Abstract
P-glycoprotein (P-gp) is a major blood-brain barrier (BBB) efflux transporter. In vitro approaches, including bi-directional efflux ratio (ER), are used to measure P-gp-mediated transport, but findings can be inconsistent across models. We propose a novel, more physiologically relevant, in vitro model: uni-directional apical efflux ratio (AP-ER) - a ratio of permeability rates at the apical side of the BBB with and without P-gp inhibitor. To test our approach, ER and AP-ER were calculated for 3227 structurally diverse compounds in porcine kidney epithelial cells (LLC-PK1) overexpressing human or mouse P-gp, and classified based on their passive transcellular P-gp permeability or charged properties. In vivo rat infusion studies were performed for selected compounds with high ER but low AP-ER. One-third of the 3227 compounds had bi-directional ER that was much higher than AP-ER; very few had AP-ER higher than ER. Compounds with a large difference between AP-ER and ER were typically basic compounds with low-to-medium passive permeability and high lipophilicity and/or amphiphilicity, leading to strong membrane binding. Outcomes in the human model were similar to those in mice, suggesting AP-ER/ER ratios may be conserved for at least two species. AP-ER predicted measured cerebrospinal fluid (CSF) concentration better than ER for the five compounds tested in our in vivo rat infusion studies. We report superior estimations of the CSF concentrations of the compounds when based on less resource-intensive AP-ER versus classical ER. Better understanding of the properties leading to high P-gp-mediated efflux in vivo could support more efficient brain-penetrant compound screening and optimization.
Significance Statement To address inconsistencies associated with the historical, bi-directional efflux ratio (ER) calculation of P-glycoprotein-mediated transport, we propose to use the novel, more physiologically relevant, uni-directional apical efflux ratio (AP-ER) model. In vitro experiments suggested that compounds with strong membrane binding showed the largest difference between AP-ER and ER, and in vivo infusion studies showed that AP-ER predicted cerebrospinal fluid concentrations of compounds better than ER; outcomes in the human model were similar to those in mice.
- Copyright © 2020 American Society for Pharmacology and Experimental Therapeutics