Population pharmacokinetics and pharmacodynamics of cefazolin using total and unbound serum concentrations in patients with high body weight

The objective of this study was to evaluate the steady state pharmacokinetics and pharmacodynamics of cefazolin in patients with a high body weight. Cefazolin was administered by 0.5-h infusions to 11 patients with total body weight (TBW) ≥120 kg receiving 3 g q8h, and 12 patients with TBW <120 kg receiving 2 g q8h. Total and unbound serum concentration–time data obtained from serial blood samples were analysed simultaneously by population pharmacokinetic modelling using NONMEM. Probability of target attainment (PTA) was calculated for various dosing regimens through Monte Carlo simulations based on the cumulative percentage of the dosing interval that the unbound concentration exceeds the minimum inhibitory concentration (MIC) value for the pathogen at steady state ( fT MIC ) ≥40%, ≥60% and 100%. A two-compartment model with non-linear protein binding and allometric scaling of the central volume of distribution using TBW best characterized both total and unbound concentration–time data. Unbound clearance was significantly associated with creatinine clearance, and maximum protein binding constant was significantly associated with serum albumin concentration and body mass index ( P <0.05). Based on unbound concentration–time profiles, all simulated regimens achieved PTA >90% at MIC values ≤2 mg/L using fT MIC ≥40%, at MIC values ≤1 mg/L using fT MIC ≥60%, and at MIC values ≤0.5 mg/L using fT MIC of 100%. At fT MIC ≥60%, 0.5-h infusion of cefazolin 1 g q8h achieved PTA <90% at MIC values ≥2 mg/L in patients with TBW≥120 kg; however, prolonged-infusion and higher-dose regimens improved PTA to >90%. Overall, cefazolin pharmacokinetics are altered considerably in obese patients. Higher-dose and/or prolonged-infusion cefazolin regimens should be considered in patients with TBW ≥120 kg, particularly those with less-susceptible Gram-negative infections.