Synthesis and structure-activity relationships of 4,5-fused pyridazinones as histamine H₃ receptor antagonists.

Three series of novel 4,5-fused pyridazinones were synthesized as histamine H(3) receptor antagonists. The 2,5,6,7-tetrahydrocyclopenta[d]pyridazin-1-one 5q and 5,6,7,8-tetrahydro-2H-phthalazin-1-one 5u displayed high affinity at both rat and human H(3) receptors, and showed potent antagonist and full inverse agonist activity in functional assays.