Effect of cimetidine on the pharmacokinetics and pharmacodynamics of quinidine

Assessment of Quinidine-Induced Torsades de Pointes Risks Using a Whole-Body Physiologically Based Pharmacokinetic Model Linked to Cardiac Ionic Current Inhibition

The lethality of torsades de pointes (TdP) by drugs is one of main reasons that some drugs were withdrawn from the market. In order to assess drug-induced TdP risks, a model of cardiac ionic current suppression in human ventricular myocytes (ToR-ORd model), combined with the maximum effective free therapeutic plasma concentration or the maximum effective free therapeutic myocyte concentration was often used, with the latter proved to be more relevant and more accurate. We aimed to develop a whole-body physiologically-based pharmacokinetic (PBPK) model, incorporated with a human cardiomyocyte pharmacodynamic (PD) model, to provide a comprehensive assessment of drug-induced TdP risks in normal and specific scenarios. Quinidine served as an example to validate the PBPK-PD model via predicting plasma quinidine concentrations and quinidine-induced changes in QT interval (ΔQTc). The predicted plasma quinidine concentrations and ΔQTc values following oral administration or intravenous administration of quinidine were comparable to clinic observations. Visual predictive checks showed that most of the observed plasma concentrations and ΔQTc values fell within the 5th and 95th percentiles of simulations. The validated PBPK-PD model was further applied to assess the TdP risks using frequencies of early afterdepolarization and long-QT syndrome occurrence in 4 scenarios, such as therapeutic dose, supra-therapeutic dose, alkalosis, and hyperkalemia in 200 human subjects. In conclusion, the developed PBPK-PD model may be applied to predict the quinidine pharmacokinetics and quinidine-induced TdP risks in healthy subjects, but also simulate quinidine-induced TdP risks under disease conditions, such as hypokalemia and alkalosis.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Drug interactions with antisecretory agents

Antisecretory agents may affect the absorption, metabolism, and renal excretion of other drugs. Inhibition of gastric acid secretion may decrease the gastrointestinal absorption of drugs such as ketoconazole that dissolve poorly in the absence of adequate acid. With anti-secretory agents, the drug interaction mechanism most likely to result in adverse effects is the inhibition of hepatic oxidative drug metabolism, primarily a problem with cimetidine. Omeprazole also appears to inhibit the hepatic metabolism of some drugs, but available evidence indicates that it interacts with fewer drugs than cimetidine and the magnitude of the inhibition is lower. Cimetidine decreases the renal clearance of procainamide and its active metabolite, N-acetylprocainamide, probably through interference with active renal tubular secretion. In therapeutic doses, other H2-receptor antagonists probably have minimal effects on renal procainamide elimination.

Cardiovascular drug-drug interactions

The drug-drug interactions discussed in this article have either documented or suspected clinical relevance for patients with cardiovascular disease and the clinician involved in the care of these patients. Oftentimes, drug-drug interactions are difficult, if not impossible, to predict because of the high degree of interpatient variability in drug disposition. Certain drug-drug interactions, however, may be avoided through knowledge and sound clinical judgment. Every clinician should maintain a working knowledge of reported drug-drug interactions and an understanding of basic pharmacokinetic and pharmacodynamic principles to help predict and minimize the incidence and severity of drug-drug interactions.

