Design of a new histamine H3 receptor antagonist chemotype: (3aR,6aR)-5-alkyl-1-aryl-octahydropyrrolo[3,4-b]pyrroles, synthesis, and structure-activity relationships.

A new histamine H3 receptor (H3R) antagonist chemotype 1 was designed by combining key pharmacophoric elements from two different precursor structural series and then simplifying and optimizing the resulting combined structural features. First, analogues were made based on a previously identified conessine-based H3R antagonist series. While the first analogues 11 and 15 showed no antagonistic activity to H3R, the mere addition of a key moiety found in the reference compound 7 (ABT-239) elevated the series to high potency at H3R. The hybrid structure (16b) was judged too synthetically demanding to enable an extensive SAR study, thus forcing a strategy to simplify the chemical structure. The resulting (3aR,6aR)-5-alkyl-1-aryl-octahydropyrrolo[3,4-b]pyrrole series proved to be highly potent, as exemplified by 17a having a human H3 Ki of 0.54 nM, rat H3 Ki of 4.57 nM, and excellent pharmacokinetics (PK) profile in rats (oral bioavailability of 39% and t1/2 of 2.4 h).