Synthesis and SAR of 5-Amino- and 5-(Aminomethyl)benzofuran Histamine H₃ Receptor Antagonists with Improved Potency

A new series of H3 receptor antagonists was discovered with nanomolar and subnanomolar affinities at human and rat H3 receptors. Starting from an earlier, more structurally limited series of benzofurans, the present series of compounds demonstrated increased structural variety and flexibility with greater in vitro potency. One compound in particular, {2-[2-(2-(R)-methylpyrrolidin-1-yl)ethyl]benzofuran-5-yl}(5-nitropyridin-2-yl)amine (7h), gave the best binding potency (human Ki of 0.05 nM, rat Ki of 0.11 nM), which represented a 9-fold (in human) and an 11-fold (in rat) improvement over ABT-239 (compound 5), a compound previously reported to have excellent in vitro potency and in vivo efficacy. The synthesis, SAR of the H3 binding affinities, in vitro assay for phospholipidosis, and pharmacokinetic properties of the new compounds are described.