Recombinant DNA Advisory Committee - 03/1-2/93 
Dr. Parkman provided background information about the multi-drug resistance (MDR) 
gene. The MDR gene encodes a protein that can export chemotherapeutic agents out of 
cells. High concentrations of MDR are present in hematopoietic stem cells; however, 
the level of MDR expression is greatly reduced following differentiation. Since the 
protection provided by MDR is diminished in differentiated cells, chemotherapy usually 
produces a significant decrease in a patient's white blood cell and platelet counts. 
The objective of this protocol is to insert the gene coding for MDR, which is linked to a 
promoter that maintains gene expression, into hematopoietic stem cells. The patient's 
cells would be relatively resistant to the effects of certain anti-tumor drugs which allow 
them to receive increased doses of these chemotherapeutic agents. Increasing the doses 
of these agents should increase the likelihood that the patient's tumor cells will be 
ablated. 
The investigators propose to use a Harvey murine leukemia virus vector; they have 
submitted preclinical data based on a murine model that demonstrates their ability to 
transduce hematopoietic stem cells with MDR and to confer a protective effect against 
several chemotherapeutic agents, particularly Taxol. 
Patient eligibility is limited to women with Stage IV breast cancer who are undergoing 
autologous bone marrow (ABM) transplantation. Approximately one-third of the 
patient's bone marrow will be enriched for CD34( + ) cells, transduced with the MDR 
retrovirus vector, and cryopreserved. At the time of transplantation, the patient will 
receive a mixture of transduced and untransduced ABM cells. The untransduced portion 
of the patient's ABM cells is sufficient for hematopoietic engraftment. Following 
transplantation, the patient will be monitored by polymerase chain reaction (PCR) for 
the MDR gene. The patient's cells will be examined for colony growth, i.e., resistance to 
the effects of various chemotherapeutic agents. 
Patients who demonstrate persistent or progressive disease at day 50 will receive Taxol 
administration. Following Taxol, patients will be monitored for any increase in the 
proportion of MDR-expressing cells resulting from the ablation of cells that do not 
express the MDR gene. 
Dr. Parkman said that there are still several remaining questions about this protocol. A 
hard copy of the complete vector sequence was not made available to all of the members 
of the committee although Dr. Miller has screened the sequence by computer analysis 
and has assured the committee that the vector is acceptable. The investigators have not 
provided any long-term culture or animal data demonstrating that the MDR gene has 
been incorporated in relatively immature hematopoietic progenitors. The submitted data 
is based on days 12 and 14 short-term cultures. The investigators have not demonstrated 
that they can transduce the gene into immature hematopoietic cells and that the progeny 
Recombinant DNA Research, Volume 17 
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