Characterization of six in vitro reactions mediated by human cytochrome P450: application to the testing of cytochrome P450-directed antibodies

Amalgamation of in-silico, in-vitro and in-vivo approach to establish glabridin as a potential CYP2E1 inhibitor

CYP2E1 is directly or indirectly involved in the metabolism of ethanol and endogenous fatty acids but it plays a major role in the bio-activation of toxic substances that produce reactive metabolites leading to hepatotoxicity. Therefore, identification of CYP2E1 inhibitor from bioflavonoids class having useful pharmacological properties has dual benefit regarding avoidance of severe food-drug/nutraceutical-drug interaction and scope to develop a phytotherapeutics through an intended pharmacokinetic interaction.In the present study, we aimed to identify CYP2E1 inhibitor from experimental bioflavonoids which are unexplored for CYP2E1 inhibition till date using in-silico, in-vitro and in-vivo approaches.Results of in-vitro CYP2E1 inhibitory studies using CYP2E1-mediated chlorzoxazone 6-hydroxylation in human liver microsomes showed that glabridin have the highest potential than fisetin, epicatechin, nobiletin, and chrysin to inhibit CYP2E1 enzyme. Mechanistic investigations indicate that glabridin is a competitive CYP2E1 inhibitor. Molecular docking study results demonstrate that glabridin strongly interacted with the active site of human CYP2E1 enzyme. Pharmacokinetics of a CYP2E1 substrate in mice model indicates a significant alteration of chlorzoxazone and 6-hydroxychlorzoxazone plasma levels in the presence of glabridin. Further studies are needed to confirm the results at clinical level.Overall, glabridin is found to be a potential CYP2E1 inhibitor.

The inhibitory activity of the extracts of popular medicinal herbs on CYP1A2, 2C9, 2C19 and 3A4 and the implications for herb-drug interaction

BACKGROUND

Studies have suggested an increasing practice of concurrent herb-drug consumption. One of the major clinical risks of such concomitant herb-drug use is pharmacokinetic herb-drug interaction (HDI). This is brought about by the ability of phytochemicals to inhibit or induce the activity of metabolic enzymes. The aim of this study was to investigate the potential of the crude aqueous extracts of three popular medicinal herbs used in South Africa to inhibit major cytochrome P450 (CYP) enzymes.

MATERIALS AND METHODS

The extracts of Bowiea volubilis, Spirostachys africana and Tulbaghia violacea were incubated with human liver microsomes (HLM) to monitor the phenacetin O-deethylation, diclofenac 4'-hydroxylation, S-mephenytoin 4'-hydroxylation and testosterone 6β-hydroxylation as respective probe reactions for CYP1A2, CYP2C9, CYP2C19 and CYP3A4. The inhibitory activity, where observed, was profiled against the extract concentration.

RESULTS

Extracts of Bowiea volubilis inhibited the metabolic activity of CYP1A2 and CYP3A4 with IC50 values of 92.3 ± 5.5 µg/mL and 8.1 ± 0.6 µg/mL respectively. Similar observation with Spirostachys africana showed inhibitory activity against CYP1A2 and CYP3A4 with respective IC50 values of 14.3 ± 0.6 µg/mL and 47.4 ± 2.4 µg/mL. Tulbaghia violacea demonstrated relatively weak inhibitory activity against CYP1A2 (767.4 ± 10.8 µg/mL) and CYP2C9 (921 ± 15.3 µg/mL).

CONCLUSION

The results suggest the potential for HDI between the herbs and the substrates of the affected enzymes, if sufficient in vivo concentration is attained.

The potential of Hypoxis hemerocallidea for herb-drug interaction

CONTEXT

Aqueous decoction of Hypoxis hemerocallidea Fisch. & C.A. Mey. (Hypoxidaceae) (Hypoxis) is widely consumed in Southern Africa by people living with HIV/AIDS, some of whom are on ARV and other medications.

