Characterization of specific cytochrome P450 enzymes responsible for the metabolism of diazepam in hepatic microsomes of adult male rats

Biodegradable and synthetic membranes for the expansion and functional differentiation of rat embryonic liver cells

The insufficient availability of donor organs for orthotopic liver transplantation worldwide has urgently increased the requirement for new therapies for acute and chronic liver disease. The creation of an unlimited source of donor cells for hepatocyte transplantation therapy and pharmaceutical applications may be the isolation and expansion of liver progenitor cells or stem cells. Here we report the expansion and functional differentiation of rat embryonic liver cells on biodegradable and synthetic polymeric membranes in comparison with traditional substrates, such as collagen and polystyrene culture dishes. Membranes prepared from chitosan and modified polyetheretherketone were used for the culture of liver progenitor cells derived from rat embryonic liver. Cells proliferated, with a significant increase in their number within 8-11 days. The cells displayed functional differentiation showing urea synthesis, albumin production and diazepam biotransformation on all substrates investigated. In particular, on a chitosan membrane liver-specific functions were expressed at significantly higher levels for prolonged times compared with other synthetic membranes, utilizing traditional substrates (collagen and PSCD) as references. These results demonstrate that chitosan membranes offer cells favourable conditions to promote the expansion and functional differentiation of embryonic liver cells that could be effectively used in liver tissue engineering and in pharmaceutical applications.

Diazepam metabolism in the kidneys of male and female rats of various strains

Previously, we have reported drastic strain differences of diazepam metabolism in the livers of a variety of rat strain. In this study, to characterize strain and sex differences of diazepam metabolism in the kidney, renal microsomal diazepam metabolic activities were determined in the Dark Agouti (DA), Sprague-Dawley (SD), Brown Norway (BN) and Wistar (WS) strains of rat. We found that the major pathway of diazepam metabolism in the kidney was diazepam N-demethylation, which is different from that in the liver, 3-hydroxylation. A Dose-course (12.5-200 muM of diazepam) study revealed that the DA and WS male rats had higher diazepam N-demethylation activity than the SD and BN rats. In contrast to the males, a lower activity of diazepam N-demethylation was observed in female BN rats. By Western blot analysis, constitutive protein expressions of cytochrome P450 (CYP) 2C11, which is responsible for diazepam N-demethylation, were detected in the 4 strain in both the male and female rats, and the BN rats had lower expression levels of CYP2C11 protein. However, we did not observe significant differences in the kinetic parameters of diazepam N-demethylation. Our results suggested that there was a strain difference in CYP-dependent diazepam N-demethylation in the rat kidney, which is different from the finding in liver microsomes.

IMPORTANCE OF CYP2D3 IN POLYMORPHISM OF DIAZEPAMP-HYDROXYLATION IN RATS

Diazepam was metabolized to three primary metabolites, 3-hydroxy-diazepam, N-desmethyl-diazepam, and p-hydroxy-diazepam. Our previous studies reported metabolic position-specific inter- or intrastrain differences in diazepam metabolism among Sprague-Dawley, Brown Norway, Dark Agouti, and Wistar rats. Especially, there were marked ( approximately 300 fold) inter- or intrastrain differences in diazepam p-hydroxylation activity at low concentration of substrate. In this study, we investigated the enzyme that catalyzes diazepam p-hydroxylation. The activity toward diazepam p-hydroxylation was inhibited by anti-cytochrome P450 2D (CYP2D) antibody, suggesting that this activity was catalyzed by CYP2D isoforms. Comparing the expression levels of the CYP2D subfamily in liver microsomes from various strains of rats using anti-CYP2D2 antibody, we found that there was a band of protein that was consistent with the phenotype of diazepam p-hydroxylation. N-terminal amino acid sequences of the specific protein exactly corresponded to those of CYP2D3, indicating that CYP2D3 might be involved in diazepam p-hydroxylation. Moreover, using rat CYP2D isoforms expressed in yeast, we tested CYP2Ds to catalyze diazepam p-hydroxylation. CYP2D1 and CYP2D2 practically did not participate in diazepam metabolism. On the other hand, diazepam p-hydroxylation was catalyzed by CYP2D3. CYP2D4 had high activity toward diazepam N-desmethylation, but not p-hydroxylation. In conclusion, the polymorphic expression of CYP2D3 caused the inter- or intrastrain differences in diazepam p-hydroxylation among rat strains or individuals.

