Qualitative and quantitative analysis of PDE-5 inhibitors in counterfeit medicines and dietary supplements by HPLC-UV using sildenafil as a sole reference.

Due to their popularity, medicinal products containing the phophodiesterase type 5 enzyme (PDE-5) inhibitors sildenafil, vardenafil and tadalafil are often subject to counterfeiting. In addition, illicit herbal dietary supplements adulterated with these substances or their analogs have appeared on the market offering an easy and anonymous sale. This paper describes an analytical method for qualitative and quantitative screening of sildenafil, vardenafil, tadalafil and 11 of their designer analogs in illegal erectile dysfunction products by high-performance liquid chromatography with UV detection (HPLC-UV). Sildenafil served as a single external standard for both identification and quantification of all analytes. Relative retentions and reference UV spectra were used for qualitative, and correction factors for quantitative analyses, respectively. The separation was performed on a Kinetex C18 reverse-phased column at 25°C using gradient elution. Mobile phase A consisted of 200mM ammonium acetate solution while mobile phase B was a 1:1 (v/v) mixture of methanol and acetonitrile with a flow rate of 0.5ml/min and injection volume of 5μl. Detection wavelength was set to 290nm. The method was validated in accordance with the appropriate guideline of the International Conference on Harmonization (ICH) in terms of specificity, selectivity, precision, linearity, limit of quantitation, limit of detection, accuracy, robustness and stability, and was successfully applied to the analysis of natural dietary supplements and herbal remedies with an indication for enhanced male sexual potency. The proposed method offers a cheap and simple alternative to LC-MS screening used by control laboratories for routine analysis of suspicious products.