Recombinant DNA Advisory Committee - 09/14-15/92 
quantitative and information based system for assuring the safety of transducing particle 
suspensions. 
Dr. Geiduschek stated his concerns regarding the issue of helper virus testing. Standards 
should be established for these assays to ensure that the probability of contamination by 
one helper virus particle is less that 10'\ where "x" is a suitable number greater than two 
or three. Currently, the S + /L' test does not meet this requirement. There is an 
extended S + /L' assay that may be more sensitive than the standard test; however, this 
extended assay was not performed by Dr. Schuening. 
Dr. Geiduschek explained that the investigators cultured the packaging cell line following 
harvest of the vector supernatant for several weeks to ensure that no helper virus 
particles were present in the original preparation. This culturing procedure is a more 
sensitive method for the detection of replication competent helper virus than the 
standard S + /L‘ assay because the entire preparation is tested, not just a fraction. In 
addition, long-term culture relies on the dynamics of helper virus appearance in a 
population resulting from an initial contamination. If the dynamics of this long-term 
culture procedure are established with regard to the specification of cell line, constructs, 
and producers, then this procedure should yield the appropriate safety criteria for 
monitoring helper virus contamination. The necessary reconstruction experiments have 
not yet been performed to determine these specific criteria. This issue should be settled 
in a quantitative and reliable manner. Since Dr. Schuening will obtain his vector 
supernatants from a source other than the one that has supplied investigators of previous 
approved protocols, perhaps the RAC should base its decision on safety data submitted 
in response to established safety criteria rather than relying on the track record of a 
particular supplier. 
Review-Dr. Krogstad 
Dr. Krogstad said that he had some of the same initial concerns regarding the helper 
virus assay that were raised by Dr. Geiduschek, but the important issues were discussed 
previously. The study proposes to treat 20 patients over a four year period. If there are 
subgroups of patients with various diagnoses, how will the investigators evaluate this 
data? If the disease process of a particular group has an impact on the outcome of the 
experiment, the data may be difficult to interpret. Dr. Schuening responded earlier that 
all of these patients should have marrow activity restored; therefore, similar positive 
results should be obtained for all groups with regard to the marking study. From a 
theoretical point, it would be preferable to employ a technique that would allow one to 
distinguish between marked cells that have and have not replicated their DNA. In 
response to concerns about quantitative PCR, Dr. Krogstad noted that the investigators 
will respond to this point during their presentation. 
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Recombinant DNA Research, Volume 16 
