Effect of phenytoin on the clinical pharmacokinetics of amiodarone

Drug Interactions in Neurocritical Care

Drug-drug interactions (DDIs) are common and avoidable complications that are associated with poor patient outcomes. Neurocritical care patients may be at particular risk for DDIs due to alterations in pharmacokinetic profiles and exposure to medications with a high DDI risk. This review describes the principles of DDI pharmacology, common and severe DDIs in Neurocritical care, and recommendations to minimize adverse outcomes. A review of published literature was performed using PubMed by searching for 'Drug Interaction' and several high DDI risk and common neurocritical care medications. Key medication classes included anticoagulants, antimicrobials, antiepileptics, antihypertensives, sedatives, and selective serotonin reuptake inhibitors. Additional literature was also reviewed to determine the risk in neurocritical care and potential therapeutic alternatives. Clinicians should be aware of interactions in this setting, the long-term complications, and therapeutic alternatives.

Amiodarone, a multi-channel blocker, enhances anticonvulsive effect of carbamazepine in the mouse maximal electroshock model

Cardiac arrhythmia may occur in the course of epilepsy. Simultaneous therapy of the two diseases might be complicated by drug interactions since antiarrhythmic and antiepileptic agents share some molecular targets. The aim of this study was to evaluate the influence of amiodarone, an antiarrhythmic drug working as a multi-channel blocker, on the protective activity of four classical antiepileptic drugs in the maximal electroshock test in mice. Amiodarone at doses up to 75 mg/kg did not affect the electroconvulsive threshold in mice. Acute amiodarone at the dose of 75 mg/kg significantly potentiated the anticonvulsive effect of carbamazepine, but not that of valproate, phenytoin or phenobarbital in the maximal electroshock-induced seizures in mice. The antiarrhythmic agent and its combinations with antiepileptic drugs did not impair motor performance or long-term memory in mice, except for the combination of amiodarone and phenobarbital. Brain concentrations of antiepileptic drugs were not changed. Despite favourable impact of amiodarone on the anticonvulsive action of carbamazepine in the maximal electroshock, co-administration of the two drugs should be carefully monitored in clinical conditions. Further studies are necessary to evaluate effects of chronic treatment with amiodarone on seizure activity and the action of antiepileptic drugs.

Updates on cytochrome p450-mediated cardiovascular drug interactions

Cytochrome P (CYP) 450 is a superfamily of hemoproteins that play an important role in the metabolism of steroid hormones, fatty acids, and many medications. Many agents used for management of cardiovascular diseases are substrates, inhibitors, or inducers of CYP450 enzymes. When two agents that are substrates, inhibitors, or inducers of CYP450 are administered together, drug interactions with significant clinical consequences may occur. This review discusses CYP450-mediated cardiovascular drug interactions as well as noncardiovascular drug interactions that produced significant cardiovascular side effects. The principles in predicting drug interactions are also discussed.

Antiepileptic drugs and other medications: what interactions may arise?

Many patients with epilepsy are on lifelong therapy with antiepileptic drugs (AEDs), and AEDs are used for other conditions such as mood stabilization and headache prophylaxis. These drugs have high potential for clinically significant interaction with nonepilepsy drugs. Interactions occur largely through altered pharmacokinetics. One drug may increase the hepatic clearance of another, leading to attenuated efficacy of the affected drug. Alternatively, inhibition of liver metabolism by one drug can cause acute toxicity by reducing clearance of another drug. To identify potential drug interactions before they lead to toxicity or therapy failure, the treating clinician should combine knowledge of the patient's overall history with a general knowledge of comorbid conditions in which significant interactions involving AEDs are most likely to occur. Treatments susceptible to interactions include anticoagulants, antiarrhythmics, antibiotics, antiretroviral drugs, immunosuppressives, antineoplastics, and contraceptives. Therefore, it is important to obtain periodically a thorough history of medical problems, use of medications or herbal remedies, and adverse effects of medications. Physicians managing epilepsy patients should also strive to avoid potential drug interactions by favoring low-interaction AEDs in patients taking many other types of drugs. There is quite a large degree of patient heterogeneity in the extent of any given interaction between an AED and another drug. Indeed, some groups of patients may have different susceptibilities to such interactions because of genetic and environmental influences on drug metabolism. Effective treatment with AEDs should include attention to drug interactions.

