Nifedipine-induced alterations in serum quinidine concentrations

Calcium channel blockers: spectrum of side effects and drug interactions

Calcium antagonists are a chemically heterogenous group of agents with potent cardiovascular effects which are beneficial in the treatment of angina pectoris, arterial hypertension and cardiac arrhythmias. The main side effects for the group are dose-dependent and the result of the main action or actions of the calcium antagonists, i.e. vasodilatation, negative inotropic effects and antiarrhythmic effects. Pronounced hypotension is reported for the main calcium antagonist drugs; verapamil, diltiazem and nifedipine. While conduction disturbances and bradycardia are seen more often after verapamil and diltiazem, tachycardia, headache and flush are more frequent after nifedipine. Constipation is relatively frequent after verapamil while nifedipine is reported to induce diarrhea in som patients. Idiosyncratic side effects are rare but have been reported from the skin, mouth, musculoskeletal system, the liver and the central nervous system. These side effects include urticarial rashes, gingival hyperplasia, arthralgia, hepathotoxicity and transistory mental confusion or akathisia. Verapamil, diltiazem and possibly also nifedipine have been reported to increase serum digoxin concentrations but the clinical relevance of these drug interactions are not clear. Furthermore, verapamil and diltiazem may potentiate the effects of beta-adrenergic blocking drugs and verapamil may also potentiate the effects of neuromuscular blocking drugs. It is concluded that side effects after calcium antagonist drugs are mostly trivial and transient although they may sometimes be relatively common. Clinically relevant drug interactions are few. Judged from the point of efficacy and safety, calcium antagonists will have a major place in the future pharmacotherapy of several cardiovascular disorders.

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.

Current drug treatment and treatment patterns with antihypertensive drugs

The 4 major classes of antihypertensive drugs are diuretics, beta-blockers, ACE inhibitors and calcium antagonists. The diuretics have recently regained prominence, largely due to the results of recent controlled trials. These trials in elderly patients demonstrated that low-dose diuretics were effective not only in preventing stroke but also in greatly reducing coronary-related events. Diuretics also decrease left ventricular mass more than the other major drug classes. In addition, they are the most effective drugs for use in combination therapy. By contrast, the safety of calcium antagonists has recently been questioned because of report of increased coronary morbidity and mortality. However, these adverse events may be restricted to the short-acting preparations, especially nifedipine, which causes cardiac stimulation. ACE inhibitors, like beta-blockers, are not only effective in reducing blood pressure, particularly when combined with a diuretic, but also improve angina and decrease postinfarction mortality. They also benefit congestive heart failure, stabilise or improve renal function in hypertensive and diabetic nephropathy and reduce albuminuria. Beta-Blockers are especially effective in reducing sudden cardiac death in patients with coronary heart disease, particularly in postinfarction patients. Final proof of the relative effectiveness of these drugs in preventing morbidity and mortality must await the outcome of large comparative trials currently under way. A recent national survey in the US found that more than 75% of hypertensive patients did not have their hypertension completely controlled. Possible reasons for this disturbing statistic are discussed along with suggestions for improvement.

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.

Evaluation of the pharmacokinetic and pharmacodynamic interaction between quinidine and nifedipine

Quinidine and nifedipine appear to be subject to metabolism by the same isozyme of cytochrome P-450. In addition, both drugs have been reported to alter the pharmacokinetics of other compounds. To investigate a potential interaction, 10 healthy subjects (five male, five female) received quinidine sulfate (200 mg orally), nifedipine (20 mg orally), or the combination of both drugs every 8 hours for 4 doses using a randomized, cross-over study design with a 2-week washout period between treatments. Drug concentration, heart rate, and mean arterial pressure were measured at frequent intervals after the final dose. Quinidine concentrations were unchanged by the co-administration of nifedipine. Nifedipine area under the curve (AUC0-8) increased 36.6% from 333 to 455 micrograms.hr/L (P < .05) after quinidine administration. Heart rate was significantly higher in the nifedipine-quinidine treatment at 0.5, 1.0, 1.5, and 2.0 hours when compared with either drug alone. The maximum increase in heart rate (17.9 beats/minute) occurred at 0.5 hours after nifedipine administration and was significantly correlated with serum concentrations at that time (r = .78). These results suggest that quinidine inhibits nifedipine metabolism, and this pharmacokinetic interaction results in enhanced pharmacologic response.

