Gepirone and 1-(2-pyrimidinyl)-piperazine in vitro: human cytochromes mediating transformation and cytochrome inhibitory effects

Biotransformation of gepirone to its principal metabolite, 1-(2-pyrimidinyl)-piperazine (1-PP), was studied in human liver microsomes and in microsomes from cDNA-transfected human lymphoblastoid cells. Formation of 1-PP from gepirone in liver microsomes proceeded with a mean apparent Km ranging from 335 to 677 microM. Coincubation with 1 microM ketoconazole reduced reaction velocity to less than 5% of control values at a gepirone concentration of 250 microM. Three other metabolites, presumed to be hydroxylated products, were also formed from gepirone. Formation of all three products was reduced to approximately 20% of control values by 1 microM ketoconazole; quinidine at 1 microM produced a small reduction in formation (91-94% of control) of two of the metabolites. 1-PP was formed from gepirone exclusively by pure P450-3A4 with a Km of 849 microM; Km values for the other metabolites were 245, 240, and 415 microM. Two of the products were also formed by P450-2D6. The results indicate that 3A4 is the principal cytochrome mediating 1-PP formation, as well as formation of the other metabolites. The properties of gepirone and 1-PP themselves as cytochrome inhibitors were tested in human liver microsomes using index reactions representing activity of P450-1A2, -2C9, -2C19, -2D6, -2E1 and -3A. Gepirone and 1-PP produced negligible inhibition of all these reactions. Thus gepirone at therapeutic doses in humans has a low likelihood of inhibiting P450-mediated drug metabolism involving these cytochromes.

Kinetic and dynamic interaction study of zolpidem with ketoconazole, itraconazole, and fluconazole

BACKGROUND

Azole antifungal agents may impair hepatic clearance of drugs metabolized by cytochrome P450-3A isoforms. The imidazopyridine hypnotic agent zolpidem is metabolized in humans in part by P450-3A, as well as by a number of other cytochromes. Potential interactions of zolpidem with 3 commonly prescribed azole derivatives were evaluated in a controlled clinical study.

METHODS

In a randomized, double-blind, 5-way, crossover, clinical pharmacokinetic-pharmacodynamic study, 12 volunteers received (A) zolpidem placebo plus azole placebo, (B) 5 mg zolpidem plus azole placebo (C) zolpidem plus ketoconazole, (D) zolpidem plus itraconazole, and (E) zolpidem plus fluconazole.

RESULTS

Mean apparent oral clearance of zolpidem when given with placebo was 422 mL/min, and elimination half-life was 1.9 hours. Clearance was significantly reduced to 250 mL/min when zolpidem was given with ketoconazole, and half-life was prolonged to 2.4 hours. Coadministration of zolpidem with itraconazole or fluconazole also reduced clearance (320 and 338 mL/min), but differences compared to the zolpidem plus placebo treatment did not reach significance. Zolpidem-induced benzodiazepine agonist effects (increased electrocardiographic beta activity, digit-symbol substitution test impairment, and delayed recall) during the first 4 hours after dosage were enhanced by ketoconazole but not by itraconazole or fluconazole.

CONCLUSION

Coadministration of zolpidem with ketoconazole impairs zolpidem clearance and enhances its benzodiazepine-like agonist pharmacodynamic effects. Itraconazole and fluconazole had a small influence on zolpidem kinetics and dynamics. The findings are consistent with in vitro studies of differentially impaired zolpidem metabolism by azole derivatives.

Human cytochromes P450 mediating phenacetin O-deethylation in vitro: validation of the high affinity component as an index of CYP1A2 activity

Phenacetin O-deethylation, widely used as an index reaction for cytochrome P450 1A2 (CYP1A2) activity, displays biphasic kinetics in human liver microsomes. CYP1A2 has been identified as contributing to the high affinity component, but is not verified as the sole contributor to the high affinity phase. In addition, the human CYP isoforms accounting for the low affinity phase have not been identified. We have used heterologously expressed human CYP isoforms to identify, kinetically characterize, and predict the relative contribution of the major human liver CYP isoforms mediating phenacetin O-deethylation. CYP1A2 (Km 31 microM) is the only high affinity phenacetin O-deethylase in human liver microsomes, while CYPs 2A6 (Km 4098 microM), 2C9 (Km 566 microM), 2C19 (Km 656 microM), 2D6 (Km 1021 microM), and 2E1 (Km 1257 microM) all contribute to the low affinity phase of the reaction. Considering the relative abundance of the various CYPs in human liver, CYP1A2 accounts for 86% of net reaction velocity at a substrate concentration of 100 microM, while CYP2C9 becomes the primary phenacetin O-deethylase at substrate concentrations of 865 microM and higher and accounts for 31% of the net Vmax of the reaction. Predictions from kinetic studies on heterologously expressed CYPs are consistent with chemical inhibition studies on human liver microsomes with sulfaphenazole and alpha-naphthoflavone that suggest a greater role for CYP2C9, and a smaller role for CYP1A2, at higher substrate concentrations. Thus CYP1A2 is the only high affinity human liver phenacetin O-deethylase, thereby validating the use of the high affinity component as an index of CYP1A2 activity in human liver microsomes.

Comparative kinetics and dynamics of zaleplon, zolpidem, and placebo

PURPOSE

This study evaluated the relationship of dose, plasma concentration, and time to the pharmacodynamics of zaleplon and zolpidem, 2 structurally distinct benzodiazepine receptor agonists.

METHOD

Ten healthy male volunteers received single oral doses of placebo, 10 mg zaleplon, 20 mg zaleplon, 10 mg zolpidem, and 20 mg zolpidem in a double-blind, 5-condition crossover study, with 48 hours elapsing between trials. Plasma drug concentrations and pharmacodynamic effects were measured during the 8 to 24 hours after administration.

RESULTS

Kinetics of zaleplon and zolpidem were not significantly related to dose. However, zaleplon had more rapid elimination (apparent elimination half-life [t1/2] of 1 hour) and higher apparent oral clearance (approximately 4300 mL/min) than zolpidem (t1/2, 2.0 to 2.2 hours; apparent oral clearance, 340 to 380 mL/min). Active treatments produced pharmacodynamic effects consistent with benzodiazepine agonist activity: self- and observer-rated sedation, impairment of digit symbol substitution test (DSST) performance, impaired memory, and increased electroencephalographic activity in the beta frequency range. The overall order of agonist potency was as follows: placebo < 10 mg zaleplon < 20 mg zaleplon < 10 mg zolpidem < 20 mg zolpidem; on a number of measures, 20 mg zaleplon was comparable to 10 mg zolpidem. Quantitative effects of zolpidem 20 mg far exceeded those of other treatments. Dynamic effects of both drugs were significantly related to plasma concentration.

CONCLUSIONS

Benzodiazepine agonist effects of zaleplon and zolpidem were dose and concentration dependent. At the usual clinically effective hypnotic dose (10 mg of either drug), agonist effects of zolpidem exceeded those of zaleplon.