OBJECTIVE

The aim of this study was to investigate the potential of the crude aqueous extracts of Hypoxis to inhibit major forms of CYP450 and transport proteins.

MATERIALS AND METHODS

Corms of Hypoxis were water-extracted and incubated (in graded concentrations: 1-100 µg/mL) with human liver microsomes (20 min) to monitor the effects on phenacetin O-deethylation, coumarin 7-hydroxylation, bupropion hydroxylation, paclitaxel 6α-hydroxylation, diclofenac 4'-hydroxylation, S-mephenytoin 4'-hydroxylation, bufuralol 1'-hydroxylation, chlorzoxazone 6-hydroxylation, midazolam 1'-hydroxylation and testosterone 6β-hydroxylation as markers for the metabolic activities of CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4/5, respectively. The generation of metabolites were monitored and quantified with the aid of LC-MS/MS. The potential of the extracts to inhibit human ATP-binding cassette transporter activity was assessed using recombinant MDCKII and LLC-PK1 cells over-expressing human breast cancer resistant protein and human P-glycoprotein , respectively (with Ko143 and cyclosporin A as positive controls). Similar assessment was performed with human organic anion transporting polypeptide (OATP1B1 and OATP1B3) using recombinant HEK293 cells over-expressing OATP1B1 and OATP1B3, respectively (with rifamycin and 10 µM atorvastatin as positive controls).

RESULTS

Extracts of Hypoxis inhibited the production of the metabolites of the substrates of the following enzymes (as compared to controls) with the indicated IC50 values (µg/mL): CYP1A2 (120.6), CYP2A6 (210.8), CYP2B6 (98.5), CYP2C8 (195.2), CYP2C9 (156) and CYP3A4/5 (185.4). The inhibition of the uptake activity of OATP1B1 and OATP1B3 were also observed with IC50 values of 93.4 and 244.8 μg/mL, respectively.

DISCUSSION

Extract concentrations higher than the estimated IC50 values are achievable in the gastrointestinal tract when traditional doses of Hypoxis are considered. This may have profound effects on presystemic metabolism of the drug substrates. If absorbed, systemic inhibition of metabolic enzymes/transporters by Hypoxis may be expected.

CONCLUSION

The result suggests that there is the potential for HDI between Hypoxis and the substrates of the affected enzymes/transporters, if sufficient in vivo concentration of Hypoxis extracts is attained.

Ritonavir and dexamethasone induce expression of CYP3A and P-glycoprotein in rats

1. The consequences of extended exposure to the human immunodeficiency viral protease inhibitor ritonavir (RIT) on the expression and function of CYP3A isoforms in the liver and in enteric mucosal cells, and on the expression of the efflux transport protein P-glycoprotein (P-gp) in enteric mucosa and in brain microvessel endothelial cells, were evaluated in rat. Dexamethasone (DEX), a known inducer of CYP3A and P-gp in rodents, served as a positive control. 2. Male CD-1 rats received RIT (20 mg kg(-1)), DEX (80 mg kg(-1)) or vehicle by oral/duodenal gavage once daily for 3 days. 3. Compared with vehicle control, CYP3A activity in liver microsomes (intrinsic clearance for triazolam hydroxylation in vitro) was increased by a factor of 2-4 by RIT, and by 10-14-fold by DEX. Similar increases were observed in expression of immunoactive CYP3A protein. Overall, maximum reaction velocity and immunoactive protein were highly intercorrelated (r2 = 0.89). Both RIT and DEX also increased function and expression of enteric CYP3A, although to a more modest extent (about 1.7-fold for RIT, about 3.3-fold for DEX). 4. Enteric P-gp expression was equally induced (by 2.8-fold) by both RIT and DEX. P-gp expressed in brain microvessel endothelial cells was increased by a factor of 1.3 by both compounds. 5. Thus, increased expression of CYP3A isoforms and of P-gp occurs with 3 days of exposure to RIT in rats. Qualitatively similar changes occur in human cell culture models and in clinical studies, and might contribute to drug interactions involving RIT (and other antiretroviral agents) in humans.

