The identification of (−)-trans-tadalafil, tadalafil, and sildenafil in counterfeit Cialis® and the optical purity of tadalafil stereoisomers

Four blisters with suspect Cialis® (tadalafil) 20 mg tablets were screened for authenticity using near infrared spectroscopy (NIRS) and for the presence of phosphodiesterase 5 (PDE-5) inhibitors using LC-DAD–MS. All samples were identified as counterfeit Cialis® and contained sildenafil or a combination of tadalafil and sildenafil. Although the tablets contained efficacious amounts of PDE-5 inhibitors, neither the active ingredient nor the dosage corresponded to the description on the blister. This is the first reported case of a diastereomeric mixture of tadalafil and trans-tadalafil (3:1) being identified in a counterfeit medicine. The LC-DAD-CD revealed that both diastereomers had a high optical purity. The optical rotation of the diastereomeric mixture was measured indicating the presence of (−)-trans-tadalafil, which is the only other stereoisomer with some PDE-5 inhibitory activity.As no safety profiles are known for the stereoisomers of tadalafil, there is a potential health risk. In addition, the optical purity of tadalafil needs to be taken into account when calculating the dosage in illegal medicines.