Verlängerung der QT-Zeit
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Eklärungen für Patienten zu den Wirkstoffen
Für die Kombination von Chinidin und Astemizol liegen uns keine zusätzlichen Warnhinweise vor. Bitte konsultieren Sie zusätzlich die jeweiligen Fachinformationen.
Die genannten Expositionsveränderungen beziehen sich jeweils auf Veränderungen der Plasmakonzentrations-Zeit-Kurve [ AUC ]. Eine Veränderung der Exposition von Chinidin haben wir nicht erkannt. Den Einfluss von Astemizol können wir aktuell nicht abschätzen. Eine Veränderung der Exposition von Astemizol haben wir nicht erkannt. Den Einfluss von Chinidin können wir aktuell nicht abschätzen.
Für die Berechnung der individuellen Expositionsveränderungen durch die Wechselwirkungen werden als Ausgangsbasis die pharmakokinetischen Parameter der durchschnittlichen Population verwendet.
Chinidin hat eine mittlere orale Bioverfügbarkeit [ F ] von 72%, weshalb die maximalen Plasmaspiegel [ Cmax ] sich bei einer Interaktion tendentiell verändern. Die terminale Halbwertszeit [ t12 ] beträgt 6.4 Stunden und konstante Plasmaspiegel [ Css ] werden ungefähr nach 25.6 Stunden erreicht. Das therapeutische Fenster ist eng und die Sicherheitsmarge daher klein. Schon kleine Expositionsänderungen können das Toxizitätsrisiko erhöhen. Die Proteinbindung [ Pb ] ist mit 84% mässig stark und das Verteilungsvolumen [ Vd ] ist mit 139 Liter sehr gross, da die Substanz eine tiefe hepatische Extraktionsrate von 0.23 besitzt, kann eine Verdrängung aus der Proteinbindung [Pb] im Rahmen einer Interaktion die Exposition erhöhen. Die Metabolisierung findet vor allem über CYP3A4 statt und der aktive Transport erfolgt zum Teil über OATP1A2 und PGP. Unter anderem ist Chinidin ein Hemmer von CYP2D6.
Astemizol hat eine tiefe orale Bioverfügbarkeit [ F ] von 3%, weshalb die maximalen Plasmaspiegel [ Cmax ] sich bei einer Interaktion tendentiell stark verändern. Die terminale Halbwertszeit [ t12 ] beträgt 22 Stunden und konstante Plasmaspiegel [ Css ] werden ungefähr nach 88 Stunden erreicht. Die Proteinbindung [ Pb ] ist mit 97% stark. Die Metabolisierung findet unter anderem über CYP2D6 und CYP3A4 statt. Unter anderem ist Astemizol ein Hemmer von CYP3A4 und PGP.
|Serotonerge Effekte a||0||Ø||Ø|
Bewertung: Gemäss unseren Erkenntnissen erhöhen weder Chinidin noch Astemizol die serotonerge Aktivität.
|Kiesel & Durán b||2||++||Ø|
Empfehlung: Insbesondere nach einer Dosiserhöhung und bei Dosierungen im oberen therapeutischen Bereich sollte vorsichtshalber auf anticholinerge Symptome geachtet werden.
Bewertung: Chinidin moduliert das anticholinerge System in moderatem Ausmass. Das Risiko für ein anticholinerge Syndrom ist bei dieser Medikation eher als gering einzustufen, wenn die Dosierung sich im üblichen Bereich befindet. Gemäss unseren Erkenntnisse erhöht Astemizol nicht die anticholinerge Aktivität.
Verlängerung der QT-Zeit
Bewertung: In Kombination können Chinidin und Astemizol potentiell ventrikuläre Arrhythmien vom Typ Torsades de pointes auslösen.