Effects of dimephosphone, xydiphone, and ionol on the content and activities of rat liver cytochromes P-450 during long-term treatment with phenobarbital

Effects of dimephosphone, xydiphone, and ionol administered in parallel with phenobarbital on the content of cytochromes P-450 in rat liver and on the rate of C-hydroxylation of diazepam, haloperidol, and prednisolone by rat liver microsomal enzymes were studied in vitro. Dimephosphone, xydiphone, and ionol exhibited similar inductive effects on C-hydroxylation reactions in the CYP P-450 system during treatment with phenobarbital. Xydiphone and ionol in a dose of 1 mmol/kg canceled phenobarbital-induced increase in P-450 cytochrome content in rat liver. Sex-dependent cytochromes P-450 are involved in the prednisolone and haloperidol C-hydroxylation reactions in rats.

Accelerated throughput metabolic route screening in early drug discovery using high-resolution liquid chromatography/quadrupole time-of-flight mass spectrometry and automated data analysis

The resource investment required to characterise the metabolic fate of a compound is relatively large, meaning that within a drug discovery environment relatively few compounds are characterised in depth. Rate-limiting steps include the setting up of a complex array of mass spectrometry experiments and the subsequent analysis of the large data sets produced. We describe here a strategy for the evaluation of metabolic routes using full-scan high-resolution liquid chromatography/quadrupole time-of-flight mass spectrometry (LC/QToFMS) with automated data analysis using Metabolynx, a commercially available software package. Data from several structurally diverse compounds taken from the literature illustrate that, with careful setting of key parameters, this approach is able to indicate the presence of a wide range of metabolites with only a limited requirement for manual intervention.

Polymorphism in Diazepam Metabolism in Wistar Rats

We observed variations in the metabolism of diazepam in Wistar rats. We studied these variations carefully, and found that the variations are dimorphic and about 17% of male rats of Wistar strain we examined showed two times higher diazepam metabolic activities in their liver microsomes than the rest of animals at the substrate concentrations less than 5 microM. We classified them as extensive metabolizer (EM) and poor metabolizer (PM) of diazepam. No sex difference was observed in the frequency of appearance of EM. Activities of the primary metabolic pathways of diazepam were examined to elucidate the cause of this polymorphism in male Wistar rats. No significant differences were observed in activities of neither diazepam 3-hydroxylation or N-desmethylation between EM and PM rats, while activity of diazepam p-hydroxylation was markedly (more than 200 times) higher in EM rats, indicating that this reaction is responsible for the polymorphism of diazepam metabolism in Wistar rats. We examined the expression levels of CYP2D1, which was reported to catalyze diazepam p-hydroxylation in Wistar rats to find no differences in the expression levels of CYP2D1 between EM and PM rats. The kinetic study on diazepam metabolism in male Wistar rats revealed that EM rats had markedly higher V(max) and smaller K(m) in diazepam p-hydroxylation than those of PM rats, indicating the presence of high affinity high capacity p-hydroxylase enzyme in EM rats. As a consequence, at low concentrations of diazepam, major pathways of diazepam metabolism were p-hydroxylation and 3-hydroxylation in male EM rats, while in male PM rats, 3-hydroxylation followed by N-desmethylation. Due to this kinetic nature of p-hydroxylase activity, EM rats had markedly higher total CL(int) of diazepam than that of PM rats. Polymorphism in diazepam metabolism in humans is well documented, but this is the first report revealing the presence of the polymorphism in diazepam metabolism in rats. The current results infer polymorphic expression of new diazepam p-hydroxylating enzyme with lower K(m) than CYP2D1 in EM Wistar rats.

QUANTITATIVE PREDICTION OF THE IN VIVO INHIBITION OF DIAZEPAM METABOLISM BY OMEPRAZOLE USING RAT LIVER MICROSOMES AND HEPATOCYTES

The diazepam (DZ)-omeprazole (OMP) interaction has been selected as a prototype for an important drug-drug interaction involving cytochrome P450 inhibition. The availability of an in vivo K(i) value (unbound K(i), 21 microM) obtained from a series of steady-state inhibitor infusion studies allowed assessment of several in vitro-derived predictions of this inhibition interaction. Studies monitoring substrate depletion with time were used to obtain in vitro K(i) values that were evaluated against the more traditional metabolite formation approach using microsomes and hepatocytes. OMP inhibited the metabolism of DZ to its primary metabolites 4'-hydroxydiazepam, 3-hydroxydiazepam, and nordiazepam to different extents over a range of concentrations (0.3-150 microM), and a competitive inhibition model best fitted the data. The K(i) values observed using the substrate depletion approach (16 +/- 3 microM and 7 +/- 2 microM in microsomes and hepatocytes, respectively) were in good agreement with the overall weighted K(i) values obtained using the standard metabolite formation approach (12 +/- 2 microM and 16 +/- 5 microM in microsomes and hepatocytes, respectively). In vitro binding and cell uptake studies as well as human serum albumin studies in hepatocytes confirmed the importance of both intracellular and extracellular unbound concentrations of inhibitor when considering inhibition predictions. Both kinetic approaches and both in vitro systems predicted the in vivo interaction well and provide a good example of the ability of in vitro inhibition studies to quantitatively predict an in vivo drug-drug interaction successfully.

