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Daugan A, Grondin P, Ruault C, Le Monnier de Gouville AC, Coste H, Linget JM, et al. (October 2003). "The discovery of tadalafil: a novel and highly selective PDE5 inhibitor. 2: 2,3,6,7,12,12a-hexahydropyrazino[1',2':1,6]pyrido[3,4-b]indole-1,4-dione analogues". Journal of Medicinal Chemistry. 46 (21): 4533–4542. doi:10.1021/jm0300577. PMID14521415.