|Ventrikuläre Arrhythmie||0.0 %||0.01||n.a.|
Basierend auf Ihren
Abstract: QRS duration, QT interval, total electromechanical systole (QS(2)), left ventricular ejection time (LVET), and preejection period (PEP) were determined in five male and two female healthy volunteers in a fasting state at hourly intervals for 7 hours during a control period and after administration of 400 mg quinidine sulfate. Changes of QRS duration (delta QRS) and rate-corrected QT interval (delta QTc) were calculated before and after quinidine. Deviations of measured QS(2), LVET, and PEP from the normal were calculated as the differences between the observed interval and those predicted from the normal regression equation. The effect of quinidine on systolic time intervals (delta QS(2), delta LVET, DELTA PEP) were expressed as the differences between the deviations from the normal regression equation during the control period and after the drug administration. After quinidine sulfate delta QRS, delta LVET, delta PEP, and delta PEP, delta LVET were slight and inconsistent. However, delta QTc and delta QS(2) were significant (at P is less than 0.05 or better) from the first hour to the 7th hour and from the 2nd hour to the 5th hour, respectively. The mean maximum delta QTc was 44.8 milliseconds and delta QS(2) was 29.9 milliseconds. The significant changes of QTc and QS(2) seemed to occur at the plasma level range of 0.75-1.9 mug/ml. This study indicates that of the various systolic time interval measurements obtained after quinidine administration, the changes of QT interval and QS(2) are most significant and that these changes seem to occur even at low plasma levels.
Abstract: The absolute bioavailability of quinidine was studied in 11 hospitalized patients. A 400-mg dose of quinidine gluconate was administered to each patient by intravenous infusion and as an oral solution. Drug treatments were separated by a 72-hr period. In 8 patients, peak plasma quinidine concentrations were reached in 65 min after the oral dose; in the remaining 3 subjects, peak concentrations were reached later. From the ratio of the total area under the plasma concentration-time curves (AUCoral/AUCir), the absolute bioavailability of quinidine ranged from 44% to 89% (mean, 72). In 8 patients, the ratio of the total amount of quinidine excreted in the urine in 48 hr (AUinfinity oral/AUinfinity ir) indicated that the extent of quinidine bioavailability varied form 47% to 96% (mean, 73). The predicted bioavailability of quindine due to first-pass effects was 76+/-11%. It is concluded that absorption after the oral solution was rapid and that the reduction of quinidine bioavailability was due to first-pass hepatic drug removal.
Abstract: No Abstract available
Abstract: Astemizole is a long-acting, highly selective histamine1-receptor antagonist with minimal central and anticholinergic effects. Comparison studies have shown astemizole to be equal or superior to currently available antihistamines, beclomethasone nasal spray, and cromolyn sodium in relieving allergic symptoms of seasonal and perennial allergic rhinitis. Other uses include treatment of allergic conjunctivitis and chronic urticaria. Astemizole is not as effective for treatment of acute allergic symptoms because of its delayed onset of action. Astemizole and its active metabolite, desmethylastemizole, have long elimination half-lives permitting once-daily dosing. The incidence of sedation is lower than with conventional antihistamines, but increased appetite and weight gain do occur. Astemizole should be useful for both maintenance and prophylactic therapy in patients with chronic allergic conditions who cannot tolerate the sedative or anticholinergic effects of conventional antihistamines.
Abstract: Astemizole is an H1-histamine receptor antagonist with a long duration of action permitting once daily administration. Its efficacy in seasonal and perennial allergic rhinitis has been convincingly demonstrated, and several comparative studies suggest that astemizole is at least as effective as some other H1-histamine receptor antagonists. A few smaller studies have shown beneficial effects on the symptoms of allergic conjunctivitis and chronic urticaria (but not atopic dermatitis). While astemizole appears to share with other H1-histamine receptor antagonists a tendency to increase appetite and cause weight gain after prolonged use, it offers the important advantage of an absence of significant central nervous system depression or anticholinergic effects with usual doses. Thus, astemizole offers a worthwhile improvement in side effect profile over 'traditional' H1-histamine receptor antagonists, especially in patients bothered by the sedative effects of these drugs.
