Clinical implications of enzyme induction and enzyme inhibition

Addison's Disease Caused by Tuberculosis with Atypical Hyperpigmentation and Active Pulmonary Tuberculosis

We herein report a case of Addison's disease caused by tuberculosis characterized by atypical hyperpigmentation, noted as exacerbation of the pigmentation of freckles and the occurrence of new freckles, that was diagnosed in the presence of active pulmonary tuberculosis. The clinical condition of the patient was markedly ameliorated by the administration of hydrocortisone and anti-tuberculosis agents. When exacerbation of the pigmentation of the freckles and/or the occurrence of new freckles are noted, Addison's disease should be considered as part of the differential diagnosis. In addition, the presence of active tuberculosis needs to be assumed whenever we treat patients with Addison's disease caused by tuberculosis, despite its rarity.

Investigation of Host-Gut Microbiota Modulation of Therapeutic Outcome

A broader understanding of factors underlying interindividual variation in pharmacotherapy is important for our pursuit of "personalized medicine." Based on knowledge gleaned from the investigation of human genetics, drug-metabolizing enzymes, and transporters, clinicians and pharmacists are able to tailor pharmacotherapies according to the genotype of patients. However, human host factors only form part of the equation that accounts for heterogeneity in therapeutic outcome. Notably, the gut microbiota possesses wide-ranging metabolic activities that expand the metabolic functions of the human host beyond that encoded by the human genome. In this review, we first illustrate the mechanisms in which gut microbes modulate pharmacokinetics and therapeutic outcome. Second, we discuss the application of metabonomics in deciphering the complex host-gut microbiota interaction in pharmacotherapy. Third, we highlight an integrative approach with particular mention of the investigation of gut microbiota using culture-based and culture-independent techniques to complement the investigation of the host-gut microbiota axes in pharmaceutical research.

RO4938581, a GABAAα5 modulator, displays strong CYP1A2 autoinduction properties in rats

Autoinduction in drug metabolism is a known phenomenon observed when a drug induces the enzymes responsible for its own metabolism. The potency, rate and extent of autoinduction following a given treatment paradigm may have therapeutic implications in clinic as well as for in vivo pharmacological assessments in animals. RO4938581, an imidazo-triazolo-benzodiazepine, is a novel GABAAα5 negative modulator recently pursued for the treatment of cognitive dysfunctions. As circulating plasma levels of RO4938581 were shown to decrease rapidly after repeated dosing in rats, with CYP1A2 being involved in the metabolism of the compound, we examined the potential role of RO4938581-mediated autoinduction of CYP1A2. Incubation of rat hepatocytes with RO4938581 revealed potent CYP1A2 induction with significant increase in enzymatic activity at concentrations of 0.1nM and RO4938581 was shown to be 700-fold more potent than β-napththoflavone. Ex vivo studies revealed a 7-fold increase in metabolic CYP1A2 activity in liver microsomes prepared from rats administered with 0.1mg/kg of RO4938581 24h before. This induction profile was reflected in vivo in pharmacokinetic studies in rats where an 8-fold reduction in plasma exposure was observed after a second dose. The reduction in plasma exposures due to CYP1A2 autoinduction were confirmed functionally in contextual fear conditioning paradigm in rats, where a positive pharmacological effect observed after acute drug administration disappeared completely after sub-chronic dosing. Together, these findings suggest that RO4938581 possesses potent CYP1A2 autoinductive properties in rats and may serve as a tool for mechanistic metabolism or drug-drug interaction studies encircling this enzyme in rats.

Therapeutic efficacy of human hepatocyte transplantation in a SCID/uPA mouse model with inducible liver disease

BACKGROUND

Severe Combined Immune Deficient (SCID)/Urokinase-type Plasminogen Activator (uPA) mice undergo liver failure and are useful hosts for the propagation of transplanted human hepatocytes (HH) which must compete with recipient-derived hepatocytes for replacement of the diseased liver parenchyma. While partial replacement by HH has proven useful for studies with Hepatitis C virus, complete replacement of SCID/uPA mouse liver by HH has never been achieved and limits the broader application of these mice for other areas of biomedical research. The herpes simplex virus type-1 thymidine kinase (HSVtk)/ganciclovir (GCV) system is a powerful tool for cell-specific ablation in transgenic animals. The aim of this study was to selectively eliminate murine-derived parenchymal liver cells from humanized SCID/uPA mouse liver in order to achieve mice with completely humanized liver parenchyma. Thus, we reproduced the HSVtk (vTK)/GCV system of hepatic failure in SCID/uPA mice.

METHODOLOGY/PRINCIPAL FINDINGS

In vitro experiments demonstrated efficient killing of vTK expressing hepatoma cells after GCV treatment. For in vivo experiments, expression of vTK was targeted to the livers of FVB/N and SCID/uPA mice. Hepatic sensitivity to GCV was first established in FVB/N mice since these mice do not undergo liver failure inherent to SCID/uPA mice. Hepatic vTK expression was found to be an integral component of GCV-induced pathologic and biochemical alterations and caused death due to liver dysfunction in vTK transgenic FVB/N and non-transplanted SCID/uPA mice. In SCID/uPA mice with humanized liver, vTK/GCV caused death despite extensive replacement of the mouse liver parenchyma with HH (ranging from 32-87%). Surprisingly, vTK/GCV-dependent apoptosis and mitochondrial aberrations were also localized to bystander vTK-negative HH.

CONCLUSIONS/SIGNIFICANCE

Extensive replacement of mouse liver parenchyma by HH does not provide a secure therapeutic advantage against vTK/GCV-induced cytotoxicity targeted to residual mouse hepatocytes. Functional support by engrafted HH may be secured by strategies aimed at limiting this bystander effect.

Altered prednisolone pharmacokinetics in patients treated with rifampicin

The pharmacokinetics and protein binding of prednisolone were studied in 7 patients before and after 3 weeks of rifampicin therapy. The elimination half-time for prednisolone decreased by 45 +/- 8.1% (p less than 0.01), and the total body clearance of prednisolone increased by 91 +/- 26% (p less than 0.01). The area under the time-concentration curve (AUC) of total (free plus protein-bound) prednisolone decreased by 48 +/- 7.3% (p less than 0.01) and of free, unbound prednisolone by 57 +/- 9.8% (p less than 0.01). The reduction in AUC was greater for free than for total prednisolone (p less than 0.05) mainly due to the non-linear nature of prednisolone protein binding. There was no significant change in the volume of distribution. Because of the marked reduction in total and especially free prednisolone, the dosage should be adjusted accordingly if prednisolone and rifampicin are prescribed concomitantly.

