Quinidine interaction with nifedipine and felodipine: pharmacokinetic and pharmacodynamic evaluation

Inhibition of Cytochrome P450 Enzymes by Drugs-Molecular Basis and Practical Applications

Drug-drug interactions are a major cause of hospitalization and deaths related to drug use. A large fraction of these is due to inhibition of enzymes involved in drug metabolism and transport, particularly cytochrome P450 (P450) enzymes. Understanding basic mechanisms of enzyme inhibition is important, particularly in terms of reversibility and the use of the appropriate parameters. In addition to drug-drug interactions, issues have involved interactions of drugs with foods and natural products related to P450 enzymes. Predicting drug-drug interactions is a major effort in drug development in the pharmaceutical industry and regulatory agencies. With appropriate in vitro experiments, it is possible to stratify clinical drug-drug interaction studies. A better understanding of drug interactions and training of physicians and pharmacists has developed. Finally, some P450s have been the targets of drugs in some cancers and other disease states.

Application of unbound liver-to-plasma concentration ratio to quantitative projection of cytochrome P450-mediated drug-drug interactions using physiologically based pharmacokinetic modelling approach

1. This study evaluated the prediction accuracy of cytochrome P450 (CYP)-mediated drug-drug interaction (DDI) using minimal physiologically-based pharmacokinetic (PBPK) modelling incorporating the hepatic accumulation factor of an inhibitor (i.e. unbound liver/unbound plasma concentration ratio [K_p,uu,liver]) based on 22 clinical DDI studies. 2. K_p,uu,liver values were estimated using three methods: (1) ratio of cell-to-medium ratio in human cryopreserved hepatocytes (C/M_u) at 37 °C to that on ice (K_p,uu,C/M), (2) multiplication of total liver/unbound plasma concentration ratio (K_p,u,liver) estimated from C/M_u at 37 °C with unbound fraction in human liver homogenate (K_p,uu,cell) and (3) observed K_p,uu,liver in rats after intravenous infusion (K_p,uu,rat). 3. PBPK model using each K_p,uu,liver projected the area under the curve (AUC) increase of substrates more accurately than the model assuming a K_p,uu,liver of 1 for the average fold error and root mean square error did. Particularly, the model with a K_p,uu,liver of 1 underestimated the AUC increase of triazolam following co-administration with CYP3A4 inhibitor itraconazole by five-fold, whereas the AUC increase projected using the model incorporating the K_p,uu,C/M, K_p,uu,cell, or K_p,uu,rat of itraconazole and hydroxyitraconazole was within approximately two-fold of the actual value. 4. The results indicated that incorporating K_p,uu,liver into the PBPK model improved the accuracy of DDI projection.

Evaluating the safety and efficacy of dextromethorphan/quinidine in the treatment of pseudobulbar affect

Pseudobulbar affect (PBA) is a common manifestation of brain pathology associated with many neurological diseases, including amyotrophic lateral sclerosis, Alzheimer's disease, stroke, multiple sclerosis, Parkinson's disease, and traumatic brain injury. PBA is defined by involuntary and uncontrollable expressed emotion that is exaggerated and inappropriate, and also incongruent with the underlying emotional state. Dextromethorphan/quinidine (DM/Q) is a combination product indicated for the treatment of PBA. The quinidine component of DM/Q inhibits the cytochrome P450 2D6-mediated metabolic conversion of dextromethorphan to its active metabolite dextrorphan, thereby increasing dextromethorphan systemic bioavailability and driving the pharmacology toward that of the parent drug and away from adverse effects of the dextrorphan metabolite. Three published efficacy and safety studies support the use of DM/Q in the treatment of PBA; significant effects were seen on the primary end point, the Center for Neurologic Study-Lability Scale, as well as secondary efficacy end points and quality of life. While concentration-effect relationships appear relatively weak for efficacy parameters, concentrations of DM/Q may have an impact on safety. Some special safety concerns exist with DM/Q, primarily because of the drug interaction and QT prolongation potential of the quinidine component. However, because concentrations of dextrorphan (which is responsible for many of the parent drug's side effects) and quinidine are lower than those observed in clinical practice with these drugs administered alone, some of the perceived safety issues may not be as relevant with this low dose combination product. However, since patients with PBA have a variety of other medical problems and are on numerous other medications, they may not tolerate DM/Q adverse effects, or may be at risk for drug interactions. Some caution is warranted when initiating DM/Q treatment, particularly in patients with underlying risk factors for torsade de pointes and in those receiving medications that may interact with DM/Q.

