gene transfer in this direct injection system is most likely due to the intimate contact of 
the tumor and producer cell lines. Any vector particles that are released in the area of 
injection will be quickly bound by the thousands of amphotropic vector receptors on 
each tumor cell and other host cells. Even if all of the vector particles produced were 
able to escape direct transfer into adjacent tumor cells and cross the blood-brain 
barrier, the number of vector particles relative to the number of receptors in any organ 
would still be very small suggesting a very small risk of injury to proliferating cells in 
any non-CNS organ. Any direct transfer into neurons by cell-to-cell contact will not 
result in HS-tk gene integration and therefore, should not pose a risk for their 
normal brain tissue is likely to be a problem (Section II.A.2.). However, if the HS-tk 
gene will be introduced into a large number of normal dividing cells within the CNS 
(such as endothelial cells and astroglial cells), vasculitis like symptoms (headaches, 
convulsions, bleeding) may develop. Such changes however will be localized to the 
immediate vicinity of the tumor as had been shown in our experiments (II. B. 2) and in 
fact may even enhance tumor eradiacation by increasing the exposure to the immune 
system. 
F. Insertional Mutagenesis 
cells. The random nature of this integration allows for the potential of an untoward 
insertional event. If the insertion disrupts a gene essential for maintaining cell function, 
that particular cell will die. Since the gene transfer will occur most predominantly in 
tumor cells, if the vector insertion site results in the death of a few tumor cells without 
GCV, that should not pose a problem. 
be accurately estimated since that has never been a documented occurrence in 
animals or man. While this is a real risk, this risk must be very low, especially in this 
protocol, where all vector containing cells will be killed by GCV. In our 189.8 months of 
cumulative patient observation in the human gene transfer clinical protocol and the 
89.9 years of cumulative observation of primates (some severely immunosuppressed 
[814] Recombinant DNA Research, Volume 15 
destruction with GCV treatment. 
E. Transduction of surrounding brain tissue. 
There is no evidence in our animal model that transduction of surrounding 
Retroviral vector DNA is inserted randomly into the genome of proliferating 
The risk of oncogeneic transformation with these retroviral vectors cannot 
