Highly selective histamine H4 receptor silent antagonist; binds with high affinity to the human H4 receptor (Ki = 26 nM). > 540-fold selective over the H3 receptor (Ki = 14.1 μM). Inhibits mast cell and eosinophil chemotaxis in vitro with IC50 values of 138 and 530 nM respectively. Orally active in vivo.