Evaluation of Pharmacokinetic and Pharmacodynamic Properties of Intravenous Colistimethate Sodium

Colistin, also known as polymixin E, is a peptide antibiotic that demonstrates concentration-dependent bactericidal killing against gram-negative pathogens including Pseudomonas and Acinetobacter. Originally discovered in the 1950s, its use has been limited due to the concern for its systemic toxicities including nephrotoxicity and neurotoxicity. However, a revival of colistin use has been seen in recent years due to the increased emergence of multidrug-resistant (MDR) gram-negative pathogens and a lack of alternative antimicrobial therapies. Colistin is administered intravenously as colistimethate sodium (CMS), an inactive prodrug that lacks antibacterial activity, and is hydrolyzed into its active form in plasma as colistin A and colistin B. Prior pharmacokinetic studies have demonstrated a wide range of pharmacokinetic/pharmacodynamics (PK/PD) findings for colistin. Variability in results have been attributed to an unreliable differentiation between colistin and CMS plasma concentrations with the use of microbiological assays, in addition to procedures that resulted in substantial in vitro hydrolysis of CMS to colistin during sample preparation. Refined laboratory methods such as high-performance liquid chromatography (HPLC) have circumvented such issues and provided opportunities for further pharmacokinetic profiling. Given the scarcity of its use and challenges with quantitative assays, the pharmacokinetic and pharmacodynamic profile of colistin has been poorly characterized particularly amongst cystic fibrosis and critically ill patients. Determination of such parameters in these patient populations is vital to optimize the maximum concentration to minimum inhibitory concentration (Cmax:MIC) ratio for maximal efficacy, minimize adverse effects and to reduce the development of bacterial resistance. Previous studies examining the steady-state pharmacokinetics of colistin in cystic fibrosis and critically ill patients have concluded that based on the in vitro pharmacodynamics against Pseudomonas aeruginosa, current dose recommendations may be inadequate to achieve desirable Cmax:MIC ratios and dose escalations may be warranted. The investigators study aims to determine steady-state colistin A and colistin B concentrations in cystic fibrosis and critically ill patients, evaluate pharmacodynamic parameters and relationship to microbiologic success, and monitor for the incidence of nephrotoxicity. The findings of this pilot study will be used to develop dosing recommendations for future pharmacokinetic studies of colistimethate sodium in cystic fibrosis and critically ill patients.