Strategies for characterizing sildenafil, vardenafil, tadalafil and their analogues in herbal dietary supplements, and detecting counterfeit products containing these drugs

The success of synthetic phosphodiesterase type-5 (PDE-5)-inhibitor drugs (viz sildenafil, vardenafil and tadalafil), which are constituents of popular brands (viz Viagra, Levitra and Cialis, respectively) for the treatment of erectile dysfunction in males, has led to their widespread use as adulterants in herbal dietary supplements (HDSs). There have been reports that not only these three approved drugs but also their unapproved analogues have been found in HDSs. The problem is becoming more complex, as concealed, structurally modified analogues are increasingly being used. Also, counterfeits of the popular brands have emerged. Fortunately, it has become possible to detect these drugs and their derivatives as adulterants and counterfeits by using modern sensitive and selective analytical techniques [e.g., liquid chromatography with tandem mass spectrometry, Fourier transform (FT) with near infrared spectrometry, and FT with Raman spectroscopy], although some conventional approaches have also been employed. We critically review the literature, and present generalized strategies, including flow charts, for characterizing adulteration of PDE-5 inhibitors in HDSs, and detecting and categorizing counterfeit products. (c) 2008 Elsevier Ltd. All rights reserved.