Analytical Separation of Closantel Enantiomers by HPLC

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.