Psychopharmacology (1984) 83: 249 - 256 Psychopharmacology 9 Springer-Verlag 1984 Evaluation of the fl-carboline ZK 93 426 as a benzodiazepine receptor antagonist Leif H. Jensen 1, Erling N. Petersen 1, Claus Braestrup 1,2, Tage Honor61, Wolfgang Kehr 3, David N. Stephens 3, Herbert Schneider 3, Dieter Seidelmann a, and Ralph Sehmiechen 3 1 A/S Ferrosan, Research Division, Sydmarken 5, DK-2860 Soeborg, Denmark 2 St. Hans Mental Hospital, DK-4000 Roskilde, Denmark 3 Schering AG, D-1000 Berlin 65 Abstract. We describe here biochemical and pharmacologi- cal effects of the fl-carboline ZK 93426, a new and potent benzodiazepine (BZ) receptor antagonist. ZK 93426 was compared with Ro 15-1788 and CGS 8216, two compounds previously described as BZ receptor antagonists. Certain effects of ZK 93426, Ro 15-1788 and CGS 8216 were quite similar (e.g., 3H-FNM displacement, "GABA ratio", "photo-shift'). In most pharmacological tests ZK 93426 and Ro 15-1788 lacked overt effects; Ro 15-1788 was a weak agonist in some paradigms, while ZK 93426 exhibited a potent proconflict effect but also a weak" anticonvulsant effect. This interesting finding with ZK 93 426 suggests that BZ receptor ligands may possess differential efficacy at BZ receptor subtypes. In contrast, CGS 8216 exhibited potent proconvulsant effects in several paradigms in addition to proconflict and pentylenetetrazol generalizing effects. ZK 93426, Ro 15-1788 and CGS 8216 were almost equally potent as antagonists of the effects of BZ receptor agonists, such as diazepam and lorazepam. However, ZK 93426 was the most potent inhibitor of the convulsions produced by the BZ receptor inverse agonist DMCM. Key words: ZK 93426 - Ro 15-1788 - CGS 8216 - Benzo- diazepine receptor agonists - Benzodiazepine receptor an- tagonist - Benzodiazepine receptor inverse agonist - Rat - Mouse Benzodiazepine (BZ) receptor ligands have previously been suggested to consist of a continuum of three overlapp- ing groups: agonists, antagonists and inverse agonists (Braestrup et al. 1982; Polc et al. 1982; Jensen et al. 1983). BZ receptor agonists comprise compounds with anticon- vulsant, anxiolytic, anti-aggressive and sedative actions, while inverse agonists include fl-carbolines like DMCM, FG 7142 and fl-CCE with anxiogenic and convulsant or proconvulsant actions (Braestrup et al. 1982; Dorow et al. 1983; Oakley and Jones 1980; Petersen et al. 1982; Jensen and Petersen 1983; Corda et al. 1983). In between these extremes BZ receptor antagonists, such as Ro 15-1788 and CGS 8216 with no or weak actions in either direction have been reported (Hunkeler et al. 1981; Czernik et al. 1981; Bonetti et al. 1982; Jensen et al. 1983). These antagonists Offprint requests to: L. H. Jensen, A/S Ferrosan, Research Division, Sydmarken 5, DK-2860 Soeborg, Denmark can inhibit the actions of both agonists and inverse agonists (Braestrup et al. 1982; Polc et al. 1982; Petersen 1983; Valin et al. 1982; Schweri et al. 1982; Jensen and Petersen 1983). Recently, weak intrinsic activity has been described for both Ro 15-1788 and CGS 8216. These reports indicate that Ro 15-1788 has weak agonist properties while CGS 8216 has weak inverse agonist properties (Bernard et al. 1981, Nutt et al. 1982; Dantzer and Perio 1982; Jensen et al. 1983). From the fl-carboline series, we have selected a potent BZ receptor antagonist, ZK 93426, with few overt activities. We describe here biochemical and pharmacological effects of ZK 93426 (Fig. 1) in comparative studies with Ro 15-1788 and CGS 8216. Materials and methods Where not stated otherwise, the following experimental ani- mals were used: mice NMRI 20-25 g; rats Wistar (Mol: Wist) 220-350 g. Room temperature was kept at 21~ and the animals had food and water available until test. ED50 values were calculated by the method of Litchfield and Wilcoxon (1949) if not stated otherwise. Biochemical experiments The membrane preparations were done at 0-3~ unless otherwise stated. Crude membranes. Cerebellum or hippocampus from rats was homogenized in 2 x 10 ml of 25 mM KHzPO4 pH 7.1 using an Ultra-Turrax tissue homogenizer, and centrifuged at 40,000 g for 15 min. The pellet was rehomogenized in 500 vol. per g original tissue of 25 mM KH2PO4 pH 7.1 and used in the binding assays. Washed membranes. Cerebellum or hippocampus from rats was homogenized in 2 x 10 ml 100 mM tris-citrate pH 7.1. H3CXc/CH3 ~) CH3 .~ .~ .... c "~ H ZK 93 426 Fig. 1, Chemical formula of ZK 93426