Second generation antiarrhythmic agents: have we reached antiarrhythmic Nirvana?

Pharmacogenetics: a laboratory tool for optimizing therapeutic efficiency

Pharmacogenetics is the study of the linkage between an individual’s genotype and that individual’s ability to metabolize a foreign compound. Differences in metabolism of therapeutics can lead to severe toxicity or therapeutic failure by altering the relation between dose and blood concentration of the pharmacologically active drug. Phenotypes exhibiting poor and ultraextensive metabolism result from genetic variance (polymorphism) of enzymes involved in metabolism. Thus, in pharmacogenetic studies one applies genotyping of polymorphic alleles encoding drug-metabolizing enzymes to the identification of an individual’s drug metabolism phenotype. This knowledge, when applied to dosing or drug selection, can avoid adverse reactions or therapeutic failure and thus enhance therapeutic efficiency. More than 25 commonly prescribed medicines are metabolized by the cytochrome P-4502D6 (CYP2D6) isoenzyme, and polymorphism of the CYP2D6 gene affects the therapeutic management of up to 17% of individuals in some ethnic groups. In this review, we summarize and update information concerning drug-metabolizing genotypes with emphasis on CYP2D6 genotyping techniques that can be applied by the clinical laboratory for linking human genetics to therapeutic management.

A consensus report on antiarrhythmic drug use

During the past few years, a number of new antiarrhythmic agents have become available for use in the United States, encainide has been withdrawn from use, and others have had indications for use modified. Therefore, a meeting of arrhythmia specialists was convened in an attempt to develop guidelines for antiarrhythmic therapy. The resultant discussions and guidelines presented in this article address general issues such as the most important antiarrhythmic drug attributes, as well as therapy for particular arrhythmias such as premature ventricular contractions, ventricular tachycardia, ventricular fibrillation, ventricular ectopy, and supraventricular tachyarrhythmias.

Drug therapy in the elderly. How to achieve optimum results

Pharmacologic management of elderly patients is difficult because of two age-related factors: (1) physiologic changes, especially in hepatic and renal function, and (2) presence of more diseases, requiring combination drug therapy and resulting in increased drug interactions and drug-induced side effects. Nevertheless, drugs have essentially the same benefits in the elderly as in younger patients and should be used for the same indications. Fewer problems will result if the clinician remembers that most drugs produce exaggerated responses, both beneficial and detrimental, in the elderly.

Use of cardiovascular drugs in the elderly. What difference does age make?

Pharmacologic treatment of cardiovascular disease is both challenging and rewarding. Multiple medications and the variable nature of most cardiac problems necessitate careful monitoring of drug regimens in patients of any age. The elderly require even more attention to optimize therapy in light of variable drug absorption, effects, metabolism, and excretion. This challenge will present itself more frequently as the US population ages.

Comparison of twice daily with thrice daily administered encainide for benign or potentially lethal ventricular arrhythmias

The antiarrhythmic efficacy of encainide administered 2 (group A) or 3 times daily (group B) was evaluated in a randomized, placebo-controlled trial involving 101 patients with benign or potentially malignant ventricular arrhythmias. In group A, encainide was titrated at dosages of 35, 50 and 75 mg twice daily and in group B at dosages of 25, 35 and 50 mg 3 times daily. Drug efficacy, as judged by repeated ambulatory monitoring, was defined as greater than 75% reduction in ventricular premature complexes combined with a greater than 90% abolition of pairs and runs of nonsustained ventricular tachycardia. In group A, 27 of 52 patients (52%) had their arrhythmia suppressed by the drug compared with 34 of 49 (69%) in group B (difference not significant). There was a trend toward better arrhythmia control in group B if a total daily dose of greater than 100 mg was necessary for arrhythmia suppression. Side effects were frequent in both groups (24 vs 28%, difference not significant). Thus, encainide administered twice daily effectively suppresses ventricular arrhythmias in this patient population.

Reevaluation of digoxin-encainide interactions using an animal model

The effects of intravenous encainide on digoxin-induced atrial ectopic tachycardia (AET) were investigated in the rat using 3-channel simultaneous limb-lead electrocardiography. Pentobarbital-anesthetized (35 mg/kg, intraperitoneal) adult male rats were given digoxin subcutaneously, 30 mg/kg. After onset of AET, rats received either saline (0.5 ml/kg) or encainide; 0.25, 0.5, 1.0, or 2.0 mg/kg intravenously in repeated doses at 15-min intervals. At all doses, encainide converted digoxin-induced AET to ventricular arrhythmias, prolonged recovery time, and increased mortality in comparison to saline-treated animals. An additional group of anesthetized rats was not given digoxin. These animals received encainide (2.0 mg/kg, intravenously) in repeated doses at 15-min interval and developed dose-related increase in the P-R interval only. Blood samples were obtained by cardiac puncture from 12 additional anesthetized, digoxin-treated rats 5 min after the fourth intravenous dose of saline (0.5 ml/kg, n = 6) or encainide (1.0 mg/kg, n = 6). Serum was prepared and analyzed by affinity column-mediated immunoassay. Digoxin levels were the same in both groups. These results suggest that encainide may exacerbate digoxin-induced arrhythmias (proarrhythmic effect) in this species. In view of our findings of digoxin-encainide interactions in the rat, we recommend caution if these drugs are coadministered in humans.

Chronic ventricular arrhythmias: which drug for which patient?

Many agents, including a number of drugs recently approved by the Food and Drug Administration, are now available for the treatment of chronic ventricular arrhythmias. The so-called first-generation agents--quinidine, procainamide and disopyramide--have been used in large numbers of patients for many years, and the safety and efficacy profiles of these drugs are well established. The "second-generation" antiarrhythmic agents recently approved by the Food and Drug Administration offer promising new alternatives; however, their safety and efficacy profiles have yet to be confirmed for broad populations over extended periods of time. Although it is recognized that the choice of agent for treatment of a particular patient is a "therapeutic trial," with an unpredictable outcome of efficacy and adverse effects, certain "descriptors," such as patient age or co-existing medical conditions, are often helpful in determining which agent is most likely to be clinically effective, and which agents are most likely to produce adverse effects. When other medical conditions such as hepatic or renal failure are present, the appropriate choice of drug and dosage is required for optimal management of the arrhythmia and for prevention of overdosage, exacerbation of other medical problems and deleterious interactions. Combination therapy with multiple antiarrhythmic agents is often quite effective for increasing arrhythmia control without increasing adverse effects. However dosage modifications are often necessary when an antiarrhythmic drug is given in conjunction with another such agent, or with agents that also have electrophysiologic activity or modify metabolic or elimination functions. The following report is one clinician's approach for optimizing efficacy and minimizing toxicity while using the difficult class of drugs called antiarrhythmic agents. It will encourage the use of certain drugs before others, based on considerations of efficacy, safety, ease of administration, follow-up, and other factors.