Blocker of IKur ultrarapid delayed rectifier potassium current and KV1.5 channels (IC50 = 0.31 μM for rKV1.5). Displays selectivity for inhibition of IKur over Ito (8-fold), IK1, IKr and IKs (20-fold) in native myocytes and selectivity for rat recombinant KV1.5 over KV3.1 (~ 15-fold). Increases action potential duration in atrial but not ventricular myocytes and prevents atrial arrhythmia.