a Enzyme Research Team, Biomass Engineering Program Cooperation Division, RIKEN Center for Sustainable Resource Science, Wako-shi, Japan. E-mail: email@example.com
The delivery of drugs or genetic material into cells is one of the emerging areas of biotechnology. Nanoparticle (NP)-based drug carriers are especially interesting due to their ability to deliver drugs inside target cells, thereby reducing the side-effects of non-specific treatments. Among the various cargoes that are transportable by NPs, genetic material allows the reprogramming of cells both temporarily as well as permanently. Several approaches are available for gene delivery, such as the use of natural vectors, like modified viruses, or artificial ones, in the form of liposomes or peptidic complexes. Among the key consideration for designing NPs to effectively overcome biological barriers are their physicochemical properties and effects they produce in living organism. This chapter discusses how the properties of nanocarriers can affect the biological response as well as the functionality of drug and gene delivery systems.
Delivery of drugs or genetic material into cells is one of the emerging areas of biotechnology.1 Nanoparticle (NP)-based drug carriers are especially interesting, due to their ability to deliver drugs inside target cells, thereby reducing the side-effects of non-specific treatments.2 Among the various cargoes that are transportable by NPs, genetic material allows the reprogramming of cells both temporarily as well as permanently.3 Several approaches are available for gene delivery, such as the use of natural vectors, like modified viruses, or artificial vectors, in the form of liposomes or peptidic complexes. Among the key considerations for designing NPs to effectively overcome biological barriers are their physicochemical properties and effects they produce in the living organism. This chapter discusses how the properties of nanocarriers can affect biological responses as well as the functionality of drug and gene delivery systems.
NPs can be classified following several parameters, size being one of the first established. In this way, NPs are defined as particles with a diameter <100 nm,1,4,5 but in practical applications6 it is common for larger particles to be used (up to 1000 nm), especially for drug4 and gene delivery. NPs can be differentiated based on composition, structure or properties; however, given their heterogeneity it is difficult to talk about pure types. Cargoes can interact with NPs either covalently, non-covalently via weak forces such as electrostatic or hydrophobic interactions, by hydrogen bonding, or can be physically entrapped in the matrix. The various types of NPs described in this chapter are summarized in Figure 1.1.
Viruses are infectious nano-sized pathogens (10–200 nm) that naturally deliver their genetic material into a determinate cell line or organ. Viruses are mainly composed of proteinaceous materials (capsid) that have provided the basis for the development of drug and gene nanocarriers.7–11 Viral-like NPs (VLPs) differ from VNPs by the absence of endogenous genetic material and their inability to replicate or to alter the host genome.7,12,13 VLPs (Figure 1.1A) can be formed by self-assembly of capsid proteins over a functionalized inorganic NP core13 into well-characterized monodisperse structures. Capsid proteins can be engineered and modified with different chemicals and proteins to promote specific targeting and improve penetration properties in VNPs/VLPs.8,9,14,15 Gene delivery is one of the most studied fields of VNPs, since viruses are natural agents that transfect their genetic material3,16–19 into cells. Although RNA and DNA viruses are potential candidates for gene delivery,3 they usually present problems of toxicity and immunogenicity, with limitations in the types of transportable cargo.20,21 VNP/VLPs have been used to improve vaccine effectiveness7,12 due to the strong immune response that is produced in the host. The development of artificial gene vectors such as polymeric NPs or liposomes22 has attracted the attention of researchers as an alternative to natural viruses, which can produce harmful side-effects when introduced into living organisms.3,17,18
