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.     

Interspecies variability and drug interactions of loxapine metabolism in liver microsomes.

Loxapine is a dibenzoxazepine neuroleptic that is metabolized by the liver in humans. In the present study, we investigated first in vitro loxapine metabolism in liver microsomes from various species including rats, mice, guinea pigs, dogs, rabbits, monkeys and humans. This enables us to choose between species to further validate drug-drug interaction studies. We observed the formation of desmethyl- and hydroxy- metabolites of loxapine after incubation of the different species liver microsomes. Hydroxylation pathway was major in all species. Wide interspecies variability of loxapine metabolism was observed. Loxapine metabolism was similar in human, guinea pig and dog microsomes. We screened in vitro effects of 67 molecules, representative of 8 therapeutic classes, on loxapine metabolism. Loxapine (100 microM) was incubated with guinea pig liver microsomes (1 mg/ml) 30 min at 37 degrees C with and without the presence of interacting drug. We found that most of psychotropics (alimemazine, cyamemazine and levomepromazine), antifungal (ketoconazole), anticancer drugs (daunorubicin, pirarubicin) and analgesic (nefopam) inhibited more than 50% of hydroxyloxapine formation in vitro. Complementary clinical and pharmacokinetic studies should be performed to confirm these results.     

Interaction of famotidine with rat liver microsomes, a study showing less inhibition of drug metabolism than with cimetidine

Interaction of famotidine with rat liver microsomes and its effect on drug metabolism in vitro were studied. Famotidine interacted with liver microsomes obtained from untreated, phenobarbital-pretreated and 3-methylcholanthrene-pretreated rats to produce characteristic type II spectral changes with peaks at 423-426 nm and troughs at 387-390 nm. The spectral dissociation constants were in the range of 0.84-0.94 mM. Famotidine inhibited aminopyrine N-demethylase activity to a much lesser extent than did cimetidine. The extent of inhibition at a concentration of 5 mM of famotidine was from 12 to 18% for the microsomes from the rats with different pretreatments. In contrast, 5 mM of cimetidine inhibited the activity 80, 59 and 80% in the microsomes from untreated, phenobarbital-pretreated and 3-methylcholanthrene-pretreated rats, respectively. Both famotidine and cimetidine inhibited aminopyrine N-demethylase in a mixed-type manner for the microsomes from phenobarbital-pretreated rats, with inhibition constants of 4.7 and 0.7 mM, respectively. These results demonstrate that famotidine is an in vitro inhibitor of microsomal drug metabolism in rats but is much less inhibitory than cimetidine.     

The effects of primaquine stereoisomers and metabolites on drug metabolism in the isolated perfused rat liver and in vitro rat liver microsomes

The effect of the antimalarial drug primaquine, its stereoisomers and its proposed metabolites, on the metabolism of substrates for mixed function oxidase, has been studied in isolated perfused rat livers (IPRL) and/or in vitro microsomal suspension. Following acute administration to an IPRL preparation, racemic primaquine produced a dose related reduction in the hepatic clearance of antipyrine which at the highest dose of primaquine (5.0 mg) represented a decrease to 46% of control values. Antipyrine clearance was reduced to a comparable extent by the (+) and (-) isomers and the racemic mixture (each at a dose of 2.5 mg) with mean reductions of 45, 49 and 47%, respectively. These changes in clearance were reflected by significant increases in half-life relative to control. The apparent volume of distribution of antipyrine was unchanged in all experiments. Racemic primaquine and its (+) and (-) isomers were equipotent in inhibiting aminopyrine N-demethylase activity, producing reductions of 56, 59 and 55%, respectively, relative to control values. These three compounds also produced corresponding reductions of 73, 58 and 73% in ethoxyresorufin O-deethylase activity. The N-acetyl and 5-hydroxy derivative of primaquine produced inhibitory effects comparable to that seen for the parent drug. In contrast the carboxylic acid metabolite of primaquine, 6-desmethylprimaquine and 5-hydroxy-6-desmethyl primaquine did not influence aminopyrine N-demethylase activity. These results indicate that the propensity to inhibit drug metabolism by these primaquine related substances, is influenced by functional group substitution rather than the optical activity of the parent drug.     