Effects of the aqueous extract from Salvia miltiorrhiza Bge on the pharmacokinetics of diazepam and on liver microsomal cytochrome P450 enzyme activity in rats

The aim of this study was to determine the effects of the aqueous extract of Salvia miltiorrhiza Bge (danshen in Chinese) on the pharmacokinetics of diazepam and on liver microsomal cytochrome P450 enzyme activity in rats. Rats (n = 5) were pretreated with danshen extract (100 mg kg(-1) per day, p.o.) for 15 consecutive days. Control rats (n = 5) received saline at the same time. Each rat was then administered a single oral dose of 15 mg kg(-1) diazepam. The pharmacokinetic parameters of diazepam were significantly different between the two groups. In the danshen pretreated group, the maximum concentration of diazepam and the area under the plasma concentration-time curve were reduced to about 72.7% and 44.4%, respectively, while the total body clearance was markedly increased by 2-fold. To help explain the results, liver microsomal suspensions were obtained from rats that were randomly divided into the control group (n = 10), and the low- (20 mg kg(-1) for 15 days, p.o., n = 10) and high-dose groups (100 mg kg(-1) for 15 days, p.o., n = 10) pretreated with danshen extract. Compared with the control rats, the microsomal protein content, cytochrome P450 enzyme level and erythromycin N-demethylase activity of pretreated rats were significantly increased. These results indicate that danshen extract can stimulate the activity of cytochrome P450 isoforms, and changes in the pharmacokinetics of diazepam resulting from danshen extract are related to an increase in metabolic activity of cytochrome P450.

Kinetics of metabolism and degradation of mometasone furoate in rat biological fluids and tissues

Mometasone furoate (MF) is a potent glucocorticoid developed for the treatment of glucocorticoid-responsive inflammatory disorders. The in-vitro and ex-vivo kinetics of the degradation and metabolism of MF were studied in selected biological fluids of rat and subcellular fractions of different rat tissues. In-vitro, MF was found to degrade slowly into four products in serum and urine, and metabolized rapidly and extensively in rat liver, minimally in extrahepatic tissues, including intestine, stomach, lung and kidney. Further investigation found that the microsomal fraction was the major intracellular site of MF 6 beta-hydroxylation in rat liver. Using chemical inhibitors, CYP3A was found to be the major enzyme involved in the in-vitro MF 6 beta-hydroxylation in rat liver microsomes. Enzyme kinetic studies in rat liver microsomes showed that the overall metabolic process of MF followed biphasic Michaelis-Menten kinetics, while 6 beta-hydroxylation obeyed monophasic Michaelis-Menten kinetics. The kinetic parameters derived from the kinetic models along with the enzyme inhibition studies suggest that MF is mainly metabolized via 6 beta-hydroxylation mediated by CYP3A primarily, and also biotransformed via other pathway(s) catalysed by other enzymes in rat liver in-vitro.

Comparative studies on in vitro methods for evaluating in vivo function of MDR1 P-glycoprotein

PURPOSE

MDR1 P-glycoprotein (P-gp) plays an important role in determining drug disposition. The purpose of the present study was to establish in vitro models to predict the in vivo function of P-gp.

METHODS

As an in vitro method, the transcellular transport of 12 compounds across the monolayer of Caco-2- and MDR1-transfected cells was examined. The ability of these compounds to stimulate the ATP hydrolysis was also determined using the isolated membrane fraction expressing P-gp. As a parameter to describe the in vivo P-gp function, we calculated the brain-to-plasma concentration ratio of compounds in mdr1a/1b knockout mice divided by the same ratio in wild type mice.

RESULTS

A good correlation was observed between the in vitro flux ratio across the monolayer and in vivo P-gp function for 12 compounds. Although all compounds that stimulated ATP hydrolysis were significantly transported by P-gp, some compounds were transported by P-gp without significantly affecting ATP hydrolysis.

CONCLUSION

Collectively, the in vitro flux ratio across monolayers of P-gp-expressing cells may be used to predict in vivo P-gp function. The extent of ATP-hydrolysis in vitro may also be a useful parameter for in vivo prediction, particularly for eliminating P-gp substrates in high-throughput screening procedures.

