Development of an LC-MS/MS method for quantification of colistin and colistin methanesulfonate in human plasma and its application to stability studies and therapeutic drug monitoring

Introduction

Colistin serves as the last line of defense against multidrug-resistant Gram-negative bacterial infections and is commonly administered in clinical practice as its prodrug, colistin methanesulfonate (CMS). However, due to its notable nephrotoxicity and narrow therapeutic window, therapeutic drug monitoring (TDM) is essential.

Objectives

To develop an optimal LC-MS/MS method for the quantification of colistin and CMS in human plasma and to apply it to stability studies and TDM.

Methods

Colistin A, colistin B, and internal standard (IS, polymyxin B2) were extracted from plasma using solid phase extraction columns. Sample separation was performed using a Welch Ultimate LP-C18 column with a 5-minute gradient elution consisting of water and acetonitrile, both supplied with 1.0% formic acid. The CMS concentration was obtained by comparing the total amount of colistin in acid-hydrolyzed and non-acid-hydrolyzed plasma.

Results

Colistin A and colistin B showed excellent linearity in the concentration range of 0.1–10.0 μg/mL (R² > 0.995) with acceptable specificity, accuracy (90.97 %–114.65 %), precision (RSD < 15 %), matrix effect (RSD < 15 %), and recovery (91.93 %–100.93 %). CMS in five commonly used clinical infusion solutions was stable when stored at room temperature for 8 h or at 4 °C for 24 h. The whole blood and plasma samples of CMS are susceptible to degradation at room temperature but are stable on ice. Plasma concentrations of colistin and CMS were accurately determined in three critically ill patients.

Conclusion

The method we have developed is robust and streamlined, and has successfully demonstrated the potential feasibility for future TDM applications of colistin and CMS in critically ill patients.