Modeling Km values using electrotopological state: Substrates for cytochrome P450 3A4-mediated metabolism

In order to determine K(m) values of substrates for CYP3A4-mediated metabolism, an in silico model has been developed in the present work. Using electrotopological state (E-state) indices, together with Bayesian-regularized neural network (BRNN), we have described an in silico method to model log(1/K(m)) values of various substrates. The relative importance of the E-state indices is analyzed by principal component analysis. By using an additional external test set, which is independent of the training set, the robustness and predictivity of the model are also validated.

Disposition of paclitaxel (Taxol®) and its metabolites in patients with advanced breast cancer (ABC) when combined with trastuzumab (Hercpetin®)

Monoclonal antibodies are capable of modulating drug metabolising enzymes resulting in unexpected plasma concentrations of a drug when given concomitantly. Therefore plasma concentration of paclitaxel (PTX) and its metabolites has been monitored in 10 patients with advanced breast cancer during treatment with PTX alone or combined with trastuzumab (TMAB, paired cross over design). Compared to the MONO regimen PTX peak plasma concentrations were about 25% lower in the TMAB schedule: cmax = 3294 +/- 1174 ng/ml (MONO: 4368 +/- 1887 ng/ml). TMAB also caused lower peak plasma concentration of the main metabolite 6-hydroxy PTX (248 +/- 89 ng/ml) compared to the MONO schedule (194 +/- 82 ng/ml). Cmax of the minor metabolites was distinctly below 100 ng/ml and consequently differed negligible in both schedules. The similar apparent formation rate of the metabolites in both schedules (range from 30 to 50 min) as well as identical tmax values (range 170-190 min) suggested that TMAB had no influence on PTX metabolism. In accordance to plasma concentrations, AUClast of PTX was lower in the MONO schedule (733 +/- 197 microg/ml*min, AUClast = 669 +/- 248 microg/ml*min for TMAB) but without significance. In summary no indices for an altered plasma disposition of PTX and its metabolites could be found when TMAB was given concomitantly.

Pharmacokinetics of Gemcitabine Combined with Trastuzumab in Patients with Advanced Breast Cancer

BACKGROUND

Combining the monoclonal antibody trastuzumab (TMAB) with chemotherapy is a new strategy in treatment of advanced breast cancer in HER+++ overexpressing patients.

PATIENTS AND METHODS

The disposition of gemcitabine has been investigated in 8 breast cancer patients (prospective cross-over design). Gemcitabine was administered as a 30-min i.v. infusion (1,000 mg/m(2) in 250 ml) on day 1 weekly for 3 weeks. On day 2 TMAB was infused with a loading dose of 4 mg/kg (90-min infusion) followed by a weekly maintenance dose of 2 mg/kg (30-min infusion). Pharmacokinetic analysis was performed after the first (= MONO) and after the third gemcitabine infusion (= TMAB).

RESULTS

Cmax was 22.2 microg/ml (t(max) = 24 min) in the MONO and 24.6 microg/ml (t(max) = 23 min) in the TMAB schedule. Gemcitabine distributed rapidly from plasma within a few minutes and was eliminated with a t1/2el of about 80 min in both arms of the study. The metabolite difluorodeoxyuridine (dFdU) appeared in plasma with t1/2appin = 12.8 min (MONO) or t1/2appin = 10.2 min (TMAB) reaching a mean peak concentration of 35.9 microg/ml (MONO) or 30.4 microg/ml (TMAB), respectively.

CONCLUSION

The results gave evidence that TMAB does not affect the disposition of gemcitabine.

