Butaprost is an EP2 selective agonist which has frequently been used to pharmacologically define the EP receptor expression profile of various human and animal tissues and cells. Prostaglandin free acids generally bind to their cognate receptors with 10 to 100 times the affinity of the corresponding ester derivative. Consistent with this trend, butaprost binds to membranes from EP2 receptor-transfected CHO cells with a Ki value of 2,400 nM, whereas butaprost (free acid) and CAY10399 (the 2-series congener of butaprost free acid) exhibit significantly lower Ki values of 73 and 92 nM, respectively. Butaprost (free acid) is therefore another useful tool for characterizing EP receptor-mediated signalling events.