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Liver Microsomal Preparation (liver + microsomal_preparation)
Kinds of Liver Microsomal Preparation Selected AbstractsGlucuronidation of olanzapine by cDNA-expressed human UDP-glucuronosyltransferases and human liver microsomesHUMAN PSYCHOPHARMACOLOGY: CLINICAL AND EXPERIMENTAL, Issue 5 2002Kristian Linnet Abstract Olanzapine is a widely used, newer antipsychotic agent, which is metabolized by various pathways: hydroxylation and N -demethylation by cytochrome P450, N -oxidation by flavin monooxygenase and direct glucuronidation. In vivo studies have pointed towards the latter pathway as being of major importance. Accordingly, the glucuronidation reaction was studied in vitro using cDNA-expressed human UDP-glucuronosyltransferase (UGT) enzymes and a pooled human liver microsomal preparation (HLM). Glucuronidated olanzapine was determined by HPLC after acid or enzymatic hydrolysis. The following UGT-isoenzymes were screened for their ability to glucuronidate olanzapine: 1A1, 1A3, 1A4, 1A6, 1A9, 2B7 and 2B15. Only UGT1A4 was able to glucuronidate olanzapine obeying saturation kinetics. The Km value was 227,,mol/l (SE 43), i.e. of the same order of magnitude as for other psychotropic drugs, and the Vmax value was 2370,pmol/(min,mg) (SE 170). Glucuronidation was also mediated by the HLM preparation, but a saturation level was not reached. The olanzapine glucuronidation reaction was inhibited by several drugs known as substrates for UGT1A4, e.g. amitriptyline, trifluoperazine and lamotrigine. Thus, competition for glucuronidation by UGT1A4 represents a possibility for drug,drug interactions in subjects receiving several of these psychotropic drugs at the same time. Whether such possible interactions are of any clinical importance may await further studies in patients. Copyright © 2002 John Wiley & Sons, Ltd. [source] Human cytochromes mediating gepirone biotransformation at low substrate concentrationsBIOPHARMACEUTICS AND DRUG DISPOSITION, Issue 2 2003David J. Greenblatt Abstract Biotransformation of gepirone to 1-(2-pyrimidinyl)-piperazine (1-PP) and 3'-OH-gepirone, as well as two other hydroxylated metabolites, was studied in vitro using a human liver microsomal preparation and heterologously expressed human CYP3A4 and CYP2D6. The focus was on a low range of gepirone concentrations (1000 nM and below). Liver microsomes formed 1-PP and 3'-OH-gepirone with similar reaction velocities. Two other hydroxylated metabolites (2-OH- and 5-OH-gepirone) were also formed, but pure reference standards were not available for purposes of quantitative analysis. The CYP3A inhibitor ketoconazole completely eliminated 1-PP formation, reduced 3'-OH-gepirone formation to less than 20% of control, and reduced 2-OH-gepirone formation to 7% of control. All metabolites were formed by expressed CYP3A4; however, CYP2D6 formed 3'-OH- and 5-OH-gepirone, but not 1-PP or 2-OH-gepirone. Based on estimated relative abundances of the two isoforms in human liver, CYP3A4 was predicted to account for more than 95% of net clearance of gepirone in vivo at low concentrations approaching the therapeutic range. CYP2D6 would account for less than 5% of net clearance. The findings are consistent with previous in vitro studies of gepirone using higher substrate concentrations. Copyright © 2003 John Wiley & Sons, Ltd. [source] Contributions of human cytochrome P450 enzymes to glyburide metabolismBIOPHARMACEUTICS AND DRUG DISPOSITION, Issue 4 2010Lin Zhou Abstract Glyburide (GLB) is a widely used oral sulfonylurea for the treatment of gestational diabetes. The therapeutic use of GLB is often complicated by a substantial inter-individual variability in the pharmacokinetics and pharmacodynamics of the drug in human populations, which might be caused by inter-individual variations in factors such as GLB metabolism. Therefore, there has been a continued interest in identifying human cytochrome P450 (CYP) isoforms that play a major role in the metabolism of GLB. However, contrasting data are available in the present literature in this regard. The present study systematically investigated the contributions of various human CYP isoforms (CYP3A4, CYP3A5, CYP2C8, CYP2C9 and CYP2C19) to in vitro metabolism of GLB. GLB depletion and metabolite formation