VALIDATED ASSAYS FOR HUMAN CYTOCHROME P450 ACTIVITIES

The measurement of the effect of new chemical entities on human cytochrome P450 marker activities using in vitro experimentation represents an important experimental approach in drug development. In vitro drug interaction data can be used in guiding the design of clinical drug interaction studies, or, when no effect is observed in vitro, the data can be used in place of an in vivo study to claim that no interaction will occur in vivo. To make such a claim, it must be assured that the in vitro experiments are performed with absolute confidence in the methods used and data obtained. To meet this need, 12 semiautomated assays for human P450 marker substrate activities have been developed and validated using approaches described in the GLP (good laboratory practices) as per the code of U.S. Federal Regulations. The assays that were validated are: phenacetin O-deethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), bupropion hydroxylase (CYP2B6), amodiaquine N-deethylase (CYP2C8), diclofenac 4'-hydroxylase and tolbutamide methylhydroxylase (CYP2C9), (S)-mephenytoin 4'-hydroxylase (CYP2C19), dextromethorphan O-demethylase (CYP2D6), chlorzoxazone 6-hydroxylase (CYP2E1), felodipine dehydrogenase, testosterone 6 beta-hydroxylase, and midazolam 1'-hydroxylase (CYP3A4 and CYP3A5). High-pressure liquid chromatography-tandem mass spectrometry, using stable isotope-labeled internal standards, has been applied as the analytical method. This analytical approach, through its high sensitivity and selectivity, has permitted the use of very low incubation concentrations of microsomal protein (0.01-0.2 mg/ml). Analytical assay accuracy and precision values were excellent. Enzyme kinetic and inhibition parameters obtained using these methods demonstrated high precision and were within the range of values previously reported in the scientific literature. These methods should prove useful in the routine assessments of the potential for new drug candidates to elicit pharmacokinetic drug interactions via inhibition of cytochrome P450 activities.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Human drug metabolism and the cytochromes P450: application and relevance of in vitro models

The cytochromes P450 (CYPs) constitute a superfamily of hemoprotein enzymes that are responsible for the biotransformation of numerous xenobiotics, including therapeutic agents. Studies of the biochemical and enzymatic properties of these enzymes and their molecular genetics and regulation of gene expression and activity have greatly enhanced our understanding of several aspects of clinical pharmacology such as pharmacokinetic variability, drug toxicity, and drug interactions. This review evaluates the major human hepatic drug-metabolizing CYP enzymes and their clinically relevant substrates, inhibitors, and inducers. Also discussed are the molecular bases and clinical implications of genetic polymorphisms that affect the CYPs. Much of the information on the specificity of substrates and inhibitors of the CYP enzymes is derived from in vitro studies using human liver microsomes and heterologously expressed CYP enzymes. These methods are discussed, and guidelines are provided for designing enzyme kinetic and reaction phenotyping studies using multiple approaches. The strengths, weaknesses, and discrepancies among the different approaches are considered using representative examples. The mathematical models used in predicting the pharmacokinetic clearance of a drug from in vitro estimates of intrinsic clearance and the principles of quantitative in vitro-in vivo scaling of metabolic drug interactions are also discussed.

Inhibition of human cytochrome P450 isoforms by nonnucleoside reverse transcriptase inhibitors

The capacity of three clinically available nonnucleoside reverse transcriptase inhibitors (NNRTIs) to inhibit the activity of human cytochromes P450 (CYPs) was studied in vitro using human liver microsomes. Delavirdine, nevirapine, and efavirenz produced negligible inhibition of phenacetin O-deethylation (CYP1A2) or dextromethorphan O-demethylation (CYP2D6). Nevirapine did not inhibit hydroxylation of tolbutamide (CYP2C9) or S-mephenytoin (CYP2C19), but these CYP isoforms were importantly inhibited by delavirdine and efavirenz. This indicates the likelihood of significantly impaired clearance of CYP2C substrate drugs (such as phenytoin, tolbutamide, and warfarin) upon initial exposure to these two NNRTIs. Delavirdine and efavirenz (but not nevirapine) also were strong inhibitors of CYP3A, consistent with clinical hazards of initial cotreatment with either of these drugs and substrates of CYP3A. The in vitro microsomal model provides relevant predictive data on probable drug interactions with NNRTIs when the mechanism is inhibition of CYP-mediated drug biotransformation. However, the model does not incorporate interactions attributable to enzyme induction.

