Pharmacokinetics of protein-unbound linezolid in the blood and the mechanism of hepatobiliary excretion in the rat

Linezolid (Zyvox), an oxazolidinones antibiotic, was developed for the treatment of infectious diseases caused by gram-positive pathogens. To investigate the mechanism of hepatobiliary excretion of linezolid, a parallel study design used two groups; in the control group, rats received linezolid alone (3 or 10 mg/kg, i.v.). In the drug-treated groups, 10 min prior to linezolid administration, cyclosporin A (CsA; 10 mg/kg, i.v.), a P-glycoprotein (P-gp) inhibitor, was given in the rats. The microdialysis probes were implanted into the jugular vein toward right atrium and the bile duct of Sprague-Dawley rats for multiple biological fluid sampling. Separation was performed using a reversed phase C₁₈ (4.6 mm × 150 mm i.d., 5 μm) with mobile phase of acetonitrile-methanol-1% 1-octanesulfonic acid in water of 30:10:60 (v/v/v) at flow rate of 1 ml/min. The UV detection for linezolid was set at a wavelength of 260 nm. Following linezolid (10 mg/kg, i.v.) administration, the concentration of linezolid in the brain was less than the limit of quantification and the area-under the concentration curve versus time curve (AUC) of blood and bile were 1780 ± 50 and 2850 ± 276 (min μg/ml), respectively. The bile-to-blood distribution ratio was 1.6 ± 0.2 (n = 6), which was defined as AUC_bile/AUC_blood. The results demonstrated that the transportation of linezolid into bile might be mediated by active transport. However, after treatment with CsA, the linezolid AUC in bile was 3060 ± 411 (min μg/ml) which did not indicate a significant difference with linezolid alone. These results suggest that the hepatobiliary excretion of linezolid might not be regulated by P-gp transportation.