Antiarrhythmic activity and unbound concentrations of disopyramide enantiomers in patients

Six patients with reproducible inducible atrial flutter randomly received, in double-blinded fashion, 77 mg of S(+)-disopyramide and R(-)-disopyramide over 20 min by intravenous infusion on two occasions separated by at least 24 h. The S(+) enantiomer prevented the inducibility of atrial flutter in five of the six patients; atrial flutter was inducible following R(-) enantiomer administration in all six patients (p less than 0.05). The mean (+/- SD) antiarrhythmic unbound serum concentration range of S(+)-disopyramide was 0.55 +/- 0.31-0.90 +/- 0.81 mg/L. The binding of disopyramide was stereoselective: the mean unbound fractions of S(+)- and R(-)-disopyramide were 0.207 +/- 0.119 and 0.338 +/- 0.214 (p less than 0.05). Binding of the individual enantiomers in some patients was markedly concentration dependent. The data suggest that the antiarrhythmic activity associated with racemic disopyramide resides in the S(+) enantiomer.