Updates on cytochrome P450-mediated cardiovascular drug interactions

Cytochrome P (CYP) 450 is a superfamily of hemoproteins that play an important role in the metabolism of steroid hormones, fatty acids, and many medications. Many agents used for management of cardiovascular diseases are substrates, inhibitors, or inducers of CYP450 enzymes.When two agents that are substrates, inhibitors, or inducers of CYP450 are administered together, drug interactions with significant clinical consequences may occur. This review discusses CYP450-mediated cardiovascular drug interactions as well as noncardiovascular drug interactions that produced significant cardiovascular side effects. The principles in predicting drug interactions are also discussed.

Effect of Amiodarone on the Serum Concentration/Dose Ratio of Metoprolol in Patients with Cardiac Arrhythmia

Amiodarone has pharmacokinetic interactions with a number of therapeutic drugs, including warfarin, phenytoin, flecainide, and cyclosporine. Metoprolol is mainly metabolized by CYP2D6, and desethylamiodarone, a metabolite of amiodarone, has a markedly greater inhibitory effect on CYP2D6 than amiodarone. Therefore, the goal of this study was to evaluate the effect of amiodarone and desethylamiodarone on the serum concentration/dose ratio (C/D) of metoprolol in 120 inpatients with cardiac arrhythmias that received either metoprolol and amiodarone (MET+AMD group, n=30) or metoprolol alone (MET group, n=90). The ratio of administered metoprolol was compared between the MET and the MET+AMD groups. The dose of metoprolol and patient age were significantly higher in the MET group when compared with the MET+AMD group (1.00+/-0.480 versus 0.767+/-0.418 mg/kg/day, p<0.050; 68.6+/-10.6 versus 57.6+/-14.1 years, p<0.001, respectively), but the C/D ratio was significantly lower in the MET group than in the MET+AMD group (90.8+/-64.0 versus 136+/-97.8, p<0.01). Furthermore, a significant correlation was found between the C/D ratio and desethylamiodarone concentration (n=30, r=0.371, p<0.01). The results suggest that there is a significant interaction between amiodarone and metoprolol via desethylamiodarone-induced inhibition of CYP2D6. Therefore, careful monitoring of metoprolol concentrations/bioactivity of CYP2D6 is required in the context of co-administration of amiodarone and metoprolol.

Pharmacokinetic Characteristics of Amiodarone in Long-Term Oral Therapy in Japanese Population

To evaluate the pharmacokinetic properties and an optimum dose schedule of amiodarone in long-term oral therapy, serum concentrations of amiodarone and its metabolite, desethylamiodarone, were monitored from 345 Japanese inpatients who received amiodarone therapy for a variety of cardiac arrhythmias. Serum amiodarone and desethylamiodarone concentrations were determined by high performance liquid chromatography system. It was observed that the amiodarone and desethylamiodarone concentrations gradually increased with time. The frequency distribution in the amiodarone clearance of 245 subjects, who received fixed maintenance amiodarone therapy for at least 6 months, was nearly a unimodal one. The variation in the ratio of desetylamiodarone to amiodarone concentration in serum was very small. Although no differences in age, dose, dose duration, amiodarone or desethyamiodarone concentration or ratio were observed between men and women: however, the mean amiodarone clearance of women was significantly higer than that of men. The laboratory data were mostly within normal values and no significant relations were observed between serum amiodarone concentration and clinical laboratory data. These results suggest that the individual variation in pharmacokinetics of amiodarone is comparatively small, which might be sufficient to decide that the maintenance dose was the same one (200 mg/d) in long-term oral amiodarone therapy.

