Optimization of the aminopyridopyrazinones class of PDE5 inhibitors: Discovery of 3-[(trans-4-hydroxycyclohexyl)amino]-7-(6-methoxypyridin-3-yl)-1-(2-propoxyethyl)pyrido[3,4-b]pyrazin-2(1H)-oneRobert Hughes, Ying Yu, Jerry W. Cubbage et al.|Bioorganic & Medicinal Chemistry Letters|2009Cited by 15