Recombinant DNA Advisory Committee - 03/1-2/93 
been demonstrated to generate helper virus in the past; however, the investigators will 
probably propose to use another version of that packaging cell line which is more 
contained. The RAC must keep in mind that any packaging cell line will generate 
helper virus at some rate, and that the breakout that occurred was a nonhomologous 
recombination. There was no overlap in the env region between the vector and the 
packaging cell line. 
Dr. Anderson stated that the precise mechanism of the breakout is unclear; the issue of 
homologous versus nonhomologous recombination has not been absolutely determined. 
Dr. McGarrity stated that if homologous recombination occurs, it will probably occur 
within the vector and packaging system. There is an overlap in the gag region of 
approximately 50 base pairs; however, there is no overlap in the env region. Dr. 
Geiduschek stated that although the recombination event should not be categorized 
prematurely, it is extremely important to understand the mechanisms of these rare events 
in order to gain an understanding of the risks. When the mechanism of recombination is 
determined, the RAC should be notified of this information immediately. 
Presentation-Dr. Seigler 
Dr. Seigler responded to the RAC members questions regarding the eradication of 
established tumor in the murine model. The murine experiments have not been 
performed because they do not accurately represent the human situation. Murine and 
human y-IFNs are species-specific; therefore, the research has focused on the human 
situation. 
Data was presented demonstrating decreased tumor growth in response to transduced 
B16 melanoma cells in immune-competent mice. Additional data indicated that a 
decrease in the growth rate of L33 tumor cells was due in part to CTL based on the fact 
that tumors do not develop in normal mice but do occur in nude mice. 
Dr. Seigler stated that endpoints will be determined by two measurements: (1) tumor 
status, and (2) the generation of CTL. Patients with stable disease will remain on the 
study, and these who demonstrate progressive disease will not continue in the protocol. 
If 10 patients demonstrate no CTL response, the study will be terminated unless there is 
clinical evidence of tumor response, i.e., stable or regressing tumor. 
Dr. Parkman asked if there are data demonstrating CTL responses from autologous PBL 
to tumor cells. Dr. Seigler said that CTL responses were generated in vitro to tumor cell 
lines. Dr. Parkman stated that established cell lines are very different from autologous 
fresh tumor cells. Dr. Darrow of Duke University Medical Center, Durham, North 
Carolina, stated that the induction of melanoma-specific CTL has been demonstrated 
with both transduced and untransduced cells against tumor cell lines and fresh 
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Recombinant DNA Research, Volume 17 
