Eslicarbazepine and the enhancement of slow inactivation of voltage-gated sodium channels: A comparison with carbamazepine, oxcarbazepine and lacosamide

Highlights

• Eslicarbazepine did not share with carbamazepine and oxcarbazepine the ability to alter fast inactivation of VGSC.

• Eslicarbazepine and lacosamide reduce VGSC availability through enhancement of slow inactivation.

• Lacosamide demonstrated higher interaction with VGSC in the resting state and with fast inactivation gating.

Abstract

This study aimed at evaluating the effects of eslicarbazepine, carbamazepine (CBZ), oxcarbazepine (OXC) and lacosamide (LCM) on the fast and slow inactivated states of voltage-gated sodium channels (VGSC). The anti-epileptiform activity was evaluated in mouse isolated hippocampal slices. The anticonvulsant effects were evaluated in MES and the 6-Hz psychomotor tests. The whole-cell patch-clamp technique was used to investigate the effects of eslicarbazepine, CBZ, OXC and LCM on sodium channels endogenously expressed in N1E-115 mouse neuroblastoma cells. CBZ and eslicarbazepine exhibit similar concentration dependent suppression of epileptiform activity in hippocampal slices. In N1E-115 mouse neuroblastoma cells, at a concentration of 250 μM, the voltage dependence of the fast inactivation was not influenced by eslicarbazepine, whereas LCM, CBZ and OXC shifted the V_0.5 value (mV) by −4.8, −12.0 and −16.6, respectively. Eslicarbazepine- and LCM-treated fast-inactivated channels recovered similarly to control conditions, whereas CBZ- and OXC-treated channels required longer pulses to recover. CBZ, eslicarbazepine and LCM shifted the voltage dependence of the slow inactivation (V_0.5, mV) by −4.6, −31.2 and −53.3, respectively. For eslicarbazepine, LCM, CBZ and OXC, the affinity to the slow inactivated state was 5.9, 10.4, 1.7 and 1.8 times higher than to the channels in the resting state, respectively. In conclusion, eslicarbazepine did not share with CBZ and OXC the ability to alter fast inactivation of VGSC. Both eslicarbazepine and LCM reduce VGSC availability through enhancement of slow inactivation, but LCM demonstrated higher interaction with VGSC in the resting state and with fast inactivation gating.

Introduction

Eslicarbazepine acetate (ESL), a once-daily antiepileptic drug (AED) (Almeida and Soares-da-Silva, 2007, Bialer and Soares-da-Silva, 2012), was approved in 2009 by the European Medicines Agency and in 2013 by the Food and Drug Administration as adjunctive therapy in adults with partial-onset seizures (POS). Following oral administration, ESL undergoes extensive first pass hydrolysis to its major active metabolite eslicarbazepine (also known as (S)-licarbazepine) (Elger et al., 2013, Falcao et al., 2007, Perucca et al., 2011), which represents approximately 95% of circulating active moieties. Though ESL, on its own, preferentially blocked voltage-gated sodium channels (VGSC) in rapidly firing neurons (Bonifacio et al., 2001), the in vivo effects of ESL may be limited to its extensive conversion to eslicarbazepine. Mechanistically, however, it is important to underline that the affinity of eslicarbazepine for VGSC in the resting state was considerably lower than that of CBZ and oxcarbazepine (OXC), a feature that may translate into an enhanced inhibitory selectivity of eslicarbazepine for rapidly firing “epileptic” neurons over those with normal activity (Hebeisen et al., 2011).

The fundamental properties that enable sodium channels to carry out their physiological roles include rapid, voltage-dependent activation, which often opens the channel, and inactivation (Vilin and Ruben, 2001). Inactivation closes the channel pore and prevents it from reopening until the cell is hyperpolarized. This makes the cell refractory to firing during a long depolarization (Eijkelkamp et al., 2012, Goldin, 2003). There are at least two distinct kinetic classes of inactivation, termed fast and slow. Fast inactivation in VGSC occurs by a “hinged lid” mechanism in which a cytoplasmic region (the inactivating particle) occludes the pore (Goldin, 2003). Slow inactivation is a separate process that does not involve the inactivating particle and may result from a structural rearrangement of the pore (Vilin and Ruben, 2001). Whereas the majority of VGSC blockers used in the treatment of epileptic seizures interfere with the fast inactivation pathway, there is limited information on the pharmacological tools that may influence the slow inactivation of VGSC (Eijkelkamp et al., 2012). Lacosamide (LCM) was shown to act by enhancing slow inactivation of VGSC (Errington et al., 2008), believed to be a new mechanism of action, as other VGSC-blocking AED (CBZ, phenytoin, lamotrigine, OXC) act on fast inactivation (Rogawski and Loscher, 2004).

The present study was aimed to determine the effects of eslicarbazepine, the major active metabolite of ESL, on the fast and slow inactivated states of VGSC endogenously expressed in N1E-115 mouse neuroblastoma cells. To enable the assessment of the antiseizure activity at spanning scales from single cells up to neural circuits, eslicarbazepine was also evaluated in an epileptiform activity model in mouse hippocampal slices where epileptiform electrical activity was induced following exposure to Mg²⁺ free media containing the potassium channel blocker 4 aminopyridine (4-AP) (Lees et al., 2006, Ross et al., 2000). To assess the in vivo pharmacodynamics/pharmacokinetic relationship of eslicarbazepine in the context of antiepileptic therapy, the effects of ESL in the mouse maximal electroshock (MES) test, the 6-Hz psychomotor test, and the plasma and brain tissue levels of eslicarbazepine were also evaluated.