Dramatic inhibition of amiodarone metabolism induced by grapefruit juice

Food-drug interaction: grapefruit juice augments drug bioavailability--mechanism, extent and relevance

More than a decade has passed since it was unintentionally discovered that grapefruit juice interacts with certain drugs. The coadministration of these drugs with grapefruit juice can markedly elevate drug bioavailability, and can alter pharmacokinetic and pharmacodynamic parameters of the drug. The predominant mechanism for this interaction is the inhibition of cytochrome P-450 3A4 in the small intestine, resulting in a significant reduction of drug presystemic metabolism. An additional mechanism is, presumably, the inhibition of P-glycoprotein, a transporter that carries drug from the enterocyte back to the gut lumen, resulting in a further increase in the fraction of drug absorbed. Some calcium channel antagonists, benzodiazepines, HMG-CoA reductase inhibitors and cyclosporine are the most affected drugs. A single exposure to one glass of the juice can usually produce the maximal magnitude of the interaction. The data available so far, concerning this interaction and its clinical implications, are reviewed in this article. It is likely that more information regarding this interaction will accumulate in the future, and awareness of such is necessary for achieving optimal drug therapy.

Interactions between grapefruit juice and cardiovascular drugs

Grapefruit juice can alter oral drug pharmacokinetics by different mechanisms. Irreversible inactivation of intestinal cytochrome P450 (CYP) 3A4 is produced by commercial grapefruit juice given as a single normal amount (e.g. 200-300 mL) or by whole fresh fruit segments. As a result, presystemic metabolism is reduced and oral drug bioavailability increased. Enhanced oral drug bioavailability can occur 24 hours after juice consumption. Inhibition of P-glycoprotein (P-gp) is a possible mechanism that increases oral drug bioavailability by reducing intestinal and/or hepatic efflux transport. Recently, inhibition of organic anion transporting polypeptides by grapefruit juice was observed in vitro; intestinal uptake transport appeared decreased as oral drug bioavailability was reduced. Numerous medications used in the prevention or treatment of coronary artery disease and its complications have been observed or are predicted to interact with grapefruit juice. Such interactions may increase the risk of rhabdomyolysis when dyslipidemia is treated with the HMG-CoA reductase inhibitors atorvastatin, lovastatin, or simvastatin. Potential alternative agents are pravastatin, fluvastatin, or rosuvastatin. Such interactions might also cause excessive vasodilatation when hypertension is managed with the dihydropyridines felodipine, nicardipine, nifedipine, nisoldipine, or nitrendipine. An alternative agent could be amlodipine. In contrast, the therapeutic effect of the angiotensin II type 1 receptor antagonist losartan may be reduced by grapefruit juice. Grapefruit juice interacting with the antidiabetic agent repaglinide may cause hypoglycemia, and interaction with the appetite suppressant sibutramine may cause elevated BP and HR. In angina pectoris, administration of grapefruit juice could result in atrioventricular conduction disorders with verapamil or attenuated antiplatelet activity with clopidrogel. Grapefruit juice may enhance drug toxicity for antiarrhythmic agents such as amiodarone, quinidine, disopyramide, or propafenone, and for the congestive heart failure drug, carvediol. Some drugs for the treatment of peripheral or central vascular disease also have the potential to interact with grapefruit juice. Interaction with sildenafil, tadalafil, or vardenafil for erectile dysfunction, may cause serious systemic vasodilatation especially when combined with a nitrate. Interaction between ergotamine for migraine and grapefruit juice may cause gangrene or stroke. In stroke, interaction with nimodipine may cause systemic hypotension. If a drug has low inherent oral bioavailability from presystemic metabolism by CYP3A4 or efflux transport by P-gp and the potential to produce serious overdose toxicity, avoidance of grapefruit juice entirely during pharmacotherapy appears mandatory. Although altered drug response is variable among individuals, the outcome is difficult to predict and avoiding the combination will guarantee toxicity is prevented. The elderly are at particular risk, as they are often prescribed medications and frequently consume grapefruit juice.

Effects of orlistat, a lipase inhibitor, on the pharmacokinetics of three highly lipophilic drugs (amiodarone, fluoxetine, and simvastatin) in healthy volunteers

