A Novel Potent and Selective Histamine H3 Receptor Antagonist Enerisant: In Vitro Profiles, In Vivo Receptor Occupancy, and Wake-Promoting and Procognitive Effects in Rodents.

Histamine H3 receptor antagonists/inverse agonists are known to enhance the activity of histaminergic neurons in the brain, thereby promoting arousal and cognition. Here, we report the in vitro and in vivo pharmacological profiles for a newly synthesized histamine H3 receptor antagonist/inverse agonist: [1-(4-{3-[(2R)-2-methylpyrrolidin-1-yl]propoxy}phenyl)-1H-pyrazol-4-yl](morpholin-4-yl)methanone monohydrochloride (enerisant hydrochloride). In vitro assays showed that enerisant was a competitive antagonist/inverse agonist with a high affinity and selectivity for human and rat histamine H3 receptors. Enerisant showed antagonist activity in vivo, as assessed using R-α-methylhistamine (a histamine H3 receptor agonist)-induced dipsogenia, and occupied the histamine H3 receptor in the frontal cortex in a dose-dependent manner. Enerisant also enhanced the extracellular levels of histamine in the posterior hypothalamus and the levels of dopamine and acetylcholine in the medial prefrontal cortex of rats. Enerisant exerted a procognitive effect or reversed scopolamine-induced cognitive impairment in a social recognition test and a novel object recognition test in rats at doses at which less than 50% of the histamine H3 receptor were occupied (0.03-0.3 mg/kg, p.o.). In contrast, higher doses (3-10 mg/kg, p.o.) at which nearly all the histamine H3 receptors were occupied were needed to exert wake-promoting effects in rats. These results indicate that enerisant is a potent and selective histamine H3 receptor antagonist/inverse agonist with the potential to promote arousal and procognition in rats. Moreover, the results also suggest that the histamine H3 receptor occupancy required to exert a pharmacological effect may vary depending on the domain that is being tested. SIGNIFICANCE STATEMENT: Enerisant is a novel histamine H3 receptor antagonist/inverse agonist that exerts wake-promoting and procognitive effects in addition to increasing the release of neurotransmitters related to these pharmacological effects in rodents. Moreover, an in vivo receptor binding study revealed that the in vivo occupancy of the histamine H3 receptor required to exert the pharmacological effects of enerisant varied, and such variations in required occupancy should be taken into account when performing dose selection in clinical studies.

Effect of TTC-909 on cerebral infarction following permanent occlusion of the middle cerebral artery in stroke prone spontaneously hypertensive rats.

We investigated the effect of TTC-909, a drug preparation of the stable prostaglandin I(2) analogue clinprost (isocarbacyclin methylester; methyl 5-[(1S,5S,6R,7R)-7-hydroxy-6-[(E)-(S)-3-hydroxy-1-octenyl] bicyclo[3.3.0]oct-2-en-3-yl] pentanoate) incorporated into lipid microspheres, on cerebral infarction 7 days after permanent occlusion of the middle cerebral artery (MCA) in stroke prone spontaneously hypertensive rats (SHRSP). Under the anesthesia, the MCA was permanently occluded above the rhinal fissure. In schedule 1, vehicle or TTC-909 was injected i.v. once daily over 7 days starting immediately after MCA occlusion. In schedule 2, vehicle or TTC-909 was infused for 3 h starting immediately after MCA occlusion. In schedule 3, vehicle or TTC-909 was infused for 3 h starting immediately after MCA occlusion followed by bolus injection once daily over 6 days. Seven days later, the infarct volume was estimated following hematoxylin and eosin staining. Cerebral infarction produced by permanent occlusion of MCA was limited to the cerebral cortex. While this volume was reduced significantly in case of schedule 3, the infarct volume was not reduced significantly in schedules 1 and 2. Ozagrel, a thromboxane A(2) synthetase inhibitor, had no effect on the infarct volume in schedule 3. These results suggest that cerebral infarction can be developed progressively not only during the first few hours but also after a permanent occlusion of MCA in SHRSP. TTC-909 inhibited cerebral infarction, maybe by improving cerebral blood flow and by protecting against neuronal damage.

Effect of TTC-909 in a middle cerebral artery thrombosis model in stroke-prone spontaneously hypertensive rats.

We investigated the effect of TTC-909, a preparation of the stable prostaglandin I(2) analogue clinprost (isocarbacyclin methylester; methyl 5-[(1S,5S,6R,7R)-7-hydroxy-6-[(E)-(S)-3-hydroxy-1-octenyl] bicyclo[3.3.0]oct-2-en-3-yl] pentanoate) incorporated into lipid microspheres, on infarct volume 24 h after photochemically induced thrombotic occlusion of the middle cerebral artery in stroke-prone spontaneously hypertensive rats (SHR). Under anesthesia, the photosensitizing dye rose bengal (20 mg/kg) was administered intravenously and photoirradiation with green light (wavelength 540 nm) on the middle cerebral artery above the rhinal fissure was achieved using a xenon lamp for 10 min. Infarct volume 24 h after the photochemically induced thrombotic occlusion of the middle cerebral artery was significantly larger in stroke-prone SHR than in Wistar rats. When TTC-909 in doses of 100, 300 and 900 ng/kg/h was intravenously infused for 3 h, starting immediately after the end of the 10-min photoirradiation, the infarct volume was dose-dependently reduced and was statistically significant at a dose of 900 ng/kg/h (p < 0.05). Ozagrel, a thromboxane A(2) synthetase inhibitor, significantly reduced the infarct volume. The model of photochemically induced thrombotic occlusion of the middle cerebral artery in stroke-prone SHR is very useful, because the cerebral infarction is large enough and reproducible. TTC-909 may be effective for the treatment of acute ischemic stroke.