Polymer-Based Vectors

Polymer-based biomaterials are becoming increasingly important, whether they are used as medical supplies (pipes, catheters, bags), prostheses, or dental materials, or in a pharmaceutical context as drug conjugates [4, 7, 158-160], protein conjugates [6, 158, 159, 161], synthetic vectors [12, 14, 18, 162, 163], or as immuno-adjuvants [164, 165]. 

In 1991, Luger et al. revealed by X-ray analysis the crystal structure of a natural DNA-histone complex. The X-ray structure shows in atomic detail how the histone protein octamer is assembled and how the base pairs of DNA are organized into a superhelix around it [74]. Since then this protein structure with cationic amino acids on the surface has acted as a model for the rational design of dendritic polymer-based gene vectors to mimic the globular shape of the natural histone complex [75-77]. 

The alternative approach is to use non-viral vectors, such lipid-based, peptide-based and polymer-based delivery systems, as described in detail in Chapter 14. Liposomes are relatively easy to manufacture, are generally non-toxic and are devoid of the capability to cause an infection (see Section 5.3.1). However, a number of limitations are associated with their use. For example, it is difficult to direct liposomes to a particular type of cell. Liposome/DNA complexes which may be internalized by the target cells are... 

Table 19.1 Representative cationic polymer-based gene delivery vectors.

Dendrimers have been investigated as a vector for nucleic acid-based therapies for nearly 20 years, and PAMAM and PPI dendrimers are the two most commonly used kinds that are commercially available.[113-115] Their unique chemical architecture with all primary, secondary, and tertiary amines enabling the proton-sponge effect described above make dendrimers one of the ideal platforms for gene delivery. Furthermore, the close-to-monodispersed chemical structure of dendrimers allows the precise control over their functionalities, which can minimize the unpredictable transfection efficiencies observed in the cases of heterogeneous liposomes and other polymer-based systems.[l 16]... 

Gene delivery vehicles can be divided into essentially one of two categories recombinant viruses and synthetic vectors. The majority of synthetic vectors, furthermore, can be divided into polymers (the subject of this review) and liposomes. Each type of material has important advantages and disadvantages. In order to best understand the problems facing polymer-based gene delivery vehicles and their current state of development, it is useful to briefly examine the alternate methodologies (Table 1). 

Paulino, A.T., Pereira, A.G.B., Fajardo, A.R., Erickson, K., Kipper, M.J., Muniz, E.C., Belfiore, L.A., Tambourgi, E.B., 2012. Natural polymer-based magnetic hydrogels potential vectors for remote-controlled dmg release. Carbohydrates Polymer 90, 1216—1225. 

Lipid- and polymer-based nonviral vectors for systemic siRNA and DNA delivery... 

Table 14.1 Current clinical trials using lipid- and polymer-based nonviral vectors containing siRNA (identification numbers of tbe clinical trial from bt s //clinicaltrials.gov/)... 

Despite the recent advances in the development of polymer-based nonviral vectors for siRNA or DNA delivery, the number of formulations that entered clinical trials has been substantially lower than lipid-based nonviral vectors. In this respect, the first-inhuman trial for siRNA delivery was a transferrin-targeted cyclodextrin, CALAA-01 (Fonseca et al., 2014). In 2008, Calando Pharmaceuticals (now part of Arrowhead Research Corporation) started a phase 1 clinical trial with CALAA-01 for the treatment of melanoma (Zuckerman et al., 2014). The rationale relied on the dowmegula-tion of ribonuclease reductase (RRM2) upon transferrin-mediated endocytosis of the siRNA-containing nanoparticle (Zuckerman et al., 2014). However, this nanoparticle was further withdrawn (Fonseca et al., 2014). 

The two classes of non-viral vector that are being investigated most actively are the cationic lipid-based vectors and the cationic polymer-based vectors. The cationic polymer-based vectors represent the newest class of non-viral delivery system to be developed and there are a number of groups working with these vectors. As this is such a new area of research there has yet to be any standardised definition of the terms involved in this field. Complexes formed between cationic polymers and DNA have been referred to as interpolyelectrolj1 e complexes, molecular conjugates, polylysine-DNA complexes, DNA-polylysine complexes, polyplexes and so on. In this review complexes formed between poly-(L-lysine) (pLL) and DNA are referred to as polylysine/DNA complexes or abbreviated to pLL/DNA complexes. Where polymers other dian poly(L-lysine) have been used the term polymer/DNA complex is used. The forward slash (/) is used to denote a non-covalent interaction, while a hyphen (-) is used to denote a covalent link. 

One of the major advantages of using cationic polymers to condense DNA is that very large genes can be used. Viral vectors are limited by the amount of genetic material that can be inserted into the viral genome. This limitation does not apply to polymer-based vectors, and even DNA... 

The ability to target polymer/DNA complexes to specific cell types, through a relatively simple attachment of cell specific ligands or antibodies, is one of the main attractions to using polymer-based vectors. Attempts have been made to target retroviruses, adenoviruses and liposomes to specific cell surface receptors however the ease with which targeting moieties... 

Cytoplasmic delivery of internalised polymer-based vectors... 

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