Pharmacokinetics of Linezolid in Children With Cystic Fibrosis

Inclusion Criteria: * Subjects \< 18 years of age inclusive, with a confirmed diagnosis of cystic fibrosis being admitted to the hospital for acute pulmonary exacerbation with MRSA isolated from sputum culture. * Female subject of childbearing potential must have a negative pregnancy test prior to the first dose of study drug, and if sexually active agrees to use an acceptable method of birth control per investigator judgment for the duration of the study. * Subjects who are receiving medications with serotonergic (such as certain types of antidepressants) and adrenergic activity that can not be discontinued based on clinical judgment of the primary physician may be enrolled. These subjects will be monitored closely for serotonin- and sympathomimetic-associated toxicity. * Subject (when able) and subject's parent /legal guardian agree to comply with the study requirements. * Subject has sufficient venous access to permit administration of the study medication, collection of pharmacokinetic samples and monitoring of safety variables. * Duration of linezolid therapy is expected to exceed 7 days. * English and Spanish-speaking subjects. Exclusion Criteria: * Subjects with clinical or laboratory evidence of severe hepatic (Child-Pugh class C) disease * Subjects with severe renal impairment (estimated creatinine clearance \<30 mL/min) * Subjects with a history of allergy to linezolid. * Pregnant and breastfeeding subjects.