Gene delivery systems viral

Kataoka, K., Harashima, H., Gene delivery systems Viral vs. non-viral vectors. Adv Drug Deliv Rev 52, 151 (2001). 

Nonviral gene transfer systems are based on a variety of technologies that employ physical/chemical means to deliver genes [6], These technologies include direct plasmid injection, bombardment with DNA coated microprojectiles, and DNA complexed with liposomes or polymers. Some nonviral transfection techniques are too inefficient (e.g., coprecipitation of DNA with calcium phosphate [7], DNA complexed with diethylaminoethyl (DEAE)-dextran [8], electroporation [9]), or laborious (e.g., microinjection of DNA [ 10 ]) for clinical use. Only those gene delivery systems (viral and nonviral) with potential for clinical application are discussed in this chapter. The main features of these technologies (Table 18.3) are described and specific examples of their applications highlighted. 

Despite improvement in nonviral gene delivery systems, viral vectors continue to have higher efficiency in most experimental systems. Adenovirus, adeno-associated virus, retrovirus, lentivirus, herpes simplex viras, and others have been used both in the laboratory, and to a much lesser extent, in human trials. Of these potential vectors, studies of adenoviras in the pulmonary circulation are the only ones in the published literature. 

Davis, M.E., Non-viral gene delivery systems. Curr. Opin. Biotechnol. 2002, 13, 128-131. 

Some 24 per cent of all gene therapy clinical trials undertaken to date have employed retroviral vectors as gene delivery systems. Retroviruses are enveloped viruses. Their genome consists of ssRNA of approximately 5-8 kb. Upon entry into sensitive cells, the viral RNA is reverse transcribed and eventually yields double-stranded DNA. This subsequently integrates into the host cell genome (Box 14.1). The basic retroviral genome contains a minimum of three structural genes ... 

Although viral-mediated gene delivery systems currently predominate, a substantial number of current clinical trials use non-viral-based methods of gene delivery. General advantages quoted with respect to non-viral delivery systems include ... 

No matter what their composition, such synthetic gene delivery systems also meet various biological barriers to efficient cellular gene delivery. Viral vector-based systems are far less prone to such problems, as the viral carrier has evolved in nature to overcome such obstacles. Obstacles relate to ... 

Some attempts have been made to rationally increase the efficiency of endosomal escape. One such avenue entails the incorporation of selected hydrophobic (viral) peptides into the gene delivery systems. Many viruses naturally enter animal cells via receptor-mediated endocytosis. These viruses have evolved efficient means of endosomal escape, usually relying upon membrane-disrupting peptides derived from the viral coat proteins. 

Figure 14.10 Overview of cellular entry of (non-viral) gene delivery systems, with subsequent plasmid relocation to the nucleus. The delivery systems (e.g. lipoplexes and polyplexes) initially enter the cell via endocytosis (the invagination of a small section of plasma membrane to form small membrane-bound vesicles termed endosomes). Endosomes subsequently fuse with golgi-derived vesicles, forming lysosomes. Golgi-derived hydrolytic lysosomal enzymes then degrade the lysosomal contents. A proportion of the plasmid DNA must escape lysosomal destruction via entry into the cytoplasm. Some plasmids subsequently enter the nucleus. Refer to text for further details...

CNTs with different characteristics, which will lead to differences in the mechanism of CNT metabolism, degradation or dissolution, clearance and bioaccumulation. On the other hand, most non-viral gene delivery systems today suffer from both limited levels of gene expression and an unfavourable toxicity profile due to their highly cationic surface character. Therefore, opportunities for CNT-based gene transfer systems are still ample. 

This section will describe the features and clinical applications for each gene delivery system. The description will start with a common feature of both viral and nonviral gene delivery systems, the expression cassette for the therapeutic protein. 

TABLE 15.2. Key features of viral and nonviral gene delivery systems... 

Kwok, K.Y., Yang, Y.S. and Rice, K.G. (2001) Evolution of cross-linked non-viral gene delivery systems. Curr. Opirt. Mol. Ther., 3,142-146. 

The addition of NLS in a non viral gene delivery system is expected to favor the nuclear import (nuclear pores allow the passage of particles of 24 nm) by using the karyopherin and importin machinery. Where should the NLS be linked, on the pDNA or on the carrier For the latter strategy, polyplexes must not dissociate before their import in the cell nucleus. Both strategies have been prospected but with only limited success. 

Tomlinson, E. and Rolland, A.P. (1996) Controlled gene therapy pharmaceutics of non-viral gene delivery systems. J. Control. Release, 39, 357-372. 

Figure 18.1 Types of non-viral gene delivery systems.

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