Antiarrhythmic agents: drug interactions of clinical significance

The management of cardiac arrhythmias has grown more complex in recent years. Despite the recent focus on nonpharmacological therapy, most clinical arrhythmias are treated with existing antiarrhythmics. Because of the narrow therapeutic index of antiarrhythmic agents, potential drug interactions with other medications are of major clinical importance. As most antiarrhythmics are metabolised via the cytochrome P450 enzyme system, pharmacokinetic interactions constitute the majority of clinically significant interactions seen with these agents. Antiarrhythmics may be substrates, inducers or inhibitors of cytochrome P450 enzymes, and many of these metabolic interactions have been characterised. However, many potential interactions have not, and knowledge of how antiarrhythmic agents are metabolised by the cytochrome P450 enzyme system may allow clinicians to predict potential interactions. Drug interactions with Vaughn-Williams Class II (beta-blockers) and Class IV (calcium antagonists) agents have previously been reviewed and are not discussed here. Class I agents, which primarily block fast sodium channels and slow conduction velocity, include quinidine, procainamide, disopyramide, lidocaine (lignocaine), mexiletine, flecainide and propafenone. All of these agents except procainamide are metabolised via the cytochrome P450 system and are involved in a number of drug-drug interactions, including over 20 different interactions with quinidine. Quinidine has been observed to inhibit the metabolism of digoxin, tricyclic antidepressants and codeine. Furthermore, cimetidine, azole antifungals and calcium antagonists can significantly inhibit the metabolism of quinidine. Procainamide is excreted via active tubular secretion, which may be inhibited by cimetidine and trimethoprim. Other Class I agents may affect the disposition of warfarin, theophylline and tricyclic antidepressants. Many of these interactions can significantly affect efficacy and/or toxicity. Of the Class III antiarrhythmics, amiodarone is involved in a significant number of interactions since it is a potent inhibitor of several cytochrome P450 enzymes. It can significantly impair the metabolism of digoxin, theophylline and warfarin. Dosages of digoxin and warfarin should empirically be decreased by one-half when amiodarone therapy is added. In addition to pharmacokinetic interactions, many reports describe the use of antiarrhythmic drug combinations for the treatment of arrhythmias. By combining antiarrhythmic drugs and utilising additive electrophysiological/pharmacodynamic effects, antiarrhythmic efficacy may be improved and toxicity reduced. As medication regimens grow more complex with the aging population, knowledge of existing and potential drug-drug interactions becomes vital for clinicians to optimise drug therapy for every patient.

Effect of cimetidine and ranitidine on pharmacokinetics and pharmacodynamics of a single dose of dofetilide

AIMS

The aim of this open-label, placebo-controlled, randomized, four-period crossover study was to determine the effects of cimetidine and ranitidine on the pharmacokinetics and pharmacodynamics of a single dose of dofetilide.

METHODS

Twenty healthy male subjects received 100 or 400 mg twice daily of cimetidine, 150 mg twice daily of ranitidine, or placebo for 4 days. On the second day, a single oral 500 microg dose of dofetilide was administered immediately after the morning doses of cimetidine, ranitidine, or placebo. Treatment periods were separated by 1-2 weeks. Pharmacokinetic parameters were determined from plasma and urinary dofetilide concentrations; prolongation of the QTc interval was determined from three-lead electrocardiograms.

RESULTS

Ranitidine did not significantly affect the pharmacokinetics or pharmacodynamics of dofetilide; however, a dose-dependent increase in exposure to dofetilide was observed with cimetidine. When dofetilide was administered with 100 and 400 mg of cimetidine, the area under the plasma concentration-time curve of dofetilide increased by 11% and 48% and the maximum plasma dofetilide concentration increased by 11% and 29%, respectively. The respective cimetidine doses reduced renal clearance of dofetilide by 13% and 33% and nonrenal clearance by 5% and 21%. Dofetilide-induced prolongation of the QTc interval was enhanced by cimetidine; the mean maximum change in QTc interval from baseline was increased by 22% and 33% with 100 and 400 mg of cimetidine, respectively. However, the relationship between the prolongation of the QTc interval and plasma dofetilide concentrations was unaffected by cimetidine or ranitidine; a 1 ng ml-1 increase in plasma dofetilide concentration produced a 17-19 ms prolongation of the QTc interval. Dofetilide was well tolerated, with no treatment-related adverse events or laboratory abnormalities.

CONCLUSIONS

These results suggest that cimetidine increased dofetilide exposure by inhibiting renal tubular dofetilide secretion, whereas ranitidine did not. This effect is not an H2-receptor antagonist class effect but is specific to cimetidine. If therapy with an H2-receptor antagonist is required, it is recommended that cimetidine at all doses be avoided; since ranitidine has no effect on dofetilide pharmacokinetics or prolongation of the QTc interval, it can be seen as a suitable alternative.