Pharmacokinetic interactions with calcium channel antagonists (Part II)

Since calcium channel antagonists are a diverse class of drugs frequently administered in combination with other agents, the potential for clinically significant pharmacokinetic drug interactions exists. These interactions occur most frequently via altered hepatic blood flow and impaired hepatic enzyme activity. Part I of the article, which appeared in the previous issue of the Journal, dealt with interactions between calcium antagonists and marker compounds, theophylline, midazolam, lithium, doxorubicin, oral hypoglycaemics and cardiac drugs. Part II examines interactions with cyclosporin, anaesthetics, carbamazepine and cardiovascular agents.

Sustained release nifedipine formulations. An appraisal of their current uses and prospective roles in the treatment of hypertension, ischaemic heart disease and peripheral vascular disorders

Nifedipine antagonises influx of calcium through cell membrane slow channels, and sustained release formulations of the calcium channel blocker have been shown to be effective in the treatment of mild to moderate hypertension and both stable and variant angina pectoris. Preliminary findings also indicate that these formulations are effective in the treatment of Raynaud's phenomenon and hypertension in pregnancy, and that they reduce the frequency of ischaemic episodes in some patients with silent myocardial ischaemia. The exact mechanism of action of nifedipine in all of these disorders has not been defined. However, its potent peripheral and coronary arterial dilator properties, together with improvements in oxygen supply/demand, are of particular importance. A major goal of sustained release therapy is to permit reductions in the frequency of nifedipine administration, preferably to once daily, and thus improve patient compliance. Two new once-daily formulations--the nifedipine gastrointestinal therapeutic system (GITS) and a fixed combination capsule comprising sustained release nifedipine 20 mg and atenolol 50 mg--have exhibited marked antihypertensive efficacy. The GITS preparation has also been used effectively in the treatment of stable angina pectoris, and both formulations appear to be well tolerated. Sustained release nifedipine formulations are generally better tolerated than their conventionally formulated counterparts, particularly with regard to reflex tachycardia. Adverse effects seem to be dose related, are mainly associated with the drug's potent vasodilatory action, and include headache, flushing and dizziness. Generally, these effects are mild to moderate in severity and transient, usually diminishing with continued treatment. Thus, sustained release nifedipine formulations are useful and established cardiovascular therapeutic agents which have demonstrable efficacy in various forms of angina, mild to moderate hypertension and Raynaud's phenomenon. Further, promising results shown by the nifedipine GITS formulation, with its advantage of once daily administration suggest that it is likely to become one of the preferred nifedipine formulations for the treatment of hypertension and the various forms of angina.

Population pharmacokinetics of quinidine

1. Population pharmacokinetic parameters of quinidine were determined based on 260 serum drug concentration measurements in 60 patients treated for arrhythmias with quinidine sulphate or quinidine bisulphate (Kinidin duriles) orally. 2. Quinidine kinetics were best described by a two compartment model with zero order absorption from the gastrointestinal tract. The pharmacokinetics are influenced by severe heart or liver failure and renal function impairment. No effect was found for mild or moderate heart failure, for age, for body weight or for coadministration of nifedipine. 3. Population pharmacokinetic parameters of quinidine (assuming 100% bioavailability of oral quinidine sulphate) were: nonrenal clearance for patients without severe heart and liver failure 12.6 l h-1, reduction in patients with severe heart or liver failure to 6.8 l h-1, renal clearance (l h-1) related to creatinine clearance (ml min-1), proportionality constant 0.0566, volume of distribution of the central compartment 161 l, maximum serum drug concentration 1.4 h after administration of quinidine sulphate and 6.0 h after administration of quinidine bisulphate. 4. The results were validated by predicting the serum drug concentration in a separate group of 30 patients. The model reliably predicted both the population average and the variability of the serum concentration of quinidine. 5. Using Monte Carlo computer simulations, an a priori dosing regimen was derived that should maximize the proportion of patients having quinidine serum concentrations within the recommended range (2-5 mg l-1): initial dose of 600 mg quinidine sulphate in all patients, 3 h later first maintenance dose of quinidine bisulphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of hepatic microsomal drug metabolism in rats by five calcium antagonists