Analysis of ritonavir in plasma/serum and tissues by high-performance liquid chromatography

A method has been developed to quantify ritonavir concentrations in human plasma and in mouse serum, liver, and brain using high-performance liquid chromatography. Extraction recoveries for ritonavir and its internal standard averaged greater than 95%. Within-day variability, expressed as a coefficient of variation, averaged 6% over the concentration range 0.5 microg/mL to 15 microg/mL ritonavir, and between-day variability averaged 5.6% over 5 microg/mL to 15 microg/mL ritonavir. The method was applied to quantitation of ritonavir in mouse serum and tissue. Measured values deviated less than 5% from the actual values in mouse serum, liver, and brain samples containing 5 microg/mL ritonavir. The slopes of calibration curves for extracted calf serum, mouse serum, mouse liver and mouse brain standards were nearly identical to the calibration slope of standards which were not extracted. All curves were linear through zero, and r2 was no less than 0.998 for any form of calibration. In addition, there was no chromatographic interference from commonly prescribed medications.

Drug interactions with newer antidepressants: role of human cytochromes P450

Selective serotonin reuptake inhibitors and related antidepressant compounds have the secondary pharmacologic property of inhibiting the activity of human cytochrome P450 enzymes responsible for the oxidative metabolism of many drugs. A number of clinically important pharmacokinetic drug interactions are a consequence of these cytochrome inhibiting effects. This review evaluates the clinical implications of the metabolic profiles of the newer antidepressants, the relative activities of various new antidepressants as inhibitors of human cytochrome P450, and the various in vivo and in vitro methodologies that can be used for identification and quantification of drug interactions.

Inhibition of desipramine hydroxylation (Cytochrome P450-2D6) in vitro by quinidine and by viral protease inhibitors: relation to drug interactions in vivo

Pharmacokinetic drug interactions with viral protease inhibitors are of potential clinical importance. An in vitro model was applied to the quantitative identification of possible interactions of protease inhibitors with substrates of cytochrome P450-2D6. Biotransformation of desipramine (DMI) to hydroxydesipramine (OH-DMI), an index reaction used to profile activity of human cytochrome P450-2D6, was studied in vitro using human liver microsomes. Quinidine and four viral protease inhibitors currently used to treat human immunodeficiency virus infection were tested as chemical inhibitors in this system. Formation of OH-DMI from DMI was consistent with Michaelis-Menten kinetics, having a mean Km value of 11.7 microM (range: 9.9-15.3 microM). Quinidine, a highly potent and relatively selective inhibitor of P450-2D6, strongly inhibited OH-DMI formation with an apparent competitive mechanism, having a mean inhibition constant of 0.16 microM (range: 0.13-0.18 microM). All four protease inhibitors impaired OH-DMI formation; the pattern was consistent with a mixed competitive-noncompetitive mechanism. Mean inhibition constants (small numbers indicating greater inhibiting potency) were as follows: ritonavir, 4.8 microM; indinavir, 15.6 microM; saquinavir, 24.0 microM; nelfinavir, 51.9 microM. In a clinical pharmacokinetic study, coadministration of ritonavir with DMI inhibited DMI clearance by an average of 59%. The in vitro findings, together with observed plasma ritonavir concentrations, provided a reasonable quantitative forecast of this interaction, whereas estimated unbound plasma or intrahepatic ritonavir concentrations yielded poor quantitative forecasts. Thus the in vitro model correctly identifies ritonavir as a potent and clinically important inhibitor of human P450-2D6. Other protease inhibitors may also inhibit 2D6 activity in humans, but with lower potency than ritonavir.

Inhibition of triazolam clearance by macrolide antimicrobial agents: in vitro correlates and dynamic consequences

BACKGROUND

Macrolide antimicrobial agents may impair hepatic clearance of drugs metabolized by cytochrome P4503A isoforms. Potential interactions of triazolam, a substrate metabolized almost entirely by cytochrome P4503A in humans, with 3 commonly prescribed macrolides were identified using an in vitro metabolic model. The actual interactions, and their pharmacodynamic consequences, were verified in a controlled clinical study.

METHODS

In an in vitro model using human liver microsomes, 250 mumol/L triazolam was incubated with ascending concentrations (0 to 250 mumol/L of troleandomycin, azithromycin, erythromycin, and clarithromycin. In a randomized, double-blind, 5-trial clinical pharmacokinetic-pharmacodynamic study, 12 volunteers received 0.125 mg triazolam orally, together with placebo, azithromycin, erythromycin, or clarithromycin. In a fifth trial they received placebo plus placebo.

RESULTS

Mean 50% inhibitory concentrations versus 4-hydroxytriazolam formation in vitro were as follows: 3.3 mumol/L troleandomycin, 27.3 mumol/L erythromycin, 25.2 mumol/L clarithromycin, and greater than 250 mumol/L azithromycin. Apparent oral clearance of triazolam when given with placebo or azithromycin was nearly identical (413 and 416 mL/min), as were peak plasma concentrations (1.25 and 1.32 ng/mL) and elimination half-life (2.7 and 2.6 hours). Apparent oral clearance was significantly reduced (P < .05) during erythromycin and clarithromycin trials (146 and 95 mL/min). Peak plasma concentration was correspondingly increased, and elimination half-life was prolonged. The effects of triazolam on dynamic measures were nearly identical when triazolam was given with placebo or azithromycin, but benzodiazepine agonist effects were enhanced during erythromycin and clarithromycin trials.

CONCLUSION

The in vitro model identifies macrolides that may impair triazolam clearance. Anticipated interactions, and their pharmacodynamic consequences in volunteer subjects, were verified in vivo.

Multiple human cytochromes contribute to biotransformation of dextromethorphan in-vitro: role of CYP2C9, CYP2C19, CYP2D6, and CYP3A

Cytochromes mediating the biotransformation of dextromethorphan to dextrorphan and 3-methoxymorphinan, its principal metabolites in man, have been studied by use of liver microsomes and microsomes containing individual cytochromes expressed by cDNA-transfected human lymphoblastoid cells. In-vitro formation of dextrorphan from dextromethorphan by liver microsomes was mediated principally by a high-affinity enzyme (Km (substrate concentration producing maximum reaction velocity) 3-13 microM). Formation of dextrorphan from 25 microM dextromethorphan was strongly inhibited by quinidine (IC50 (concentration resulting in 50% inhibition) = 0.37 microM); inhibition by sulphaphenazole was approximately 18% and omeprazole and ketoconazole had minimal effect. Dextrorphan was formed from dextromethorphan by microsomes from cDNA-transfected lymphoblastoid cells expressing CYP2C9, -2C19, and -2D6 but not by those expressing CYP1A2, -2E1 or -3A4. Despite the low in-vivo abundance of CYP2D6, this cytochrome was identified as the dominant enzyme mediating dextrorphan formation at substrate concentrations below 10 microM. Formation of 3-methoxy-morphinan from dextromethorphan in liver microsomes proceeded with a mean Km of 259 microM. For formation of 3-methoxymorphinan from 25 microM dextromethorphan the IC50 for ketoconazole was 1.15 microM; sulphaphenazole, omeprazole and quinidine had little effect. 3-Methoxymorphinan was formed by microsomes from cDNA-transfected lymphoblastoid cells expressing CYP2C9, -2C19, -2D6, and -3A4, but not by those expressing CYP1A2 or -2E1. CYP2C19 had the highest affinity (Km = 49 microM) whereas CYP3A4 had the lowest (Km = 1155 microM). Relative abundances of the four cytochromes were determined in liver microsomes by use of the relative activity factor approach. After adjustment for relative abundance, CYP3A4 was identified as the dominant enzyme mediating 3-methoxymorphinan formation from dextromethorphan, although CYP2C9 and -2C19 were estimated to contribute to 3-methoxymorphinan formation, particularly at low substrate concentrations. Although formation of dextrorphan from dextromethorphan appears to be sufficiently specific to be used as an in-vitro or in-vivo index reaction for profiling of CYP2D6 activity, the findings raise questions about the specificity of 3-methoxymorphinan formation as an index of CYP3A activity.

