Potent histamine H4 receptor antagonist (pKi values are 8.24 and 8.47 in human and rat H4 receptors respectively). Displays 162-fold, 620-fold, and > 1600-fold selectivity over human H3, H1 and H2 receptors. Blocks zymosan-induced neutrophil reflux and attenuates thermal hypersensitivity in vivo (ED50 = 42 μmol/kg, ip).