Biopharmaceutical Parameters to Consider in Order to Alter the Fate of Nanocarriers after Oral Delivery

Emilie Roger; Frederic Lagarce; Emmanuel Garcion; Jean-Pierre Benoit

Disclosures

Nanomedicine. ;5(2):287-306. 

In This Article

Properties of Nanocarriers

The biodistribution of a nanocarrier depends on its physical and chemical properties. To perform pharmacological targeting after oral administration, the nanocarrier has to conserve its structure.

Hussain et al. have shown that over 10% of orally administered latex nanoparticles were found in the systemic circulation of rats.[21] Improving this percentage may be interesting but, in order to do that, nanocarriers have to avoid several stress factors and, consequently, should possess several properties.[68,117] Factors increasing particle uptake have been extensively described,[123] including size and surface charge. In the last part of this review, the discussion will be focused on the possible optimization that could enhance intact particle transport across the GI epithelium.

Size

Particle size and size distribution are the most important characteristics of nanoparticles related to their biodistribution properties. Nanoformulations increase the dissolution rate of the drug compound, improve bioavailability and reduce variability and the food effect.[161] Thus, nanocrystal dispersion (particles of 100–300 nm size) improves dissolution rate and oral bioavailability[162,163] in comparison with micronized suspension. Size is also an important parameter governing the entry of the nanocarriers in the cell and their fate in subcellular domains.[164] Generally, nanoparticles have relatively high cell uptake when compared with microparticles.[165] Particles with a diameter below 50 nm showed a higher degree of uptake by mammalian cells than large particles.[166,167] Nanoparticles have been shown to greatly improve cellular uptake over microspheres, which were only taken up by phagocytotic M-cells in the Peyer's patches: 100 nm nanoparticles experienced a 2.5-fold greater uptake rate by Caco-2 cells than 1 µm microparticles.[123,167] However, Win and Feng established that the uptake of 100–200 nm polystyrene nanoparticles was optimal, whereas nanoparticles with a size of 50 nm showed a lower uptake by enterocytes: 100 nm particles experienced a 2.3-fold greater uptake compared with that of 50 nm particles.[112]

Delivery of the drug polymer complex was achieved by producing nanospheres having a size compatible with caveolae-coated vesicles or clathrine-coated pits. Rejman et al. demonstrated that particles with diameter of less than 200 nm are internalized via CCPs and larger particles (200 nm–1 µm) are internalized by a clathrin-independent pathway, more especially by a caveolae-mediated endocytosis.[168] Therefore, receptor-mediated endocytosis is a size-dependent mechanism. Thus, whereas smaller particles (<200 nm) are internalized through a relatively rapid process by clathrin-mediated endocytosis, larger particles can be taken up via a caveolae-mediated receptor pathway and thereby escape degradation in the lysosomal compartment. This result, found by Rejman et al., is not in agreement with the observed diameter of caveolae-coated pits (50–80 nm), which implicates a modification of the CCP to allow the larger particles to be internalized by this particular pathway. This shows that the relationship between size and transport may be difficult to forecast and should be assayed case by case.

Size is a parameter governing the fate of a nanocarrier after oral administration and can be more or less easily modulated. In the case of polymer nanoparticles, the preparation process, the assembly of monomer and the molecular weight of the polymer (higher than 10,000 Da in general) are key parameters used to obtain the sought size.[169] For example, polyalkylcyanoacrylate nanoparticles produced by anionic polymerization had sizes ranging from 20 to 770 nm with dextran as a stabilizing agent (the size was 193 and 585 nm with dextran 70 and dextran 10, respectively).[170] If produced by interfacial polymerization, polyalkylcyanoacrylate nanoparticles had a size of less than 500 nm.[117,171,172]

The size of lipid nanoparticles can also be modulated. These particles are classified into two mains groups: SLNs and LNCs. SLNs are generally larger than 150 nm.[173] The particle size depends on the lipid, the surfactant type and percentage, and the production method. Internalization via CCPs is not likely. By contrast, LNCs are larger than 20 nm, but this can be modified by varying the excipient proportions.[174] Recent work demonstrated that LNCs were internalized via clathrin- and caveolae-dependent pit endocytosis.[91]

The size of liposomes depends on their structure, which is related to their preparation process: multilamellar (> 100 nm), small unilamellar (10–50 nm) and large unilamellar (LUV; 50–1000 nm) liposomes can be obtained.[175] Maintani et al. have shown that 100 nm liposomes were absorbed more effectively than those of 200 nm in diameter.[46]

Surface Properties

In order to increase the percentage nanocarrier absorption by the intestinal cells, nanocarriers need to remain intact following GI transit. Hydrophilic polymers such as PEG form a sterically stabilizing crown on the surface of the nanocarriers that protects the particles against GI degradation. In the case of LNCs developed in our laboratory, PEG660 (Solutol®HS15) is located on the external surface and makes the LNCs relatively stable in GI fluids.[176] To stabilise liposomes or some polymer nanoparticle formulations that are unstable, PEG can be covalently linked to the surface during preparation of formulation or by a postinsertion method.[177]

Nanocarriers also have to overcome intestinal transit and cross mucus to be taken up by enterocytes. Bioadhesion could be a solution to increase the uptake levels of particles by enterocytes rather than M-cells, which are less protected by mucus. The bioadhesion of colloidal drug delivery systems has been reviewed[178] and some polymers have mucoadhesive properties.[179] More particularly, Takeuchi et al. demonstrated that chitosan and Carbopol® exhibited superior adhesion ability (1.6- to 4.6-fold) than other polymers such as poly(vinyl alcohol) and poly(acrylic acid).[180] The coating can be obtained by using a fluidized bed; the nanocarriers can be mixed with the polymer solution, and sometimes the coating is added during the preparation of the particles.

