AT-56 is a selective, competitive, and highly bioavailable inhibitor of L-PGDS (lipocalin-type prostaglandin D synthase) (Ki = 75 µM). It inhibits the production of PGD2 by L-PGDS purified from human CSF and recombinant mouse cells with an IC₅₀ value of 95 µM. At concentrations as high as 100 µM in vitro or 30 mg/kg in vivo, AT-56 does not affect the production of PGE2, PGF2α, or H-PGDS-catalyzed PGD2.