Pharmacokinetic interaction between voriconazole and efavirenz at steady state in healthy male subjects

A randomized, placebo-controlled (with respect to voriconazole), 2-period, multiple-dose intragroup fixed-dose sequence study was conducted in 34 healthy male subjects to evaluate the interactions between voriconazole (triazole antifungal agent) and efavirenz (reverse transcriptase inhibitor). In period 1, subjects received 200 mg twice-daily (bid) voriconazole (n = 17) or placebo (n = 17) for 3 days (400-mg bid loading doses on day 1). In period 2, following a 7-day washout, subjects received 400 mg once-daily (qd) efavirenz alone for 10 days (days 11-20). Then efavirenz was coadministered with 200 mg bid voriconazole or placebo for the next 9 days (days 21-29). Serial plasma voriconazole and efavirenz concentrations were measured on days 3, 19, and 29, and the safety data were collected throughout the study. The 400-mg qd efavirenz dose substantially reduced the steady-state mean voriconazole area under the curve over the dosing interval (AUC0-12) by 80% (90% confidence interval [CI], 75%-84%) and peak concentration (Cmax) by 66% (90% CI, 57%-73%). The decrease in voriconazole exposure during coadministration is probably mainly due to the induction of CYP2C19 and CYP2C9 by efavirenz. The 200 mg bid voriconazole increased the steady-state mean AUC0-24 and Cmax of efavirenz by 43% (90% CI, 36%-51%) and 37% (90% CI, 29%-46%), respectively. The increase in efavirenz exposure during coadministration is probably due to the inhibition of CYP3A4 by voriconazole. Coadministration of 200 mg bid voriconazole with 400 mg (or higher) qd efavirenz is contraindicated due to the clinically significant effect of efavirenz on voriconazole pharmacokinetics.

Women with epilepsy: hormonal issues from menarche through menopause

Epilepsy is a multilayered disorder complicated by numerous comorbid conditions and hormonal changes. More than 1.5 million girls and women with epilepsy face side effects that are compounded at different ages by menstruation, fertility, pregnancy, fetal health, bone health, and other health issues. Changes in hormonal balance during maturation, from menarche through menopause, affect seizure thresholds and antiepileptic drugs, and vice versa. This overview provides physicians with a background on the multiple issues relevant to women of all ages in the reproductive years, including those planning to conceive and those who are pregnant, and beyond the childbearing years.

Ritonavir 100 mg does not cause QTc prolongation in healthy subjects: a possible role as CYP3A inhibitor in thorough QTc studies

To assess the QTc prolongation by ritonavir (RTV) 100 mg and explore its potential use as CYP3A inhibitor in thorough QTc (TQT) studies. Randomized, crossover study of single-dose RTV 100 mg, placebo, and moxifloxacin (MFLX) 400 mg in 65 healthy subjects with serial triplicate electrocardiograms obtained for 12 h post-dose. Largest mean placebo-adjusted QTcF increase from baseline (90% confidence interval (CI)) for RTV 100 mg was noninferior to placebo (0.16 ms (-1.38, 1.69)). Study sensitivity was validated by detecting the largest mean placebo-adjusted QTcF increase from baseline (90% CI) for MFLX of 8.31 ms (6.44, 10.18). A single dose of RTV 100 mg does not cause QTc prolongation in healthy subjects. Based on the potent CYP3A4 inhibition, lack of QTc effect and better safety profile, RTV 100 mg could replace ketoconazole as the CYP3A4 inhibitor in TQT studies.

Liver enzyme induction and inhibition: implications for anaesthesia

Recent breakthroughs in molecular biology have enabled a reclassification of drug metabolising enzymes based on their amino acid sequence. This has led to a better understanding of drug metabolism and drug interactions. The majority of these drug metabolising enzymes may be either induced or inhibited by drugs or by extraneous substances including foodstuffs, cigarette smoke and environmental pollutants. Virtually all drugs used in anaesthesia are metabolised by either hepatic phase 1 or phase II enzymes. This review considers the classification of drug metabolising enzymes, explains the mechanisms of enzyme induction and inhibition, and also considers how the action of drugs commonly used by anaesthetists, including opioids and neuromuscular blocking drugs, may be altered by this mechanism.

Lack of pharmacokinetic interaction between oxcarbazepine and lamotrigine

Epilepsy and bipolar disorder are commonly treated by combination drug therapy, such as lamotrigine and oxcarbazepine. To ensure the safety of this combination, information on pharmacokinetics and tolerability must be available. The objective of study was to evaluate the pharmacokinetics and tolerability of coadministered lamotrigine and oxcarbazepine in healthy subjects. This randomized, single-blind, parallel-group study comprised three cohorts: lamotrigine (200 mg daily) plus oxcarbazepine (600 mg twice daily), lamotrigine (200 mg daily) plus placebo, and oxcarbazepine (600 mg twice daily) plus placebo. Serial blood samples were collected at steady state to determine serum concentrations of lamotrigine and plasma concentrations of oxcarbazepine and its active metabolite 10-monohydroxy metabolite (MHD). Pharmacokinetic parameters were determined by noncompartmental methods. Tolerability was monitored through adverse event reports, clinical laboratory results, vital signs, and electrocardiograms. A total of 47 male volunteers received study drugs. At steady state, lamotrigine AUC((0-24)) and C(max) were not significantly affected by oxcarbazepine co-therapy, nor were MHD AUC((0-12)) and C(max) significantly affected by lamotrigine co-therapy. The most common adverse events, headache, dizziness, nausea, and somnolence, occurred more frequently during lamotrigine and oxcarbazepine combination therapy than during the monotherapy. No significant changes in clinical laboratory parameters, vital signs, or electrocardiograms were reported. In conclusion, the combination of lamotrigine and oxcarbazepine does not require dose adjustments based on pharmacokinetic data. However, it is important to recognize that the combination therapy was associated with more frequent adverse events.

Population pharmacokinetics of APOMINE: a meta-analysis in cancer patients and healthy males

AIMS

1) To characterize the population pharmacokinetics of apomine in healthy males and in male and female patients with solid tumours and 2) to understand more fully the influence of induction and between- and within-subject variability on exposure to drug using Monte Carlo simulation.

METHODS

Apomine was administered once- or twice-daily with or without food in single and multiple oral doses of 30-2100 mg to healthy males (n = 19) and patients with solid tumours (n = 19). The data were divided into model development and validation sets. Models were developed using standard population methods. These were the identification of an appropriate base model, calculation of the empirical Bayes estimates of the primary pharmacokinetic parameters, covariate screening, forward stepwise addition of covariates using the likelihood ratio test as a model selection criteria, and backwards elimination to obtain the final model. To study the influence of data from individual subjects, the model development dataset was subjected to the delete-1 jack-knife and the final model was fitted to each jack-knifed dataset. Principal components analysis of the jack-knifed matrix of model parameters identified two influential subjects who were removed from the dataset, and the final model contained data from the remaining subjects. Model validation was examined using goodness of fit statistics and relative error measures using independent datasets from cancer patients. The model provided a reasonable approximation to the pharmacokinetic measurements in the validation datasets. Computer simulations were undertaken to understand further the pharmacokinetics of apomine in otherwise healthy females, a population not yet studied.