The effects of five calcium antagonists, verapamil, diltiazem, nifedipine, darodipine and isradipine, on rat liver microsomal drug metabolism in vitro and in vivo were studied. All compounds prolonged hexobarbital-induced sleeping time in a dose-dependent manner (doses 3.0 and 30.0 mg/kg, except nifedipine 0.3 and 3.0 mg/kg) and inhibited cytochrome P450-dependent N-demethylation of aminopyrine in vitro in rat liver microsomes. The incubation of all compounds with microsomes resulted in the apparent formation of formaldehyde, suggesting either N- or O-demethylation. Diltiazem, isradipine and darodipine gave rise to a type I spectral change. Nifedipine seemed to produce a type II spectral change. A spectrum of verapamil changed from a type I to a type II as concentration increased. These results indicate that all calcium antagonists studied interact with P450 and are in vitro inhibitors of microsomal drug metabolism in the rat and the inhibition brings out pharmacokinetic drug-drug interactions in vivo.

Combination therapy with diltiazem and nifedipine in patients with effort angina pectoris

The antianginal effects of diltiazem and nifedipine alone and in combination were evaluated in a double-blind, randomized, placebo-controlled trial in 11 patients (nine men and two women, 57 +/- 8 years old) with stable effort angina. Each patient received placebo, 30 mg of diltiazem, 10 mg of nifedipine, and 30 mg of diltiazem plus 10 mg of nifedipine four times daily for 1 week each. Antianginal efficacy was assessed by means of a treadmill exercise test. The exercise tolerance time was significantly prolonged from 235.1 +/- 52 (placebo period) to 342.2 +/- 101 sec by diltiazem (p less than .01) and to 325.6 +/- 73 sec by nifedipine (p less than .01). The drug combination further prolonged exercise time to 451.1 +/- 103 sec, which was significantly longer than the interval attained with either diltiazem (p less than .01) or nifedipine (p less than .01) alone. The plasma concentration of diltiazem was unaffected by the addition of nifedipine, whereas the plasma nifedipine concentration was significantly increased from 34.8 +/- 11 to 106.4 +/- 37 ng/ml (p less than .001) by the concomitant administration of diltiazem. These data suggest that exercise tolerance in patients with effort angina is increased by the concomitant administration of diltiazem and nifedipine associated with an increase in the nifedipine plasma concentration.

Calcium antagonists--adverse drug interactions

In the clinical management of heart disease, calcium channel blockers are generally prescribed in combination with one or more anti-angina, antiarrhythmic, or antihypertensive agents. Two different mechanisms are involved in drug interactions: pharmacokinetic and pharmacodynamic. In the former, the disposition of one drug is altered by the action of another, causing an increase or decrease in its absorption or its modified distribution, metabolism, or excretion. In pharmacodynamic interactions, the physiologic effects of one drug interfere either directly or indirectly with those of another, for instance, by alterations in fluid or electrolyte balance. This effect may be antagonistic or additive. The present work outlines the possible adverse interactions between the three main calcium antagonists and other therapeutic agents, including digoxin, beta blockers and antiarrhythmic, anesthetic, antihypertensive, antiasthmatic, and antidiabetic drugs and contrast media. Knowledge of these effects is of major clinical importance in the treatment of cardiac patients.

Individualization of calcium entry-blocker dosage for systemic hypertension

The calcium entry blockers are used in a wide variety of clinical situations. Coexisting disease states, such as renal or hepatic dysfunction, may require individualized dosing of these agents. The physiologic changes associated with aging may also affect the pharmacokinetic properties of the drugs. If calcium entry blockers are used concurrently with other medications, dosage adjustment or selection of an alternative drug may be needed. Drug interactions between calcium entry blockers and cimetidine, digoxin and quinidine appear to be clinically significant. Individualized dosing in patients who have coexisting disease or who are using other medications is essential to achieve an adequate therapeutic response and avoid adverse effects. Considerations to attain an optimal response in such situations are presented.

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.