3. Toxicity studies of HS-tk vector-producer cells with and without GCV in normal brain. 
Rats were inoculated with 3x1 0 6 HS-tk vector-producer cells into the deep white matter 
of the cerebral hemisphere. The contralateral hemisphere served as control with 3x1 0 6 
BAG-transduced 3T3 cells. Cells were injected in a volume of 50pL. Rats were treated 
with GCV at a dose of 15mg/Kg BID for 7 days and sacrificed 3 days later for 
histological evaluation of the brain. The deep injection site was evident in both 
hemispheres with local changes secondary to the infusion of cells. No difference was 
seen between the HS-tk vector-producer cell injection site and the BAG-transduced 
3T3 cell injection site. Surrounding brain tissue did not show evidence of inflammation 
or destructive chamges in either group. A repeat experiment with unilateral injection of 
a mixture of HS-tk-producer cells (90%) and 3T3-BAG (10%) followed by GCV 15mg/Kg 
BID for 7 days showed similar results. On day 5 following injection of the HS-tk- 
producer cells, all the rats appeared ill and dehydrated. Similar symptoms were 
observed when control BAG producer cells alone were injected. All the rats recovered 
completely after 24 hours of treatment with subcutaneous saline administration. The 
rats were sacrificed after cessation of GCV. Histology showed mild edema around the 
injected cells. No viable BAG cells were seen and there was some degree of 
neutrophilic infiltrate localized to the injection site. In the following repeat toxicity 
studies, dexamethasone was administered to the rats as an oral dose of 0.5 mg/Kg/day, 
starting on the third day post injection of the vector-producer cells. In this experiment, as 
well as in the subsequent experiments in the 9L rat brain tumor model, no evidence of 
clinical toxicity was observed in the dexamethasone treated rats. It appears that 
dexamethosone pretreatment diminishes the non-specific symptoms related to the 
surgery and implantaion of cells in the brain. 
In an ongoing toxicity study in Rhesus monkeys, IxlO 7 HS-tk-producer cells 
were injected into the right frontal hemisphere of 5 monkeys. All animals were 
pretreated with high-dose dexamethosone. MRI scans 5 days after inoculation of the 
cells demonstrated the small (5mm) inoculation site without any evidence of edema or 
mass effect within the brain. GCV was given to two monkeys on the seventh 
postoperative day. Again, no neurological ill effects were observed. MRI scan in the 
monkeys receiving GCV showed the inoculation site without any evidence of edema or 
mass effect within the brain. Additional data is pending from histological assessment of 
the brains and organs of three monkeys who will be sacrificed at the termination of the 
Recombinant DNA Research, Volume 15 
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