荧光光谱法研究青蒿素及双氢青蒿素与肝微粒体的相互作用

目的研究抗疟药青蒿素(ART)与双氢青蒿素(DHA)和大鼠肝微粒体(RLM)以及人肝微粒体(HLM)之间的相互作用。方法在模拟人体生理条件下,采用荧光光谱法,得出青蒿素、DHA与RLM和HLM的结合参数。结果青蒿素和DHA对RLM和HLM的猝灭机制不同,它们对RLM的猝灭机制均为静态猝灭,而对HLM的猝灭作用为混合猝灭;其相互间作用力均为氢键和范德华力,其结合反应均为自发的热力学反应。结论青蒿素和DHA对大鼠和人肝微粒体均有结合,且结合力存在种属差异,同时青蒿素和DHA与同种肝微粒体的结合能力也存在差异。     

Inhibition of in-vitro simvastatin metabolism in rat liver microsomes by bergamottin, a component of grapefruit juice

Grapefruit juice can modify the pharmacokinetic parameters of many drugs, in particular simvastatin, an orally active cholesterol-lowering agent. The exact components in grapefruit juice responsible for drug interactions are not perfectly known. However, it seems that bergamottin, a furocoumarin derivative, is one of the main active components within grapefruit juice. The objective of this paper was to quantify and to characterize in-vitro the inhibitory effect of bergamottin on simvastatin metabolism by using rat and human liver microsomes. In rat liver microsomes, the incubation conditions (+/-NADPH) of bergamottin were found to influence its inhibiting capacity. In co-incubation with simvastatin, the Ki value (the equilibrium dissociation constant for the enzyme-inhibitor complex) was higher (Ki = 174 +/- 36 microM) than in pre-incubation (Ki = 45 +/- 6 microM and 4 +/- 2 microM, without and with NADPH, respectively). It thus seems that the pre-incubation of bergamottin (in particular with NADPH) increases its inhibiting capacity on simvastatin metabolism. Bergamottin metabolism study in rat liver microsomes showed the formation of two metabolites that were CYP-450 dependent. In contrast, in human liver microsomes, the incubation conditions of bergamottin did not influence its inhibiting capacity of simvastatin metabolism (Ki = 34 +/- 5 microM, Ki = 22 +/- 5 microM, Ki = 27 +/- 11 microM in coincubation and pre-incubation without and with NADPH, respectively). In rat and man, bergamottin was found to be a mixed-type inhibitor of simvastatin hepatic metabolism. However, in rat, bergamottin was partially a mechanism-based inhibitor by involvement of either bergamottin alone or one of its metabolites. The results highlight the importance of validating in-vitro models to help verify the suitability of the in-vitro model for predicting the nature and degree of metabolic drug interactions.     

Enantioselective metabolism of camazepam by rat liver microsomes

Camazepam [3-(N,N-dimethyl)carbamoyloxy-7-chloro-1-methyl-1,3-dihydro-5-phenyl-2H-1,4-benzodiazepin-2-one, CMZ] possesses anxiolytic, anticonvulsant, muscle relaxant and hypnotic properties. CMZ is clinically used as a racemate. The enantioselective metabolism of racemic CMZ by rat liver microsomes was studied. Major metabolites were isolated by normal-phase and reversed-phase liquid chromatography (LC) and further characterized by UV absorption, mass, and circular dichroism spectral analyses, and by chiral stationary phase LC analysis. Following anin vitro incubation of rac-CMZ, the unmetabolized CMZ was found to be enriched in the (S)-CMZ, indicating that the Renantiomer was enantioselectively metabolized. Two of the most abundant metabolites, formed by hydroxylation and demethylation of a methyl group of theN,N-dimethylcarbamyloxy side chain, were found to be enriched in the Renantiomer. The results indicated that the (R)-CMZ was metabolized at a faster rate than (S)-CMZ by rat liver microsomes.     