In general, transport of hydrophilic compounds is more favorable in biological aqueous conditions.23 Hydrophobic cargoes can be transported by amphipathic NPs forming the classical core–shell carriers.5,24–28 For example, in micelles (Figure 1.1B), the hydrophilic part is exposed to medium while forming a hydrophobic core.29 Amphipathic interactions can produce NPs with several layers, the transport of hydrophilic cargoes in liposomes being possible24,30 (Figure 1.1C). Liposomes can easily penetrate the cytoplasmic membrane6 and promote cellular uptake.31 Micellization of chemotherapeutic agents can reduce their cytotoxic effect and increase their effectiveness.28,32–35 Liposomes formed with cationic lipids36 are extensively used in gene delivery, being one of the most efficient strategies.37 Cationic lipids can spontaneously combine and condense negatively charged DNA molecules, thus allowing penetration of DNA into cells while being protected from nuclease attack.38 Although liposomes have high loading capacity, their low stability and strong interaction with cargo can produce a non-stable release.39,40In vivo employment of cationic lipids is limited by their high cytotoxicity and their unspecific absorption by phagocytic cells of the reticulo-endothelial system.20 A drawback of liposomes is their low stability, which can result in fusion, aggregation, or leakage of the encapsulated drug substance during storage.41 An alternative to increase the stability of lipid NPs is the use of solid lipid NPs that mainly consist of solid lipid stabilized with surfactants.5,24,42 These NPs show good physical stability and biocompatibility,25,42,43 although, similar to other hydrophobic NPs, they have a short half-life in vivo and are removed from the blood circulation by the reticulo-endothelial system, particularly in the liver and the spleen.40
Polymeric NPs consist of non-biodegradable and/or biodegradable polymers.42 Biodegradable polymers, which are widely used for drug delivery,25,44,45 can be divided into two groups, namely biopolymers (protein, peptide and polysaccharide) and synthetic polymers [poly(lactic acid) (PLA) and poly(ε-carpolactone) (PCL)]. Polymer composition and physical properties are factors that influence the effectiveness of these NPs.24 The high versatility of these NPs, produced by the precipitation of linear polymers into colloidal nanoparticles solution, is due to the countless numbers of available polymers and their combinations6 (Figure 1.1F).
The more commonly used non-degradable synthetic polymers are N-(2-hydroxypropyl)-methacrylamide copolymer (HPMA),24,46 poly(vinylpirrolidone) (PVP)39 and polyethylene glycol (PEG),25 since they do not induce a significant cytotoxicity within biological systems.5,24 Poly(N-isopropylacrylamide) [poly(NIPAAm)] is a thermosensitive polymer and exhibits a low critical solution temperature at body temperature.47 These characteristics allow poly(NIPAAm) to be employed extensively as a drug carrier21,48,49 for thermally controlled release. Natural polymers including albumin, silk, chitosan, and heparin have been employed for the delivery of oligonucleotides, proteins, and drugs.45,46,50 A substantial part of the studies on polymeric NPs focus on the encapsulation of larger molecules like DNA or proteins rather than drugs.23 Nanospheres (Figure 1.1E) are NPs in which the cargo is dispersed through the polymeric matrix.25 Biodegradable nanospheres are especially suitable for the controlled release of drugs,4,51 because the choice of polymer for NP formation promotes a different degradation rate under selected conditions.23 Biodegradable polymeric NPs52 comprise PLA, poly(glycolic acid), poly(lactic-glycolic acid) (PLGA),4 poly(methyl methacrylate) (PMMA), or poly(l-glutamic acid) (PGA).25,53 These NPs are advantageous because they can undergo hydrolysis to form biodegradable metabolites in biological systems. For applications that require a long-term biocompatible stay in the host organism, PCL can be used for NP synthesis due to its slower degradation rate in comparison with other biodegradable NPs.52 Polycationic polymers such as poly(l-lysine)54 (PLL) or linear poly(ethylenimine) (PEI) have been employed for the condensation of DNA to form poly-ion complexes. PLL has been shown to mediate gene transfer by compacting pDNA into a tight toroid structure of ∼100 nm and rendering it resistant to DNase digestion.54 Even so, the high cationic nature of PLL and PEI produces cytotoxicity and triggered an immune response by the activation of complement.55 Another cationic polymer is the natural polysaccharide chitosan,56 which has shown positive attributes of biocompatibility and degradability.21 This linear polymer, which is a soluble derivative of chitin (the main compound of arthropod shells), is composed of randomly distributed d-glucosamine and N-acetyl-d-glucosamine.56 Chitosan NPs have the ability to adhere to mucosal surfaces and penetrate into cells;20,57 the presence of hydroxyl and amine groups allows chemical modification to increase its bioactivity.20,21,50,57–60 For all the above, chitosan derivatives have been studied as non-viral vectors, since its cationic charges allows complexation with DNA or RNA.21,56 Cyclodextrins61–63 are cyclic oligosaccharides with a lipophilic inner cavity and a hydrophilic outer surface. Their amphipathic nature allows formation of non-covalent inclusion complexes with drugs,63 although inorganic compounds are not generally suitable for complexation.62 These versatile molecules have been employed to increase the loading capacity of NPs, since they are able to enhance the number of hydrophobic sites in NPs structure.62,64 Cyclodextrins can mask drug cytotoxicity65 and even form the backbone of more complex structures for the transport of genetic materials.21,55,66
