Primary and secondary oxidative metabolism of dextromethorphan. In vitro studies with female Sprague-Dawley and Dark Agouti rat liver microsomes

Rat brain CYP2D enzymatic metabolism alters acute and chronic haloperidol side-effects by different mechanisms

Risk for side-effects after acute (e.g. parkinsonism) or chronic (e.g. tardive dyskinesia) treatment with antipsychotics, including haloperidol, varies substantially among people. CYP2D can metabolize many antipsychotics and variable brain CYP2D metabolism can influence local drug and metabolite levels sufficiently to alter behavioral responses. Here we investigated a role for brain CYP2D in acutely and chronically administered haloperidol levels and side-effects in a rat model. Rat brain, but not liver, CYP2D activity was irreversibly inhibited with intracerebral propranolol and/or induced by seven days of subcutaneous nicotine pre-treatment. The role of variable brain CYP2D was investigated in rat models of acute (catalepsy) and chronic (vacuous chewing movements, VCMs) haloperidol side-effects. Selective inhibition and induction of brain, but not liver, CYP2D decreased and increased catalepsy after acute haloperidol, respectively. Catalepsy correlated with brain, but not hepatic, CYP2D enzyme activity. Inhibition of brain CYP2D increased VCMs after chronic haloperidol; VCMs correlated with brain, but not hepatic, CYP2D activity, haloperidol levels and lipid peroxidation. Baseline measures, hepatic CYP2D activity and plasma haloperidol levels were unchanged by brain CYP2D manipulations. Variable rat brain CYP2D alters side-effects from acute and chronic haloperidol in opposite directions; catalepsy appears to be enhanced by a brain CYP2D-derived metabolite while the parent haloperidol likely causes VCMs. These data provide novel mechanistic evidence for brain CYP2D altering side-effects of haloperidol and other antipsychotics metabolized by CYP2D, suggesting that variation in human brain CYP2D may be a risk factor for antipsychotic side-effects.

The time-dependent effects of St John's wort on cytochrome P450, uridine diphosphate-glucuronosyltransferase, glutathione S-transferase, and NAD(P)H-quinone oxidoreductase in mice

Hypericum perforatum [St. John’s wort (SJW)] is known to cause a drug interaction with the substrates of cytochrome P450 (P450, CYP) isoforms, mainly CYP3A. This study aims to determine the dose response and time course of the effects of SJW extract on P450s, UDP-glucuronosyltransferase (UGT), glutathione S-transferase (GST), and NAD(P)H-quinone oxidoreductase (NQO) in mice. The oral administration of SJW extract to male mice at 0.6 g/kg/d for 21 days increased hepatic oxidation activity toward a Cyp3a substrate nifedipine. By extending the SJW treatment to 28 days, hepatic nifedipine oxidation (NFO) and warfarin 7-hydroxylation (WOH) (Cyp2c) activities were increased by 95% and 34%, respectively. Immunoblot analysis of liver microsomal proteins revealed that the Cyp2c protein level was elevated by the 28-day treatment. However, the liver microsomal activities of the oxidation of the respective substrates of Cyp1a, Cyp2a, Cyp2b, Cyp2d, and Cyp2e1 remained unchanged. In the kidney, SJW increased the NFO, but not the WOH activity. The extended 28-day treatment did not alter mouse hepatic and renal UGT, GST, and NQO activities. These findings demonstrate that SJW stimulates hepatic and renal Cyp3a activity and hepatic Cyp2c activity and expression. The induction of hepatic Cyp2c requires repeated treatment for a period longer than the initial induction of Cyp3a.

Effects of aqueous extract of Ruta graveolens and its ingredients on cytochrome P450, uridine diphosphate (UDP)-glucuronosyltransferase, and reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H)-quinone oxidoreductase in mice

Ruta graveolens (the common rue) has been used for various therapeutic purposes, including relief of rheumatism and treatment of circulatory disorder. To elucidate the effects of rue on main drug-metabolizing enzymes, effects of an aqueous extract of the aerial part of rue and its ingredients on cytochrome P450 (P450/CYP), uridine diphosphate (UDP)-glucuronosyltransferase, and reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H):quinone oxidoreductase were studied in C57BL/6JNarl mice. Oral administration of rue extract to males increased hepatic Cyp1a and Cyp2b activities in a dose-dependent manner. Under a 7-day treatment regimen, rue extract (0.5 g/kg) induced hepatic Cyp1a and Cyp2b activities and protein levels in males and females. This treatment increased hepatic UDP-glucuronosyltransferase activity only in males. However, NAD(P)H:quinone oxidoreductase activity remained unchanged. Based on the contents of rutin and furanocoumarins of mouse dose of rue extract, rutin increased hepatic Cyp1a activity and the mixture of furanocoumarins (F_mix) increased Cyp2b activities in males. The mixture of rutin and F_mix increased Cyp1a and Cyp2b activities. These results revealed that rutin and F_mix contributed at least in part to the P450 induction by rue.

