DRUG-DELIVERY VIA ACTIVE-TRANSPORT AT THE BLOOD-BRAIN-BARRIER - AFFINITY OF A PRODRUG OF PHOSPHONOFORMATE FOR THE LARGE AMINO-ACID TRANSPORTER

Due to its hydrophilic nature, the antiviral agent phosphonoformate (PFA) is excluded from the CNS by the blood-brain barrier (BBB). Lipophilic triesters of PFA, designed to penetrate the BBB have been found to be unsuitable as prodrugs due to their rapid and complicated hydrolysis. Hydrophilic drugs, such as L-dopa, are known to cross the blood-brain barrier by means of an active amino acid transporter. Thus, the possibility that a PFA-amino acid conjugate may be actively transported at the BBB was investigated. A PFA-L-tyrosine conjugate [sodium 4-(2'-carboxyl-2'-aminoethyl)phenyl methoxycarbonylphosphonate] was synthesised and characterised. Active amino acid transport was studied in vitro using monolayers of porcine brain microvessel endothelial cells. Confluent monolayers were obtained after 4-5 days in culture, and alkaline phosphatase activity and the presence of Factor VIII antigen were demonstrated histochemically. The transport of L-[H-3]tyrosine was shown to be temperature- and concentration-dependent and transport constants were calculated to be K-m = 0.149 mM and V-max = 3.07 nmol/h per insert by non-linear regression. L-Dopa and other large amino acids were found to inhibit the transport of L-[H-3]tyrosine, consistent with their transport by the amino acid transporter in vivo. The PFA-L-tyrosine conjugate, which was stable under the experimental conditions, also inhibited L-[H-3]tyrosine transport indicating that it may be a substrate for active transport at the BBB.