Ketoconazole inhibition of triazolam and alprazolam clearance: differential kinetic and dynamic consequences

BACKGROUND

Kinetic and dynamic consequences of metabolic inhibition were evaluated in a study of the interaction of ketoconazole, a P4503A inhibitor, with alprazolam and triazolam, two 3A substrate drugs with different kinetic profiles.

METHODS

In a double-blind, 5-way crossover study, healthy volunteers received (A) ketoconazole placebo plus 1.0 mg alprazolam orally, (B) 200 mg ketoconazole twice a day plus 1.0 mg alprazolam, (C) ketoconazole placebo plus 0.25 mg triazolam orally, (D) 200 mg ketoconazole twice a day plus 0.25 mg triazolam, and (E) 200 mg ketoconazole twice a day plus benzodiazepine placebo. Plasma concentrations and pharmacodynamic parameters were measured after each dose.

RESULTS

For trial B versus trial A, alprazolam clearance was reduced (27 versus 86 mL/min; P < .002) and apparent elimination half-life (t1/2) prolonged (59 versus 15 hours; P < .03), whereas peak plasma concentration (Cmax) was only slightly increased (16.1 versus 14.7 ng/mL). The 8-hour pharmacodynamic effect areas for electroencephalographic (EEG) beta activity were increased by a factor of 1.35, and those for digit-symbol substitution test (DSST) decrement were increased by 2.29 for trial B versus trial A. For trial D versus trial C, triazolam clearance was reduced (40 versus 444 mL/min; P < .002), t1/2 was prolonged (18.3 versus 3.0 hours; P < .01), and Cmax was increased (2.6 versus 5.4 ng/mL; P < .001). The 8-hour effect area for EEG was increased by a factor of 2.51, and that for DSST decrement was increased by 4.33. Observed in vivo clearance decrements due to ketoconazole were consistent with those anticipated on the basis of an in vitro model, together with in vivo plasma concentrations of ketoconazole.

CONCLUSION

For triazolam, an intermediate-extraction compound, impaired clearance by ketoconazole has more profound clinical consequences than those for alprazolam, a low extraction compound.

Appetite suppressant drugs as inhibitors of human cytochromes P450: in vitro inhibition of P450-2D6 by D- and L-fenfluramine, but not phentermine

The activity of D-fenfluramine, L-fenfluramine, and phentermine as inhibitors of five human cytochromes P450 was evaluated using human liver microsomes in vitro. All three compounds produced negligible inhibition of P450-1A2, -2C9, -2E1, and -3A. Phentermine also did not inhibit P450-2D6. However, D- and L-fenfluramine significantly inhibited P450-2D6 activity as measured by dextromethorphan O-demethylation, with mean 50% inhibitory concentrations (15.1 microM) within one order of magnitude of that for fluoxetine (2.7 microM). Findings from the in vitro assay are consistent with clinical studies showing significant inhibition of desipramine clearance by coadministration of fenfluramine.

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.

Kinetic characterization and identification of the enzymes responsible for the hepatic biotransformation of adinazolam and N-desmethyladinazolam in man

The kinetics of the N-demethylation of adinazolam to N-desmethyladinazolam (NDMAD), and of NDMAD to didesmethyladinazolam (DDMAD), were studied with human liver microsomes using substrate concentrations in the range 10-1000 microM. The specific cytochrome P450 (CYP) isoforms mediating the biotransformations were identified using microsomes containing specific recombinant CYP isozymes expressed in human lymphoblastoid cells, and by the use of CYP isoform-selective chemical inhibitors. Adinazolam was demethylated by human liver microsomes to NDMAD, and NDMAD was demethylated to DDMAD; the substrate concentrations, Km, at which the reaction velocities were 50% of the maximum were 92 and 259 microM, respectively. Another metabolite of yet undetermined identity (U) was also formed from NDMAD (Km 498 microM). Adinazolam was demethylated by cDNA-expressed CYP 2C19 (Km 39 microM) and CYP 3A4 (Km 83 microM); no detectable activity was observed for CYPs 1A2, 2C9, 2D6 and 2E1. Ketoconazole, a relatively specific CYP 3A4 inhibitor, inhibited the reaction; the concentration resulting in 50% of maximum inhibition, IC50, was 0.15 microM and the inhibition constant, Ki, was < 0.04 microM in five of six livers tested. Troleandomycin, a specific inhibitor of CYP 3A4, inhibited adinazolam N-demethylation with an IC50 of 1.96 microM. The CYP 2C19-inhibitor omeprazole resulted in only partial inhibition (IC50 21 microM) and sulphaphenazole, alpha-naphthoflavone, quinidine and diethyldithiocarbamate did not inhibit the reaction. NDMAD was demethylated by cDNA-expressed CYP 3A4 (Km 220 microM, Hill number A 1.21), CYP 2C19 (Km 187 microM, Hill number A 1.29) and CYP 2C9 (Km 1068 microM). Formation of U was catalysed by CYP 3A4 alone. Ketoconazole strongly inhibited NDMAD demethylation (IC50 0.14 microM) and formation of U (IC50 < 0.1 microM) whereas omeprazole and sulphaphenazole had no effect on reaction rates. These results show that CYP 3A4 is the primary hepatic CYP isoform mediating the N-demethylation of adinazolam and NDMAD. Co-administration of adinazolam with CYP 3A4 inhibitors such as ketoconazole or erythromycin might lead to reduced efficacy, since adinazolam by itself has relatively weak benzodiazepine agonist activity, with much of the pharmacological activity of adinazolam being attributable to its active metabolite NDMAD.

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.

In vitro approaches to predicting drug interactions in vivo

In vitro metabolic models using human liver microsomes can be applied to quantitative prediction of in vivo drug interactions caused by reversible inhibition of metabolism. One approach utilizes in vitro Ki, values together with in vivo values of inhibitor concentration to forecast in vivo decrements of clearance caused by coadministration of inhibitor. A critical limitation is the lack of a general scheme for assigning intrahepatic exposure of enzyme to inhibitor or substrate based only on plasma concentration; however, the assumption that plasma protein binding necessarily restricts hepatic uptake is not tenable. Other potential limitations include: flow-dependent hepatic clearance, "mechanism-based" chemical inhibition, concurrent induction, or a major contribution of gastrointestinal P450-3A isoforms to presystemic extraction. Nonetheless, the model to date has provided reasonably accurate forecasts of in vivo inhibition of clearance of several substrates (desipramine, terfenadine, triazolam, alprazolam, midazolam) by coadministration of selective serotonin reuptake-inhibitor antidepressants and azole antifungal agents. Such predictive models deserve further evaluation, since they may ultimately yield more cost-effective and expeditious screening for drug interactions, with reduced human drug exposure and risk.

Inhibition of CYP2C9 by selective serotonin reuptake inhibitors in vitro: studies of phenytoin p-hydroxylation

AIMS

Inhibition of cytochrome P450 (CYP) activity by selective serotonin reuptake inhibitors (SSRIs) has frequently been reported with regard to pathways mediated by CYP2D6, CYP3A4/5, and CYP1A2. Little data exist on the capability of SSRIs to inhibit CYP2C9.

