Although the majority of gene therapy studies have relied on a surgical delivery of genetically altered cells or of viral vectors to transmit the desired genes, a recent line of research has experimented with engineered liposomes that are immunologically targeted to be taken up by neurons. Plasmid DNA containing the gene intended for transfer (such as tyrosine hydroxylase) is packaged in a 100-nm liposome. The structure is stabilized by the incorporation of several thousand polyethylene glycol residues. A percentage of these are conjugated to monoclonal antibodies that target either the insulin or transferrin receptors. The antibody targeting causes the liposomes to be selectively taken up in the brain. Association of the desired gene with a glial fibrillary acidic protein (GFAP) promoter further specifies the location of gene activity (112).
Genes packaged by this technique can be delivered by a simple intravenous administration. Pardridge et al. (112) have demonstrated the ability of this technique to deliver a desired gene specifically to the brain in both rodents and in rhesus monkeys. They have further shown that the same intravenous delivery of the tyrosine hydroxylase gene can reverse rotational behavior in 6-OHDA-lesioned rats (112-114). Although these studies are still quite preliminary, they offer the additional hope that the genetic modifications described earlier might eventually be delivered with decreased risk and discomfort to the patient. This might also allow a more measured and gradual titration of delivery to maximize benefit and limit side effects.
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