High-affinity blockade of human ether-a-go-go-related gene human cardiac potassium channels by the novel antiarrhythmic drug BRL-32872

Human ether-a-go-go-related gene (HERG) potassium channels are one primary target for the pharmacological treatment of cardiac arrhythmias by class III antiarrhythmic drugs. These drugs are characterized by high antiarrhythmic efficacy, but they can also initiate life-threatening "torsade de pointes" tachyarrhythmias. Recently, it has been suggested that combining potassium and calcium channel blocking mechanisms reduces the proarrhythmic potential of selective class III antiarrhythmic agents. BRL-32872 is a novel antiarrhythmic drug that inhibits potassium and calcium currents in isolated cardiomyocytes. In our study, we investigated the effects of BRL-32872 on cloned HERG channels heterologously expressed in Xenopus oocytes. Using the two-microelectrode voltage clamp technique, we found that BRL-32872 caused a high-affinity, state-dependent block of open HERG channels (IC(50) = 241 nM) in a frequency-dependent manner with slow unbinding kinetics. Inactivated channels mainly had to open to be blocked by BRL-32872. The HERG S620T mutant channel, which has a strongly reduced degree of inactivation, was 51-fold less sensitive to BRL-32872 block, indicating that BRL-32872 binding was enhanced by the inactivation process. In an additional approach, we studied HERG channels expressed in a human cell line (HEK 293) using the whole-cell patch-clamp technique. BRL-32872 inhibited HERG currents in HEK 293 cells in a dose-dependent manner, with an IC(50) value of 19.8 nM. We conclude that BRL-32872 is a potent blocker of HERG potassium channels, which accounts for the class III antiarrhythmic action of BRL-32872.

Antiarrhythmic drug carvedilol inhibits HERG potassium channels

OBJECTIVE

The aryloxypropanolamine carvedilol is a multiple action cardiovascular drug with blocking effects on alpha-receptors, beta-receptors, Ca(2+)-channels, Na(+)-channels and various native cardiac K(+) channels, thereby prolonging the cardiac action potential. In a number of clinical trials with patients suffering from congestive heart failure, carvedilol appeared to be superior to other beta-blocking agents in reducing total mortality. Given the multiple pharmacological actions of carvedilol, this may be due to specific channel blockade rather than beta-antagonistic activity. Since human ether-a-go-go related gene (HERG) K(+)channels play a critical role in the pathogenesis of cardiac arrhythmias and sudden cardiac death, the effects of carvedilol on HERG K(+)channels were investigated.

METHODS

Double-electrode voltage-clamp experiments were performed on HERG potassium channels which were expressed heterologously in Xenopus oocytes.

RESULTS

Carvedilol at a concentration of 10 microM blocked HERG potassium tail currents by 47%. The electrophysiological characteristics of HERG, i.e. activation, steady-state inactivation and recovery from inactivation were not affected by carvedilol. Inhibition of current gradually increased from 0% immediately after the test pulse to about 80% at 600 ms with subsequent marginal changes of current kinetics during the resting 29 s, indicating a very fast open channel block by carvedilol as the major blocking mechanism.

CONCLUSION

This is the first study demonstrating that carvedilol blocks HERG potassium channels. The biophysical data presented in this study with a potentially antiarrhythmic effect may contribute to the positive outcome of clinical trials with carvedilol.