rendered resistant to chemotherapeutic myelosuppression following transplantation of bone 
marrow from MDR-1 transgenic mice (14). 
Our work in a murine model has established that retroviral gene transfer may be used to 
introduce and express the MDR-1 cDNA in hematopoietic cells (15). Similar results have 
been reported by others (16). The level of expression of the retrovirally transduced human 
MDR-1 cDNA in primary hematopoietic cells is substantially higher than the level of 
expression from the endogenous murine multidrug resistance genes mdr-1 and mdr-3 (B. 
Sorrentino, unpublished observations, Appendix I). Furthermore, we have shown that the 
MDR-1 cDNA confers preferential survival on retrovirally transduced hematopoietic cells 
following treatment with the chemotherapeutic drug taxol (15). Preferential survival may be 
traced to retrovirally transduced, long-lived primitive hematopoietic cells that can be 
transplanted to secondary recipients (B. Sorrentino, unpublished observations, Appendix I). 
Patients with Stage IV breast cancer are currendy treated with high dose ifosfamide, 
carboplatin, and etoposide (ICE) followed by autologous bone marrow transplantation 
(ABMT) on active NCI Medicine Branch protocols at NIH. Patients requiring further 
treatment after ABMT have been treated with taxol and vinblastine on other protocols. Taxol 
is an active drug in breast cancer with a 56 percent response rate in previously treated 
metastatic breast cancer (17). Vinblastine has a 20-30 percent response rate in the salvage 
therapy of breast cancer (18). 
We propose to use retroviral gene transfer to introduce the MDR-1 cDNA into the 
hematopoietic cells of these patients who are receiving ICE chemotherapy followed by ABMT. 
These experiments will allow us to address two basic questions. First, the feasibility of 
introducing and expressing drug-resistance genes in the bone marrow of patients receiving 
chemotherapy for cancer can be explored. Second, in the patients requiring post-transplant 
therapy with taxol or vinblastine, to test if the MDR-1 gene can be used as a dominant 
selectable marker in vivo and therefore potentially overcome current limitations in gene 
transfer efficiency into hematopoietic stem cells. Based on our pre-clinical studies, we 
anticipate that transduced hematopoietic cells constitutively expressing retrovirally encoded 
Pgp will have a survival advantage over non-transduced cells following chemotherapy. This 
will be reflected in an increase in the proportion of hematopoietic cells carrying and expressing 
the proviral MDR-1 cDNA. If significant enrichment of MDR-1 expressing cells occurs, it is 
possible that patients’ bone marrow will become more resistant to the myelosuppressive 
effects of chemotherapy. We will monitor the nadir blood counts of each patient receiving 
salvage chemotherapy for evidence of myeloprotection and correlate this data with changes in 
the mean proviral copy number. Preliminary findings suggestive of myeloprotection would 
need to be confirmed in a later controlled study. 
These studies represent the initial steps toward a long-term goal of creating a fully drug 
resistant bone marrow. If combined with more efficient approaches for transduction of 
hematopoietic cells, these experiments may contribute to the development of new, intensive 
chemotherapy regimens for the treatment of malignant disease. In addition, our studies have 
important potential applications in the selection of genetically modified hematopoietic cells and 
the treatment of non-malignant disorders. 
The design of this study is analogous to our protocol entitled, "Pilot Study of High-Dose ICE 
(Ifosfamide, Carboplatin, Etoposide) Chemotherapy and Autologous Bone Marrow 
Transplant (ABMT) with neoR-transduced Bone Marrow Stem Cells" and its Amendment 
"Genetic Marking with Retroviral Vectors to Study the Feasibility of Stem Cell Gene Transfer 
and the Biology of Hematopoietic Reconstitution after Autologous Transplantation in Multiple 
Myeloma, Chronic Myelogenous Leukemia or Metastatic Breast Cancer", which have been 
approved by the Institutional Review Board (IRB), the Institutional Biosafety Committee 
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Recombinant DNA Research, Volume 18 
