Classic histamine H1 receptor antagonists: a critical review of their metabolic and pharmacokinetic fate from a bird's eye view.

The so-called "classic" histamine H(1) receptor antagonists are highly lipophilic compounds associated with significant biotransformation and tissue distribution. They are categorized according to their chemical structure into ethanolamines, alkylamines, ethylenediamines, piperazines, phenothiazines and piperidines, all of which have characteristic metabolic fates. The former four categories undergo primarily cytochrome P450-mediated oxidative N-desalkylations and deamination whereas the aromatic rings of the latter two undergo P450-mediated oxidative hydroxylation and/or epoxide formation. The common tertiary amino group is susceptible to oxidative metabolism by flavin containing monooxygenases forming N-oxides, and the alicyclic tertiary amines produce small amounts (up to 7%) of N-glucuronides in humans. Species, sex and racial differences in the metabolism and pharmacokinetics of antihistamines are known. Specific P450-isozymes implicated in the metabolism were identified in a few cases, such as CYP2D6 that contributes to the metabolism of promethazine, diphenhydramine and chlorpheniramine. Low circulating plasma concentrations of antihistamines are in part explained by significant first-pass effect and tissue distribution. Antihistaminic effects last up to 6 hours though some compounds exhibit a longer duration of action due to circulating active metabolites. Importantly, diphenhydramine inhibited CYP2D6 leading to a clinically significant drug-drug interaction with metoprolol. Other classic antihistamines were shown to be potent in vitro inhibitors of CYP2D6 and CYP3A4. The prescription-free access to most classic antihistamines can easily lead to their co-administration with other drugs metabolized by the same enzyme system thereby leading to drug accumulation and adverse effects. In depth knowledge of the metabolic pathways of classic antihistamines and the enzymes involved is crucial to prevent the high incidence of drug interactions in humans, which are predictable based on pre-clinical data but unexpected when such data is unavailable.

Diphenhydramine alters the disposition of venlafaxine through inhibition of CYP2D6 activity in humans.

CYP2D6 is the major enzyme involved in the metabolism of venlafaxine. Subjects with a low CYP2D6 activity have increased plasma concentrations of venlafaxine that may predispose them to cardiovascular side effects. In vitro and in vivo studies showed that diphenhydramine, a nonprescription antihistamine, can inhibit CYP2D6 activity. Therefore, the authors investigated in this study a potential drug interaction between diphenhydramine and venlafaxine. Fifteen male volunteers, nine with the extensive metabolizer (EM) and six with the poor metabolizer (PM) phenotype of CYP2D6, received venlafaxine hydrochloride 18.75 mg orally every 12 hours for 48 hours on two occasions (1 week apart): once alone and once during the concomitant administration of diphenhydramine hydrochloride (50 mg every 12 hours). Blood and urine samples were collected for 12 hours under steady-state conditions. In EMs, diphenhydramine decreased venlafaxine oral clearance from 104+/-60 L/hr to 43+/-23 L/hr (mean +/- SD; p < 0.05) without any effect on renal clearance (4+/-1 L/hr during venlafaxine alone and 4+/-2 L/hr during venlafaxine plus diphenhydramine). In PMs, coadministration of diphenhydramine did not cause significant changes in oral clearance and partial metabolic clearances of venlafaxine to its various metabolites. Diphenhydramine disposition was only slightly affected by genetically determined low CYP2D6 activity or concomitant administration of venlafaxine. In conclusion, diphenhydramine, at therapeutic doses, inhibits CYP2D6-mediated metabolism of venlafaxine in humans. Clinically significant interactions could be encountered during the concomitant administration of diphenhydramine and other antidepressant or antipsychotic drugs that are substrates of CYP2D6.

Effect of gender, sex hormones, time variables and physiological urinary pH on apparent CYP2D6 activity as assessed by metabolic ratios of marker substrates.

