Correlation between ex Vivo Receptor Occupancy and Wake-Promoting Activity of Selective H3Receptor Antagonists

The histamine H₃ receptor (H₃R) modulates the release of neurotransmitters that are involved in vigilance, cognition, and sleep-wake regulation. H₃R antagonism has been proposed as a novel approach to the treatment of cognitive and attention deficit as well as sleep disorders. It is apparent that H₃R antagonists produce pharmacological effects in preclinical animal models across a wide dose range. Several H₃R antagonists were reported to be effective at producing cognitive enhancing effects at low doses, while producing robust wake enhancement at higher doses. To better understand the effect of H₃R antagonists across a broad dose range, an ex vivo receptor binding assay has been used to estimate the degree of H₃R occupancy in vivo. The H₃R antagonists ciproxifan, thioperamide, GSK189254 (6-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy]-N-methyl-3-pyridinecarboxamide hydrochloride), and ABT-239 ([4-(2-{2-[(2R)-2-methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile) produced wake-promoting activity in vivo and a dose-dependent inhibition of H₃R binding ex vivo. For ciproxifan, thioperamide, and GSK189254, a relatively low level of cumulative wake activity was linearly correlated with up to 80% of the receptor occupancy. In contrast, an abrupt break from linearity and a robust increase of waking activity was observed at doses that produce greater than 80% occupancy. Our results suggest a relatively small increase of waking activity at low levels of receptor occupancy that may be consistent with reported enhancement of attention and cognitive function. Robust waking activity at higher levels of H₃R occupancy may be mechanistically different from activities at low levels of H₃R occupancy.