Inhibition Of CYP Enzymes Research Articles

CYP450 enzyme inhibition of berberine in pooled human liver microsomes by cocktail probe drugs

To investigate the effect of CYP450 enzyme inhibition of berberine in pooled human liver microsomes by cocktail probe drugs. Cocktail probe drugs method has been established, an LC-MS/MS analytical method has been established to determine the five probes of midazolam, phenacetin, dextromethorphan, tolbutamide, chlorzoxazone and the internal standard was benzhydramine to evaluate the effect of CYP450 activity following administration of berberine in pooled human liver microsomes. Compared with control group, the pharmacokinetics of midazolam, phenacetin and tolbutamide were no significant differences, but the pharmacokinetics of chlorzoxazone was significantly decreased. There were no significant differences for the pharmacokinetics of dextromethorphan when the concentration of berberine was 50 microg x L(-1). The pharmacokinetics of dextromethorphan was significantly decreased when the concentration of berberine was exceed 200 microg x L(-1). Berberine has no influence on the activities of CYP3A4, CYP1A2 and CYP2C19 below 2 000 microg x L(-1), but can inhibit the activity of CYP2E1 and CYP2D6 in concentration-dependent.

The relationship between clinical pharmacokinetics of aripiprazole and CYP2D6 genetic polymorphism: effects of CYP enzyme inhibition by coadministration of paroxetine or fluvoxamine

PurposeTo investigate the effects of coadministration of paroxetine or fluvoxamine on the pharmacokinetics of aripiprazole in healthy adult Japanese with different CYP2D6 genotypes.MethodsFourteen CYP2D6 extensive metabolizer (EM) and 14 CYP2D6 intermediate metabolizer (IM) subjects were coadministered a single oral dose of aripiprazole 3 mg after steady-state plasma concentrations of the SSRIs paroxetine (20 mg/day) or fluvoxamine (100 mg/day) were reached by repeated oral doses for 6–7 days. The pharmacokinetics of aripiprazole with and without coadministration of SSRIs were compared according to CYP2D6 genotypes.ResultsCoadministration of paroxetine, a potent CYP2D6 inhibitor, decreased systemic clearance (CL/F) of aripiprazole by 58 and 23% in CYP2D6 EMs and IMs, respectively, demonstrating that the percentage inhibition of CYP2D6 activity by coadministration of paroxetine was apparently greater in CYP2D6 EMs than in IMs. Coadministration of fluvoxamine, a less potent CYP3A4 inhibitor, decreased the CL/F of aripiprazole by 39% in CYP2D6 EMs and 40% in IMs, indicating the same inhibitory effect on CYP enzymes, regardless of the CYP2D6 genotype. Percent contribution of CYP2D6 to total CL/F (CYP2D6 plus CYP3A4) of aripiprazole estimated as a reduced percentage of CL/F by CYP enzyme inhibition was 62% for CYP2D6 EMs and 24% for IMs in paroxetine coadministration, and 40% for CYP2D6 EMs and 18% for IMs in fluvoxamine coadministration.ConclusionsThere were marked differences in the degree of influence of paroxetine coadministration on the pharmacokinetics of aripiprazole between CYP2D6 EMs and IMs, but no apparent differences were found between two CYP2D6 genotypes in fluvoxamine coadministration. Aripiprazole can be used safely in combination with SSRIs that have a CYP enzyme-inhibitory action.

Discovery of potent and selective histamine H 3 receptor inverse agonists based on the 3,4-dihydro-2 H-pyrazino[1,2- a]indol-1-one scaffold

A novel series of potent histamine H 3 receptor inverse agonists based on the 3,4-dihydro-2 H-pyrazino[1,2- a]indol-1-one scaffold has been discovered. Several compounds display high selectivity over other histamine receptor subtypes and have favorable physicochemical properties, low potential for CYP450 enzyme inhibition and high metabolic stability in microsomal preparations. ( R)-2-Cyclopropylmethyl-8-(1-isopropyl-piperidin-4-yloxy)-3-methyl-3,4-dihydro-2 H-pyrazino[1,2- a]indol-1-one ( 8t) showed good in vivo efficacy after per os application in an acute rat dipsogenia model of water intake.

