Abstracts

Rationale: De novo gain of function mutations in GRIN2B encoding the GluN2B subunits of the N-methyl-D-aspartate (NMDA) receptor have been linked with epileptic encephalopathies, including infantile spasms.  Methods: We investigated the effects of radiprodil, a selective GluN2B negative allosteric modulator and other non-selective NMDA receptor inhibitors on glutamate currents mediated by NMDA receptors containing mutated GluN2B subunits. The experiments were performed in Xenopus oocytes co-injected with the human mRNAs coding for the mutated GRIN2B variants R540H, N615I and V618G.  Results: Inhibition by Mg2+ was completely abolished in N615I and V618G variants, indicating a gain of function. In fact, Mg2+ enhanced glutamate responses in those variants. The potency of radiprodil to block glutamate-evoked currents was not affected in any of the variants, while the effects of non-selective NMDA inhibitors were greatly reduced in some of the variants. Additionally, in the Mg2+ insensitive variants, radiprodil blocked glutamate-activated currents with the same potency as in the absence of Mg2+.The GRIN1 variants coding for N552E, M641I and E662K were also investigated. In GRIN1-M641I, Mg2+ inhibition was significantly decreased versus wild-type GRIN1. In the three variants, a selective GluN2B negative allosteric modulator blocked glutamate-evoked currents with the same potency as in the wild-type.  Conclusions: The gain of function observed in the reported GRIN2B and GRIN1 variants could be a key pathophysiological factor leading to neuronal hyper-excitability in epileptic encephalopathies. GluN2B-selective inhibitors fully retained their pharmacological profile under these conditions, while other non-selective NMDA receptor antagonists lost their potency. Consequently, our data suggest that radiprodil may be a valuable therapeutic option for treatment of pediatric epileptic encephalopathies associated with GRIN2B and GRIN1 mutations. Funding: Study funded by UCB Pharma.