No side effects have been seen in animal studies nor in human clinical use to 
date. However, there are theoretical risks. First the LNL-6 vector preparation 
used could be contaminated with replication-competent murine retrovirus (see 
4.3 below). Secondly, even though the virus used to insert the gene into the 
bone marrow cells cannot grow and is considered harmless, it is possible that 
oncogenic events could occur within a transduced cell. Finally, the inserted 
gene may produce a protein that inactivates some aminoglycoside antibiotics. 
Satisfactory alternative antibiotics are available. 
4.3 Replication Competent Virus 
The LNL-6 vector has been modified so that it entirely lacks viral genes. The 
only coding genetic material that will be transferred to the patient’s cells will 
be the marker gene (neo^). Since LNL-6 has no remaining viral genes, it is 
incapable of producing the virion proteins necessary to package its RNA into 
an intact infectious virus. 
Since retroviral vectors are replication-defective, their genome must be 
"packaged" into a virion so that they may transfer their genetic material to the 
target cell. This is accomplished by use of a "packaging cell line" which 
supplies the necessary proteins for virion formation (Figure 2). The cell to be 
used, PA317, contains a modified murine leukemia virus (MLV) genome with 
intact packaging genes, but with deletions which make the virus incapable of 
inserting its own RNA genome into a virion. Thus, the patient’s cells are 
never exposed to a replication-competent virus, but only to a viral "coat" within 
which is contained the neomycin vector. The LNL-6/PA317 combination has 
been tested extensively over several years for replicating retrovirus, which 
could theoretically develop by recombination between vector and viral 
sequences in the packaging cell lines. To date, the multiple modifications in 
the vector and the packaging cell lines aimed at preventing recombination 
appear successful since replication virus has been detected only rarely after 
long periods of incubation. As a further check, each batch of supemate 
intended for clinical use is extensively screened for replication-competent virus. 
Even if this screening were to fail, data from primates have shown that intact 
MLV replicates poorly and transiently in primates in vivo even when these 
animals are injected with massive doses of virus. To date no clinical illness or 
sequelae has developed in these animals. 
Additional information exists in four animals exposed to high titer replication- 
competent retrovirus along with retroviral vector at the time of severe 
immunosuppression, and autologous bone marrow transplant. The 
transplanted monkeys were exposed to virus and vector at approximately equal 
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Recombinant DNA Research, Volume 14 