First demonstration that brain CYP2D-mediated opiate metabolic activation alters analgesia in vivo

The response to centrally acting drugs is highly variable between individuals and does not always correlate with plasma drug levels. Drug-metabolizing CYP enzymes in the brain may contribute to this variability by affecting local drug and metabolite concentrations. CYP2D metabolizes codeine to the active morphine metabolite. We investigated the effect of inhibiting brain, and not liver, CYP2D activity on codeine-induced analgesia. Rats received intracerebroventricular injections of CYP2D inhibitors (20 μg propranolol or 40 μg propafenone) or vehicle controls. Compared to vehicle-pretreated rats, inhibitor-pretreated rats had: (a) lower analgesia in the tail-flick test (p<0.05) and lower areas under the analgesia-time curve (p<0.02) within the first hour after 30 mg/kg subcutaneous codeine, (b) lower morphine concentrations and morphine to codeine ratios in the brain (p<0.02 and p<0.05, respectively), but not in plasma (p>0.6 and p>0.7, respectively), tested at 30 min after 30 mg/kg subcutaneous codeine, and (c) lower morphine formation from codeine ex vivo by brain membranes (p<0.04), but not by liver microsomes (p>0.9). Analgesia trended toward a correlation with brain morphine concentrations (p=0.07) and correlated with brain morphine to codeine ratios (p<0.005), but not with plasma morphine concentrations (p>0.8) or plasma morphine to codeine ratios (p>0.8). Our findings suggest that brain CYP2D affects brain morphine levels after peripheral codeine administration, and may thereby alter codeine's therapeutic efficacy, side-effect profile and abuse liability. Brain CYPs are highly variable due to genetics, environmental factors and age, and may therefore contribute to interindividual variation in the response to centrally acting drugs.

Concentration-dependent inhibitory effects of baicalin on the metabolism of dextromethorphan, a dual probe of CYP2D and CYP3A, in rats

Baicalin has been shown to possess many pharmacological effects, including antiviral, antioxidant, anti-cancer and anti-inflammatory properties. In the current study, we reveal the inhibitory effects of baicalin on the metabolism of dextromethorphan (DXM), a dual probe substrate of CYP2D and CYP3A, in rats. Lineweaver-Burk plots demonstrated that baicalin inhibited the activities of CYP2D and CYP3A in a non-competitive manner in rat liver microsomes (RLMs). Concomitant administration of baicalin (0.90 g/kg, i.v.) and DXM (10 mg/kg, i.v.) increased the maximum drug concentration (C(max)) (37%) and the area under concentration-time curve (AUC) (42%) and decreased the clearance (CL) (27%) of DXM in a randomised, crossover study in rats (P < 0.01). The change in the AUC of DXM was significantly correlated with the C(max) and AUC of baicalin (P < 0.05). The inhibitory effects of multiple doses of baicalin (0.90 g/kg, i.v., 12 days) on the metabolism of DXM were similar to those observed following a single dose in rats. The activity of CYP3A in excised liver samples from rats following multiple baicalin treatment was significantly decreased compared to that of the control group (P < 0.05), whereas multiple doses of baicalin had no obvious effect on the activity of CYP2D. Taken together, these data demonstrate that baicalin inhibits the metabolism of DXM in a concentration-dependent manner in rats, possibly through inhibiting hepatic CYP2D and CYP3A activities.

Rat CYP2D2, not 2D1, is functionally conserved with human CYP2D6 in endogenous morphine formation

The assumption that CYP2D1 is the corresponding rat cytochrome to human CYP2D6 has been revisited using recombinant proteins in direct enzyme assays. CYP2D1 and 2D2 were incubated with known CYP2D6 substrates, the three morphine precursors thebaine, codeine and (R)-reticuline. Mass spectrometric analysis showed that rat CYP2D2, not 2D1, catalyzed the 3-O-demethylation reaction of thebaine and codeine. In addition, CYP2D2 incubated with (R)-reticuline generated four products corytuberine, pallidine, salutaridine and isoboldine while rat CYP2D1 was completely inactive. This intramolecular phenol-coupling reaction follows the same mechanism as observed for CYP2D6. Michaelis-Menten kinetic parameters revealed high catalytic efficiencies for rat CYP2D2. These findings suggest a critical evaluation of other commonly accepted, however untested, CYP2D1 substrates.

Chiral analyses of dextromethorphan/levomethorphan and their metabolites in rat and human samples using LC-MS/MS

In order to develop an analytical method for the discrimination of dextromethorphan (an antitussive medicine) from its enantiomer, levomethorphan (a narcotic) in biological samples, chiral analyses of these drugs and their O-demethyl and/or N-demethyl metabolites in rat plasma, urine, and hair were carried out using LC-MS/MS. After the i.p. administration of dextromethorphan or levomethorphan to pigmented hairy male DA rats (5 mg/kg/day, 10 days), the parent compounds and their three metabolites in plasma, urine and hair were determined using LC-MS/MS. Complete chiral separation was achieved in 12 min on a Chiral CD-Ph column in 0.1% formic acid-acetonitrile by a linear gradient program. Most of the metabolites were detected as being the corresponding O-demethyl and N, O-didemethyl metabolites in the rat plasma and urine after the hydrolysis of O-glucuronides, although obvious differences in the amounts of these metabolites were found between the dextro and levo forms. No racemation was observed through O- and/or N-demethylation. In the rat hair samples collected 4 weeks after the first administration, those differences were more clearly detected and the concentrations of the parent compounds, their O-demethyl, N-demethyl, and N, O-didemethyl metabolites were 63.4, 2.7, 25.1, and 0.7 ng/mg for the dextro forms and 24.5, 24.6, 2.6, and 0.5 ng/mg for the levo forms, respectively. In order to fully investigate the differences of their metabolic properties between dextromethorphan and levomethorphan, DA rat and human liver microsomes were studied. The results suggested that there might be an enantioselective metabolism of levomethorphan, especially with regard to the O-demethylation, not only in DA rat but human liver microsomes as well. The proposed chiral analyses might be applied to human samples and could be useful for discriminating dextromethorphan use from levomethorphan use in the field of forensic toxicology, although further studies should be carried out using authentic human samples.

