Pharmacokinetics and distribution of flucloxacillin in pacemaker patients

Summary

The pharmacokinetics of flucloxacillin in plasma and tissue fluid after i.v. infusion of 1 g was analyzed according to an open two-compartment model in 19 patients with bradyarrhythmias (mean age 70.8 years) admitted for implantation or replacement of a permanent pacemaker system. After the first infusion of flucloxacillin (5 min), the distribution phase was rapid (t_1/2α=0.13 h). The plasma half-life of elimination (t_1/2β) was 1.51 h, which is almost twice as long as reported in healthy volunteers. Total plasma clearance (93.1 ml/min) was also lower than is usually found in healthy individuals, due to low renal clearance of flucloxacillin (60.2 ml/min). The total apparent volume of distribution during the β-phase (Vd_area) was 0.172 l/kg and distribution in the central compartment (V_c) 0.064 l/kg. In each patient plasma protein binding and drug distribution to plasma water, proteins and blood cells in whole blood were determined. Binding in plasma to proteins was 91.0% and distribution to blood cells in whole blood 13.8%. The mean distribution volume of free flucloxacillin during the β-phase (V_{dβ free}) was 2.18 l/kg, which exceeds total body water, suggesting possible intracellular distribution and substantial tissue binding. Plasma concentrations of flucloxacillin after the fourth dose (1 g t.i.d.) were very similar to those obtained after the first infusion and those predicted from the single dose kinetics. The concentration of flucloxacillin in fluid from the pacemaker pockets in 5 patients averaged 12.1 µg/ml and 9.5 µg/ml at 1 and 5 h, respectively, which was more than ten times the MIC-values for Staphylococcus aureus and S. epidermidis. The average concentration ratio (tissue fluid/plasma) was 0.57. Thus the pharmacokinetics of flucloxacillin in these elderly patients exhibited marked differences from what has been found in healthy volunteers. Despite the high degree of plasma protein binding, flucloxacillin appears to distribute rapidly and efficiently to extravascular compartments, such as a pacemaker pocket.