. The enzyme kinetics was characterized in rat liver microsomes (RLM). A study based on sandwich-cultured rat hepatocytes (SCRH) was carried out to confirm the results from liver microsomes. An inducer and inhibitor of CYP3A were then used to reveal the role of CYP3A enzyme in the TP toxicity and to clarify the risk of drug–drug interaction.
Materials and methods
Results
Discussion An important feature of CYP enzymes is that they may be easily induced or inhibited. Many drugs commonly used in the clinic have the potential for CYP enzyme induction or inhibition. Therefore, when two or more drugs are administered simultaneously, this may lead to the metabolism based drug interactions. Liver microsomes and cultured primary hepatocytes are commonly used in vitro to investigate potential drug–drug interactions (Gonzalez, 1989, Guengerich, 1992). It is particularly important for the drug with a narrow therapeutic index, such as TP, because it is often co-administered with other drugs. The toxicity caused by TP has created extensive attention for many years. Recently it was discovered that CYP3A is involved in the metabolism of TP, and therefore CYP3A may be associated with TP-induced liver injury (Ye et al., 2010, Xue et al., 2011). However, the mechanism underlying the CYP3A enzyme mediated drug interaction of TP has not been reported, especially the correlation of CYP3A activity with the drug toxicity in the presence of CYP3A inducer and inhibitor has not been investigated at the cellular level.