To investigate the effect of orlistat on the pharmacokinetics of three highly lipophilic drugs (amiodarone, fluoxetine, and simvastatin), the authors performed double-blind, placebo-controlled, randomized two-period crossover (for fluoxetine and simvastatin) or parallel (for amiodarone) studies in healthy volunteers ages 18 to 65 years of a body mass index between 18 and 30 kg/m2. During treatment with orlistat or matching placebo for 5 to 13 1/3 days, a single oral dose of highly lipophilic drug was administered, followed by obtaining serial blood samples for measuring plasma (for fluoxetine and simvastatin) or serum (for amiodarone) concentrations of the lipophilic drug and its active metabolite. Treatments were compared for the pharmacokinetic parameters AUC0-infinity, Cmax, tmax, and t 1/2 of highly lipophilic drugs and active metabolites. Analysis of variance was performed to assess the significance of the sequence effect and provide the variance estimate for the 90% confidence intervals. Subjects were also evaluated for adverse events, vital signs, and clinical and laboratory safety. The absorption of amiodarone (and active metabolite) was significantly reduced by approximately one-quarter using parameters of Cmax and AUC, while no inhibition of absorption was observed for fluoxetine and simvastatin as well as their active metabolites. There were no clinically significant differences in t 1/2 and tmax for all three drugs tested. Due to expected gastrointestinal adverse events known to occur with orlistat, there was a higher incidence of adverse events under regimen B (highly lipophilic drugs and orlistat) than under regimen A (highly lipophilic drugs and placebo). Other adverse events were sporadic and unremarkable. There were no clinically relevant changes in vital signs or laboratory values. In conclusion, except for amiodarone, there was no effect of orlistat on the pharmacokinetics of highly lipophilic drugs when these drugs were taken concomitantly with orlistat.

Prescription of QT-prolonging drugs in a cohort of about 5 million outpatients

Many drugs prolong the QT interval and increase the risk of torsade de pointes. Concurrent use of two or more of these drugs can further increase the risk, but the prevalence of concurrent prescription of QT-prolonging drugs is not known. Using the administrative claims database of a national pharmaceutical benefit manager, we conducted a retrospective cohort study in 4,825,345 subjects aged 18 years or older. After identifying 50 drugs with QT-prolonging potential, and an additional 26 drugs that inhibit the metabolic clearance of QT-prolonging drugs, we measured the frequency of overlapping prescriptions for two or more of these drugs in the outpatient setting in 1999. Nearly 1.1 million subjects (22.8%) filled 4.4 million prescriptions for QT-prolonging drugs. Of these, 103,119 subjects (9.4%) filled overlapping prescriptions for two or more of the drugs or for a QT-prolonging drug and another drug that inhibits its clearance; 7249 subjects (0.7%) filled overlapping prescriptions for three or more of these drugs. Twenty-two percent of subjects who filled overlapping prescriptions were aged 65 or older; 74% were women. Antidepressants were involved in nearly 50% of the cases. Concurrent prescription of QT-prolonging drugs is common in the outpatient setting, and antidepressants are involved in half of these cases. Large pharmaceutical claims databases are useful for detecting potentially harmful prescribing behaviors, but better clinical evidence on medication safety is needed before such a system can be implemented fully.

Effects of liquid and freeze-dried grapefruit juice on the pharmacokinetics of praziquantel and its metabolite 4′-hydroxy praziquantel in beagle dogs

Grapefruit juice changes the pharmacokinetic parameters of a variety of drugs metabolized primarily by cytochrome P450 3A. In a three-phase crossover study, six male beagle dogs were administered 100 ml of water (control), 100 ml of commercial liquid grapefruit juice, or 10 g of freeze-dried grapefruit juice (equivalent to 100 ml of liquid grapefruit juice) with 100 ml of water, followed after 2 h by single oral dose of praziquantel (30 mg kg(-1)). After treatment, the dogs were sampled at different times. Determination of praziquantel and its metabolite 4'-hydroxy praziquantel (identified by GC/MS) was performed by HPLC. Liquid and freeze-dried grapefruit juice preadministration increased the C(max) of praziquantel about three-fold and the AUC 2.5- and 2.3-fold, respectively. The T(max) (0.75 h) was unaffected by liquid or freeze-dried grapefruit juice, while T(1/2) was 2.3- and 1.7-fold higher compared with controls. The amount of 4'-hydroxy praziquantel was also affected by both liquid and freeze-dried grapefruit juice administration: the AUC and C(max) increased four- and three-fold, respectively and the T(max) was significantly enhanced. These findings demonstrate that both freeze-dried grapefruit juice and commercial liquid grapefruit juice significantly increase plasma concentrations and T(1/2) of praziquantel in dogs.

Interactions of grapefruit juice and cardiovascular medications: A potential risk of toxicity

Recently, drug interactions with grapefruit juice (GFJ) have received considerable attention from basic scientists, physicians, industry and drug regulatory agencies. GFJ has been shown to inhibit cytochrome P-450 3A4 isoenzyme and P-glycoprotein transporters in the intestine and liver. The GFJ-induced inhibitory effects are considered to be responsible for alterations in drug bioavailability, and pharmacokinetic and pharmacodynamic changes when drugs are ingested concurrently with GFJ. However, little or no interaction is observed when GFJ is taken concomitantly with parentally administered drugs. It is well known that risk factors for cardiovascular disease increase with advancing age, while hepatic metabolic activity decreases in elderly individuals. It is, therefore, possible that the combination of GFJ and cardiovascular medications may pose a health risk, especially in elderly patients. A number of studies have shown interactions of GFJ with cardiovascular drugs such as calcium-channel blockers, angiotensin II receptor antagonists, beta-blockers, and statins. Such interactions are likely to change the pharmacokinetics and pharmacodynamics of these drugs, consequently causing undesirable health effects. Therefore, health care professionals and the public need to be advised of the potential risks associated with the concomitant use of GFJ and interacting medications, especially cardiovascular drugs and agents with a narrow therapeutic index. This review focuses on the adverse interactions of GFJ and cardiovascular medications, and the proposed underlying mechanisms of such interactions.

