Clobazam and Its Active Metabolite, N-Desmethylclobazam, Are Partial Benzodiazepine Receptor Agonists at Cloned GABAA Receptors Expressed in Xenopus leavis Oocytes (P05.085)

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Objective: We investigated functional effects of clobazam and its active metabolite, N-desmethylclobazam (N-CLB), on recombinant GABA A receptors by electrophysiology. Background Clobazam, a 1,5-benzodiazepine, is in FDA review for Lennox-Gastaut syndrome. Design/Methods: To better understand the observed clinical profile of clobazam, we first characterized its GABA A -receptor-subtype–dependent positive allosteric modulation. Using two electrode voltage clamps, we determined that clobazam and N-CLB were both positive allosteric modulators at Xenopus leavis oocytes injected with cRNA encoding different human GABA A -receptor subunits. GABA concentration-response curves were generated at combinations of α (α 1 , α 2 , α 3 , and α 5 ), β 2, and γ 2 subunits injected at 1:1:1 ratios. At each oocyte, maximum response to GABA and EC 50 and EC 20 values were detemined. Modulatory effects of the two 1,5-benzodiazepines to GABA EC 20 response was determined by co-application of different concentrations of the individual benzodiazepines with a GABA EC 20 concentration. Potentiation of GABA EC 20 response exerted by the respective benzodiazepines was related to maximum responses elicited by GABA on the GABA A -receptor–expressing oocyte. Results: Clobazam and N-CLB both acted as potent partial agonists at α 1 β 2 γ 2 and α 2 β 2 γ 2 receptors. Clobazam exhibited an EC 50 value of 120 nM and a maximum effect of 51% of that of the maximum response of GABA at α 1 β 2 γ 2 receptors, while it displayed an EC 50 value of 110 nM and a maximum effect of 50% at α 2 β 2 γ 2 receptors. Similarly, N-CLB displayed an EC 50 value of 130 nM and maximum effect of 39% at α 1 β 2 γ 2 , and an EC 50 value of 110 nM and maximum effect of 51% at α 2 β 2 γ 2 . Conclusions: Clobazam and N-desmethylclobazam were shown to be partial agonists acting through the benzodiazepine site of the GABA A receptor. This partial agonism may contribute to clobazam9s overall clinical effects and raises the possibility that clobazam9s tolerance profile in animal models may be different than those of 1,4-benzodiazepines such as diazepam. Supported by: H. Lundbeck A/S. Disclosure: Dr. Jensen has received research support from Lundbeck A/S. Dr. Hammer has received research support from Lundbeck Research USA, Inc. Dr. Ebert has received personal compensation for activities with Lundbeck Research USA as an employee. Dr. Sindal Jensen has received personal compensation for activities with H. Lundbeck A/S as an employee.