Abstract: The elimination of quinidine is accomplished by a combination of renal excretion of the intact drug (15 to 40% of total clearance) and hepatic biotransformation to a variety of metabolites (60 to 85% of total clearance). Many of the metabolites appear to be pharmacologically active. Typical ranges for kinetic properites of quinidine in healthy persons are: apparent volume of distribution 2.0 to 3.5 litres/kg; elimination half-life 5 to 12 hours; clearance, 2.5 to 5.0 ml/min/kg. Quinidine clearance is reduced in the elderly, in patients with cirrhosis, and in those with congestive heart failure. Oral quinidine is available either as relatively rapidly absorbed conventional tablets (usually quinidine sulphate) or as a variety of slowly absorbed sustained release preparations. Absolute systemic availability generally is 70% or greater. Quinidine is 70 to 95% bound to plasma protein, primarily to albumin but also to a number of other plasma constituents. Binding is reduced in patients with cirrhosis, partly because of hypoalbuminaemia, but is not influenced by renal insufficiency. Clinical interpretation of total serum or plasma quinidine concentrations must be altered in patients with reduced or increased binding, since it is the unbound fraction which is pharmacologically active.
Abstract: No Abstract available
Abstract: We studied the interaction of disopyramide, quinidine, and procainamide with cardiac muscarinic receptors. In electrophysiological experiments, the effects of disopyramide, quinidine, procainamide, and atropine were determined on spontaneously depolarizing guinea pig right atria (GPRA) both in the presence and absence of pharmacologically induced (physostigmine) cholinergic stimulation. All four agents demonstrated a concentration-dependent antagonism of the negative chronotropic effects of physostigmine. The order of anticholinergic potency was atropine greater than disopyramide greater than quinidine greater than procainamide. The ability of disopyramide to antagonize the physostigmine induced slowing was stereoselective, (+)disopyramide greater than (-)disopyramide. In contrast, the ability of quinidine to antagonize the negative chronotropic effects of physostigmine was non-stereoselective, quinidine = quinine. In parallel experiments, we studied the ability of disopyramide, quinidine, procainamide, and atropine to compete with the radiolabeled muscarinic receptor antagonist [3H] quinuclidinyl benzilate ([3H]QNB) for binding to muscarinic receptors in crude homogenates of GPRA and membrane vesicles from canine ventricular myocardium. All four agents inhibited [3H]QNB binding to muscarinic receptors. The order of anticholinergic potency determined by the receptor binding studies was identical to that determined by the physiological studies. The interaction of disopyramide with muscarinic receptors was stereoselective, (+)disopyramide > (-)disopyramide. Quinidine was only slightly more potent than quinine in inhibiting [3H]QNB binding to muscarinic receptors. Interaction of antiarrhythmic drugs with muscarinic receptors satisfied criteria for a competitive interaction. The data from this study localize the anticholinergic effects of disopyramide and quinidine to the muscarinic receptor.
Abstract: An overdose of astemizole predisposes the myocardium to ventricular dysrhythmias, including torsades de pointes. Herein we describe a case of astemizole-induced torsades de pointes ventricular tachycardia and also review previous case reports in the literature. All the patients were young, and dysrhythmias developed only in those with corrected QT intervals greater than 500 ms. Although several mechanisms have been postulated, no clear explanation has been provided for why astemizole promotes myocardial dysrhythmias. Treatment of astemizole-induced torsades de pointes includes discontinuing use of astemizole, intravenous administration of magnesium sulfate and isoproterenol, temporary cardiac pacing, and, when necessary, direct current cardioversion. A cardiac cause of syncope or convulsions must not be overlooked, especially in patients taking H1 antagonists because they often have these symptoms before hospitalization or detection of torsades de pointes (or both).
Abstract: No Abstract available
Abstract: A 26 year-old woman was admitted to the hospital two hours after astemizole overdose. Electrocardiograph showed a prolonged QT interval. Torsade de pointes occurred 13 h after ingestion. Plasma levels of astemizole plus hydroxylated metabolites showed an apparent plasma half-life of 17 h. The possible occurrence of torsade de pointes in astemizole overdose, and the long elimination time of astemizole and hydroxylated metabolites, makes it necessary to maintain ECG monitoring until QT interval has returned to normal.
