Endocytosis enhancement

The acidification of endosomal compartments, as they evolve toward lysosomes is a well-described phenomenon (1) that can be exploited to design drug delivery systems capable of releasing their contents after endocytosis. Enhanced cytoplasmic drug concentrations can therefore be achieved with smart formulations, which are sensitive to acidic pHs. For this purpose, liposomal formulations are attractive, because their deformable phospholipid bilayers can be rapidly disrupted to trigger drug release. In this section, ionizable copolymers of ISTisopropylacrylamide (NIPAM) are anchored in the phospholipid membrane and used to destabilize the bilayer upon acidification of the environment. 

Figure 7.22b shows that hydrophilic molecules, those with log Kj < 1, are much more permeable in octanol than in olive oil. The same may be said in comparison to 2% DOPC and dodecane. Octanol appears to enhance the permeability of hydrophilic molecules, compared to that of DOPC, dodecane, and olive oil. This is dramatically evident in Fig. 7.7, and is confirmed in Figs. 7.8c and 7.22b. The mechanism is not precisely known, but it is reasonable to suspect a shuttle service may be provided by the water clusters in octanol-based PAMPA (perhaps like an inverted micelle equivalent of endocytosis). Thus, it appears that charged molecules can be substantially permeable in the octanol PAMPA. However, do charged molecules permeate phospholipid bilayers to any appreciable extent We will return to this question later, and will cite evidence at least for a partial answer. 

The polyvalent nature of dendrimers has been investigated as vehicles for carrying multiple chelator groups to enhance signals in various imaging applications (Barthand Soloway, 1994 Yoo et al., 1999 Kobayashi et al., 2000, 2001 Sato et al., 2001). In addition, in certain chelate-dendrimer constructs, excess amines on the dendrimer surface can aid in the cellular uptake process through charge-mediated endocytosis. 

Farge, E., Ojcius, D.M., Subtil, A. and Dautry-Varsat, A., 1999, Enhancement of endocytosis due to aminophosphohpid transport across the plasma membrane of living cells. Am. J. Physiol CellPhysiol, 276 C725-C733. 

Mahnke K. Guo M, Lee S, Sepulveda H, Swain SL, Nussenzweig M, Steinman RM The dendritic cell receptor for endocytosis, DEC-205, can recycle and enhance antigen presentation via major histocompatibility complex class Il-positive lysosomal compartments. J Cell Biol 2000 151 673-684. 

Integrin receptor-binding peptides have been used to enhance liposome binding, uptake, and expression (25,47 9). The inclusion of an 0(5pi integrin-targeted peptide into a liposomal complex enhanced transfection efficiency four- to five-fold in Jurkat cells and 10- to 13-fold in TF-1 cells (48). Confocal and electron microscopy revealed that the mechanism of cell entry conferred by RGD peptides on liposomes is predominantly by clathrin-coated endocytosis rather than by phagocytosis (50). 

Figure 2 Proposed pathways for liposomal entry into the cell enhanced by peptides. These include direct cell entry suggested as the mechanism of entry by cell-penetrating peptides and receptor-mediated endocytosis by caveolae- and clathrin-dependent endocytosis.

Receptor mediated endocytosis, on the other hand, requires recognition by the receptor before internalization of the molecule. So far, two receptor mediated endocytosis protocols have mainly been studied for potential applications in MRI contrast enhancement, namely the transferrin and folic acid receptor systems. 

Cationic polymer is also frequently examined to increase the potential of a gene drug. Large molecular weight cationic polymers can condense pDNA more efficiently than cationic liposomes. They include poly-L-lysine (PLL), poly-L-omithine, polyethyleneimine (PEI), chitosan, starburst dendrimer and various novel synthetic polymers. These polymers can enhance the cellular uptake of pDNA by nonspecific adsorptive endocytosis. 

The most efficient rectal absorption enhancers, which have been studied, include surfactants, bile acids, sodium salicylate (NaSA), medium-chain glycerides (MCG), NaCIO, enamine derivatives, EDTA, and others [45 17]. Transport from the rectal epithelium primarily involves two routes, i.e., the paracellular route and the transcellular route. The paracellular transport mechanism implies that drugs diffuse through a space between epithelial cells. On the other hand, an uptake mechanism which depends on lipophilicity involves a typical transcellular transport route, and active transport for amino acids, carrier-mediated transport for (3-lactam antibiotics and dipeptides, and endocytosis are also involved in the transcellular transport system, but these transporters are unlikely to express in rectum (Figure 8.7). Table 8.3 summarizes the typical absorption enhancers in rectal routes. 

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