Abstract 3785: Isothiocyanates vs. isoselenocyanates: Reactivity toward glutathione and cysteinyl residues in cell and cell-free systems

Abstract Isothiocyanates (ITCs) are naturally occurring compounds found in cruciferous vegetables. Both naturally occurring phenylalkyl ITCs (BITC, PEITC) and longer chain synthetic ITCs (PBITC, PHITC) have been studied extensively and are shown to inhibit carcinogenesis due to inhibition of CYP450 enzymes, as well as induction of Phase II detoxification enzymes. These actions are thought to be mediated by the reactivity of the ITC functional group (N=C=S) with sulfhydryl groups of cell thiols, mainly glutathione and proteins (cysteinyl residues). Our lab has modified both the naturally occurring and synthetic phenyl alkyl ITCs by isosterically replacing sulfur with selenium to make isoselenocyanates (ISCs): ISC1, ISC2, ISC4, and ISC6. We have found these compounds to be more potent in cancer cell viability assays and in xenograft mouse model at inhibiting tumor growth than corresponding ITCs. Our hypothesis is the better potency of ISC compounds may be due to their modified reactivity rates toward cell thiols. To test this, A549 lung cancer cells were treated with our panel of ITC compounds or corresponding ISC compounds from 500 nM to 15 µM for 1 hour. GSH depletion appeared to be dose dependent for all compounds except for hexyl chain compounds PHITC and ISC6. For the remaining compounds, ISC compounds depleted GSH in cells to a greater extent than corresponding ITCs. 15 µM of ISC1, ISC2, and ISC4 depleted GSH levels to 58%, 53%, and 63% of control, respectively, whereas BITC, PEITC, and PBITC only depleted GSH to 69%, 86%, and 86%, respectively. These results may be related to redox cycling capabilities which measures the reactivity of compounds with GSH and O2 to form superoxide. In a chemiluminesence assay measuring redox cycling, ISC compounds showed a much greater ability to redox cycle compared to corresponding ITCs. The redox cycling capabilities were as follows: ISC2≫PEITC>ISC1=ISC4=ISC6>PHITC>PBITC>BITC. In a cell free system, overall binding of ITC and ISC compounds were analyzed for GSH using the Ellman assay. In this cell free system both ITCs and ISCs showed a trend of increasing binding with decreasing chain length: BITC>PEITC>PBITC>PHITC and ISC1>ISC2>ISC4>ISC6. However, corresponding ITC and ISC compounds bound similarly to GSH. Similar experiment using tubulin protein exhibited the same trend of reactivity with ITCs, as with GSH. However, among ISC compounds, ISC2 showed the greatest binding followed by ISC1=ISC4>ISC6. These results suggest that cell environment is playing a role in the reactivity of these compounds with glutathione and proteins. In conclusion, ISC compounds depleted GSH in cells to a greater extent than corresponding ITC compounds and were also better able to redox cycle. Future studies will analyze the exact rate of reactivity of ISC compounds toward GSH, identifying protein targets of these compounds, and binding preferences of different proteins. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3785.

Impact of Ritonavir, Atazanavir and Their Combination on the CYP3A4 Induction by Efavirenz in Primary Human Hepatocytes

Currently used combinations of anti-HIV drugs, known as Highly Active Antiretroviral Therapy (HAART), have considerably reduced the mortality in patients with AIDS. However, HAART medications such as efavirenz (EFV), atazanavir (ATV) and ritonavir (RTV) often cause adverse drug-drug interactions (DDIs) that result from changes in the expression and activity of drug metabolizing enzymes. Since EFV is most commonly used with ATV and RTV, the known CYP inhibitors, we evaluated the effects of combinations of these agents on the CYP3A4 induction by EFV. We determined the induction of CYP3A4 by EFV, RTV, ATV, EFV+RTV, EFV+ATV, EFV+RTV+ATV and rifampicin (RIF) employing primary human hepatocytes from 3 donors. Also, concentration dependent activation of human Pregnane X-receptor (hPXR) which is key transcriptional regulator of CYP3A4 by EFV, RIF and RTV was estimated in transiently transfected LS180 cells. CYP3A4 activity (testosterone-6beta-hydroxylation) was induced by EFV (3 fold) and RIF (4 fold), but was significantly suppressed in the presence of RTV and ATV. All treatments significantly induced the CYP3A4 transcripts (3-25 fold) as quantitated by RT-PCR. hPXR activation data in LS180 cells were consistent with the induction of transcripts and the estimated EC(50) values were 0.87 microM, 0.44 microM and 3.7 microM for RIF, RTV and EFV, respectively. However, in primary hepatocytes the net effect was suppression of EFV mediated CYP3A4 induction by RTV and ATV. This observation corresponds to the clinical observations of attenuated CYP3A4 induction by EFV induction in the presence of RTV and other protease inhibitors (PIs). Our results underscore the limitation of transcriptional activation assays in predicting the net outcome for compounds that exhibit complex interactions resulting from induction and inhibition of CYP enzymes.