Abstract: Conflicting findings suggest that serum quinidine concentrations may be decreased or increased by nifedipine. We performed a double-blind, placebo-controlled trial of Latin-square design. Twelve healthy men received 3 days of pretreatment with nifedipine prolonged action (20 mg twice a day) or felodipine extended release (10 mg every day), another dihydropyridine calcium antagonist, followed by coadministration of quinidine (400 mg). Quinidine pharmacokinetics were not changed by either dihydropyridine. However, 3-hydroxyquinidine area under the concentration-time curve (AUC) and 3-hydroxyquinidine/quinidine AUC ratio were decreased by felodipine, consistent with reduced metabolite formation. Heart rates and adverse events were higher with felodipine, demonstrating lack of bioequivalence with nifedipine. The QTc interval did not deviate from that expected for the observed quinidine concentration, suggesting the pharmacokinetics of active quinidine metabolites were not markedly altered among treatments. Quinidine disposition did not appear to be changed sufficiently to be clinically important by sustained-release nifedipine and felodipine.
Abstract: OBJECTIVES: To examine the pharmacokinetic and pharmacodynamic interactions between quinidine and diltiazem because both drugs can inhibit drug metabolism. METHODS: Twelve fasting, healthy male volunteers (age, 24 +/- 5 years; weight, 75 +/- 10 kg) received a single oral dose of diltiazem (60 mg) or quinidine (200 mg), alone and on a background of the other drug, in a crossover study. Background treatment consisted of 100 mg quinidine twice a day or 90 mg sustained-release diltiazem twice a day for 2 day before the study day. RESULTS: Pretreatment with diltiazem significantly (p < 0.05) increased the area under the curve of quinidine from 7414 +/- 1965 to 11,213 +/- 2610 ng.hr/ml and increased its terminal elimination half-life (t1/2) from 6.8 +/- 1.1 to 9.3 +/- 1.5 hours. Its oral clearance was decreased from 0.39 +/- 0.1 to 0.25 +/- 0.1 L/hr/kg, whereas the maximal concentration was not significantly affected. Diltiazem disposition was not significantly affected by pretreatment with quinidine. Diltiazem pretreatment increased QTc and PR intervals and decreased heart rate and diastolic blood pressure. No significant pharmacodynamic differences were shown for diltiazem alone versus quinidine pretreatment. CONCLUSION: Diltiazem significantly decreased the clearance and increased the t1/2 of quinidine, but quinidine did not alter the kinetics of diltiazem with the dose used. No significant pharmacodynamic interaction was shown for the combination that would not be predicted from individual drug administration.
Abstract: AIMS: The aim of this study was to investigate the influence of chronic itraconazole treatment on the pharmacokinetics and cardiovascular effects of single dose astemizole in healthy subjects was studied. METHODS: Twelve male volunteers were taking orally 200 mg twice daily itraconazole or placebo for 14 days with a washout period of 4 weeks in between. Approximately 2 h after the morning dose of itraconazole or placebo on day 11, 10 mg astemizole was orally administered. The plasma concentrations of astemizole and desmethylastemizole were measured by radioimmunoassay up to 504 h after administration; electrocardiograms with analysis of the QTc interval were recorded up to 24 h post administration. RESULTS: Itraconazole treatment did not significantly change the peak concentration of astemizole (0.74 vs 0.81 ng ml-1) but it increased the area under the curve from 0 to 24 h (5.46 to 9.95 ng ml-1 h) and from 0 to infinity (17.4 to 48.2 ng ml-1 h), and the elimination half-life (2.1 to 3.6 days). The systemic bioavailability of desmethylastemizole was also increased. The QTc interval did not increase after astemizole administration and there was no difference in the QTc intervals between the itraconazole and placebo session. CONCLUSIONS: Chronic administration of itraconazole influences the metabolism of single dose astemizole in normal volunteers without changes of cardiac repolarization during the first 24 h after astemizole administration. However, the reduction in astemizole clearance under concomitant administration of itraconazole may result in a marked increase in astemizole plasma concentrations and QTc alterations during chronic combined intake of astemizole with itraconazole.
