To determine the apparent KD for ab118174 at histamine H1 receptors, cells were treated with varying concentrations of histamine agonist alone, or in the presence of 1 µM ab118174, and the serum-induced expression of SPAP measured. The apparent KD at H1 was calculated from the rightward shift of the agonist response curve in the presence of ab118174, compared to the response curve for the agonist alone. KD value for H1 = 8.07 Antagonist –log KD values