tumor with a cell line actively producing a retroviral vector carrying a gene conferring 
drug sensitivity to the tumor. A retroviral vector is a mouse retrovirus genetically 
engineered to replace its own genes with a new gene. Such vectors are capable of 
“infecting” mammalian cells and stably incorporate their new genetic material into the 
genome of the infected host. The producer cell is an NIH 3T3 cell that has been 
genetically engineered to continually produce retroviral vectors.The new gene is 
incorporated into the genome of the tumor cells and expresses the protein which is 
encoded by the new gene. This protein (the herpes simplex virus enzyme thymidine 
kinase, HS-tk) sensitizes the tumor cells to an antiviral drug (ganciclovir, GCV) which is 
a natural substrate for HS-tk. The enzymatic process induced by GCV leads to death of 
the cell expressing the herpes TK activity, i.e., death of the tumor cells. Since the HS-tk 
enzyme which is normally present in mammalian cells has very low affinity for GCV, 
systemic toxicity related to this mechanism is not observed. This type of in vivo gene 
transfer has several unique features. First, these retroviral-vectors will only integrate 
and express their genes in cells which are actively synthesizing DNA. Therefore, 
surrounding non-proliferating normal brain tissue should not acquire the HS-tk gene 
and will remain insensitive to GCV. Second, all of the transduced tumor cells (and 
retroviral vector producing cells) will be killed by the host immune response and/or 
GCV treatment eliminating potential concern about insertional mutagenesis giving rise 
to malignant cells. 
This is the first clinical attempt to treat malignant tumors in human beings by 
in-vivo genetic manipulation of the tumor’s genome. 
Over al l S tud y Design 
The study was designed to provide information on three aspects of this 
therapy: 1 . Is there significant toxicity associated with our approach, despite the safety 
which was shown in the various animal studies. 2. Does in-vivo transduction take place 
in the brain tumor, and if so, what is the extent of transduction, and 3. What is the 
efficacy (biological effect) of our approach, i.e., the tumoricidal effect in human patients. 
Adult patients (>18 years) with brain tumors will be evaluated for the extent 
and location(s) of their disease. Accordingly, the patients will be divided into surgically 
accessible and inaccessible groups. In both groups of patients, the tumors will be 
injected with the HS-tk-producing vector using an MRI guided stereotaxic approach. 
Recombinant DNA Research, Volume 15 
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