Population pharmacokinetics of unbound ceftriaxone in a critically ill population

Objective: To develop a reliable 2-compartment population pharmacokinetic (PK) model for unbound ceftriaxone in a critically ill population and determine an optimal dosing regimen. Materials and methods: This was a prospective, single-center, observational study of critically ill patients treated with ceftriaxone. Unbound serum ceftriaxone concentrations were measured using validated ultrafiltration and ultra-performance liquid chromatography-tandem mass spectrometry. PK analysis and dosing simulations were performed using an iterative 2-stage Bayesian fitting procedure and Monte Carlo simulations. The PK/pharmacodynamics (PD) target was attained when unbound serum ceftriaxone concentrations exceeded 4 times the minimum inhibitory concentration (MIC) for ≥ 60% of the dosing interval (ƒT>4xMIC ≥ 60%). Results: 91 patients were enrolled, and 173 unbound ceftriaxone concentrations were acquired. The population PK parameter estimates were hepatic clearance 5.2 ± 0.9 L/h/1.85m2, the unbound renal clearance of ceftriaxone divided by the creatinine clearance 0.61 ± 0.24, lean body mass corrected volume of distribution of the central compartment 0.82 ± 0.21 L/kg, and intercompartmental distribution rate constant from central to peripheral compartment 0.18 ± 0.08 h–1. Dosing simulations predicted ƒT>4 mg/L of 88% (95% CI: 69 – 100%) for 2,000 mg ceftriaxone once daily and ƒT>4 mg/L of 100% (95% CI: 100 – 100%) both for 1,000 mg twice daily and continuous infusion of 2,000 mg daily. Conclusion: We developed a reliable population PK model for unbound ceftriaxone in a critically ill population. Dosing simulations revealed ƒT>4 mg/L ≥ 60% for 1,000 mg twice daily and 2,000 mg once daily or by continuous infusion.