Pharmacokinetics and efficacy of linezolid in a gerbil model of Streptococcus pneumoniae-induced acute otitis media.

The oxazolidinone linezolid represents a new antibacterial class of potential benefit in managing multidrug-resistant gram-positive infections, including those caused by Streptococcus pneumoniae. In a gerbil model of acute otitis media (AOM) induced by either penicillin-resistant S. pneumoniae (PRSP; amoxicillin MIC = 8 micro g/ml, linezolid MIC = 1 micro g/ml) or penicillin-susceptible S. pneumoniae (PSSP; amoxicillin MIC = 0.015 micro g/ml, linezolid MIC = 1 micro g/ml), we explored the plasma and ear fluid levels of linezolid required to demonstrate efficacy. Threshold pathogen doses required to induce bilateral AOM (1,500 CFU/ear with PRSP; 30 CFU/ear with PSSP) were administered to gerbils by intrabullar injection on day 0. At peak infection ( approximately 10(6) to 10(7) CFU/ear flush; day 2 for PRSP-AOM and day 3 for PSSP-AOM), twice-a-day oral doses of linezolid, amoxicillin, or vehicle were administered over 4.5 days prior to collection and assay of middle ear effluents for S. pneumoniae content. Linezolid doses of >/=10 mg/kg of body weight induced significant cure rates of >/=72% versus both PRSP and PSSP infections, whereas amoxicillin at </=100 mg/kg was consistently effective only versus PSSP-AOM. Plasma and ear fluid levels of linezolid necessary to elicit pneumococcal eradication from the middle ear were measured by high-performance liquid chromatography-tandem mass spectrometry and found to be similar both within and between each infection protocol. The plasma-ear fluid pharmacodynamic profile associated with linezolid efficacy was a T>MIC of >/=42%, a C(max)/MIC ratio of >/=3.1, and a (24-h area under the curve)/MIC ratio of >/=30 h. Application of this model will be useful in defining preclinical pharmacodynamic relationships of novel antibiotics necessary to cure S. pneumoniae-induced AOM.