Stereoselective Effect of Amiodarone on the Pharmacokinetics of Racemic Carvedilol

We investigated whether there was a stereoselective effect of amiodarone on the pharmacokinetics of carvedilol. Among a series of 106 inpatients with heart failure, 52 received carvedilol monotherapy (carvedilol group) and 54 received carvedilol plus amiodarone (carvedilol+amiodarone group). The serum carvedilol concentration administered/dose ratio was compared between the two groups based on HPLC measurement of the serum levels of carvedilol, amiodarone, and desethylamiodarone. In 6 patients from the carvedilol group, serum carvedilol levels were compared before and after coadministration of amiodarone. There was no significant between-group difference of the serum concentration to dose (C/D ratio) for the R-enantiomer carvedilol, however, the C/D ratio for the S-enantiomer and the serum S-carvedilol to R-carvedilol (S/R) ratio were both significantly lower in the carvedilol group than in the carvedilol+amiodarone group(47.8+/-56.7 versus 95.3+/-105 ng/mg/kg, P=0.0048 and 0.460+/-0.207 versus 0.879+/-0.377 ng/mg/kg, P<0.001), respectively. Furthermore, the mean S-carvedilol concentration over 14 days of coadministration with amiodarone was higher than that before coadministration (6.54+/-1.73 ng/mL versus 3.03+/-0.670 ng/mL, P<0.001). These results suggest that metabolism of S-carvedilol was markedly inhibited by coadministration of amiodarone.

Potentially significant drug interactions of class III antiarrhythmic drugs

Class III antiarrhythmic drugs, especially amiodarone (a broad-spectrum antiarrhythmic agent), have gained popularity for use in clinical practice in recent years. Other class III antiarrhythmic drugs include bretylium, dofetilide, ibutilide and sotalol. These agents are effective for the management of various types of cardiac arrhythmias both atrial and ventricular in origin. Class III antiarrhythmic drugs may interact with other drugs by two major processes: pharmacodynamic and pharmacokinetic interactions. The pharmacodynamic interaction occurs when the pharmacological effects of the object drug are stimulated or inhibited by the precipitant drug. Pharmacokinetic interactions can result from the interference of drug absorption, metabolism and/or elimination of the object drug by the precipitant drug. Among the class III antiarrhythmic drugs, amiodarone has been reported to be involved in a significant number of drug interactions. It is mainly metabolised by cytochrome P450 (CYP)3A4 and it is a potent inhibitor of CYP1A2, 2C9, 2D6 and 3A4. In addition, amiodarone may interact with other drugs (such as digoxin) via the inhibition of the P-glycoprotein membrane transporter system, a recently described pharmacokinetic mechanism of drug interactions. Bretylium is not metabolised; it is excreted unchanged in the urine. Therefore the interactions between bretylium and other drugs (including other antiarrhythmic drugs) is primarily through the pharmacodynamic mechanism. Dofetilide is metabolised by CYP3A4 and excreted by the renal cation transport system. Drugs that inhibit CYP3A4 (such as erythromycin) and/or the renal transport system (such as triamterene) may interact with dofetilide. It appears that the potential for pharmacokinetic interactions between ibutilide and other drugs is low. This is because ibutilide is not metabolised by CYP3A4 or CYP2D6. However, ibutilide may significantly interact with other drugs by a pharmacodynamic mechanism. Sotalol is primarily excreted unchanged in the urine. The potential for drug interactions due to hepatic enzyme induction or inhibition appears to be less likely. However, a number of drugs (such as digoxin) have been reported to interact with sotalol pharmacodynamically. If concurrent use of a class III antiarrhythmic agent and another drug cannot be avoided or no published studies for that particular drug interaction are available, caution should be exercised and close monitoring of the patient should be performed in order to avoid or minimise the risks associated with a possible adverse drug interaction.

