Carrier-mediated delivery of metabotropic glutamate receptor ligands to the central nervous system: Structural tolerance and potential of the L-system amino acid transporter at the blood-brain barrier

The brain endothelial large neutral amino acid carrier (L-system) is well suited for facilitated drug transport to the brain because of its high transport capacity and relatively broad structural substrate tolerance. The authors have examined the potential of this transporter for central nervous system (CNS) delivery of a new family of compounds derived from the large neutral amino acid phenylglycine. These compounds are highly selective for specific isoforms of metabotropic glutamate receptors (mGluRs) but will only become effective therapeutics for CNS diseases such as ischemic disorders, stroke, and epilepsy if they can effectively cross the blood-brain barrier. Using the immortalized rat brain endothelial cell line RBE4 as in vitro, blood-brain barrier model, the authors have studied the interaction of phenylglycine and selected derivatives with the L-system-mediated transport of L-[H-3]-histidine. The transport of L-histidine was characteristic of the L-system in vivo with the following kinetic parameters: K-m 135 +/- 18 mu mol/L, V-max 15.3 +/- 1.13 nmol/min/mg protein, and K-D 2.38 +/- 0.84 mu L/min/mg protein. The affinities of the L-system for phenylglycine and the derivatives investigated increased in the order S-4-carboxy-phenylglycine (K-i = 16 mmol/L) <R-phenylglycine (2.2 mmol/L) <S-3-hydroxy-phenylglycine (48 mu mol/L) <S-phenylglycine (34 mu mol/L), suggesting that a negative charge at the side chain or R-configuration is detrimental for carrier recognition, whereas neutral side chain substituents are well tolerated. The authors have further shown (1) that the mode of interaction with the L-system of S-phenylglycine and S-3-hydroxy-phenylglycine is competitive, and (2) that the transporter carries these two agents into the cell as shown by high-performance liquid chromatography (HPLC) analysis of the RBE3 cell contents. The study provides the first evidence for the potential of S-phenylglycine derivatives for carrier-mediated delivery to the CNS and outlines the substrate specificity of the L-system at the blood-brain barrier for this class of mGluR ligands. As the affinities of S-phenylglycine and S-3-hydroxy-phenylglycine for the L-system carrier are even higher than those of some natural substrates, these agents should efficiently enter CNS via this route. Possible strategies for a synergistic optimization of phenylglycine-derived therapeutics with respect to desired activity at the CNS target combined with carrier-mediated delivery to overcome the blood-brain barrier are discussed.