Abstract

Utilization of chemotherapy for treatment of tumors is mainly limited by its hematological toxicity. Because of the low level expression of drug resistance genes, transduction of hematopoietic progenitors with multidrug resistance 1 (MDR1) or multidrug resistance-associated protein 1 (MRP1) genes should provide protection from chemotherapy toxicity. Successful transfer of drug resistance genes into hematopoietic cells might allow the administration of higher doses of chemotherapy and, therefore, increase regression of chemosensitive tumors. In addition, this approach can be used to select in vivo transduced cells by their enrichment after administration of cytotoxic drugs. Our group has studied the potential value of MRP1 to protect hematopoietic cells. The interest in the use of MRP1 as an alternative to MDR1 gene transfer for bone marrow protection lies in its different modulation. Indeed, classical P-gp reversal agents, tested in clinic to decrease MDR1 tumor resistance, have little or no effect on MRP1 function. This would allow, in the same patient, the use of reversal agents to decrease P-gp tumor resistance without reversing bone marrow protection of the transduced hematopoietic cells provided by MRP1. We constructed two different MRP1-containing vectors with either the Harvey retroviral long terminal repeat (LTR) or phosphoglycerate kinase (PGK) as promoters and generated ecotropic producer cells. MRP1 transduced fibroblasts were more resistant to doxorubicin, vincristine, and etoposide and their chemoprotection was increased after selection with chemotherapeutic agents in the presence of glutathione, a co-factor for MRP1 function. Lethally irradiated mice were engrafted with bone marrow (BM) cells transduced with MRP1 vectors (PGK promoter). We demonstrated that high expression of MRP1 in murine hematopoietic cells reduces doxorubicin-induced leukopenia and mortality. In addition, in vivo selection of MRP1-transduced BM cells was achieved following doxorubicin administration and allowed a better chemoprotection after the second chemotherapy cycle. This article reviews the data of chemoprotection and selection with MRP1 gene transfer.