Identifying blood-brain-barrier selective single-chain antibody fragments.

The blood-brain barrier (BBB) represents an obstacle in targeting and delivering therapeutics to the central nervous system. In order to discover new BBB-targeting molecules, we panned a phage-displayed nonimmune human single-chain antibody fragment (scFv) library against a representative BBB model comprised of hydrocortisone-treated primary rat brain endothelial cells. Parallel screens were performed with or without pre-subtraction against primary rat heart and lung endothelial cells in an effort to identify antibodies that may have binding selectivity toward brain endothelial cells. After three rounds of screening, three unique scFvs, scFv15, scFv38, and scFv29, were identified that maintained binding to primary rat brain endothelial cells, both in phage and soluble scFv format. While scFv29 and to a lesser extent, scFv15, exhibited some brain endothelial cell specificity in tissue culture, scFv29 did not appear to bind a BBB antigen in vivo. In contrast, both scFv15 and scFv38 were capable of immunolabeling rat brain vessels in vivo and displayed brain vascular selectivity with respect to all peripheral organs tested other than heart. Taken together, scFv15 and scFv38 represent two new antibodies that are capable of binding antigens that are expressed at the BBB in vivo.