Abstract: BACKGROUND: Quinidine is eliminated mainly by CYP3A4-mediated metabolism. Itraconazole interacts with some but not all of the substrates of CYP3A4; it is therefore important to study the possible interaction of itraconazole with quinidine. METHODS: A double-blind, randomized, two-phase crossover study design was used with nine healthy volunteers. Itraconazole (200 mg) or placebo was ingested once a day for 4 days. A single 100 mg oral dose of quinidine sulfate was ingested on day 4. Plasma concentrations of quinidine, itraconazole, and hydroxyitraconazole, as well as cumulative excretion of quinidine into urine, were determined up to 24 hours. The ECG, heart rate, and blood pressure were also recorded up to 24 hours. RESULTS: On average the peak plasma concentration of quinidine increased to 1.6-fold (p < 0.05), and the area under the concentration-time curve of quinidine increased to 2.4-fold (p < 0.01) by itraconazole. The elimination half-life of quinidine was prolonged 1.6-fold (p < 0.001), and the area under the 3-hydroxyquinidine/quinidine ratio-time curve decreased to one-fifth (p < 0.001) by itraconazole. The renal clearance of quinidine decreased 50% (p < 0.001) by itraconazole, whereas the creatinine clearance was unaffected. The QTc interval correlated with the concentrations of quinidine during both itraconazole and placebo phases (r2 = 0.71 and r2 = 0.79, respectively; p < 0.01), although only minor changes between the phases were observed in other pharmacodynamic variables. CONCLUSIONS: Itraconazole increases plasma concentrations of oral quinidine, probably by inhibiting the CYP3A4 isozyme during the first-pass and elimination phases of quinidine. The decreased renal clearance of quinidine might be the result of the inhibition of P-glycoprotein-mediated tubular secretion of quinidine by itraconazole. The concentrations of quinidine should be closely monitored if itraconazole or some other potent CYP3A inhibitors are used with quinidine.
Abstract: Second-generation histamine H1 receptor antagonists (antihistamines) have been developed to reduce or eliminate the sedation and anticholinergic adverse effects that occur with older H1 receptor antagonists. This article evaluates second-generation antihistamines, including acrivastine, astemizole, azelastine, cetirizine, ebastine, fexofenadine, ketotifen, loratadine, mizolastine and terfenadine, for significant features that affect choice. In addition to their primary mechanism of antagonising histamine at the H1 receptor, these agents may act on other mediators of the allergic reaction. However, the clinical significance of activity beyond that mediated by histamine H1 receptor antagonism has yet to be demonstrated. Most of the agents reviewed are metabolised by the liver to active metabolites that play a significant role in their effect. Conditions that result in accumulation of astemizole, ebastine and terfenadine may prolong the QT interval and result in torsade de pointes. The remaining agents reviewed do not appear to have this risk. For allergic rhinitis, all agents are effective and the choice should be based on other factors. For urticaria, cetirizine and mizolastine demonstrate superior suppression of wheal and flare at the dosages recommended by the manufacturer. For atopic dermatitis, as adjunctive therapy to reduce pruritus, cetirizine, ketotifen and loratadine demonstrate efficacy. Although current evidence does not suggest a primary role for these agents in the management of asthma, it does support their use for asthmatic patients when there is coexisting allergic rhinitis, dermatitis or urticaria.
Abstract: The aim of this study was to evaluate the (3S)-3-hydroxylation and the N-oxidation of quinidine as biomarkers for cytochrome P-450 (CYP)3A4 activity in human liver microsome preparations. An HPLC method was developed to assay the metabolites (3S)-3-hydroxyquinidine (3-OH-Q) and quinidine N-oxide (Q-N-OX) formed during incubation with microsomes from human liver and from Saccharomyces cerevisiae strains expressing 10 human CYPs. 3-OH-Q formation complied with Michaelis-Menten kinetics (mean values of Vmax and Km: 74.4 nmol/mg/h and 74.2 microM, respectively). Q-N-OX formation followed two-site kinetics with mean values of Vmax, Km and Vmax/Km for the low affinity isozyme of 15.9 nmol/mg/h, 76.1 microM and 0.03 ml/mg/h, respectively. 3-OH-Q and Q-N-OX formations were potently inhibited by ketoconazole, itraconazole, and triacetyloleandomycin. Isozyme specific inhibitors of CYP1A2, -2C9, -2C19, -2D6, and -2E1 did not inhibit 3-OH-Q or Q-N-OX formation, with Ki values comparable with previously reported values. Statistically significant correlations were observed between CYP3A4 content and formations of 3-OH-Q and Q-N-OX in 12 human liver microsome preparations. Studies with yeast-expressed isozymes revealed that only CYP3A4 actively catalyzed the (3S)-3-hydroxylation. CYP3A4 was the most active enzyme in Q-N-OX formation, but CYP2C9 and 2E1 also catalyzed minor proportions of the N-oxidation. In conclusion, our studies demonstrate that only CYP3A4 is actively involved in the formation of 3-OH-Q. Hence, the (3S)-3-hydroxylation of quinidine is a specific probe for CYP3A4 activity in human liver microsome preparations, whereas the N-oxidation of quinidine is a somewhat less specific marker reaction for CYP3A4 activity, because the presence of a low affinity enzyme is demonstrated by different approaches.
