Abstracts

Rationale: An ideal antiepileptic drug (AED) would inhibit hyper-excitability associated with seizures in excitatory circuits while sparing inhibitory circuits. Voltage-gated sodium channel inhibitors, like carbamazepine, are effective anticonvulsant drugs. These drugs inhibit the sodium channels that drive action potential (AP) firing in excitatory neurons (NaV1.2 and NaV1.6 channels) as well as those primarily linked to inhibitory interneuron firing (NaV1.1). Gain of function mutations in both SCN8A (encoding NaV1.6 channels) and SCN2A (encoding NaV1.2 channels) cause early infantile epileptic encephalopathy in humans (EIEE13, and EIEE11 respectively). Conversely, loss of function mutations in SCN1A (encoding NaV1.1) cause Dravet Syndrome (EIEE6) and generalized epilepsy with febrile seizures plus (GEFS+), and non-selective sodium channel inhibitory are known to exacerbate seizures in Dravet patients. Selective sodium channel inhibitors that reduce AP firing in excitatory neurons, while sparing inhibitory interneurons should provide a better pharmacologic profile for new anticonvulsant drugs. Methods: We created novel small molecule inhibitors that target NaV1.6 selectively (XPC’7224) or NaV.12 and NaV1.6 in combination (XPC’5462) while sparing NaV1.1 and other voltage gated sodium channels. We then tested the relative efficacy of XPC’7224 and XPC’5462 to inhibit action potential firing in current clamped mouse brain slice neurons. Activity on excitatory pyramidal neurons versus inhibitory interneurons was compared for both compounds relative to that of carbamazepine, a nonselective inhibitor of voltage-gated sodium channels. Results: Selective inhibition of NaV1.6 with bath application of 500 nM XPC-7224 (3X IC50 for NaV1.6 stably expressed in HEK293 cells) inhibits firing of pyramidal neurons in cortical layer 2/3 pyramidal neurons in brain slices from adult CF-1 mice, as measured by current clamp electrophysiology. However, the same concentration of XPC-7224 had markedly less impact on AP firing in cortical inhibitory interneurons. Similarly, inhibition of NaV1.2/1.6 with bath application of 150 nM XPC-5462 (3X NaV1.6 IC50) impairs AP firing in cortical pyramidal neurons with only modest effect on AP firing in cortical inhibitory interneurons. In contrast, bath application of 100 μM carbamazepine (3X NaV1.6 IC50), a commonly prescribed anti-seizure drug, impairs AP firing in both pyramidal neurons and interneurons in brain slices from age-matched mice. Conclusions: Selectively targeting the specific sodium channel isoforms expressed in excitatory neurons, NaV1.2 and NaV1.6, enables selective reduction of action potential firing in those neurons, and prevents the simultaneous impairment of the activity of inhibitory interneurons. This profile provides a new, mechanistically differentiated, class of voltage-gated sodium channel inhibitors with the potential to provide improved seizure control and side effect profile for epilepsy patients. Funding: No funding