Polymer-based delivery systems

Recently, siRNA conjugates have shown promise as therapeutic agents. Despite the high therapeutic potential of siRNA, its application in the clinical setting is still limited. This is due in part to the lack of efficient delivery systems that can deliver siRNA to its intended site of action (Jeong JH et al. 2007, Iorns EC et al. 2007, Guo P et al. 2010, Wang Y et al. 2010). The following carrier systems have been used for siRNA transport in the past:

Polymer- and liposome-based transport systems: These have been best studied and further developed for siRNA transport (Dana H et al. 2017). As non-viral siRNA and plasmid vectors, many polymers have been investigated due to their physical properties and diverse chemical properties, as well as their well-characterized structural flexibility. These allow easy modification to fine-tune their physiochemical properties.

Chitosan: Chitosan is a copolymer of N-acetyl-D-glucosamine (GlcNAc) and D-glucosamine (GlcN) and acts as a weak base due to the presence of the D-glucosamine residue. For decades, chitosan has been widely used in various fields, especially in pharmaceuticals. The versatility of chitosan stems from its high natural occurrence, non-allergenicity, biocompatibility and biodegradability. In addition to these advantageous properties, the cationic nature makes chitosan derivatives a potent vector for nucleic acids. Although chitosan-mediated pDNA transfer has been widely researched, its application in siRNA transfer has not gained much importance (Mao et al. 2010).

Cyclodextrin is another polymer that is also used as an siRNA delivery system (Felber AE et al. 2012). Cyclodextrins (CD) are naturally occurring cyclic oligosaccharides composed of 6 (α-CD), 7 (β-CD), or 8 (γ-CD) D(+)-glucose units linked by α-1,4 linkages (Felber AE et al. 2012). CD-containing polymers (CDPs) are valued for their lack of immunogenicity. Moreover, CD possesses unique geometrical and structural properties. The hydrophilic outer surface and hydrophobic inner cavity allow it to accommodate insoluble molecules to form inclusion complexes. Taking advantage of their special structural properties, CDPs have also been used in the transport of nucleic acid payloads such as pDNA, siRNA, and DNAzymes (Bartlett et al. 2007

LGA (poly(dl-lactide-co-glycolide) is a copolymer of glycolic acid (GA) and lactic acid (LA) linked by ester bonds. PLGAs with different compositions of LA and GA have been commercially developed and are being investigated for a wide range of biomedical applications. PLGA-based nanoparticles (NPs) have attracted considerable attention as non-viral polymeric vectors in nucleic acid transport research due to their small particle size, advantageous safety profile and sustained release characteristics. siRNA-loaded PLGA NPs have shown great promise in in vitro gene silencing (Yuan et al. 2006). In general, 2 different approaches are used to load nucleic acids into PLGA NPs: encapsulation into the core of the NPs and adsorption onto the surface of modified cationic PLGA NPs via electrostatic interactions.

Polyethylenimines (PEI) or polyaziridines are polymers with a repeating unit consisting of the amine group and an aliphatic CH2-CH2 moiety. Linear polyethylenimines contain all secondary amines, unlike branched PEIs which contain primary, secondary and tertiary amino groups. Polyethyleneimines are good delivery systems. They are manufactured on an industrial scale.

Polypeptides: For several reasons including their lower cytotoxicity, polypeptides are considered as an alternative to cationic polymers for siRNA transport: their advantage is pH-based membranes, their efficient packaging and their efficient membrane transport. Their simple synthesis and the stability of the peptide-oligonucleotide complex make the low molecular weight polypeptides a particularly suitable efficient transport vehicle for siRNA (Adami RC et al.1999, Choi SW et al. 2010). However, their condensation ability is less pronounced. Similarly, the release of the nucleic acid payload at the cytosol is a problem. In this respect, several functional and structural modifications are usually carried out.