METHODS

We investigated the effect of SSRIs on p-hydroxylation of phenytoin (PPH), an established index reaction reflecting CYP2C9 activity, in an in vitro assay using liver tissue from six different human donors.

RESULTS

In control incubations (without inhibitor), 5-(p-hydroxy-phenyl)-5-phenylhydantoin (HPPH) formation rates were: Vmax 0.023 nmol min(-1) mg(-1); Km 14.3 microM. Average inhibition constants (Ki) differed significantly among the SSRIs, with fluvoxamine having the lowest Ki (6 microM) followed by R-fluoxetine (13 microM), norfluoxetine (17 microM), RS-fluoxetine (19 microM), sertraline (33 microM), paroxetine (35 microM), S-fluoxetine (62 microM), and desmethylsertraline (66 microM). Thus, assuming comparable molar concentrations at the site of inhibition, fluvoxamine can be expected to have the highest probability of interfering with the metabolism of CYP2C9 substrates. S-fluoxetine is on average a 5 fold weaker CYP2C9 inhibitor than either R-fluoxetine or the racemic mixture.

CONCLUSIONS

These findings are consistent with published case reports describing SSRI-related increments in plasma phenytoin levels. Because phenytoin has a narrow therapeutic index, plasma levels should be closely monitored when SSRIs are coadministered.

Human cytochromes mediating N-demethylation of fluoxetine in vitro

Biotransformation of the selective serotonin reuptake inhibitor antidepressant, fluoxetine, to its principal metabolite, norfluoxetine, was evaluated in human liver microsomes and in microsomes from transfected cell lines expressing pure human cytochromes. In human liver microsomes, formation of norfluoxetine from R,S-fluoxetine was consistent with Michaelis-Menten kinetics (mean K(m) = 33 microM), with evidence of substrate inhibition at high substrate concentrations in a number of cases. The reaction was minimally inhibited by coincubation with chemical probes inhibitory for P450-2D6 (quinidine), -1A2 (furafylline, alpha-naphthoflavone), and -2E1 (diethyldithiocarbamate). Substantial inhibition was produced by coincubation with sulfaphenazole (Ki = 2.8 microM), an inhibitory probe for P450-2C9, and by ketoconazole (Ki = 2.5 microM) and fluvoxamine (Ki = 5.2 microM). However, ketoconazole, relatively specific for P450-3A isoforms only at low concentrations, reduced norfluoxetine formation by only 20% at 1 microM, and triacetyloleandomycin (> or = 5 microM) reduced the velocity by only 20-25%. Microsomes from cDNA-transfected human lymphoblastoid cells containing human P450-2C9 produced substantial quantities of norfluoxetine when incubated with 100 microM fluoxetine. Smaller amounts of product were produced by P450-2C19 and -2D6, but no product was produced by P450-1A2, -2E1, or 3A4. Cytochrome P450-2C9 appears to be the principal human cytochrome mediating fluoxetine N-demethylation. P450-2C19 and -3A may make a further small contribution, but P450-2D6 is unlikely to make an important contribution.

Metabolism of dextromethorphan in vitro: involvement of cytochromes P450 2D6 and 3A3/4, with a possible role of 2E1

Dextromethorphan (DMO), a cough suppressing synthetic analog of codeine, undergoes parallel O-demethylation to dextrorphan (DOP), and N-demethylation to 3-methoxymorphinan (MEM), in humans. 3-hydroxymorphinan, a didemethylated metabolite, is formed secondarily. O-demethylation activity is well established as an index reaction for CYP2D6. However, this pathway appears to be mediated by at least two different enzymes in vitro. N-demethylation activity has recently been proposed to reflect CYP3A3/4 activity. We investigated both pathways in vitro with microsomal preparations from three human livers to assess the value of DMO as a probe drug for CYP2D6 and CYP3A3/4, DMO O-demethylation displayed a biphasic pattern with a high-affinity site reflecting CYP2D6 activity (mean Ki for quinidine, 0.1 +/- 0.13 microM). Kinetic parameters for the two O-demethylation mediating enzymes predict an average relative intrinsic clearance (Vmax/K(m) ratio) of 96% of total O-demethylation mediated via the high-affinity enzyme. Thus, in vitro data confirms the usefulness of DMO O-demethylation as an index reaction to monitor CYP2D6 activity. The Eadie-Hofstee plot of DMO N-demethylation was consistent with single-enzyme Michaelis-Menten kinetics (Vmax varying from 3.3 to 6.8 nmol mg-1 min-1, K(m) from 231 to 322 microM). However, ketoconazole, a CYP3A3/4 inhibitor, reduced N-demethylation only by 60% and had a mean Ki an order of magnitude higher (0.37 microM) compared to other pure CYP3A3/4 mediated reactions. Troleandomycin, a mechanism based CYP3A3/4 inhibitor, inhibited MEM formation by an average maximum of 46%, with an IC50 varying from 1 to 2.6 microM. A polyclonal rat liver CYP3A1 antibody inhibited MEM formation only by approximately 50%. Diethyldithiocarbamate (DDC), a mechanism based CYP2E1 inhibitor, reduced MEM formation at concentrations up to 150 microM between 33 and 43%. Chemical inhibitors of CYP2d6 (quinidine), CYP1A1/2 (alpha-naphthoflavone), and CYP2C9 (sulfaphenazole), as well as a goat rat liver CYP2C11 polyclonal antibody (inhibitory against human CYP2C9 and CYP2C19), had minimal effect on MEM formation rate, thus excluding an involvement of any of these enzymes. DMO N-demethylation is only partly mediated by CYP3A3/4, and therefore is not a reliable index reaction for CYP3A3/4 activity either in vitro or probably in vivo.

Alprazolam hydroxylation by mouse liver microsomes in vitro: the effect of age and phenobarbital induction

The effects of age on hepatic microsomal enzyme induction were studied in male CD-1 mice. Six week old and 1 year old animals were treated with either phenobarbital (80 mg kg-1) or saline once daily for 3d. Twenty-four hours after the last treatment, animals were sacrificed and livers were harvested. Hepatic microsomal fractions were isolated and incubated with alprazolam, a triazolobenzodiazepine metabolized by cytochrome P-450-3A isoforms in humans. Metabolites were identified and quantitated by HPLC. All microsomal preparations produced two principal metabolites (alpha-OH- and 4-OH-alprazolam) while microsomes from phenobarbital-treated animals also produced a third metabolite (alpha, 4-dihydroxyalprazolam). Vmax, K(m), and intrinsic clearance (Vmax/K(m) ratio) for both alpha-OH- and 4-OH-alprazolam in the saline-treated control animals were not significantly different between age groups. Vmax and intrinsic clearance for both metabolites were more than three times greater in phenobarbital-treated animals than in the control mice (p < 0.001). Age did not influence the extent of induction, and both pathways were induced to approximately an equal extent. Thus the present in vitro study of liver microsomal preparations from male CD-1 mice does not delineate a mechanism for impaired alprazolam clearance in aging organisms in vivo. There is no evidence that age alters susceptibility to induction by phenobarbital.