Selective effects of nitric oxide on the disposition of chlorzoxazone and dextromethorphan in isolated perfused rat livers

The rapid and direct effects of nitric oxide (NO) donors sodium nitroprusside (SNP) and isosorbide dinitrate (ISDN) on the hepatic and biliary disposition of chlorzoxazone (CZX), a marker of CYP2E1, and dextromethorphan (DEM), a marker of CYP2D1, were studied in a single-pass isolated perfused rat liver model. Livers (n = 30) were perfused with constant concentrations of NO donors (0-120 min) in addition to infusion of CZX or DEM (60-120 min), and periodical outlet and bile samples were collected. Both ISDN and SNP significantly reduced (30 and 60%, respectively) the hepatic extraction ratio of CZX and decreased (50 and 70%, respectively) the recovery of the CYP2E1-mediated metabolite, 6-hydroxychlorzoxazone, in the outlet perfusate and bile. As for DEM, both NO donors increased (up to 3.5-fold) the recovery of the CYP2D1-mediated metabolite dextrorphan (DOR) in the outlet perfusate. However, this was associated with a simultaneous decrease (50-75%) in the excretion of the metabolite into the bile, thus resulting in no change in the overall recovery of DOR as a result of NO donor treatment. The decrease in the biliary excretion of DOR was caused by NO-induced simultaneous reductions in both the conjugation of DOR and biliary clearance of DOR conjugate. Additionally, both SNP and ISDN significantly reduced the metabolism of DEM to 3-hydroxymorphinan, which is mostly regulated by CYP3A2. These studies in an intact liver model confirm the selectivity of the inhibitory effects of NO donors on cytochrome P450 enzymes, which was recently reported in microsomal studies, and expand these inhibitory effects to conjugation pathways.

Drug disposition of chiral and achiral drug substrates metabolized by cytochrome P450 2D6 isozyme: case studies, analytical perspectives and developmental implications

The concepts of drug development have evolved over the last few decades. Although number of novel chemical entitities belonging to varied classes have made it to the market, the process of drug development is challenging, intertwined as it is with complexities and uncertainities. The intention of this article is to provide a comprehensive review of novel chemical entities (NCEs) that are substrates to cytochrome P450 (CYP) 2D6 isozyme. Topics covered in this review aim: (1) to provide a framework of the importance of CYP2D6 isozyme in the biotransformation of NCEs as stand-alones and/or in conjunction with other CYP isozymes; (2) to provide several case studies of drug disposition of important drug substrates, (3) to cover key analytical perspectives and key assay considerations to assess the role and involvement of CYP2D6, and (4) to elaborate some important considerations from the development point of view. Additionally, wherever applicable, special emphasis is provided on chiral drug substrates in the various subsections of the review.

Effect of methamphetamine on the pharmacokinetics of dextromethorphan and midazolam in rats

Methamphetamine is the fourth most frequently reported compound associated with drug abuse on admission of patients to treatment centres after cocaine, heroin and marijuana. It is metabolized in the organism with a reaction that is catalyzed by cytochrome P450, mainly by the CYP2D and CYP3A subfamily, 4-hydroxyamphetamine and amphetamine being dominant metabolites. The present pharmacokinetic study was undertaken to investigate the possible influence of methamphetamine (10 mg/kg, i.p., once daily for six days) on the pharmacokinetics of dextromethorphane as a model substrate for rat cytochrome P-4502D2 and midazolam as a model substrate for CYP3A1/2. Animals received a single injection of dextromethorphane (10 mg/kg) or midazolam (5 mg/kg) in the tail vein 24 h after the last dose of methamphetamine or administration of placebo. The results of pharmacokinetic analysis showed a significantly increased rate of dextrorphane and 3-hydroxymorphinan formation, and a marked stimulatory effect of methamphetamine on CYP2D2 metabolic activity. Similarly, the kinetics of midazolam's metabolic conversion to hydroxy derivates of midazolam indicated a significant increase in CYP3A1/2 activity. The results showed that the administration of methamphetamine significantly stimulated the metabolic activity of CYP2D2 as well as that of CYP3A1/2. With regard to the high level of homology between human and rat CYP isoforms studied, the results may have a clinical impact on future pharmacotherapy for methamphetamine abuse.

Dextromethorphan differentially affects opioid antinociception in rats

Opioid drugs such as morphine and meperidine are widely used in clinical pain management, although they can cause some adverse effects. A number of studies indicate that N-methyl-D-aspartate (NMDA) receptors may play a role in the mechanism of morphine analgesia, tolerance and dependence. Being an antitussive with NMDA antagonist properties, dextromethorphan (DM) may have some therapeutic benefits when coadministered with morphine. In the present study, we investigated the effects of DM on the antinociceptive effects of different opioids. We also investigated the possible pharmacokinetic mechanisms involved. The antinociceptive effects of the mu-opioid receptor agonists morphine (5 mg kg(-1), s.c.), meperidine (25 mg kg(-1), s.c.) and codeine (25 mg kg(-1), s.c.), and the kappa-opioid agonists nalbuphine (8 mg kg(-1), s.c.) and U-50,488H (20 mg kg(-1), s.c.) were studied using the tail-flick test in male Sprague-Dawley rats. Coadministration of DM (20 mg kg(-1), i.p.) with these opioids was also performed and investigated. The pharmacokinetic effects of DM on morphine and codeine were examined, and the free concentration of morphine or codeine in serum was determined by HPLC.It was found that DM potentiated the antinociceptive effects of some mu-opioid agonists but not codeine or kappa-opioid agonists in rats. DM potentiated morphine's antinociceptive effect, and acutely increased the serum concentration of morphine. In contrast, DM attenuated the antinociceptive effect of codeine and decreased the serum concentration of its active metabolite (morphine). The pharmacokinetic interactions between DM and opioids may partially explain the differential effects of DM on the antinociception caused by opioids.

