Abstracts

Rationale: Fenfluramine (FFA) provides a unique, unexpected antiepileptic activity in Dravet syndrome (DS) with high degrees of seizure freedom and prolonged duration of efficacy. Its antiepileptic activity is thought to act by agonist activity of neuronally released serotonin (5-HT) or by a direct agonist effect on 5-HT receptors. However, other receptors may be involved as evidenced by the observation that FFA is a positive allosteric modulator of the sigma 1 receptor (S1R) in vitro. Here we describe non-seizure-related in vivo experiments designed to investigate the nature of any serotonin-S1R interaction. Methods: Synergistic effect of 5-HT1A and S1R ligands: The 5-HT1A receptor agonist 8-OH-DPAT (0.1, 3 mg/kg IP) and the S1R agonist igmesine (10, 30 mg/kg IP) were tested alone and in combination in male Swiss mice submitted to the Forced Swim Test (n=12/group).Positive Allosteric Modulation of FFA on S1R: FFA alone (0.1, 1.0 mg/kg IP) or FFA plus the S1R agonist PRE-084 (0.1, 0.3 mg/kg IP) were administered to the mice before the NMDA receptor antagonist dizocilpine (0.15 mg/kg IP). Learning ability was analyzed using the spontaneous alteration test and passive avoidance, which assess for spatial working memory and contextual long term memory, respectively. Results: Synergistic effect of 5-HT1A and S1R ligands: 8-OH-DPAT reduced immobility at 3 mg/kg but not at lower doses. Igmesine decreased immobility duration at 30, but not 10, mg/kg. Combination of the maximal non-active doses of 8-OH-DPAT and igmesine led to a significant reduction of immobility duration. Calculation of the combination index (CI) based on isobologram representation of the mix revealed a synergy between the two drugs (CI < 1).Positive Allosteric Modulation of FFA on S1R receptors: Dizocilpine produced drastic alterations of spontaneous alternation and passive avoidance learning. FFA significantly attenuated both deficits and the most effective doses were 0.3 and 1 mg/kg IP. PRE-084 attenuated dizocilpine-induced deficits at 0.3 but not 0.1 mg/kg. In both tests, PRE-084 plus a low dose of FFA (0.1 mg/kg) showed synergy between the two drugs (with CI < 1) while PRE-084 (0.1 mg/kg) and a higher dose of FFA (0.3 mg/kg) showed additivity between the two drugs (CI=1). Conclusions: These in vivo studies in mice reveal that a combined pharmacologic action at 5-HT1A and S1R is synergistic in behavioral tests unrelated to seizures. Moreover, these results showed that FFA potentiated the effects of the S1R agonist PRE-084. Recently, allosteric modulation of the S1R was reported to have antiepileptic activity in vivo (Vavers et al, Brain Behav Res 2017). Together these new data suggest that FFA’s antiepileptic activity may not be due solely to serotoninergic effects, but could involve a synergistic effect on the S1R. Funding: This study was funded by Zogenix, Inc. Medical writing assistance was provided by PharmaWrite (Princeton, NJ) and was funded by Zogenix Inc.