Structurally novel histamine H3 receptor antagonists GSK207040 and GSK334429 improve scopolamine-induced memory impairment and capsaicin-induced secondary allodynia in rats

GSK207040 (5-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy]-N-methyl-2-pyrazinecarboxamide) and GSK334429 (1-(1-methylethyl)-4-({1-[6-(trifluoromethyl)-3-pyridinyl]-4-piperidinyl}carbonyl)hexahydro-1H-1,4-diazepine) are novel and selective non-imidazole histamine H3 receptor antagonists from distinct chemical series with high affinity for human (pKi = 9.67 ± 0.06 and 9.49 ± 0.09, respectively) and rat (pKi = 9.08 ± 0.16 and 9.12 ± 0.14, respectively) H3 receptors expressed in cerebral cortex. At the human recombinant H3 receptor, GSK207040 and GSK334429 were potent functional antagonists (pA2 = 9.26 ± 0.04 and 8.84 ± 0.04, respectively versus H3 agonist-induced changes in cAMP) and exhibited inverse agonist properties (pIC50 = 9.20 ± 0.36 and 8.59 ± 0.04 versus basal GTPγS binding). Following oral administration, GSK207040 and GSK334429 potently inhibited cortical ex vivo [3H]-R-α-methylhistamine binding (ED50 = 0.03 and 0.35 mg/kg, respectively). Functional antagonism of central H3 receptors was demonstrated by blockade of R-α-methylhistamine-induced dipsogenia in rats (ID50 = 0.02 and 0.11 mg/kg p.o. for GSK207040 and GSK334429, respectively). In more pathophysiologically relevant pharmacodynamic models, GSK207040 (0.1, 0.3, 1 and 3 mg/kg p.o.) and GSK334429 (0.3, 1 and 3 mg/kg p.o.) significantly reversed amnesia induced by the cholinergic antagonist scopolamine in a passive avoidance paradigm. In addition, GSK207040 (0.1, 0.3 and 1 mg/kg p.o.) and GSK334429 (3 and 10 mg/kg p.o.) significantly reversed capsaicin-induced reductions in paw withdrawal threshold, suggesting for the first time that blockade of H3 receptors may be able to reduce tactile allodynia. Novel H3 receptor antagonists such as GSK207040 and GSK334429 may therefore have therapeutic potential not only in dementia but also in neuropathic pain.