Synthetic Nucleic Acid Delivery Systems in Gene Therapy

Synthetic nucleic acid delivery systems also known as synthetic nonviral vectors, are prepared primarily using synthetic organic chemistry expertise for the delivery of therapeutic nucleic acids to cells in vitro, ex vivo and in vivo. In clinical trials, simple cationic liposomes/micelles or simple cationic polymers have been the most widely used synthetic nucleic acid delivery systems. Problems were encountered and solutions are now in development. Consequently, there is no currently marketed gene therapy that makes use of synthetic nucleic acid delivery systems (synthetic nonviral vectors). Given latest developments, this situation should be reversed given time.

Keywords: polycations; liposomes; polymers; nucleic acids; gene therapy

Figure 1. Structures of well-known neutral-colipids, cytofectins (B-layer) and polyethyleneglycol (PEG)-lipids (BC-layer molecules) used in the preparation of synthetic nonviral vector systems; DOPE, dioleoyl-l--phosphatidylethanolamine; Chol, cholesterol; DSPC, distearoyl-l--phosphatidylcholine; DOTMA, N-[1-(2,3-dioleyloxy)propyl]-N,N,N-trimethylammonium chloride; DOTAP, 1,2-dioleoyloxy-3-(trimethylammonio)propane; DMRIE, 1,2-dimyristyl-oxypropyl-3-dimethylhydroxyethyl ammonium bromide; DC-Chol, 3-[N-(N¢,N¢-dimethyl-aminoethane)carbamoyl]cholesterol; DDAB, dimethyldioctadecylammonium bromide; EDMPC, 3-ethyl-1,2-dimyristoyl-l--phosphatidylcholine; DOTIM, 1-[2-(oleoyloxy)ethyl]-2-oleyl-3-(2-hydroxyethyl)imidazolinium chloride; GL-67, lipid 67; DODMA, N-[1-(2,3-dioleyloxy)propyl]-N,N-dimethyl ammonium chloride; PEG-DSG, polyethyleneglycol conjugate of distearoyl glycerol; DMPE-PEG, polyethyleneglycol conjugate of dimyristoyl-l--phosphatidyl-ethanolamine; pLL, poly-l-lysine; PEI, polyethylenimine and PAMAM, polyamidoamine dendrimer.
Figure 2. Diagram showing the process of cationic liposome/micelle–DNA (lipoplex, LD) particle [or cationic polymer–DNA (polyplex, PD) particle] cell entry. LD or PD particles that have not succumbed to aggregation associate with the cell surface and enter usually by endocytosis. The majority in early endosomes become trapped in late endosomes (path A) and the DNA fails to reach the cytosol. A minority of particles are able to release their bound DNA into the cytosol. DNA then subsequently follows path C to enter the nucleus to act. The diagram is drawn on the assumption that plasmid DNA (pDNA) has been delivered and is expressed in an epichromosomal manner. If instead LR or PR particles, formulated from RNA are involved, then delivered RNA follows path B so as to act directly in the cytosol post delivery. Reproduced from Miller (1999) with the kind permission of Bios Scientific Publishers Ltd.
Figure 3. Nucleic acids (A) are condensed within functional concentric layers of chemical components designed for delivery into cells and intracellular trafficking (B – lipid layer), protection against degradation (C – stealth/biocompatibility polymer layer) and delivery to target cells (D – biological targeting ligand layer). Reproduced from Kostarelos and Miller (2005) by kind permission from ImuThes Ltd.
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 Further Reading
    Fletcher S, Ahmad A, Perouzel E et al. (2006) In vivo studies of dialkynoyl analogues of DOTAP demonstrate improved gene transfer efficiency of cationic liposomes in mouse lung. Journal of Medicinal Chemistry 49: 349–357.
    Fletcher S, Ahmad A, Perouzel E, Jorgensen MR and Miller AD (2006) A dialkanoyl analogue of DOPE improves gene transfer of lower-charged, cationic lipoplexes. Organic & Biomolecular Chemistry 4: 196–199.
    Lu C, Tej SS, Luo S et al. (2005) Elucidation of the small RNA component of the transcriptome. Science 309: 1567–1569.
    Mattick JS (2005) The functional genomics of noncoding RNA. Science 309: 1527–1528.
    Miller AD (1998) Cationic liposomes for gene therapy. Angewandte Chemie (International ed. in English) 37: 1768–1785.
    Wente SR (2000) Gatekeepers of the nucleus. Science 288: 1374–1377.
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Miller, Andrew David(Mar 2008) Synthetic Nucleic Acid Delivery Systems in Gene Therapy. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0005745.pub2]