in human liver microsomes were most significantly inhibited by the CYP3A inhibitor ketoconazole compared with the inhibitors of other CYP isoforms. Furthermore, multiple correlation analysis between GLB depletion and individual CYP activities was performed, demonstrating a significant correlation between GLB depletion and the CYP3A probe activity in 16 individual human liver microsomal preparations, but not between GLB depletion and the CYP2C19, CYP2C8 or CYP2C9 probe activity. By using recombinant supersomes overexpressing individual human CYP isoforms, it was found that GLB could be depleted by all the enzymes tested; however, the intrinsic clearance (Vmax/Km) of CYP3A4 for GLB depletion was 4,17 times greater than that of other CYP isoforms. These results confirm that human CYP3A4 is the major enzyme involved in the in vitro metabolism of GLB. Copyright © 2010 John Wiley & Sons, Ltd. [source] Equally potent inhibitors of cholesterol synthesis in human hepatocytes have distinguishable effects on different cytochrome P450 enzymesBIOPHARMACEUTICS AND DRUG DISPOSITION, Issue 9 2000L.H. Cohen Abstract Six 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (the present cholesterol-lowering drugs known as statins), lovastatin (L), simvastatin (S), pravastatin (P), fluvastatin (F), atorvastatin (A) and cerivastatin (C) are shown to be potent inhibitors of cholesterol synthesis in human hepatocytes, the target tissue for these drugs in man. All six inhibited in the nM range (IC50 values: 0.2,8.0 nM). As daily used cholesterol-lowering drugs they are likely coadministered with other drugs. While several cytochrome P450 (CYP) enzymes are involved in drug metabolism in the liver and thus play an important role in drug,drug interaction it was investigated which of these enzymes are influenced by the active forms of the six statins. These enzyme activities were studied in human liver microsomal preparations, and in simian and human hepatocytes in primary culture. The following CYP reactions were used: nifedipine aromatization (CYP3A4), testosterone 6,-hydroxylation (CYP3A4), tolbutamide methylhydroxylation (CYP2C9), S -mephenytoin 4-hydroxylation (CYP2C19), bufuralol 1,-hydroxylation (CYP2D6), aniline 4-hydroxylation (CYP2E1), coumarin 7-hydroxylation (CYP2A6) and 7-ethoxyresorufin O -dealkylation (CYP1A1/2). In the human liver microsomes the statins (concentrations up to 400 µM) did not influence the CYP1A1/2 activity and hardly the CYP2A6 and CYP2E1 activities. Except P, the other five statins were stronger inhibitors of the CYP2C19 activity with IC50 values around 200 µM and the same holds for the effect of A, C and F on the CYP2D6 activity. L and S were weaker inhibitors of the latter enzyme activity, whereas P did not influence both activities. About the same was observed for the statin effect on CYP2C9 activity, except that F was a strong inhibitor of this activity (IC50 value: 4 µM). Using the assay of testosterone 6,-hydroxylation the CYP3A4 activity was decreased by L, S and F with IC50 values of about 200 µM and a little more by C and A (IC50 around 100 µM). P had hardly an effect on this activity. To a somewhat less extent the same trend was seen when CYP3A4 activity was measured using nifedipine as substrate. The inhibitory effects observed in microsomes were verified in suspension culture of freshly isolated hepatocytes from Cynomolgus monkey (as a readily available model) and of human hepatocytes. In general the same trends were seen as in the human microsomes, except that in some cases the inhibition of the CYP activity was less, possibly by the induction of the particular CYP enzyme by incubation of the cells with a particular statin. F remained a strong inhibitor of CYP2C9 activity in human and monkey hepatocytes. A induced the CYP2C9 in monkey hepatocytes but was an inhibitor of the CYP2C9 in human hepatocytes. A, S, L and C were moderate inhibitors in both cellular systems of CYP3A4. P was not affecting any of the CYP activities in the three systems studied. It is concluded that different CYP enzymes interact with different statins and therefore differences in between these drugs are to be expected when drug,drug interaction is considered. Copyright © 2000 John Wiley & Sons, Ltd. 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