Oxadiazolylindazole Sodium Channel Modulators are Neuroprotective toward Hippocampal Neurones

We report the discovery of a new class of neuroprotective voltage-dependent sodium channel modulators exemplified by (5-(1-benzyl-1H-indazol-3-yl)-1,2,4-oxadiazol-3-yl)methanamine 11 (CFM1178). The compounds were inhibitors of [(14)C]guanidinium ion flux in rat forebrain synaptosomes and displaced binding of the sodium channel ligand [(3)H]BW202W92. 11 and the corresponding N(2)-benzyl isomer, 38 (CFM6058), demonstrated neuroprotective activity in hippocampal slices comparable to sipatrigine. CYP450 enzyme inhibition observed with 11 was reduced with 38. In electrophysiological experiments on dissociated hippocampal neurons, these two compounds caused use- and voltage-dependent block of sodium currents. Sodium channel isoform profiling against Na(v)1.1-1.8 demonstrated that the standard sodium channel blocker lamotrigine had modest activity against Na(v)1.1, while sipatrigine was generally more potent and less selective. 11 and 38 showed potent activity against Na(v)1.6, pointing to pharmacological block of this isoform being consistent with the neuroprotective effect. 38 also showed use dependent block of Na(v)1.6 in HEK cells.

Sulforaphane and its analogues inhibit CYP1A1 and CYP1A2 activity induced by benzo[a]pyrene

CYP1A1 and CYP1A2 enzymes metabolize polycyclic aromatic hydrocarbons (PAHs) to the reactive oxyderivatives. PAHs can induce the activity of both enzymes, which increases its conversion and enhances risk of carcinogenesis. Thus, the inhibition of CYP enzymes is recognized as a cancer chemoprevention strategy. A well-known group of chemopreventive agents is isothiocyanates, which occur naturally in Brassica vegetables. In this paper, a naturally occurring sulforaphane and its two synthetic analogues isothiocyanate-2-oxohexyl and alyssin were investigated. The aim of the study was to determine whether the differences in the isothiocyanate structure change its ability to inhibit CYP1A1 and CYP1A2 activity induced by benzo[a]pyrene in HepG2 and Mcf7 cells. Also a mechanistic study was performed including isothiocyanates' influence on CYP1A1 and CYP1A2 catalytic activity, enzymatic protein level, and AhR translocation. It was shown that both enzymes were significantly induced by benzo[a]pyrene, and isothiocyanates were capable of decreasing the induced activity. The inhibitory properties depend on the types of isothiocyanate and enzyme. In general, CYP1A2 was altered in the more meaningful way than CYP1A1 by isothiocyanates. Sulforaphane exhibited weak inhibitory properties, whereas both analogues were capable of inhibiting BaP-induced activity with the similar efficacy. The mechanistic study revealed that analogues decreased the CYP1A2 activity via the protein-level reduction and CYP1A1 directly. The results indicate that isothiocyanates can be considered as potent chemopreventive substances and the change in the sulforaphane structure increases its chemopreventive potency.

Pharmacovigilance: effects of herbal components on human drugs interactions involving cytochrome P450.