A novel incubation direct injection LC/MS/MS technique for in vitro drug metabolism screening studies involving the CYP 2D6 and the CYP 3A4 isozymes

A direct injection LC/MS/MS method involving a novel incubation technique was developed for the inhibition screening of CYP 2D6 and CYP 3A4 isoenzymes using dextromethorphan and midazolam as probe substrates. Both assays were performed using an electrospray ionization source in the positive ion mode. Direct injection was possible by using a short C 18, LC column (2 mm x 20 mm) with large particle diameter packing (10 microm). Analytical characteristics of the direct injection technique were studied by examining matrix effects, which showed suppression of the ESI signal between 0.20 and 0.65 min. The retention times for analytes were adjusted to approximately 0.8 min (k'>3), resulting in no matrix effect. Column lifetime was evaluated and determined to be approximately 160 direct injections of the matrix. The precision and accuracy of the control samples for the quantitation of dextromethorphan was between -0.53 and -12.80, and 3.73 and 6.69% respectively. Unlike conventional incubation techniques, incubations were carried out in an autosampler equipped with a heating accessory. This novel incubation method, which involved no stirring of the incubation mixture, estimated the Cl(int in vitro) for dextromethorphan and midazolam in human liver microsomes to be 1.65+/-0.22 ml/(hmg) and 0.861 ml/(min mg) respectively. The autosampler tray maintained uniform temperature and was sensitive to changes in temperature between 33 and 41 degrees C. High-throughput screening was performed using known inhibitors of the CYP 2D6 isozyme, and the system was evaluated for its ability to differentiate between these inhibitors. The strong inhibitor quinidine resulted in a 25.6% increase in t(1/2), the medium potency inhibitor chlorpromazine resulted in an increase of 6.14% and the weak inhibitor primaquine had no significant effect on half-life. This technique involves no sample preparation, demonstrated run times of 2 min per injection and can be fully automated. The method should therefore prove to be a valuable tool in the drug discovery process.

ENZYME-SELECTIVE EFFECTS OF NITRIC OXIDE ON AFFINITY AND MAXIMUM VELOCITY OF VARIOUS RAT CYTOCHROMES P450

Nitric oxide (NO) has recently been shown to decrease cytochrome P450 (P450) enzyme activity rapidly (< or =30 min), concentration dependently, and enzyme-selectively in the rat liver. Interestingly, among all the studied P450 enzymes, only CYP2D1 was not affected by NO donors. However, these studies were conducted using only a single concentration of the substrates, thus lacking information about the possible simultaneous changes in both maximum velocity (Vmax) and affinity (Km) of the enzymes. In the present study, we systematically evaluated the effects of NO on the enzyme kinetic parameters of marker substrates for a range of P450 enzymes, including 2D1. Livers were perfused (1 h) in the absence (control) or presence of two NO donors with different mechanisms of NO release. At the end of the perfusion, microsomes were prepared and used for kinetic analysis. Except for 2D1, NO reduced the Vmax of all the model reactions studied, although to a varying degree. However, the effects of NO donors on Km were more diverse. Whereas the Km values for testosterone 6beta-hydroxylation (3A2) and 16alpha-hydroxylation (2C11) significantly decreased, the values for chlorzoxazone 6-hydroxylation (2E1), dextromethorphan N-demethylation (3A2), and high affinity ethoxyresorufin O-dealkylation (1A1/2) significantly increased in the presence of NO donors. Furthermore, the Km values for the high-affinity component of dextromethorphan O-demethylation and benzyloxyresorufin O-dealkylation remained unchanged. These results indicate that NO can potentially change both the Vmax and Km of various substrates selectively and confirm our previous findings that the activity of CYP2D1 is not affected by NO donors.

Influence of concomitant quinidine administration on dextromethorphan disposition in rats

High doses of dextromethorphan (DM) have been clinically investigated for the treatment of multiple neuronal disorders including neuropathic pain. Several authors have suggested the concomitant administration of DM and a CYP2D6 reversible inhibitor in order to enhance the exposure of DM and limit the exposure to total dextrorphan (DX). The present study proposes to determine whether or not a single dose of quinidine is sufficient to enhance the plasma concentrations of DM in rats and keep those of DX at a minimal level. Oral doses of DM (50 mg/kg) were administered with increasing dose levels of quinidine (0, 2, 20, and 50 mg/kg) to male Sprague-Dawley rats and blood samples were collected over 24 h. Plasma concentrations of DM and total DX were determined using ESI-LC/MS/MS. Quinidine coadministration resulted in a more than twofold increase in the area under the curve of DM with an ED(50) of approximately 2 mg/kg whereas those of total DX were only increased by 21%. These results support the working hypothesis that a single dose of quinidine may enhance the plasma concentrations of DM relative to those of total DX and may therefore improve the treatment of neuropathic pain.

Demethylation of radiolabelled dextromethorphan in rat microsomes and intact hepatocytes

