A Novel Class of Orally Active Non-Peptide Bradykinin B2 Receptor Antagonists. 3. Discovering Bioisosteres of the Imidazo[1,2-a]pyridine Moiety

Recently we reported on overcoming the species difference of our first orally active non-peptide bradykinin (BK) B2 receptor antagonists, incorporating an 8-[[3-(N-acylglycyl-N-methylamino)-2,6-dichlorobenzyl]oxy]-3-halo-2-methylimidazo[1,2-a]pyridine skeleton, leading to identification of the first clinical candidate 4a (FR167344). With this potent new lead compound in hand, we then investigated further refinement of the basic framework by replacement of the imidazo[1,2-a]pyridine moiety and discovered several bioisosteric heterocycles. Extensive optimization of these new heteroaromatic derivatives revealed the detailed structure−activity relationships (SAR) around the imidazo[1,2-a]pyridine ring and the 2,6-dichlorobenzyl moiety, leading to the discovery of our second clinical candidate 87b (FR173657) which inhibited the specific binding of [3H]BK to recombinant human B2 receptors expressed in Chinese hamster ovary (CHO) cells and guinea pig ileum membrane preparations expressing B2 receptors with IC50's of 1.4 and 0.46 nM, respectively. This compound also displayed excellent in vivo functional antagonistic activity against BK-induced bronchoconstriction in guinea pigs with an ED50 value of 0.075 mg/kg by oral administration. Further modifications of the terminal substituents on the pyridine moiety led to a novel pharmacophore and resulted in the identification of 99 (FR184280), whose IC50 value for human B2 receptors (0.51 nM) was comparable to that of the second-generation peptide B2 antagonist Icatibant.