The effects of gender, time variables, menstrual cycle phases, plasma sex hormone concentrations and physiologic urinary pH on CYP2D6 phenotyping were studied using two widely employed CYP2D6 probe drugs, namely dextromethorphan and metoprolol. Phenotyping on a single occasion of 150 young, healthy, drug-free women and men revealed that the dextromethorphan: dextrorphan metabolic ratio (MR) was significantly lower (P < 0.0001) in 56 female extensive metabolizers (0.008+/-0.021) compared to 86 male extensive metabolizers (0.020 +/-0.040). Urinary pH was a significant predictor of dextromethorphan: dextrorphan MRs in men and women (P < 0.001). Once-a-month phenotyping with dextromethorphan of 12 healthy young men (eight extensive metabolizers and four poor metabolizers) over a 1-year period, as well as every-other-day phenotyping with dextromethorphan of healthy, pre-menopausal women (10 extensive metabolizers and 2 poor metabolizers) during a complete menstrual cycle, did not follow a particular pattern and showed similar intrasubject variability ranging from 24.1% to 74.5% (mean 50.9%) in men and from 20.5% to 96.2% (mean 52.0%) in women, independent of the CYP2D6 phenotype (P = 0.342). Using metoprolol as a probe drug, considerable intrasubject variability (38.6+/- 12.0%) but no correlation between metoprolol: alpha-hydroxymetoprolol MRs and pre-ovulatory, ovulatory and luteal phases (mean +/- SD metoprolol: a-hydroxymetoprolol MRs: 1.086+/- 1.137 pre-ovulatory; 1.159+/-1.158 ovulatory and 1.002+/-1.405 luteal phase; P> 0.9) or 17beta-oestradiol, progesterone or testosterone plasma concentrations was observed. There was a significant inverse relationship between physiologic urinary pH and sequential dextromethorphan: dextrorphan MRs as well as metoprolol: alpha-hydroxymetoprolol MRs in men and women, with metabolic ratios varying up to six-fold with metoprolol and up to 20-fold with dextromethorphan (ANCOVA P < 0.001). We conclude that apparent CYP2D6 activity is highly variable, independent of menstrual cycle phases, sex hormones, time variables or phenotype. Up to 80% of the observed variability can be explained by variations of urinary pH within the physiological range. An apparent phenotype shift as a result of variations in urinary pH may be observed in individuals who have metabolic ratios close to the population antimode.

Significant interaction between the nonprescription antihistamine diphenhydramine and the CYP2D6 substrate metoprolol in healthy men with high or low CYP2D6 activity.

The prototype "classic" over-the-counter antihistamine diphenhydramine was shown to interact with the polymorphic P450 enzyme CYP2D6. This project was undertaken to investigate (1) whether diphenhydramine inhibits the biotransformation of the clinically relevant CYP2D6 substrate metoprolol in vitro and (2) whether this in vitro interaction results in a clinically significant pharmacokinetic and pharmacodynamic drug interaction in vivo. In vitro incubations were carried out with microsomes obtained from lymphoblastic cells transfected with CYP2D6 complementary deoxyribonucleic acid to determine the type and extent of inhibition. We then randomized 16 subjects with genetically determined high (extensive metabolizers) or low (poor metabolizers) CYP2D6 activity to receive metoprolol (100 mg) in the presence of steady-state concentrations of diphenhydramine or placebo. In vitro, diphenhydramine was a potent competitive inhibitor of metoprolol alpha-hydroxylation, exhibiting an inhibitory constant of 2 micromol/L and increasing the Michaelis-Menten constant of metoprolol sixfold. In vivo, diphenhydramine decreased metoprolol oral and nonrenal clearances twofold and metoprolol-->alpha-hydroxymetoprolol partial metabolic clearance 2.5-fold in extensive metabolizers (all P < .05) but not in poor metabolizers (P > .2). Although the hemodynamic response to metoprolol was unaltered by diphenhydramine in poor metabolizers (P > .05), metoprolol-related effects on heart rate, systolic blood pressure, and Doppler-derived aortic blood flow peak velocity were more pronounced and lasted significantly longer in extensive metabolizers receiving diphenhydramine compared with poor metabolizers and extensive metabolizers receiving placebo. We conclude that diphenhydramine inhibits the metabolism of metoprolol in extensive metabolizers, thereby prolonging the negative chronotropic and inotropic effects of the drug. Clinically relevant drug interactions may occur between diphenhydramine and many CYP2D6 substrates, particularly those with a narrow therapeutic index.

CYP2D6 mutations and therapeutic outcome in schizophrenic patients.

STUDY OBJECTIVE: To investigate whether a relationship exists between the most common known cytochrome P450 (CYP) isozyme 2D6 mutations and schizophrenia. Because most antipsychotic and antidepressant agents interact with CYP2D6, we also investigated clinical outcomes in schizophrenic poor metabolizers (PMs) and extensive metabolizers (EMs). DESIGN: Prospective, observational study. SETTING: Two psychiatric hospitals and a university-affiliated nonpsychiatric hospital. SUBJECTS: Thirty-nine consecutive schizophrenic patients (POP 1), 89 schizophrenics of French Canadian origin (POP 2), and 384 healthy French Canadians (POP 3). INTERVENTION: All study subjects were genotyped for CYP2D6 mutant alleles. POP 1 patients were evaluated before and after 21 or more days of treatment with antipsychotic drugs metabolized at least in part by CYP2D6. MEASUREMENTS AND MAIN RESULTS: Whole blood was collected to determine CYP2D6 alleles *1, *3, *4, *5, *6, and *7 using standard restriction fragment length polymorphisms and polymerase chain reaction techniques. In comparison, CYP2D6 genotypes were determined in POP 2 and POP 3. Twenty-three (59.0%) of 39 patients in POP 1 were genotypically EM homozygotes, 15 (38.4%) were EM heterozygotes, and 1 (2.6%) was a PM. Similar genotype distributions were determined in POP 2 and in POP 3. Genotype distributions for all three populations were in Hardy-Weinberg equilibrium (p>0.05), and there was no significant difference among them (p=0.857). In POP 1, no differences were seen among genotypes in disease symptom severity, number and severity of adverse drug effects, or attitudes toward drug treatment at baseline and at the end of the study. In fact, all patients improved significantly during their hospital stay (all p<0.05), although independent of the CYP2D6 genotype. CONCLUSION: Common CYP2D6 mutant alleles were not associated with schizophrenia or with disease symptoms, antipsychotic-related adverse effects, or attitudes toward treatment.

