Human Gene Therapy Subcommittee - 11/21-22/91 
numbers of tumor cells. 
Dr. Nabel described another mouse model, the VL-6 melanoma model, developed by 
Drs. Chang, Shu, and Fox at the University of Michigan. Injection of the tumor with the 
H-2K S vector resulted in complete tumor regression. Subsequent tumor challenge on the 
contralateral side demonstrated no tumor growth at the site of injection. However, no 
regression was observed in pre-established tumors. 
Dr. Nabel said that one aspect of his system was that existing tumors actually regressed, 
whereas other approaches have merely attempted to provide immunization. If tumor 
regression is occurring, future approaches to cancer therapy may include accessing the 
tumor by catherization in order to deliver the gene to cells that are not transfectable by 
direct injection of liposomes. 
Dr. Nabel presented another murine experiment that would further address the issue of 
systemic immunity. Melanoma cells that migrate specifically to the lung were injected 
intravenously into mice. Three days following tumor injection, the mice received an 
injection of H-2K S transfected autologous lymphocytes. Resistance to pulmonary 
metastases was observed. Preliminary data suggest that with additional parameters, 
systemic immunity might be established. 
Dr. Nabel responded to Dr. Mclvor's question concerning germ line tissue analysis. 
Extensive PCR assays have been performed on germ line as well as other tissues from 
more than 25 animals. No PCR positivity was demonstrated in ovaries or testes. No 
evidence of pathologic changes was observed in the heart muscle of these animals. 
Dr. Nabel presented additional data in which DNA was radiolabelled with S 35 in the 
liposome complex and injected intravenously. The majority of the label went to the liver. 
Since DNA is degraded rapidly, no label was detected after 24 hours. Following 
intratumor injection of these DNA-liposome complexes, fewer than 1 in 10 6 cells are 
transduced in the heart compared to 1 in 10 tumor cells. Since injected DNA traffics to 
areas other than the injection site, PCR analysis will be performed on all available 
tissues obtained from autopsy. This analysis will provide important data. 
Dr. Nabel stated that the three vectors referred to in the current protocol were 
constructed in response to comments offered by the HGTS members at the July 29-30, 
1991, meeting. Vector I allows for the comparison of liposome-mediated vector insertion 
versus standard retroviral infection techniques. Vectors II and III are both derivatives of 
the RSV vector. The RSV vector was chosen because it is a well documented plasmid 
with a known sequence; it has no untoward effects. The RSV vector expresses well in a 
variety of tissues, and it contains a stable enhancer. Vector II incorporates the RSV 
SV40 polyadenylation (poly A) site. Vector III contains a p-globin poly A site. 
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