Recombinant DNA Advisory Committee - 06/1-2/92 
treatment is approximately two weeks. This data corresponds with the results obtained 
in other animal models. 
Dr. Krogstad asked how the levels of ganciclovir necessary to produce this effect 
compare to serum levels. What is the effect on the blood /brain barrier? Dr. Ram 
responded that the cerebral spinal fluid (CSF) plasma ratio ranges between 0.2 and 0.7. 
In order to observe the effects of ganciclovir on the TK expressing cells, higher levels 
need to be obtained in the serum to observe this effect. While serum levels may peak at 
approximately 9 micrograms per milliliter, the levels of ganciclovir on the TK expressing 
cells are approximately 1 microgram per milliliter at the site of injection. 
Dr. Ram addressed Dr. Leventhal's question regarding non-tumoral tissue transduction. 
Dr. Ram said that the other mitotically active cells in the brain, the glial and endothelial 
cells, were evaluated for their capacity to replicate. Dr. Ram stated that in general, glial 
cells cannot be transduced. However, some astrocytes and microglia may have been 
transduced at levels too low to detect. 
Dr. Ram stated that he has been able to consistently transduce endothelial cells; 
however, transduction has always been observed as a localized event within the region of 
the injection. Dr. Ram hypothesized that it is this localized transduction of endothelial 
cells which may be responsible for the tumoricidal effect. Capillaries may respond to 
angiogenic factors resulting in new vascularization of the tumor. This factor may be very 
significant in eliminating the tumor. 
Dr. Leventhal asked if the investigators had examined other tissues in the animal models 
for evidence of toxicity. Dr. Ram said that several species (up to 50 animals per group) 
had been examined. Up to 10 s producer cells were injected intravascularly and 10 7 cells 
intraperitoneally. No evidence of toxicity has been observed. Dr. Ram noted that some 
of these animals have survived for six months. In animals that were sacrificed, small 
bowel, large bowel, spleen, liver, thymus, and bone marrow were examined. 
In response to Dr. Leventhal's comments regarding the statement "there are no dividing 
cells in the brain except for tumor cells ", Dr. Culver referred Dr. Leventhal to a revised 
statement "the tumor is the most proliferative tissue in the brain". 
Dr. Post referred to earlier comments regarding the transduction of endothelial cells in 
the vicinity of the injection. How wide is the area of transduction? Dr. Ram stated that 
endothelial transduction occurred no further than two to three cell layers surrounding the 
tumor. 
Dr. Parkman suggested that the RAC should receive the toxicity data that Drs. Culver 
and Ram have presented. It is important for the RAC to see the actual numbers and 
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Recombinant DNA Research, Volume 15 
