The OX26 mouse monoclonal antibody to the rat transferrin receptor undergoes transcytosis through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo, owing to high concentrations of transferrin receptor on the BBB. This property allows the OX26 antibody to serve as a brain drug transport vector. To simplify coupling of therapeutics to the OX26 antibody, the present studies examine the use of the avidin/biotin system to promote coupling of biotin and biotinylated drugs to brain transport vectors. The OX26 antibody was affinity purified from ascites fluid and was covalently coupled via a thioether linkage to avidin, and the conjugate was purified to homogeneity by gel filtration fast protein liquid chromatography. The biotin binding capacity of the avidin-OX26 conjugate was measured, and 2.3 biotin binding sites per avidin-OX26 (1:1) conjugate were detected. Transcytosis through the BBB of [3H]biotin bound to either unconjugated avidin or to the avidin-OX26 conjugate was measured with an internal carotid artery perfusion/capillary depletion technique. [3H]Biotin bound to the avidin-OX26 conjugate was transported through the BBB at rates that equaled the rate of transcytosis of the unconjugated OX26 antibody. The clearance from plasma of [3H]biotin bound to the avidin-OX26 conjugate approximated the rate of clearance of the unconjugated OX26 antibody, and not the rate of clearance of [3H]biotin bound to avidin, which was cleared from plasma at much faster rates.(ABSTRACT TRUNCATED AT 250 WORDS)