Synthesis and characterization of related substances and metabolite of tadalafil, a PDE-5 inhibitor

Tadalafil (Cialis) is a potent and selective inhibitor of cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type-5 (PDE-5) and it is administered orally for the treatment of erectile dysfunction (ED). Two synthetic schemes were evaluated to study the impurity profile of tadalafil. Six impurities were detected in the bulk substance (prepared by two methods) at a level of 0.1-0.15%. Identification, synthesis, characterization of impurities (related substances) and metabolite and origin of their formation is described .