RESULTS

Apomine pharmacokinetics were complex and consistent with a two-compartment model with a lag-time. Apparent oral clearance at baseline and apparent volume of distribution at steady-state were larger in healthy males than in cancer patients (41 ml h(-1) and 14.1 l vs 10 ml h(-1) and 8.9 l, respectively, for a 75 kg person). Clearance was time-variant showing a maximal increase with full induction of 320 ml h(-1), independent of patient type. The time to reach 50% maximal induction was about 2 days. The fraction of drug absorbed was relatively constant at doses less than 100-200 mg once daily but decreased at higher doses. Food also decreased relative bioavailability by 36%. Patient characteristics had no effect on apomine pharmacokinetics except for weight, which was proportional to the volume of the central compartment. Between-subject variability (68% for clearance, 30% for central volume, and 141% for peripheral volume) was moderate to large and independent of patient type. Inter-occasion variability was small (18% for both clearance and central volume). Residual variability was modelled with an additive and proportional error model. Cancer patients had slightly higher plasma concentrations than healthy males but this difference was probably not clinically significant. Steady-state was reached in about 3-4 days after once-daily drug administration. The half-life of apomine after three weeks of once-daily dosing was 41 h in cancer patients and 32 h in healthy males.

CONCLUSIONS

A population model for apomine has been developed has been developed that characterizes its pharmacokinetics in cancer patients and healthy subjects under a variety of conditions.

Human Parathion Poisoning

The mortality rate of suicidal parathion poisoning is particularly high, the onset of fulminant cholinergic signs, and the patients frequently present to the emergency physician with life-threatening symptoms. Despite this uniformity, subsequent clinical course differs significantly among patients, mostly not as a result of different delays in treatment or insufficiency of primary care. Probably, the differences depend on the amount of poison absorbed and/or the disposition of the active poison, paraoxon. We followed the toxicokinetics of parathion and tried to quantify the actual poison load. To this end, we monitored parathion-intoxicated patients (patients requiring artificial ventilation) for plasma levels of parathion and paraoxon along with the activity of erythrocyte acetylcholinesterase and its reactivatability. Plasma obidoxime concentrations were followed as well as the cumulative urinary para-nitrophenol conjugate excretion as a measure of total poison load. All patients received a standard obidoxime scheme of a 250 mg bolus dose intravenously, followed by continuous infusion with 750 mg per 24 hours as long as reactivation could be expected (usually 1 week). All other treatment was instituted as judged by the physician. It was recommended to use atropine at low doses to achieve dry mucous membranes, no bronchoconstriction and no bradycardia. Usually 1-2 mg/h were sufficient. Seven selected cases are presented exemplifying toxicokinetic peculiarities. All patients were severely intoxicated, while the amount of parathion absorbed varied widely (between 0.12 and 4.4 g; lethal dose 0.02-0.1 g) and was generally much lower than anticipated from the reports of relatives. It remains open whether the discrepancies between reports and findings were due to exaggeration or to effective decontamination (including spontaneous vomiting, gastric lavage and activated charcoal). Absorption of parathion from the gastrointestinal tract was sometimes retarded, up to 5 days, resulting in fluctuating plasma profiles. The volume of distribution at steady-state (Vdss) of parathion was around 20 L/kg. Post-mortem analysis in one patient revealed a 66-fold higher parathion concentration in fat tissue compared with plasma, 16 days after ingestion. Biotransformation of parathion varied widely and was severely retarded in one patient receiving fluconazole during worsening of renal function, while phenobarbital (phenobarbitone) sedation (two cases) had apparently no effect. The proportion of plasma parathion to paraoxon varied from 0.3-30, pointing also to varying paraoxon elimination, as illustrated by one case with particularly low paraoxonase-1 activity. Obidoxime was effective at paraoxon concentrations below 0.5 microM, provided aging was not too advanced. This concentration correlated poorly with the paration concentration or the poison load. The data are discussed in light of the pertinent literature.

Interactions between antiepileptic drugs and hormonal contraception

An interaction between antiepileptic drugs (AEDs) and the combined oral contraceptive pill was first proposed when the dose of estradiol in the oral contraceptive pill was reduced from 100 to 50 microg. There was a higher incidence of breakthrough bleeding and contraceptive failure among women with epilepsy compared with women in general. Since then, interaction studies have been undertaken to look for possible interactions between AEDs and the combined oral contraceptive pill. Phenobarbital (phenobarbitone), phenytoin, carbamazepine, oxcarbazepine, felbamate and topiramate have been shown to increase the metabolism of ethinylestradiol and progestogens. Therefore, if a women is on one of the AEDs and wishes to take the oral contraceptive pill, she will need to take a preparation containing at least 50 microg of ethinylestradiol. Levonorgestrel implants are contraindicated in women receiving these AEDs because of cases of contraceptive failure. It is recommended that medroxyprogesterone injections be given every 10 rather than 12 weeks to women who are receiving AEDs that induce hepatic microsomal enzymes. There are no interactions between the combined oral contraceptive pill, progesterone-only pill, medroxyprogesterone injections or levonorgestrel implants and the AEDs valproic acid (sodium valproate), vigabatrin, lamotrigine, gabapentin, tiagabine, levetiracetam, zonisamide, ethosuximide and the benzodiazepines. Therefore, normal dose contraceptive preparations can be used in patients receiving these AEDs.

Time course of lamotrigine de-induction: impact of step-wise withdrawal of carbamazepine or phenytoin

OBJECTIVE

The objective of the present analysis is to examine lamotrigine (LTG) pharmacokinetics both during polytherapy with enzyme inducing antiepileptic drugs and to evaluate the time course of de-induction following the step-wise withdrawal of enzyme inducers.

BACKGROUND

LTG pharmacokinetics can be significantly influenced by concomitant AEDs, and the addition of enzyme inducers can markedly increase LTG clearance, thereby reducing serum concentrations. A clinically relevant question is how will LTG clearance and resulting plasma concentrations be altered during concomitant enzyme inducer withdrawal/conversion process.

DESIGN/METHOD

As part of a previously published, active-control, LTG monotherapy trial, dose and plasma concentration data for LTG, carbamazepine (CBZ) or phenytoin (PHT) were obtained. Following the attainment of a LTG target dose of 500 mg/day, CZB or PHT were withdrawn in weekly 20% decrements. Following inducer withdrawal, LTG was then continued as monotherapy for an additional 12 weeks. Plasma concentrations and daily doses of LTG, CBZ, or PHT were obtained at regularly scheduled study visits during inducer withdrawal and during LTG monotherapy. Pharmacokinetic analysis of the plasma concentration data was done to determine the time-course and effect of inducer plasma concentration on LTG oral clearance (Cl(o)), where LTG Cl(o) was estimated as the dose/concentration ratio.

RESULTS

Of the 156 patients enrolled in this trial, 76 were assigned to LTG arm, 43 completed the withdrawal to monotherapy phase with 28 successfully completing the study. In a subset analysis of completers, LTG Cl(o) determined prior to withdrawal of the inducers was significantly greater in patients (n=28) on LTG+PHT (160% increase) than in those (n=48) receiving LTG+CBZ (62% increase): 1.77+/-0.77 vs. 1.06+/-0.41 ml/min/kg, respectively, p=0.017. The significant reduction in LTG Cl(o) occurred only when CBZ plasma concentrations reached approximately 2 microg/ml or PHT plasma concentrations reached zero.

