Blood—Brain Barrier Transport and Protein Binding of Flumazenil and Iomazenil in the Rat: Implications for Neuroreceptor Studies

The calculated fraction of receptor ligands available for blood–brain barrier passage in vivo (favail) may differ from in vitro (feq) measurements. This study evaluates the protein–ligand interaction for iomazenil and flumazenil in rats by comparing feq and favail. Repeated measurements of blood–brain barrier permeability for two benzodiazepine antagonists were performed in 44 rats by the double-indicator technique. Cerebral blood flow was measured by intracarotid Xe-injection. The apparent permeability–surface product (PSapp) was measured while CBF or bolus composition was changed. Comparison of PSapp obtained in the absence and presence of 5% albumin in the injectate yielded favail, whereas feq was measured by equilibrium dialysis. Iomazenil and flumazenil favail was 62% and 82%, respectively, whereas feq was significantly lower, 42% and 61%. The PSapp for iomazenil and flumazenil increased significantly by 89% and 161% after relative CBF increases of 259% and 201%, respectively. The results demonstrate that application of feq in neuroreceptor studies underestimates the plasma input function to the brain. Model simulations render possible that the differences between favail and feq as well as the effect of CBF on PSapp can be caused by capillary heterogeneity.