Immunoassay for Colistin Monitoring in Critically Ill Patients Receiving Colistin Methanesulfonate Therapy

Background/Objectives: Colistin (COL), administered as a prodrug colistimethate sodium (CMS), is commonly used to treat infections caused by multidrug-resistant Gram-negative bacteria in critically ill patients. Given high CMS instability, very complex and variable pharmacokinetics (PK) and high incidence of toxicity, therapeutic drug monitoring (TDM) of active COL might play an important role. This study aimed to develop and validate an accessible immunoassay-based approach for COL monitoring in human serum. Methods: A direct competitive enzyme-linked immunosorbent assay (dcELISA) was developed using polyclonal (pAb) anti-polymyxin antibody alongside a polymyxin B-horseradish peroxidase conjugate. CMS conversion to COL along with serum deproteinization was achieved using 5% trichloroacetic acid (TCA) treatment at 37 °C. Assay accuracy and precision were assessed by spike-and-recovery experiments in healthy volunteer serum. The assay was applied to serum samples from critically ill patients with burns or pneumonia receiving CMS therapy. The reliability of the measurements was confirmed by parallel dcELISA based on a reference monoclonal antibody (mAb) against fragmented polymyxin molecule. Results: Both ELISA formats demonstrated high sensitivity, with limits of detection of 0.053 ng/mL (pAb) and 0.047 ng/mL (mAb). TCA treatment achieved maximal CMS hydrolysis under tested conditions within one hour. Clinical sample analysis showed excellent agreement between the two assays (R² = 0.996), with Bland-Altman analysis revealing a minimal bias of 3.7%. Exploratory PK analysis in burn patients demonstrated increased total drug volume of distribution (45.7-64.9 L) and clearance (8.3-16.3 L/h). Conclusions: This is the first report of ELISA for COL TDM in critically ill patients. The method offers acceptable analytical performance and practical simplicity, with potential to broaden TDM access beyond specialist centers.