Nanocarriers' entry into the cell: relevance to drug delivery

Abstract

Nanocarriers offer unique possibilities to overcome cellular barriers in order to improve the delivery of various drugs and drug candidates, including the promising therapeutic biomacromolecules (i.e., nucleic acids, proteins). There are various mechanisms of nanocarrier cell internalization that are dramatically influenced by nanoparticles' physicochemical properties. Depending on the cellular uptake and intracellular trafficking, different pharmacological applications may be considered. This review will discuss these opportunities, starting with the phagocytosis pathway, which, being increasingly well characterized and understood, has allowed several successes in the treatment of certain cancers and infectious diseases. On the other hand, the non-phagocytic pathways encompass various complicated mechanisms, such as clathrin-mediated endocytosis, caveolae-mediated endocytosis and macropinocytosis, which are more challenging to control for pharmaceutical drug delivery applications. Nevertheless, various strategies are being actively investigated in order to tailor nanocarriers able to deliver anticancer agents, nucleic acids, proteins and peptides for therapeutic applications by these non-phagocytic routes.

Fig. 1

Principal types of nanocarriers for drug delivery. a Liposomes are formed by one (or several) phospholipid bilayers surrounding an aqueous core. They can be PEGylated and decorated with targeting ligands. b Polymeric nanospheres are designed using biodegradable polyesters or poly(alkylacyanoacrylate), or natural polymers, like albumin. They can also be PEGylated and decorated with targeting ligands. c Polymeric nanocapsules are formed by a polymer membrane (same materials as for nanospheres) surrounding either an oily or an aqueous core. d Polymeric micelles are formed by the assembly of amphiphilic polymers, generally exhibiting a PEG shell that can be functionalized by targeting ligands

Fig. 2

Principal nanocarrier internalization pathways in mammalian cells. a Phagocytosis is an actin-based mechanism occurring primarily in professional phagocytes, such as macrophages, and closely associated with opsonization. b Clathrin-mediated endocytosis is a widely shared pathway of nanoparticle internalization, associated with the formation of a clathrin lattice and depending on the GTPase dynamin. c Caveolae-mediated endocytosis occurs in typical flask-shaped invaginations of the membrane coated with caveolin dimers, also depending on dynamin. d Macropinocytosis is an actin-based pathway, engulfing nanoparticles and the extracellular milieu with a poor selectivity. e Other endocytosis pathways can be involved in the nanoparticle internalization, independent of both clathrin and caveolae

Fig. 3

Nanocarrier internalization by opsonization and phagocytosis. a Unless specifically designed, nanocarriers generally undergo extensive opsonization in the bloodstream, i.e., adsorption of immunoglobulins (mainly IgG), complement components (mainly C3) and other proteins like fibronectin. b The opsonized nanoparticles bind to the cell surface through specific recognition of the opsonins, trigerring actin assembly and particle engulfment. c The resulting phagosome matures, fuses with lysosomes and becomes acidified, leading to the enzyme-rich phagolysosomes (d) prone to particle degradation

Fig. 4

Vesicle formation during clathrin-mediated endocytosis. a The assembly of clathrin triskelions (based on three clathrin heavy chains) into a polygonal lattice helps deform the overlying plasma membrane into a coated pit. b After assembly of the basket-like clathrin lattice, dynamin is recruited at the neck of the pit to mediate the membrane fission. c This leads to the cytosolic release of the clathrin-coated vesicle. d The following uncoating of the vesicle allows the recycling of clathrin triskelia

Fig. 5

Intracellular nanocarrier trafficking following macropinocytosis, clathrin-mediated endocytosis and caveolae-mediated endocytosis. a Macropinocytosis leads to the formation of a macropinosome, which is thought to eventually fuse with lysosomes or recycle its content to the surface. b Clathrin-mediated endocytosis of a nanocarrier leads to the formation of an early endosome, which is acidified and fuses with prelysosomal vesicles containing enzymes (in red) to give rise to a late endosome and finally a lysosome, an acidic and enzyme-rich environment prone to nanocarrier and drug degradation. Unless a lysosomal delivery is desired, strategies for a cytosolic drug delivery by this route will focus on the drug escape from the endosome as early as possible. c Caveolae-mediated endocytosis of a nanocarrier gives rise to a caveolar vesicle that can be delivered to caveosome, avoiding a degradative acidic and enzyme-rich environment

Fig. 6

Examples of multifunctional nanocarriers. Such systems, also called “double-targeted,” can exhibit different surface properties depending on the external stimuli. In particular, their hydrophilic PEG coating allows a prolonged circulation time in normal tissues (where pH ~7.4) and a specific extravasation in tumoral tissue through the EPR effect; but once in the tumoral tissue (where pH decreases to around 6.5–7), the targeting ligands get the upper hand and promote nanocarrier internalization in the target cells. a Multifunctional polymeric micelles can be formulated using a pH-sensitive poly(histidine) actuator. Unionized, this actuator maintains the biotin ligand close to the hydrophobic core, hidden within the PEG chains. Once ionized, the actuator allows the exposition of biotin out of the PEG chain [162]. b Multifunctional liposomes can be PEGylated using an acid-labile hydrazone bond, which releases the PEG chains in an acidic medium, thus exposing the TAT targeting ligands coated directly on the surface of the liposomes [163]