Biotransformation of mestranol to ethinyl estradiol in vitro: the role of cytochrome P-450 2C9 and metabolic inhibitors

Mestranol, the estrogen component of some oral contraceptive formulations, must be demethylated to its active metabolite, 17 alpha-ethinyl estradiol, to produce estrogenic activity. To investigate the transformation of mestranol to ethinyl estradiol, an in vitro assay was used with human liver microsomes from four different donors. Incubation of a fixed concentration of mestranol (3 mumol/L) with varying concentrations of CYP inhibitors revealed strong inhibition of ethinyl estradiol formation by sulfaphenazole, a specific CYP2C9 inhibitor, with an average inhibitor concentration at one half of Emax (IC50) of 3.6 mumol/L (range, 1.8-8.3 mumol/L) and an average maximal inhibitory capacity (Emax) of 75% (range, 60-91%). Troleandomycin (a CYP3A3/4 inhibitor) and quinidine (a CYP2D6 inhibitor), however, produced no substantial inhibitory activity. alpha-Naphthoflavone (a CYP1A1/2 inhibitor only at concentrations < 2 mumol/L and a CYP2C9 inhibitor at higher concentrations) had a weak inhibitory effect on ethinyl estradiol formation (< 20% decrease in mestranol demethylation activity). Of the three antifungal azoles tested, miconazole strongly inhibited mestranol demethylation, with an average IC50 of 1.5 mumol/L (range, 0.7-3.2 mumol/L) and an average Emax of 90% (range, 77-100%), whereas fluconazole displayed relatively weak inhibition only at the highest concentration of 50 mumol/L (mean reduction in demethylation activity was 29%). Itraconazole produced no meaningful inhibition. Strong inhibition of ethinyl estradiol formation by sulfaphenazole suggests a major contribution of CYP2C9 to this reaction.

Phenacetin O-deethylation by human liver microsomes in vitro: inhibition by chemical probes, SSRI antidepressants, nefazodone and venlafaxine

Biotransformation of phenacetin via O-deethylation to acetaminophen, an index reaction reflecting activity of Cytochrome P450-1A2, was studied in microsomal preparations from a series of human livers. Acetaminophen formation was consistent with a double Michaelis-Menten system, with low-Km (mean Km1 = 68 microM) and high-Km (mean Km2 = 7691 microM) components. The low-K(m) enzyme accounted for an average of 96% of estimated intrinsic clearance, and was predicted to contribute more than 50% of net reaction velocity at phenacetin concentrations less than 2000 microM. Among index inhibitor probes, alpha-naphthoflavone was a highly potent inhibitor of the low-Km enzyme (Ki1 = 0.013 microM); furafylline also was a moderately active inhibitor (Ki1 = 4.4 microM), but its inhibiting potency was increased by preincubation with microsomes. Ketoconazole was a relatively weak inhibitor (Ki1 = 32 microM); quinidine and cimetidine showed minimal inhibiting activity. Among six selective serotonin reuptake inhibitor (SSRI) antidepressants, fluvoxamine was a potent inhibitor of 1A2 (mean Ki1 = 0.24 microM). The other SSRIs were more than tenfold less potent. Mean Ki1 values were: fluoxetine, 4.4 microM; norfluoxetine, 15.9 microM; sertraline, 8.8 microM; desmethylsertraline, 9.5 microM; paroxetine, 5.5 microM. The antidepressant nefazodone and four of its metabolites (meta-chloro-phenylpiperazine, two hydroxylated derivatives, and a triazoledione) were very weak inhibitors of P450-1A2. Venlafaxine and its O- and N-desmethyl metabolites showed minimal inhibitory activity.

Influence of polyethylene glycol and acetone on the in vitro biotransformation of tamoxifen and alprazolam by human liver microsomes

The use of in vitro hepatic microsomal models as a method of studying the biotransformation of xenobiotics can be complicated by the insolubility of a lipophilic substrate. Addition of solvent to an in vitro system is an approach used to increase solubility of such substrates, but solvents may interact with the microsomal enzymes and cause changes in the kinetic parameters. This study focused on the solvents polyethylene glycol-400 (PEG) and acetone and their influence on the human microsomal in vitro biotransformation of the antineoplastic agent tamoxifen (TAM). The antianxiety agent alprazolam (ALP), a verified P450 3A substrate, was also studied. TAM is a nonpolar drug that is metabolized to two oxidative metabolites, N-desmethyltamoxifen (DMT) and 4-hydroxytamoxifen (4-OH-TAM), by cytochrome P450 isoforms. DMT is formed primarily by 3A isoforms, and the pathway(s) responsible for 4-OH-TAM formation are unknown. Biotransformation of ALP is mediated by P450 3A isoforms, which form alpha-hydroxyalprazolam (alpha-OH-ALP) and 4-hydroxyalprazolam (4-OH-ALP). Both PEG and acetone at 5% of the total microsomal incubation volume were found to solubilize TAM in the in vitro system. However, both solvents had an effect on the P450 mediated metabolism of ALP and TAM. For ALP, PEG was a noncompetitive inhibitor of alpha-OH-ALP (mean Ki = 2.06%) and 4-OH-ALP (mean Ki = 2.37%) formation. Acetone stimulated the production of alpha-OH-ALP, but had no apparent influence on 4-OH-ALP formation. The solvent's influence on TAM metabolism varied. PEG decreased the amount of DMT formed by the microsomes as compared to the system containing no solvent (control); however, 4-OH-TAM formation in the presence of PEG was 146-226% of controls. Samples containing acetone produced smaller quantities of both DMT (39-63%) and 4-OH-TAM (45-69%) as compared to controls. Since PEG and acetone increased the solubility of TAM in the incubation buffer but inhibited or accelerated enzymatic reactions compared to buffer alone, actual solubility in buffer was not a determinant of the rate of TAM metabolism. TAM appeared to be taken up into the microsomal fraction, making it available for biotransformation by the P450 isoforms. Although solvents may increase the solubility of nonpolar agents in aqueous systems, careful evaluation of the effects of solvent on metabolite formation, as compared to buffer controls, is needed in order to properly evaluate an in vitro metabolic model.

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.

Inhibition of terfenadine metabolism in vitro by azole antifungal agents and by selective serotonin reuptake inhibitor antidepressants: relation to pharmacokinetic interactions in vivo

Biotransformation of the H-1 antagonist terfenadine to its desalkyl and hydroxy metabolites was studied in vitro using microsomal preparations of human liver. These metabolic reactions are presumed to be mediated by Cytochrome P450-3A isoforms. The azole antifungal agent ketoconazole was a highly potent inhibitor of both reactions, having mean inhibition constants (Ki) of 0.037 and 0.34 microM for desalkyl- and hydroxy-terfenadine formation, respectively. Itraconazole also was a potent inhibitor, with Ki values of 0.28 and 2.05 microM, respectively. Fluconazole, on the other hand, was a weak inhibitor. Six selective serotonin reuptake inhibitor antidepressants tested in this system were at least 20 times less potent inhibitors of terfenadine metabolism than was ketoconazole. An in vitro-in vivo scaling model used in vitro Ki values, typical clinically relevant plasma concentrations of inhibitors, and presumed liver:plasma partition ratios to predict the degree of terfenadine clearance impairment during coadministration of terfenadine with these inhibitors in humans. The model predicted a large and potentially hazardous impairment of terfenadine clearance by ketoconazole and, to a slightly lesser extent, by itraconazole. However, fluconazole and the six selective serotonin reuptake inhibitors (SSRIs) at usual clinical doses were not predicted to impair terfenadine clearance to a degree that would be of clinical importance. Caution is nonetheless warranted with the coadministration of SSRIs and terfenadine when high doses of SSRIs (particularly fluoxetine) are administered. Also, some individuals may be unusually susceptible to metabolic inhibition for a variety of reasons.

