Stereoselectivity of tocainide pharmacodynamics in vivo and in vitro

Racemic tocainide [R,S-T], which is used in the treatment of ventricular arrhythmias, consists of an R(-)-enantiomer [R-T] and an S(+)-enantiomer [S-T]. The present experiments examined these substances for pharmacodynamic differences. The ability of various doses of R-T, S-T, and R,S-T to both induce ataxia (A) and protect against chloroform-induced ventricular arrhythmias (P) in mice were compared at 20 min after subcutaneous treatment. For A effects, the ED50S (95% Fieller Limits) of R-T, S-T, and R,S-T were 125 (78-802), 125 (84-283), and 90 (61-149) mg/kg, respectively. For P effects, the ED50S were 40 (11-71), 116 (82-204), and 86 (55-187) mg/kg. The ratios of A/P, a measure of the margin of safety, were 3.1, 1.1, and 1.0. Effects upon intracardiac conduction in isolated rabbit hearts also exhibited, qualitatively, similar patterns of selectivity. At 1 X 10(-4) M intraatrial, His-Purkinje, intraventricular conduction times, and the QT interval were prolonged with the following order of potency: R-T greater than R,S-T greater than S-T; A-V nodal conduction was unchanged. R-T also evoked the greatest decrease in contractility. With respect to separation of antiarrhythmic action and the production of ataxia, R-T has a greater margin of safety than either S-T or R,S-T.