Cell-penetrating peptides (CPPs): Cell-penetrating peptides, also known as membrane translocation peptides or protein transduction domains (PTDs), were first discovered several decades ago when the HIV-1 Tat protein crossed the plasma membrane and transactivated transcription of the HIV-1 genome. CPPs are the most studied peptides for siRNA transport. CPPs are classified into three classes: synthetic CPPs, chimeric peptides CPPs naturally derived peptides. Versch. Research groups conjugated the cholesterol moiety and cationic cell-penetrating peptide oligo-D-arginine to develop a hydrophobically modified protein transduction domain, cholesteryl-oligo-D-arginine (Chol-R9) (Kim et al. 2006). The efficient regression of a subcutaneous tumor in a mouse model after local administration of complexed VEGF-targeting siRNA could be attributed to the enhanced cellular uptake and stability of Chol-R9/siRNA complexes induced by the cationic cell-penetrating peptides and the cholesterol moiety, respectively.

Lipid-based delivery systems: Various lipid-based delivery systems have been developed for the in vivo application of siRNA. Lipid-based systems include liposomes, micelles, emulsions, and solid lipid nanoparticles (Zhang J et al.2013; Semple SC et al. 2010). Liposomes have been used as transport vehicles for a wide range of therapeutics for nearly 30 years. More recently, they have also been used for siRNA. The interaction of lipids with nucleic acid leads to the formation of either coated vesicles with nucleic acid in the core or aggregates. The combination of two or more types of delivery vehicles can lead to the formation of complex combinations, These include: peptide polymers, liposome peptide polymers and others (Lee SK et al. 2012).

Dendrimers: Dendrimers are regular and highly branched monodisperse and usually highly symmetric spherical synthetic macromolecules with tunable structure, molecular size and surface charge. The unique structural properties such as high chemical and structural homogeneity, high ligand and functional density enable them to load therapeutics by internal encapsulation, surface adsorption or chemical conjugation. Polycationic dendrimers such as poly(amidoamine) (PAMAM) and poly(propylene imine) (PPI) dendrimers have been extensively studied as efficient vehicles for gene and therapeutic drug delivery. However, there are few reports describing their potential for siRNA delivery.

Nanogel: Nanogels are nanoscale, colloidally stable hydrogel particles composed of chemically or physically cross-linked polymer networks. High water contents, high loading capacity, high stability, biocompatibility and protection of loaded biomolecules, including siRNA, make nanogels potential delivery systems for therapeutic carriers compared to hydrogels. Moreover, nanogels can be internalized by cells and transfer siRNA to the cytosol, where siRNA-mediated gene silencing occurs. Therefore, nanogels are promising as siRNA transporters (Kabanov AV et al. 2009).

Nanomaterial-mediated siRNA transport vectors.

• Nanomaterials such as gold nanoparticles, carbon nanotubes, and silica nanoparticles have attracted much attention in their pharmaceutical applications due to their biocompatibility and ability to facilitate the delivery of therapeutic cargos. Thus, the fusion of nanomaterials and polymer has the potential and has been applied for gene and siRNA delivery. Gold nanoparticles (AuNPs) have shown promise as siRNA delivery vehicles. They are easy to synthesize and offer well-defined surface chemistry and possess good biocompatibility. The conventional strategy uses immobilization of siRNA on the AuNP surface to develop siRNA-AuNP conjugates (Giljohann et al., 2009), which have a longer half-life in serum and prolonged ability for gene knockdown.
• Mesoporous silica nanoparticles and carbon nanotubes also belong to this group. The relatively new mesoporous silica nanoparticles (MSNs) suitable for siRNA delivery have a good chance to assert themselves as delivery systems for siRNAs due to their unique structural and functional properties such as the chemically stable mesoporous structures, large surface areas, adjustable pore sizes, encapsulation of small molecules, and well-defined surface chemistry. For example, a PEI-functionalized MSN-based doxorubicin/siRNA codelivery system induces apoptosis of drug-resistant cancer cell lines by a synergistic method.
• Carbon nanotubes: Another recent development in nanoparticle-mediated siRNA delivery is the incorporation of carbon nanotubes into the polymeric composite system due to the chemical inertness, ease of surface functionalization, and unique structure of CNTs. The inherent hydrophobicity of the original CNTs necessitates functionalization of the CNTs to impart water solubility and biocompatibility.