Interaction of drugs and Chinese herbs: pharmacokinetic changes of tolbutamide and diazepam caused by extract of Angelica dahurica

The inhibitory effects of Angelica dahurica root extract on rat liver microsomal cytochrome P450 and drug-drug interactions were studied. The 2alpha- and 16alpha-hydroxylase activity of testosterone were most strongly inhibited, with 17.2% and 28-5% of their activity remaining, respectively, after oral administration of A. dahurica extract at a 1 g kg(-1) dose. 6beta-Hydroxylase activity was also inhibited, with 70% of its activity remaining, under the same conditions. In addition, treatment with the extract inhibited the metabolism of tolbutamide, nifedipine and bufuralol. These results showed that the extract inhibited the various isoforms of cytochrome P450 such as CYP2C, CYP3A and CYP2D1. The A. dahurica extract delayed elimination of tolbutamide after intravenous administration at a 10 mg kg(-1) dose to rats. Thus, the extract altered the liver intrinsic clearance. It had little effect, however, on the pharmacokinetic parameters of diazepam after intravenous administration at 10 mg kg(-1). Since diazepam showed high clearance, it underwent hepatic blood flow rate-limited metabolism. Therefore, the change of intrinsic clearance had little effect on hepatic clearance. However, the Cmax value after oral administration of diazepam with extract treatment was four times that with non-treatment. It was suggested that the first-pass effect was changed markedly by the extract. High-dose (1 g kg(-1)), but not low dose (0.3 g kg(-1)), administration of A. dahurica extract increased significantly the duration of rotarod disruption following intravenous administration of diazepam at 5 mg kg(-1). It was concluded that administration of A. dahurica extract has the potential to interfere with the metabolism, by liver cytochrome P450, of other drugs.

Effects of imidazole antimycotics on the liver microsomal cytochrome P450 isoforms in rats: comparison of in vitro and ex vivo studies

We have studied the effects of three imidazole derivatives, clotrimazole (CLO), ketoconazole (KET) and miconazole (MIC) on the liver microsomal diazepam (DZ) metabolism. In in vitro experiments using rats and human liver microsomes, significant inhibition of CYP3A in terms of DZ-3-hydroxylase activity was observed. The inhibition of DZ metabolism was seen 1 h after CLO dosing. On the other hand, the induction of certain cytochrome P450 (CYP) isozymes was observed in in vivo studies 24 h after dosing. That is, CYP1A, CYP2B and CYP3A2, but not CYP2E, were observed 24 h after CLO or KET or MIC treatment. Under these conditions, CLO was the most potent inducer of CYP3A and MIC was a more potent inducer of CYP1A and CYP2B. KET induced CYP1A and CYP2B whereas the inducibility of KET was less than those of CLO and MIC. All of the imidazole derivatives tested here showed significant inhibition of CYP isozymes which overcame the induction of the CYP isozymes by those drugs in the data of Western blot analysis.

Effects of subacutely administered saiboku-to, an oriental herbal medicine, on pharmacodynamics and pharmacokinetics of diazepam in rodents

Subacute treatment with saiboku-to (2000 mg/kg, p.o., once a day) for 7 days induced an anxiolytic-like effect in rats. It did not, however, produce any other effects, such as sedative and hypnotic effects, anticonvulsive and muscle relaxant effects except for anxiolytic effect observed in diazepam-injected rats or mice. Diazepam (1.0 mg/kg, s.c.) induced anxiolytic-like effect was enhanced in saiboku-to treated rats as an additional effect of that induced by saiboku-to. To elucidate whether the enhancement of the anxiolytic-like effect following combined administration of diazepam and saiboku-to is due to the inhibition of hepatic drug-metabolizing enzymes, the pharmacokinetics of diazepam were further investigated in saiboku-to treated rats. The pharmacokinetic studies clearly demonstrated that subacute treatment with saiboku-to did not affect plasma concentration and protein binding rate of diazepam, and the activities of hepatic drug-metabolizing enzymes related to diazepam metabolism. These results, taken together, suggest that the enhancement of diazepam-induced anxiolytic-like effect observed in saiboku-to-treated rats is not due to an inhibition of diazepam metabolism.

