Interspecies differences in coumarin metabolism in liver microsomes examined by capillary electrophoresis

1. A simple, rapid method was developed for studying xenobiotic metabolism by cytochrome P450 in liver microsome preparations. Capillary electrophoresis was used to separate the metabolite from the metabolic mixture.

2. Coumarin is metabolized to 7-hydroxycoumarin by a cytochrome P450 isoenzyme. Human, bovine, gerbil, mouse (Schofield, CD1), rat, rabbit, porcine, and cynomologus monkey microsomal preparations were investigated for coumarin metabolism by determining the content of 7-hydroxycoumarin present after metabolism.

3. Separation of 7-hydroxycoumarin from the reaction mixture was carried out in 50 mM phosphate buffer, pH 6.8, on a fused silica capillary at 25°C and 15 kV. The metabolic matrix consisted of an NADPH regeneration system, 205.5 μM coumarin, and the microsomal preparation. Standard curves were prepared in the microsomal preparation and the limit of quantification was 6.17 μM, with a linear range from 0 to 308.5 μM.

4. The reaction was initiated by the addition of the microsomes. An aliquot of the reaction mixture was removed at specific timed intervals over 2?h and injected directly onto a capillary electrophoresis column and the concentration of 7-hydroxycoumarin determined. The metabolism of coumarin to 7-hydroxycoumarin is greatest in human and monkey microsomes.     

Drug metabolism and drug-induced spectral interactions in human fetal liver microsomes

Properties of a mono-oxygenase system in human fetal liver microsomes were studied. The levels of cytochrome P-450 and NADPH-cytochrome c reductase were 20 and 30 per cent, respectively of rat liver microsomal levels. Corresponding percentages for homogenates were 13 and 18 per cent, respectively. Hepatic 12,000 g supernatants from human fetuses were found to catalyze the hydroxylation of 3,4-benzpyrene and aniline and the N-demethylation of aminopyrine. These activities were 2, 12 and 11 per cent of those in adult rat liver. The cytochrome P-450- and NADPH-cytochrome c reductase-related turn-over-numbers for aminopyrine and aniline were of the same order of magnitude in human fetal and adult livers and in rat liver. The fetal turn-over-number for 3,4-benzpyrene was small compared with adult and rat values. Spectral changes induced by the addition of various compounds to liver microsomes were studied. Aminopyrine and hexobarbital were found to yield type I spectral changes with rat or adult human microsomes, but with fetal microsomes these compounds yielded a type II spectral change except in some cases when hexobarbital in low concentration yielded a type I change. Aniline and n-octylamine induced type II spectral changes with both adult and fetal microsomes. The relative magnitudes of spectral changes differed greatly between fetal and adult microsomes.     

Effects of synthetic progestagens on drug metabolism in rat liver microsomes

The inhibitory action of four progestagens on oxidative drug metabolism was studied. p-Nitroanisole demethylation and aniline hydroxylation in phenobarbital stimulated rat liver musomes were inhibited by norethisterone acetate, d-norgestrel, lynestrenol and allylestrenol. Inhibitor constants were between 19 μM and 49 μM for p-nitroanisole demethylation and between 45 μM and 112 μM for aniline hydroxylation. The four steroids tested exhibited type I difference spectra in musomes under aerobic conditions. The spectral dissociation constants were in the same order of magnitude as the inhibitor constants.     

An in-vitro study on the metabolism of rokitamycin and possible interactions of the drug with rat liver microsomes

This in-vitro study was designed to identify the enzyme(s) involved in the major metabolic pathway of rokitamycin, i.e. the formation of leucomycin A7, and to assess possible interactions of the drug with rat liver microsomes. Formation of leucomycin A7 was NADPH-independent and was not appreciably inhibited by anti-rat NADPH cytochrome P-450 reductase serum or cimetidine, a nonspecific inhibitor of cytochrome P-450 isoforms. Eadie-Hofstee plots for the formation of leucomycin A7 were indicative of apparently monophasic behaviour for six rat liver microsomes tested. The mean (+/- s.d.) kinetic parameters, Km, Vmax and Vmax/Km, for the formation of leucomycin A7 from rokitamycin were 47+/-13 microM, 390+/-56 nmol min(-1) (mg protein)(-1) and 8.6+/-1.6 mL min(-1) (mg protein)(-1), respectively. Three esterase inhibitors (100 microM), bis-nitrophenylphosphate, physostigmine and metrifonate inhibited the formation of leucomycin A7 by more than 60%. Metabolism of rokitamycin was inhibited by terfenadine, but not by mequitazine, whereas chlorpheniramine and theophylline activated the formation of leucomycin A7. Rokitamycin, leucomycin A7, leucomycin V, erythromycin and clarithromycin were weak inhibitors of CYP3A-catalysed 3-hydroxylation of quinine with mean IC50 values ranging from 71 to >100 microM. It is concluded that in rat liver microsomes the formation of leucomycin A7 from rokitamycin is catalysed mainly by an esterase (possibly cholinesterase, EC3.1.1.8), but not by cytochrome P-450 enzyme(s). Although in this in-vitro animal study CYP3A activity was barely inhibited by rokitamycin, the possibility cannot be totally discounted in man when rokitamycin is co-administered with drugs metabolized by CYP3A.     