Effects of herbal components on cDNA-expressed cytochrome P450 enzyme catalytic activity

We evaluated the effects of 25 purified components of commonly used herbal products on the catalytic activity of cDNA-expressed cytochrome P450 isoforms in in vitro experiments. Increasing concentrations of the compounds were incubated with a panel of recombinant human CYP isoforms (CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A4) and their effects on the conversion of specific surrogate substrates measured fluorometrically in a 96-well plate format. For each test substance, the IC50 (the concentration required to inhibit metabolism of surrogate substrates by 50%) was estimated and compared with IC50's for the positive control inhibitory drugs furafylline, sulfaphenazole, tranylcypromine, quinidine, and ketoconazole. Constituents of Ginkgo biloba (ginkgolic acids I and II), kava (desmethoxyyangonin, dihydromethysticin, and methysticin), garlic (allicin), evening primrose oil (cis-linoleic acid), and St. John's wort (hyperforin and quercetin) significantly inhibited one or more of the cDNA human P450 isoforms at concentrations of less than 10 uM. Some of the test compounds (components of Ginkgo biloba, kava, and St. John's wort) were more potent inhibitors of the isoforms 1A2, 2C19, and 2C19 than the positive controls used in each assay (furafylline, sulfaphenazole, and tranylcypromine, respectively), which are known to produce clinically significant drug interactions. The enzyme most sensitive to the inhibitory of effects of these compounds was CYP2C19, while the isoform least effected was CYP2D6. These data suggest that herbal products containing evening primrose oil, Ginkgo biloba, kava, and St. John's Wort could potentially inhibit the metabolism of co-administered medications whose primary route of elimination is via cytochrome P450.

Amiodarone -- waxed and waned and waxed again

Amiodarone has been used as an anti-arrhythmic drug since the 1970s and has an established role in the treatment of ventricular tachyarrhythmias. Although considered to be a class III anti-arrhythmic, amiodarone also has class I, II and IV actions, which gives it a unique pharmacological and anti-arrhythmic profile. Amiodarone is a structural analogue of thyroid hormone and some of its anti-arrhythmic properties and toxicity may be attributable to interactions with nuclear thyroid hormone receptors. The lipid solubility of amiodarone gives it an exceptionally long half-life. Oral amiodarone takes days to work in ventricular tachyarrhythmias, but iv. amiodarone has immediate effect and can be used in life threatening ventricular arrhythmias. Intravenous amiodarone administered after out-of-hospital cardiac arrest due to ventricular fibrillation improves survival to hospital admission. Many survivors of myocardial infarction (MI) die during the subsequent year, probably due to ventricular arrhythmia. Amiodarone reduces sudden death after MI and this benefit is predominantly observed in patients with preserved cardiac function. Sudden cardiac death, predominantly due to ventricular arrhythmias, is also commonly seen in patients with heart failure. The Grupo de Estudio de la Sobrevida en lsuficiencia Cardiaca en Argentina (GESICA) and Estudio Piloto Argentino de Muerte Subita y Amiodarona (EPAMSA) trials showed survival benefit of amiodarone in heart failure, whereas Congestive Heart Failure-Survival Trial of Anti-arrhythmic Therapy (CHF-STAT) did not. Subsequent meta-analysis established a survival benefit of amiodarone in heart failure. Implanted Cardioverter Def ibrillators (ICDs) also give survival benefit to patients at risk of sudden death. In patients with a history of ventricular fibrillation or haemodynamically-compromising ventricular tachycardia, ICDs have been shown to be superior to anti-arrhythmic drugs, principally amiodarone. Further analysis has been undertaken to ascertain which patients are most likely to benefit from ICDs, as these are more expensive than treatment with amiodarone. Patients with severely depressed ejection fractions should be the first to be considered for ICDs. A new indication for amiodarone is atrial fibrillation or flutter. Amiodarone is effective in chronic and recent onset atrial fibrillation and orally or iv. for atrial fibrillation after heart surgery. In atrial fibrillation amiodarone is more than or equi-effective with flecainide, quinidine, racemic sotalol, propafenone and diltiazem and therefore should be considered for first line therapy. Amiodarone is also safe and effective in controlling refractory tachyarrhythmias in infants and is safe after cardiac surgery.

Drug interactions

The prevalence of undesirable drug interactions is substantial but largely unknown. Although drug interactions are certainly an important cause of drug toxicity, their clinical significance may have been exaggerated. This review presents a brief overview of possible drug interactions.