Abstract: OBJECTIVE: To investigate the possible involvement of cytochromes CYP1A2 and CYP2C19 in the in vivo oxidative metabolism of quinidine. METHODS: This was an open study of six healthy young male volunteers. The pharmacokinetics of a 200-mg single oral dose of quinidine were studied before and during daily treatment with 100 mg fluvoxamine. Biomarkers of other isozyme activities in the form of caffeine, sparteine, mephenytoin, tolbutamide and cortisol metabolism were applied. RESULTS: The results showed a statistically significant median reduction of 2944% in the quinidine total apparent oral clearance, partial clearances by 3-hydroxylation and N-oxidation and residual clearance during fluvoxamine treatment. Renal clearance was unaffected by fluvoxamine. CONCLUSIONS: The effect of fluvoxamine on the formation clearances of 3-hydroxyquinidine and quinidine-N-oxide most likely reflects inhibition of cytochrome P4503A4 by fluvoxamine at clinically relevant doses. The results of this study do not rule out a possible involvement of CYP1A2 and CYP2C19 in the in vivo oxidative metabolism of quinidine.
Abstract: AIMS: In vitro studies suggest that the oxidation of quinidine to 3-hydroxyquinidine is a specific marker reaction for CYP3A4 activity. To assess the possible use of this reaction as an in vivo marker of CYP3A4 activity, we studied the involvement of cytochromes CYP2C9, CYP2E1 and CYP3A4 in the in vivo oxidative metabolism of quinidine. METHODS: An open study of 30 healthy young male volunteers was performed. The pharmacokinetics of a 200 mg single oral dose of quinidine was studied before and during daily administration of 100 mg diclofenac, a CYP2C9 substrate (n=6); 200 mg disulfiram, an inhibitor of CYP2E1 (n=6); 100 mg itraconazole, an inhibitor of CYP3A4 (n=6); 250 ml single strength grapefruit juice twice daily, an inhibitor of CYP3A4 (n=6); 250 mg of erythromycin 4 times daily, an inhibitor of CYP3A4 (n=6). Probes of other enzyme activities, caffeine (CYP1A2), sparteine (CYP2D6), mephenytoin (CYP2C19), tolbutamide (CYP2C9) and cortisol (CYP3A4) were also studied. RESULTS: Concomitant administration of diclofenac reduced the partial clearance of quinidine by N-oxidation by 27%, while no effect was found for other pharmacokinetic parameters of quinidine. Concomitant administration of disulfiram did not alter any of the pharmacokinetic parameters of quinidine. Concomitant administration of itraconazole reduced quinidine total clearance, partial clearance by 3-hydroxylation and partial clearance by N-oxidation by 61, 84 and 73%, respectively. The renal clearance was reduced by 60% and the elimination half-life increased by 35%. Concomitant administration of grapefruit juice reduced the total clearance of quinidine and its partial clearance by 3-hydroxylation and N-oxidation by 15, 19 and 27%, respectively. The elimination half-life of quinidine was increased by 19%. The caffeine metabolic index was reduced by 25%. Concomitant administration of erythromycin reduced the total clearance of quinidine and its partial clearance by 3-hydroxylation and N-oxidation by 34, 50 and 33%, respectively. Cmax was increased by 39%. CONCLUSIONS: The results confirm an important role for CYP3A4 in the oxidation of quinidine in vivo, and this applies particularly to the formation of 3-hydroxyquinidine. While a minor contribution of CYP2C9 to the N-oxidation of quinidine is possible, a major involvement of the CYP2C9 or CYP2E1 enzymes in the oxidation of quinidine in vivo is unlikely.
