Paclitaxel chemotherapy after autologous stem-cell transplantation and engraftment of hematopoietic cells transduced with a retrovirus containing themultidrug resistance complementary DNA (MDR1) in metastatic breast cancer patients

The MDR1 multidrug resistance gene confers resistance to natural-product an ticancer drugs including paclitaxel. We conducted a clinical gene therapy s tudy to determine whether retroviral-mediated transfer of MDR1 in human hem atopoietic cells would result in stable engraftment, and possibly expansion , of cells containing this gene after treatment with myelosuppressive doses of paclitaxel, Patients with metastatic breast cancer who achieved a compl ete or partial remission after standard chemotherapy were eligible for the study. Hematopoietic stem cells (HSCs) were collected by both peripheral bl ood apheresis and bone marrow harvest after mobilization with a single dose of cyclophosphamide (4 g/m(2)) and daily filgrastim therapy (10 mu g/kg/da y). After enrichment for CD34+ cells, one-third of each collection was incu bated ex vivo for 72 h with a replication-incompetent retrovirus containing the MDR1 gene (G1MD) in the presence of stem-cell factor, interleukin 3, a nd interleukin 6, The remaining CD34+ cells were stored without further man ipulation. All of the CD34+ cells were reinfused for hematopoietic rescue a fter conditioning chemotherapy with ifosfamide, carboplatin, and etoposide regimen,. After hematopoietic recovery, patients received six cycles of pac litaxel (175 mg/m(2) every 3 weeks). Bone marrow and serial peripheral bloo d samples were obtained and tested for the presence of the MDR1 transgene u sing a PCR assay. Six patients were enrolled in the study and four patients received infusion of genetically altered cells. The ex vivo transduction e fficiency, estimated by the PCR assay, ranged from 0.1 to 0.5%. Three of th e four patients demonstrated engraftment of cells containing the MDR1 trans gene. The estimated percentage of granulocytes containing the MDR1 transgen e ranged from a maximum of 9% of circulating nucleated cells down to the li mit of detection of 0.01%, One patient remained positive for the MDR1 trans gene throughout all six cycles of paclitaxel therapy, whereas the other 2 p atients showed a decrease in the number of cells containing the transgene t o undetectable levels. Despite the low level of engraftment of MDR1-marked cells, a correlation was observed between the relative number of granulocyt es containing the MDR1 transgene and the granulocyte nadir after paclitaxel therapy. No adverse reactions to the genetic manipulation procedures were detected. Therefore, engraftment of human HSCs transduced with the MDR1 gen e can be achieved. However, the overall transduction efficiency and stable engraftment of gene-modified HSCs must be improved before MDR1 gene therapy and ill vivo selection with anticancer drugs can be reliably used to prote ct cancer patients from drug-related myelosuppression.