Protocol 
Retroviral Mediated Transfer of the Human Multidrug Resistance Gene (MDR-1) 
into Hematopoietic Stem Cells during Autologous Transplantation after Intensive 
Chemotherapy for Breast Cancer 
1.0 Objectives 
1.1 To study the feasibility of obtaining engraftment after high dose chemotherapy with 
retrovirally transduced hematopoietic stem cells expressing a cDNA for the human 
multi-drug resistance- 1 (MDR-1) gene. 
1.2 To determine whether the transduced MDR- 1 gene will confer drug resistance to 
hematopoietic cells and act as a in vivo dominant selectable marker. This will be 
defined as amplification of the transduced hematopoietic cell population in patients 
receiving post-transplantation chemotherapy. 
2.0 Background 
Stage IV breast cancer is currently incurable. The median survival for patients with metastatic 
disease is 18-24 months. Both hormonal and chemotherapy are considered palliative and have 
not been definitively shown to improve either disease-free (DFS) or overall survival (OS) (1). 
Because many chemotherapy drugs have steep tumoricidal dose-response curves, a substantial 
increase in cell kill may be achieved by significantly increasing the dose. Bone marrow 
suppression is a major dose limiting toxicity of intensive chemotherapy. The use of 
autologous peripheral blood and/or bone marrow cells combined with administration of 
recombinant hematopoietic growth factors restores bone marrow function, thereby allowing 
dose-escalation over conventional chemotherapy. Studies of high dose chemotherapy and 
autologous bone marrow transplantation (ABMT) following 3-4 cycles of induction therapy in 
chemotherapy responsive breast cancer have shown very high complete response rates of 50% 
(2-4). While the median time to treatment failure and median survival appear to be no better 
than that achieved with conventional chemotherapy, three trials have reported that 15-20% of 
all patients undergoing ABMT are alive and disease-free at 3 years (2-4). This compares 
favorably with the results of conventional chemotherapy where fewer than 10% of patients are 
generally disease-free at 3 years (5,6). Many patients will relapse following ABMT, and bone 
marrow suppression remains as a major dose-limiting complication of post-transplantation 
chemotherapy. New approaches are needed to circumvent the bone marrow toxicity 
associated with high dose chemotherapy. 
One approach would be to make normal bone marrow cells resistant to the toxic effects of 
chemotherapy. This might be accomplished by introducing specific drug resistance genes into 
hematopoietic cells. Retroviral mediated gene transfer of the dihydrofolate reductase gene into 
murine hematopoietic stem cells resulted in relative resistance to the myelosuppressive effects 
of the antineoplastic drug methotrexate (7). Another candidate drug resistance gene is the 
human MDR-1 gene which encodes for a 170-kilodalton transmembrane glycoprotein called P- 
glycoprotein (Pgp). Pgp expression in primary tumor cells confers pleiotropic resistance to a 
number of naturally occurring chemotherapeutic agents by enhancing their elimination from 
the cell (8,9). Transfer of the MDR-1 gene to protect the bone marrow cells of breast cancer 
patients is attractive because several drugs with clinical activity in the treatment of breast 
cancer, including taxol and vinblastine, are effluxed by Pgp (10). 
The endogenous MDR-1 gene is expressed in primitive hematopoietic cells at low levels, and 
expression is extinguished with myeloid maturation (11-13). Transgenic mice in which the 
human MDR- 1 cDNA is constitutively expressed in hematopoietic cells acquire resistance to 
the myelosuppressive effects of chemotherapeutic drugs (10). Drug sensitive mice can be 
Recombinant DNA Research, Volume 18 
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