Inhibition of cytochrome P450 by nefazodone in vitro: studies of dextromethorphan O- and N-demethylation

Nefazodone (NEF), a 5-HT2A/2C antagonist antidepressant, is extensively metabolized in the human body to hydroxy NEF (OH-NEF), p-hydroxy NEF (pOH-NEF), a dione metabolite, and via cleavage of the molecule to m-chlorophenyl-piperazine (mCPP) and BMY-33604. The latter is further metabolized to BMS-183695-01 (BMSa) and BMS-183562-01 (BMSb). To investigate the potential of NEF and its metabolites to interfere with the metabolism of other drugs, we tested these compounds for their ability to alter dextromethorphan (DMO) O-demethylation to dextrorphan (DOP; an index reaction for CYP2D6) and N-demethylation to 3-methoxy morphinan (MEM, a recently proposed index reaction of CYP3A3/4). The assay was performed in an in vitro system with human liver microsomes from three different donors. NEF, OH-NEF, pOH-NEF, mCPP and BMSb were weak inhibitors of DMO O and N-demethylation, with average Ki values ranging from 18 to 50 microM for DOP formation, and from 21 to > 200 microM for MEM formation. The dione metabolite and BMSa did not produce detectable inhibition of either pathway. The findings for DMO O-demethylation, well-established as a CYP2D6-mediated reaction, indicate that NEF and metabolites are weak inhibitors of this reaction, with Ki values at least 100 times higher than fluoxetine (Ki = 0.1 microM +/- 0.09). The implications of results on DMO N-demethylation are not clear. In vivo data, as well as in vitro data based on "pure' CYP3A3/4 substrates, provide evidence for clinically relevant CYP3A3/4 inhibition by NEF, OH-NEF, and pOH-NEF. Thus, formation of MEM by N-demethylation of DMO may not constitute a suitable index reaction to probe CYP3A3/4 activity.

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

The identification of cytochrome P450 (CYP) isozymes mediating metabolic pathways of drugs has become increasingly important in anticipating pharmacokinetic drug interactions. The activity of individual CYPs can be monitored in vitro with human liver microsomes by means by relatively specific metabolic reactions: for CYP1A1/2, phenacetin O-deethylation; for CYP2C9, phenytoin 4-hydroxylation; for CYP2C19, S-mephenytoin 4'-hydroxylation; for CYP2D6, dextromethorphan O-demethylation; for CYP3A3/4, alprazolam 4-hydroxylation, and for CYP2E1, chlorzoxazone 6-hydroxylation. We determined the kinetic parameters (Vmax, Km) of these reactions and utilized them to test a monoclonal rat liver CYP1A1 antibody, a monoclonal rat liver CYP2E1 antibody, a polyclonal rabbit anti-rat liver CYP3A1 antibody, and a polyclonal goat anti-rat liver CYP2C11 antibody for their specificity and inhibitory capacity. The CYP1A1 monoclonal antibody (MAb), the CYP2E1 MAb, and the CYP3A1 polyclonal antibody (PAb) inhibited only their respective index reactions. The CYP2C11 PAb inhibited both phenytoin 4-hydroxylation and S-mephenytoin 4'-hydroxylation. At a microsomal versus antibody protein mass ratio of 1:15, 4-hydroxyalprazolam formation was reduced by 73.4% of control with the CYP3A1 PAb; 4'-hydroxymephenytoin formation decreased by 66.3% and 4-hydroxyphenytoin formation by 43.4% with the CYP2C11 PAb; phenacetin O-deethylation was reduced by 39.7% with the CYP1A1 MAb, and 6-hydroxychlorzoxazone formation decreased by 30.0% with the CYP2E1 MAb. Thus, all antibodies tested are at least CYP subfamily specific. The PAbs exhibited greater than 60% inhibition versus the CYP3A3/4- and the CYP2C19-mediated reactions, whereas the MAbs produced less than 50% inhibition for their respective index reactions. Because of their limited inhibitory capacity, MAbs may be correspondingly limited as tools for identification of human CYP enzymes via immunoinhibition studies.

Triazolam biotransformation by human liver microsomes in vitro: effects of metabolic inhibitors and clinical confirmation of a predicted interaction with ketoconazole

Biotransformation of the triazolobenzodiazepine triazolam to its hydroxylated metabolites, alpha-hydroxy (OH)- and 4-OH-triazolam, was studied in vitro using microsomal preparations of human liver. Mean values of Vmax (10.3 nM/min/mg of protein) and Km (304 microM) for the 4-OH pathway exceeded values for the alpha-OH pathway (2.4 and 74, respectively). However the mean Vmax/Km ratios for the two pathways were nearly identical, indicating that both contribute approximately equally to intrinsic clearance. Ketoconazole was a powerful inhibitor of triazolam biotransformation, having mean competitive Ki values of 0.006 and 0.023 microM for the alpha-OH and 4-OH pathways. This is consistent with the role of P450-3A isoforms in mediating triazolam biotransformation. The serotonin2 antagonist antidepressant nefazodone inhibited the alpha-OH and 4-OH pathways (Ki = 0.6 and 1.7 microM, respectively), but with considerably less activity than ketoconazole. Among six selective serotonin reuptake-inhibitor antidepressants, norfluoxetine was the most potent inhibitor (Ki = 2.7 and 8.0 microM) and fluoxetine itself was the weakest (Ki = 7.0 and 44.3 microM). In a double-blind clinical pharmacokinetic-pharmacodynamic study, administration of triazolam (0.125 mg) preceded by ketoconazole, compared to triazolam preceded by placebo, produced a nearly 9-fold reduction in apparent oral clearance of triazolam (41 vs. 337 ml/min) and a 4-fold prolongation of half-life (13.5 vs. 3.4 hr). Pharmacodynamic testing indicated enhancement of electroencephalographic beta activity, and enhanced decrements in digit-symbol substitution test performance, attributable to coadministration of ketoconazole. Plasma ketoconazole concentrations measured in the clinical study ranged from 0.02 microgram/ml (projected minimum) to 4.95 micrograms/ml (maximum). An in vitro-in vivo scaling model, using these plasma ketoconazole concentrations together with liver partition ratios and the in vitro Ki values, predicted a decrement of triazolam clearance due to ketoconazole coadministration that was consistent with the 88% decrement in clearance actually observed in vivo.

N-demethylation of amitriptyline in vitro: role of cytochrome P-450 3A (CYP3A) isoforms and effect of metabolic inhibitors

Biotransformation of amitriptyline (AMI) to its demethylated product nortriptyline (NT) was studied in vitro with human liver microsomes from four different donors, preselected to reflect a range of metabolic rates. Reaction velocity versus substrate concentration was consistent with a sigmoid Vmax model. Vmax varied from 0.42 to 3.42 nmol/mg/min, Km from 33 to 89 microM AMI. Ketoconazole was a highly potent inhibitor of N-demethylation, with a mean Ki value of 0.11 +/- 0.013 microM (+/- S.D.), whereas quinidine (up to 50 microM), a CYP2D6 inhibitor, and alpha-naphthoflavone (up to 5 microM), a CYP1A2 inhibitor only at low concentrations, showed no effect. All selective serotonin reuptake inhibitors (SSRIs) tested had an inhibitory effect on the formation of NT, with mean Ki values of 4.37 (+/- 3.38) microM for sertraline, 5.46 (+/- 1.95) microM for desmethylsertraline, 9.22 (+/- 3.69) microM for fluvoxamine, 12.26 (+/- 5.67) microM for norfluoxetine, 15.76 (+/- 5.05) microM for paroxetine, and 43.55 (+/- 18.28) microM for fluoxetine. A polyclonal rabbit antibody against rat liver CYP3A1, in antibody/microsomal protein ratios varying from 1:1 to 10:1, inhibited N-demethylation of AMI to an asymptotic maximum of 60%. These results are consistent with several case reports describing impairment of AMI metabolism by SSRIs. Inhibition of AMI demethylation by low concentrations of ketoconazole and by anti-3A antibody supports an important role for CYP3A isoforms in mediating this reaction.

