Nifekalant (MS-551), a Class III antiarrhythmic agent, functions as an IKr potassium channel blocker with an IC50 of 10 μM. It is utilized in the research of rapid supraventricular arrhythmias.

Nifekalant, a pure K+ channel blocker, selectively interacts with cardiac M2 and peripheral M3 receptors, exhibiting K i values of 27 mM and 74 mM, respectively. It dose-dependently inhibits HERG channels, achieving an IC 50 of 7.9 mM, and blocks these channels primarily in their open state in a frequency-dependent manner. Notably, Nifekalant does not impede minK currents in the Xenopus oocyte expression system. Unlike amiodarone, Nifekalant lacks negative inotropic effects associated with β-blocking actions and does not alter cardiac conduction.

Before coronary ligation in rats (a species deficient in functional cardiac IK), administration of MS-551 at doses of 3 mg/kg and 10 mg/kg resulted in reductions in heart rate by 6% and 12%, while mean arterial pressure (MAP) increased by 14% and 33%, respectively. Additionally, MS-551 extends the QT interval and decreases the occurrence of sustained ventricular fibrillation (VF) following reperfusion.