CONCLUSIONS

Mean LTG plasma concentrations will approximately double following the withdrawal PHT; however increases of only 60% may occur following the withdrawal of CBZ. Importantly, these data suggest that LTG concentrations would not be expected to increase until the concomitant inducer is almost completely removed.

Altered corticosteroid metabolism differentially affects pituitary corticotropin response

To evaluate the effects of altered corticosteroid metabolism on the hypothalamic-pituitary-adrenal axis, we examined rats treated with glycyrrhizic acid (G rats) or rifampicin (R rats) for 7 days. The half-life of exogenously administered hydrocortisone as a substitute for corticosterone was longer in G rats and shorter in R rats, with no differences in basal plasma levels of ACTH or corticosterone. The ACTH responses to human corticotropin-releasing factor (CRF) or insulin-induced hypoglycemia were greater in G rats and tended to be smaller in R rats compared with those in the control rats, whereas the corticosterone response was similar. No difference was observed in the content and mRNA level of hypothalamic CRF among the groups. The number and mRNA level of CRF receptor and type 1 11 beta-hydroxysteroid dehydrogenase (11-HSD1) mRNA level in the pituitary were increased in G rats but not changed in R rats, suggesting that chronically increased intrapituitary corticosterone upregulates pituitary CRF receptor expression. In contrast, CRF mRNA levels in the pituitary were increased in R rats. Our data indicate novel mechanisms of corticosteroid metabolic modulation and the involvement of pituitary 11-HSD1 and CRF in glucocorticoid feedback physiology.

Carbamazepine decreases antihypertensive effect of nilvadipine

A 59-year-old male who had suffered from hypertension for 21 years was admitted because of manic and delusional symptoms. He was treated with 12 mg/day of haloperidol for psychotic symptoms and 8 mg/day of nilvadipine for hypertension. Due to insufficient effect of haloperidol on the patient's manic symptoms, carbamazepine was added to these medications. Abnormally high blood pressure was observed during carbamazepine coadministration, and it returned gradually to normal range after its discontinuation. Retrospective analyses revealed that the plasma concentrations of nilvadipine were undetectable during carbamazepine treatment. The clinical course and laboratory findings suggest that carbamazepine decreased the plasma concentration and hence the antihypertensive effect of nilvadipine probably via CYP3A induction. This interaction between nilvadipine and carbamazepine should be kept in mind when these drugs are administered concomitantly.

Possible clinically significant interaction of itraconazole plus rifampin

We report two patients treated with the combination of itraconazole plus rifampin for more than 4 months. While on itraconazole plus rifampin, patient 1 lost weight at a rate of 30 g/d. After stopping rifampin, he gained 14 g/d. While on itraconazole plus rifampin, patient 2 lost 41 grams/day. After stopping rifampin, he gained 33 g/d. Weight loss while taking the combination of itraconazole plus rifampin, followed by weight gain after stopping rifampin, suggests the possibility of a clinically significant drug interaction between itraconazole and rifampin.

Epidemiology, outcomes research, and drug interactions

Drug interactions are one of several outcomes (typically adverse) associated with drug therapy. The bulk of the clinical literature that has been published about drug interactions has focused either on the mechanisms, pharmacokinetic studies, or cases observed by clinicians. However, when it comes to the true risk of a drug interaction, or the probability of such an interaction occurring in a given population, a dearth of information is available. This area is opportune for the application of epidemiological and outcomes-based studies.

Characterization of cytochrome P450 (CYP3A12) induction by rifampicin in dog liver

1. Effects of rifampicin (Rif) on the contents of cytochrome P450 (P450) enzymes (CYP1A1/2, 2B11, 2C21 and 3A12) assessed by enzyme-linked immunosorbent assay and catalytic activities (ethoxyresorufin O-deethylase, and testosterone 6 beta-, 16 alpha- and 16 beta-hydroxylase; 6 beta-, 16 alpha- and 16 beta-OHT) in dog liver microsomes were compared between liver lobes of both the male and female dogs. 2. In the control dogs, the contents of individual P450 enzymes and their activities showed no significant differences between individual liver lobes and between the sexes. 3. Rif treatment (10 mg/kg/day, p.o. for 7 days) induced substantial increases in the content of CYP3A12 and 6 beta- and 16 beta-OHT activities, and slight increases in the content of CYP2B11 and 16 alpha-OHT activity, and their elevated levels were virtually the same between liver lobes. The magnitudes of the elevation of the CYP3A12 level and 6 beta- and 16 beta-OHT activities compared with control levels appeared to be greater in the female dogs. However, the ratios of their magnitudes (CYP3A content/6 beta-OHT activity and CYP3A content/16 beta-OHT activity) showed no differences between the sexes. 4. In both the control and Rif-treated dogs, the activities of 6 beta- and 16 beta-OHT were specifically inhibited by anti-CYP3A12 antiserum, and 16 alpha-OHT activity was specifically inhibited by anti-CYP2B11 and anti-CYP2C21 antiserum. 5. These results indicate that Rif treatment induces the expression of CYP3A12 protein, and correlates well with the elevation of its catalytic activity (6 beta- and 16 beta-OHT), and that the female dog is more responsive to Rif treatment as compared with the male.

Effect of multiple doses of rifampin on the [14C N-methyl] erythromycin breath test in healthy male volunteers

The erythromycin breath test (EBT), which measures 14CO2 produced from [14C N-methyl] erythromycin, is one of the most frequently employed measures to examine drug interactions involving cytochrome P450 3A4 (CYP3A). However, the reproducibility and reliability of this test, and the effects of drugs that alter CYP3A activity, continue to be defined. In this study, the reproducibility of the EBT was evaluated in eight healthy volunteers before and after oral administration of 600 mg of rifampin daily for 8 days. Two sequential EBT determinations performed 5 days apart before rifampin administration were highly reproducible. Rifampin induced CYP3A, reflected in a mean percent (+/- standard deviation) increase in EBT values of 86 +/- 30%. Recovery of enzyme function after discontinuation of rifampin for 17 days was manifested as a return of EBT values to preinduction levels. These results support the utility of EBT as a valid, reproducible, and reliable measure of CYP3A activity in vivo.

Urinary excretion of 6 beta-hydroxycortisol as an in vivo marker for CYP3A induction: applications and recommendations

OBJECTIVE

To evaluate the usefulness of 6 beta-hydroxycortisol as a screen for CYP3A induction in early-phase drug development.

METHODS

Five groups of 12 healthy elderly men were randomized to one of five treatment regimens: (1) 600 mg rifampin (INN, rifampicin) once daily, (2) placebo once daily, (3) 40 mg SB 216469 twice a day, (4) 60 mg SB 216469 twice a day, or (5) 40 mg SB 216469 three times a day. All medications were taken orally and administered for 7 consecutive days. Urine was collected over a 24-hour period for each subject before administration and on the last day of administration for each respective regimen for measurement of 6 beta-hydroxycortisol and 17-hydroxycorticosteroid concentrations.

