Retrovirus-mediated gene transfer of the human multidrug resistance-associated protein into hematopoietic cells protects mice from chemotherapy-induced leukopenia

Utilization of chemotherapy for the treatment of tumors is mainly limited b y its hematological toxicity, Because of the low-level expression of drug r esistance genes, transduction of hematopoietic progenitors with multidrug r esistance 1 (MDR1) or multidrug resistance-associated protein (MRP) genes s hould provide protection from chemotherapeutic agent toxicity, Successful t ransfer of drug resistance genes into hematopoietic cells may allow the adm inistration of higher doses of chemotherapy and, thus, increase regression of chemosensitive tumors. The interest in the use of MRP as an alternative to MDR1 for bone marrow protection lies in its different modulation. This w ould allow, in the same patient, the use of MDR1 reversal agents to decreas e MDR1 tumor resistance without reversing bone marrow (BM) protection of th e MRP-transduced hematopoietic cells, since MRP expression is not reversed by these agents. We have constructed MRP-containing retroviral vectors usin g the phosphoglycerate kinase promoter and generated ecotropic producer cel ls. Lethally irradiated mice were engrafted with BM cells transduced by coc ulture with MRP producer cells, Evidence of long-term (9 months) gene trans fer was provided by PCR of peripheral blood from MRP-transduced mice. South ern blot analysis confirmed the integrity of the provirus in the MRP-transd uced mice. Long-term MRP expression (>5 months) was detected by RT-PCR and fluorescence-activated cell sorting of blood from living mice. High-level e xpression of MRP in murine hematopoietic cells reduces doxorubicin-induced leukopenia and mortality, Furthermore, we show in vivo selection of MRP-tra nsduced cells following doxorubicin administration, with better and more si gnificant chemoprotection after the second chemotherapy cycle. These data i ndicate that MRP retroviral gene transfer may be useful for chemoprotection and selection.