Dendrimers are radially hyperbranched polymers with regular repeat units.6 They are attractive systems for drug delivery due to their highly defined dispersity, nanometer size range, spheroid-like shape and multi-functionality.47,67,68 Aside from their ease of preparation, dendrimers have multiple copies of functional groups on the molecular surface which enables derivatization for biological recognition processes.65 Even so, dendrimers are reported to cause hematological toxicity,69 especially in the case of non-functionalization. Examples of typical dendrimers are poly(propyleneimine) (PPI), poly(amido amine) (PAMAM),70,71 poly(2,2-bis(hydroxymethyl)propionic acid (bis-MPA), poly(glycerol-succinic acid) (PGLSA-OH)72 or epsilon derivatives of PLL.73 Although their high charge density allows easy insertion into membranes, and can facilitate endosomal escape,74 dendrimers have low water solubility and exhibit elevated cytotoxicity.75 For example, PAMAM NPs have limited applications in medicine due to their original toxicity.55,69,70 PAMAM is known to induce nephrotoxicity as well as hepatotoxicity, and its cationic charge can cause platelet aggregation by disruption of membrane integrity.76 Nevertheless, it is possible to reduce the cytotoxicity by chemical modification and the combination of different polymeric ends.47,55,69,75,77,78 Branched PEI is one of the most studied and commonly used branched polycationic polymers6,21,70,79–83 for gene delivery, due to its cationic charges and lower cytotoxicity compared to PAMAM, although its cytotoxic effect is higher than other cationic polymers.84 In general, the buffering capacity of polyamines promotes endosomal escape of NPs, since osmotic swelling occurs by the accumulation of chloride ions in the endosome.85,86
Peptidic NPs are based on the use of peptide sequences that promote cellular internalization known as cell-penetrating peptides (CPPs) or protein transduction domains.29,87 CPPs are short peptides (6–30 aa) that are able to cross the cellular membrane for intracellular trafficking of cargoes.6,29,50,87 It has been postulated that the ability of CPPs to be internalized by cells is related to their strong affinity for lipid bilayers.88 CPP sequences are based on natural protein-transduction domains87 such as transactivator of transcription of human immunodeficiency virus (HIV-1 TAT peptide),6,26,29 or penetratin (pAnt), which is derived from the third helix of the Drosophila antennapedia homeodomain.26,89 Low molecular weight protamine (LMWP) is derived from the natural protein protamide, an arginine-rich nuclear protein that replaces histones during spermatogenesis.90,91 LMWP has demonstrated comparable performance to TAT peptide for cellular translocation while being neither as antigenic, mutagenic nor cytotoxic as other cationic peptides.90 Similar to other NPs, CPPs can be covalently linked to the cargo, forming a conjugate that promotes transport and internalization of the complex25,29,87via cellular pathways, but this covalent modification may alter the biological activity of cargoes. To circumvent this limitation, a non-covalent strategy for the attachment of cargo29,87,92,93 without the necessity of chemical cross-linking or modification is preferred. The presence of cationic amino acids such as lysine or arginine in CPPs seems to be one of the factors that help to improve their transfection efficiency.88,94 Meanwhile histidine-rich regions can enhance endosomal escape through the pH buffering, or proton sponge effect.45 In general, cationic CPPs can form complexes with negatively charged DNA molecules based on electrostatic interactions.50,94 Besides CPPs, cationic peptides have been used for gene delivery, and consist of consecutive basic amino acid sequences, which compact DNA into spherical complexes, or chromatin-like components such as histones or protamine, which compact DNA in a structured manner.3 In this way, oligo-arginine95–97 has demonstrated similar characteristics to CPP in cell translocation, being superior to other polycationic homopolymers.98,99 The peptide motive Arg-Gly-Asp (RGD) is able to recognize and bind to the ανβ3/ανβ5 integrins that are expressed in certain cell types such as endothelial cells, osteoclasts, macrophages, and platelets.44,100 Integrins are transmembrane glycoproteins that interact with the cellular matrix and promote receptor-mediated endocytosis. ανβ3/ανβ5 integrins are overexpressed in angiogenic endothelial cells, also being suitable markers for neoplasms.38 Modifications to increase the specificity of NPs 97,101 for targeted delivery to a specific organelle within a cell6 include the use of signal peptides87,101 such as nuclear localization signals102 or mitochondrial-targeting peptides.103 NPs composed of amphipathic peptides6,87,93,104 can also be used for the delivery of hydrophobic drugs.27