RESULTS

Subjects in the rifampin group had a significant increase from predose value in the 24-hour urinary excretion of 6 beta-hydroxycortisol and the ratio of 6 beta-hydroxycortisol to 17-hydroxycorticosteroid. All 12 subjects in the rifampin group had increases in 6 beta-hydroxycortisol excretion, whereas 11 of 12 had an increase in the ratio. The placebo and three active treatment groups did not show significant changes in either parameter.

CONCLUSIONS

Urinary excretion of 6 beta-hydroxycortisol may be useful as a screening tool in early-phase development to assess the potential for an investigational drug to induce CYP3A.

Experimental models for evaluating enzyme induction potential of new drug candidates in animals and humans and a strategy for their use

Experimental models that have application for evaluating enzyme induction potential have been described in order of increasing complexity. The main focus was on models that have had wide application thus far. However, many new models are currently being developed that may have future applications in evaluating enzyme induction potential. A strategy to evaluate the enzyme induction potential of drug candidates was outlined. This scheme uses a combination of new and established techniques to evaluate data in a stepwise manner that is appropriate to the drug's current stage of development.

Pharmacokinetic study of the interaction between rifampin and delavirdine mesylate

OBJECTIVE

To study the effect of rifampin (INN, rifampicin), a potent inducer of cytochrome P450, on the steady-state pharmacokinetics of delavirdine.

METHODS

Twelve patients who were positive for human immunodeficiency virus, with CD4 counts ranging from 110 to 483/mm3, were randomized to two groups and studied in parallel. Both the control group (n = 5) and the rifampin group (n = 7) received 400 mg delavirdine mesylate every 8 hours for 30 days; subjects in the rifampin group took a 600 mg once-daily dose of rifampin on days 16 through 30. Harvested plasma from serial blood samples collected after dosing on days 15, 16, and 30 was assayed for delavirdine and its N-desalkyl metabolite concentrations with a reversed-phase HPLC method. Blood samples obtained on days 16 and 30 were also assayed for rifampin by HPLC.

RESULTS

Delavirdine mesylate alone and in combination with rifampin was well tolerated. On day 30, statistically significant differences between groups were observed for all delavirdine pharmacokinetic parameters (p < 0.049). In the rifampin group, delavirdine oral clearance increased by about 27-fold (p = 0.022), resulting in virtually negligible (< 0.09 mumol/L) steady-state through drug concentrations in all patients after 2 weeks of concurrent dosing of delavirdine mesylate and rifampin. The ratio of metabolite formation to elimination clearance for desalkyldelavirdine was significantly higher (3.9 +/- 1.2 versus 0.23 +/- 0.10) and delavirdine elimination half-life was significantly shorter (1.7 +/- 1.4 versus 4.3 +/- 1.3 hours) when delavirdine mesylate was taken with rifampin. Rifampin pharmacokinetic parameters on days 16 and 30 were similar to those previously reported for normal volunteers.

CONCLUSIONS

The findings of this study indicate that rifampin induces the metabolism of delavirdine. Therefore therapy with rifampin is contraindicated in patients receiving delavirdine mesylate.

Assessment of doxylamine influence on mixed function oxidase activity upon multiple dose oral administration to normal volunteers

The primary purpose of this study was to assess the influence of doxylamine and phenobarbital on antipyrine/metabolites pharmacokinetics and 6 beta-hydroxycortisol urinary excretion. This study was conducted in 48 healthy male human volunteers (16 per treatment group) using a parallel study design. Treatment groups consisted of 12.5 mg of doxylamine succinate, placebo, or 30 mg of phenobarbital administered orally every 6 h for 17 days. Results indicate that no statistically significant differences were observed between the doxylamine and placebo groups that are indicative of enzyme induction. For the phenobarbital group, a significant increase for antipyrine total (36 versus 45 mL/h/kg) and nonrenal (35 versus 44 mL/h/kg) clearances and 6 beta-hydroxycortisol excretion (338 versus 529 micrograms) and a significant decrease in the terminal exponential half-life (11 versus 9 h) of antipyrine were observed.

Quantitation of cytochrome P450 enzymes (CYP1A1/2, 2B11, 2C21 and 3A12) in dog liver microsomes by enzyme-linked immunosorbent assay

1. An enzyme-linked immunosorbent assay (ELISA) using specific antisera has been developed to quantify individual cytochrome P450 (P450) enzymes (1A1/2, 2B11, 2C21 and 3A12) in dog liver microsomes. 2. The specific contents of CYP1A1/2, 2B11, 2C21 and 3A12 in untreated male dog liver microsomes determined by the ELISA were 17, 48, 160 and 69 pmol/mg protein respectively, corresponding to 4, 10, 34 and 15% of total optically determined P450 respectively. These P450 enzymes in untreated female dog liver microsomes showed almost similar amounts and relative proportions to those observed in male dog liver microsomes. 3. The oral treatment of male dogs with phenobarbital (PB), rifampicin (Rif) or beta-naphthoflavone (beta-NF) induced significant increases in the contents of CYP1A1/2 (12-fold by beta-NF), 2B11 (16-fold by PB), 2C21 (2-fold by PB) and 3A12 (5-fold by PB and Rif), resulting in marked proportional alterations of the P450 enzymes in dog liver microsomes. 4. This ELISA method will be a useful tool for investigating possible influences (induct on/suppression) of xenobiotics on the expression of P450 enzymes in dog liver.

Urinary glucaric acid excretion in rheumatoid arthritis: influence of disease activity and disease modifying drugs

OBJECTIVE

To examine if a correlation exists between cytochrome P-450 enzyme induction and disease activity in patients with rheumatoid arthritis (RA), measuring urinary excretion of D-glucaric acid (GA) as an index of phase II drug metabolism.

METHODS

Patients with RA were treated with sulphasalazine, sodium aurothiomalate, or D-penicillamine in standard dose regimens, for 24 weeks. Patients with ankylosing spondylitis (AS) or non-inflammatory arthritis (NIA) acted as controls. The urinary GA:creatinine ratio was measured at 0, 12, and 24 weeks of treatment.

RESULTS

Patients with RA had a slightly greater urinary GA:creatinine ratio than patients with AS or NIA at baseline; this increased during treatment with disease modifying antirheumatic drugs (DMARDs). Sulphasalazine treatment had a greater effect on GA excretion than sodium aurothiomalate or D-penicillamine; this difference was statistically significant between weeks 0 and 12 (p = 0.01). Gamma glutamyltranspeptidase concentration showed a weak correlation with GA excretion between weeks 0 and 12 (p = 0.03), but all other measurements of changes in disease activity (plasma viscosity, C reactive protein, platelets, and articular index) were found not to correlate with GA excretion between weeks 0-12 or 0-24.

CONCLUSION

The increased excretion of GA in patients with RA receiving DMARD treatment is probably the result of an indirect effect on hepatic metabolism bearing no relationship to disease activity.