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.

Drug-drug interactions: effect of quinidine on nifedipine binding to human cytochrome P450 3A4

Quinidine is a known inhibitor of cytochrome P450-mediated nifedipine metabolism. The interactions of nifedipine and quinidine with human cytochrome P450 3A4, which metabolizes these drugs, were examined using the kinetics of CO binding to this P450 as a rapid kinetic probe of protein conformation and dynamics. This approach showed that nifedipine and quinidine bind to different P450 3A4 species, respectively termed species I and II, with distinct conformations. When both drugs were present simultaneously, nifedipine interacted with the quinidine-bound P450 species II, but not species I. These findings indicate that quinidine acts as an allosteric inhibitor by switching nifedipine binding from nifedipine-metabolizing species I to the nonmetabolizing species II.

Calcium antagonists. Drug interactions of clinical significance

The interaction of calcium antagonists, including the dihydropyridine calcium antagonists (e.g. nifedipine), verapamil and diltiazem, with drugs from other classes has major clinical ramifications as the use of drug combinations increases in frequency. Combinations are used in the treatment of disorders ranging from hypertension to cardiac rhythm disturbances, angina pectoris and peripheral vasospastic disease. In this era of organ transplantation, drugs like cyclosporin are coming into potential conflict with an ever-growing list of drugs. Drug combinations used as part of long term therapies are also making their appearance in toxic drug reactions, including antituberculous and anticonvulsant agents. Bronchodilators and H2-blockers also fall into this category of potential culprits of combined drug toxicity, and the interactions of calcium antagonists with beta-blockers and antiarrhythmic agents are also becoming a matter of concern.

Nimodipine. Potential for drug-drug interactions in the elderly

Nimodipine is indicated for a variety of conditions in elderly patients. Elderly patients often have multiple morbidity and receive treatment with a variety of drugs. Therefore, it is important to investigate the possible pharmacokinetic and pharmacodynamic interactions of nimodipine with various drugs commonly prescribed for elderly patients. There were no clinically relevant interactions of nimodipine with any of the following specific agents studied: the antiarrhythmics mexiletine, propafenone, disopyramide or quinidine, digoxin, the beta-adrenoceptor antagonists propranolol or atenolol, nifedipine, warfarin, diazepam, indomethacin, ranitidine or glibenclamide (glyburide). However, there were some notable interactions. In epileptic patients taking the anticonvulsants carbamazepine, phenobarbital (phenobarbitone) and/or phenytoin, there was a 7-fold decrease in the area under the plasma concentration versus time curve (AUC) and an 8- to 10-fold decrease in the maximum plasma concentration of nimodipine. These effects were to be expected, considering the hepatic enzyme-inducing properties of these anticonvulsant drugs. Therefore concomitant use of these agents with oral nimodipine is not recommended. In contrast, epileptic patients treated with nimodipine and valproic acid (sodium valproate) showed an increase in both the AUC (approximately 50%) and maximum plasma concentrations (approximately 30%) of nimodipine, which may be explained by valproic acid inhibiting the presystemic oxidative metabolism of nimodipine. Concomitant administration of cimetidine produced an approximate doubling of the bioavailability of nimodipine. This again was to be expected, considering the known inhibitory effect of cimetidine on cytochrome P450. However, no changes in haemodynamics, clinical or laboratory status or tolerability were observed, and dose adjustment did not appear to be clinically necessary.

Possible inhibition of hepatic metabolism of quinidine by erythromycin

OBJECTIVE

To present and analyze a patient case illustrating a possible drug interaction between quinidine and erythromycin.

METHODS

This is a case report of one hospitalized patient. The setting for this analysis was a university hospital. Through a MEDLINE search of all English medical literature (1966 to 1994) documenting possible interactions between quinidine and erythromycin, retrospective patient chart review, and analysis of the relationship between serum quinidine concentrations and significant clinical events, deduce the possibility of a quinidine and erythromycin pharmacokinetic and pharmacodynamic interaction in this particular patient case.

RESULTS

This case demonstrated a probable erythromycin-quinidine pharmacokinetic interaction that led to a decrease in quinidine apparent clearance, an increase in quinidine serum concentrations, and a possible quinidine toxicity.

CONCLUSION

Serum quinidine concentrations, electrocardiograms, and other factors that may predispose patients to torsades de pointes, such as hypokalemia and hypomagnesemia, should be monitored closely if quinidine is coadministered with erythromycin.