Mechanism-based inhibition of rat liver microsomal diazepam C3-hydroxylase by mifepristone associated with loss of spectrally detectable cytochrome P450

Since initial studies with the steroids norethindrone and ethynylestradiol, reported by White and Muller-Eberhard in 1977 (Biochem. J. 166, 57-64), there has been continuing interest in xenobiotics that bear terminal or sub-terminal acetylenic groups which can cause catalysis-dependent inhibition of CYP monooxygenases associated either with loss of prosthetic group heme or protein adduct formation. Mifepristone is a synthetic steroid bearing a propyne substitution on carbon 17 and this suggested to us that it may act as a mechanism-based inhibitor of the CYP isoforms responsible for its metabolism. In human and rat liver, CYP3A isoforms have been implicated in mifepristone clearance and mifepristone administration to rats has also been shown to induce CYP3A enzymes and the associated diazepam C3-hydroxylase activity (Cheesman, Mason and Reilly, J. Steroid Biochem. Mol. Biol., 58, 1996, 447-454). With microsomes prepared from the livers of untreated female rats and others in which diazepam C3-hydroxylase has been induced, we show here that mifepristone can cause catalysis-dependent inhibition of this monooxygenase. In addition, incubation of microsomes with mifepristone in the presence, but not in the absence, of NADPH caused loss of spectrally detectable cytochrome P450. These results suggest that heme adduct formation may result from mifepristone metabolism by CYP3A monooxygenases which undergo self-catalysed irreversible inactivation with this drug as substrate. Since mifepristone administration in vivo is able also to cause induction of the synthesis of hepatic CYP3A apoprotein, mifepristone may have the potential in human medicine for complex interactions with other co-administered drugs which are also substrates for CYP3A monooxygenases.

Human, rat and crocodile liver microsomal monooxygenase activities measured using diazepam and nifedipine: effects of CYP3A inhibitors and relationship to immunochemically detected CYP3A apoprotein

Nifedipine oxidase and diazepam C3-hydroxylase were tested as activities for selectively measuring CYP3A enzymes using liver microsomes from male and female human organ donors, male and female Wistar rats and male and female estuarine crocodiles. The association between CYP3A enzymes and these monooxygenations was confirmed for the human samples. Male rat samples had lower specific contents of CYP3A apoprotein than the human samples but had equivalent (nifedipine) or higher (diazepam) monooxygenase specific activities. CYP3A apoprotein was undetectable in female rat samples which had very low activities towards both substrates. Enzyme inhibition studies showed that diazepam C3-hydroxylase of male rat liver was attributable to CYP3A but corresponding results for female rats suggested a contribution from non-CYP3A enzyme. Western blotting with immunochemical detection using anti-CYP3A4 IgG suggested the presence of putative CYP3A apoprotein in male and female crocodile liver samples and inhibition studies with diazepam as substrate suggested the presence of CYP3A subfamily monooxygenase activity in these enzyme preparations. Results for nifedipine oxidase with male and female rat liver and male crocodile liver suggested major contributions to catalysis from non-CYP3A enzymes. Inhibition studies suggested that a higher proportion of nifedipine oxidase in female crocodile liver may be attributable to the putative CYP3A enzyme(s) than in male crocodile liver. These results show the need for care in the assessment of CYP3A activity of fractionated tissues when using these substrates in cross-species studies and where gender is a variable.

Differential inducibility of specific mRNA corresponding to five CYP3A isoforms in female rat liver by RU486 and food deprivation: comparison with protein abundance and enzymic activities

The induction of cytochrome P450 3A (CYP3A) protein and mRNA by RU486 [17beta-hydroxy-11beta-(4-dimethylaminophenyl)-17alpha-1-pro pyl-estra-4,9-dien-3-one] treatment and food deprivation in female rat liver was studied using Western blotting and competitive reverse transcription-polymerase chain reaction (RT-PCR). CYP3A apoprotein levels increased in response to food deprivation and to RU486 treatment, and the combination of RU486 treatment plus food deprivation had an apparent additive effect. Food deprivation and RU486 treatment also caused increases in CYP3A1, CYP3A18, and CYP3A23 mRNA, and the combined effects of these treatments on each of these mRNA forms were synergistic. CYP3A2 mRNA was not detected in any of the treatment groups, and there was a lack of concordance between CYP3A9 mRNA levels and the specific messages corresponding to the other CYP3A isoforms. CYP3A9 mRNA levels were highest in food-deprived animals, whereas RU486 inhibited CYP3A9 mRNA expression and suppressed the induction effect of food deprivation. Food deprivation and RU486 treatment each separately caused increased microsomal diazepam C3-hydroxylase activity, and the combined effects of these treatments on this monooxygenase were additive. In contrast, the [N-methyl-14C]erythromycin demethylase activity of the fasted, RU486-treated group of rats did not differ from that of the untreated group, and kinetic analyses revealed that both groups of animals exhibited similar Km and Vmax values. These results suggest that CYP3A9 may be primarily responsible for erythromycin N-demethylation and that the isoforms induced by the combination of fasting and RU486 administration are CYP3A1, CYP3A23, and, to a lesser extent, CYP3A18.