Liver microsomal preparations are routinely used to predict drug interactions that can occur in vivo as a result of inhibition of cytochrome P450 (CYP)-mediated metabolism. However, the concentration of free drug (substrate and inhibitor) at its intrahepatic site of action, a variable that cannot be directly measured, may be significantly different from that in microsomal incubation systems. Intact cells more closely reflect the environment to which CYP substrates and inhibitors are exposed in the liver, and it may therefore be desirable to assess the potential of a drug to cause CYP inhibition in isolated hepatocytes. The objective of this study was to compare the inhibitory potencies of a series of CYP2D inhibitors in rat liver microsomes and hepatocytes. For this, we developed an assay suitable for rapid analysis of CYP-mediated drug interactions in both systems, using radiolabelled dextromethorphan, a well-characterized probe substrate for enzymes of the CYP2D family. Dextromethorphan demethylation exhibited saturable kinetics in rat microsomes and hepatocytes, with apparent Km and Vmax values of 2.1 vs. 2.8 microM and 0.74 nM x min(-1) per mg microsomal protein vs. 0.11 nM x min(-1) per mg cellular protein, respectively. Quinine, quinidine, pyrilamine, propafenone, verapamil, ketoconazole and terfenadine inhibited dextromethorphan O-demethylation in rat liver microsomes and hepatocytes with IC50 values in the low micromolar range. Some of these compounds exhibited biphasic inhibition kinetics, indicative of interaction with more than one CYP2D isoform. Even though no important differences in inhibitory potencies were observed between the two systems, most inhibitors, including quinine and quinidine, displayed 2-3-fold lower IC50 in hepatocytes than in microsomes. The cell-associated concentrations of quinine and quinidine were found to be significantly higher than those in the extracellular medium, suggesting that intracellular accumulation may potentiate the effect of these compounds. Studies of CYP inhibition in intact hepatocytes may be warranted for compounds that concentrate in the liver as the result of cellular transport.

Characterization of moclobemide N-oxidation in human liver microsomes

1. Moclobemide underdergoes morpholine ring N-oxidation to form a major metabolite in plasma Rol2-5637. 2. The kinetics of moclobemide N-oxidation in human liver microsomes (HLM) (n = 6) have been investigated and the mixed-function oxidase enzymes catalysing this reaction have been identified using inhibition, enzyme correlation, altered pH and heat pretreatment experiments. 3. N-oxidation followed single enzyme Michealis-Menten kinetics (0.02-4.0 mm). Km app and Vmax ranged from 0.48 to 1.35 mM (mean +/- SD) 0.77 +/- 0.34 mM) and 0.22 to 2.15 nmol mg(-1) min(-1) (1.39 +/- 0.80 nmol mg(-1) respectively. 4. The N-oxidation of moclobemide strongly correlated with benzydamine N-oxidation a probe reaction for flavin-containing monoxygenase (FMO) activity (0.1 mM moclobemide, rs = 0.81, p < 0.005; 4 mM moclobemide, rs = 0.94, p = 0.0001). Correlations were observed between moclobemide N-oxidation and specific cytochromre P450 (CYP) activities at both moclobemide concentrations (0.1 mM moclobemide, CYP2C19 0.66, p < 0.05; 4 mM moclobemide, CYP2E1 rs = 0.56, p < 0.05). 5. The general P450 inhibitor, N-benzylimidazole, did not affect the rate of Rol2-5637 formation (0% inhibition versus control) (at 1.3 mM moclobemide. Furthermore, the rate of Ro12-5637 formation in HLM was unaffected by inhibitors Or substrates of specific P450s (< 10% inhibition versus control). 6. Heat pretreatment of HLM in the absence of NADPH (inactivating FMOs) resulted in 97% inhibition of Ro12-5637 formation. N-oxidation activity was greatest when incubated at pH 8.5. These results ilre consistent with the reaction being FMO medialtetd . 7. In conclusion, moclobemide N-oxidation activity has been observed in HLM in vitro and the reaction is predominantly catalysed by FMOs with a potentially small contribution from cytochrome P450 isoforms.

Characterization of biotransformation enzyme activities in primary rat proximal tubular cells

The proximal tubule is a frequent target for nephrotoxic compounds due to it's ability to transport and accumulate xenobiotics and their metabolites, as well as by the presence of an organ-selective set of biotransformation enzymes. The aim of the present study was to characterize the activities of different biotransformation enzymes during primary culturing of rat proximal tubular cells (PT cells). Specific marker substrates for determining cytochrome P450 (CYP450) activity of primary cultured PT cells include 7-ethoxyresorufin (CYP1A1), caffeine (CYP1A), testosterone (CY2B/C, CYP3A), tolbutamide (CYP2C) and dextromethorphan (CYP2D1). Activities of the CYP450 isoenzymes decreased considerably during culture with the greatest loss in activity within 24 h of culture. In addition, expression of CYP450 apoprotein, including CYP1A, CYP2C, CYP2D, CYP2E and CYP4A, was detected in microsomes from freshly isolated PT cells by immunoblotting using specific antibodies. CYP2B and CYP3A apoprotein could not be detected. Activity of the phase II biotransformation enzymes GST, GGT, beta-lyase and UGT was determined with 1-chloro-2,4-dinitrobenzene, L-glutamic acid gamma-(7-amido-4-methyl-coumarin), S-(1,1,2,2-tetrafluoroethyl)-L-cysteine and 1-naphthol, respectively, as marker substrates. Activity of the phase II enzymes remained more stable and, in contrast to CYP450 activity, significant activity was still expressed after 1 week of PT cell culture. Thus, despite the obvious advantages of PT cells as an in-vitro model for studies of biotransformation mediated toxicity, the strong time dependency of especially phase I and, to a lesser extent, phase II biotransformation activities confers limitations to their application.

The effect of repeat administration of GTS-21 on mixed-function oxidase activities in rat

The effect of repeat administration of GTS-21 on hepatic microsomal enzymes was determined in rats administered the drug at levels of 3, 60 and 300 mg/kg/day for 7 days. Liver weight and cytochrome P450 (CYP) contents were not changed. Cytochrome b5 contents were increased at the mid and high doses of GTS-21, as the contents increased with increasing dose, but were unchanged at the low dose. Five selective activities of CYP isoforms, acetanilide hydroxylase (CYP1A2), tolbutamide hydroxylase (CYP2C6), dextromethorphan O-demethylase (CYP2D1), p-nitrophenol hydroxylase (CYP2E1) and erythromycin N-demethylase (CYP3A) were examined. Enzyme activities were changed only at the highest dose; the activity of CYP1A2 was increased by 71% and the activities of CYP2C6 and CYP2D1 were decreased by 37 and 19%, respectively. At low and mid doses of GTS-21, all activities were unchanged. These data indicate that GTS-21 is not a strong modulator of the mixed-function oxidase system.