3'-azido-3'-deoxythymidine drug interactions. Screening for inhibitors in human liver microsomes.

Zidovudine is a widely used antiretroviral drug active against human immunodeficiency virus. The drug interactions of this compound, which are primarily eliminated as a glucuronide, have not yet been extensively studied. Because zidovudine is frequently combined with other drugs, complete knowledge of interactions is essential to optimize AIDS therapy. We therefore screened the effect of 55 molecules, representative of 20 different therapeutic classes, on 3'-azido-3'-deoxythymidine (AZT) glucuronidation by human liver microsomes. We demonstrate that many drugs caused more than 15% inhibition of AZT glucuronidation in vitro, whereas major antibiotics (ceftazidine, isoniazid, aminoglycosides, macrolides, and sulfamides), antivirals (2',3'-dideoxycytidine, 2',3'-dideoxyinosine, and acyclovir), flucytosine, metronidazole, acetaminophen, and ranitidine had no effect. For compounds that appeared to inhibit AZT glucuronidation, extrapolation to the clinical situation must take into account both the in vitro apparent Ki values and the usual expected plasma level for the coadministered drug. By considering these parameters, this work indicates that clinically relevant inhibition of AZT glucuronidation may be observed with the following drugs: cefoperazone, penicillin G, amoxicilin, piperacillin, chloramphenicol, vancomycin, miconazole, rifampicin, phenobarbital, carbamazepine, phenytoin, valproic acid, quinidine, phenylbutazone, ketoprofen, probenecid, and propofol. Complementary clinical and pharmacokinetic studies should be performed to validate these assumptions.     

Inhibitory effects of psychotropic drugs on mexiletine metabolism in human liver microsomes: prediction of in vivo drug interactions

Mexiletine, an anti-arrhythmic agent, is used for the control of ventricular arrhythmias and for neuropathic pain from cancer or diabetes mellitus. It is sometimes used together with psychotropic drugs in patients with depression, schizophrenia or sleep disorder. It is metabolized mainly by cytochrome P450 (CYP) 2 D 6 and, to a minor extent, by CYP1A2. To predict possible drug interactions between mexiletine and psychotropic drugs, the inhibitory effects of 14 psychotropic drugs (phenytoin, carbamazepine, fluvoxamine, paroxetine, fluoxetine, citalopram, sertraline, imipramine, desipramine, haloperidol, thioridazine, olanzapine, etizolam, and quazepam) on mexiletine metabolism in human liver microsomes were determined. Fluoxetine (Ki=0.6+/- 0.1 microM), sertraline (Ki=7.6+/- 0.8 microM) and desipramine (Ki=3.2+/- 0.5 microM) competitively inhibited the mexiletine p-hydroxylation in human liver microsomes. Thioridazine (Kis=0.5+/- 0.2 microM; Kii =3.6+/-1.6 microM) and paroxetine (Kis=1.7+/- 0.7 microM; Kii=3.6+/- 0.9 microM) exhibited a mixed-type inhibition (competitive and non-competitive) toward mexiletine p-hydroxylation in human liver microsomes. The changes of the in vivo clearance of mexiletine by the psychotropic drugs were predicted by 1+(I/Ki) using the in vitro Ki and unbound inhibitor concentrations in liver. The values were calculated as 2.4 for paroxetine, 5.5 for fluoxetine, 1.1 for sertraline, 2.8 for desipramine and 2.2 for thioridazine. In addition, paroxetine exhibited a mechanism-based inactivation with Ki=0.7 microM and Kinact=0.15 min(-1). The present study predicted the possibility of drug interactions between mexiletine and paroxetine, fluoxetine, desipramine, and thioridazine in clinical use.     