Involvement of CYP2D1 in the metabolism of carteolol by male rat liver microsomes

1. The metabolism of carteolol, a beta-adrenoceptor blocking drug, was investigated in male Sprague-Dawley rat liver microsomes. 2. The formation of 8-hydroxycarteolol was the principal metabolic pathway of carteolol in vitro and followed Michaelis-Menten kinetics with a K(m) = 11.0 +/- 5.4 microM and a Vmax = 1.58 +/- 0.64 nmol/min/nmol P450 respectively (mean +/- SD, n = 5). Eadie-Hofstee plot analysis of carteolol 8-hydroxylase activity confirmed single-enzyme Michaelis-Menten kinetics. 3. The cytochrome P450 isoforms involved in 8-hydroxylation of carteolol were investigated using selective chemical inhibitors and polyclonal anti-P450 antibodies. Quinine (Ki = 0.06 microM) and quinidine (Ki = 2.0 microM), selective inhibitors of CYP2D1, competitively inhibited 8-hydroxycarteolol formation. Furthermore, only anti-human CYP2D6 antibody inhibited this reaction. 4. These results suggest that carteolol is metabolized to 8-hydroxycarteolol by CYP2D1. The K(m) of carteolol for CYP2D1 in male rat liver microsomes was much greater than those of propranolol or bunitrolol, indicating that carteolol has a lower affinity for CYP2D1 compared with these other beta-adrenoceptor blocking drugs.

Pharmacokinetics and pharmacodynamics in toxicology

1. Pharmacokinetics aids interpretation of the dose-response relationship in individual toxicology studies. 2. When used to compare across studies, even in a single species other factors, including variation in pharmacodynamic response, must be taken into account. Variation in pharmacodynamic response becomes more profound when one compares across species. 3. Examples do occur where plasma concentration-response relationships are constant across species, particularly when corrected for unbound drug. These examples should not be taken as support, however, of a general universal principle. 4. Owing to multiple factors such as species differences in receptors, enzymes and ion channels, dose or plasma concentration-response relationships can vary enormously across species. In the light of this, the results of toxicology studies should be viewed as qualitative rather than quantitative. Once sufficient clinical experience is gained the human database is the overriding measure of drug safety.

Evidence for CYP2D1-mediated primary and secondary O-dealkylation of ethylmorphine and codeine in rat liver microsomes

The purpose of the present study was to investigate the role of specific CYPs responsible for the O-dealkylation of ethylmorphine (EM) and codeine (CD) to morphine (M), as well as that of norethylmorphine (NEM) and norcodeine (NCD) to normorphine (NM) in rat liver microsomes. Liver microsomes metabolize EM and CD to M, and NEM and NCD to NM, in the presence of an NADPH-generating system. The metabolites of EM and CD were determined by HPLC with UV and electrochemical detection. In the present study, the role of CYP2D1 in O-dealkylation of EM/NEM and CD/NCD was investigated by use of specific antiCYP antibodies. When testing rabbit antirat CYP2D1, 2E1, 2C11, and 3A2 antibodies, only the antiCYP2D1 antibody inhibited the EM/NEM and CD/NCD O-dealkylase activities significantly. The maximum inhibition achieved was approximately 80% at a protein ratio (IgG to microsomes) of 10:1, p = 0.001. The contribution of CYP2D1 to the O-dealkylation of EM/NEM and CD/NCD was further confirmed by use of the specific CYP2D1 inhibitors quinine and propafenone. Five microM of quinine inhibited the EM/NEM and CD/NCD O-dealkylase activities by approximately 80%. The CYP3A inhibitor troleandomycin (TAO) failed to inhibit the CYP2D1 catalyzed reaction, but did inhibit the N-demethylation of EM and CD. The O-dealkylation of NEM and NCD was also impaired in Dark Agouti rat (DA) liver microsomes. Taken together, the immunoinhibition and chemical-inhibitor studies of rat liver microsomes provided convincing evidence for the involvement of CYP2D1, the rat counterpart of human CYP2D6, in the metabolism of EM/NEM and CD/NCD to the corresponding O-dealkylated metabolites.

