Pharmacokinetic drug interactions between digoxin and antiarrhythmic agents and calcium channel blocking agents: an appraisal of study methodology.

While preliminary screening for interactions between new cardiovascular pharmacotherapeutic agents and digoxin can be efficiently and safely conducted in normal healthy volunteers, it is particularly important to detect and quantify drug interactions in patients with varying degrees of cardiac, hepatic and/or renal dysfunction. Much of the previously published literature provides only minimal data to guide clinical practice because of limitations of study design including sample size and measurement techniques. Important factors that determine the ability of a particular study design to detect a drug interaction with digoxin include the accuracy and precision of the assay method for serum digoxin concentrations, intrasubject and intersubject variability in serum digoxin concentration, and sample size. The format of the trial (chronic versus single digoxin dosing in cardiac patients; chronic versus single digoxin dosing in normal subjects) and the method of assessment of alterations in digoxin handling (formal determination of digoxin clearance, comparison of multiple or single digoxin measurements during various phases of trial) also impact greatly on the clinical relevance of such investigations. Guidelines for future studies of drug interactions with digoxin in cardiac patients are proposed with particular emphasis on laboratory methods; measurement techniques during baseline, placebo, and active drug phases; calculation of the statistical power of the study; time course of the trial; and assessment of the clinical significance of the findings.