Nanocrystals are associations of molecules in a crystalline form,105 composed of pure drug with only a thin coating of surfactants25 (Figure 1.1I). Drug nanocrystals can be generated by “bottom-up” (intermolecular association) or “top-down” (milling of crystals) technologies.64 Nanocrystals NPs are composed of 100% drug without the addition of carrier materials such as polymeric NPs.64 Nanocrystals have been more studied for material science than for drug delivery, given that not all therapeutic compounds can be easily crystallized.105 However, they are the usual choice for the oral administration of drugs,25,64,106 since their nano-scale size improves drug solubility and dissolution rate as well as increasing adhesion to the intestinal wall and capillary uptake.64,107 Nanocrystals have also been successfully employed in the parenteral delivery of compounds such as antibiotics or insulin.5
Metallic NPs (Figure 1.1J) are heavily utilized in biomedical sciences because they can be prepared and surface-functionalized in many different ways.108 These NPs can be used in diagnostics as well as for drug and gene delivery.24 Metallic NPs can be easily synthesized over a broad range of sizes and shapes, and are usually composed of gold, platinum, titanium dioxide, copper, iron oxides [as magnetite (MxFe3−xO4, M=Mn, Ni, Co, Fe) or maghemite (Fe2O3)]105 and can be functionalized via thiol-metal chemistry.6 Metallic NPs can combine properties as surface plasmon resonance, magnetism, or anti-oxidant capabilities.109 Among the most used are the colloidal gold NPs,4,24,109,110 given that cells can intake gold NPs without apparent cytotoxicity. Shelling NPs with gold will reduce their cytotoxicity while increasing their stability.110 Gold NPs are one of the most successful inorganic carriers in oncology, where they have shown applicability as drug carriers and in the thermal ablation of tumors.4,111 However, the gold NPs with quantum sizes (1.5 nm diameter) can be toxic, because of their ability to penetrate into the cellular nucleus and bind irreversibly to DNA.112 Moreover, metallic NPs have shown the production of reactive oxygen species (ROS) and oxidative stress,40 although this effect has been shown ubiquitously in several types of NPs.69 The safety of the use of metallic NPs in vivo is under debate, considering that divalent cations and heavy metals are toxic.40,69
Silicon dioxide NPs (Figure 1.1K) are multifunctional structures available in micro- or mesoporous forms suitable for the encapsulation of various cargoes.6,105,113,114 Their low cytotoxicity and easy derivatization with different surface chemistries make them versatile tools for cargo delivery25,105,111,113–119 with excellent physicochemical stability. Silica NPs can be prepared as hollow or multichannel structures, which can be used for drug/gene cargoes. Mesoporous silica NPs present a high surface area and large pore volumes for functionalization.118,111 For example, by using complexes that limit the release of the cargo (nanovalves), it is possible to induce drug release following an intracellular event or external stimulus.111,114 Silica NPs have shown cytotoxicity caused by ROS generation, which could be correlated with size (20 nm being more toxic than 100 nm) and surface charge.120
Fullerenes and nanotubes are hollow, carbon-based cage-like NPs (Figure 1.1L) that have been employed for drug and gene delivery. These materials are constructed within a sheet of graphene arranged into small cylindrical or spherical structures.25,69 Carbon nanotubes can be formed using single or multi-walled graphene sheets and have different sizes and diameters depending on the synthesis conditions.121–123 Unmodified carbon NPs are insoluble, therefore requiring modification of their surface to improve solubility and reduce cytotoxicity.122,123 Carbon NPs can be functionalized by the addition of compatible functional groups for the delivery of various biomolecular cargoes.121 The efficiency of their transport across cells is related to their hydrophobic nature, which allows penetration without damaging the cellular membranes. However, these carriers have been reported to cause cellular apoptosis due to ROS production in the mitochondria69 and, similar to other fullerene compounds, they reduce the systemic immune response.12
