Mexiletine Block of Voltage-Gated Sodium Channels: Isoform- and State-Dependent Drug-Pore Interactions.

Mexiletine is a class Ib antiarrhythmic drug and is also used clinically to reduce or prevent myotonia. Mexiletine also has neuroprotective effects in the models of brain ischemia. We compared state-dependent affinities of mexiletine for Nav1.2, Nav1.4 and Nav1.5, and examined the effects of mutations of S6 residues in Nav1.5 affecting local anesthetic binding on mexiletine block. Three channel isoforms had similar affinities of mexiletine for the rested state, and Nav1.4 and Nav1.5 had similar affinities for the open and inactivated states, while Nav1.2 had lower affinity for these states than Nav1.4 and Nav1.5. In the mutational studies of Nav1.5, the largest affinity changes were observed in Ala substitution of Phe in domain IV S6, which is a key residue for local anesthetic binding. In our homology modeling based on the bacterial Na+ channel and ligand docking, mexiletine was located at a different place in the pore depending on the state of the channel irrespective of the channel isoform. Mexiletine occurred upper in the pore in the open state and lower in the closed state. High affinity binding of mexiletine in the open states of Nav1.4 and Nav1.5 was caused by a π-π interaction with Phe, whereas mexiletine was located away from Phe in the open state of Nav1.2. These results provide crucial information on the mechanism of isoform differences in state-dependent block by local anesthetics and related drugs. Mexiletine at upper locations in the open state may effectively cause an electrostatic mechanism of block.