The delivery of therapeutic agents through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo, is enabled by coupling drugs to brain drug delivery transport vectors, such as the OX26 monoclonal antibody to the transferrin receptor located on the BBB. Drug conjugation to delivery vectors is possible by the use of avidin/biotin technology, and the production of avidin/vector conjugates potentially allows for the delivery through the BBB of many biotinylated therapeutics. However, the use of avidin causes reduced brain delivery of avidin/vector conjugates, because of the rapid systemic clearance of such conjugates from the bloodstream. Because previous studies have shown that this rapid elimination is due to avidin's cationic nature, the present studies describe the production of neutral avidin-OX26 antibody conjugates. Isoelectric focusing demonstrated the pls of avidin and neutral avidin were > 9 and 5 to 6, respectively. Neutral avidin and the OX26 antibody, which was purified from serum-free hybridoma-conditioned supernatants, were conjugated with a thio-ether linkage. The area under the plasma concentration curve of [3H] biotin/neutral avidin-OX26 was more than 5-fold greater than that for [3H] biotin/avidin-OX26. The mean residence time of [3H] biotin/neutral avidin-OX26 in plasma was 11.3 +/- 0.2 hr. The BBB permeability-surface area product was not significantly different for either [3H] biotin/neutral avidin-OX26 or [3H] biotin/avidin-OX26. The delivery of [3H] biotin to brain reached 0.20 to 0.25% of injected dose per gram brain by 2-6 hr after single intravenous injection, whereas the brain delivery of [3H] biotin/avidin-OX26 did not exceed 0.05% injected dose per g.(ABSTRACT TRUNCATED AT 250 WORDS)