Nickel(II)-assisted enantiomeric differentiation and quantitation of tadalafil by direct electrospray ionization mass spectrometry

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.

Diastereomer recognition of three pairs of tetracyclines by electrospray ionization mass spectrometry

RATIONALE

Stereoisomer profiling is always a difficult issue. Based on the difference between diastereomers, usually because of steric hindrance, isomers can be differentiated by mass spectrometry (MS), although it is often not an easy task. In the current study, tetracycline, chlortetracycline and doxycycline could be distinguished from their respective 4-epimers by MS.

METHODS

The electrospray ionization tandem mass spectrometry (ESI-MSⁿ ) analyses were carried out on a Bruker 3000^plus ion trap mass spectrometer. For MS/MS experiments, the collision energy was set between 0.18 and 0.45 V to perform energy-resolved mass spectrometry (ERMS). Test solutions were prepared in methanol/water (90:10, v/v) at a concentration of 10 μg/mL.

RESULTS

Compared with the collision-induced dissociation (CID) spectrum of protonated tetracycline, the most abundant peak changed from m/z 427 to m/z 410 for 4-epitetracycline. For chlortetracycline and its 4-epimer, differences in relative abundance were observed too. In the CID spectrum of a fragment ion of doxycycline, the abundance of m/z 154 was relatively higher than for the 4-epimer, showing the same trend as in the CID spectra of the other two pairs of tetracyclines.

CONCLUSIONS

The CID spectra of tetracycline and chlortetracycline were different from those of their 4-epimers. The CID spectra of protonated doxycycline and its 4-epimer showed only a subtle difference, but the m/z 154 fragment ion in the CID spectra of the fragment ion at m/z 428 offers the possibility to differentiate both epimers.

Interpol review of controlled substances 2016-2019

This review paper covers the forensic-relevant literature in controlled substances from 2016 to 2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/content/download/14458/file/Interpol%20Review%20Papers%202019.pdf.

Direct differentiation of stereoisomers of ezetimibe/ambrisentan/atorvastatin and their mechanism study by electrospray ionization quadrupole time-of-flight mass spectrometry

A mass spectrometric method is introduced for rapid and accurate chiral quantification by examining a trimeric metal complex into which a chiral reference is incorporated with the analyte. Several metal ions (Cu^II , Ni^II , Mg^II , Mn^II , Co^II , and Zn^II ) were selected as the central metal ion, and chiral drugs ezetimibe (EZM) and ambrisentan (AMB) were used as the reference to each other for isomeric differentiation by using electrospray ionization quadrupole time-of-flight mass spectrometry. Doubly charged trimeric cluster ions instead of the singly charged clusters were applied in this study. Kinetic method (KM) and chiral recognition (CR) method were used for construction of a calibration curve for chiral quantitation. The results from the two methods were found to be complementary to each other, which improved quantitative analysis of stereoisomers for EZM. Furthermore, we have successfully used S-AMB as reference for the chiral differentiation of enantiomeric atorvastatin (ATO), which is frequently combined with EZM as a codrug. Experimental results showed that the binary mixture of EZM and ATO enantiomers can be determined simultaneously without prior separation steps. The direct measurement of chiral purity within 5% was demonstrated. This mass spectrometric method represents an effective alternative to commonly used chromatographic techniques as means of chiral purity determination and is of potential use in rapid screening experiments.

Chiral detection of entecavir stereoisomeric impurities through coordination with R-besivance and ZnII using mass spectrometry

In this study, a mass spectrometry (MS)-based kinetic method (KM) is shown to be successful at analyzing a multichiral center drug stereoisomer, entecavir (ETV), both qualitatively and quantitatively. On the basis of the KM, the bivalent complex ion [M^II (A)(ref*)₂ ]²⁺ (M^II  = divalent metal ion, A = analyte, and ref* = chiral reference) was set as precursor ion in MS/MS. The experiment results suggest strong chiral selectivity between ETV and its isomers when using Zn^II coordinated with the chiral reference R-besivance (R-B). The logarithm of the fragment ion abundance ratio and the enantiomeric percentage (%) exhibits a strong linear relation because of the competitive loss of the reference and analyte. The product ion pair [Zn^II (R-B)A-H]⁺ (m/z 733) and [Zn^II (R-B)₂ -H]⁺ (m/z 849), together with [R-B + H]⁺ (m/z 394) and [A + H]⁺ (m/z 278), can realize the identification of ETV and all of its chiral isomers. Theoretical calculation were also performed using the B3LYP functional with the 6-31G* and LanL2DZ basis set to clarify the mechanism of structural difference of these bivalent complex ions. The results reveal that MS-KM can be used to detect optical impurities without a chiral chromatographic column and fussy sample pretreatment. The established method has been used to determine stereoisomeric impurities of less than 0.1% in ETV crude drug, a demonstration of its simple and effective nature for rapid detection of stereoisomeric impurities.

Chiroptical and vibrational spectroscopic study of genuine and counterfeit medicines containing tadalafil

The production and distribution of counterfeit pharmaceuticals present a serious problem worldwide. This is true especially in case of phosphodiesterase type 5 inhibitors for treating erectile dysfunction, where consumers often prefer buying them anonymously from unverified sources. In this study, genuine and counterfeit Cialis® 20 mg tablets were analyzed by electronic circular dichroism, vibrational circular dichroism, and infrared spectroscopy. The characteristic spectral patterns were identified by comparison with the spectra of tadalafil standard as an active pharmaceutical ingredient, and its presence was confirmed in all samples. The amount of tadalafil, however, was markedly lower in the case of counterfeit tablet as the observed band intensities were considerably lower. No other significant differences between the genuine and counterfeit tablets were revealed. Ab initio density functional theory calculations provided a detailed description of the stable conformers of tadalafil in a solution and enabled thorough interpretation of the experimental spectra.