Treprostinil, a stable prostacyclin analogue used in the treatment of pulmonary arterial hypertension, is in development as a sustained release oral tablet, treprostinil diolamine (United Therapeutics Corp, Research Triangle Park, NC). As combination therapy yields additional benefit in pulmonary arterial hypertension, treprostinil diolamine may be used with sildenafil, a phosphodiesterase-5 inhibitor. This study was designed to evaluate the presence of a pharmacokinetic drug interaction between treprostinil diolamine and sildenafil. Treprostinil is primarily metabolized by cytochrome (CYP) P450 2C8 with minor contribution from CYP2C9. Sildenafil is metabolized by CYP3A4 with minor contribution from CYP2C9. Eighteen healthy volunteers were randomized to receive 4.5 days each of (1) treprostinil diolamine alone, (2) sildenafil alone, and (3) combination treprostinil diolamine and sildenafil in an open-label, 3-period, 3-sequence crossover study. The geometric mean ratio (90% confidence intervals) for combination/agent alone of steady state area under the concentration-time curve and peak concentration (Cmax) were 0.972 (0.824-1.145) and 1.030 (0.900, 1.1-9), respectively, for treprostinil diolamine and were 0.881 (0.804-0.966) and 0.910 (0.876-0.946), respectively, for sildenafil. The results suggest lack of a metabolic interaction between treprostinil diolamine and sildenafil, as geometric mean ratio 90% confidence intervals were within 0.8-1.25. Combination therapy was well tolerated but had slightly higher rates of nausea, headache, and extremity pain than monotherapy.