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. In addition, mexiletine has neuroprotective effects in models of brain ischemia. We compared state-dependent affinities of mexiletine for Na_v1.2, Na_v1.4, and Na_v1.5 and examined the effects of mutations of transmembrane segment S6 residues on mexiletine block of Na_v1.5. Three channel isoforms had similar affinities of mexiletine for the rested state, and Na_v1.4 and Na_v1.5 had similar affinities for the open and inactivated states, while Na_v1.2 had lower affinity for these states than Na_v1.4 and Na_v1.5. Mutational studies revealed that the largest affinity change was observed for an Ala substitution of Phe in domain IV S6. In our homology modeling based on the bacterial Na⁺ channel, mexiletine changed its location and orientation in the pore depending on the state of the channel, irrespective of the channel isoform. Mexiletine occurred in the upper part in the pore in the open state and lower in the closed state. High-affinity binding of mexiletine in the open states of Na_v1.4 and Na_v1.5 was caused by a π-π interaction with Phe, whereas mexiletine was located away from Phe in the open state of Na_v1.2. 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.