Inhibitory effects of trospium chloride on cytochrome P450 enzymes in human liver microsomes

Trospium chloride, an atropine derivative used for the treatment of urge incontinence, was tested for inhibitory effects on human cytochrome P450 enzymes. Metabolic activities were determined in liver microsomes from two donors using the following selective substrates: dextromethorphan (CYP2D6), denitronifedipine (CYP3A4), caffeine (CYP1A2), chlorzoxazone (CYP2E1), S-(+)-mephenytoin (CYP2C19), S-(-)-warfarin (CYP2C9) and coumarin (CYP2A6). Incubations with each substrate were carried out without a possible inhibitor and in the presence of trospium chloride at varying concentrations (37-3000 microM) at 37 degrees in 0.1 M KH2PO4 buffer containing up to 3% DMSO. Metabolite concentrations were determined by high-performance liquid chromatography (HPLC) in all cases except CYP2A6 where direct fluorescence spectroscopy was used. First, trospium chloride IC50 values were determined for each substrate at respective K(M) concentrations. Trospium chloride did not show relevant inhibitory effects on the metabolism of most substrates (IC50 values considerably higher than 1 mM). The only clear inhibition was seen for the CYP2D6-dependent high-affinity O-demethylation of dextromethorphan, where IC50 values of 27 microM and 44 microM were observed. Therefore, additional dextromethorphan concentrations (0.4-2000 microM) were tested. Trospium chloride was a competitive inhibitor of the reaction with Ki values of 20 and 51 microM, respectively. Thus, trospium chloride has negligible inhibitory effects on CYP3A4, CYP1A2, CYP2E1, CYP2C19, CYP2C9 and CYP2A6 activity but is a reasonably potent inhibitor of CYP2D6 in vitro. Compared to therapeutic trospium chloride peak plasma concentrations below 50 nM, the 1000-times higher competitive inhibition constant Ki however suggests that inhibition of CYP2D6 by trospium chloride is without any clinical relevance.

Homology modeling and substrate binding study of human CYP2C9 enzyme

The CYP2C subfamily of human liver P450 isozymes is of major importance in drug metabolism. The most abundant 2C isozyme, CYP2C9, regioselectively hydroxylates a wide variety of substrates. A major obstacle to understanding this specificity in human CYP2C9 is the absence of a 3D structure. A 3D model of CYP2C9 was built, assessed, and used to characterize explicit enzyme-substrate complexes using methods previously developed in our laboratory. The 3D model was assessed by determining its stability to unconstrained molecular dynamics and by comparison of specific properties with those of known protein structures. The CYP2C9 model was then used to characterize explicit enzyme complexes with three structurally and chemically diverse substrates: (S)-naproxen, phenytoin, and progesterone. Each substrate was found to bind to the enzyme with a favorable interaction energy and to remain in the binding site during unconstrained molecular dynamics. Moreover, the mode of binding of each substrate led to calculated preferred hydroxylation sites consistent with experiment. Binding-site residues identified for the models included Arg 105 and Arg97 as key cationic residues, as well as Asn 202, Asp 293, Pro 101, Leu 102, Gly 296, and Phe 476. Site-specific mutations are proposed for further integrated computational and experimental study.

Citalopram and desmethylcitalopram in vitro: human cytochromes mediating transformation, and cytochrome inhibitory effects

BACKGROUND

Biotransformation of citalopram (CT), a newly available selective serotonin reuptake inhibitor antidepressant, to its principal metabolite, desmethycitalopram (DCT), and the capacity of CT and DCT to inhibit human cytochromes P450, were studied in vitro.

METHODS

Formation of DCT from CT was evaluated using human liver microsomes and microsomes from cDNA-transfected human lymphoblastoid cells. Cytochrome inhibition by CT and DCT in liver microsomes was studied using isoform-specific index reactions.

