Distribution volume of radiolabeled large neutral amino acids in brain tissue.

Variations in the cerebellum to plasma ratio at late times in 6-[18F]fluoro-L-DOPA studies are shown to be consistent with competitive binding of large neutral amino acids for a common transporter in the blood-brain barrier and the stability of brain tissue large neutral amino acid level in the presence of plasma level changes. The distribution volume of an inert large neutral amino acid can be estimated from plasma and tissue large neutral amino acid levels and apparent half-saturation concentrations (Km) of the transporter in the blood-brain barrier. Stability of brain large neutral amino acid levels is supported by literature findings and can be explained by high saturation of the large neutral amino acid transporter at physiologic conditions.