The recently described potent and selective GABAA antagonist SR 95531 (gabazine) is compared to six other GABAA antagonists: (+)-bicuculline, (-)-securinine, (+)-tubocurarine, iso-THAZ, R-5135, and pitrazepine. Starting from ab initio molecular orbital calculations performed on crystal atomic coordinates, attempts were made to identify in each structure the functional groups that are involved in receptor recognition and binding. A molecular modeling study revealed that (a) all compounds possess accessible cationic and anionic sites separated by an 4.6-5.2 A intercharge distance, (b) the antagonistic nature of the compounds can be explained by the presence of additional binding sites, (c) the correct spatial orientation of the additional binding sites is crucial for GABAA selectivity, and (d) the criteria determining the potency of the antagonist effect are an accurate intercharge distance (greater than 5 A) and the existence of hydrogen-bonding functionalities on one of the additional ring system. The presented pharmacophore accounts also for the inactivity of closely related compounds such as (-)-bicuculline, adlumidine, virosecurinine, allosecurinine, and the 4,6-diphenyl analogue of gabazine.

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The crystal structure of pitrazepin has been solved by direct methods from single-crystal X-ray diffraction data and refined b y full-matrix least squares: monoclinic, space group P2,/c, a = 18.845( 2), b = 15.424( 5), c = 16.950(4) A, p = 104.51 ( 2 ) ', Z = 4, two pitrazepin and three water molecules per asymmetric unit, final R factor is 0.042. This compound binds with high affinity to pharmacological GABA-A receptors; it acts as an antagonist. A b initio molecular orbital calculations for pitrazepin and GABA show that the piperazine and triazole rings of pitrazepin might be considered as the GABA-mimetic moieties.