Cytochrome P450 (CYP P450) enzymes are a superfamily of mono-oxygenases that are found in all kingdoms of life. The CYP P450 enzymes constitute a large superfamily of haem-thiolate proteins involved in the metabolism of a wide variety of both exogenous and endogenous compounds. The CYP activities have been shown to be involved in numerous interactions especially between drugs and herbal constituents. The majority of serious cases of drug interactions are as a result of the interference of the metabolic clearance of one drug by yet another co-administered drug, food or natural product. Gaining mechanistic knowledge towards such interactions has been accepted as an approach to avoid adverse reactions. The inductions and inhibition of CYP enzymes by natural products in the presence of a prescribed drug has led to adverse effects. Herbal medicines such as St. John's wort (Hypericum perforatum), garlic (Allium sativa), piperine (from Piper sp.), ginseng (Ginseng sp.), gingko (Gingko biloba), soya beans (Glycine max), alfalfa (Medicago sativa) and grape fruit juice show clinical interactions when co-administered with medicines. This review documents the involvement of CYP enzymes in the metabolism of known available drugs and herbal products. We also document the interactions between herbal constituents & CYP enzymes showing potential drug-herb interactions. Data on CYP450 enzymes in activation (i.e. induction or inhibition) with natural constituents is also reviewed.

Granatapfelpolyphenole gegen Prostatakarzinom

Granatapfelpolyphenole zeigten in In-vitro-, In-vivo- und klinischen Studien eine überzeugende Hemmung der Kanzerogenese (Initiation, Promotion, Progression), des Wachstums und der Invasion durch antiangiogenetische, redifferenzierende, antiproliferative und proapoptotische Effekte. Hierfür verantwortlich ist das natürliche Zusammenspiel der antiinflammatorischen, antioxidativen, phytohormonellen Polyphenole, die das Immunsystem, das Epigenom und die Zellsignalwege modulieren. Bei einer großen Vielzahl präklinischer Studien gibt es bislang nur eine kontrollierte klinische Studie mit Prostatakrebspatienten, die eine durchschnittliche Vervierfachung der PSA-Verdoppelungszeit ergab. Besonders interessant sind neue präklinische Ergebnisse, die auf mögliche Synergieeffekte mit Hormonablation, Chemo- und Strahlentherapie bei der Therapie des hormonrefraktären Prostatakarzinoms hinweisen. Zu beachten sind Unterschiede in Bezug auf Qualität und Polyphenolgehalt von Granatapfelprodukten, wobei fermentierte Granatapfelpolyphenole eine höhere Bioaktivität und vermutlich auch Bioverfügbarkeit aufweisen. Insbesondere werden neue Wirkmechanismen in Bezug auf die Hemmung von CYP-Enzymen, die mögliche Konzentrationsteigerung von 3β-Adiol und Aktivierung des antiproliferativen Östrogenrezeptors β sowie eine Antagonisierung des proliferativen Östrogenrezeptors α durch Granatapfelpolyphenole vorgestellt.

Generalized Proteochemometric Model of Multiple Cytochrome P450 Enzymes and Their Inhibitors

Cytochrome P450 enzymes are a superfamily of heme-containing enzymes responsible for the oxidation of structurally diverse chemical compounds. Inhibition of CYP enzymes is probably the most common mechanism underlying acute drug toxicity, loss of therapeutic drug efficacy, and drug-drug interactions. The presence of polymorphic genetic variants of CYPs among the population makes it difficult to foresee undesired effects of drugs and is a common cause of drug candidate failure. Computational models that can predict early drug failures due to the inhibition of CYP isoforms can substantially reduce the cost of drug development. Although several computational models for CYP inhibition have been developed recently, all were constructed for one CYP isoform at a time, thus limiting their use for comprehensive analysis and generalizations to other CYP isoforms and polymorphisms. Here we report a novel approach based on the principles of proteochemometrics for the generalized concomitant modeling of multiple CYP isoforms and their inhibitors. We created a predictive and statistically valid proteochemometric model for CYP enzymes by combining data from a large number of publicly available reports that describe the interactions of 14 CYP enzyme subtypes and 375 structurally diverse inhibitors. Our results demonstrate that our model is capable of predicting the potential of new drug candidates to inhibit multiple CYP enzymes. Analysis of the CYP model also revealed molecular properties of CYP enzymes and xenobiotics that are important for CYP inhibition. This approach may aid in the selection of novel drug candidates that are unlikely to inhibit multiple CYP subtypes.

Cytochrome P450 Inhibition Assays Using Traditional and Fluorescent Substrates

A key liability in transitioning a new chemical entity (NCE) to a development candidate is NCE-related inhibition (or induction) of cytochrome P450 enzymes, a superfamily of heme-containing oxygenases that are the major route of first-pass metabolism for the majority of marketed drugs. The drawback of a drug/NCE that modulates CYP450 enzyme activity occurs when the compound is co-administered with another drug that relies on the same P450 enzyme for its metabolism. This could result in overdose of the second drug in the case of inhibition, or more rapid metabolism of one or both drugs accompanied by loss of efficacy in the case of enzyme induction. Screening for the inhibition of CYP450 enzymes is now routine in the early stages of evaluating NCEs. This unit describes two inhibition assays using traditional and fluorescent substrates.

