Enhancement of transport of D-melphalan analogue by conjugation with L-glutamate across bovine brain microvessel endothelial cell monolayers

In this paper, the L-glutamate (L-Glu) transport system was targeted to imp rove the delivery of a model compound, p-di(hydroxyethyl)-amino-D-phenylala nine (D-MOD), through the blood-brain barrier (BBB) in vitro cell culture m odel. D-MOD is an analogue of an antitumor agent D-melphalan. To target the L-Glu transport system, D-MOD was conjugated to L-Glu to give D-MOD-L-Glu conjugate. D-MOD and D-MOD-L-Glu transport properties were evaluated using the bovine brain microvessel endothelial cell (BBMEC) monolayers. The resul ts suggest that D-MOD-L-Glu conjugate permeates through the BBMEC monolayer s more readily than the parent D-MOD. The improve ment of transport may be due to the recognition of D-MOD-L-Glu by the L-Glu transport system. The tr ansport mechanism was evaluated using several different experiments includi ng: (a) concentration-dependent studies; (b) temperature-dependent studies; (c) substrate inhibition studies; and (d) metabolic inhibitor studies. The D-MOD-L-Glu transport was inhibited by the change of temperature from 37 d egrees C to 4 degrees C. At higher concentrations, the transport of D-MOD-L -Glu reached plateau due to saturation. Furthermore, some amino acids (i.e. , L-Glu, L-Asp, D-Asp, and L-Gln) inhibited the transport of D-MOD-L-Glu; p resumably the conjugate was competing with these amino acids for the same t ransport system. Metabolic inhibitors (i.e., 2,4-dinitrophenol and sodium a zide) suppressed the transport of the conjugate. However, the conjugate was not transported by monocarboxylic acid, dipeptide and neutral amino acid t ransporters. In conclusion, the L-Glu transport system can be utilized to f acilitate a non-permeable drug across the BBB by conjugating the drug with L-Glu amino acid.