Clinically important drug interactions in epilepsy: interactions between antiepileptic drugs and other drugs

Antiepileptic drugs (AEDs) are commonly prescribed for long periods, up to a lifetime, and many patients will require treatment with other agents for the management of concomitant or intercurrent conditions. When two or more drugs are prescribed together, clinically important interactions can occur. Among old-generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone are potent inducers of hepatic enzymes, and decrease the plasma concentration of many psychotropic, immunosuppressant, antineoplastic, antimicrobial, and cardiovascular drugs, as well as oral contraceptive steroids. Most new generation AEDs do not have clinically important enzyme inducing effects. Other drugs can affect the pharmacokinetics of AEDs; examples include the stimulation of lamotrigine metabolism by oral contraceptive steroids and the inhibition of carbamazepine metabolism by certain macrolide antibiotics, antifungals, verapamil, diltiazem, and isoniazid. Careful monitoring of clinical response is recommended whenever a drug is added or removed from a patient's AED regimen.

Amiodarone Interaction Time Differences with Warfarin and Digoxin

Background:

Amiodarone has pharmacokinetic and pharmacodynamic interactions with various therapeutic agents. The mechanism of interaction between warfarin and amiodarone is the inhibition of warfarin metabolism by amiodarone, and that between digoxin and amiodarone is the inhibition of digoxin transport by amiodarone.

Objective:

To investigate the pharmacokinetic magnitude of the time differences between amiodarone–warfarin and amiodarone–digoxin interactions.

Methods:

Amiodarone was administered concomitantly to 79 inpatients who had been receiving fixed-maintenance doses of warfarin or digoxin. Seventy-seven inpatients were prescribed warfarin therapy, and 54 inpatients were prescribed digoxin therapy. To determine serum concentrations of the warfarin enantiomers digoxin, amiodarone, and desethylamiodarone blood samples were obtained with coadministration of amiodarone. Serum S- and R-warfarin, amiodarone, and desethylamiodarone concentrations were measured by HPLC methods, and serum digoxin concentrations were measured by a fluorescence polarization immunoassay.

Results:

A remarkable decrease of S-warfarin clearance was observed within approximately the first 2 weeks after coadministration of amiodarone. Only a small decrease in R-warfarin clearance was observed. Digoxin clearance was gradually decreased with time, and a good reverse correlation was obtained between amiodarone or desethylamiodarone concentrations and digoxin clearance.

Conclusions:

Relatively short-term monitoring of warfarin clearance is required when amiodarone is coadministered. Long-term monitoring of digoxin serum amiodarone and desethylamiodarone concentrations is necessary to detect the amiodarone–digoxin interaction.

Lack of interaction between amiodarone and mexiletine in cardiac arrhythmia patients

Amiodarone has pharmacokinetic interactions with various therapeutic agents, including phenytoin, flecainide, and cyclosporine. Mexiletine is metabolized by CYP2D6 and CYP1A2. The objective of this study is to evaluate the effect of amiodarone on the pharmacokinetics of mexiletine through its inhibition of various cytochrome P450 (CYP) subtypes. In a series of 181 inpatients with supraventricular tachyarrhythmias, 26 inpatients received mexiletine and amiodarone therapy (MEX + AMD group), and the others received mexiletine therapy (MEX group). In 10 inpatients of the MEX + AMD group, the mexiletine clearance (CL(MEX)/F) before and after coadministration of amiodarone was compared. CL(MEX)/F was also compared in the MEX and MEX + AMD groups after the start of amiodarone therapy. Serum mexiletine, amiodarone, and desethylamiodarone concentrations were measured by an HPLC method. The CL(MEX)/F was estimated by the Bayesian method using population pharmacokinetic analysis. There was no significant difference in CL(MEX)/F before and after 1-month coadministration of amiodarone in 10 inpatients of the MEX + AMD group. Although serum amiodarone and desethylamiodarone concentrations gradually increased with time after the start of amiodarone therapy in these patients, CL(MEX)/F showed no change at 3 and 5 months after the start of amiodarone therapy. There was no significant difference in CL(MEX)/F of the MEX group and the MEX + AMD group. The results suggest that the pharmacokinetics of mexiletine is not affected by amiodarone in patients with cardiac arrhythmias.

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.