SYNTHESIS, INVITRO KINETICS, AND INVIVO STUDIES ON PROTEIN CONJUGATES OF AZT - EVALUATION AS A TRANSPORT-SYSTEM TO INCREASE BRAIN DELIVERY

One method to increase the delivery of polar drugs to the central nervous system is via drug-protein conjugates with proteins that interact with and cross brain capillary endothelial cells. As a model for drugs containing a reactive hydroxyl group, AZT was conjugated via a succinate linker to two such protein carriers, the highly cationic histone H1 and an anti-transferrin receptor antibody, OX-26. The protein carriers were selected on the basis of their ability to interact with brain capillary endothelial cells by absorptive or receptor-mediated events, respectively. An in vitro pH profile of the rate of AZT release indicated that the observed hydrolysis proceeds by a specific base-catalysis mechanism. At 37-degrees-C, the release of AZT proceeded at a rate approximately 10-fold faster (K(obs) approximately 8 X 10(-4) min-1) than expected for a simple ester (AZT succinate; K(obs) approximately 1.25 X 10(-4) min-1). Using simple model systems, product analysis revealed that intramolecular cyclization of the succinate linker accounts for the observed rate enhancement. Drug delivery in vivo was assessed using immunohistochemical techniques and quantitative brain uptake measurements with singly and doubly labeled AZT-OX-26 conjugates. Immunohistochemical staining of brain sections showed the colocalization of AZT and OX-26 in the brain vasculature. Therefore, drug can be linked to the antibody without affecting the targeting property of the antibody. Furthermore, an in vivo time course using radiolabeled conjugate showed that AZT is delivered to the brain capillaries but is not transported into the brain parenchyma with the antibody. These results demonstrate that drugs conjugated via a succinate linker to the anti-transferrin receptor antibody OX-26 could provide a useful method of selective drug delivery to brain capillary endothelial cells.