氯米帕明在人肝微粒体中的N—去甲基代谢

目的：研究ＣＹＰ４５０选择性抑制剂对体外氯米帕明（Ｃｌｏ）Ｎ－去甲基代谢的影响。方法：应用米氏方程计算肝微粒体中Ｃｏｌ Ｎ－去甲基代谢的动力学参数，比较加入抑制剂前后这些参数的改变。结果：Ｋｍ１，Ｋｍ２，Ｖｍａｘ１，Ｖｍａｘ２，Ｖａｍｘ２／Ｖｍ１和Ｖａｍｘ２／Ｋｍ２分别为（０．１１±０．０６），（２４±１１）μｍｏｌ·Ｌ＾－１，（１１４±４７），（４２８±１８８）ｎｍｏｌ·ｇ＾－１·ｍｉｎ＾－１。     

An in vitro study on the metabolism and possible drug interactions of rokitamycin, a macrolide antibiotic, using human liver microsomes.

This in vitro study was designed to identify the enzyme(s) involved in the two major metabolic pathways of rokitamycin [formations of leucomycin A7 (LMA7) from rokitamycin and of leucomycin V (LMV) from LMA7] and to assess possible drug interactions using human liver microsomes. Formation of LMA7 or LMV was NADPH-independent. Anti-rat NADPH cytochrome P-450 (CYP) reductase serum, specific inhibitors, or substrates of CYP isoforms showed no effects on the formation of LMA7 or LMV. The mean Vmax and Vmax/Km for the formation of LMA7 from rokitamycin were much greater (P <.01) than those for the formation of LMV from LMA7. Two esterase inhibitors, bis-nitro-phenylphosphate and physostigmine (100 microM), inhibited the formation of LMA7 or LMV by more than 85%, whereas no appreciable inhibition occurred by several substrates of carboxylesterase (EC 3.1.1.1). Except the moderate inhibition produced by promethazine and terfenadine, theophylline, mequitazine, chlorpheniramine, and diphenhydramine showed little or no inhibition for the formation of LMA7 or LMV. Rokitamycin, LMA7, LMV, erythromycin, and clarithromycin (up to 500 microM) had no appreciable inhibition for CYP1A2-, 2C9-, and 2D6-mediated catalytic reactions. However, rokitamycin, LMA7, erythromycin, and clarithromycin inhibited the CYP3A4-catalyzed triazolam alpha-hydroxylation with IC50 (Ki) values of 5.8 (2.0), 40, 33 (20), and 56 (43) microM, respectively. It is concluded that the formations of LMA7 from rokitamycin and of LMV from LMA7 are catalyzed mainly by human esterase enzyme [possibly cholinesterase (EC3.1.1.8)]. However, whether rokitamycin would inhibit the CYP3A-mediated drug metabolism in vivo requires further investigations in patients.     

Oxidative metabolism of cinnarizine in rat liver microsomes.

The oxidative metabolism of cinnarizine (CZ) [1-(diphenylmethyl)-4-(3-phenyl-2-propenyl)-piperazine] to 1-(diphenylmethyl)piperazine (M-1), 1-(diphenylmethyl)-4-[3-(4'-hydroxyphenyl)-2-propenyl]piperazine (M-2), benzophenone (M-3) and 1-[4'-hydroxyphenyl)-phenylmethyl]-4-(3- phenyl-2-propenyl)piperazine (M-4) has been studied in rat liver microsomes. In Wistar rats, kinetic analysis revealed sex differences (male > female) in the Km values for formation of all the metabolites and the Vmax values for the formation of M-1, M-3 and M-4. The reactions required NADPH, and were inhibited by carbon monoxide and SKF 525-A. Only M-2 formation was suppressed by sparteine or metoprolol, and was significantly lower in female Dark Agouti rats than in Wistar rats of both sexes. The results suggest that CZ is oxidized by cytochrome P450, and M-2 formation is related to debrisoquine/sparteine-type polymorphic drug oxidation.     

Characterisation of theophylline metabolism in human liver microsomes.

