Selective stabilization of the intermediate inactivated Na⁺ channel by the new-generation anticonvulsant rufinamide

Na⁺ channels undergo multiple inactivated states with different kinetics, which set the refractory period of neuronal discharges, but isolating the intermediate inactivated state has been challenging. Most classical Na⁺channel-inhibiting anticonvulsants bind to the fast inactivated state to reduce Na⁺currents and cellular excitability. These anticonvulsants have the slow binding kinetics and thus necessitate long depolarization for drug action, a "use-dependent" effect sparing most normal activities. Rufinamide is a new anticonvulsant targeting Na⁺channels, and has a therapeutic effect on Lennox-Gastaut syndrome (LGS) which is refractory to classicalNa⁺channel inhibitors. The efficacy on LGS, whose epileptiform discharges largely involve short depolarization or bursts, is primarily due to the very fast binding kinetics of rufinamide. Could the very fast kinetics of rufinamide lead to indiscriminate inhibition of neuronal activities ? Onhippocampal neurons from male and female mice, wefound that rufinamide most effectively shifts the Na⁺channel inactivation curve if the inactivating pulse is 1 s, rather than 0.1 or 18 s, in duration. Rufinamide also shows a maximal slowing effect on the recovery kinetics from the inactivation driven by modest depolarization (e.g. -60 mV) of intermediate length (e.g. 50-300 ms). Consistently, rufinamide selectively inhibits the burst discharges at 50-300 ms on a plateau of ∼-60 mV. This is mechanistically ascribable to selective binding of rufinamide to an intermediate inactivated state withan apparent dissociation constantof ∼40 μM. Being the first molecule embodying the evasive transitional gating state, rufinamide could have a unique anti-seizure profile with a novel form of use-dependent action.