Pharmacokinetics and brain uptake of biotinylated basic fibroblast growth factor conjugated to a blood-brain barrier drug delivery system

Human basic fibroblast growth factor (bFGF) is a potent neuroprotective agent. The clinical efficacy of this neurotrophin, however, is restricted by poor permeability across the blood-brain barrier (BBB). This study was designed to test the hypotheses that bFGF will retain its biological activity and have an enhanced BBB transport after re-formulation and conjugation to a BBB peptide drug delivery vector. The BBB delivery vector is comprised of a conjugate of streptavidin (SA) and the murine OX26 monoclonal antibody against the rat transferrin receptor, and the conjugate of biotinylated bFGF (bio-bFGF) bound to a vector is designated bio-bFGF/OX26-SA. A radioreceptor binding assay shows that the native bFGF, bio-bFGF, and bio-bFGF/OX26-SA conjugate have IC50 values of 0.12, 0.40, and 0.56 nM, respectively. After an IV bolus injection to the rat, [125I]-bio-bFGF is avidly taken up by peripheral organs, with low brain uptake at 60 min, 0.010+/-0.004% of injected dose (ID)/g brain. By contrast, the brain uptake of the [125I]-bio-bFGF/OX26-SA is increased 5-fold to 0.050+/-0.011%ID/g, although the uptake of the conjugate by peripheral tissues was decreased relative to the unconjugated bio-bFGF. In conclusion, conjugation of bio-bFGF to a BBB drug delivery vector (a) causes only a minor decrease in affinity for the bFGF receptor, (b) decreases the peripheral organ uptake of the bFGF, and (c) increases the brain uptake of the neurotrophin. The re-formulation of bFGF to enable receptor-mediated transcytosis across the BBB may improve the therapeutic index of this neurotrophin as a neuroprotective agent.