Cellular and molecular neurosurgery: Pathways from concept to reality .2. Vector systems and delivery methodologies for gene therapy of the central nervous system

DIFFERENT VECTOR SYSTEMS that have been used and/or specifically developed for central nervous system (CNS) gene transfer studies are briefly discussed along with their advantages and disadvantages with respect to potential clinical application. These include retroviruses, recombinant herpes simplex virus, adenoviruses, adenoassociated viruses, encapsulation of plasmid deoxyribonucleic acid into cationic liposomes, and neural and oliogodendroglial stem cells. Particular attention has been paid to relate the modality of a specific CNS gene therapy to the strategy for adequate delivery of genetic material to the brain for either global or localized CNS neurodegenerative chronic disorder, as well as for CNS tumors and stroke. Techniques to circumvent the ''impermeable'' blood-brain barrier and how to breach the more versatile blood-brain-tumor barrier to deliver the genetic material to the target CNS cells are reviewed and include the following: 1) local stereotactic (CNS injection/infusion of viral vectors, administration of vector producer cells, or cell replacement; 2) local administration of genetic material into the cerebrospinal fluid ventriculocisternal system; 3) osmotic opening of the blood-brain barrier; 4) local intra-arterial infusion; and 5) administration of blood-brain-tumor barrier permeabilizers, such as a bradykinin B2 agonist RMP-7. It is concluded that gene therapy for several brain disorders holds great potential, as suggested mainly by in vitro experiments and, to some extent, by a limited number of animal experiments. However, several drawbacks currently hamper the application of gene therapy under the clinical setting. The problems associated with gene therapy that still present major obstacles are as follows: 1) inefficient transfection of host cells by viral vectors; 2) restricted delivery of genetic material across vascular barriers of the CNS and brain tumors; 3) nonselective expression of the transgene; and 4) in situ CNS regulation of the transgene expression in a therapeutically controlled manner, as imposed by the course and phenotype of the CNS disease.