Effects of cimetidine on the pharmacokinetics of proguanil in healthy subjects and in peptic ulcer patients

The pharmacokinetics of orally administered proguanil and its metabolites were determined in six healthy volunteers and in six peptic ulcer patients, before and after a 3-day course of cimetidine (400 mg given two times daily for 2 days and 400 mg on the third day 1 h before proguanil). Cimetidine significantly increased Cmax (P < 0.05), AUCo-alpha (P < 0.005) and elimination half-life t 1/2b of proquanil in plasma of healthy subjects. In ulcer patients, cimetidine significantly increased, AUCo-alpha (P < 0.05), elimination half life (P < 0.005) and Cmax. Cimetidine significantly reduced (P < 0.05) Total body clearance in both healthy subjects and in peptic ulcer patients. The Cmax and AUCo-alpha of the active metabolite cycloguanil was significantly decreased (P < 0.05) in both the healthy subjects and in the peptic ulcer patients. The Cmax of the inactive metabolite, 4-CPB was significantly decreased in healthy subjects and AUCo-alpha significantly decreased in peptic ulcer patients.

Itraconazole increases plasma concentrations of quinidine

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.

Possible pharmacokinetic interaction with quinidine: ciprofloxacin or metronidazole?

OBJECTIVE

To discuss a potential pharmacokinetic interaction between quinidine, ciprofloxacin, and metronidazole.

CASE SUMMARY

A 51-year-old black woman was admitted for shortness of breath, abdominal pain, and atrial fibrillation. Procainamide and diltiazem were begun for the atrial fibrillation and ciprofloxacin and metronidazole for suspected diverticulitis. The therapy was switched to quinidine on day 5 because of adverse events associated with procainamide. A trough serum quinidine concentration (SQC) on day 7 was 6.3 micrograms/mL (normal 2-5) with normal QT and QTc intervals. On day 8, the patient was discharged in normal sinus rhythm. She took her last doses of antibiotics on day 15 and a follow-up SQC on day 18 was 2.3 micrograms/mL.

DISCUSSION

The possible explanations for the changes in SQCs include: (1) laboratory error, (2) compliance with medication regimen, and (3) altered hepatic metabolism. The first two are not likely in this case. The laboratory verified the elevated SQC and the patient had her prescriptions refilled within appropriate time limits. The third explanation seems more plausible. Quinidine is metabolized by the hepatic mixed-function oxidase system, specifically cytochrome P450 (CYP) 3A4. We found that metronidazole has been shown to inhibit CYP3A activity and ciprofloxacin has been shown to inhibit certain isozymes in the cytochrome P450 system as well.

CONCLUSIONS

When metronidazole and ciprofloxacin are administered concomitantly with quinidine, clinicians should be aware of this potential interaction. Quinidine concentrations should be monitored and patients should be assessed for signs and symptoms of quinidine toxicity.

Clinical relevance of cimetidine drug interactions

The excellent efficacy and tolerability profiles of H2-antagonists have established these agents as the leading class of antiulcer drugs. Attention has been focused on drug interactions with H2-antagonists as a means of product differentiation and because many patients are receiving multiple drug therapy. The main mechanism of most drug interactions involving cimetidine appears to be inhibition of the hepatic microsomal enzyme cytochrome P450, an effect which may be related to the different structures of H2-antagonists. Ranitidine appears to have less affinity than cimetidine for this system. There have been many published case reports and studies of drug interactions with cimetidine, but many of these have provided pharmacokinetic data only, with little information concerning the clinical significance of these findings. Nevertheless, the coadministration of cimetidine with drugs that have a narrow therapeutic margin (such as theophylline) may potentially result in clinically significant adverse effects. The monitoring of serum concentrations of drugs coadministered with cimetidine may reduce the risk of adverse events but does not abolish the problem. However, for most patients, concomitant administration of cimetidine with drugs possessing a wide therapeutic margin is unlikely to pose a significant problem.