Inhibition of alprazolam and desipramine hydroxylation in vitro by paroxetine and fluvoxamine: comparison with other selective serotonin reuptake inhibitor antidepressants

In vitro preparations of human liver microsomes were used to study the inhibiting effects of two selective serotonin reuptake inhibitor (SSRI) antidepressants, paroxetine and fluvoxamine, on metabolism via hydroxylation of alprazolam and of desipramine. These reactions are mediated by Cytochromes P450-3A4 and P450-2D6, respectively. Paroxetine was a highly potent inhibitor of desipramine hydroxylation; the inhibition constant (Ki) value of 2.0 microM indicated greater inhibiting potency than fluoxetine or norfluoxetine. The in vitro data predicted in vivo impairment of desipramine clearance by coadministration of paroxetine which was in the same range as observed in a clinical study. Fluvoxamine, by contrast, was a much weaker inhibitor of desipramine hydroxylation, having a Ki value (16.6 microM) similar to those of sertraline and desmethylsertraline. For hydroxylation of alprazolam, paroxetine was a relatively weak inhibitor, approximately comparable to fluoxetine, whereas fluvoxamine showed inhibiting capacity similar to that of norfluoxetine. The in vitro data predicted the degree of impairment of alprazolam clearance observed in vitro model can therefore provide clinically relevant data on prediction of potential drug interactions with SSRIs.

Metabolism of drugs by cytochrome P450 3A isoforms. Implications for drug interactions in psychopharmacology

Members of the P450 3A subfamily are the most abundant of the human hepatic cytochromes. CYP3A isoforms mediate the biotransformation of many drugs, including a number of psychotropic, cardiac, analgesic, hormonal, immunosuppressant, antineoplastic, and antihistaminic agents. Activity of CYP3A in humans is variable among individuals, but there is no evidence of genetic polymorphism. Significant amounts of CYP3A are present in the gastrointestinal tract, and may contribute to presystemic extraction of drugs such as cyclosporin. The azole antifungal agents ketoconazole and itraconazole are potent inhibitors of human CYP3A isoforms. Selective serotonin reuptake inhibitor (SSRI) antidepressants are also CYP3A inhibitors, but much less potent than ketoconazole or itraconazole. In vitro models can provide important information on the qualitative and quantitative activity of potential inhibitors of human cytochromes. However, in vitro inhibition constant (Ki) values alone do not predict the magnitude of an in vivo interaction, nor whether an interaction will be of clinical importance. For example, SSRIs are predicted to impair clearance of the antihistamine terfenadine in humans. However, the magnitude of this effect is much less than would be associated with a pharmacokinetic interaction of clinical importance.

In vitro prediction of the terfenadine-ketoconazole pharmacokinetic interaction

Biotransformation of the peripherally acting H-1 histamine antagonist, terfenadine, to its desalkyl and hydroxy metabolites was studied in vitro using microsomal preparations from six separate human livers. These metabolic reactions are mediated by the specific cytochrome P450-3A4. Addition of ketoconazole to the reaction mixtures reduced the rate of formation of both metabolites in a manner consistent with competitive inhibition. Ketoconazole inhibition constants (Ki) averaged 0.024 microM for the desalkyl terfenadine pathway, and 0.237 microM for the hydroxy terfenadine pathway. A mathematical model, based on the in vitro Ki values and the usual clinical range of plasma ketoconazole concentrations (1-5 micrograms/mL; 1.88-0.94 microM), predicted that plasma terfenadine levels during coadministration of ketoconazole would increase by a factor ranging from 13-fold to 59-fold relative to the same dose of terfenadine given without ketoconazole. Actual plasma terfenadine levels during terfenadine-ketoconazole coadministration in a clinical pharmacokinetic study were close to those predicted by the model. These plasma levels were associated with prolongation of the corrected QT interval, thereby explaining the potentially life-threatening ventricular arrhythmias reportedly associated with terfenadine-ketoconazole cotherapy. Thus, data from studies of drug metabolism in vitro can be used to predict and thereby possibly avoid clinically important drug interactions.

Inhibitors of alprazolam metabolism in vitro: effect of serotonin-reuptake-inhibitor antidepressants, ketoconazole and quinidine

1. The biotransformation of the triazolobenzodiazepine alprazolam (ALP) to its hydroxylated metabolites (4-OH-ALP and alpha-OH-ALP) was evaluated in human, monkey, rat, and mouse liver microsomes. 2. In all species 4-OH-ALP was the principal metabolite, accounting for 84% of clearance in human microsomes compared with 16% for alpha-OH-ALP. 3. Among the serotonin-specific reuptake inhibitors fluoxetine (FLU) and sertraline (SERT), and their respective demethylated metabolites norfluoxetine (NOR) and desmethylsertraline (DES), NOR was the most potent inhibitor (mean Ki for 4-OH-ALP formation in humans: 11 microM), FLU the weakest (Ki = 83 microM), with SERT and DES falling in between (Ki = 24 and 20 microM). 4. The in vitro data predict 29% inhibition of ALP clearance at mean FLU and NOR plasma concentrations of 77 ng ml-1 and 72 ng ml-1, respectively, after correction for liver:water partition ratios in the range of 12-14. The observed mean degree of inhibition in a previous in vivo study was 21%. 5. Ketoconazole was a potent inhibitor of ALP metabolism in vitro (Ki = 0.046 microM), suggesting that ALP hydroxylation is mediated by the cytochrome P450-3A sub-family. Quinidine was a weak inhibitor (Ki = 626 microM).

Inhibition of desipramine hydroxylation in vitro by serotonin-reuptake-inhibitor antidepressants, and by quinidine and ketoconazole: a model system to predict drug interactions in vivo

Biotransformation of the tricyclic antidepressant desipramine (DMI) to its metabolite 2-hydroxy-desipramine (2-OH-DMI) was studied in vitro using microsomal preparations from human, monkey, mouse and rat liver. In all species 2-OH-DMI was the principal identified metabolite. Mean (+/- S.E.) reaction parameters in six human liver samples were: Vmax, 0.11 +/- .02 nmol/ml/min/mg protein; Km, 16.1 +/- 4.2 microM. Quinidine was a highly potent inhibitor of 2-OH-DMI formation (mean Ki = 0.053 microM), consistent with the presumed role of Cytochrome P450-2D6 in mediating this reaction. Ketoconazole was a much less potent inhibitor (mean Ki = 10.3 microM). Two serotonin-specific reuptake inhibitor (SSRI) antidepressants, and their respective metabolites, were evaluated as potential inhibitors of 2-OH-DMI formation. Fluoxetine (FLU) and norfluoxetine (NOR) were the most potent inhibitors (mean Ki values: 3.0 and 3.5 microM, respectively). Sertraline (SERT) and its metabolite desmethylsertraline (DES) also inhibited the reaction (mean Ki: 22.7 and 16.0 microM), but were significantly less potent than FLU or NOR. Values of Ki and Km measured in vitro were used to generate a theoretical prediction of the degree of clearance inhibition in vivo at any given concentration of substrate and inhibitor. The model was applied to a clinical study in which DMI clearance in humans was impaired by coadministration of FLU (yielding FLU and NOR in plasma) or by SERT (yielding SERT and DES in plasma). Use of plasma SSRI concentrations in the predictive model underestimated the actual impairment of DMI clearance.(ABSTRACT TRUNCATED AT 250 WORDS)

