Structural and functional characterization of HQL-79, an orally selective inhibitor of human hematopoietic prostaglandin D synthase

We determined the crystal structure of human hematopoietic prostaglandin (PG) D synthase (H-PGDS) as the quaternary complex with glutathione (GSH), Mg2+, and an inhibitor, HQL-79, having anti-inflammatory activities in vivo, at a 1.45-angstrom resolution. In the quaternary complex, HQL-79 was found to reside within the catalytic cleft between Trp(104) and GSH. HQL-79 was stabilized by interaction of a phenyl ring of its diphenyl group with Trp104 and by its piperidine group with GSH and Arg(14) through water molecules, which form a network with hydrogen bonding and salt bridges linked to Mg2+. HQL-79 inhibited human H-PGDS competitively against the substrate PGH(2) and non-competitively against GSH with K-i of 5 and 3 mu(M), respectively. Surface plasmon resonance analysis revealed that HQL-79 bound to H-PGDS with an affinity that was 12-fold higher in the presence of GSH and Mg2+ (K-d, 0.8 mu(M)) than in their absence. Mutational studies revealed that Arg14 was important for the Mg2+-mediated increase in the binding affinity of H-PGDS for HQL-79, and that Trp(104), Lys(112), and Lys(198) were important for maintaining the HQL-binding pocket. HQL-79 selectively inhibited PGD(2) production by H-PGDS-expressing human megakaryocytes and rat mastocytoma cells with an IC50 value of about 100 mu(M) but only marginally affected the production of other prostanoids, suggesting the tight functional engagement between H-PGDS and cyclooxygenase. Orally administered HQL-79 (30 mg/kg body weight) inhibited antigen-induced production of PGD(2), without affecting the production of PGE(2) and PGF(2 alpha), and ameliorated airway inflammation in wild-type and human H-PGDS-overexpressing mice. Knowledge about this structure of quaternary complex is useful for understanding the inhibitory mechanism of HQL-79 and should accelerate the structure-based development of novel anti-inflammatory drugs that inhibit PGD2 production specifically.