Role of CYP3A in bromperidol metabolism in rat in vitro and in vivo

1. The aim was to identify whether CYP3A metabolizes bromperidol (BP), an antipsychotic drug, to form 4-fluorobenzoyl-propionic acid (FBPA) in hepatic microsomes from 8-week-old male Sprague-Dawley rats and to investigate whether an inhibitor or an inducer of CYP3A affects BP pharmacokinetics in rat. 2. In an in vitro study, only troleandomycin showed marked inhibition of FBPA formation among several specific CYP isozyme inhibitors studied including troleandomycin, diethyldithiocarbamate, furafylline and quinine. Anti-rat CYP3A2 serum inhibited FBPA formation by 80%, whereas other anti-rat CYP sera (1A1, 1A2, 2B1, 2C11, 2E1) only slightly inhibited it. 3. In a pharmacokinetic study, BP half-life was prolonged to 137% of the average control value by 7-day treatment with erythromycin, a CYP3A inhibitor, and shortened to 58% of the control by 2-day treatment with dexamethasone, a CYP3A inducer. BP clearance was reduced to 68% of the control by erythromycin and was increased to 145% of control by dexamethasone. 4. These results suggested that BP biotransformation is catalysed mainly by CYP3A to form FBPA in rat and that the modification of this enzyme activity would affect the pharmacokinetics of BP.

Role of CYP3A in haloperidol N-dealkylation and pharmacokinetics in rats

Haloperidol (HP), an antipsychotic drug, is N-dealkylated by cytochrome P450 (CYP) to 4-fluorobenzoylpropionic acid (FBPA). The purpose of this study was to identify whether CYP3A metabolizes HP to FBPA in hepatic microsomes of rats and to investigate whether an inhibitor or an inducer of CYP3A affects HP pharmacokinetics in rats. The rate of FBPA formation was determined in hepatic microsomes from 8-week-old male Sprague-Dawley rats. Among several specific CYP isozyme inhibitors including troleandomycin (TAO), diethyldithiocarbamate, furafylline and quinine, only TAO showed marked inhibition of FBPA formation. Anti-rat CYP3A serum inhibited FBPA formation by 76.4%, while other anti-rat CYP sera (1A1, 1A2, 2B1, 2C11, 2E1) only slightly did. In a pharmacokinetic study, 8-week-old male Sprague-Dawley rats were given 0.5 mg/kg HP intravenously after treatment with 100 mg/kg erythromycin, a CYP3A inhibitor, or 80 mg/kg dexamethasone, a CYP3A inducer, intraperitoneally once a day for 7 days or 2 days, respectively or untreated. HP half-life was prolongated to 171% of the average control value by erythromycin and shortened to 49% of control by dexamethasone. HP clearance was reduced to 63% of control by erythromycin and was increased to 167% of control by dexamethasone. These results suggested that CYP3A mainly catalyzed HP to FBPA in rats, and the modification of this enzyme activity would affect the pharmacokinetics of HP.

Fully automated analysis of activities catalysed by the major human liver cytochrome P450 (CYP) enzymes: assessment of human CYP inhibition potential

1. Fully automated inhibition screens for the major human hepatic cytochrome P450s have been developed and validated. Probe assays were the fluorometric-based ethoxyresorufin O-deethylation for CYP1A2 and radiometric analysis of erythromycin N-demethylation for CYP3A4, dextromethorphan O-demethylation for CYP2D6, naproxen O-demethylation for CYP2C9 and diazepam N-demethylation for CYP2C19. For the radiometric assays > 99.7% of 14C-labelled substrate was routinely extracted from incubations by solid-phase extraction. 2. Furafylline, sulphaphenazole, omeprazole, quinidine and ketoconazole were identified as specific markers for the respective CYP1A2 (IC50 = 6 microM), CYP2C9 (0.7 microM), CYP2C19 (6 microM), CYP2D6 (0.02 microM) and CYP3A4 (0.2 microM) inhibition screens. 3. For the radiometric methods, a two-point IC50 estimate was validated by correlating the IC50 obtained with a full (seven-point) assay (r2 = 0.98, p < 0.001). The two-point IC50 estimate is useful for initial screening, while the full IC50 method provides more definitive quantitation, where required. 4. IC50 determined for a series of test compounds in human liver microsomes and cytochrome P450 cDNA-expressed enzymes were similar (r2 = 0.89, p < 0.001). In particular, the CYP1A2, CYP2D6 and CYP3A4 screens demonstrated the flexibility to accept either enzyme source. As a result of incomplete substrate selectivity, expressed enzymes were utilized for analysis of CYP2C9 and CYP2C19 inhibition. Good agreement was demonstrated between IC50 determined in these assays to IC50 published by other laboratories using a wide range of analytical techniques, which provided confidence in the universality of these inhibition screens. 5. These automated screens for initial assessment of P450 inhibition potential allow rapid determination of IC50. The radiometric assays are flexible, sensitive, robust and free from analytical interference, and they should permit the identification and eradication of inhibitory structural motifs within a series of potential drug candidates.

