Clinical pharmacokinetics of linezolid, a novel oxazolidinone antibacterial.

Linezolid is the first antibacterial to be approved from the oxazolidinone class. The drug has substantial antimicrobial activity against Gram-positive organisms such as streptococci, staphylococci and enterococci, including species resistant to conventional antibacterial treatment. Linezolid is fully bioavailable following oral administration when compared with intravenous administration. Maximum plasma linezolid concentrations are usually achieved between 1 and 2 hours after oral administration. Food slightly decreases the rate, but not the extent, of absorption. The distribution of linezolid is approximately equivalent to total body water, and there is low protein binding (31%) to serum albumin. The elimination half-life of linezolid is 5-7 hours, and twice-daily administration of 400-600 mg provides steady-state concentrations in the therapeutic range. Linezolid is mainly cleared by non-renal clearance to two metabolites and renal clearance of the parent compound. Approximately 50% of an administered dose appears in the urine as the two major metabolites, and approximately 35% appears as parent drug. A small degree of nonlinearity has been observed, with a 30% decrease in clearance after a 5-fold increase in dose. The nonlinearity is not relevant over the therapeutic dosage range. Plasma linezolid concentrations in elderly patients, patients with mild to moderate hepatic impairment or mild to severe renal impairment are similar to those achieved in young or healthy volunteers. Higher concentrations are observed in women as compared with men, but the difference is not sufficient to warrant an adjustment in dosage. In patients with severe renal impairment requiring haemodialysis, the exposure to the two primary metabolites was 7 to 8-fold higher than in patients with normal renal function. Therefore, linezolid should be used with caution in patients with severe renal insufficiency. A higher clearance of linezolid was found in children as compared with adults, and therefore higher daily dosages per kg bodyweight are required in children. There is no pharmacokinetic interaction when linezolid is coadministered with aztreonam, gentamicin or warfarin. Linezolid is a mild, reversible, inhibitor of monoamine oxidases A and B. Coadministration of linezolid with the adrenergic agents pseudoephedrine and phenylpropanolamine resulted in increases in blood pressure relative to these agents alone or to placebo. The degree of the change in blood pressure was within that associated with normal daily activities. No interaction was observed when linezolid was coadministered with the serotonergic agent dextromethorphan.