The significance of different formulations of aerosolized colistin

There are two different forms of colistin available for clinical use. Colistin sulfate is administered orally for bowel decontamination and is administered topically as a powder for the treatment of bacterial skin infections; and colistimethate sodium (also called colistin methanesulfate, pentasodium colistimethanesulfate, colistin sulfamethate, and colistin sulfonyl methate) is administered intravenously and intramuscularly [3]. It is obvious that the terminology regarding the different formulations of colistin may be confusing. Colistimethate sodium is produced by a sulfomethylation reaction of colistin in which the primary amine groups of L-α-γ-diaminobutyric acid are reacted with formaldehyde followed by sodium bisulfite [4].Both formulations of colistin (colistin sulfate and colistimethate sodium) have been used for aerosol treatment. However, colistimethate sodium is associated with fewer adverse effects such as chest tightness, throat irritation, and cough compared with colistin sulfate [5]. The formulation of colistin that was administered to our patients was therefore colistimethate sodium (i.e. the less toxic form of the drug), not colistin sulfate. In fact, the exact trade names of colistin that were administered to our patients are stated in our paper [2].S Mubareka and E RubinsteinWe would like to thank the authors for their response to our editorial [1]. We acknowledge that colistimethate may be better tolerated from a respiratory point of view. What remains unknown, however, is the systemic absorption of inhaled colistin in critically ill patients, particularly in those with pneumonia in whom the barrier between the alveolar cell layer and the vascular system may be damaged. It is appreciated that this parameter may not be determined in a retrospective study, and these preliminary results suggest that further research is called for.High-pressure liquid chromatography has been used to measure serum levels of colistin in humans [6]. Appreciable limitati