Estimating Organ-to-Plasma Ratios in physiologically based PK modeling: a simplified approach for early drug discovery

Physiologically based pharmacokinetics (PBPK) modeling is a valuable tool in drug development and candidate selection. However, its widespread adoption in early stages of drug discovery remains limited. This study proposes a novel simplified approach to estimate organ-to-plasma ratio (Kp) values based on measured or estimated volume of distribution (VDss) to eliminate the reliance on additional in vitro data such as blood to plasma partition, LogP and pKa, which may not be available for newly synthesized molecules. The study includes 11 simulations across 4 compounds and 5 species. When simulations were based on ideal VDss and clearance (CL) values determined through non-compartmental analysis (NCA), the observed average absolute fold error (AAFE) aligned with experimental variability in the in vivo experiments (AAFE=1.3). However, when in vitro to in vivo correlation approaches were used to predict VDss and CL starting from in vitro intrinsic CL, plasma protein binding and microsomal binding, the AAFE increased to 1.7, reflecting the additional error introduced by the use of in vitro data to determine PK properties. Overall, this work provides a starting point for the facile implementation of PBPK models to meet the needs of early drug discovery projects.