Use of salivary caffeine tests to assess the inducer effect of a drug on hepatic metabolism

OBJECTIVE

To validate the use of successive salivary caffeine tests in evaluating how long inducer drugs affect hepatic metabolism. The time course of the inducer effect of rifampin found in other studies using different methodologies was chosen as the time course of reference.

DESIGN

Open-label, prospective, longitudinal study.

SETTING

A university hospital.

PARTICIPANTS

Five healthy volunteers.

MAIN OUTCOME MEASURES

Rifampin 600 mg/d was administered for 21 days. Anhydrous caffeine 300 mg was concurrently administered on each study day. Salivary caffeine tests were carried out on the following days: predose (baseline), and days 1, 5, 9, 13, and 17. Salivary tests were performed for up to 13 days after the last dose of rifampin (study days 21, 25, 29, and 33).

RESULTS

The mean systemic caffeine clearance was increased for up to 17 days after the intake of rifampin, reaching the maximum inducer effect between days 5 and 9, and returning to previous values progressively during several days after rifampin was discontinued.

CONCLUSIONS

Our results suggest that successive salivary caffeine measurements could be a safe, reliable, noninvasive, and suitable test for exploring the time course of the inducer effect of drugs on hepatic metabolism activity.

Urinary 6 beta-hydroxycortisol excretion in Hong Kong Chinese patients with hepatocellular carcinoma and other chronic liver diseases

BACKGROUND

The biotransformation of xenobiotics into toxic metabolites by cytochrome P-450 has been implicated in carcinogenesis. This study investigated CYP3A4 activity, which metabolically activates procarcinogens such as aflatoxin B1, by measuring the urinary 6 beta-hydroxycortisol (6 beta OHF) to free cortisol (F) ratio in patients with hepatocellular carcinoma (HCC) and other chronic liver diseases.

METHODS

One hundred forty-three controls and 150 patients with different liver diseases, including chronic liver disease (due to alcoholism and/or chronic hepatitis B virus infection), cirrhosis (any cause), and resectable and nonresectable HCC, were recruited. Twenty-four hour urine samples were collected for measurement of 6 beta OHF and free cortisol by an enzyme-linked immunosorbent assay (ELISA) and a radioimmunoassay, respectively.

RESULTS

Patients with nonresectable HCC showed a significant increase in 6 beta OHF excretion as well as their 6 beta OHF/F ratio (P < 0.05) when compared with the controls and other liver disease groups including patients with resectable HCC. The nonresectable HCC group showed a bimodal distribution in the 6 beta OHF/F ratio. Using a ratio of 9 or more in all HCC patients, the sensitivity and specificity of using the 6 beta OHF/F ratio to predict nonresectability of HCC was 48.8% and 92.6%, respectively.

CONCLUSIONS

Our results show an increase in mean CYP3A4 enzyme activity, reflected as an increase in the 6 beta OHF/F ratio, in Hong Kong Chinese with nonresectable HCC compared with those with resectable HCC and other liver diseases. Although the role of increased CYP3A4 activity in the aetiology of HCC is not known, our specificity and sensitivity estimates suggest that a high 6 beta OHF/F ratio indicates probable inoperability. However, a normal level is a poor predictor of resectability.

Phenobarbital minimally alters plasma concentrations of losartan and its active metabolite E-3174

Losartan, a selective angiotensin II (AT1) receptor antagonist for hypertension, is metabolized to an active carboxylic acid metabolite, E-3174, which has a longer half-life. To investigate the effects of induction of cytochrome P450 on the metabolism of losartan, we evaluated the effects of phenobarbital on the plasma profiles of losartan and E-3174 in 15 healthy male subjects. Ten subjects received a single 100 mg oral dose of losartan before and during phenobarbital administration (100 mg/day for 16 days), and five subjects received losartan before and during placebo. Urinary excretion of 6-beta-hydroxycortisol (relative to 17-hydroxycorticosteroids) was measured as an endogenous marker of cytochrome P450 induction. The geometric mean area under the plasma concentration-time curve ratios (with/without phenobarbital and 90% confidence intervals) for losartan and its metabolite (E-3174) were 0.795 (0.723, 0.875) and 0.799 (0.778, 0.820), respectively, indicating that phenobarbital treatment significantly but to a clinically minor extent reduced plasma concentrations of losartan and E-3174 (p<0.01). Half-life values of losartan and E-3174 were unchanged. The ratio of 6-beta-hydroxycortisol to 17-hydroxycorticosteroids doubled in the phenobarbital group (p < 0.001) and did not change appreciably in the placebo group.

Clinically significant drug interactions with cyclosporin. An update

Since its approval in 1983 for immunosuppressive therapy in patients undergoing organ and bone marrow transplants, cyclosporin has had a major impact on organ transplantation. It has significantly improved 1-year and 2-year graft survival rates, and decreased morbidity in kidney, liver, heart, heart-lung and pancreas transplantation. Several studies have supported the efficacy of cyclosporin in preventing graft-versus-host disease in bone marrow transplantation. Cyclosporin is also possibly effective in treating diseases of autoimmune origin and as an antineoplastic agent. The introduction of therapeutic drug monitoring of cyclosporin was extremely useful because of the wide inter- and intraindividual variability in the pharmacokinetics of cyclosporin after oral or intravenous administration. Optimal long term use of cyclosporin requires careful monitoring of the blood (or plasma) concentrations. Sustained and clinically significant drug-drug interactions can occur during long term therapy with cyclosporin. The coadministration of multiple drugs with cyclosporin could result in graft rejection, renal dysfunction or other undesirable effects. Any interaction that leads to modified cyclosporin concentrations is of potential clinical importance. Cyclosporin itself may have significant effects on the pharmacokinetics and/or pharmacodynamics of coadministered drugs, such as digoxin, HMG-CoA reductase inhibitors and antineoplastic drugs affected by multidrug resistance. Many drugs have been shown to affect the pharmacokinetics and/or pharmacodynamics of cyclosporin. Interactions between cyclosporin and danazol, diltiazem, erythromycin, fluconazole, itraconazole, ketoconazole, metoclopramide, nicardipine, verapamil, carbamazepine, phenobarbital (phenobarbitone), phenytoin, rifampicin (rifampin) and cotrimoxazole (trimethoprim/sulfamethoxazole) are well documented in a large number of patients. Other interactions (such as those with aciclovir, estradiol and imipenem) are documented only in isolated case studies.

A prospective study of rectal methohexital: efficacy and side effects in 648 cases

Rectal methohexital has been used for nearly 30 yr in pediatric anesthesia. Despite this long and increasingly varied use, no large prospective series has been published detailing safety and efficacy. This study prospectively evaluated the efficacy, safety, and side effects of this medication in a series of 648 cases. On 553 of 648 occasions (85%), the child fell asleep after a single 30-mg/kg dose of 10% methohexital. Sleep was less likely in patients with myelomeningocele or who were receiving oral phenobarbital or phenytoin. When sleep occurred, the average time to onset of sleep was 6 min. Most patients who remained awake 15 min after drug were sedated. Defecation (10%) and hiccups (13%) were common but benign side effects. Partial airway obstruction and/or desaturation to Spo2 < or = 93% occurred in 26 patients (4%), but was resolved with blow-by oxygen and/or jaw-thrust in all but two cases. These two patients (0.3% of total) required aggressive airway intervention by the supervising anesthesiologist. Apnea did not occur in any patient. Methohexital has a high efficacy rate for sleep (85%) or sedation (96%), and has a relatively rapid onset. Significant respiratory side effects occur infrequently, but can be life threatening if not properly managed.

