the u.s. fda approved sildenafil, a selective inhibitor of cgmp specific phosphodiesterase type 5 (pde5), for the treatment of pah in 2005. it is marketed for pah as revatio. in 2009, they also approved tadalafil, another pde5 inhibitor, marketed under the name adcirca. pde5 inhibitors are believed to increase pulmonary artery vasodilation, and inhibit vascular remodeling, thus lowering pulmonary arterial pressure and pulmonary vascular resistance.