Biochemical, Functional, and Pharmacological Characterization of AT-56, an Orally Active and Selective Inhibitor of Lipocalin-type Prostaglandin D Synthase

We report here that 4-dibenzo[a,d] cyclohepten-5-ylidene-1[4-(2H-tetrazol-5-yl)-butyl]-piperidine (AT-56) is an orally active and selective inhibitor of lipocalin-type prostaglandin (PG) D synthase (L-PGDS). AT-56 inhibited human and mouse L-PGDSs in a concentration (3-250 mu M)-dependent manner but did not affect the activities of hematopoietic PGD synthase (H-PGDS), cyclooxygenase-1 and -2, and microsomal PGE synthase1. AT-56 inhibited the L-PGDS activity in a competitive manner against the substrate-PGH(2) (K-m = 14 mu M) with a K-i value of 75 mu M but did not inhibit the binding of 13-cis-retinoic acid, a nonsubstrate lipophilic ligand, to L-PGDS. NMR titration analysis revealed that AT-56 occupied the catalytic pocket, but not the retinoid-binding pocket, of L-PGDS. AT-56 inhibited the production of PGD(2) by L-PGDS-expressing human TE-671 cells after stimulation with Ca2+ ionophore (5 mu M A23187) with an IC50 value of about 3 mu M without affecting their production of PGE(2) and PGF(2 alpha) but had no effect on the PGD2 production by H-PGDS-expressing human megakaryocytes. Orally administered AT-56 (< 30 mg/ kg body weight) decreased the PGD2 production to 40% in the brain of H-PGDS-deficient mice after a stab wound injury in a dose-dependent manner without affecting the production of PGE2 and PGF(2 alpha) and also suppressed the accumulation of eosinophils and monocytes in the bronco-alveolar lavage fluid from the antigen-induced lung inflammation model of human L-PGDS-transgenic mice.