Adverse effects of established and new antiepileptic drugs: an attempted comparison

Seizures are but one aspect of the negative impact epilepsy has on patients' lives. Adverse effects of antiepileptic treatment may affect the patient's quality of life to an even greater extent than the occurrence of seizures. Adverse effects of antiepileptic drugs (AEDs) are common, and because the differences in efficacy are often marginal, adverse effects may be the most important factor in choosing the best AED for the patient. The search for more efficient and less toxic agents is constantly ongoing. Current evidence suggests that the new generation of AEDs is as efficient as the established AEDs and exhibits fewer adverse effects, but the scientific evidence from randomised clinical trials comparing established and new AEDs with each other is still pending.

Induction and autoinduction properties of rifamycin derivatives: a review of animal and human studies

Animal studies have demonstrated that the mouse and rabbit are far more responsive to the inductive properties of rifamycin derivatives than the rat and guinea pig. The rat hepatic cytochrome P450 system seems to be resistant to the action of rifampicin unless very high doses are used. Mouse hepatic microsomal mixed-function oxidase activity is markedly increased by repeated dosing with rifampicin, whereas administration of rifabutin may be ineffective. In humans, both rifampicin and rifabutin are extensively metabolized and induce their own metabolism. The induced metabolic pathways remain essentially unknown. Under autoinduction conditions, the elimination half-life of rifampicin decreases, whereas that of rifabutin is not altered. Although the effects of repeated administration of rifampicin and rifabutin on the various forms of cytochrome P450 in humans have not been extensively examined, there is convincing evidence that the P4503A subfamily is induced by either drug, whereas the P4501A subfamily and P4502D6 do not appear to be affected by rifampicin. Limited reliable information is available concerning the induction of human glucuronyltransferase activities by rifampicin and rifabutin which, however, do not seem to influence zidovudine glucuronide formation in healthy subjects.

Simultaneous determination of urinary free cortisol and 6 beta-hydroxycortisol by high-performance liquid chromatography to measure human CYP3A activity

The ratio of the hydrophilic metabolite 6 beta-hydroxycortisol to its parent compound cortisol has recently been demonstrated to be a specific marker for human CYP3A oxygenase activity. We have developed a sensitive and simple single-run high-performance liquid chromatographic method for the quantification of urinary free cortisol and 6 beta-hydroxycortisol using dexamethasone as internal standard. The urine samples (1 ml) are applied to Sep-Pak cartridges, which are washed with water and eluted with ethyl acetate-diethyl ether (4:1, v/v). The organic extracts are washed sequentially with alkaline and acidic solutions saturated with sodium sulfate and subsequently concentrated to dryness. After reconstitution in ethanolic water, the samples are analyzed on a reversed-phase gradient system using ultraviolet absorbance detection at 254 nm. The within- and between-day coefficients of variation (C.V.) for the assay where both in the range of 5-10%. The reference interval for the 6 beta-hydroxycortisol/cortisol ratio of eleven healthy non-smoking subjects was 2.77-26.88 with an average of 10.09 +/- 6.89 (S.D.). The method constitutes an improvement over previous methods and is suitable for routine assessment of the 6 beta-hydroxycortisol/cortisol ratio requiring only 1 ml of urine or less.

Enzyme induction by moricizine: time course and extent in healthy subjects

Moricizine.HCl, a novel phenothiazine derivative with oral antiarrhythmic activity, was examined for its potential to induce its own hepatic metabolism and to alter the pharmacokinetics of the test substrate, antipyrine, in 12 healthy male subjects. Antipyrine oral clearance increased from a starting value of .74 mL/minute/kg to .98 (+32%, P < .01) after 7 days of moricizine administration (250 mg every 8 hours) and to 1.15 mL/minute/kg after 14 days (+47%, P < .05); t1/2 was correspondingly reduced. Moricizine oral clearance increased from a baseline of 3.01 L/hour/kg to 3.62 (+20%, P < .05) after 6 days of oral moricizine and 4.66 (+51%, not significant) after 13 days. Moricizine t1/2 was marginally, but consistently, increased (+23%, P < .05) instead of decreased as one would expect because of enzyme induction, presumably due to a decrease in systemic bioavailability and its influence on the oral volume of distribution. In half of the subjects who discontinued moricizine after 7 days, antipyrine pharmacokinetic values returned to near baseline 7 days later. Although moricizine was able to induce its own hepatic metabolism and that of antipyrine after 6 or 7 days of continuous administration, the electrocardiographic properties of moricizine did not appear to be altered by continuous dosing.

Effect of phenobarbital and other model inducers on cytochrome P450 isoenzymes in primary culture of dog hepatocytes

1. The effects of phenobarbital (PB), beta-naphthoflavone (beta-NF), omeprazole (Omep) and rifampicin (Rif) on drug-metabolizing activities in dog hepatocytes, cultured with William's medium E, were examined. 2. The drug metabolizing activities of the hepatocytes decreased during culture; 7-ethoxycoumarin O-deethylase (ECOD) activity was nearly 70% of initial value at 72 h, but 7-methoxycoumarin O-demethylase (MCOD), 7-propoxycoumarin O-depropylase (PCOD), progesterone 6 beta-hydroxylase (6 beta-OH-P), progesterone 16 alpha-hydroxylase (16 alpha-OH-P), progesterone 21-hydroxylase (21-OH-P), 7-ethoxyresorufin O-deethylase (EROD) activities and total cytochrome P450 content were approx. 50%. 3. When the hepatocytes were cultured with PB, the enzyme activities increased time- and dose-dependently. MCOD, ECOD and PCOD activities increased 5-8 fold with 2 mM PB in 96 h. Similar results were obtained for 6 beta-OH-P, 16 alpha-OH-P and 21-OH-P activities, and total cytochrome P450. The effect of PB was abolished when 2.5 microM cycloheximide or 0.1 microM actinomycin D was included in the culture. 4. Treatment of hepatocytes with 40 microM beta-NF for 72 h resulted in 25-fold elevation of EROD activity. beta-NF enhanced PCOD activity approx. six-fold, while ECOD increased only slightly, and 7-MCOD negligibly. 5. Omep (100 microM) increased EROD activity nearly 10-fold, and 25 microM Rif increased 6 beta-OH-P activity approx. 8-fold, but ECOD only slightly. 6. Western blot analysis of microsomes from cultured dog hepatocytes with anti-rat CYP 2B1 antibodies indicated that PB increased an immunochemically-reactive protein. The protein showed the same mobility as the major dog P450 isozyme (cytochrome P450 PBD-2 or CYP 2B11) purified from liver microsomes of PB-treated male beagle dog. In a similar manner, induction of cytochrome P450 PBD-1 (CYP 3A12) by PB was confirmed.