RESULTS

Formation of DCT from CT in liver microsomes had a mean apparent K(m) of 174 mumol/L. Coincubation with 1 mumol/L ketoconazole reduced reaction velocity to 46 to 58% of control values, while omeprazole, 10 mumol/L, reduced velocity to 80% of control. Quinidine produced minimal inhibition. DCT was formed from CT by heterologously expressed human P450-2D6, -2C19, -3A4. After accounting for the relative abundance of individual cytochromes, 3A4 and 2C19 were estimated to make major contributions to net reaction velocity, with a possible contribution of 2D6 at therapeutic CT concentrations. CT and DCT themselves produced negligible inhibition of 2C9, 2E1, and 3A, and only weak inhibition of 1A2, 2C19, and 2D6.

CONCLUSIONS

Formation of DCT from CT is mediated mainly by P450-3A4 and 2C19, with an additional contribution of 2D6. CT at therapeutic doses in humans may produce a small degree of inhibition of P450-1A2, -2C19, and -2D6, but negligible inhibition of P450-2C9, -2E1, and -3A.

Biotransformation of alprazolam by members of the human cytochrome P4503A subfamily

1. To aid in the prediction of drug interactions with alprazolam, the human CYP involved in the 1'- and 4-hydroxylation of alprazolam were characterized using human liver microsomes, expressed enzymes and selective chemical inhibitors. 2. The formation of 4-hydroxyalprazolam and 1'-hydroxyalprazolam at an alprazolam concentration of 62.5 microM were reduced by the prototypic CYP3A inhibitor, troleandomycin (50 microM), by 97 and 9900 respectively. Only microsomes from B-lymphoblastoid cells expressing CYP3A4 were capable of catalysing the 1'- and 4-hydroxylation of alprazolam. 3. The formation rates of 1'-hydroxyalprazolam and 4-hydroxyalprazolam at an alprazolam concentration of 1 mM were significantly correlated (n = 19, r = 0.95, p<0.01) indicating that the same enzyme(s) mediated these biotransformations. A significant (p<0.01) correlation was observed between alprazolam 4- and 1'-hydroxylase activity and CYP3A-mediated midazolam 4-hydroxylase, midazolam 1'-hydroxylase, dextromethorphan N-demethylase and erythromycin N-demethylase activities. 4. In conclusion, in adult human liver the CYP3A subfamily members are the principal enzymes involved in the 1'- and 4-hydroxylation of alprazolam. Thus, clinically significant drug drug interactions between alprazolam and other CYP3A substrates are to be expected.

Extrapolating in vitro data on drug metabolism to in vivo pharmacokinetics: evaluation of the pharmacokinetic interaction between amitriptyline and fluoxetine

Recently, models have been proposed to extrapolate in vitro data on the influence of inhibitors on drug metabolism to in vivo decrement in drug clearance. Many factors influence drug clearance such as age, gender, habits, diet, environment, liver disease, heredity, and other drugs. In vitro investigation of hepatic cytochrome P450 activity has generally centered on genetic influences and interactions with other drugs. This group of enzymes is involved in many, although not all, drug interactions. The interaction of amitriptyline and fluoxetine is an example. Of the different in vitro paradigms, interaction studies utilizing human liver microsomal preparations have proved to be the most generally applicable for in vitro scaling models. Assuming Michaelis-Menten conditions and applying nonlinear regression, a hybrid inhibition constant (Ki) can be generated that allows classification of the inhibitory potency of an inhibitor toward a specific reaction. This constant is largely independent of the substrate concentration, but in vivo relevance is critically dependent on the inhibitor concentration in the site of metabolic activity, the liver cell cytosol. Many lipophilic drugs are extensively bound to plasma protein but, nonetheless, demonstrate extensive partitioning into liver tissue. This is not compatible with diffusion only of the unbound drug fraction into liver cells. The introduction of a partition factor, based on data from a number of possible sources, provided a reasonable basis for the scaling of in vitro data to in vivo conditions. Many interactions could be reconstructed or predicted with greater accuracy and clinical relevance for interactions such as terfenadine or midazolam and ketoconazole. Even for less marked interactions such as amitriptyline and fluoxetine, this model provides a forecast consistent with the clinically observed range of 22-45% reduction in oral clearance, although this interaction is complicated by the presence of two inhibitors, fluoxetine and norfluoxetine. The concept of in vitro-in vivo scaling is promising and might ultimately yield a fast and more cost-effective screening for drug interactions with reduced human drug exposure and risk.