Prediction of CYP2D6-mediated polymorphic drug metabolism (sparteine type) based on in vitro investigations

Discovery of genetic polymorphism in drug metabolism has contributed a great deal to understanding the variability in dose-concentration relationships introduced by genetic factors, thereby elucidating the mechanisms responsible for unexpected drug reactions. This knowledge should find its way into clinical practice in order to make therapy more efficient and safe. Moreover, genetic factors in drug metabolism should be taken into account during drug development. Therefore, in vitro methods for identifying the metabolic pattern of new compounds during early stages of drug development should be improved. This review summarizes in vitro methods available to identify genetic polymorphism in drug oxidation, in particular the CYP2D6-related polymorphism.

Gliclazide hydroxylation by rat liver microsomes

1. The metabolism of gliclazide to hydroxygliclazide has been investigated in Sprague-Dawley rat liver microsomes. 2. The kinetics of hydroxygliclazide formation are consistent with Michaelis-Menten kinetics (mean (+/- SD, n = 3) apparent K(m) and Vmax = 256 +/- 27 microM and 1.85 +/- 0.10 nmol/ min/mg respectively). 3. Tolbutamide competitively inhibited hydroxygliclazide formation (Ki = 840 microM) and gliclazide competitively inhibited hydroxytolbutamide formation (Ki = 240 microM) with Ki similar to K(m). Therefore gliclazide and tolbutamide may be metabolized by the same enzyme in the rat. In nine livers the formation of hydroxygliclazide correlated with the formation of hydroxytolbutamide (rs = 0.82, p < 0.01). 4. Diclofenac (Ki = 64 microM), phenytoin (Ki = 38 microM), mephenytoin (Ki = 66 microM), glibenclamide (Ki = 14 microM) and glipizide (Ki = 189 microM) were fully competitive inhibitors of gliclazide hydroxylation. The rank order of Ki constants differed for gliclazide and tolbutamide suggesting that gliclazide and tolbutamide hydroxylases are not identical enzymes. 5. Quinine (Ki = 0.3 microM) and quinidine (Ki = 4.3 microM) were partially competitive inhibitors of hydroxygliclazide formation. Hydroxylation of gliclazide was related to the activity of CYP2D1 as assessed by dextrorphan production from dextromethorphan (rs = 0.83, p = 0.01). 6. In the rat gliclazide is metabolized to hydroxygliclazide by at least two cytochrome P450 isoforms, including tolbutamide hydroxylase and 2D1, which have similar affinities for gliclazide.

Effect of cyclophosphamide administration on the activity and relative content of hepatic P4502D1 in rat

1. The effects of the administration of the anticancer and immunosuppressive drug, cyclophosphamide, to the rat on hepatic P4502D1 activity and content in the microsomal fraction have been examined. 2. Liver microsomes were obtained from male Hooded Wistar rats administered a single dose (i.p.) of saline or cyclophosphamide (200 mg/kg). Rats receiving cyclophosphamide were killed 1, 4, 7, 10 or 14 days after cyclophosphamide administration. The O-demethylation of dextromethorphan to dextrorphan was used to monitor 2D1 activity. 3. The mean Vmax for dextrorphan formation was reduced significantly (p < 0.0001) 7, 10 and 14 days after cyclophosphamide administration compared with the control group (control, 0.32 +/- 0.07; 7-day, 0.20 +/- 0.08; 10-day, 0.11 +/- 0.02; and 14-day group, 0.15 +/- 0.02 nmol/mg/min). 4. Western blotting revealed that there was a significant reduction (p < 0.0005) in the microsomal relative 2D1 content 10 days after cyclophosphamide administration compared with the control group (control, 1.25 +/- 0.44; and 10-day group, 0.65 +/- 0.14). 5. The activity of reduced nicotinamide adenine dinucleotide phosphate P450 reductase was significantly reduced (p < 0.0001) 7, 10 and 14 days following cyclophosphamide administration (control, 215 +/- 24; 7-day, 102 +/- 20; 10-day, 59 +/- 4 and 14-day group, 76 +/- 8 nmol/mg/min). Cytochrome b5 content was significantly reduced (p < 0.0001) 7 and 10 days following cyclophosphamide administration (control, 0.46 +/- 0.13; 7-day, 0.28 +/- 0.07 and 10-day group, 0.20 +/- 0.03 nmol/mg). 6. The significant reductions in the activity of rat hepatic microsomal 2D1 following cyclophosphamide administration, as seen by the alterations in mean Vmax for dextrorphan formation, do not appear to be due to a single factor, but may result from a combination of several events, including reductions in relative 2D1 content, reduced nicotinamide adenine dinucleotide phosphate P450-reductase activity and cytochrome b5 content.

A radiometric TLC assay of liver microsomal dextromethorphan O-demethylation

A simple and sensitive assay for in vitro analysis of dextromethorphan O-demethylation, a marker for P450 2D deficiency in both humans (2D6) and rats (2D1), has been devised. Commercially available [N-methyl-3H]-dextromethorphan was used to develop a radiometric TLC assay for dextromethorphan O-demethylation. Hexane-triethylamine efficiently extracted dextromethorphan and metabolites from rat liver microsomes, and a solvent system of cyclohexane-toluene-diethylamine (65:15:20, v/v/v) provided sufficient separation (approximately 2 cm) between the two radioactive bands, dextromethorphan and dextrorphan, and no interference from the unlabeled N-demethylation products, 3-methoxymorphinan and 3-hydroxymorphinan. The recovery of dextrorphan from TLC plates increases with microsomal protein and incubation time. An eight-fold decrease in activity was noted in female Dark Agouti relative to the male Sprague-Dawley rats, respective models for poor and extensive P450 2D metabolizers. The assay, even with an approximately 100-fold dilution of radiolabeled substrate, had an approximate limit of detection of 100 pmol. Within- and between-run imprecision was 12.4% and 7.2%, respectively. The radiometric TLC assay for dextromethorphan O-demethylation was sensitive and easy, and used readily available equipment.

