Increased exposure w/ CYP450, UGT1A1 & 1A9, P-glycoprotein/BCRP eg, ketoconazole, itraconazole & HIV PIs eg, ritonavir. Potentiated blood lowering effect of SL nitroglycerin. Additive systemic BP lowering effect w/ sildenafil or vardenafil. Caution w/ cyclosporine A. Lowered oral bioavailability w/ drugs that increase upper GI pH. Reduced mean AUC & Cmax
by antacids eg, Al & Mg hydroxide. Decreased steady-state plasma conc by bosentan & strong CYP3A4 inducers eg, phenytoin, carbamazepine, phenobarb or St. John's wort.