In vitro mixing experiments were set up using irradiated TK positive cells (HCT- 
STK, human colon carcinoma) and non-irradiated TK negative cells (kbalb-LNL). 
Cells were plated at varying ratios of irradiated TK positive to TK negative cells with a 
total of 2 x 10 6 cells plated in a 100mm culture dish and GCV (50 uM) was 
immediately added. The results approximate those seen in the above experiments 
using non-irradiate TK positive tumor cells. When the TK positive cells made up as 
few as 10% of the cells virtually all the irradiated HCT-STK and non-irradiated kbalb- 
LNL cells were killed by GCV, while irradiated HCT tumor cells had no effect on the 
kbalb-LNL cells (figure 24). 
24 
■ kbalb-LNL + HCT(irrad) 
■ kbalb-LNL + HCT-STK(irrad.) 
1000 -i 
% STK 
We next determined the affect of irradiated murine and human tumors on mice with 
preexisting murine (kbalb) tumor. Mice received 2 x 10 5 kbalb-LNL tumor cells on day 
0 and 5 x 10 6 kbalb-LNL or kbalb- STK cells on day 1 and day 2. GCV therapy was 
begun on day 3 (7 doses). Figure 25 shows results which demonstrate that injection of 
irradiated kbalb-STK cells approximate those seen using non-irradiated kbalb-STK 
cells. The mean survival was 19 days for the controls and 38 days for the mice 
receiving TK positive cells (p <0.05). We also repeated the experiment by using the 
HCT colon carcinoma cell line. 2 x 10 5 kbalb-LNL cells were injected on day 0 and 2 
x 1 0 7 HCT or HCT-STK cells were injected on day 2 followed by 7 doses of GCV 
beginning 12 hours later. We demonstrated long term survival (> 70 days) in 
approximately 25% (figure 26). 
Recombinant DNA Research, Volume 15 
[587] 
