14. A. 2). Nevertheless, as these patients will be uninephric, special 
attention will focus on detecting early evidence of renal deterioration. A 
uninephric baseline DPG renal scan (as the most sensitive test of GFR 
currently available clinically) will be obtained before day 0 of Stage 2 in 
order to follow and quantitate the degree of renal injury induced if it does 
occur . 
Potential Toxicity from MFG Retroviral Vector 
The generation of genetically modified tumor cells capable of secreting 
the desired quantity of human GM-CSF will be accomplished by retroviral 
mediated gene transfer, a highly efficient means of transferring genes into 
mammalian cells. This method involves the use of genetically engineered 
viruses which are capable of entering cells and inserting their genetic 
material into host cell chromosomal DNA at high efficiency, yet unlike normal 
replication competent retroviruses, are not capable of producing any progeny 
virus after transduction of the target cell. 
Previous clinical protocols which have involved the use of such 
replication-deficient recombinant viruses have not revealed any toxicities 
associated with the gene transfer process. However, several areas of concern 
about the gene transfer process have been theorized and are worthy of mention. 
A first concern relates to the possibility that the recombinant viral stocks 
used for transduction of the tumor cells may contain low levels of 
replication-competent virus. In animal models, the chronic infection of host 
cells by certain retroviruses has been shown to contribute to the development 
of neoplastic disease, by virtue of the capacity of specific insertions of 
proviral DNA into chromosomal DNA to alter the expression of growth control 
and other genes. Accordingly, the presence of replication competent virus in 
the virus stocks used to transduce tumor cells could lead to the chronic 
propagation of replication competent virus in vivo , after transplantation of 
the transduced cells, and potentially contribute to the development of disease 
in the patient. The presence of replication competent virus in vivo would 
also make possible the transfer of the GM-CSF gene to other cells. Finally, 
even in the absence of replication competent virus, the transmission of the 
GM-CSF sequences to either tumor cells on contaminating normal cells could 
affect the growth properties of those cells in an undesirable way. 
While aware of these issues, we believe they pose no significant risk in 
this particular clinical protocol. First, the system for generating 
recombinant virus that we will employ has been designed to eliminate the 
possibility of helper virus generation, and we have developed extremely 
sensitive assays for monitoring the purity of our virus stocks that can detect 
the presence of a single virus particle in the stocks, should it exist. To 
date, we have no evidence of helper virus production. It is also worth noting 
that the packaging cell system we will employ possesses additional features 
from those engineered in the packaging cell system that has been used most 
frequently in the past for the generation of clinical grade viruses (Appendix 
14. B) . 
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Recombinant DNA Research, Volume 17 
