## 6 PLGA Nanoparticles Functionalization
### BBB Transcytosis
#### 6.2.2 Receptor-Mediated Transcytosis  
CNS endothelial cells express receptors, such as TfR, low-density lipoprotein receptor (LDLR), insulin-like growth factor receptor, insulin receptor, diphtheia toxin receptor, scavenger receptor class B type and nicotinic acetylcholine receptor (nAChR), onto their surfaces. Thus, delivery systems can be decorated with targeting ligands that specifically recognise these receptors to mediate drug penetration to the brain [165]. Polysorbate 80 (P80, also known as Tween 80) is particularly interesting for brain delivery, since PBCA NPs coated with P80 have already facilitated brain delivery of several drugs that were unable to cross the BBB in a free form [166]. Poloxamer 188 (P188, also known as Pluronic F-68) was also found to be effective as an NP coating material for brain targeting [97]. This effect is attributed to the enhanced adsorption of certain plasma proteins (especially the apolipoproteins E, A-I and B) onto the NPs. Indeed, after being exposed to serum or plasma, various proteins quickly adsorb onto foreign NPs [167]. This effect is known as differential protein adsorption, and was first described in 1989 [168]. The postulated hypothesis is that the adsorption of apolipoproteins onto P80/P188-coated NPs is responsible for the subsequent interaction with their respective receptors expressed by the endothelial cells forming the BBB, and thus promotes receptor-mediated endocytosis of NPs, facilitating their delivery into the brain [169]. Therefore, all these nanoparticle DDS appear to act like Trojan horses that would transport the drugs into the brain endothelial cells and, in this way, represent a novel platform technology for the treatment of neurodegenerative diseases [170]. Moreover, the coating of particles with P80 causes a rearrangement of the proteins composing the tight junctions at the BBB which results in the increase of paracellular crossing of NPs into the brain [171, 172]. Another reason is attributed to the role of P80 in blocking the efflux system, reducing the pump-off effect of the P-glycoprotein, and thereby achieving a high drug concentration in the brain [173]. [PERSON] et al. [102] studied different surface-modified PLGA NPs formulations based on P80, P188 and CS for targeting CNS diseases. The resulting PLGA NPs had sizes ranging from 250 to 400 nm depending on the type of surface modifier used, and after systemic administration, NP concentration in the brain increased with the surface modification of the particles, in particular, CS and P80 PLGA NP. However, CS-PLGA NPs were only adsorbed on endothelial cells and not transferred into brain tissue, whereas a higher brain distribution was seen with P80-PLGA NPs, which were seen in the parenchyma beyond the cerebral blood vessel endothelial cells. This suggested that P80-PLGA NPs could not only adsorb to the endothelial membrane of cerebral blood vessels but could also be internalized by endothelial cells and cross the BBB. Moreover, P80-PLGA NPs exhibited prolonged circulation in the blood compared to the other NPs evaluated, which might be a reason for the increased brain distribution by avoiding uptake by RES. Therefore, P80-PLGA NPs have high potential as effective drug carriers for CNS delivery [102]. This was further confirmed by [PERSON] et al. [174]. Indeed, they demonstrated the capability of galantamine-PLGA NPs to cross the BBB because of the permeabilizing and targeting effect of P80, leading to a formulation with interesting properties to be used as advanced DDS for the symptomatic treatment of AD. As already mentioned, receptors that are highly expressed on CNS endothelial cells include TfR. Lactoferrin (Lf) is a naturally occurring iron-binding glycoprotein belonging to the transferrin family. Its receptor (LfR) is highly expressed in brain endothelial cells and in neurons, being especially overexpressed in capillaries and neurons associated with age-related neurodegenerative diseases [175, 176]. This fact makes Lf a promising targeting molecule for the treatment of AD.