Transport Studies Using Blood-Brain Barrier In Vitro Models: A Critical Review and Guidelines

Permeation is one of the most evaluated parameters using preclinical in vitro blood-brain barrier models, as it has long been considered to be one of the major factors influencing central nervous system drug delivery. Blood-brain barrier permeability can be defined as the speed at which a compound crosses the brain endothelial cell barrier and is employed to assess barrier tightness, which is a crucial feature of brain capillaries in vivo. In addition, it is used to assess brain drug penetration. We review traditionally used methods to assess blood-brain barrier permeability in vitro and summarize often neglected in vivo (e.g., plasma protein and brain tissue binding) or in vitro (e.g., culture insert materials or methodology) factors that influence this property. These factors are crucial to consider when performing BBB permeability assessments, and especially when comparing permeability data obtained from different models, since model diversification significantly complicates inter-study comparisons. Finally, measuring transendothelial electrical resistance can be used to describe blood-brain barrier tightness; however, several parameters should be considered while comparing these measurements to the blood-brain barrier permeability to paracellular markers.