Relative quantities of catalytically active CYP 2C9 and 2C19 in human liver microsomes: application of the relative activity factor approach

The relative catalytic activities of CYP2C9 and CYP2C19 in human liver microsomes has been determined using the approach of relative activity factors (RAFs). Tolbutamide methylhydroxylation and S-mephenytoin 4'-hydroxylation were used as measures of CYP2C9 and CYP2C19 activity, respectively. The kinetics of these reactions were studied in human liver microsomes, in microsomes from human lymphoblastoid cells, and in insect cells expressing CYP2C9 and CYP2C19. RAFs were calculated as the ratio of Vmax (reaction velocity at saturating substrate concentrations) in human liver microsomes of the isoform-specific index reaction divided by the Vmax of the reaction catalyzed by the cDNA expressed isoform. RAFs were also determined for SUPERMIX, a commercially available mixture of cDNA expressed human drug metabolizing CYPs formulated to achieve a balance of enzyme activities similar to that found in human liver microsomes. Lymphoblast RAF2C9 in human liver microsomes ranged from 54 to 145 pmol CYP/mg protein (mean value: 87), while a value of 251 pmol CYP/mg protein was obtained for SUPERMIX. Insect cell RAF2C9 in human liver microsomes ranged from 1.6 to 143 pmol CYP/mg protein (mean value: 49), while a value of 201 pmol CYP/mg protein was obtained for SUPERMIX. Both lymphoblast and insect cell RAF2C19 in human liver microsomes ranged from 4 to 45 pmol CYP/mg protein (mean values: 29 and 28, respectively), while a value of 29 pmol CYP/mg protein was obtained for SUPERMIX. The nature of the cDNA expression system used had no effect on the kinetic parameters of CYP2C9 as a tolbutamide methylhydroxylase, or of CYP2C19 as a S-mephenytoin hydroxylase. However insect cell expressed CYP2C19 (which includes oxidoreductase) had substantially greater activity as a tolbutamide methylhydroxylase when compared to lymphoblast expressed CYP2C19. The ratio of mean lymphoblast-determined RAF2C9 to RAF2C19 in human livers was 3.0 (range 1.6-17.9; n = 10), while this ratio for SUPERMIX was 8.6. The ratio of mean insect cell-determined RAF2C9 to RAF2C19 in human livers was 1.7 (range 0.04-16.2; n = 10), while this ratio for SUPERMIX was 7.0. Neither ratio is in agreement with the 20:1 ratio of immunoquantified levels of CYP2C9 and 2C19 in human liver microsomes reported in previous studies. SUPERMIX may contain catalytically active CYP2C9 in levels higher than those in human liver microsomes.

Protease inhibitors as inhibitors of human cytochromes P450: high risk associated with ritonavir

Four protease inhibitor antiviral agents (ritonavir, indinavir, nelfinavir, saquinavir) were evaluated as in vitro inhibitors of the activity of six human cytochromes using an in vitro model based on human liver microsomes. Ritonavir was a highly potent inhibitor of P450-3A activity (triazolam hydroxylation), having inhibitory potency slightly less than ketoconazole. Indinavir was also a potent 3A inhibitor, while nelfinavir and saquinavir were less potent. Ritonavir had high inhibition potency against cytochrome P450-2C9 (tolbutamide hydroxylation), -2C19 (S-mephenytoin hydroxylation), and -2D6 (dextromethorphan O-demethylation and desipramine hydroxylation), while the other protease inhibitors had one or more orders of magnitude lower inhibitory activity against these reactions. None of the protease inhibitors had important inhibitory potency against P450-1A2 (phenacetin O-deethylation) or -2E1 (chlorzoxazone hydroxylation). Thus, among available protease inhibitors, ritonavir carries the highest risk of incurring drug interactions due to inhibition of cytochrome P450 activity.