Metabolite complex formation of orphenadrine with cytochrome P450. Involvement of CYP2C11 and CYP3A isozymes

Expression and inhibition of cytochrome P450 (CYP) isozymes capable of forming an orphenadrine metabolite complex were studied in microsomes of untreated and inducer-treated male and female rats. High levels of complex-forming isozymes were found in microsomes of untreated male as compared to female rats. Treatment of male rats with several P450 inducers did not considerably increase the extent of in vitro complex formation. In female rats, however, phenobarbital or dexamethasone treatments led to pronounced induction. The isozyme specificity of complex formation was investigated by several approaches including: 1. inhibition by orphenadrine of isozyme-specific P450 activities, such as hydroxylation of testosterone, O-dealkylation of pentoxy-and ethoxyresorufin and complex formation with triacetyloleandomycin (TAO), 2. inhibition of orphenadrine complex formation by metyrapone, TAO, and cimetidine, and 3. correlation of complex levels with immunochemically, enzymatically, or spectroscopically determined amounts of P450 isozymes. Our data suggest that CYP2C11, a CYP3A isozyme and an unidentified P450 species are involved in complex formation with orphenadrine, but exclude the involvement of CYP1A1/2 and CYP2B1/2. The capability of CYP2C11 to form a metabolite complex with orphenadrine is strongly suggested for the following reasons: 1. Efficient inhibition of testosterone 2 alpha- and 16 alpha-hydroxylation by complex formation with orphenadrine in microsomes of untreated male rats, 2. high expression of orphenadrine-complexing isozymes in untreated male compared to female rats, 3. specific inhibition of in vitro complex formation by cimetidine, 4. suppression of complex-forming isozymes by 3-methylcholanthrene and beta-naphthoflavone, and 5. concomitant induction of complex-forming isozymes, immunodetectable CYP2C11, and testosterone 2 alpha-hydroxylase by stanozolol. That at least one, but not all, CYP3A isozymes is involved in complex formation is concluded from inhibition experiments with TAO that show that orphenadrine complexation can be significantly inhibited in microsomes of dexamethasone-treated, but not in microsomes of untreated rats. Furthermore, complex formation with TAO is not inhibited by orphenadrine in microsomes of phenobarbital (PB)-treated rats. In PB-treated female rats, a further unidentified complex-forming isozyme can be detected that is not inhibited by complex formation with TAO.

Effects of food deprivation and adrenalectomy on CYP3A induction by RU486 in female rats

We have studied the effects of food deprivation and adrenalectomy on the induction by RU486 of female rat liver microsomal CYP3A apoprotein, erythromycin N-demethylase and diazepam C3-hydroxylase activities. RU486 was a potent inducer of CYP3A apoprotein in intact animals and food deprivation enhanced this response. Food deprivation alone caused only weak CYP3A apoprotein induction suggesting a synergistic interaction in the regulation of protein expression. These results were reflected in the measurements of diazepam C3-hydroxylase activity. This confirms diazepam C3-hydroxylase as a useful and easily measured index of CYP3A monooxygenase content in female rat liver microsomes. Erythromycin N-demethylase did not show concordance with this pattern; this monooxygenase was much more strongly induced by food deprivation alone than by RU486 administration and, in addition, adrenalectomy abolished the induction response to food deprivation. The lack of correspondence between the apoprotein and erythromycin N-demethylase results suggests that non-CYP3A or novel, hitherto uncharacterized CYP3A isoforms may contribute to erythromycin N-demethylation in female rats. The close agreement between the results for CYP3A apoprotein and diazepam C3-hydroxylase indicates that although RU486 possesses a terminal acetylenic moeity it does not, at the dosages used here, cause mechanism-based inactivation of the CYP3A monooxygenase protein it induces. Current studies are directed to characterizing the particular CYP3A isoform(s) whose production is stimulated by RU486.

Characterization of a primary hepatocyte culture system for toxicological studies

An hepatocyte culture system was developed for potential use in toxicological studies in vitro. Rat hepatocytes were isolated by two-step collagenase perfusion and cultured on Vitrogen-coated Permanox dishes in a modified Chee's medium containing 1 microM dexamethasone and 1% dimethylsulfoxide. The cells remained highly viable for at least 10 d as determined by lactate dehydrogenase release and total protein levels. Albumin secretion into the medium, as a measure of differentiated function, was maintained at elevated levels over the course of 10 d in culture. A number of CYP activities were determined by the analysis of testosterone metabolism in freeze-thawed cells, diazepam metabolism in live cells, and specific assays for CYP 1A1/2, 2B1/2, 2E1, and 3A. Results of these assays indicated that a wide range of CYP isozymes were maintained, some activities were enhanced under the conditions of culture and some activities were inducible. Activities of the phase II enzymes, glutathione S-transferase and UDP-glucuronosyltransferase, and glutathione levels were also maintained in the cultured hepatocytes for at least 6 d. These results strongly support the use of this hepatocyte culture system for in vitro toxicological studies.

