Synthesis and evaluation of a spiro-isobenzofuranone class of histamine H3 receptor inverse agonists

Spiro-isobenzofuranones 1a and 1b were discovered as potent, selective, and brain-penetrable non-imidazole H3 receptor inverse agonists. Our corporate sample collection was screened to identify 2a as a lead. Recognizing the right-hand portion of 2a as an essential pharmacophore, an extensive screen of the left-hand piperidine portion was carried out to yield the potent spiro-derivatives 2t−x. Spiro-isobenzofuranone 2x, the most potent among the derivatives, was converted to the corresponding amide 1a, which possessed dramatically improved H3 activity (IC50 = 0.72 nM; more than 20-fold improvement over 2x). Further elaboration led to the identification of 1b, a 5-methoxy derivative with an IC50 of 0.54 nM. Our studies demonstrated that derivatives 1a and 1b to be potent, selective, and brain-penetrable H3 inverse agonists.