NPs have a high surface area to mass ratio, which allows maximization of the functional surface while minimizing the biological response to the amount of exogenous compounds in the organism. The biological properties and interactions of nano-scale materials dramatically change in comparison with bulk material.5 Thereby, NP design must take into account some critical parameters65 such as a cytotoxic effect, transport and release of NPs into cells6,124 or target tissues4,26 (Figures 1.2 and 1.3). With the current advances in material design it is possible to modify or custom-make particles; in the design of NPs attention must be paid to the physicochemical properties of nanomaterials such as size, shape, or surface charge, and the biological response they induce. In addition to the specific advantages that a type of NP may present (Table 1.1), it should be considered that, due to their optimal properties, the applications and routes of administration will depend on whether the cargo is cytotoxic and this will determine if they are administered via oral, intravenous, cutaneous or mucosal routes.3,4,6,26,64
|Virus||High efficiency||Trigger immune response Poor drug-loading capacity|
|Micellar||Efficient loading of hydrophobic drugs Excellent cell penetration||Poor stability Liver accumulation Cytotoxic|
|Polymeric||Excellent functionalization High versatility||Polydisperse Cytotoxic|
|Dendrimers||Facilitate endosomal escape Controllable generation Easy functionalization||Cytotoxic|
|Peptidic||Increased cell penetration High specificity||Poor stability Self-aggregation|
|Nanocrystals||High loading capacity Good dissolution||Limited applicability|
|Metallic||Tunable shape and size||Cytotoxic Not biodegradable|
|Sillica||High loading capacity Easy functionalization Good biocompatibility||Poor solubility|
|Carbon-based||Stable Good cell penetration Tunable size and shape||Cytotoxic Immune suppressors Poor reactivity|
The size of NPs is known to affect the mechanism of absorption and residence time in the organism.105,125,126 It was observed that NPs of 20 nm can penetrate into cells without the contribution of endocytic mechanisms.4 However, it is generally described that NPs with a diameter <100 nm present a favored entrance into cells.124 Even 500 nm NPs can enter the cells via caveolae-mediated internalization, smaller NPs enter by phagocytosis, macropinocytosis or clathrin-mediated endocytosis.124,126 The absorption of large NPs can be reduced, since they will be recognized by neutrophils.105 However, for non-phagocytic cells, small size correlates with increased cytotoxicity.85 In the case of gold and silica NPs, ROS formation as well as cytotoxicity increases with a decrease in particle size.112,120 The NPs with smaller sizes have a higher tendency to aggregate and interact with biological components, thereby increasing their cytotoxicity.69 The size of NPs also influences their biodistribution, as demonstrated in a study using gold NPs. Upon administration of different-sized particles in rats, NPs with a diameter of 10 nm were localized in the main part of the organs, while those with diameters of 50 and 250 nm were only present in the liver and the spleen.107 Relatively small particles with diameters in the range 1–20 nm have a long circulatory residence.105 The mobility of NPs inside the cell depends on their size and interaction with the uptake pathway,6 for example NPs with a diameter of 40 nm can enter the cellular nucleus.85 The effect of size on the internalization of NPs varies depending on cell type, and it is also influenced by the shape of NP, varying between nanospheres and nanotubes.124
NP shape is a parameter that can significantly influence time of residence and mechanisms of cellular uptake.127–129 The cellular immune system respond to the shape of exogenous particles;125 macrophages phagocytose rod-shaped NPs faster than spherical ones, and internalization velocity depends on the orientation of the NP.127 Rod-like shape (aspect ratio) is one of the factors that intracellular pathogens use to enter into cells, even in non-phagocytic cells; for instance, rod-like NPs showed the highest uptake in HeLa cells, followed by spheres, cylinders, and cubes.125 Even though the overall process of phagocytosis is a result of the complex interplay between shape and size, the effect of shape becomes less evident when the size of NPs is <100 nm.124 Engineering the shape of NPs can tailor them to either avoid or promote phagocytosis.127 Conversely, an increase in phagocytosis will help to induce immune response112 and enhance cellular uptake, which will consequently reduce the bio-availability of NPs by accumulation in the reticulo-endothelial system.