The effect of three H2 receptor antagonists on the disposition of cyclosporin A in the in situ perfused rat liver model

The in situ perfused rat liver model was used to investigate the effect of three H2 receptor antagonists on the disposition of cyclosporin A (CyA) and the major human metabolite, AM1. Perfusion experiments, using standard techniques, were carried out on four groups (one control and three H2-receptor antagonist-treated groups) of male Sprague-Dawley rats (300-350 g). All animals received CyA, 2.5 mg; the three treated groups received cimetidine (8 mg), ranitidine (3 mg), or famotidine (0.4 mg). Perfusate and bile samples were collected and assayed for CyA, AM1, and the H2 receptor antagonists by HPLC. Results indicated that CyA perfusate concentrations in the controls and cimetidine and ranitidine-treated groups were not significantly different, although levels in the famotidine group were significantly higher at all times (p < 0.05), except 30 min, compared to the controls. However, examination of the AM1 perfusate and bile data and the apparent metabolic clearance data indicated that CyA metabolism was still occurring, despite the presence of the H2 receptor antagonist. It is suggested that the absence of a interaction may be attributed to a lack of specificity of the H2 receptor antagonists for CYP3A, the isoenzyme responsible for CyA metabolism.

Kinetics of diazepam metabolism in rat hepatic microsomes and hepatocytes and their use in predicting in vivo hepatic clearance

1. The rates of diazepam (DZ) metabolism to the primary metabolites 3-hydroxydiazepam, 4'-hydroxydiazepam and nordiazepam were studied in vitro using rat hepatic microsomes and hepatocytes. 4'-hydroxydiazepam had the largest intrinsic clearance (Vmax/Km ratio, CL(int)) in both microsomes and hepatocytes representing 49 and 70% of total metabolism respectively. Whereas the contribution of 3-hydroxydiazepam was similar in both systems (21-24%), the N-demethylation pathway was greater in microsomes (27%) than hepatocytes (9%). 2. The pharmacokinetics of DZ were determined in vivo using the intraportal route to avoid blood flow limitations due to the high clearance of DZ. No dose dependency was observed in either clearance or steady state volume of distribution, which were estimated to be 38 ml/min/SRW (where SRW is a standard rat weight of 250 g) and 1.3 L/SRW respectively. Blood binding of DZ was concentration independent, the unbound fraction being 0.22. 3. Scaling factors were used to relate the in vitro CL(int) to the in vivo unbound clearance. Hepatocytes (123 ml/min/SRW) produced a more realistic prediction for the in vivo value (174 ml/min/SRW) than microsomes (41 ml/min/SRW). This situation is believed to arise from the quantitative differences in the three metabolic pathways in the two in vitro systems. It is speculated that end product inhibition is responsible for reduced total metabolism in microsomes whereas hepatocytes operate kinetically in a manner close to in vivo.

The effect of three H2-receptor antagonists on the disposition of midazolam in the rat in-situ perfused liver model

The rat in-situ perfused liver model was used to investigate the effect of three H2-receptor antagonists on the pharmacokinetic disposition of the short-acting benzodiazepine, midazolam. Perfusion experiments, using standard techniques, were carried out on four groups (one control and three H2-receptor antagonist-treated groups) of male Sprague-Dawley rats (300-350 g). All animals received midazolam 1 mg; the three treated groups received cimetidine (8 mg), ranitidine (3 mg) or famotidine (0.4 mg). Perfusate and bile samples were collected and assayed for midazolam using gas chromatography. The perfusate data indicated that midazolam disposition was impaired at 10, 50 and 60 min of the experimental period following the addition of cimetidine, whereas ranitidine and famotidine produced an effect at 10 min only; midazolam levels in bile were not affected by the presence of an H2-receptor antagonist. It was concluded that the limited inhibitory effect of cimetidine may be attributed to its lack of specificity for CYP3A, the isoenzyme responsible for the metabolism of midazolam.

Involvement of CYP2D6, CYP3A4, and other cytochrome P-450 isozymes in N-dealkylation reactions

Metabolic N-dealkylation is a commonly observed biotransformation with tertiary and secondary amine drugs and related N-alkylated amides, but surprisingly little is known about the cytochrome P-450 isozymes involved in these dealkylation reactions. In this review, evidence is provided that supports the involvement of various P-450 isozymes, but especially CYP3A4 and other isozymes of the CYP3A subfamily. Although CYP2D6 is generally not considered to be capable of catalyzing the N-dealkylation of basic drugs, some examples of the involvement of this important isozyme in N-dealkylation reactions are identified. Procedures used to identify individual P-450 isozymes involved in N-dealkylation reactions are discussed.