Continuous infusion simulations in PBPK and QSP models reveal steady-state properties and rate-limiting steps.

Objectives: We aimed to increase our understanding of large-scale models and to develop efficient tools that can be applied to large-scale models without losing insight into mechanistic details. We aimed to develop a numerical model analysis toolbox that utilizes continuous infusion simulations to provide a numerical steady-state analysis. Methods: The analysis toolbox presented here utilizes the standard sensitivity analysis spider plots as present in the esqlabsR package (v5.1.3) and the ospsuite package (v12.0.0) in R (v4.3.1). Simulation models were created using a whole-body Physiologically-Based Pharmacokinetic model (PK-Sim® v11.2) and extended in MoBi® (v11.2) to include TMDD or PD models. The created models were saved as .pkml files and analyzed in R using dedicated R code for the developed analysis toolbox. Results: 3 types of continuous infusion analyses were developed for various applications: firstly, if a long terminal half-life is observed in a single simulation, comparing that same simulation with a continuous infusion in plasma gives the relative concentrations in different organs in steady-state and reveals which concentration is not in steady-state and causing the long terminal half-life in the original simulation. Secondly, a repeated local sensitivity analysis combined with a continuous infusion in different compartments and for various molecular species provided a comprehensive steady-state analysis and the predominant parameters of various elements of large-scale models. These model elements included the partition coefficient calculation methods in the small-molecule whole-body PBPK model, FcRn-mediated endosomal uptake and recycling in the large-molecule whole-body PBPK model, the two-pore model in the large-molecule whole-body PBPK model and a tissue TMDD-PBPK model. Finally, applying the continuous infusion repeated sensitivity analysis with infusion in both compartments of a two-compartment model provided a comprehensive insight into this model with a clear distinction between two possible steady states, depending on the rate-limiting step. Conclusion: Our study demonstrates how continuous infusion simulations can provide numerical model analysis tools that greatly enhance our understanding of large-scale models. This understanding will improve efficiency in developing and communicating these models and their applications.