The surface charge of NPs in suspension is usually determined by zeta (ζ)-potential analysis. When charged NPs are in suspension, surface electric charge attracts the surrounding counter-ions to form an electrical double layer, with an inner region (Stern layer) where the ions are strongly bound. ζ-potential is the electric potential that is produced at the surface of hydrodynamic shear between the ions of a solvent and the surface of a particle. Higher ζ-potential contributes greater resistance to flocculation of NPs due to their high self-repulsion.130,131 The surface charge of NPs plays a crucial role in cell association,40 where, generally, positively charged NPs show a higher internalization rate than neutral or negatively charged NPs.43,85,124,125 Positively charged carriers are frequently employed for gene transport, as they are able to form complexes with the negatively charged DNA molecule.29,87,92,93 Polarity of surface charge can promote the specificity of cellular uptake, given that phagocytic cells tend to preferably interact with negatively charged NPs.85 In addition to cellular uptake and stability of NPs, polarity of surface charge affects toxicity; for example, polycations are generally recognized as cytotoxic and hemolytic and can activate the innate immune system,55 whereas polyanions can cause anticoagulant activity and stimulate cytokine release.39 Positively charged NPs were generally found to be more apt to cause inflammation compared to a neutral or negatively charged particle.24 Cationic charged NPs produce toxicity in non-phagocytic cells by plasma-membrane disruption, while anionic NPs produce toxicity in phagocytic cells by apoptosis.85 When NPs are introduced into a living organism they readily associate with serum proteins and lipids, forming an external corona.40,43,132,133 The protein corona can affect the efficiency of cargo delivery, by affecting circulation lifetime, signaling, kinetics, and transport, and triggering the immune response133 (Figure 1.3). Formation of the corona can influence the size and shape of NPs, and in the case of strongly charged NPs proteins can be denatured, increasing the aggregation of NPs. The nature of the proteins forming the corona will depend on the NP's composition and surface charge, in this way, cationically charged NPs absorb proteins with an isoelectric point <5.5, such as albumin, while anionic NPs promote the absorption of proteins with an isoelectric point of >5.5, such as immunoglobulin G.133 Interaction with serum protein can completely alter the ζ-potential of NPs,134 changing the theoretical values predicted in protein-free solutions and alter or suppress the intended function of NPs.135
The surface properties of NPs are known to be fundamental in determining NP–cell interaction, hence the conjugation of various chemical or biological ligands to NP surfaces has been explored. PEGylation (modification of the surface with PEG) is the predominant method used to reduce toxicity6,26,35,38,39,65,136–138 as the resultant “stealth” brushes mimic the cell's glycocalyx.139,140 PEGylation increases a colloidal carrier's stability in vivo by its steric effect, which acts as a barrier to aggregation24,105 and the formation of the protein corona.43 Nevertheless, PEGylation reduces uptake38,124,141,142 and increases the circulation time of NPs.4 In an attempt to mimic viral particles, stearylation of peptidic NPs has dramatically increased cell transfection efficiency for gene delivery,97,143,144 albeit with a mild increase in cytotoxicity.145 Molecules attached to the NP surface can act as a homing sequence and change the biological distribution of the NP. For instance, it is known that sigma receptors are overexpressed in many human cancers and benzamides exhibit a high affinity for these sigma receptors.35 Hence, modification of the NP surface by the addition of benzamide (i.e. anisamide) enables the targeting of cancerous cells.35,146,147 Another method, folate receptor-mediated targeting, is extensively employed in the targeting of NPs into tumor cells.34,57,83,118,148–151 The folate receptor is overexpressed in many types of cancers (breast, ovary, endometrium, kidney, lung, head and neck, brain, and myeloid) and is