Class-selective histone deacetylase (HDAC) inhibitors were designed to improve safety profiles and therapeutic effectiveness in the treatment of multiple cancers relative to pan-HDAC inhibitors. However, the underlying mechanisms for their therapeutic and cardiotoxic potentials remain poorly understood. Cardiac sodium currents (INa) and gap junction conductance (gj) were measured by whole cell patch clamp techniques on primary cultures of neonatal cardiomyocytes. Cardiac NaV1.5 sodium channel and connexin43 (Cx43) gap junction protein levels were assessed by Western blot analyses. Panobinostat produced concentration-dependent reductions in ventricular gj, peak INa density, and NaV1.5 protein expression levels. Membrane voltage (Vm)-dependent activation of INa was shifted by +3 to 6 mV with no effect on inactivation. Entinostat (1 μM) did not affect ventricular gj, peak INa density, or INa activation. However, the INa half-inactivation voltage (V½) was shifted by -3.5 mV. Ricolinostat had only minor effects on ventricular gj and INa properties, though INa activation was shifted by -4 mV. Cx43 and NaV1.5 protein expression levels were not altered by class-selective HDAC inhibitors. The lack of effects of class-selective HDAC inhibitors on ventricular gj and INa may help explain the improved cardiac safety profile of entinostat and ricolinostat.
Keywords: NaV1.5; cardiac sodium channel; cardiotoxicity; class-selective; connexin43; gap junction; histone deacetylase inhibitors.