Thioridazine lengthens repolarization of cardiac ventricular myocytes by blocking the delayed rectifier potassium current.

Proarrhythmia has been observed with the antipsychotic agent thioridazine (THIO). The mechanisms underlying these effects are unknown. The objectives of this study were 1) to characterize the effects of THIO on cardiac repolarization and 2) to determine whether lengthening of the Q-T interval could be explained by blocking major K+-repolarizing currents. Isolated, buffer-perfused guinea pig hearts (n = 32) were stimulated at various pacing cycle lengths (150-250 ms) and exposed to THIO at concentrations ranging from 300 nM to 3 microM. THIO increased monophasic action potential duration at 90% repolarization (MAPD90) in a concentration-dependent manner from 14.9 +/- 1.8 at 300 nM to 37.1 +/- 3.2 ms at 3 microM. Increase in MAPD90 was also reverse frequency-dependent; THIO (300 nM) increased MAPD90 by 14.9 +/- 1.8 ms at a pacing cycle length of 250 ms, but by only 7.7 +/- 1.2 ms at a pacing cycle length of 150 ms. Patch-clamp experiments demonstrated that THIO decreases the time-dependent outward K+ current elicited by short depolarizations (250 ms; IK250) in a concentration-dependent manner. Estimated IC50 for IK250, which mostly underlies IKr, was 1.25 microM. Time-dependent outward K+ current elicited in tsA201 cells expressing high levels of HERG protein was also decreased approximately 50% by 1.25 microM THIO. On the other hand, THIO was less potent (IC50 of 14 microM) to decrease time-dependent K+ current elicited by long pulses (5000 ms; IK5000). Under the latter conditions, IK5000 corresponds mainly to IKs. Thus, these results demonstrate block of K+ currents and lengthening of cardiac repolarization by THIO in a concentration-dependent manner. This may provide an explanation of Q-T prolongation observed in some patients treated with THIO.

Block of potassium currents in guinea pig ventricular myocytes and lengthening of cardiac repolarization in man by the histamine H1 receptor antagonist diphenhydramine.

Treatment with second generation histamine H1 receptor antagonists has been associated with lengthening of the Q-T interval and proarrhythmia. Similarly, lengthening of the Q-T interval has been reported in patients after overdosing with diphenhydramine (DPH), a first generation agent. Therefore, our study was designed 1) to assess effects of DPH on cardiac repolarization and 2) to characterize effects of the drug on major voltage-dependent cardiac K+ currents. First, we noticed that oral administration of DPH at usual dosages to healthy volunteers or to patients (prior to angioplasty) was associated with prolongation of the Q-Tc interval. Although this effect was modest in most individuals, Q-Tc was increased more than 20 ms in 7 of 20 patients. Second, we noticed that exposure of isolated guinea pig hearts to DPH 10(-5) M caused a lengthening of monophasic action potential duration. This effect was potentiated by the combined perfusion of other K+ channel blockers such as indapamide. Finally, experiments performed with the patch-clamp technique demonstrated unequivocal block of the rapid component of the delayed rectifier (IKr) by DPH; however, IC50 determined for block of IKr (3 x 10(-5) M) is approximately 40-fold greater than plasma concentrations of the drug measured at usual dosages (7 x 10(-7) M). Consequently, in agreement with the long-term clinical use of the drug, prolongation of cardiac repolarization should be minimal in most patients at usual dosages but may be observed with overdosing. Nevertheless, caution remains since excessive lengthening of cardiac repolarization may occur after administration of DPH with other drugs due to 1) concomitant block of other ionic currents or 2) pharmacokinetic interactions leading to toxic concentrations of DPH.

Influence of CYP2D6 activity on the disposition and cardiovascular toxicity of the antidepressant agent venlafaxine in humans.

