Abstracts

Rationale: Dravet syndrome is a catastrophic childhood epilepsy where patients suffer from drug resistant seizures often within the first year of life. Loss-of-function de novo mutations in the α subunit of the voltage-gated sodium channel gene SCN1A is the primary genetic cause of Dravet syndrome. Zebrafish larvae with a mutation in the homologous gene scn1lab recapitulate the spontaneous seizure activity and mimic the convulsive behavioral movements observed in Dravet syndrome patients. Importantly, scn1lab mutant zebrafish also show pharmacoresistance to several different antiepileptic drugs, emulating the persistent drug resistant seizures observed in human patients and making them ideal for discovering and developing new antiepileptics for Dravet syndrome.  Methods: By using the scn1lab mutant zebrafish, we developed an in vivo drug screening platform to identify new antiepileptic compounds.  Screening is performed in a blinded and un-biased manner and rapidly identifies drugs with the capacity to reduce the rapid swimming and convulsive behaviors associated with seizure activity in the Dravet syndrome larvae. Local field potential recordings from individual larvae are subsequently used to confirm a reduction in the abnormal electrical events in the brain that are the hallmark of epilepsy. Results: We have previously identified the antihistamine clemizole as exhibiting potent antiepileptic properties (Baraban 2013). Further characterization of clemizole revealed in addition to its known binding to the histamine H1 receptor it also binds to the serotonin receptors HTR2A and HTR2B (Griffin 2017). We hypothesize the  HTR2 receptor agonist activity of clemizole is responsible for its antiepileptic activity. Using clemizole as our hit compound, we have synthesized a library of 28 clemizole analogs with drug-like properties. These compounds aim to improve the antiepileptic activity of clemizole by enhancing the HTR2 activity. By using our scn1lab mutant zebrafish drug screening platform we have performed structure-activity relationship studies and identify several ‘clemalogs’ capable of reducing the seizure activity observed in the Dravet syndrome larvae. Furthermore, these hit clemalogs also showed strong binding to the human HTR2 receptors. Additionally, a targeted screen of serotonin modulating compounds was performed. Two HTR2 receptor binding compounds, lorcaserin and trazodone, were identified as having antiepileptic activity. Our comparative zebrafish studies between clemizole, loracaserin, trazodone and the serotonin reuptake blocker fenfluramine, suggests agonists of HTR2 receptors is therapeutic for Dravet syndrome.     Conclusions: The recapitulation of the human syndrome at the genetic, physiological and pharmacological levels make the scn1lab mutants ideal for the discovery and development of novel antiepileptics for Dravet syndrome. We conclude that zebrafish-based drug discovery offers a powerful system for accelerating the time between discovery, development and the clinical application for personalized treatments for epilepsy. Funding: NIH/NINDS: R01NS096976 Dravet syndrome fellowship: A128618UCSF Catalyst Award