The metabolic disposition of flucloxacillin in patients with impaired kidney function

The fate of flucloxacillin and its active metabolite hydroxyflucloxacillin was studied in a group of patients with impaired kidney function. Flucloxacillin was administered orally or intravenously. Peak levels of hydroxyflucloxacillin were obtained between 150 and 250 min after the administration of flucloxacillin. The plasma concentrations obtained after a therapeutic dose of flucloxacillin were well above the concentration (i.e. 1-2 microgram/ml) generally considered to be the effective minimum for isoxalyl penicillins. The plasma half life of the metabolite was twice as long as that of flucloxacillin (295 min and 154 min, respectively). The nonprotein-bound fraction of hydroxyflucloxacillin in plasma from patients was twice as large as that of its parent compound (16.2 vs. 8.1%). This was also observed in normal human plasma, although protein binding in the latter was higher than in uraemic plasma. Some accumulation of hydroxyflucloxacillin may occur during flucloxacillin therapy with dosage intervals of 6 h.