BICYCLOORTHOCARBOXYLATE CONVULSANTS - POTENT GABAA RECEPTOR ANTAGONISTS

  • 1 January 1985
    • journal article
    • research article
    • Vol. 28  (3) , 246-253
Abstract
4-t-Butyl-1-(4-bromophenyl)-bicycloorthocarboxylate antagonizes .gamma.-aminobutyric acid (GABA)-mediated relaxation at a functional insect nerve-muscle synapse, mimicking the action of picrotoxinin, suggesting that it causes GABA antagonism through blockade of the chloride ionophore. It is also a potent GABAA receptor antagonist, inhibiting the binding of [35S]t-butyl-bicyclophosphorothionate ([35S]TBPS) to EDTA/water-dialyzed human brain P2 membranes. Structure-activity relationships of 74 1,4-bis-substituted bicycloorthocarboxylates, mostly new compounds, reveal that for high potency as a GABAA receptor antagonist the optimal 4-substituent is a C4 to C6 branched chain alkyl or cycloalkyl group (e.g., t-butyl, s-butyl, or cyclohexyl) and the optimal 1-substituent is a phenyl moiety with one or more electron-withdrawing groups (e.g., 4-cyano, 4-bromo, 4-chloro, 3,5-dichloro, or pentafluoro). Bicycloorthocarboxylate inhibitors of [35S]TBPS binding with IC50 values of 5-10 nM exceed by several-fold the potency of any GABAA receptor antagonist previously reported. The 4-t-butyl-1-(4-azidophenyl) analog, synthesized as a candidate photoaffinity label, gives an IC50 of 315 nM. The potency of bicycloorthocarboxylates for decreasing [35S]TBPS binding generally correlates with their toxicity, i.e., compounds without inhibitory activity in this brain receptor assay are of low toxicity on intraperitoneal administration to mice, and the analogs most potent as inhibitors are generally those most toxic to mice (e.g., IC50 of 5 nM and LD50 of 0.06 mg/kg for 4-t-butyl-1-(4-cyanophenyl)-bicycloorthocarboxylate). The effects of phenyl substituents on the potency of the orthobenzoates as GABAA receptor antagonists are similar to those on toxicity. In contrast to the 1-substituted phenyl compounds, 4-t-butyl-1-ethynyl-bicycloorthocarboxylate and its 4-i-propyl analog are very toxic (LD50 0.4-2 mg/kg) but have only moderate inhibitory potency (IC50 480-2900 nM), a pattern noted for many 1-alkyl-bicycloorthocarboxylates, suggesting that even within this series there may be different types of receptor-inhibitor interactions. 1-(4-Chlorophenyl)-4-cyclohexylbicycloorthocarboxylate is particularly sensitive to oxidative detoxification based on its 10-fold synergism of toxicity by piperonyl butoxide and marked potency loss in a coupled [35S]TBPS receptor/microsomal oxidase assay. Some benzodiazepines and phenobarbital protect against poisoning by 1-(4-bromophenyl)- and 1-ethynyl-4-t-butyl-bicycloorthocarboxylates and their 1-(4-bromophenyl)-4-cyclohexyl analog. The exceptional potency and high structural specificity of the bicycloorthocarboxylates indicate they are useful probes in further studies of the GABA receptor/ionophore complex.