Bioequivalence and Pharmacokinetic Study of Sildenafil in Healthy Thai Male Volunteers

Background: Sildenafil is a popular drug used for improving penile erectile function that has been commercially available through several manufacturers and distributors in Thailand. Therefore, it is necessary to study bioequivalence of the drugs obtained from the original manufacture and from a local manufacturer to ascertain that they can be medicated interchangeably.Objective: To determine whether two sildenafil preparations: Test (Erec®, Unison Laboratories, Co., Ltd., Thailand) and reference, (Viagra®, Pfizer Pty Limited., Australia) are bioequivalent.Design: Single oral dose and double-blind randomized two-way crossover.Population and samples: Fifteen healthy Thai male volunteers.Setting: Department of pharmacology, and Srinagarind Hospital, Faculty of Medicine, Khon Kaen University.Methods: The subjects received either 100 mg of the reference or test formulation. Blood samples were collected from catheter at several time points after sildenafil administration up to 12 hours. The bioequivalence between the two formulations was assessed by comparison of the peak plasma concentrations (Cmax) and area under the curve of time, from 0 to the last measurable concentration (AUC0-t last).Results: All subjects were well tolerated and presented no serious side effect. Statistical analysis revealed that the 90% confident intervals (CI) for the ratios between test and reference drugs of the log transformed the Cmax (0.8377-1.1985) and AUC0-t last (0.8610-1.1590), are within the Food and Drug Administration Guideline range of bioequivalence (0.80 to 1.25).Conclusions: It can be concluded that the 100 mg formulation of Test (Erec®) is bioequivalent to the Reference.Keywords: sildenafil, bioequivalence