According to in-vitro studies with microsomes from human livers and from yeast expression systems with high CYP2D6 activity, the major oxidation pathway of venlafaxine is catalysed by CYP2D6. In this study, we investigated the role of the CYP2D6 polymorphism and the effects of low-dose quinidine, a selective inhibitor of, CYP2D6, on the disposition of venlafaxine. Fourteen healthy men, eight with the extensive metabolizer and six with the poor metabolizer phenotype were administered venlafaxine hydrochloride 18.75 mg orally every 12 h for 48 h on two occasions (1 week apart); once alone and once during the concomitant administration of quinidine sulfate 100 mg every 12 h. Blood and urine samples were collected under steady-state conditions over one dosing interval (12 h). When venlafaxine was administered alone, the oral clearance of venlafaxine was more than fourfold less in poor metabolizers compared to extensive metabolizers (P < 0.05). This was mainly due to a decreased capability of poor metabolizers to form O-desmethylated metabolites at the position 4 of the aromatic moiety. In extensive metabolizers, quinidine decreased venlafaxine oral clearance from 100 +/- 62 l/h to 17 +/- 5 l/h (mean +/- SD; P < 0.05) without any effects on renal clearance (4 +/- 1 l/h during venlafaxine alone and 4 +/- 1 l/h during venlafaxine plus quinidine). In these individuals, the sequential metabolism of venlafaxine to O-desmethylvenlafaxine and to N,O-didesmethylvenlafaxine was inhibited by quinidine coadministration so that metabolic clearances to O-desmethylated metabolites decreased from 43 +/- 32 l/h to 2 +/- 1 l/h (P < 0.05). In poor metabolizers, coadministration of quinidine did not cause significant changes in oral clearance and partial metabolic clearances of venlafaxine to its various metabolites. Decreased CYP2D6 activity could also be associated with cardiovascular toxicity as observed in four patients during treatment with the drug. Thus, genetically determined or pharmacologically altered CYP2D6 activity represents a major determinant of venlafaxine disposition in humans.

Involvement of CYP2D6 activity in the N-oxidation of procainamide in man.

Occurrence of a lupus-like syndrome in a significant number of patients treated with procainamide has limited the clinical use of this antiarrhythmic drug. In-vitro studies conducted in our laboratory have demonstrated that CYP2D6 is the major cytochrome P450 isozyme involved in the formation of N-hydroxyprocainamide, a metabolite potentially involved in the drug-induced lupus erythematosus syndrome observed with procainamide. In the current study, we evaluated the role of CYP2D6 activity in the in-vivo oxidation of procainamide in man. Nineteen healthy individuals, 13 with high (extensive metabolizers) and six with low (poor metabolizers) CYP2D6 activity, received a single 500 mg oral dose of procainamide hydrochloride on two occasions, once alone (period 1) and once during the concomitant administration of the selective inhibitor quinidine (50 mg four times daily; period 2). Blood and urine samples were collected over 36 h after drug administration of procainamide and analysed for procainamide and its major metabolites (N-acetylprocainamide, desethylprocainamide, N-acetyl-desethylprocainamide, p-aminobenzoic acid and its N-acetylated derivative, and nitroprocainamide). No differences were observed in the oral and renal clearances of procainamide between extensive metabolizers and poor metabolizers during either study period. However, partial metabolic clearance of procainamide to desethylprocainamide was significantly greater in extensive metabolizers than in poor metabolizers during both periods. Most importantly, the urinary excretion of nitroprocainamide during period 1 was measurable in 7/13 extensive metabolizers but in none of the poor metabolizers. During the concomitant administration of quinidine, nitroprocainamide could not be detected in the urine of any individuals tested. Therefore, our results suggest that CYP2D6 is involved in the in-vivo aliphatic amine deethylation and N-oxidation of procainamide at its arylamine function in man. Further studies are needed to demonstrate whether a low CYP2D6 activity, either genetically determined or pharmacologically modulated, could prevent drug-induced lupus erythematosus syndrome observed during chronic therapy with procainamide.

In vitro characterization of cytochrome P450 2D6 inhibition by classic histamine H1 receptor antagonists.

