2-(4-Aminophenyl)-1,2-dihydro-1-oxo-7-(2-pyridinylmethoxy)-4-(3,4,5-trimethoxypheny Tl)-3-isoquinolinecarboxylic Acid Dihydrochloride, is a selective substrate for cytochrome P450 3A5 (CYP3A5). These compounds are also 4-Aryl-1-isoquinolinone Derivatives, which are shown to be novel, potent, and selective Phosphodiesterase 5 (PDE5) Inhibitors.