GABA(A) AGONISTS - RESOLUTION AND PHARMACOLOGY OF (-ISOGUVACINE AND (-)-ISOGUVACINE OXIDE())

(3SR, 4RS)-3, 4-Epoxypiperidine-4-carboxylic acid (isoguvacine oxide) is a potent and specific GABA(A) receptor agonist. Isoguvacine oxide, originally designed as a potentially alkylating agonist, turned out to interact with the GABA(A) receptor in a fully reversible manner. The protected form of isoguvacine oxide, benzyl (3SR,4RS)-1-(benzyl-oxycar bonyl)-3, 4-epoxypiperidine-4-carboxylate (1) (Scheme 1), has now been resolved by chiral chromatography using cellulose triacetate as a chi ral stationary phase. The enantiomers of 1 tee greater than or equal t o 98.8%) were subsequently deprotected by hydrogenolysis. Whereas both enantiomers of isoguvacine oxide were inactive as inhibitors of the b inding of [H-3]GABA to GABA(B) receptor sites (IC50 > 100 mu M), (+)-i soguvacine oxide (IC50 = 0.20 +/- 0.03 mu M) and (-)-isoguvacine oxide (IC50 = 0.32 +/- 0.05 mu M) showed comparable potencies as inhibitors of the binding of [H-3]GABA to GABA(A) receptor sites. Furthermore, ( +)-isoguvacine oxide (EC(50) = 6 mu M;33% relative efficacy) and (-)-i soguvacine oxide (EC(50) = 5 mu M; 38% efficacy relative to 10 mu M mu scimol) were approximately equipotent and equieficacious as stimulator s of the binding of [H-3]diazepam to the GABA(A) receptor associated b enzodiazepine site. This latter effect is an in vitro estimate of GABA (A) agonist efficacy. These pharmacological data for isoguvacine oxide and its enantiomers do not seem to support our earlier conception of the topography of the GABA(A) recognition site(s), derived from extens ive structure-activity studies on GABA(A) agonists. Thus, the model of the GABAA recognition site(s) comprising a narrow cleft or pocket, in which the anionic moiety of the zwitterionic GABA, agonists is assume d to be embedded during receptor activation, may have to be revised. ( C) 1995 Wiley-Liss, Inc.