Herein is reported a fusion polypeptide comprising i) at least one binding site, e.g. which comprises an antibody heavy chain variable domain and an antibody light chain variable domain, and which binds to an internalizing cell surface receptor, and ii) at least one pharmaceutically active compound, whereby the EC50-value of the binding pair that binds to an internalizing cell surface receptor determined at pH 5.5 is higher than the EC50-value of the same binding pair determined at pH 7.4 and its use for delivering a pharmaceutically active compound across the blood-brain-barrier.