Postulated pathogenic pathway in triazole fungicide induced dysmorphogenic effects

Triazole fungicides are used in medicine as well as in agricultural treatment of mycoses. The pharmacological mechanism is related to the inhibition of CYP enzymes involved in the formation of the fungal walls. A similar inhibition of human CYP enzymes has been suggested as the cause of triazole side effects in humans. An importent role of some CYP isoforms (CYP26 isoforms) expressed during mammalian development is the catabolism of retinoic acid, a known morphogen in vertebrates and invertebrates. The adverse effects on morphogenesis, observed after exposure of mammalian, anfibian and ascidiacea, are compared to the reported effects of triazole in humans. The possible pathogenic pathway in triazole-related teratogenesis is discussed on the basis of different experimental approaches.

Metabolism of Repaglinide by CYP2C8 and CYP3A4 in vitro: Effect of Fibrates and Rifampicin

Repaglinide is an antidiabetic drug metabolised by cytochrome P450 (CYP) 2C8 and CYP3A4 enzymes. To clarify the mechanisms of observed repaglinide drug interactions, we determined the contribution of the two enzymes to repaglinide metabolism at different substrate concentrations, and examined the effect of fibrates and rifampicin on CYP2C8, CYP3A4 and repaglinide metabolism in vitro. We studied repaglinide metabolism using pooled human liver microsomes, recombinant CYP2C8 and recombinant CYP3A4 enzymes. The effect of quercetin and itraconazole on repaglinide metabolism, and of gemfibrozil, bezafibrate, fenofibrate and rifampicin on CYP2C8 (paclitaxel 6alpha-hydroxylation) and CYP3A4 (midazolam 1-hydroxylation) activities and repaglinide metabolism were studied using human liver microsomes. At therapeutic repaglinide concentrations (<0.4 microM), CYP2C8 and CYP3A4 metabolised repaglinide at similar rates. Quercetin (25 microM) and itraconazole (3 microM) inhibited the metabolism of 0.2 microM repaglinide by 58% and 71%, and that of 2 microM repaglinide by 56% and 59%, respectively. The three fibrates inhibited CYP2C8 (Ki: bezafibrate 9.7 microM, gemfibrozil 30.4 microM and fenofibrate 92.6 microM) and repaglinide metabolism (IC50: bezafibrate 37.7 microM, gemfibrozil 111 microM and fenofibrate 164 microM), but had no effect on CYP3A4. Rifampicin inhibited CYP2C8 (Ki 30.2 microM), CYP3A4 (Ki 18.5 microM) and repaglinide metabolism (IC50 13.7 microM). In conclusion, both CYP2C8 and CYP3A4 are important in the metabolism of therapeutic concentrations of repaglinide in vitro, but their predicted contributions in vivo are highly dependent on the scaling factor used. Gemfibrozil is only a moderate inhibitor of CYP2C8 and does not inhibit CYP3A4; inhibition of CYP-enzymes by parent gemfibrozil alone does not explain its interaction with repaglinide in vivo. Rifampicin competitively inhibits both CYP2C8 and CYP3A4, which can counteract its inducing effect in humans.

Effects of Two Fungicides with Multiple Modes of Action on Reproductive Endocrine Function in the Fathead Minnow (Pimephales promelas)