Alprazolam metabolism in vitro: studies of human, monkey, mouse, and rat liver microsomes

Biotransformation of the triazolobenzodiazepine alprazolam (ALP) was studied in vitro using hepatic microsomal preparations from human, monkey, mouse, and rat liver tissue. Two principal hydroxylated metabolites were identified: 4-hydroxy- and alpha-hydroxy-alprazolam (4-OH-ALP and alpha-OH-ALP). In all species, rates of 4-OH-ALP formation exceeded those of alpha-OH-ALP. In human liver microsomes, ratios of 4-OH-ALP/alpha-OH-ALP reaction velocities calculated at clinically relevant plasma concentrations of ALP ranged from 7 to 17, qualitatively consistent with, but numerically larger than, the ratio of the plasma levels of the two metabolites during clinical use of ALP in humans. Km values for both 4-OH-ALP (170-305 microM) and alpha-OH-ALP (63-441 microM) considerably exceeded the usual maximum plasma concentration observed in humans (200 ng/ml, 0.65 microM), consistent with the linear (dose-independent) pharmacokinetic characteristics of ALP observed in humans. Thus formation of 4-OH-ALP via hydroxylation is the major route of ALP metabolism. This pathway is probably mediated by the cytochrome P-450-3A subfamily. Factors that impair the activity of this cytochrome subtype are likely to impair clearance of ALP in vivo.

Alprazolam pharmacokinetics, metabolism, and plasma levels: clinical implications

The triazolobenzodiazepine alprazolam is biotransformed by hepatic microsomal oxidation, yielding two hydroxylated metabolites (4-hydroxy- and a-hydroxy-alprazolam) as the principal metabolic products. Both metabolites have lower benzodiazepine receptor affinity than the parent compound and at steady state appear in plasma at concentrations considerably lower than intact alprazolam. Thus, clinical activity during treatment with alprazolam is essentially entirely attributable to intact alprazolam. The cytochrome P450 IIIA subfamily appears to mediate alprazolam metabolism in humans. This cytochrome subfamily is not subject to variation due to genetic polymorphism. Ketoconazole, cimetidine, macrolide antibiotics, and serotonin-reuptake-inhibitor antidepressants impair alprazolam biotransformation in vitro. Reduced clearance of alprazolam in vivo has been demonstrated for drugs in this group that have been studied in humans; for those not yet studied, impaired alprazolam clearance should be anticipated during coadministration. Studies of plasma alprazolam concentration versus clinical response during short-term treatment of panic disorder indicate that therapeutic response at steady-state plasma levels of 20 to 40 ng/mL is significantly greater than at levels less than 20 ng/mL. Substantial additional benefit from plasma levels greater than 40 ng/mL is not consistently demonstrated. However, side effects attributable to benzodiazepine agonist activity (e.g., drowsiness, sedation) increase in frequency with increasing steady-state plasma levels. Concentration-response data indicate that monitoring of alprazolam plasma levels can be of considerable clinical value during treatment of panic disorder.

Inhibition of acetaminophen and lorazepam glucuronidation in vitro by probenecid

The effect of probenecid on glucuronidation of acetaminophen and lorazepam in hepatic microsomes from various species was studied to see if in vitro results were consistent with previous in vivo observations. Mouse, rat, and human microsomes were incubated with acetaminophen and probenecid while monkey microsomes were incubated with lorazepam and probenecid. Glucuronidation rates in all species varied with substrate, protein, and detergent concentrations. Mice exhibited faster rates of glucuronidation than rats or humans. All species showed inhibition of glucuronidation of acetaminophen or lorazepam when probenecid was added. Analysis suggested competitive inhibition. Thus, in vitro studies support in vivo results and confirm that the inhibition takes place at the hepatic level.

Clinical pharmacokinetics of antidepressants in the elderly. Therapeutic implications

The prevalence of depression in the elderly suggests that a substantial number of older patients will be treated with an antidepressant medication such as one of the tricyclics, trazodone, fluoxetine or lithium. The physiological changes that accompany aging raise the possibilities of altered pharmacokinetics, patterns of efficacy and adverse effect profiles. The literature addressing the subject of antidepressant use in the elderly has not provided a clear, consistent picture of how these drugs behave in this population in comparison with younger patients. Particularly in the case of the tricyclic antidepressants (TCAs), a large degree of interindividual variation in drug clearance (CL) confounds attempts to find differences attributable to age per se. Study design, however, is also a problem in that very few investigators include a young control group, choosing instead to compare their data with previously reported outcomes. Designations of statistical significance and positive correlation also differ among investigators, and the clinical significance of any finding is not always addressed. The available data suggest that imipramine CL is reduced in the elderly and that amitriptyline CL may be reduced. Desipramine CL does not appear to be affected by age, although decreased renal function in the elderly may lead to accumulation of the hydroxylated metabolite, the clinical importance of which is not known. Nortriptyline is the most thoroughly studied TCA in the elderly. CL seems decisively lower only in elderly patients with concurrent medical illness. The hydroxylated metabolite probably accumulates with diminishing renal function. Not enough data are available on doxepin to make a conclusion. Trazodone CL is diminished somewhat in elderly men. Lithium CL appears to diminish with the declining renal function associated with aging. Fluoxetine data are sparse. Available data do not show any decrease in CL of the parent drug; more information is needed on the metabolite norfluoxetine. Although knowledge of CL changes with aging can help the clinician more accurately achieve the desired steady-state concentration of a drug during long term therapy, much work is still needed to evaluate the relationships among drug concentrations at steady-state, efficacy and adverse effects in the elderly.

Antipyrine kinetics in patients with primary biliary cirrhosis

Fourteen antimitochrodrial antibody-positive patients (13 women, 1 man) with biopsy-proven primary biliary cirrhosis, aged 40 to 71 years (mean, 57 years) weighing 43 to 102 kg (mean, 63 kg), along with 14 age- and sex-matched healthy controls, received a single 1.0- to 1.2-g dose of intravenous antipyrine. Plasma antipyrine levels were determined during a 12- to 24-hour period. Patients' mean serum chemistry values were: albumin, 3.9 g/dL (range, 3.1-4.4) and total bilirubin, 1.9 mg/dL (range, 0.3-10.9). Seven of the fourteen patients had cirrhosis. Mean kinetic variables for antipyrine in controls and primary biliary cirrhosis patients were: Vd, .54 versus .49 L/kg; half-life, 12.0 versus 15.1 hours (P < .07); clearance, .55 versus .41 mL/min/kg (P < .04). Within the primary biliary cirrhosis group, there was no correlation between total bilirubin and clearance (r = .09), nor did clearance vary significantly among histologic categories. Clearance of antipyrine in primary biliary cirrhosis patients is reduced by an average of 25%, but the clinical prognosticators of serum bilirubin levels and histologic grade do not correlate with or predict the degree of clearance impairment.