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NEUROPHARMACOLOGY

Fenobam: A Clinically Validated Nonbenzodiazepine Anxiolytic Is a Potent, Selective, and Noncompetitive mGlu5 Receptor Antagonist with Inverse Agonist Activity

Pharma Division, Discovery Research CNS (R.H.P.P., W.S., T.M.B., A.M., E.P., A.M., S.G., V.M., P.M.) and Chemistry (G.J., B.B., S.K., J.-U.P., J.W., E.V.), F. Hoffmann-La Roche, Basel, Switzerland

Fenobam [N-(3-chlorophenyl)-N'-(4,5-dihydro-1-methyl-4-oxo-1H-imidazole-2-yl)urea] is an atypical anxiolytic agent with unknown molecular target that has previously been demonstrated both in rodents and human to exert anxiolytic activity. Here, we report that fenobam is a selective and potent metabotropic glutamate (mGlu)5 receptor antagonist acting at an allosteric modulatory site shared with 2-methyl-6-phenylethynyl-pyridine (MPEP), the protypical selective mGlu5 receptor antagonist. Fenobam inhibited quisqualate-evoked intracellular calcium response mediated by human mGlu5 receptor with IC₅₀ = 58 ± 2 nM. It acted in a noncompetitive manner, similar to MPEP and demonstrated inverse agonist properties, blocking 66% of the mGlu5 receptor basal activity (in an over expressed cell line) with an IC₅₀ = 84 ± 13 nM. [³H]Fenobam bound to rat and human recombinant receptors with K_d values of 54 ± 6 and 31 ± 4 nM, respectively. MPEP inhibited [³H]fenobam binding to human mGlu5 receptors with a K_i value of 6.7 ± 0.7 nM, indicating a common binding site shared by both allosteric antagonists. Fenobam exhibits anxiolytic activity in the stress-induced hyperthermia model, Vogel conflict test, Geller-Seifter conflict test, and conditioned emotional response with a minimum effective dose of 10 to 30 mg/kg p.o. Furthermore, fenobam is devoid of GABAergic activity, confirming previous reports that fenobam acts by a mechanism distinct from benzodiazepines. The non-GABAergic activity of fenobam, coupled with its robust anxiolytic activity and reported efficacy in human in a double blind placebo-controlled trial, supports the potential of developing mGlu5 receptor antagonists with an improved therapeutic window over benzodiazepines as novel anxiolytic agents.

Address correspondence to: Dr. Richard Porter, F. Hoffmann-La Roche, PRBD-N, Bldg. 69/452, CH-4070 Basel, Switzerland. E-mail: richard.porter{at}roche.com