internalized into cells after ligand binding.34 Another common strategy for the modification of NPs is the attachment of peptidic sequences, which can help with the improvement of translocation and absorption properties such as gastric mucoadhesivity152,153 and blood-vessel specificity.154 For example, LMWP,90 TAT peptide,26,155 and neurotensin-targeting peptide142 have been successfully employed for NP functionalization to enable the delivery of specific cargoes into the brain, while the RGD sequence can be employed for selectively targeting angiogenesis.50,156 Lactoferrin,157 lectins,158,159 or transferrin30,160,161 have also been employed for delivery of NPs into the brain. Arginine-based or derivatized NPs have shown to increase cellular uptake without increasing cytotoxicity,95,137,143,162,163 while the addition of histidine residues to the NP improved intracellular release of cargo by the proton sponge effect.58,70,164 Functionalization of NPs with tumor-homing peptides has shown that is possible to deliver NPs specifically to some types of tumors.136,165
Specific delivery of NPs is a crucial point when they transport chemotherapeutic drugs for treatment, for instance to cancerous cells. The first molecules that specifically recognized tumor antigens were monoclonal antibodies.83 Antibodies are able to specifically recognize target molecules (antigens) by multiple weak interactions between complementary three-dimensional surfaces.135 However, despite their high specificity, direct ligation of antibodies to drugs resulted in non-significant improvements in treatment,46 since their recognition properties are reduced after drugs are loaded. Even so, antibody modification can be employed for liposomes,30,70,157,166 cationic peptides,167,168 polymeric NPs,169,170 magnetic NPs,171 or mesoporous silica.113,172 Despite the demonstration of antibody application in vitro, its in vivo application is limited owing to their high cost, limited shelf-life, potential immunogenicity and retention in the reticulo-endothelial system.34,108
The nano-technological approach to improve gene/drug delivery has been a dynamic and promising research field for a few decades. In the design of nanocarriers, the intrinsic properties of NPs to select their composition for controlled release of cargo. Therefore, one of the main functions of NPs must be to serve as a transport system without increasing endogenous toxicity of the transported substance (Figure 1.3). The different NPs described in this chapter present advantages and drawbacks depending on the application and route of administration (Table 1.1). Since NPs represent the maximization of surface by unit of mass, improvement of the transporting capacities of NPs can be performed by functionalization of their surface (Table 1.2). NPs administered systemically tend to accumulate in tumors due to the enhanced permeability and retention effect.28,173 Nevertheless, NPs also tend to accumulate in the reticulo-endothelial system. In order to reduce the side-effects of a systemic treatment,83 design of chemotherapeutical drug nanocarriers must be focused into the specific release of cargo into the target cells/organs. One of the most promising fields of research in NPs is based on their synergic effect with cytotoxic drugs in the treatment of cancer.174 The studies carried out with NPs incorporating signaling molecules, such as benzamides, folate, or tumor-homing sequences showed their potential to improve target delivery functions. Furthermore, the large bioactive surface of NPs becomes fundamental to improve their efficiency and reduce their toxicity by tailoring signaling molecules.
|PEG||Reduced cytotoxicity||Increased circulation time Reduced cellular uptake|
|Cell-penetrating peptides||Increase cellular uptake||Cytotoxic|
|Folate||Tumor homing tag||Limited application|
|Anisamide||Tumor homing tag||Limited application|
|Tumor-homing peptides||Tumor homing tag||Less transfection efficiency|
|Arginine||Increase cellular uptake|
|Histidine||Increase endosomal escape|
|Stearyl (group)||Increase transfection|
|Gold shelling||Reduce cytotoxicity||Non-biodegradable|
|Antibodies||Increase specificity||Increases immune response|
© The Royal Society of Chemistry 2016 (2016)