Quinidine single dose pharmacokinetics and pharmacodynamics are unaltered by omeprazole

Omeprazole has been shown in previous studies to inhibit the hepatic metabolism of selected drugs. Quinidine is an antiarrhythmic and antimalarial agent with a low therapeutic index. We therefore examined the effect of 40 mg omeprazole daily for one week or placebo on the pharmacokinetics and pharmacodynamics of a single 400 mg dose of quinidine in 8 healthy volunteers in a double-blind crossover study. During placebo and omeprazole treatment, there was no significant difference in area under the time-plasma quinidine concentration curve, (17.0 +/- 4.83 micrograms.h/ml, 18.6 +/- 4.43 micrograms.h/ml, respectively; P greater than 0.2) or renal clearance of quinidine (56.2 +/- 26.0 ml/min, 55.6 +/- 12.7 ml/min, respectively; P greater than 0.5). Quinidine unbound fraction in plasma (0.170 +/- 0.041 vs. 0.166 +/- 0.041 in the presence of omeprazole; P greater than 0.5) was not altered by omeprazole. Peak plasma quinidine concentration and the time this occurred did not differ. Omeprazole also had no effect on these parameters for the metabolite 3-hydroxyquinidine. There was no significant difference in the change in the corrected Q-T interval on the electrocardiogram due to quinidine (mean area under the time versus delta Q-Tc curve = 351 +/- 192 ms.h, placebo; 414 +/- 303 ms.h, omeprazole) showing that quinidine pharmacodynamics were unaltered by omeprazole. We conclude that omeprazole does not affect the pharmacokinetics of quinidine.

The effect of ciprofloxacin on the pharmacokinetic and ECG parameters of quinidine

Ciprofloxacin decreases the clearance of antipyrine and other drugs which, in part, undergo oxidative metabolism. Based on these findings, the authors hypothesized that ciprofloxacin may decrease the clearance of quinidine, a drug which also undergoes oxidative metabolism. The purpose of this study was to evaluate the effect of ciprofloxacin on the pharmacokinetic and ECG parameters of quinidine in seven healthy men. Oral quinidine sulfate 400 mg was administered alone (Phase A) and after oral ciprofloxacin pretreatment (Phase B) in a randomized crossover fashion with a 2-week washout period between each phase. During Phase B, ciprofloxacin pretreatment (750 mg every 12 hours) was administered for 5 days before and 24 hours after quinidine administration. Quinidine serum samples were obtained over a 24-hour period. QRS and QTc intervals were measured over a 12-hour period. There were no significant differences in clearance (20.3 +/- 3.3 L/hr vs 20.1 +/- 2.3 L/hr, P = .836), half-life (7.9 +/- 1 hr vs 7.8 +/- 0.8 hr, P = 0.8), maximum concentration (1.4 +/- 0.6 mg/L vs 1.5 +/- 0.6 mg/L, P = 0.613), or time to maximum concentration (1.5 +/- 0.2 hr vs 1.5 +/- 0.1 hr, P = 0.571) for quinidine between Phase A and Phase B, respectively. The largest decrease in clearance observed for Phase B compared to Phase A was 10%. There was also no significant difference in the degree of QRS and QTc prolongation between Phase A and Phase B. From these results, it appears that ciprofloxacin in the dose given does not alter the pharmacokinetic or ECG parameters of quinidine. Therefore, no adjustment in the dose of quinidine is needed when coadministered with ciprofloxacin.

The effects of cimetidine on the oxidative metabolism of estradiol

Cimetidine, a histamine H2-receptor antagonist widely used to treat peptic ulceration, is known to cause gynecomastia and sexual dysfunction in some men. Since cimetidine inhibits the cytochrome P-450-dependent biotransformation of numerous drugs, we investigated the possibility that it might also inhibit the cytochrome P-450--dependent metabolism of estradiol. Radiometric analysis of urine and serum samples from nine normal male volunteers showed that the extent of 2-hydroxylation of estradiol was significantly reduced from a mean (+/- SEM) of 31.7 +/- 2.3 percent to 19.7 +/- 2.3 percent (P less than 0.0001) after two weeks of oral treatment with cimetidine (800 mg twice a day); the 16 alpha-hydroxylation of estradiol was unaffected. At the same time, the urinary excretion of 2-hydroxyestrone decreased by approximately 25 percent (P less than 0.0002), and the serum concentration of estradiol increased by approximately 20 percent (P less than 0.04). The mean percentage of estradiol 2-hydroxylation was also rapidly reduced, from 36.8 +/- 4.4 percent to 24.5 +/- 3.4 percent in six men after one week of oral cimetidine at a lower dosage (400 mg twice a day; P less than 0.0006). In a separate study of seven men, ranitidine, a second-generation H2-receptor antagonist, was found to have no effect on the 2-hydroxylation of estradiol. This study demonstrates that the administration of cimetidine to men decreases the 2-hydroxylation of estradiol and results in an increase in the serum estradiol concentration. This mechanism may help to account for the signs and symptoms of estrogen excess reported with the long-term use of cimetidine.

