ENHANCED UPTAKE OF RSCD4 ACROSS THE RODENT AND PRIMATE BLOOD-BRAIN-BARRIER AFTER CONJUGATION TO ANTITRANSFERRIN RECEPTOR ANTIBODIES

The delivery to the brain of nonlipophilic therapeutic compounds, espe cially proteins, is severely hindered by the presence of the blood-bra in barrier, which is formed by the tightly apposed brain capillary end othelial cells. However, brain endothelial cells do possess specific r eceptor-mediated transport mechanisms so that substances required by t he brain can cross the blood-brain barrier. By use of monoclonal antib odies that bind to the transferrin receptor present on the luminal sur face of brain capillary endothelial cells, we have taken advantage of the transport system responsible for the delivery of iron to the brain to deliver recombinant human soluble CD4 (rsCD4), a potential anti-HI V therapeutic, across the blood-brain barrier. Anti-transferrin recept or antibody-rsCD4, conjugates were synthesized with a disulfide linkag e and characterized in vitro. Experiments that use immunohistochemistr y to localize these conjugates after intravenous administration into t he tail vein of rats have shown that both the carrier antibody and the protein ''passenger'' accumulate in brain capillaries. The carrier-me diated delivery of radiolabeled protein across the blood-brain barrier in vivo was also examined in both rodents and primates. With use of t he technique of capillary depletion in rats, the amount of rsCD4 in th e capillary fraction of the brain, which reaches a maximal value withi n 1 hr postinjection, was shown to decrease with time, whereas the amo unt in the brain parenchyma increased, which suggests that the protein was delivered across the blood-brain barrier. In primates, rsCD4 leve ls in the brain were increased 5-fold when the protein was administrat ed intravenously in the form of an anti-transferrin receptor antibody- rsCD4 conjugate.