Classic antihistamines, namely diphenhydramine, chlorpheniramine, clemastine, perphenazine, hydroxyzine, and tripelennamine, share structural features with substrates and inhibitors of the polymorphic cytochrome P450 (CYP) isozyme CYP2D6. Therefore, the current study was undertaken to characterize the in vitro inhibition of CYP2D6 by these commonly used, histamine H1 receptor antagonists. Microsomal incubations were performed using bufuralol as a specific CYP2D6 substrate and microsomes derived from human cells transfected with CYP2D6 cDNA. Reaction velocities were assessed in the absence and presence of antihistamines (20 microM) at 11 substrate concentrations (1, 2.5, 5, 7.5, 10, 15, 20, 25, 50, 75, and 100 microM), as well as at three nonsaturating substrate concentrations (2.5, 5, and 20 microM) and three inhibitor concentrations (5, 20, and 50 microM). In the presence of all antihistamines, the Vmax and KM of bufuralol 1'-hydroxylation were significantly altered, compared with the uninhibited reaction (p < 0.05). Lineweaver-Burke plots suggested competitive inhibition of the reaction by diphenhydramine and mixed inhibition by all other antihistamines tested. Diphenhydramine and chlorpheniramine, with estimated Ki values of approximately 11 microM, were the weakest inhibitors of CYP2D6 in vitro. Whereas tripelennamine, promethazine, and hydroxyzine were similar in their inhibitory capacities (Ki approximately 4-6 microM), clemastine appeared to be significantly more potent, with a Ki of approximately 2 microM. These data demonstrate that classic histamine H1 receptor antagonists, available in over-the-counter preparations, inhibit CYP2D6 in vitro. Furthermore, the CYP2D6-inhibitory concentrations of these antihistamines are in the range of their expected hepatic blood concentrations, suggesting that, under specific circumstances, clinically relevant interactions between classic antihistamines and CYP2D6 substrates might occur.

Hydrophilicity/lipophilicity: relevance for the pharmacology and clinical effects of HMG-CoA reductase inhibitors.

The recent development of specific competitive inhibitors of the hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase such as lovastatin, simvastatin, pravastatin and fluvastatin has provided an important new and effective approach to the treatment of hyperlipidaemia and atherosclerosis. These agents are designed to be hepatoselective because the primary site of cholesterol synthesis is the liver and peripheral inhibition of cholesterol synthesis would be more likely to cause adverse drug effects. In this review, Bettina Hamelin and Jacques Turgeon discuss how specific physico-chemical and pharmacological properties (first-pass effect or carrier-mediated uptake) confer hepatoselectivity to either lipophilic or hydrophilic HMG-CoA reductase inhibitors.

The effect of timing of a standard meal on the pharmacokinetics and pharmacodynamics of the novel atypical antipsychotic agent ziprasidone.

STUDY OBJECTIVE: To evaluate the influence of a high-fat meal on the pharmacokinetics and pharmacodynamics of the novel atypical antipsychotic drug ziprasidone. DESIGN: Open, randomized, three-way crossover study. SETTING: University-based research facility. SUBJECTS: Eight healthy male volunteers. INTERVENTIONS: Ziprasidone 20 mg was administered under fasting conditions (treatment A), and directly after (treatment B) and 2 hours after (treatment C) a standard high-fat breakfast. MEASUREMENTS AND MAIN RESULTS: Serial blood samples were obtained over 36 hours. Three objective psychometric tests were employed to evaluate daytime vigilance at baseline and 2 hours after each dose. Ziprasidone had a significant effect on area under the curve (AUC0-infinity), maximum serum concentration, and half-life (analysis of variance all p<0.05), with the mean AUC0-infinity being significantly greater (627.2 +/- 206.4 vs 371.0 +/- 126.5 ng x hr/ml, ANOVA with Bonferroni's criteria p<0.016) and half-life significantly shorter (4.7 +/- 0.8 vs 6.6 +/- 1.3 hrs, ANOVA with Bonferroni's criteria p<0.016) after treatment B compared with treatment A. Although similar trends were observed after treatment C compared with treatment A, the differences did not reach statistical significance when Bonferroni's correction criteria were applied (p>0.016). CONCLUSION: These data suggest an increase in systemic exposure to the highly lipophilic compound ziprasidone when taken after fatty foods, possibly due to improved drug dissolution and solubilization. The drug's longer half-life under fasting conditions may reflect dissolution-limited absorption, although this could not be directly assessed. Despite postprandial increases in ziprasidone AUC0-infinity and maximum concentration, daytime vigilance was not affected.

Improved high-performance liquid chromatographic assay for the determination of procainamide and its N-acetylated metabolite in plasma: application to a single-dose pharmacokinetic study.

An improved high-performance liquid chromatographic assay for the determination of procainamide and N-acetylprocainamide (NAPA) at concentrations observed up to 32 h after a single oral dose administration of procainamide to human subjects is reported. Following liquid-liquid extraction of plasma samples, procainamide, NAPA, and the internal standard (N-propionylprocainamide) are separated on a reversed-phase C8 column with retention times of 4.0, 6.7, and 13.2 min, respectively. The ultraviolet detection limit (wavelength, 280 nm) of procainamide and NAPA is 2 ng/mL (signal-to-noise ratio, 3:1), and the quantitation limit is 4 ng/mL (signal-to-noise ratio, 5:1). Intra- and interday coefficients of variation are less than 8% in the range of 20-500 ng/mL.

