Screening of Phosphodiesterase-5 Inhibitors and Their Analogs in Dietary Supplements by Liquid Chromatography-Hybrid Ion Trap-Time of Flight Mass Spectrometry

A novel standard-free detection of adulteration method for sildenafil derivatives in dietary supplements

The rapid and accurate detection of illegal adulteration of chemical drugs into dietary supplements is a big challenge in the food chemistry field. Detection of compounds without a standard reference is even more difficult; however, this is a common situation. Here in this study, a novel "standard-free detection of adulteration" (SFDA) method was proposed and phosphodiesterase-5 inhibitor derivatives were used as an example to figure out the possibility and reliability of this SFDA method. After analysis by quadrupole coupled time of flight-tandem mass spectrometry detection and multivariable statistics, six common fragment ions were chosen to indicate whether adulteration was present or not, while 20 characteristic fragment ions indicated whether adulteration was by nitrogen-containing heterocycles or by anilines. Furthermore, the quantitative methods were conducted by high-performance liquid chromatography-tandem mass spectrometry. In a word, this strategy allows for a quick determination of dietary supplement adulteration without any need for standard materials, improving the efficacy of food safety testing.

Isolation and characterization of a novel tadalafil analogue adulterant, N-cyclohexyl nortadalafil, in a dietary supplement

A novel tadalafil analogue was detected during routine screening of dietary supplements suspected to be adulterated with an erectile dysfunction drug(s) by using ultra-high-performance liquid chromatography coupled to Quadrupole-Exactive Orbitrap mass spectrometry (UPLC‑Q‑Exactive Orbitrap/MS). The MS² spectrum of the compound was highly similar to that of tadalafil within the range of < 300 m/z. The UV spectrum of the compound was almost identical to that of tadalafil. The analogue was purified by semi-preparative HPLC and structurally elucidated by FT-IR, MS and NMR analysis. The analogue was finally determined as N-cyclohexyl nortadalafil, featuring the cyclohexyl group instead of the N-methyl group on the piperazinedione ring of tadalafil.

Hierarchical clustering of liquid chromatography-tandem mass spectrometry data for screening of phosphodiesterase type 5 inhibitors and their analogues in adulterated dietary supplements

Sexual enhancement dietary supplements have often been adulterated with phosphodiesterase type 5 (PDE-5) inhibitors used for treatment of erectile dysfunction, and widely distributed through online markets. As the illegal adulterants, the original PDE-5 inhibitor drugs and a numerous number of synthetized analogues, more than 80, have already been found. Therefore, analytical methods that detect various PDE-5 inhibitors and uncover newly synthesized analogues are needed. In this study, we have developed a rapid and reliable screening method for PDE-5 inhibitors and their structural analogues by using liquid chromatography-tandem mass spectrometry (LC-MS/MS) followed by hierarchical clustering based on similarity of MS/MS spectra. Forty reference standards of PDE-5 inhibitors/analogues were measured using a quadrupole-orbitrap mass spectrometer in data-dependent mode. The 60 most intense fragment ions were extracted from each MS/MS spectra, and the ions observed within 1.5 mDa mass tolerance were considered to be the same ion. Based on fragment ion tables representing detected ions for each compound, hierarchical clustering was performed. The resulting dendrogram showed that the reference standards were separated into seven clusters according to their characteristic structures. Subsequently, two additional standards spiked into a herbal sample were analyzed. While herbal components were clearly separated from the clusters of the reference standards, the spiked standards were clustered closely with the structurally similar standards. Furthermore, application of our method to dietary supplements allowed for detection of sildenafil and tadalafil as adulterants. These results suggest that our screening method facilitates discovery of adulterant PDE-5 inhibitors/analogues by illustrating their structural similarity.