Five distinct human cytochromes mediate amitriptyline N-demethylation in vitro: dominance of CYP 2C19 and 3A4

The human cytochromes P450 (CYPs) mediating amitriptyline N-demethylation have been identified using a combination of enzyme kinetic and chemical inhibition studies. Amitriptyline was N-demethylated to nortriptyline by microsomes from cDNA transfected human lymphoblastoid cells expressing human CYPs 1A2, 2C9, 2C19, 2D6, and 3A4. CYP 2E1 showed no detectable activity. While CYP 2C19 and CYP 2D6 showed high affinity, CYP 3A4 showed low affinity; CYP 2C9 and 1A2 showed intermediate affinities. Based on these kinetic parameters and estimated relative abundance of the different CYPs in human liver, CYP 2C19 was identified as the major amitriptyline N-demethylase at low (therapeutically relevant) amitriptyline concentrations, whereas CYP 3A4 may be more important at higher amitriptyline concentrations. Chemical inhibition studies with ketoconazole and omeprazole indicate that CYP 3A4 is the major amitriptyline N-demethylase at 100 mumol/L amitriptyline, while CYP 2C19 is equally important at a substrate concentration of 5 mumol/L. The CYP 1A2 inhibitor alpha-naphthoflavone and the CYP 2C9 inhibitor sulfaphenazole produced much less inhibition of amitriptyline N-demethylation at both substrate concentrations. Quinidine produced no detectable inhibition. The kinetics of amitriptyline N-demethylation by human liver microsomes were consistent with a two enzyme model, with the high affinity component exhibiting Michaelis Menten kinetics and the low affinity component exhibiting Hill enzyme kinetics. No difference was apparent in the kinetics of amitriptyline N-demethylation in two liver samples with low levels of CYP 2C19 activity compared with two other samples with relatively normal 2C19 activity. This may reflect the importance of higher substrate concentration values in estimation of kinetic parameters in vitro.

Midazolam hydroxylation by human liver microsomes in vitro: inhibition by fluoxetine, norfluoxetine, and by azole antifungal agents

Biotransformation of the imidazobenzodiazepine midazolam to its alpha-hydroxy and 4-hydroxy metabolites was studied in vitro using human liver microsomal preparations. Formation of alpha-hydroxy-midazolam was a high-affinity (Km = 3.3 mumol/L) Michaelis-Menten process coupled with substrate inhibition at high concentrations of midazolam. Formation of 4-hydroxy-midazolam had much lower apparent affinity (57 mumol/L), with minimal evidence of substrate inhibition. Based on comparison of Vmax/Km ratios for the two pathways, alpha-hydroxy-midazolam formation was estimated to account for 95% of net intrinsic clearance. Three azole antifungal agents were inhibitors of midazolam metabolism in vitro, with inhibition being largely consistent with a competitive mechanism. Mean competitive inhibition constants (Ki) versus alpha-hydroxy-midazolam formation were 0.0037 mumol/L for ketoconazole, 0.27 mumol/L for itraconazole, and 1.27 mumol/L for fluconazole. An in vitro-in vivo scaling model predicted inhibition of oral midazolam clearance due to coadministration of ketoconazole or itraconazole; the predicted inhibition was consistent with observed interactions in clinical pharmacokinetic studies. The selective serotonin reuptake inhibitor (SSRI) antidepressant fluoxetine and its principal metabolite, norfluoxetine, also were inhibitors of both pathways of midazolam biotransformation, with norfluoxetine being a much more potent inhibitor than was fluoxetine itself. This finding is consistent with results of other in vitro studies and of clinical studies, indicating that fluoxetine, largely via its metabolite norfluoxetine, may impair clearance of P450-3A substrates.