Pharmacokinetics
One of the advantages of linezolid is its high bioavailability (close to 100%) when given by mouth: the entire dose reaches the bloodstream, as if it had been given intravenously. This means that people receiving intravenous linezolid may be switched to oral linezolid as soon as their condition allows it, whereas comparable antibiotics (such as vancomycin and quinupristin/dalfopristin) can only be given intravenously. Taking linezolid with food somewhat slows its absorption, but the area under the curve is not affected.
Linezolid has low plasma protein binding (approximately 31%, but highly variable) and an apparent volume of distribution at steady state of around 40–50 liters. Peak serum concentrations (Cmax) are reached one to two hours after administration of the drug. Linezolid is readily distributed to all tissues in the body apart from bone matrix and white adipose tissue. Notably, the concentration of linezolid in the epithelial lining fluid of the lower respiratory tract is at least equal to, and often higher than, that achieved in serum (some authors have reported bronchial fluid concentrations up to four times higher than serum concentrations), which may account for its efficacy in treating pneumonia. Cerebrospinal fluid (CSF) concentrations vary; peak CSF concentrations are lower than serum ones, due to slow diffusion across the blood–brain barrier, and trough concentrations in the CSF are higher for the same reason. The average half-life is three hours in children, four hours in teenagers, and five hours in adults.
Linezolid is metabolized in the liver, by oxidation of the morpholine ring, without involvement of the cytochrome P450 system. This metabolic pathway leads to two major inactive metabolites (which each account for around 45% and 10% of an excreted dose at steady state), one minor metabolite, and several trace metabolites, none of which accounts for more than 1% of an excreted dose. Clearance of linezolid varies with age and gender; it is fastest in children (which accounts for the shorter half-life), and appears to be 20% lower in women than in men.
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