Biopharmaceutical Characterization of Nebulized Antimicrobial Agents in Rats: 2. Colistin

The purpose of this study was to investigate the pharmacokinetic properties of colistin following intrapulmonary administration of colistin sulfate in rats. Colistin was infused or delivered in nebulized form at a dose of 0.35 mg/kg of body weight in rats, and plasma drug concentrations were measured for 4 h after administration. Bronchoalveolar lavages (BAL) were also conducted at 0.5, 2, and 4 h after intravenous (i.v.) administration and administration via nebulized drug to estimate epithelial lining fluid (ELF) drug concentrations. Unbound colistin plasma concentrations at distribution equilibrium (2 h postdosing) were almost identical after i.v. infusion and nebulized drug inhalation. ELF drug concentrations were undetectable in BAL samples after i.v. administration, but they were about 1,800 times higher than unbound plasma drug levels at 2 h and 4 h after administration of the nebulized drug. Simultaneous pharmacokinetic modeling of plasma and ELF drug concentrations was performed with a model characterized by a fixed physiological volume of ELF (V-ELF), a passive diffusion clearance (Q(ELF)) between plasma and ELF, and a nonlinear influx transfer from ELF to the central compartment, which was assessed by reducing the nebulized dose of colistin by 10-fold (0.035 mg kg(-1)). The k(m) was estimated to be 133 mu g ml(-1), and the V-max, in-to-K-m ratio was equal to 2.5 x 10(-3) liter h(-1) kg(-1), which was 37 times higher than the QELF (6.7 x 10(-5) liter h(-1) kg(-1)). This study showed that with the higher ELF drug concentrations after administration via nebulized aerosol than after intravenous administration, for antibiotics with low permeability such as colistin, nebulization offers a real potential over intravenous administration for the treatment of pulmonary infections.