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Antibiotic dosing strategies are generally based on systemic drug concentrations. However, drug concentrations at the infection site drive antimicrobial effect, and efficacy predictions and dosing strategies should be based on these concentrations. We set out to review different translational pharmacokinetic-pharmacodynamic (PK/PD) approaches from a target site perspective. The most common approach involves calculating the probability of attaining animal-derived PK/PD index targets, which link PK parameters to antimicrobial susceptibility measures. This approach is time efficient but ignores some aspects of the shape of the PK profile and inter-species differences in drug clearance and distribution, and provides no information on the PD time-course. Time–kill curves, in contrast, depict bacterial response over time. In vitro dynamic time–kill setups allow for the evaluation of bacterial response to clinical PK profiles, but are not representative of the infection site environment. The translational value of in vivo time–kill experiments, conversely, is limited from a PK perspective. Computational PK/PD models, especially when developed using both in vitro and in vivo data and coupled to target site PK models, can bridge translational gaps in both PK and PD. Ultimately, clinical PK and experimental and computational tools should be combined to tailor antibiotic treatment strategies to the site of infection.

Background High protein binding (PB) of antibiotics has an impact on their antimicrobial activity. It has been questioned whether in vitro PB determination can capture the dynamic and concentration-dependent PB of highly bound antibiotics. Objectives This clinical study compared in vitro ultrafiltration (UF) and in vivo IV microdialysis (MD) methods to determine ceftriaxone PB. Methods Six healthy male volunteers received a single IV 2 g dose of ceftriaxone. Unbound ceftriaxone plasma concentrations were measured with MD and venous plasma sampling with subsequent UF. Pharmacokinetic parameters were determined using non-compartmental pharmacokinetic analysis. Non-linear mixed-effects modelling was used to quantify the PB. The PTA was estimated. Results The Cmax of ceftriaxone total plasma concentration (297.42 ± 21.0 mg/L) was approximately 5.5-fold higher than for free concentrations obtained with UF (52.83 ± 5.07 mg/L), and only 3.5-fold higher than for free concentrations obtained with MD (81.37 ± 26.93 mg/L). Non-linear, saturable PB binding was confirmed for both UF and MD. Significantly different dissociation constants (Kd) for the albumin/ceftriaxone complex were quantified: in UF it was 23.7 mg/L (95% CI 21.3–26.2) versus 15.9 mg/L (95% CI 13.6–18.6) in MD. Moreover, the estimated number of binding sites (95% CI) per albumin molecule was 0.916 (0.86–0.97) in UF versus 0.548 in MD (0.51–0.59). The PTA obtained with MD was at most 27% higher than with UF. Conclusions In vitro UF versus in vivo intravasal MD revealed significantly different PB, especially during the distribution phase. The method of PB determination could have an impact on the breakpoint determination and dose optimisation of antibiotics.

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