Imidazenil: a new partial positive allosteric modulator of gamma-aminobutyric acid (GABA) action at GABAA receptors

Positive allosteric modulators of gamma-aminobutyric acid (GABA)A receptors, including benzodiazepines and congeners, can be classified into three categories: 1) full allosteric modulators (i.e., triazolam and alprazolam) that act with high potency and efficacy at many GABAA receptors; 2) selective allosteric modulators (i.e., diazepam) that act with high potency and high efficacy at selected GABAA receptors; and 3) partial allosteric modulators (i.e., bretazenil) that act with high potency but low efficacy at many GABAA receptors. Imidazenil, an imidazobenzodiazepine carboxamide, has been characterized as a novel representative of the partial allosteric modulator class. When tested on a broad spectrum (native and recombinant) of GABAA receptors, imidazenil positively modulates the GABA-elicited Cl- currents with a 4- to 5-fold higher potency but an efficacy (30-50%) lower than that of diazepam, and it antagonizes the effects of the latter drug. Imidazenil in vitro (Ki = 5 x 10(-10) M) and in vivo (ID50 = 0.2 mumol/kg i.v.) displaces [3H]flumazenil from its brain binding sites and in vivo it possesses a marked anticonflict profile in the rat Vogel conflict-punishment test and is 10 times more potent than bretazenil and 100 times more potent than diazepam or alprazolam in antagonizing bicuculline- and pentylenetetrazol-induced seizures. Unlike diazepam and alprazolam, which induce sedation and ataxia and potentiate the effects of ethanol and thiopental at doses similar to those that produce anticonflict effects and occupy 50% of brain flumazenil binding sites, imidazenil does not produce ataxia or sedation in rats nor does it potentiate the effects of ethanol or thiopental in doses 30- to 50-fold higher than those required for the anticonflict effect and for 100% occupancy of brain flumazenil binding sites. Furthermore, when administered with diazepam, imidazenil blocks in a dose-related fashion the sedative, ataxic effects of this drug and thus acts on these unwanted responses as an antagonist (i.e., like flumazenil). In all tests, imidazenil has the pharmacological profile of a partial allosteric modulator, but is more potent than bretazenil, has a longer biological half-life and, in rodents, is virtually unable to cause sedation, ataxia or to potentiate ethanol toxicity.