Analytical Separation of Closantel Enantiomers by HPLC

Dual SPE-HPLC-MS/MS Platform for Cross-Class Antiparasitic Surveillance: Simultaneous Quantification of Oxyclozanide and Levamisole Hydrochloride in Ovine Tissues with Applications to Withdrawal Period Optimization

This study presents a novel high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method for simultaneous determination of oxyclozanide (OXY) and levamisole hydrochloride (LEV) residues in ovine tissues, addressing the critical gap in cross-class antiparasitic drug monitoring. Leveraging dual solid-phase extraction strategies-MAX anion-exchange for lipophilic OXY and MCX cation-exchange for hydrophilic LEV-we achieved efficient purification of these pharmacokinetically divergent compounds from complex matrices (muscle, liver, kidney, and perirenal adipose). The method demonstrated superior sensitivity with limits of detection (1.5 μg/kg) and quantification (2.5 μg/kg) below international maximum residue limits (MRLs), validated through Codex Alimentarius guidelines (CAC/GL 71-2009). Linear responses (2.5-1000 μg/kg, R2 > 0.9900) and robust precision (intra-day RSD: 1.44-12.51%; inter-day RSD: 0.29-17.70%) were maintained across spiked concentrations (LOQ, 0.5×, 1×, and 2 × MRLs), with recoveries of 80.94-115.36% confirming matrix-agnostic accuracy. Stability assessments under diverse storage conditions further validated method reliability. Applied to pharmacokinetic profiling in medicated sheep, this protocol established a 28-day withdrawal period for edible tissues, reconciling regulatory compliance with food safety requirements. As the first reported simultaneous quantification platform for OXY and LEV antiparasitics, our methodology advances veterinary residue analytics by enabling efficient multi-class surveillance and evidence-based withdrawal period optimization.

Design of novel chromatographic stationary phases based on long-chain ionic liquid-derived carbon quantum dots, with embedded polar groups and imidazoles, for the separation of multiple compounds

Carbon quantum dots (CQDs) were synthesized from brominated 1-hexadecyl-3-vinylimidazolium and citric acid, and a mixed-mode chromatographic (MMC) stationary phase, which can be used in the RPLC/HILIC/IEC chromatographic modes with a good separation efficiency, was successfully synthesized by click-chemistry bonding to a sulfhydryl-modified silica surface. The successful synthesis of the chromatographic stationary phase was demonstrated by its characterization. The effects of organic phase ratio, column temperature and buffer salts on the column were investigated, established the van't Hoff equation, and verified the ion-exchange effect of the column by separating benzoic acid and phenolic compounds under different pH conditions. The reversed-phase performance of the column was characterized by Tanaka test. Compounds such as alkylbenzenes, polycyclic aromatic hydrocarbons, and nucleosides can all be separated on the column in a short period of time. The reproducibility and stability of the columns were verified under the corresponding chromatographic conditions, and the results showed good reproducibility and stability of the columns. The chromatographic column was used to separate the base nucleosides in apples, and four base nucleosides were successfully isolated from apples.

Synergistic antibacterial effects of closantel and its enantiomers in combination with colistin against multidrug resistant gram-negative bacteria

Drug combinations and repurposing have recently provided promising alternatives to cope with the increasingly severe issue of antibiotic resistance and depletion of natural drug molecular repertoires that undermine traditional antibacterial strategies. Closantel, an effective adjuvant, reverses antibiotic resistance in gram-negative bacteria. Herein, the combined antibacterial enantioselectivity of closantel is presented through separate enantiomer studies. Despite yielding unexpected differences, two closantel enantiomers (R, S) increased colistin activity against gram-negative bacteria both in vitro and in vivo. The fractional inhibitory concentration indices of R-closantel and S-closantel combined with colistin against Pseudomonas aeruginosa, Klebsiella pneumoniae, and Escherichia coli ranged from 0.0087 to 0.5004 and from 0.0117 to 0.5312, respectively. This difference was further demonstrated using growth inhibition assays and time-killing curves. Mechanistically, a higher intracellular concentration of R-CLO is more effective in enhancing the antimicrobial activity of combination. A mouse cutaneous infection model confirmed the synergistic stereoselectivity of closantel. This discovery provides novel insights for developing precision medication and containment of increasing antibiotic resistance.

Chiral separation of racemic closantel and ultratrace detection of its enantiomers in bacteria by enhanced liquid chromatography-tandem mass spectrometry combined with postcolumn infusion of ammonia

Reliable analysis of ultratrace antibiotics in bacterial cells may become a new means to elucidate the antibacterial mechanism, drug resistance and environmental fate. In this work, an ultrahigh-sensitive, accurate and enhanced liquid chromatography-tandem mass spectrometric method was first developed for chiral separation and detection of racemic closantel, as an antibacterial adjuvant. Optimizing acetonitrile-water-formic acid system that is compatible with mass spectrometry as a mobile phase, the baseline separation of two enantiomers was achieved by using EnantioPak® Y1-R chiral column, and the resolution of the two analytes was more than 1.95. Further adopt the strategy of postcolumn infusion of ammonia, the mobile phase pH was reversed from acidic condition suitable for the optimal chromatographic separation of R- and S-closantel to alkaline, so that closantel could realize efficient electrospray ionization under the preferred negative ion mode. The bacterial cells were subjected to be frozen-cracked, and the analytes were extracted with acetonitrile after clipping the pointed bottom of the Eppendorf tube into a new tube. The method was linear over concentration ranges of 0.5-50 pg/mL (r²≥0.99) for R- and S-closantel. The detection limits of target analytes were all 0.15 pg/mL in bacterial cells. The average recoveries of two enantiomers ranged from 81.2% to 107.8% with relative standard deviations below 15%. The method proposed might be important support for the deep research of the stereoselectivity of biological activity, toxicity and metabolism of closantel enantiomers.