Timapiprant (OC000459) is a potent, selective, and orally active D prostanoid receptor 2 (DP2, also known as CRTH2) antagonist. Timapiprant (OC000459) potently displaces [3H] PGD2 from human recombinant DP2 (Ki=13 nM), rat recombinant DP2 (Ki=3 nM), and human native DP2 (Ki=4 nM). Timapiprant (OC000459) inhibits mast cell activation of Th2 lymphocytes and eosinophils.IC50 & Target: Ki: 0.013 μM (human recombinant DP2); 0.003 μM (rat recombinant DP2) and 0.004 μM (human native DP2)In Vitro: Timapiprant (OC000459) (0.0001 μM-10 μM; 5 hours) inhibits chemotaxis (IC50=0.028 μM) of human Th2 lymphocytes and cytokine production (IC50=0.019 μM) by human Th2 lymphocytes.Timapiprant (OC000459) (1 μM) inhibits the activation of Th2 cells and eosinophils in response to supernatants from IgE/anti-IgE-activated human mast cells.Timapiprant (OC000459) (1 nM–1000 nM; 16 hours) inhibits the anti-apoptotic effect of PGD2 on Th2 cells with an IC50 of 0.035 uM.In Vivo: Timapiprant (OC000459) (gavage; 2 mg/kg, 10 mg/kg) inhibits blood eosinophilia induced by 13,14-dihydro-15-keto-PGD2 (DK-PGD2) in Rat (ED50=0.04 mg/kg). Timapiprant (OC000459) (gavage; 0.01 mg/kg, 0.1 mg/kg, 1.0 mg/kg) inhibits airway eosinophilia in response to an aerosol of DK-PGD2 in guinea pigs (ED50=0.01 mg/kg).