Ethanol inhibits in-vitro metabolism of nifedipine, triazolam and testosterone in human liver microsomes

Although extended exposure to ethanol induces CYP3A metabolism in-vivo, the acute effects of ethanol on CYP3A metabolism have not been fully evaluated in-vitro. We assessed the effect of ethanol on CYP3A-mediated biotransformation using human liver microsomes in-vitro with three prototypic CYP3A-mediated reactions: nifedipine to oxidized nifedipine, triazolam to its 1-hydroxy (1-OH TRZ) and 4-hydroxy (4-OH TRZ) metabolites, and testosterone to 6beta-hydroxytestosterone (6beta-OH TST). Ethanol inhibited metabolism of nifedipine (oxidized nifedipine IC50 3 mg dL(-1), where the IC50 value is the inhibitor concentration corresponding to a 50% reduction in metabolite formation velocity), triazolam (1-OH TRZ IC50 1.1 mg dL(-1), 4-OH TRZ IC50 2.7 mg dL(-1)) and testosterone (6beta-OH TST IC50 2.4 mg dL(-1)). The inhibitory potency of ethanol was similar for the three substrates representing the three hypothetical CYP3A substrate categories. The IC50 values obtained were lower than clinically relevant blood alcohol concentrations. In conclusion, ethanol is an inhibitor of human CYP3A metabolism and may contribute to clinically important interactions.     

In vitro metabolism study of saikosaponin d and its derivatives in rat liver microsomes

1.?Saikosaponins, one of the representative bioactive ingredients in Radix Bupleuri, possess hepatoprotective, anti-inflammatory, antiviral, antitumor, and other pharmacological activities. Up to now, few studies focused on the further metabolism of saikosaponins and their secondary metabolites absorbed into the circulatory system.

2.?To understand the in vivo efficacy of saikosaponin d, the in vitro metabolism of saikosaponin d, and its two derivatives formed in the gastrointestinal tract, prosaikogenin G and saikogenin G was investigated in rat liver microsomes, respectively.

3.?Fifteen metabolites were detected using high-performance liquid chromatography hybrid ion trap and time-of-flight mass spectrometry and triple-quadrupole mass spectrometry, and the predominant metabolic reactions were hydroxylation, carboxylation and combinations of these steps on the aglycone moiety.

4.?The metabolic pathways of saikosaponin d, prosaikogenin G, and saikogenin G were proposed in vitro and the results contribute to the understanding of saikosaponins in vivo metabolism.     

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.     

藁本内酯在大鼠肝微粒体中代谢的酶动力学

研究体外大鼠肝微粒体藁本内酯代谢的酶动力学及选择性CYP450酶抑制剂对其代谢的影响。建立测定肝微粒体孵育液中藁本内酯含量的LC-MS法,以尼群地平为内标,二者的定量离子m/z分别选择173和315。考察确定最佳温孵条件,进行藁本内酯代谢的酶促反应动力学研究,通过特异性抑制试验,探讨参与藁本内酯代谢的主要同工酶。结果显示,酮康唑、甲氧苄啶、α-萘黄酮显著抑制藁本内酯的体外代谢,而奥美拉唑、4-甲基吡唑、奎尼丁对其体外代谢影响不大。可见CYP3A4、CYP2C9和CYP1A2是参与藁本内酯代谢的主要代谢酶,CYP2C19、CYP2E1和CYP2D6没有明显参与催化其代谢。

     

Characterization of deltamethrin metabolism by rat plasma and liver microsomes

Deltamethrin, a widely used type II pyrethroid insecticide, is a relatively potent neurotoxicant. While the toxicity has been extensively examined, toxicokinetic studies of deltamethrin and most other pyrethroids are very limited. The aims of this study were to identify, characterize, and assess the relative contributions of esterases and cytochrome P450s (CYP450s) responsible for deltamethrin metabolism by measuring deltamethrin disappearance following incubation of various concentrations (2 to 400 microM) in plasma (esterases) and liver microsomes (esterases and CYP450s) prepared from adult male rats. While the carboxylesterase metabolism in plasma and liver was characterized using an inhibitor, tetra isopropyl pyrophosphoramide (isoOMPA), CYP450 metabolism was characterized using the cofactor, NADPH. Michaelis-Menten rate constants were calculated using linear and nonlinear regression as applicable. The metabolic efficiency of these pathways was estimated by calculating intrinsic clearance (Vmax/Km). In plasma, isoOMPA completely inhibited deltamethrin biotransformation at concentrations (2 and 20 microM of deltamethrin) that are 2- to 10-fold higher than previously reported peak blood levels in deltamethrin-poisoned rats. For carboxylesterase-mediated deltamethrin metabolism in plasma, Vmax=325.3+/-53.4 nmol/h/ml and Km=165.4+/-41.9 microM. Calcium chelation by EGTA did not inhibit deltamethrin metabolism in plasma or liver microsomes, indicating that A-esterases do not metabolize deltamethrin. In liver microsomes, esterase-mediated deltamethrin metabolism was completely inhibited by isoOMPA, confirming the role of carboxylesterases. The rate constants for liver carboxylesterases were Vmax=1981.8+/-132.3 nmol/h/g liver and Km=172.5+/-22.5 microM. Liver microsomal CYP450-mediated biotransformation of deltamethrin was a higher capacity (Vmax=2611.3+/-134.1 nmol/h/g liver) and higher affinity (Km=74.9+/-5.9 microM) process than carboxylesterase (plasma or liver) detoxification. Genetically engineered individual rat CYP450s (Supersomes) were used to identify specific CYP450 isozyme(s) involved in the deltamethrin metabolism. CYP1A2, CYP1A1, and CYP2C11 in decreasing order of importance quantitatively, metabolized deltamethrin. Intrinsic clearance by liver CYP450s (35.5) was more efficient than that by liver (12.0) or plasma carboxylesterases (2.4).     

