&NA;E 4031, a potent class III antiarrhythmic agent, selectively blocks the rapidly activating component of the delayed rectifier potassium channel current (IKr). The drug is undergoing phase II trials with Eisai in Europe, and preclinical development in the USA. E 4031 may be most useful for the treatment of paroxysmal atrial fibrillation in patients with Wolff-Parkinson-White syndrome and for the prevention of malignant ventricular arrhythmias and sudden heart arrest.E 4031 is one of several ‘pure’ methanesulfonamide class III agents derived from dexosotalol hydrochloride, a group which includes dofetilide, ibutilide, MK 499, risotilide and UK 66914.