Functional estimation of endothelin-1 receptor antagonism by bosentan, macitentan and ambrisentan in human pulmonary and radial arteries in vitro

Background: Endothelin receptor antagonists are approved for pulmonary arterial hypertension. Development of selective ETA-receptor antagonists over mixed or dual receptor antagonists has depended on a range of receptor binding assays, second messenger assays and functional blood vessel assays. This study compared the 3 clinically-approved endothelin receptor antagonists in assays of human isolated pulmonary and radial arteries in vitro. Methods: Human isolated pulmonary (i.d. 5.5 mm) and human radial (i.d. 3.23 mm) artery ring segments were mounted in organ baths for isometric force measurement. Single concentration-contraction curves to endothelin-1 were constructed in the absence or presence of bosentan (1-10 mu M), macitentan (0.03-0.3 mu M) or ambrisentan (0.1-1 mu M). Results: All 3 endothelin antagonists caused competitive rightward shifts in the endothelin-1 concentration response curves in both arteries. The Clark plot and analysis gave the following pK(B) values: bosentan, pulmonary artery 6.28 +/- 0.13 and radial artery 6.04 +/- 0.10; macitentan, pulmonary artery 8.02 +/- 0.13 and radial artery 7.49 +/- 0.08; and ambrisentan, pulmonary artery 7.38 +/- 0.13 and radial artery 6.96 +/- 0.10. Conclusions: Noting the maximum plasma levels attained from recommended oral doses of each antagonist in volunteers, the pK(B) findings here show that there would be significant antagonism of endothelin-1 contraction in the pulmonary and radial arteries at therapeutic plasma levels. This functional assay confirms in human tissue that much higher plasma concentrations of endothelin-1 receptor antagonists are required to be effective than those predicted from binding or other biochemical assays.