Sematilide hydrochloride is an investigational class III antiarrhythmic agent that selectively blocks the rapid component of the delayed rectifier potassium current (I_Kr) in cardiac cells. It was primarily developed and studied for the treatment and prevention of supraventricular arrhythmias, such as atrial fibrillation and atrial flutter, by prolonging the action potential duration and effective refractory period in atrial tissue while minimizing effects on ventricular tissue. Due to concerns over potential proarrhythmic effects, including torsades de pointes, it has not been approved for widespread clinical use and is mainly employed in preclinical and clinical research to investigate cardiac electrophysiology, ion channel function, and arrhythmia mechanisms. Its use requires stringent safety monitoring in controlled settings.