Retroviral transfer of the hENT2 nucleoside transporter cDNA confers broad-spectrum antifolate resistance in murine bone marrow cells

Antifolate drugs such as methotrexate are commonly used in cancer chemother apy, It may be possible to increase the antitumor activity of antifolates b y the coadministration of drugs that inhibit nucleoside transport, thereby blocking the capacity of tumor cells to salvage nucleotide precursors. An i mportant limitation of this approach is severe myelosuppression caused by m any of these drug combinations. For this reason, we have developed a gene t herapy strategy to protect bone marrow cells against combined treatment wit h antifolates and nitrobenzylmercaptopurine riboside (NBMPR), a potent inhi bitor of the es nucleoside transporter. A retroviral vector (MeiIRG) was co nstructed that expressed the NBMPR-insensitive ei transporter, hypothesizin g that transduced bone marrow cells would survive drug treatment because of the preservation of nucleoside salvage pathways. In vitro clonogenic assay s confirmed that the MeiIRG vector did protect myeloid progenitors against the toxic effects of 3 different antifolates when each was combined with NB MPR, On testing this system in vivo, decreased myelosuppression was observe d in mice transplanted with MeiIRG-transduced bone marrow cells and subsequ ently treated with trimetrexate and NBMPR-P, In these mice, significant inc reases were noted in absolute neutrophil count nadirs, reticulocyte indices , and the numbers of myeloid progenitors in the bone marrow. Furthermore, a survival advantage was associated with transfer of the MeiIRG vector, indi cating that significant dose intensification was possible with this approac h. In summary, the MeiIRG vector can decrease the toxicity associated with the combined use of antifolates and NBMPR-P and thereby may provide a strat egy for simultaneously sensitizing tumor cells while protecting hematopolet ic cells. (Blood.2000;95:2356-2363) (C) 2000 by The American Society of Hem atology.