The effect of oral cimetidine on total and unbound serum lidocaine concentrations in patients with suspected myocardial infarction

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.

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.

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.