1. A method of delivering a pharmaceutically active compound across the blood-brain barrier in an individual, comprising administering to the individual an effective amount of a fusion polypeptide comprising at least one binding pair that comprises an antibody heavy chain variable domain and an antibody light chain variable domain, and which binds to an internalizable receptor the surface of the cell, and at least one pharmaceutically active compound, wherein I internalized cell surface receptor, determined at pH 5.5, the value Ekta same binding pair with the same receptor, determined at pH 7.4 is 10 or more for the delivery of pharmaceutically active compounds across the blood brain barer.2. A method of transcytosis through epithelial cells of a subject, comprising administering to the subject a fusion polypeptide comprising at least one binding pair that comprises an antibody heavy chain variable domain and an antibody light chain variable domain, and which binds to an internalizable cell surface receptor, and at least at least one pharmaceutically active compound, wherein the ratio of the value of the E-binding pair that binds to the internalizable receptor on the cell surface is determined at pH 5.5, the ECT value of the same binding pair with the same receptor, determined at pH 7.4, is 10 or more. 3. A method according to any one of claims 1 and 2, characterized in that the ratio is 15 or more. The method according to any one of claims 1 and 2, characterize1. Способ доставки фармацевтически активного соединения через гематоэнцефалический барьер у индивидуума, включающий введение индивидууму эффективного количества слитого полипептида, содержащего- по меньшей мере одну связывающуюся пару, которая содержит вариабельный домен тяжелой цепи антитела и вариабельный домен легкой цепи антитела, и которая связывается с интернализуемым рецептором поверхности клетки, и- по меньшей мере одно фармацевтически активное соединение,при этом отношение значения ЭКсвязывающейся па
