Troglitazone Thiol Adduct Formation in Human Liver Microsomes: Enzyme Knietics and Reaction Phenotyping
Wen C. Shyu,
A. David Rodrigues,
I do hope the DML Reader will find all twelve papers included in this issue, to be quite interesting: Urs A. Boelsterli et al.. studied the bioactivation of leflunomide and the metabolic degradation to its major metabolite, A771726. Leflunomide was rapidly metabolized in human hepatocytes to A771726, but its toxicity was dependent on other, CYP-dependent intermediates. James F. Rusling and colleagues used rat liver microsomes attached to nanoparticles for LC-MS studies of CYP3A and 2E1 enzymes in metabolism of N-nitroso compounds. Using these biocolloids, turnover rates were measured within 2 min. Inhibitor IC50 values for ketoconazole (KET) and 4-methylpyrazole (4-MEP) were also estimated. Lee Jia et al. investigated the effects of CsA and ITZ on 1) intestinal permeability of amlodipine (a calcium channel blocker used as a cardiovascular agent) in isolated rat everted gut sac model, and 2) biliary excretion and pharmacokinetics of amlodipine in rats. Pretreatment of rats with ITZ increased plasma levels and biliary excretion of amlodipine in a dose-dependent manner. In contrast, pretreatment with CsA slightly decreased biliary excretion of amlodipine and made no changes in its plasma levels. Michael W. Sinz et al. demonstrated that the humanized SXR mouse can be used as a model to predict human CYP3A4 induction and the resulting pharmacokinetic changes of CYP3A4 substrates in humans. Jasminder Sahi et al. compared AO activity in cytosol and cryopreserved hepatocytes from human, monkey, rat and mouse livers to assess species differences. They also evaluated possible species differences in drug interactions using seven drugs known to inhibit human cytosolic AO i.e. raloxifene, perphenazine, menadione, maprotiline, ketoconazole, erythromycin, and estradiol. Their data showed major differences in the rate of AO metabolism, and inhibition of AO across species, indicating that results from animal studies cannot be safely extrapolated to humans. David Rodrigues et al. worked on troglitazone (TGZ) induced hepatotoxicity, which has been linked to cytochrome P450 (CYP)- catalyzed reactive metabolite formation. They concluded that both CYP3A4/5 and CYP2C8 play a major role in the formation of TGZ adduct in HLM. However, the contribution of these CYPs varies depending on their relative expression and the concentration of TGZ.
Keywords: Troglitazone, reactive metabolite, reaction phenotyping, CYP3A4, CYP2C8
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