Ambrisentan is a selective antagonist of ETA-receptor with Ki value of 1nM [1].

Ambrisentan is an orally active diphenyl propionic acid derivative. It is usually used to treat for pulmonary arterial hypertension. In the in vitro assay, ambrisentan shows selective affinity with ETA-receptor over ETB-receptor expressed in CHO cells. The Ki values of ETA- and ETB-receptor are 1nM and 195nM, respectively. This selectivity is much more higher for the recombinant human ET-receptors in intact cells. The Ki values of ETA- and ETB-receptor are 0.63nM and 48.7nM, respectively. As a selective antagonist of ETA-receptor, ambrisentan is preferential to non-selective receptor antagonism as it permitting maintenance of vasodilator and clearance functions specific to ETB- receptors on the endothelial cells. Moreover, ambrisentan also has possible use in the prevention of reperfusion injury and is appropriate to treat for cerebrovascular disorders [1, 2].

References:
[1] Vatter H, Seifert V. Ambrisentan, a Non-peptide Endothelin Receptor Antagonist. Cardiovascular drug reviews, 2006, 24(1): 63-76.
[2] Barst R J. A review of pulmonary arterial hypertension: role of ambrisentan. Vascular health and risk management, 2007, 3(1): 11.