The role of permeability in drug ADME/PK, interactions and toxicity--presentation of a permeability-based classification system (PCS) for prediction of ADME/PK in humans.

PURPOSE: The objective was to establish in vitro passive permeability (Pe) vs in vivo fraction absorbed (fa)-relationships for each passage through the human intestine, liver, renal tubuli and brain, and develop a Pe-based ADME/PK classification system (PCS).
MATERIALS AND METHODS: Pe- and intestinal fa-data were taken from an available data set. Hepatic fa was calculated based on extraction ratios of the unbound fraction of drugs (with support from animal in vivo uptake data). Renal fa (reabsorption) was estimated using renal pharmacokinetic data, and brain fa was predicted using animal in vitro and in vivo brain Pe-data. Hepatic and intestinal fa-data were used to predict bile excretion potential.
RESULTS: Relationships were established, including predicted curves for bile excretion potential and minimum oral bioavailability, and a 4-Class PCS was developed: I (very high Pe; elimination mainly by metabolism); II (high Pe) and III (intermediate Pe and incomplete fa); IV (low Pe and fa). The system enables assessment of potential drug-drug transport interactions, and drug and metabolite organ trapping.
CONCLUSIONS: The PCS and high quality Pe-data (with and without active transport) are believed to be useful for predictions and understanding of ADME/PK, elimination routes, and potential interactions and organ trapping/toxicity in humans.