1. A radiometric high performance liquid chromatographic method is described for the assay of theophylline metabolism in vitro by the microsomal fraction of human liver. 2. Formation of the three metabolites of theophylline (3-methylxanthine, 1-methylxanthine and 1,3-dimethyluric acid) were linear with protein concentrations to 4 mg ml-1 and with incubation times up to 180 min. 3. The coefficients of variation for the formation of 3-methylxanthine, 1-methylxanthine and 1,3-dimethyluric acid were 1.2%, 1% and 1.6%, respectively. 4. Theophylline is metabolised by microsomal enzymes with a requirement for NADPH. 5. The mean (n = 7) Km values for 1-demethylation, 3-demethylation and 8-hydroxylation were 545, 630 and 788 microM, respectively, and the mean Vmax values were 2.65, 2.84 and 11.23 pmol min-1 mg-1, respectively. 6. There was a high correlation between the Km and Vmax values for the two demethylation pathways suggesting that the demethylations are performed by the same enzyme. 7. Overall the in vitro studies are consistent with the in vivo results which suggest the involvement of two cytochrome P-450 isozymes in the metabolism of theophylline.     

Inhibition of drug metabolism in human liver microsomes by nizatidine, cimetidine and omeprazole

1. The inhibitory effects of cimetidine, nizatidine and omeprazole on the metabolic activity of CYP2C9, 2C19, 2D6 and 3A were investigated in human liver microsomes. Both cimetidine and omeprazole inhibited each of the CYP subfamily enzymes; in particular, omeprazole extensively inhibited the hydroxylation of S-mephenytoin (CYP2C19, Ki = 7.1 microM). Nizatidine exhibited no inhibition of any of the CYP isoforms examined. 2. Cimetidine inhibited the hydroxylation of tolbutamide but not of diclofenac, whereas omeprazole inhibited the hydroxylation of diclofenac but not that of tolbutamide. The ability to inhibit CYP2C9 varied with incubation time, as measured by the metabolic rate constant for the substrates. Therefore, suitable substrates and incubation times must be selected in inhibition studies examining metabolic clearance and the mechanism of inhibition of these drugs. 3. Nizatidine did not inhibit the metabolism of cisapride, glibenclamide, benidipine and simvastatin. Omeprazole inhibited the metabolism of cisapride (Ki = 0.4 microM), glibenclamide (11.7 microM) and benidipine (6.5 microM), whereas cimetidine inhibited the metabolism of glibenclamide (11.6 microM). To avoid drug-drug interactions, care needs to be taken to select suitable medicines for co-administration with anti-ulcer drugs.     

Felbamate in vitro metabolism by rat liver microsomes.

Incubation of [14C]felbamate at 37 degrees C for 60 min with liver microsomes from untreated Sprague-Dawley rats converted 10% of the drug to the p-hydroxy (6%) and 2-hydroxy (4%) metabolites. With microsomes from phenobarbital-pretreated rats, 21% of the drug was metabolized to the p-hydroxy (7.5%) and 2-hydroxy (13.5%) metabolites. With microsomes from felbamate-pretreated rats, up to 25% of drug was metabolized to the p-hydroxy (5%) and 2-hydroxy (20%) metabolites. In addition, a small amount of the monocarbamate metabolite was also present, but no other metabolites were formed. Coincubations of [14C]phenytoin with felbamate had no effect on the metabolism of phenytoin, whereas the amount of [14C]felbamate metabolized in the presence of phenytoin decreased by 30-38%.     

Benzene metabolism in mouse liver microsomes

Mouse liver microsomes metabolized benzene more rapidly than microsomes prepared from rat and rabbit liver. Treatment of mice with benzene increased the metabolism of benzene in vitro without increasing cytochrome P-450 concentrations. Conversely, treatment of mice with phenobarbital increased cytochrome P-450 values but did not increase benzene metabolism. Benzene metabolism was inhibited by compounds known to interact with the mixed function oxidase system, e.g., aniline, metyrapone, aminopyrine, SKF-525A and cytochrome c, but not by KCN or 3-amino-1,2,4-triazole. CO also inhibited benzene metabolism and the Warburg partition coefficient was similar to that obtained for other drugs metabolized by cytochrome P-450. Addition of benzene to mouse liver microsomes yielded a type I binding spectrum. Induction with benzene increased the magnitude of the type I spectral change (ΔE_max) by a factor approximately equal to the increase in benzene metabolism. The evidence suggests that benzene metabolism is mediated by the mixed function oxidase and binding of benzene to cytochrome P-450 is a significant factor in determining the rate of benzene metabolism.     

