Relationship between in vitro transendothelial permeability and in vivo single-pass brain extraction.

Unlabelled: In vitro transendothelial permeability was compared to in vivo rat single-pass cerebral extractions to evaluate which method would best estimate the blood-brain barrier (BBB) permeability of several SPECT imaging agents.

Method: Six 99mTc complexes and seven non-Tc complexes were tested in vitro using monolayers of primary bovine brain microvessel endothelial cells and in vivo using the rat single-pass cerebral extraction model. In vitro transendothelial permeability indices (PI) were determined by measuring the average percent of radioactivity traversing the monolayers as a function of time. In vivo single-pass cerebral extractions were determined using an indicator fractionation method.

Results: A positive correlation between extraction and PI was found for the non-Tc complexes (r2 = 0.96). The CBF imaging agents 99mTc-ECD and 99mTc-PnAO have high values for E and PI, demonstrating that these agents penetrate the BBB and have a high membrane permeability, while the heart imaging agent 99mTc-sestamibi had low values for both E and PI. The low PI and E values for 99mTc-sestamibi are consistent with a low brain uptake for this agent, except in cases of disruption of the BBB. In contrast to 99mTc-ECD, 99mTc-PnAO and 99mTc-sestamibi, which had concordant values for E and PI, two highly lipophilic boronic acid adducts of technetium dioxime (BATOs), 99mTc-teboroxime and 99mTcCl(DMG)3(2)MP, had low negative values for PI, but high values for E. In addition, after 3 hr of incubation, the monolayer-to-medium concentration ratio of the BATOs was 642:1 and 744:1, respectively. This compares with values of 89:1 (99mTc-PnAO), 25:1 (99mTc-ECD) and 34:1 (99mTc-sestamibi).

Conclusion: These data suggest that the high in vivo single-pass extraction of the BATOs may be explained by a hydrophobic interaction with the luminal surface of the capillary endothelial cell plasma membrane. We conclude that a high single-pass extraction cannot necessarily be used to infer high BBB or membrane permeability.