Block of the rapid component of the delayed rectifier potassium current by the prokinetic agent cisapride underlies drug-related lengthening of the QT interval.

BACKGROUND: Lengthening of the QT interval and torsades de pointes resulting in cardiac arrests and deaths have been noticed during treatment with cisapride, a newly developed gastrointestinal prokinetic agent. The rapid (I[Kr]) and slow (I[Ks]) components of the delayed rectifier current (I[K]) are candidate ionic currents to explain cisapride-related toxicity because of their role in repolarization of cardiac ventricular myocytes. Our objectives were to (1) characterize effects of cisapride on two major time-dependent outward potassium currents involved in the repolarization of cardiac ventricular myocytes, I(Kr) and I(Ks), and (2) determine action potential-prolonging effects of cisapride on isolated hearts. METHODS AND RESULTS: A first set of experiments was performed in isolated guinea pig ventricular myocytes with the whole-cell configuration of the patch-clamp technique. Cells were held at -40 mV while time-dependent outward currents were elicited by depolarizing pulses lasting either 250 ms (I[K250]) or 5000 ms (I[K5000]). Effects of cisapride on the I(Kr) component were assessed by measurement of time-dependent activating currents elicited by short pulses (250 ms; I[K250]) to low depolarizing potentials (-20, -10, and 0 mV). Time-dependent activating currents elicited by long pulses (5000 ms; I[K5000]) to positive potentials (>+30 mV) were recorded to assess effects of the drug on the I(Ks) component. A second set of experiments was conducted in isolated guinea pig hearts buffer-perfused in the Langendorff mode to assess effects of the drug on monophasic action potential duration measured at 90% repolarization (MAPD90). Hearts were exposed to cisapride 100 nmol/L at decremental pacing cycle lengths of 250, 225, 200, 175, and 150 ms to determine reverse frequency-dependent effects of the drug. Overall, 112 myocytes were exposed to seven concentrations of cisapride (10 nmol/L to 10 micromol/L). Cisapride inhibited I(Kr), the major time-dependent outward current elicited by short pulses (I[K250]) to low depolarizing potentials, in a concentration-dependent manner with an IC50 of 15 nmol/L (therapeutic levels, 50 to 200 nmol/L). Conversely, block of I(Ks) by the drug was less potent (estimated IC50 >10 micromol/L). In isolated hearts (n=9 experiments), cisapride 100 nmol/L increased MAPD90 by 23+/-3 (P<.05) at a basic cycle length of 250 ms but by only 7+/-1 ms (P<.05) at a basic cycle length of 150 ms. CONCLUSIONS: Block of I(Kr) gives an explanation to lengthening of cardiac repolarization observed in isolated guinea pig hearts. Potent block of I(Kr) is also likely to underlie prolongation of the QT interval observed in patients receiving clinically recommended doses of cisapride as well as severe cardiac toxicity (torsades de pointes) observed in patients with increased plasma concentrations of the drug.

Role of CYP2D6 in the N-hydroxylation of procainamide.

Sequential oxidations at the arylamine moiety of the procainamide molecule leading to the formation of N-hydroxyprocainamide and its nitroso derivative may be responsible for lupus erythematosus observed in patients treated with the drug. The objective of the present study was to characterize major cytochrome P450 isozyme(s) involved in the N-hydroxylation of procainamide. Firstly, incubations were performed with microsomes from either lymphoblastoid cells or yeast transfected with cDNA encoding for specific human cytochrome P450 isozymes. Experiments performed with these enzyme expression systems indicated that the highest formation rate of N-hydroxyprocainamide was observed in the presence of CYP2D6 enriched microsomes. Additional experiments demonstrated that the formation rate of N-hydroxyprocainamide by CYP2D6 enriched microsomes was decreased from 45 +/- 4% to 93 +/- 1% by quinidine at concentrations ranging from 30 nM to 100 microM (all p < 0.05 vs control) and by approximately 75% by antibodies directed against CYP2D6. Secondly, incubations were performed with microsomes prepared from 15 human liver samples. Using this approach, an excellent correlation was observed between the formation rate of N-hydroxyprocainamide and dextromethorphan O-demethylase activity (CYP2D6; r = 0.9305; p < 0.0001). In contrast, no correlation could be established between N-hydroxyprocainamide formation rate and caffeine N3-demethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), S-mephenytoin N-demethylase (CYP2B6), tolbutamide methlhydroxylase (CYP2C9), S-mephenytoin 4'-hydroxylase (CYP2C19), chlorzoxazone 6-hydroxylase (CYP2E1), dextromethorphan N-demethylase (CYP3A4), testosterone 6 beta-hydroxylase (CYP3A4/5) or lauric acid 12-hydroxylase (CYP4A11) activities. Furthermore, formation rate of N-hydroxyprocainamide was decreased in a concentration-dependent manner by quinidine (300 nM to 100 microM) and by antibodies directed against CYP2D6 but not by furafylline 20 microM (CYP1A2), ketoconazole 1 microM (CYP3A4), sulfaphenazole 10 microM (CYP2C9) or antibodies directed against CYP1A1/1A2, CYP2C, CYP2A6, CYP2E1 or CYP3A4/3A5. In conclusion, the results obtained in the present study demonstrate that CYP2D6 is the major human cytochrome P450 isozyme involved in the formation of the reactive metabolite of procainamide, namely N-hydroxyprocainamide.