Effect of cimetidine and ranitidine on cardiovascular drugs

A compilation of drug interactions between H2 antagonists and cardiovascular drugs is found in Table I. Cimetidine's potency, lipophilicity, and affinity for binding to the P-450 cytochrome system can probably be attributed to the drug interactions that have been identified with the H2 antagonists. The mechanism for most cimetidine drug interactions is inhibition of hepatic metabolism. There is conflicting evidence regarding significance of altered liver blood flow for both cimetidine and ranitidine and their influence on other agents. Cimetidine may increase propranolol's blood concentrations and potentiate beta blocking effects through inhibition of hepatic microsomal enzymes and possibly through reduction of hepatic blood flow. Ranitidine has no effect on propranolol. Cimetidine, when administered concurrently with metoprolol, could possibly cause an increase in plasma metoprolol concentrations or bioavailability through inhibition of hepatic P-450 metabolizing enzymes. No effect of cimetidine on metoprolol pharmacodynamics was evident. Ranitidine has no effect on metoprolol pharmacokinetics or pharmacodynamics. Neither H2 antagonist altered the kinetics or physiologic effects of atenolol. Atenolol is the drug of choice in patients receiving H2 antagonists, since no interaction has been observed. Metoprolol could probably be used safely in most patients, as no change in pharmacodynamics has been evident. Concurrent administration of cimetidine and nifedipine may result in alterations in heart rate and blood pressure. The mechanism is inhibition of oxidative liver metabolism. Ranitidine has no effect on nifedipine. Studies are needed to investigate the interaction between the H2 antagonists and diltiazem or verapamil. Cimetidine, given concomitantly with lidocaine, may increase lidocaine concentrations and clinical symptoms of lidocaine toxicity. The mechanism involved is probably a reduction in oxidative drug metabolism or liver blood flow. Ranitidine has no significant effects on lidocaine pharmacokinetics. Cimetidine may increase quinidine levels and symptoms of quinidine toxicity. Additionally, enhanced arrhythmic effects may be observed. The interaction probably caused by an inhibition of hepatic drug metabolism of quinidine by cimetidine would be most significant in patients with liver disease and in the elderly. Ranitidine may enhance quinidine's arrhythmic effect. Cimetidine can possibly increase procainamide and NAPA serum concentrations, especially in the elderly and in patients with renal dysfunction, predisposing them to adverse side effects. The interaction is mediated by a reduction of tubular secretion of procainamide and NAPA.

Effect of cimetidine on quinidine bioavailability

Elevations in quinidine steady-state serum concentrations have been reported in patients who received cimetidine concurrently. Studies in normal volunteers have shown that areas under the serum concentration-time curve of orally administered quinidine are higher when quinidine is given during chronic cimetidine therapy as compared to under control conditions. The mechanism for this interaction is generally ascribed to decreased hepatic clearance as a consequence of enzyme inhibition. In this study, we show that cimetidine also decreases the bioavailable fraction of quinidine.

