Sildenafil, trade name VIAGRA^TM, chemical name 5-[2-ethoxy-5-(4-methylpiperazin-1-ylsulfonyl)phenyl]-1- methyl-3-propyl-1,6-dihydro-7H-pyrazolo[4,3-d]pyrimidin-7-one, formula C₂₂H₃₀N₆O₄S, was discovered through a rational drug design programme by the pharmaceutical company Pfizer. (N. K. Terrett, A. S. Bell, D. Brown, P. Ellis Bioorganic & Medicinal Chem. Lett., 1996, 6, pp.1819-1824).