Modeling of macitentan (A) and bosentan (B) binding to the active site of the ET_A receptor.

The amino acid residues predicted to contact macitentan and/or bosentan are grouped into the categories “nearest neighbors”, “ERA charge interaction” and “extended ERA binding pocket”. (C) Conformations of macitentan (red) and bosentan (green) bound to ET_A as proposed by molecular modeling. Relevant structural differences are (1) the different head groups, (2) the length of the central rigid axis spanning over the head group and either one (macitentan) or two (bosentan) pyrimidines, (3) the presence or absence of the 5-bromo-pyrimidine stacking onto the core pyrimidine.