Pharmacokinetic interactions of cimetidine 1987

The number of studies on drug interactions with cimetidine has increased at a rapid rate over the past 5 years, with many of the interactions being solely pharmacokinetic in origin. Very few studies have investigated the clinical relevance of such pharmacokinetic interactions by measuring pharmacodynamic responses or clinical endpoints. Apart from pharmacokinetic studies, invariably conducted in young, healthy subjects, there have been a large number of in vitro and in vivo animal studies, case reports, clinical observations and general reviews on the subject, which is tending to develop an industry of its own accord. Nevertheless, where specific mechanisms have been considered, these have undoubtedly increased our knowledge on the way in which humans eliminate xenobiotics. There is now sufficient information to predict the likelihood of a pharmacokinetic drug-drug interaction with cimetidine and to make specific clinical recommendations. Pharmacokinetic drug interactions with cimetidine occur at the sites of gastrointestinal absorption and elimination including metabolism and excretion. Cimetidine has been found to reduce the plasma concentrations of ketoconazole, indomethacin and chlorpromazine by reducing their absorption. In the case of ketoconazole the interaction was clinically important. Cimetidine does not inhibit conjugation mechanisms including glucuronidation, sulphation and acetylation, or deacetylation or ethanol dehydrogenation. It binds to the haem portion of cytochrome P-450 and is thus an inhibitor of phase I drug metabolism (i.e. hydroxylation, dealkylation). Although generally recognised as a nonspecific inhibitor of this type of metabolism, cimetidine does demonstrate some degree of specificity. To date, theophylline 8-oxidation, tolbutamide hydroxylation, ibuprofen hydroxylation, misonidazole demethylation, carbamazepine epoxidation, mexiletine oxidation and steroid hydroxylation have not been shown to be inhibited by cimetidine in humans but the metabolism of at least 30 other drugs is affected. Recent evidence indicates negligible effects of cimetidine on liver blood flow. Cimetidine reduces the renal clearance of drugs which are organic cations, by competing for active tubular secretion in the proximal tubule of the kidney, reducing the renal clearances of procainamide, ranitidine, triamterene, metformin, flecainide and the active metabolite N-acetylprocainamide. This previously unrecognised form of drug interaction with cimetidine may be clinically important for both parent drug, and metabolites which may be active.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of cimetidine and ranitidine on the pharmacokinetics of quinine

The pharmacokinetics of orally administered quinine were determined in six normal volunteers before and after a 7-day course of cimetidine (1 g day-1) or ranitidine (300 mg day-1). Peak plasma quinine concentration and the time of peak concentration were not altered after cimetidine or ranitidine pretreatment. After cimetidine pretreatment there was a significant reduction in the apparent oral clearance of quinine, from 0.182 +/- 0.063 (mean +/- s.d.) to 0.133 +/- 0.055 1 h-1 kg-1 (P less than 0.05). This was reflected in a 49% (range 17 to 90%) increase in the mean elimination half-life from 7.6 +/- 1.3 to 11.3 +/- 3.7 h (P less than 0.05). In contrast to cimetidine, ranitidine had no significant effect on the clearance or half-life of quinine. The apparent interaction between quinine and cimetidine may have therapeutic implications. Special care should be taken in patients taking these two common drugs concomitantly. Additionally, to avoid unnecessary risks due to drug interaction, the use of ranitidine may be preferable in the patients in whom it is desirable to administer an H2-receptor antagonist together with quinine.

The effect of cimetidine on serum concentrations of piroxicam

To evaluate the effect of cimetidine on serum concentrations of piroxicam, we administered a single 20-mg oral dose of piroxicam to 10 healthy male volunteers on 2 occasions. The first was given on day 1 of the study and the second on day 15, 7 days after starting cimetidine 300 mg orally 4 times a day. Nineteen blood samples were drawn for 7 days after each piroxicam dose to characterize its pharmacokinetics. Piroxicam was analyzed by high-performance liquid chromatography. The mean piroxicam elimination rate constants (Kel), elimination half-lives, and area under the serum concentration-time curves (AUC) were as follows (mean +/- standard deviation): (formula; see text) Data were analyzed with a Wilcoxon matched-pairs, signed-ranks, two-tailed statistical test. Although the increase in AUC was statistically significant, it was of low amplitude (mean 15%) and is probably not clinically significant. The results of this study suggest that cimetidine does not significantly alter the elimination kinetics of a single dose of piroxicam in young healthy males. Additional investigation is needed to confirm these findings in other patient populations.

Quinidine-nifedipine interaction

Quinidine pharmacokinetics are known to be altered by a number of drugs. We present a case where dose-related increases in quinidine serum concentrations were significantly suppressed by concurrent nifedipine therapy. Clinicians should be alert to the possibility of an alteration in quinidine serum concentrations when instituting or discontinuing nifedipine in patients receiving quinidine.

