3. The Proposed Treatment of Human Brain Tumors 
The central nervous system has several advantages of safety and efficacy 
for in-vivo gene transfer. First, retroviral vectors only integrate and therefore express 
vector genes in proliferating cells. In the brain, the tumor is the most mitotically active 
cell, with only macrophage-derived cells, blood cells and endothelial cells at minimal 
risk. Therefore, the possibility of specific transduction of the tumor is enhanced. 
Second, the brain is a partially immunologic privileged site, which should allow a 
somewhat longer survival of the xenogeneic murine cells in the brain and a greater 
transduction frequency of the growing tumor. A special feature of human gliomas is 
their ability to depress local immunity. This is thought to be secondary to down 
regulation of IL-2 secretion and diminished expression of high affinity IL-2 receptors on 
T-lymphocytes (9). The murine cells should survive longer allowing for the transduction 
of greater numbers of tumor cells. However, this period of survival will be limited since 
all cells that integrate and express HS-tk will be destroyed by the GCV. 
B. Clinical Gene Transfer 
1. Gene Transfer Methods 
There are 3 major methods of gene transfer: 1) chemical (e.g. calcium- 
phosphate precipitation), 2) physical (e.g. microinjection, electroporation, fusion- 
liposomes) and 3) viral (e.g. herpes simplex, adenovirus, retrovirus) (10). Physical and 
chemical methods have poor gene integration efficiency, ranging from 1 :1,000 to 
1 :1 00,000, making them impractical for most clinical applications. The adenoviruses 
and herpes simplex viruses have been used to transfer the vector genes into cells 
where they replicate in an extrachromosomal location. Therefore, adenoviruses and 
herpes simplex viruses can transfer genes into non-replicating tissues (e.g. lung, brain), 
unlike the murine retroviruses. 
2. Retroviral-mediated Gene Transfer (11). 
In contrast to chemical and physical methods, murine retroviral vectors 
have proven to be extremely efficient for gene transfer into mammalian cells, with 
efficiencies as high as 90% in cultured murine fibroblast cell lines. Murine retroviral 
vectors differ from the adenoviruses and herpes simplex viruses in that the retroviruses 
will only integrate and subsequently express their genes in proliferating tissues (e.g. 
tumor). This feature of the retroviral vectors may be particularly advantageous in the 
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