Structure-internalization relationship for adsorptive-mediated endocytosis of basic peptides at the blood-brain barrier

For the purpose of the brain delivery of peptides, the structural specificity of adsorptive-mediated endocytosis at the blood-brain barrier was studied by measuring transport of a newly synthesized basic peptide 001-C8, H-MeTyr-Arg-MeArg-D-Leu-NH(CH2)(8)NH2, using primary cultured bovine brain capillary endothelial cells. The apparent uptake of [I-125]001-C8 increased time-dependently and reached a steady-state at 60 min. The steady-state uptake of [I-125]001-C8 was temperature and concentration dependent and was significantly decreased in the presence of dansylcadaverine, protamine or poly-L-lysine. Uptakes of peptides modified by 1,8-octanediamine, 1,5-pentanediamine, 1,2-ethanediamine or ethylamide and peptides with a free carboxyl terminal were significantly higher than, and similar to, that of [H-3]PEG900, respectively. The half-saturation constants and the maximal uptake capacities of these peptides were in the ranges of 0.2 to 134 mu M and 1.1 to 408 pmol/mg protein, respectively. These values were correlated with the basicity of the molecules. In conclusion, not the number of constituent amino acids of peptides, but rather the C-terminal structure and the basicity of the molecules are the most important determinants for the uptake by the adsorptive-mediated endocytosis system at the blood brain barrier.