Acute pharmacological blockade of central histamine H 3 receptors (H 3 Rs) enhances arousal/attention in rodents. However, there is little information available for other behavioral domains or for repeated administration using selective compounds. ABT-239 [4-(2-{2-[(2R)-2-methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile] exemplifies such a selective, nonimidazole H 3 R antagonist with high affinity for rat (pK i ϭ 8.9) and human (pK i ϭ 9.5) H 3 Rs. Acute functional blockade of central H 3 Rs was demonstrated by blocking the dipsogenia response to the selective H 3 R agonist (R)-␣-methylhistamine in mice. In cognition studies, acquisition of a five-trial, inhibitory avoidance test in rat pups was improved with ABT-239 (0.1-1.0 mg/kg), a 10-to 150-fold gain in potency, with similar efficacy, over previous antagonists such as thioperamide, ciproxifan, A-304121 [(4-(3-(4 -furamide], and A-349821 [(4Ј-(3-((R,R)2,5-dimethyl-pyrrolidin-1-yl)-propoxy)-biphenyl-4-yl)-morpholin-4-yl-methanone]. Efficacy in this modelwas maintained for 3 to 6 h and following repeated dosing with ABT-239. Social memory was also improved in adult (0.01-0.3 mg/kg) and aged (0.3-1.0 mg/kg) rats. In schizophrenia models, ABT-239 improved gating deficits in DBA/2 mice using prepulse inhibition of startle (1.0 -3.0 mg/kg) and N40 (1.0 -10.0 mg/kg). Furthermore, ABT-239 (1.0 mg/kg) attenuated methamphetamineinduced hyperactivity in mice. In freely moving rat microdialysis studies, ABT-239 enhanced acetylcholine release (0.1-3.0 mg/kg) in adult rat frontal cortex and hippocampus and enhanced dopamine release in frontal cortex (3.0 mg/kg), but not striatum. In summary, broad efficacy was observed with ABT-239 across animal models such that potential clinical efficacy may extend beyond disorders such as ADHD to include Alzheimer's disease and schizophrenia.

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Pharmacological blockade of central histamine H 3 receptors (H 3 Rs) enhances cognition in rodents and offers promise for the clinical treatment of neurological disorders. However, many previously characterized H 3 R antagonists are either not selective for H 3 Rs or have potentially significant tolerability issues. Here, we present in vivo behavioral and neurophysiological data for two novel and selective H 3 R antagonists with improved safety indices. (1 mg/kg). Social memory was also significantly enhanced in the adult rat with A-304121 (3, 10 mg/kg) and A-317920 (1, 3 mg/kg) at doses that produced no significant change in electroencephalogram slow-wave amplitude activity. Relative therapeutic indices (TIs) of 30 and 42 were estimated for A-304121 and A-317920, respectively, by comparing doses producing adverse effects in general observation studies with potency in inhibitory avoidance, which were superior to TIs of 8, 10, and 18 observed for the reference antagonists thioperamide, ciproxifan, and GT-2331, respectively. A-304121 and A-317920 represent a series of novel, H 3 R-selective piperazine amides that enhance cognition in vivo, which could offer advantages over existing H 3 R antagonists or cognition-enhancing agents.

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