Block of IKs by the diuretic agent indapamide modulates cardiac electrophysiological effects of the class III antiarrhythmic drug dl-sotalol.

Indapamide is a diuretic agent with direct electrophysiological effects on ionic currents involved in cardiac repolarization. In particular, indapamide blocks the slow component of delayed rectifier potassium current. In contrast, most class III antiarrhythmic agents, such as dl-sotalol, block the rapid component of delayed rectifier potassium current. Computer simulations have suggested potentiation of drug effects on cardiac repolarization by the combined block of the rapid component of delayed rectifier potassium current and the slow component of delayed rectifier potassium current. Therefore, the objective of our study was to evaluate the modulation of cardiac electrophysiological effects of dl-sotalol by indapamide. Two indices of cardiac repolarization, monophasic action potential duration at 90% repolarization and effective refractory period, at two basic cycle lengths (800 and 400 msec) were determined in 24 anesthetized open-chest dogs. In two treatment groups (n = 6/group), data were obtained at base line and every 2 min during steadily increasing concentrations of dl-sotalol (0-40 microg/ml) either alone or in the presence of indapamide (500 ng/ml). Data were also obtained in dogs receiving either a low-dose (500 ng/ml) or a high-dose (up to 7.5 microg/ml) infusion regimen of indapamide alone. Administration of dl-sotalol was associated with concentration-dependent increases in monophasic action potential duration at 90% repolarization and effective refractory period, whereas repolarization was only slightly altered by the administration of indapamide alone. However, concentration-response curves of dl-sotalol were shifted to the left in dogs treated with the combination of dl-sotalol and indapamide, and the EC50 values of dl-sotalol estimated for the prolongation of monophasic action potential duration at 90% repolarization and effective refractory period were decreased 3-fold during the coadministration of both drugs (P < .05 vs. dl-sotalol alone). Thus, under conditions of normal K+ levels, clinically relevant concentrations of indapamide modulate dl-sotalol effects on cardiac repolarization. Additional block of cardiac K+ currents, especially the rapid component of delayed rectifier potassium current and the slow component of delayed rectifier potassium current could explain these observations.

The disposition of fluoxetine but not sertraline is altered in poor metabolizers of debrisoquin.

BACKGROUND: Substrates and inhibitors of the cytochrome P450 isozyme CYP2D6 have overlapping structural characteristics. Two prototype serotonin uptake inhibitors, sertraline and fluoxetine, share these structural criteria and have been identified as potent inhibitors of CYP2D6 in vitro. The current study was undertaken to investigate whether genetically determined CYP2D6 activity alters the disposition of sertraline or fluoxetine or both. METHODS: Single doses of sertraline (50 mg) and fluoxetine (20 mg) were administered successively to 20 young men with high (extensive metabolizers; n = 10) and low (poor metabolizers; n = 10) CYP2D6 activity. Blood and urine samples were collected for 5 to 7 half-lives and sertraline, desmethylsertraline, fluoxetine, and norfluoxetine were determined by GC and HPLC techniques. RESULTS: Poor metabolizers had significantly greater fluoxetine peak plasma concentrations (Cmax; increases 57%), area under the concentration versus time curve (AUCzero-->infinity; increases 290%), and terminal elimination half-life (increases 216%) compared with extensive metabolizers. The total amount of fluoxetine excreted in the urine during 8 days was almost three times higher in poor metabolizers than in extensive metabolizers (719 versus 225 micrograms; p < 0.05), whereas the total amount of norfluoxetine excreted in urine of poor metabolizers was about half of that of extensive metabolizers (524 versus 1047 micrograms; p < 0.05). Norfluoxetine Cmax and AUCzero-->t were significantly smaller in poor metabolizers (decreases 55% and decreases 53%, respectively), and the partial metabolic clearance of fluoxetine into norfluoxetine was 10 times smaller in this group (4.3 +/- 1.9 versus 0.4 +/- 0.1 L/hr; p < 0.05). No significant differences between extensive and poor metabolizers were found for sertraline and desmethylsertraline pharmacokinetics. CONCLUSION: These data indicate that poor metabolizers accumulate fluoxetine but not sertraline and that CYP2D6 plays an important role in the demethylation of fluoxetine but not of sertraline.