Many chemicals that adversely affect reproduction and/or development do so through multiple pathways within the reproductive tract and hypothalamic-pituitary-gonadal axis. Notable in this regard are fungicides, such as prochloraz or fenarimol, which in mammals have the potential to impact endocrine function through inhibition of CYP enzymes involved in steroid metabolism, as well as through antagonism of the androgen receptor(s). The objective of our studies was to assess the effects of prochloraz and fenarimol on reproductive endocrine function in a model small fish species, the fathead minnow (Pimephales promelas), using both in vitro and in vivo assays. The two fungicides inhibited in vitro CYP19 aromatase activity in brain and ovarian homogenates from the fish, with prochloraz exhibiting a greater potency than fenarimol. Prochloraz and fenarimol also bound competitively to the cloned fathead minnow androgen receptor expressed in COS-1 cells. The two fungicides significantly reduced fecundity of the fish in a 21-day reproduction assay at water concentrations of 0.1 (prochloraz) and 1.0 (fenarimol) mg/l. The in vivo effects of prochloraz on plasma steroid (17beta-estradiol, testosterone, 11-ketotestosterone) and vitellogenin (an estrogen-responsive protein) concentrations, as well as on gonadal histopathology, were consistent with inhibition of steroidogenesis. Fenarimol also affected several aspects of endocrine function in vivo; however, the suite of observed effects did not reflect either aromatase inhibition or androgen receptor antagonism. These studies contribute to a better mechanistic understanding of the extrapolation of effects of endocrine-disrupting chemicals across vertebrate classes.

Clinical significance of pharmacokinetic interactions between antiepileptic and psychotropic drugs.

As antiepileptic drugs (AEDs) and psychotropic agents are increasingly used in combination, the possibility of pharmacokinetic interactions between these compounds is relatively common. Most pharmacokinetic interactions between AEDs and psychoactive drugs occur at a metabolic level, and usually involve changes in the activity of the cytochrome P450 mixed-function oxidases (CYP) involved in their biotransformation. As a consequence of CYP inhibition or induction, plasma concentrations of a given drug may reach toxic or subtherapeutic levels, and dosage adjustments may be required to avoid adverse effects or clinical failure. Enzyme-inducing AEDs, such as carbamazepine (CBZ), phenytoin (PHT), and barbiturates, stimulate the oxidative biotransformation of many concurrently prescribed psychotropics. In particular, these AEDs may decrease the plasma concentrations of tricyclic antidepressants, many antipsychotics, including traditional compounds, i.e., haloperidol and chlorpromazine, and newer agents, i.e., clozapine, risperidone, olanzapine, quetiapine, and ziprasidone, and some benzodiazepines. Conversely, new AEDs appear to have a lower potential for interactions with all psychotropic drugs. While antipsychotics and anxiolytics do not significantly influence the pharmacokinetics of most AEDs, some newer antidepressants, such as viloxazine, fluoxetine, and fluvoxamine, may lead to higher serum levels of some AEDs, namely CBZ and PHT, through inhibition of CYP enzymes. No significant pharmacokinetic interactions have been documented between AEDs and lithium. Information about CYP enzymes responsible for the biotransformation of individual agents and about the effects of these compounds on the activity of specific CYP enzymes may help in predicting and avoiding clinically significant interactions. Apart from careful clinical observation, serum level monitoring of AEDs and psychotropic drugs can be useful in determining the need for dosage adjustments, especially if there is any change in seizure control, or possible toxicity.

The inhibition of CYP enzymes in mouse and human liver by pilocarpine

1. Pilocarpine is a cholinomimetic natural alkaloid. Its interactions with testosterone hydroxylations, coumarin 7-hydroxylase (COH), dimethylnitrosamine N-demethylase (DMNA), pentoxyresorufin O-dealkylase (PROD) and 7-ethoxyresorufin O-deethylase (EROD), which are indicative of the activities of cytochrome P4502A5 (CYP2A5) or 6, 2E1, 2B, 1A, were examined in mouse and human liver microsomes. 2. In mouse liver microsomes the IC50 values of pilocarpine were 6 microM for COH and testosterone 15 alpha-hydroxylase (T15 alpha OH) activities, 4 microM for PROD, approximately 100 microM for DMNA and testosterone 6 beta-hydroxylase (T6 beta OH) activities and > 1 mM for EROD activity. 3. In human liver microsomes, the IC50 value for COH was 6 microM and for DMNA 10 microM; T15 alpha OH and PROD activities were not detectable but T6 beta OH and testosterone 16 beta/2 beta-hydroxylase activities were moderately inhibited (IC50 70 microM). 4. These results suggest that pilocarpine has (i) a high affinity towards phenobarbitone-inducible CYP2A4/5 and CYP2B activities in mouse liver, (ii) a high affinity towards CYP2A6 in human liver microsomes and (iii) a moderate affinity towards CYP3A enzyme(s) in both microsomal preparations. 5. The low IC50 concentrations in vitro indicate potential metabolic interactions between pilocarpine and several P450 enzymes.