JNJ303 is a potent, specific delayed rectifier potassium current I(Ks) blocker with IC50 of 64 nM, shows no effects on other cardiac channels (IC50=3.3, >10, 11.1 and 12.6 uM for Na, ICa, Ito and IKr currents); prolongs QT, strongly increases field potential (FP) duration.
