Application of the double-indicator technique for measurement of blood-brain barrier permeability in humans

This review examines and evaluates the double-indicator technique for utilization in quantitative measurements of the transport of substances across the human blood-brain barrier (BBB). The classic double-indicator method and its limitations are described along with a new approach for correction of capillary heterogeneity and tracer backflux. This approach considers the total course of the venous outflow curves and involves a short-time experiment model that incorporates calculations of parameters for transport from the blood into the brain and from the brain back to the blood, for the uptake of neurons and glia cells, and for the tracer distribution volume. A modification of the double-indicator technique with intravenous instead of intracarotid bolus injection is discussed along with advantages and limitations of this technique. The application of the method is described and examples are given for D-glucose as well as for some large neutral amino acids and flow tracers. On the basis of the model, it is demonstrated that after crossing the BBB, D-glucose distributes in the brain interstitial fluid volume, and already at the peak of the glucose outflow curves, the apparent extraction is significantly influenced by backflux from the brain. For large neutral amino acids, the permeability from the interstitial fluid space back to the blood is approximately 10 times higher than the permeability from the blood into the brain. Such a difference in permeabilities across the BBB can almost entirely be ascribed to the effect of a nonlinear transport system combined with a relatively small brain amino acid metabolism. This high and rapid backflux causes methodological problems when estimating blood-to-brain transfer of amino acids with traditional in vivo methods. The method is also evaluated for high-permeable substances. Water and the two flow tracers ethyl cysteinate dimer and hexamethylpropyleneamine oxime and the obtained values for brain extraction and distribution volume compare well with those obtained by other methods. Finally, ethical aspects and the future role and possibilities of the double-indicator technique are discussed and related to other methods for determination of BBB permeabilities in the living human brain.