Inter-species variability of haplamine metabolism and identification of its phase I metabolites from liver microsomes

Haplamine, a pyranoquinoline alkaloid, was isolated from the genus Haplophyllum. The inter-species variability of haplamine metabolism was determined by reversed phase high performance liquid chromatography (HPLC) with UV detection. Microsomes from the liver of rats, mice, rabbits, guinea-pigs and humans were incubated with haplamine. After incubation, samples were extracted with a mixture of ethyl acetate and isopropyl alcohol (90 : 10; v/v). Haplamine and its metabolites were separated by HPLC using Nucleosil C18 Nautilus (5 microm) connected with a precolumn of the same type. The HPLC mobile phase consisted of water (A) and a mixture of methanol and acetonitrile (85 : 15; v/v) (B) used in a gradient mode (17 to 27 % B for 10 min, 27 to 90 % B for 37 min, 90 to 17 % B for 3 min, and finally 17 % B for 3 min) at 1 mL/min. Quantitative and qualitative results showed significant inter-species differences in haplamine metabolism. Qualitative similarities were found between guinea-pigs, rabbits, and humans. The metabolites were isolated by HPLC and identified by GC/MS after silylation. The phase I metabolites identified in human liver microsomes were TRANS/CIS-3,4-dihydroxy-9-O-desmethylhaplamine, TRANS/CIS-3,4-dihydroxyhaplamine and 9-O-desmethylhaplamine.     

Interspecies metabolic diversity of harmaline and harmine in in vitro 11 mammalian liver microsomes

The β‐carboline alkaloids harmaline and harmine are widely present in hallucinogenic plants with great potential for treating depression, Parkinson's disease, and Alzheimer's disease. The present study was to elucidate metabolic difference of harmaline and harmine in 11 mammalian liver microsomes in order to quantitate species‐specific metabolic profiles. Using the probe substrate reaction, the enzymatic activities for 8 CYP450 isozymes of 11 liver microsomes were characterized. Combining ultra performance liquid chromatography combined with electrospray ionization quadrupole time‐of‐flight tandem mass spectrometry (UPLC‐ESI‐Q/TOF‐MS) and ultra performance liquid chromatography combined with electrospray ionization quadrupole tandem mass spectrometry (UPLC‐ESI‐MS/MS) methods, 18 metabolites for harmaline and 11 for harmine were identified. The metabolism patterns differences of them presented discrepancy in the quality and quantity of metabolites. It was notable that O ‐sulfate conjugation was detected in all species except sheep. The intrinsic clearance CL _{int , LM} values for the metabolites harmine and harmol in rabbits (37.5 and 42.4 μL/min/mg) were higher than those in other animals, while dogs (16.2 and 16.7 μL/min/mg) and humans (16.0 and 16.3 μL/min/mg) exhibited similar in vitro metabolic clearance. These observations suggested that harmaline and harmine were rapidly metabolized in liver microsomes of rat, mouse, and rabbit; moderately metabolized in human and dog; while weakly metabolized in sheep. Comprehensive analysis of the metabolism indicated that dogs and humans showed considerable similarity in the elimination of parent drugs, metabolic profiles, and catalytic processes. To summarize, these findings illustrated that in vitro studies of harmaline and harmine metabolic profiles in different species are helpful for the proper selection and interpretation of animal models for pharmacological and toxicological evaluation, and will ultimately provide useful guidance for the development of β‐carboline alkaloids. Copyright © 2016 John Wiley & Sons Ltd.     

Inhibitors of imipramine metabolism by human liver microsomes.

1. The aromatic 2-hydroxylation of imipramine was studied in microsomes from three human livers. The kinetics were best described by a biphasic enzyme model. The estimated values of Vmax and Km for the high affinity site ranged from 3.2 to 5.7 nmol mg-1 h-1 and from 25 to 31 microM, respectively. 2. Quinidine was a potent inhibitor of the high affinity site for the 2-hydroxylation of imipramine in microsomes from all three human livers, with apparent Ki-values ranging from 9 to 92 nM. This finding strongly suggests that the high affinity enzyme is CYP2D6, the source of the sparteine/debrisoquine oxidation polymorphism. 3. The selective serotonin reuptake inhibitors (SSRI), paroxetine, fluoxetine and norfluoxetine were potent inhibitors of the high affinity site having apparent Ki-values of 0.36, 0.92 and 0.33 microM, respectively. Three other SSRIs, citalopram, desmethylcitalopram and fluvoxamine, were less potent inhibitors of CYP2D6, with apparent Ki-values of 19, 1.3 and 3.9 microM, respectively. 4. Among 20 drugs screened, fluvoxamine was the only potent inhibitor of the N-demethylation of imipramine, with a Ki-value of 0.14 microM. 5. Neither mephenytoin, citalopram, diazepam, omeprazole or proguanil showed any inhibition of the N-demethylation of imipramine and the role of the S-mephenytoin hydroxylase for this oxidative pathway could not be confirmed.