Replacing In Vivo Experiments for PK/PD Target Determination Through In Vitro Time-Kill Experiments and PK/PD Modelling Incorporating Inter-strain Variability: Application to Meropenem Against Pseudomonas aeruginosa

Background. Optimal antibiotic dosing regimens depend on the pharmacokinetic/pharmacodynamic (PK/PD) index that best predicts antibacterial efficacy. PK/PD targets are traditionally determined using murine infection models based on a limited number of bacterial isolates. Objective. This study aimed to investigate whether animal experiments could be replaced by in vitro time-kill experiments performed on a large collection of clinical isolates and analyzed using a modelling approach accounting for inter-strain variability. The proposed framework was evaluated using meropenem against Pseudomonas aeruginosa. Materials and Methods. In vitro time-kill experiments were performed on 66 clinical isolates of P. aeruginosa. A population pharmacodynamic model was developed from experimental data. A murine pharmacokinetic model was reproduced from literature and combined with the pharmacodynamic model to simulate in vivo bacterial burden over time. The relationships between simulated bacterial counts at 24 h and the three main PK/PD indices (fCmax/MIC, fAUC/MIC and %fT>MIC) were characterized using nonlinear mixed-effects Imax models. Results. The PK/PD index showing the strongest correlation with meropenem efficacy at 24 h was %fT>MIC (R2 = 0.989), compared with fAUC/MIC (R2 = 0.373) and fCmax/MIC (R2 = 0.284). These findings are consistent with previous studies using murine thigh infection models. The %fT>MIC target required to achieve a 2-log CFU reduction was estimated at 44%, with substantial inter-strain variability (10th and 90th percentiles: 27% and 71%, respectively). Conclusions. Using meropenem against P. aeruginosa as a proof of concept, we demonstrate that in vitro time-kill experiments combined with pharmacometric modelling can identify the same PK/PD efficacy targets as animal infection models. Moreover, performing experiments on a large panel of clinical isolates enables the quantification of inter-strain variability in PK/PD targets, providing information that may improve their translation to clinical dosing optimization.