Abstract: We investigated the effect of cytochrome P450 induction by rifampicin on the in vivo oxidative metabolism of quinidine. The pharmacokinetics of a 200 mg oral single dose quinidine were studied before and after one week of daily treatment with 600 mg rifampicin in six healthy young male volunteers. Biomarker reactions of cytochrome P450 isozyme activities in the form of caffeine, sparteine, mephenytoin, tolbutamide and cortisol metabolism were applied. The median total apparent oral clearance and partial clearance by 3-hydroxylation of quinidine increased 9 times. The partial clearance by N-oxidation increased 6 times. The Cmax and the elimination half life were reduced 3 times. No statistically significant changes were found for quinidine tmax and renal clearance. The cortisol metabolic ratio increased 5 times, while no statistically significant effects were seen for other CYP marker reactions. The results indicate that the inductive effect of rifampicin is likely to be of clinical relevance particularly when used concomitantly with drugs metabolized by CYP3A4.
Abstract: AIMS: The aims of the present study were to investigate the metabolism of astemizole in human liver microsomes, to assess possible pharmacokinetic drug-interactions with astemizole and to compare its metabolism with terfenadine, a typical H1 receptor antagonist known to be metabolized predominantly by CYP3A4. METHODS: Astemizole or terfenadine were incubated with human liver microsomes or recombinant cytochromes P450 in the absence or presence of chemical inhibitors and antibodies. RESULTS: Troleandomycin, a CYP3A4 inhibitor, markedly reduced the oxidation of terfenadine (26% of controls) in human liver microsomes, but showed only a marginal inhibition on the oxidation of astemizole (81% of controls). Three metabolites of astemizole were detected in a liver microsomal system, i.e. desmethylastemizole (DES-AST), 6-hydroxyastemizole (6OH-AST) and norastemizole (NOR-AST) at the ratio of 7.4 : 2.8 : 1. Experiments with recombinant P450s and antibodies indicate a negligible role for CYP3A4 on the main metabolic route of astemizole, i.e. formation of DES-AST, although CYP3A4 may mediate the relatively minor metabolic routes to 6OH-AST and NOR-AST. Recombinant CYP2D6 catalysed the formation of 6OH-AST and DES-AST. Studies with human liver microsomes, however, suggest a major role for a mono P450 in DES-AST formation. CONCLUSIONS: In contrast to terfenadine, a minor role for CYP3A4 and involvement of multiple P450 isozymes are suggested in the metabolism of astemizole. These differences in P450 isozymes involved in the metabolism of astemizole and terfenadine may associate with distinct pharmacokinetic influences observed with coadministration of drugs metabolized by CYP3A4.
Abstract: Permeability glycoprotein (P-gp) mediates the export of drugs from cells located in the small intestine, blood-brain barrier, hepatocytes, and kidney proximal tubule, serving a protective function for the body against foreign substances. Intestinal absorption, biliary excretion, and urinary excretion of P-gp substrates can therefore be altered by either the inhibition or induction of P-gp. A wide spectrum of drugs, such as anticancer agents and steroids, are known P-gp substrates and/or inhibitors, and many cardiovascular drugs have recently been observed to have clinically relevant interactions as well. We review the interactions among commonly prescribed cardiovascular drugs that are P-gp substrates and observe interactions involving P-gp that may be relevant to clinical practice. Cardiovascular drugs with narrow therapeutic indexes (e.g., antiarrhythmic agents, anticoagulant agents) have demonstrated large increases in concentrations when coadministered with potent P-gp inhibitors, thus increasing the risk for drug toxicity. Therefore, dose adjustment or use of alternative agents should be considered when strong P-gp-mediated drug-drug interactions are present. Finally, interactions between novel drugs and known P-gp inhibitors are now being systematically evaluated during drug development, and recommended guidelines for the administration of P-gp substrate drugs will be expanded.
Abstract: Rituximab can cause late-onset neutropenia that may result in serious life-threatening complications. The author describes the pathophysiology, incidence, and management of this adverse reaction and presents two case histories.