MECHANISM OF BLOCK OF A HUMAN CARDIAC POTASSIUM CHANNEL BY TERFENADINE RACEMATE AND ENANTIOMERS

1 The cardiac toxicity of racemic terfenadine (marked QT prolongation and polymorphic ventricular arrhythmias) is probably due to potassium channel blockade. To test whether one of its enantiomers would be a le ss efficient potassium channel blocker, we compared the mechanism of a ction of the racemate with that of the individual enantiomers. 2 We sy nthesized the individual enantiomers of terfenadine and examined under whole cell voltage-clamp conditions the mechanism of action of the ra cemate, both enantiomers and a major metabolite on a cloned human card iac potassium channel, hKv1.5. This delayed rectifier is sensitive to quinidine, clofilium and other 'class III' antiarrhythmic drugs at cli nically relevant concentrations. 3 Upon depolarization, racemic terfen adine and its enantiomers induced a fast decline of hKv1.5 current tow ards a reduced steady state current level. During subsequent repolariz ation the tail currents deactivated more slowly than the control, resu lting in a 'crossover' phenomenon. 4 The voltage-dependence of block w as biphasic with a steep increase in block over the voltage range of c hannel opening (-30 to 0 mV) and a more shallow phase positive to 0 mV (where the channel is fully open). The latter was consistent with a b inding reaction sensing 21% of the transmembrane electrical field (wit h reference to the cell interior). 5 The EC(50) for hKv1.5 block by ra cemic terfenadine was 0.88 mu M, while the values for R- and S-terfena dine were 1.19 mu M and 1.16 mu M, respectively. In contrast, the acid metabolite reduced hKv1.5 current by only 5% at a concentration of 50 mu M. 6 These findings suggest that terfenadine blocks the hKv1.5 cha nnel after it opens by entering into the internal mouth of the channel . We have previously shown that quinidine blocks hKv1.5 in a similar m anner but with an apparent affinity of similar to 6 mu M. Thus, terfen adine and its enantiomers are approximately equipotent open state bloc kers of this human K+ channel and about 6 times more potent than quini dine. The similar state-, time-, and voltage-dependence of hKv1.5 bloc k by both enantiomers also indicates that the chiral centre does not s ignificantly constrain the orientation of critical binding determinant s of terfenadine with respect to the receptor site.