Establishing a liquid chromatography-mass spectrometry method for the quantification of colistin A, a last line of defense against antibiotic resistance

Antibiotic resistance has become an increasingly pressing issue in recent times. With a dwindling discovery rate of new antibacterials, gram-negative bacteria threaten to return humanity to the pre-antibiotic era where mere paper cuts could result in fatal outcomes. The antimicrobial peptide colistin is our present-day last line of defense against gram-negative infections. Nevertheless, despite the recent surge in use and research on colistin, research on its biological effects and chemical behavior remains somewhat scarce. Colistin was initially abandoned due to numerous reports of neuro- and nephrotoxicity. Slow leakage from colistin-loaded liposomes may reduce this toxicity significantly. In this work, an LC-MS method for separating and quantifying colistin and polymyxin B1 was developed to measure colistin leakage from loaded liposomes over time. Isocratic elution provided adequate separation of polymyxin E1 and the internal standard, polymyxin B1, and measurements were done using a triple-quadrupole MS. Widely unreported and uninvestigated adsorption and degradation mechanisms of colistin were observed and addressed by adding a 100 mM ammonium formate buffer at pH 3 to solutions of colistin. The LC-MS method provides a reliable and accurate approach for measuring colistin at lower concentrations (< 10 µg/mL) in ammonium formate buffered solutions, and the future challenges and potential approaches for further research upon colistin are discussed. The LC-MS method described serves as a solid platform for further research on colistin, colistin toxicity negation, and more may be built upon