Yin et al. have also shown that WGA-conjugated PLGA nanoparticles increased the bioadhesion of nanoparticles (~1.5–4.8-times) via lectin binding sites over the rat M-cells and enterocytes.[181] WGA is a specific ligand, demonstrating an affinity for N-acetyl-D-glucosamine and sialic acid, which is present in both M-cells and enterocytes. Thus, coating nanocarrriers with mucoadhesive polymers or polysaccharides allows increased contact with the apical membrane of enterocytes and thus these vectors can be absorbed favorably by the paracellular or transcellular route.

Surface coating can be confirmed by measuring the ζ-potential. For example, polystyrene nanoparticles are uncharged particles (ζ potential equal to +1.1 ± 1 mV) and very hydrophobic. When coated with hydrophilic polycation chitosan, these particles display a ζ potential of +17.5 ± 3.6 mV. This potential becomes negative (–23.9 ± 1.2 mV) after coating with PEG–PLA (hydrophilic PEG-coat). The cell association of nanoparticles with Caco-2 cells and MTX-E12 cells depends on this coating (the rank order of association was found to be: polystyrene > chitosan >> PEG-PLA with Caco-2 cells and chitosan > polystyrene >> PLA-PEG with MTX-E12 cells).[70] Thus, the surface charge, and especially the surface properties of nanocarriers appear to influence first the transport across the mucus. More hydrophobic particles prepared with poly(lactide-co-glycolides) or polyanhydrides polymers increase mucus transport in comparison to more hydrophilic particles, for example alginate.[182] Moreover, the surface properties and surface charge seem to influence the transport of nanocarriers by intestinal cells. The addition of positive charges to nanocarriers may improve their potential to be internalized by endocytosis and more especially, by the clathrin-mediated endocytosis pathway.[183] Moreover, positive charges avoid endolysosomal degradation. These positive charge can be acquired by the incorporation of chitosan into nanocarriers[77] or cationic lipids such as N[1-(2,3-dioleyloxy) propyl]-N,N,N-trimethylammonium chloride or 1,2-dioleyl-3-trimethylammonium-propane. Stearylamine or oleylamine can be added to nanoparticle formulation to confer a net positive surface charge.[4,184]

Targeting Nanocarrier

Specific ligands can be added to the nanocarrier's surface in order to target specific surface receptors on enterocytes. It is well documented that several proteins are expressed in caveolae, such as the folic acid receptor, the glycophosphatidylinositol anchor, gp60, the autocrine motility factor receptor, the IL-2 receptor, GM1 gangliosides, sialic acid, α2β1-integrin, PDGF, EGF, bradykinin and the CCK receptor.[185] Consequently, in order to promote this endocytosis pathway, nanocarriers conjugated to specific ligands could be formulated (for example with folic acid, albumin or the new peptide, IRQ[185]).

Clathrin-mediated endocytosis is receptor-mediated endocytosis, and includes transferrin, asiloglycoprotein, EGF, and CCR2 chemokine.[186] Thus, transferrin-coated nanoparticles[187] or ICAM-1-coated nanocarriers[188] can target clathrin-mediated endocytosis. Nevertheless, peptides that interact with receptors are generally unstable in the GI environment, resulting in the degradation of the compound prior to absorption. Consequently, it may be important to confirm that the conjugated ligand keeps its properties after GI transit.[189] Interestingly, tomato lectin was shown to resist the digestion process[190] and provided specific binding to biological surfaces bearing sugar residues located at the surface of epithelial cells. Consequently, tomato lectin-conjugated nanoparticles enhanced the bioadhesive and endocytic potential of latex particles.[191,192]

Another possibility is to coat nanocarriers with polymers that enhance endocytosis pathways. For example, the presence of polyethylenimine or WGA[86] allow endocytosis by both clathrin and caveolae-mediated endocytoses.[193] Indeed, WGA is also a specific binder for N-acetylglucosamine and sialic acid.[84] However, polymers (e.g., PEG–PLA) demonstrate better lysosome escape ability than WGA, and may thus protect certain loaded drugs from lysosomal degradation.[86] A mixture of WGA and PEG–PLA may increase endocytosis and escape lysosomal vesicles activity. Russel-Jones et al. also demonstrated an increase in internalization and transport of vitamin B12-coated nanoparticles.[194] Moreover, the modification of PLGA nanoparticles with vitamin E (TPGS; bioadhesive material) improved cellular uptake.[119] Nevertheless, coating nanoparticles can increase particle size and in this case modified the endocytosis pathway.

Finally, due to the presence of enzymes and acidity, most nanocarriers aim at avoiding lysosomal trafficking,[186] however, currently no specific targeting for caveolae endocytosis can be administered orally due the GI tract environment

Co-administration

In order to increase the oral bioavailability of drugs encapsulated in nanocarriers, it may be interesting to administer nanocarriers with a simultaneous physiological system that can favor the GI absorption. Samstein et al. demonstrated the potential of bile salts for improving the oral delivery of PLGA nanoparticles.[195] The co-administration of nanoparticles and deoxycholic acid emulsion improved the bioavailability of encapsulated model agent (rhodamine B). In fact, the presence of bile salts delayed transit time, thus increasing absorption, and bile salts can also disrupt tight junctions, hence improving paracellular transport. In the same way, exposure to Yersinia pseudotuberculosis affects the paracellular route in model epithelial cells, and improves the transcytosis of nanoparticles. To be more precise, bacteria induced active energy-dependent transport, possibly macropinocytosis, across the intestinal epithelium.[141]

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