Ethylmorphine O-deethylation in isolated rat hepatocytes. Involvement of codeine O-demethylation enzyme systems

The O-dealkylation of ethylmorphine (EM) and codeine (CD) to morphine (M) co-segregates with debrisoquine/sparteine genetic polymorphism in man. CD O-demethylation is catalysed by cytochrome P450 2D1 (CYP2D1) in rats. In the present study, the O-deethylation of EM was examined and compared with that of CD in suspensions of freshly-isolated hepatocytes prepared by a collagenase method from Wistar rats with and without CYP2D1 inhibitors. Isolated hepatocytes were also prepared from Dark Agouti (DA) rats deficient in CYP2D1, and were incubated with EM or CD. EM, CD and their metabolites were quantified by HPLC with UV detection. EM had a similar pattern of metabolism to that of CD in suspensions of hepatocytes from Wistar rats. Both EM and CD were O-dealkylated to form M plus morphine-3-glucuronide (M3G) and N-demethylated to form norethylmorphine (NEM) or norcodeine (NCD), respectively, which were further metabolized to normorphine (NM) and finally glucuronidated to normorphine-3-glucuronide (NM3G). As compared to hepatocytes from Wistar rats, DA rats were characterized by a markedly decreased formation (70 approximately 75% reduction) of M plus M3G from both EM and CD. Quinine, quinidine, propafenone and sparteine all inhibited EM O-deethylation as well as CD O-demethylation. Quinine was the most potent inhibitor of both these O-dealkylations (Ki = 0.2 microM for both EM and CD, respectively). Quinine as well as the other inhibitors inhibited both EM and CD O-dealkylation competitively and with small differences in Ki versus EM and CD, respectively. The metabolism of EM to M plus M3G and that of CD to M plus M3G was highly correlated when results from the various separate cell suspensions were plotted. In conclusion all findings indicated that the enzyme responsible for O-demethylation of CD, CYP2D1 was also responsible for the O-deethylation of EM to M.

The role of CYP2D6 in primary and secondary oxidative metabolism of dextromethorphan: in vitro studies using human liver microsomes

1. The enzyme kinetics of dextromethorphan O-demethylation in liver microsomes from three extensive metabolisers (EM) with respect to CYP2D6 indicated high (Km1 2.2-9.4 microM) and low (Km2 55.5-307.3 microM) affinity sites whereas microsomes from two poor metabolisers (PM) indicated a single site (Km 560 and 157 microM). Similar differences were shown for 3-methoxymorphinan O-demethylation to 3-hydroxymorphinan (Km 6.9-9.6 microM in EM subjects; Km 307 and 213 microM in PM subjects). 2. Dextromethorphan O-demethylation was inhibited competitively by quinidine (Ki 0.1 microM), rac-perhexiline (Ki 0.4 microM), dextropropoxyphene (Ki 6 microM), rac-methadone (Ki 8 microM), and 3-methoxymorphinan (Ki 15 microM). These compounds were also potent inhibitors of 3-methoxymorphinan O-demethylation with IC50 values ranging from 0.02-12 microM. Anti-LKM1 serum inhibited both dextromethorphan and 3-methoxymorphinan O-demethylations in a titre-dependent manner. 3. The Michaelis-Menten constant for dextromethorphan N-demethylation to 3-methoxymorphinan (Km 632-977 microM) and dextrorphan N-demethylation to 3-hydroxymorphinan (Km 1571-4286 microM) did not differ between EM and PM microsomes. These N-demethylation reactions were not inhibited by quinidine and rac-methadone or LKM1 antibodies. 4. Dextromethorphan and 3-methoxymorphinan are metabolised by the same P450 isoform, CYP2D6, whereas the N-demethylation reactions are not carried out by CYP2D6.

Dextromethorphan metabolism in rat: interstrain differences and the fate of individually administered oxidative metabolites

1. Dextromethorphan undergoes O- and N-demethylation, with the resultant metabolites being further N- and O-demethylated respectively to 3-hydroxymorphinan. The polymorphically expressed O-demethylation reaction is catalysed by P4502D1 in the Sprague-Dawley (SD) rat. The Dark-Agouti (DA) rat lacks this enzyme. 2. The aims were: (1) to determine if there were strain differences also in the Hooded Wistar (HW) and Albino Wistar (AW) rats with respect to the four demethylation reactions after dextromethorphan 20 mg/kg intraperitoneally; (2) to investigate the inhibition of the demethylation reactions by quinine and quinidine (each 40 mg/kg i.p.) in the above strains; and (3) to investigate the fate of separately administered metabolites (5 mg/kg i.p.) of dextromethorphan in the SD strain. 3. The total recovery of dextromethorphan and metabolites in the four strains ranged from 38 to 64% of the dose. The O-demethylation ratios (expressed as the ratio of urinary total dextrorphan divided by dextromethorphan) in the AW and DA strains were similar but less than in the SD/HW strains; the N-demethylation ratios (expressed as the ratio of urinary total 3-hydroxymorphinan plus 3-methoxymorphinan divided by dextromethorphan) in the DA and SD strains were similar but greater than in the AW and HW strains. Quinine and quinidine significantly reduced the O-demethylation ratio in the SD and DA rat strains, and the N-demethylation ratio in the SD strain. 4. In the SD rat the major metabolic route was via O-demethylation to dextrorphan. The source of 3-hydroxymorphinan is primarily from N-demethylation of dextromethorphan to 3-methoxymorphinan and its subsequent O-demethylation to 3-hydroxymorphinan. The O-demethylation metabolic ratio for dextromethorphan should be calculated as the quotient of urinary total dextrorphan divided by dextromethorphan.