safety-modified retroviruses which carry a single MDR-1 cDNA in a 
transcription unit (6). In addition, animal model experiments in 
dogs, mice and in primates, have shown that it is feasible to modify 
the hematopoietic stem cells with safety-modified retroviruses which 
carry the MDR-1 cDNA in man or the MDR-3 cDNA in the mouse such that 
the hematopoietic cells following bone marrow transplant retain 
significant levels of cells which are genetically modified, and 
express the MDR-1 cDNA (5,7-8). Furthermore, these studies have shown 
that delivery of conventional dose chemotherapy to these animals 
results in increases in the levels of the MDR-1 cDNA and GP170 protein 
in the marrow and peripheral blood cells of these animals, and in the 
percentage of cells which carry the retroviral transgenome. In other 
words, these studies have shown that it is possible to increase the 
resistance of hematopoietic cells to the effects of MDR-1 drugs and 
that this increased resistance, which the MDR- 1 retrovirus 
transduction confers upon the hematopoietic stem cells, results in the 
ability to select and maintain the genetically modified cells in the 
marrow of animals which have been transplanted with these modified 
cells by autologous bone marrow transplantation. No alteration in the 
maturation of hematopoietic cells has been seen in mice transplanted 
with MDR modified cells (8,14) . Several groups have now shown that the 
presence of viral MDR-1 in transplanted marrow is resistant to Taxol 
and other MDR drugs (11-14). 
We have proposed the use of peripheral blood CD34 positive cells for 
the transplant. Our institution has had experience with transplantation 
with the CD3 4 selected cells. We have seen recovery of the absolute 
neutrophil count to 500/cu mm in 10, 14, 14, and 17 days in breast 
cancer patients who have been infused with peripheral blood cells and 
14, 17, 18, 18, 24, and 27 days in advanced CML patients who have been 
transplanted with CD34 selected peripheral blood and marrow cells. The 
University of Denver has observed an average of 17 days to 500 | 
neutrophils/cu mm in advanced breast cancer patients. 
As shown in Figure lA, our own laboratory (see Appendix C) has shown 
that introduction of the MDR-1 cDNA containing safety-modified 
retrovirus into the marrow cells of Balb-C mice before transplant shows 
that the marrow cells of the mice transplanted with the MDR-1 modified 
marrow are more resistant to Taxol than are mice transplanted with 
unmodified marrow. In addition, as shown in Figure IB, sequential 
transplantation of these MDR-1 modified cells generates hematopoietic 
recovery which is Taxol-resistant through three successive transplants 
(see Appendix C) . This shows that early progenitor cells can be 
modified by our MDR-1 procedure. In addition, in vitro data with CD34 ; 
selected progenitor cells shows that the long-term culture initiating 
cell is transduced by this virus, that the transduction frequency is 
10%, and that the levels of MDR-1 mRNA from the viral MDR-1 are much 
higher than the endogenous MDR-1, and that this translates into Taxol 
resistance . 
The marrow becomes resistant to Taxol chemotherapy using transduction 
procedures which give integration of the virus into only 1-10% of the 
cells suggesting that in vivo selection of the MDR-1 modified cells 
occurs during chemotherapy. During the past two years, considerable 
progress has been made in the introduction of additional sequences into 
the somatic cells of patients in both constitutional as well as 
acquired molecular diseases (9-13) . This has been accomplished with a 
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Recombinant DNA Research, Volume 19 
