Non-Viral Vectors for Gene Transfer
Non-viral vectors were not the agents of choice for gene transfer protocols but problems in clinical trials with recombinant viruses renewed interest in this method of gene transfer. Non-viral vectors generally consist of double-stranded recombinant DNA plas-mids alone or encapsulated in cationic polymer or lipid-based formulations. Non-viral vectors offer several important advantages over virus-based meth-ods for gene transfer (Table 5). Their unlimited cloning capacity significantly expands the types of therapeutic transgenes and expression cassettes that can be used for gene transfer. Plasmids, unlike their viral counterparts, are generally non-immunogenic and can easily be readministered multiple times without induction of a prohibitive immune response. Non-viral vectors have a reduced capacity for inser-tional mutagenesis and a limited ability to produce unwanted by-products in vivo due to homologous recombination. Recombinant DNA plasmids for gene transfer are also relatively easy to manipulate using standard techniques and do not require specialized skills or equipment for large-scale production. They are also inexpensive to produce, especially on a large scale in contrast to viral vectors. Despite all the advantages that non-viral vectors have to offer, their clinical utility is significantly hindered by low transduction efficiency, which stems from non-specificuptake of the vector and poor delivery to the therapeutic target. They also have a limited capacity to override cellular gene silencing mechanisms and, as a result, cannot achieve sustained gene expression. Refinement of delivery methods for non-viral gene transfer and reconstruction of vectors at the molecular level have addressed these issues.