Gene therapy is the emerging trend in biomedical science for treatment of life-threatening diseases. This involves delivery of a therapeutic gene to the nucleus of an affected cell by a suitable vector. Gene delivery using non-viral vectors such as cationic polymer and lipid is gaining attention due to their favourable properties, including lack of immunogenicity, low toxicity, and potential for tissue specificity when compared with viral vectors. A variety of non-viral vectors have been proposed, most of which facilitate gene delivery by electrostatic interactions, encapsulation, and in some cases, condensing nucleic acids into nano-sized particles which can then be taken up by cells. Successful gene delivery within a cell is the nanocarrier's ability to protect its contents from degradation in the extracellular environment. A well-designed nanocarrier will promote cellular uptake and intracellular release of the nucleic acid. This chapter highlights different polymers, lipids, and their nanocarriers employed for gene delivery along with clinical trials.
Top1. Introduction
Gene therapy has gained significant attention over the past two decades as a potential method for treating life-threatening diseases (Swami, 2013). The idea of gene medicine is based on the utilization of nucleic acids as drugs for gene therapy with the hope of repairing or shutting down of a specific cellular function associated with the target gene (Cavazzana-Calvo, Thrasher, & Mavilio, 2004). It is now possible to treat diseases of genetic origin by administering healthy copies of mutated genes or promote a protective immune response by administering genes encoding for specific antigens. Currently, the research focused on designing effective carrier vectors that compact and protect oligonucleotides for gene therapy as free oligonucleotides are rapidly degraded by serum nucleases in the blood when injected intravenously (Anderson, 1985). Gene expression results when nucleic acid is transported inside the cell nucleus of the target cell and there is still a need for carriers, which perform this work safely and efficiently. An ideal carrier should be specific to the target cells and easy to enter the cell body. It should be able to express abundantly and continuously. Furthermore, it has to be safe, without showing any side effects, and be capable of large-scale production. Initial research concentrated on using viral carriers, including both retroviruses and adenoviruses, as these vectors exhibited high efficiency at delivering nucleic acids to numerous cell lines (Anderson, 1985). However, fundamental problems associated with viral vector systems, including toxicity, immunogenicity, and limitations with respect to scale-up procedures, encouraged the investigation of other potential carriers for the delivery of nucleic acids into targeted tissue (Verma, & Somia, 1997).
Non-viral gene delivery involves formulation of liposomes complexed with nucleic acid (lipoplexes) (Song, 1997), cationic polymers complexed with nucleic acid (polyplexes) (Ogris, 1999), polymeric vesicles complexed with nucleic acid (Brown, 2000) or a combination of both cationic lipids and cationic polymers complexed with nucleic acid (lipopolyplexes) (Kircheis, 1999). Figure 1 gives an overview of formation mechanism of gene-nonviral vector complex and release of gene followed by expression. After administration, these complexes were taken up by cells through endocytosis (Luo, & Saltzman, 2000). Cellular entry of these complexes is followed by endosomal escape and dissociation, which release nucleic acid into the nucleus from nonviral vectors complex for gene expression (Smyth, 2002). The lipids/polymers used are generally less toxic and immunogenic than the viral counterparts. Other advantages of nonviral approaches include ease of production and the potential for repeat administration. Nonviral gene delivery methods are generally viewed as less efficacious than the viral methods in terms of gene expression however, these methods are considered safer than methods involving viral vectors. The purpose of this chapter is to provide a concise overview of different polymers, lipids and their nanocarriers employed for gene delivery.
Figure 1.
Various nonviral gene delivery vectors and sequential step in gene delivery. (A) siRNA complex formation by surface adsorption mechanism, (B) siRNA complex formation by entrapment mechanism (C) entry of gene-vector complex into cell, (D) endosomal escape, (E) entry of gene into cell nucleus and (F) gene expression.