Use of single sample clearance estimates of cytochrome P450 substrates to characterize human hepatic CYP status in vivo

1. Single sample clearance estimates (CL/F) of orally administered ethosuximide were obtained in four groups of healthy adult subjects. One group was treated with phenobarbital to induce CYP2B/2C and CYP3A activity; one group was treated with rifampin to induce CYP3A activity; one group consisted of cigarette smokers (increased CYP1A activity), and one group was untreated (controls). Ethosuximide CL/F values were slightly, though not significantly, increased among cigarette smokers (12.5% increase) and the phenobarbital group (25% increase), but rifampin treatment resulted in a significant increase (65%). 2. The influence of rifampin treatment on the single sample oral clearances of antipyrine, theophylline, phenytoin, carbamazepine, ethosuximide, quinidine, valproic acid, and lorazepam was investigated to determine whether rifampin induces only CYP3A. Rifampin treatment significantly increased the oral clearance of each drug from 1.4-fold (valproic acid) to 3.4-fold (quinidine). These findings indicate that the inductive effect of rifampin extends well beyond CYP3A.

A system approach to pharmacodynamics. Input-effect control system analysis of central nervous system effect of alfentanil

Virtually all biological variables, including those affected by drugs, are subject to adaptive self regulation. In the description of the pharmacodynamics (PD) of drugs, it may be necessary to consider the endogenous control system (ECS) as an integral part of the PD. A PDECS model based on system analysis principles is presented and tested on PD data for alfentanil considering the central nervous system activity quantified by a power spectrum analysis of the electroencephalogram. The model was tested in terms of a proposed relative prediction performance criterion that measures the accuracy of future predictions relative to how well the model describes (fits) the past effect data. A mean value of 80% (standard deviation, 28) for relative prediction performance indicates that the model performs well when challenged by the complex multiple infusion scheme used in the test. The overshoot phenomenon observed in the data is considered by the PDECS model as a ECS-based tolerance phenomenon. The proposed development of tolerance is modeled as a variable gain in the ECS processing that influences the effect. Although the development and loss of tolerance is determined by a single rate constant in the tolerance model, the rates of increase and decrease of tolerance may be substantially different. Contrary to other PD tolerance models, the proposed PDECS approach models the tolerance in terms of an effect deviation from an ECS set point. The intrinsic (no tolerance) effect of the drug is isolated in terms of an open loop (no feedback) effect.

The effect of enzyme induction on the cytochrome P450-mediated bioactivation of carbamazepine by mouse liver microsomes

Predisposition to idiosyncratic toxicity with carbamazepine is thought to be due to a deficiency of the detoxication enzyme, microsomal epoxide hydrolase, although in some cases, concurrent administration of enzyme inducers might be a contributory risk factor, by altering the critical balance between bioactivation and detoxication. In this study, a mouse model has been used to determine the factors affecting carbamazepine bioactivation, using covalent binding and cytotoxicity as markers of bioactivation in vitro. Microsomes prepared from mice pre-treated with phenobarbitone increased (relative to the control microsomes) the formation of cytotoxic (12.3% vs 3.2%), protein-reactive (3.0% vs 2.0%) and stable (33.8% vs 18.1%) metabolites of carbamazepine. Similarly, pre-treatment with dexamethasone also increased the formation of the cytotoxic (24.8% vs 6.7%), protein-reactive (2.8% vs 1.5%) and stable (38% vs 19.8%) metabolites of carbamazepine, while beta-naphthoflavone pretreatment did not increase the formation of either the toxic or stable metabolites of carbamazepine when compared with its control microsomes. Co-incubation with gestodene (10-250 microM) resulted in a dose-dependent inhibition of both the bioactivation of carbamazepine and the formation of its stable 10,11-epoxide. SDS-PAGE and immunoblotting of the microsomes with anti-CYP3A antibody revealed the presence of a 52 kDa protein band in each preparation of microsomes, but the relative intensities of the bands, as measured by laser densitometry, were highest with the phenobarbitone and dexamethasone microsomes. The microsomal oxidation of cortisol to 6 beta-hydroxycortisol was also enhanced by pretreatment of mice with phenobarbitone (6.5% vs 2.7%) and dexamethasone (8.2% vs 4.3%), but not beta-naphthoflavone (2.2% vs 1.6%), when compared with their respective control microsomes, and was inhibited (range 25-68% inhibition), with all the microsomes by gestodene (50 microM). Taken collectively, the data in this study demonstrate that in the mouse, induction of the CYP3A subfamily significantly increases carbamazepine bioactivation. It is likely that in humans inducers of the orthologous form of this enzyme, most notably anticonvulsants, may increase the bioactivation of carbamazepine.

Oxcarbazepine does not affect the anticoagulant activity of warfarin

The possible interaction of the antiepileptic drug oxcarbazepine (OCBZ) on the anticoagulant effect of warfarin was investigated in 10 healthy male volunteers. After reaching steady-state conditions by repeated administration of warfarin, the prothrombin time (Quick value) was assessed before and after single (600 mg) and multiple dosing (450 mg twice daily in 1 week) of OCBZ. In 7 of the 10 volunteers with evaluable data, the prothrombin time was not significantly different (paired t test) from baseline either after single (p = 0.299) or repeated dosing (p = 0.333), indicating that OCBZ does not interact to any relevant extent with the hypothrombinemic effect of warfarin.

Two- and four-day rifampin chemoprophylaxis regimens induce oxidative metabolism

The effects of two short-term chemoprophylaxis regimens of rifampin (2 or 4 days) on oxidative metabolism were investigated in 14 healthy subjects. Seven subjects received 600 mg of rifampin twice daily on study days 6 and 7 (group A), and seven subjects received 600 mg of rifampin once daily on days 4, 5, 6, and 7 (group B). Antipyrine (18 mg/kg of body weight) was administered orally on days 1, 8, and 15. Short-term rifampin regimens increased oral clearance of antipyrine in both groups compared with the baseline value (P less than 0.05), and group B displayed a larger percent increase over the baseline value than group A did (70.5 +/- 14.3 versus 33.1 +/- 18.1; P less than 0.05). The partial metabolic clearance (CLM) of antipyrine to 3-hydroxymethylantipyrine (HMA) on day 8 increased 71 and 108% for regimens A and B, respectively (P less than 0.05 for both). The corresponding increases in CLM to norantipyrine (NORA) were 57 and 98% (P less than 0.05 for both). CLM to 4-hydroxyantipyrine (OHA) on day 8 increased 64% for regimen A (P = 0.08) and 97% for regimen B (P less than 0.05) compared with the baseline. Although CLM to HMA and OHA on day 15 remained greater than 50% over the baseline with both regimens, CLM to NORA on day 15 was less than 25% over the baseline with both regimens. Thus, both short-term rifampin chemoprophylaxis regimens increased antipyrine clearance for at least 1 week. The increase tended to be higher with the 4-day regimen. The pattern observed for the CLMS suggests that more than one P-450 enzyme is affected.