(3S)-hydroxy Quinidine is an active quinidine metabolite. Quinidine, an antiarrhythmic agent, undergoes rapid first-pass metabolism by the cytochrome P450 (CYP) isoforms CYP3A4, CYP2C9, CYP2E1, and CYP3A5, with CYP3A4 being the most active enzyme in the (3S)-hydroxy quinidine formation. (3S)-hydroxy Quinidine prolongs repolarization of canine Purkinje fibers in vitro and prevents ventricular fibrillation and ventricular tachycardia after coronary reperfusion in isolated rat hearts in a dose-dependent manner.