Usefulness and safety of cimetidine in patients receiving mexiletine for ventricular arrhythmia

Cimetidine, a commonly used H2 receptor antagonist, was found to adversely interact with many drugs metabolized by the liver, including class I antiarrhythmic agents, lidocaine and quinidine. Mexiletine is a new class I antiarrhythmic agent similar to lidocaine which when used orally may have significant gastric side effects. Since some patients with peptic ulcer disease or gastric hyperacidity on mexiletine may benefit from the addition of cimetidine, it was important to rule out any significant adverse interaction between the two drugs in such patients. Eleven patients currently receiving long-term oral mexiletine for the treatment of complex ventricular arrhythmia underwent a double-blind crossover trial where they were maintained on their usual dose of mexiletine, and cimetidine, 300 mg orally every 6 hours, or placebo were added for a 1-week period each. Peak and trough mexiletine blood levels were not significantly altered by cimetidine. Similarly, there was no significant change in the frequency and severity of ventricular arrhythmia when cimetidine was added to mexiletine. Cimetidine reduced gastric side effects of mexiletine in 50% of patients who had complained of such symptoms on mexiletine alone or on mexiletine and placebo. We conclude that cimetidine can effectively reduce gastric side effects of mexiletine in many patients without adversely affecting the plasma concentration or the efficacy of the drug.

Effects of cimetidine on carbamazepine auto- and hetero-induction in man

The effect of cimetidine (CMT; 400 mg twice daily) and matching placebo on the enzyme-inducing properties of carbamazepine (CBZ; 200 mg at night for 15 days) was studied in seven healthy male volunteers. CMT alone had no significant effect on antipyrine kinetics, urinary 6 beta-hydroxycortisol excretion or leucocyte delta-aminolaevulinic acid synthase (ALA.S) activity. CBZ increased leucocyte ALA.S activity by 204% following 1 week's treatment (P less than 0.001). Thereafter, ALA.S activity fell despite continued CBZ administration. Concomitant CMT did not influence this response. Antipyrine clearance and urinary 6 beta-hydroxycortisol excretion were both increased by CBZ after 2 weeks' treatment (P less than 0.01). CMT blocked CBZ induction of antipyrine metabolism but the rise in urinary 6 beta-hydroxycortisol excretion was unaffected. Plasma CBZ concentrations 10, 14 and 18 h following the 8th and 15th doses were higher when CMT was taken concurrently (P less than 0.05). CBZ half-life fell by 36% and clearance rose by 29% (both P less than 0.001) with placebo and by 10% and 7% (both NS) when CMT was taken concurrently. CMT inhibits CBZ auto- and hetero-induction in man. Epileptic patients receiving CBZ chronically may be at risk of toxicity if CMT is also prescribed.

Arrhythmias in patients with drug toxicity, electrolyte, and endocrine disturbances

The common rhythm disturbances related to electrolyte imbalance are due predominantly to abnormalities of potassium. An understanding of the mechanism underlying these abnormalities is facilitated by a brief review of normal electrical activity during impulse propagation in cardiac tissue. Also discussed are the actions of all cardioactive and antiarrhythmic drugs on membrane permeability to ions. Lastly, the nonspecific arrhythmias associated with endocrine disturbances are outlined.

New directions in antiarrhythmic drug therapy

Cardiac arrhythmia causing sudden cardiac death is a serious worldwide public health problem. Antiarrhythmic agents have been available for therapy, but the conventional agents cause a high degree of intolerable side effects. The recent development of many new experimental antiarrhythmic agents has increased our capacity to effectively treat cardiac arrhythmias. Using a multifaceted approach of programmed electrical stimulation studies, drug level determinations, exercise testing and 24-hour ambulatory Holter monitoring, it can reasonably be decided which patient needs therapy and if therapy is going to be effective. Both aspects of the sudden death equation, ectopy frequency (triggering mechanism) and the ability to propagate sustained ventricular tachycardia (substrate), may be examined. Careful follow-up is needed to determine continued drug efficacy and the presence of side effects that may compromise patient compliance with therapy. If side effects intervene that may cause continued therapy to be intolerable, changing the antiarrhythmic agent, as opposed to decreasing the dosage to an ineffective range, may be appropriate. A comprehensive approach to arrhythmia management may begin to reduce the high incidence of sudden death due to fatal arrhythmias.