Influence of diluent volume of colistimethate sodium on aerosol characteristics and pharmacokinetics in ventilator-associated pneumonia caused by MDR bacteria

Objectives: Nebulized colistimethate sodium (CMS) can be used to treat ventilator-associated pneumonia caused by MDR bacteria. The influence of the diluent volume of CMS on aerosol delivery has never been studied. The main objectives of the study were to compare aerosol particle characteristics and plasma and urine pharmacokinetics between two diluent volumes in patients treated with nebulized CMS. Methods: A crossover study was conducted in eight patients receiving nebulized CMS every 8 h. After inclusion, nebulization started with 4 million international units (MIU) of CMS diluted either in 6mL (experimental dilution) or in 12 mL (recommended dilution) of normal saline in a random order. For each diluent volume, CMS aerosol particle sizes were measured and plasma and urine samples were collected every 2 h. Nebulization time and stability of colistin in normal saline were assessed. Results: The mass median aerodynamic diameters were 1.4 +/- 0.2 versus 0.9 +/- 0.2 mu m (P < 0.001) for 6 and 12 mL diluent volumes, respectively. The plasma area under the concentration-time curve from 0 to 8 h (AUC(0-8)) of colistin(A+B) was 6.6 (4.3-17.0) versus 6.7 (3.6-14.0) mu g.h/mL (P = 0.461) for each dilution. The total amount of colistin and CMS eliminated in the urine represented, respectively, 17% and 13% of the CMS initially placed in the nebulizer chamber for 6 and 12mL diluent volumes (P = 0.4). Nebulization time was shorter [66 (58-75) versus 93 (69-136) min, P = 0.042] and colistin stability was better with the 6 mL diluent volume. Conclusions: Nebulization with a higher concentration of CMS in saline (4 MIU in 6 mL) decreases nebulization time and improves colistin stability without changing plasma and urine pharmacokinetics or aerosol particle characteristics for lung deposition.