Comparison of the ex vivo receptor occupancy profile of ketamine to several NMDA receptor antagonists in mouse hippocampus

NMDA receptor antagonists, particularly these targeting the GluN2B subunit are of therapeutic interest for the treatment of severe mood disorders. The receptor occupancy profiles of several NMDA receptor antagonists (30 mg/kg, s.c.) were compared in mouse hippocampus by ex vivo autoradiography using [(3)H]MK-801, a non-selective NMDA channel blocker, and [(3)H]ifenprodil a selective GluN2B antagonist. Subcutaneous administration of ketamine ((RS)-2-(2-Chlorophenyl)-2-(methylamino)cyclohexanone) and memantine (3,5-dimethyladamantan-1-amine) inhibited [(3)H]MK-801 but not [(3)H]ifenprodil binding in mouse hippocampus. Ketamine reached maximal occupancy of [(3)H]MK-801 binding sites after 15 min and rapidly cleared from the brain with no significant level of occupancy measured at the 1h time point. Memantine significantly occupied [(3)H]MK-801 binding sites throughout the 6h time course. The selective GluN2B antagonist CP101,606 ((1S,2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol) and Ro 25-6981 ((αR,βS)-α-(4-Hydroxyphenyl)-β-methyl-4-(phenylmethyl)-1-piperidinepropanol maleate) inhibited [(3)H]ifenprodil but not [(3)H]MK-801 binding and significant levels of occupancy (above 50%) were measured throughout the 6h time course. These data highlight the unique quick pulse target engagement profile of ketamine compared to other NMDA receptor antagonists.