Caffeine metabolism in cystic fibrosis: enhanced xanthine oxidase activity.

STUDY OBJECTIVE: To characterize the activities of the P450 mixed-function oxidase CYP1A2 as well as the cytosolic enzymes N-acetyltransferase and xanthine oxidase using caffeine as a probe in children with cystic fibrosis compared to age-matched healthy control subjects. METHODS: After administration of caffeine (cola beverage) to 12 children with cystic fibrosis (age range, 5 to 11 years) and 12 healthy control subjects (age range, 5 to 12 years), urine was collected for 4 hours. Caffeine metabolites were determined by HPLC, and urinary caffeine metabolite ratios were computed to determine liver enzyme activities. In addition, a blood sample was used to detect cystic fibrosis mutant alleles by polymerase chain reaction. RESULTS: The indexes for CYP1A2, N-acetyltransferase, and 8-hydroxylation were similar in both groups of subjects. In contrast, there was a significant difference in the frequency distribution of the xanthine oxidase activity between the two groups. Nine of 12 patients with cystic fibrosis but only one of 12 healthy volunteers had xanthine oxidase activities above 0.42 (Kolmogorov-Smirnov two-sample test, p < 0.01). CONCLUSIONS: Differences in xanthine oxidase may have clinical implications with regard to interindividual variation in xenobiotic biotransformation and the exposure to lung tissue-damaging oxygen radicals. Hepatic enzyme activities appear to be selectively altered in patients with cystic fibrosis.

Cefepime pharmacokinetics in cystic fibrosis.

STUDY OBJECTIVE: To determine the disposition of cefepime in patients with cystic fibrosis compared with healthy controls. DESIGN: Open-label, single-dose study. SETTING: Laboratoire de Pharmacocinétique Clinique, Université Laval, Québec, Canada. PATIENTS AND SUBJECTS: Twelve patients with the confirmed diagnosis of cystic fibrosis (CF) and 12 healthy volunteers. One subject with CF withdrew for personal reasons; the data of another patient were excluded from the evaluation of renal values due to incomplete urine collection. INTERVENTIONS: A single 2000-mg dose of cefepime was administered as a 30-minute intravenous infusion. Healthy subjects did not use any other drugs throughout the study. Those with CF refrained from taking prophylactic antibiotics prior to and during the study, but continued to use pancreatic enzymes, multivitamins, and beta-agonist and/or steroid inhalers. One patient continued insulin treatment. MEASUREMENTS AND MAIN RESULTS: Cefepime's maximum concentration was approximately 150 micrograms/ml at the end of the infusion, half-life 2-2.5 hours, and urinary recovery 80% in both groups. No statistically significant difference was seen in any of the pharmacokinetic values between the groups, except for the mean residence time (2.03 +/- 0.26 vs 2.39 +/- 0.37 hrs; p < 0.02). Total clearance was 19% higher in patients with CF than in healthy volunteers (119.7 +/- 20.1 vs 103.5 +/- 19.8 ml/min), perhaps due to higher renal (95.1 +/- 12.4 vs 85.1 +/- 12.0 ml/min) and/or nonrenal (25.4 +/- 13.1 vs 18.4 +/- 12.0 ml/min) clearances in subjects with CF. CONCLUSIONS: The disposition of cefepime is not significantly affected by CF, and dosage adjustment appears not to be necessary in these patients.

Fleroxacin pharmacokinetics in patients with liver cirrhosis.

In this open-label study, the disposition of fleroxacin in liver disease in 12 healthy male volunteers, 6 male cirrhotics without ascites (group A), and 6 male cirrhotics with ascites (group B) was evaluated. Fleroxacin (400 mg) was administered orally and intravenously to each subject in a random crossover fashion. Fleroxacin was completely absorbed and achieved similar peak concentrations in plasma in all three study groups (P greater than 0.05). The volume of distribution exceeded 1 liter/kg in healthy controls and was not affected by liver impairment (P greater than 0.05). Only group B demonstrated differences in the pharmacokinetic parameters evaluated: the systemic and renal clearances of fleroxacin and the renal clearances and clearances of the two major metabolites of fleroxacin formed, N-demethyl fleroxacin and fleroxacin N-oxide, were significantly lower and the half-lives of the parent drug and its metabolites were significantly longer in group B than in healthy controls and group A (P less than 0.05). The elimination of the two metabolites appeared to be formation rate limited in all three study groups. It was concluded from this study that a 50% reduction in the fleroxacin maintenance dose in patients with liver disease appears justified only in patients with ascites. However, no change in the fleroxacin loading dose is needed in patients with compromised liver function.