that a single protein, which was an efflux pump, conferred resistance 
to these drugs through generating a reduction in the intracellular 
concentrations of any one of these agents. Work designed to isolate 
this efflux pump protein culminated in the isolation of the pl70 
glycoprotein and the MDR-1 cDNA and gene. 
Subsequent laboratory work on tumor cell lines which exhibited 
resistance to these drugs, or on tumor cell lines into which the cDNA 
for the MDR-1 cDNA was introduced by transduction, led to the 
conclusion that the MDR-1 cDNA conferred resistance to the following 
drugs: actinomycin D, VP-16, Velban, vincristine, adriamycin, and 
Taxol. Although there are multiple forms of the MDR cDNA both in 
mouse and man (MDR-1 and 3 in man and MDR 1-3 in the mouse) , only one 
of the group in each species is functionally active in conferring 
resistance to these drugs (MDR-1 in man and MDR- 3 in mouse) . In 
addition, exposure of the cells of patients often leads to increases 
in the levels of the MDR-1 mRNA and in the protein GP170 for which it 
is the coding sequence. 
Studies of patients with ovarian cancer have shown that the elevation 
or even the presence of detectable levels of the MDR-1 mRNA or protein 
in ovarian cancer cells is very uncommon (3-5) , and that even after 
exposure to the MDR-1 drugs, that the levels of elevated MDR-1 (which 
are 2 to 3-fold higher than those which are detectable before 
treatment) , are at least 3-fold lower than the levels which are 
achievable in hematopoietic stem cells which have been transfected 
with 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 and marrow CD34 positive 
cells for the transplant. Our institution has had experience with 
transplantation with the CD34 selected cells. We have seen recovery 
of the absolute neutrophil count to 500/cu mm in 10, 14, 14, and 17 
[664] 
Recombinant DNA Research, Volume 17 
