ent-Tadalafil (ent-IC-351), compound (6S,12aS), is a inactive cis-enantiomer of compound (6R,12aS). compound (6R,12aS) is a potent PDE5 inhibitor with an IC50 of 0.090 μM, while ent-Tadalafil is inactive at concentrations up to 10 µM[1].
[1]. Daugan A, et al. 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.J Med Chem. 2003 Oct 9;46(21):4533-42.