柠檬苦素在大鼠肝微粒体中代谢的性别差异

目的：研究柠檬苦素在大鼠肝微粒体中代谢的性别差异以及引起性别差异的可能的CYP450亚型。方法：柠檬苦素与雌雄大鼠肝微粒体共同孵育,抑制实验为在肝微粒体中加入特异性CYP450抑制剂后,再加入柠檬苦素共同孵育,温孵后的样品乙醚提取后进行HPLC分析。结果：柠檬苦素在雌雄大鼠肝微粒体中代谢的表观酶动力学参数分别为：米氏常数K㎡：（12±3）、（16±4）ug／mL,最大反应速度Vmax：（49±6）、（93±14）ng·min^-1·mg^-1。柠檬苦素在雌雄大鼠肝微粒体中孵育30min的代谢率分别为（33．9±4．7）9／6和（53．8±2．8）％,代谢速度分别为（4．46±0．62）和（7．08±0．37）ng·min^-1·mg^-1。CYP3A2特异性抑制剂酮康唑、CYP2C11特异性抑制剂西咪替丁对柠檬苦素的代谢抑制作用明显,CYPlA2特异性抑制剂a-萘黄酮和CYP2D1特异性抑制剂奎尼丁对柠檬苦素的代谢也有不同程度的抑制作用。结论：柠檬苦素在大鼠肝微粒体中代谢存在明显的性别差异,CYP3A2和CYP2C11在雌雄大鼠肝微粒体中的性别差异可能是引起代谢性别差异的主要原因。     

染料木黄酮在大鼠肝微粒体代谢的酶动力学

目的：体外研究大鼠肝微粒体中染料木黄酮代谢的酶动力学，及选择性细胞色素(CYP)酶抑制剂对其代谢的影响。方法：用大鼠肝微粒体研究染料木黄酮代谢的酶动力学，探讨CYP酶的选择性抑制剂对其代谢的影响及参与其代谢的CYP酶。结果：CYP1A2抑制剂呋喃茶碱可以显地抑制染料木黄酮代谢，使染料木黄酮的代谢速率下降。而其它CYP特异性抑制剂对染料木黄酮代谢没有明显的影响。结论：CYP1A2参与了染料木黄酮的代谢，CYP1A2的抑制剂可能会与染料木黄酮发生代谢相互作用，从而降低染料木黄酮的代谢速率。     

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.     

Acute effect of bromodichloromethane on lipid metabolism in rat liver microsomes

Acute effects of CHBrCl2 on microsomal lipid metabolism were studied in the liver of male Wistar rats administered a single oral dose (215 mg/kg). 3 h after administration, the modulation of phospholipid metabolism and the accumulation of triacylglycerol (TG) were noted in the microsomal membranes. A marked decrease in the synthetic rate of phospatidylcholine (PC) could be explained by a fall in the microsomal activity of CTP:phosphocholine cytidylyltransferase. A decrease in glycerophosphate acyltransferase activity and an enhancement in phosphatidate phosphatase activity seemed to offset each other, and consequently no significant change was observed in the synthetic rate of microsomal TG. These alterations in microsomal lipid metabolism can be regarded as one of the manifestations of hepatotoxicity of CHBrCl2.