Amsacrine modifies action potential configuration without its prolongation via the blockade of multiple ion currents in canine ventricular cells.

Amsacrine is an antineoplastic agent for acute lymphoblastic leukemia. It contains methanesulfonamide group -similar to hERG/IKr inhibitors like E-4031 and dofetilide-therefore may exert cardiac arrhythmias. Although amsacrine inhibits expressed hERG channels, its effects on native cardiac electrophysiology remain unclear. We investigated the effects of 1 and 10 µM amsacrine on action potentials (APs) and underlying ionic currents in isolated canine ventricular cardiomyocytes. Amsacrine reduced phase 0 parameters: maximal upstroke velocity (∼50-60%), action potential amplitude (∼15%), and peak potential (∼55%), suggesting inhibition of sodium current. Moreover, the maximal rates of phase 1 and 3 repolarizations, and plateau potentials were reduced, suggesting effects on multiple ionic currents. Despite these, AP duration (APD20-APD90) remained unaffected. AP voltage-clamp analysis revealed a complex amsacrine-sensitive current consisting of an early inward component consistent with sodium current inhibition, a second early outward component suggesting transient outward current suppression, a mid-plateau inward component likely reflecting inhibition of inward plateau currents, and a late outward component compatible with IKr inhibition. 10 µM amsacrine reduced L-type calcium, transient outward potassium and expressed NaV1.5 channel currents by 22, 38, and 10%, respectively. Native IKr was also reduced in a concentration-dependent manner (152 nM IC50 and 1.21 Hill coefficient). These findings demonstrate that amsacrine modulates multiple cardiac ion currents. The lack of AP prolongation despite IKr inhibition can be due to concomitant suppression of inward currents. As the tested concentrations overlap with therapeutic plasma levels (3-18 µM), amsacrine may carry a non-negligible risk of cardiac electrophysiological effects, particularly in susceptible patients.