"One-shot" analysis of PDE-5 inhibitors and analogues in counterfeit herbal natural products using an LC-DAD-QTOF system

A highly selective and robust method for simultaneous screening and confirmation of target and non-target phosphodiesterase type 5 (PDE-5) inhibitor analogues within a single chromatographic run in counterfeit herbal products was developed. The protocol, based on an easy and rapid extraction with a water/acetonitrile 1 % formic acid solution, followed by sonication and centrifugation, exploits an LC-diode array detector-quadrupole-time-of-flight (DAD-QTOF) system. The extraction method was optimized both at high concentrations and at trace levels. These two situations are typically encountered in pharmaceutical formulations and herbal food supplements. Carryover effects, never reported before and occurring mainly for vardenafil, were overcome using a polymer-based column. An in-house validation was carried out using five blanks of different bulk matrices spiked with seven standard analytes, namely yohimbine, sildenafil, vardenafil, tadalafil, homosildenafil, pseudovardenafil and hydroxyhomovardenafil. Reliable quantitation was possible using a conventional standard solution for all the pharmaceutical and herbal samples considered, as matrix effects were eliminated. Accuracy ranged from 80.9 to 108.1 % with overall relative standard deviation (RSD) <11 % (N = 15), measured at 1.0, 5.0 and 10.0 μg/g. Limits of detection (LODs) obtained ensured the determination of cross contaminations at nanogram per gram levels. A database with 82 PDE-5 inhibitor analogues was implemented for automatic non-target analysis. Among the 26 samples of dietary supplements and herbal remedies bulk marketed for erectile dysfunctions, three samples were found to be contaminated with both registered and unregistered synthetic PDE-5 inhibitors, i.e. yohimbine, sildenafil, dimethylsildenafil and thiodimethylsildenafil or thiomethisosildenafil. The occurrence of such contaminations, both at trace levels and at pharmaceutical dosage, indicates the illicit use of synthetic PDE-5 analogues. Graphical Abstract Examples of pharmaceutical formulations and herbal natural products marketed for the erectile dysfunction.