Figure 3.

BBB-permeation mechanisms of P/LLL(40%) (purple) and P/LLL(40%)/vector (red). LLL refers to P/LLL(40%). (A) Suggested dual constitutive mechanism. The LLL-portion and the vector-portion of the nanoconjugates bind freely to the LLL-specific receptor (blue) and the vector-specific receptor (red), respectively, on the surface of the endothelial membrane. Both receptors ferry the bound ligands through BBB, and the combined results would indicate that the permeation efficacy is additive. (B) Each of the LLL and vector portions of the nanoconjugates bind to independent receptors and proceed directly to transcytosis as shown in (A). However, in mechanism B, the binding of the LLL-portion would allosterically induce the exposure of receptor sites previously hidden inaccessible in the membrane, and a “boost” phenomenon is observed. While the receptor remains exposed, it carries out one or more rounds of vector binding and transcytosis. Thereby, the BBB-permeation efficacy could be increased in comparison with the constitutional system (mechanism A) and is manifested as the “boost” phenomenon. If multiple LLL-binding sites contribute to boosted BBB-permeability, the permeation efficacy is likely to follow a supra-linear dose dependence. Moreover, binding of free P/LLL(40%) molecules would not function as competitive inhibitors but rather as BBB-permeation activators. The indication of “boosts”, supra-linear dose dependence, and insensitivity against the competition are hallmarks of the BBB permeation by the P/LLL/vector conjugate.