The hematological effects of folate analogs: implications for using the dihydrofolate reductase gene for in vivo selection.

Previous studies have shown that dihydrofolate reductase (DHFR) gene transfer protects marrow from methotrexate (MTX)-mediated toxicity; however, MTX treatment in vivo has not convincingly been shown to enrich DHFR-transduced progenitors or stem cells. Experiments were performed to better characterize the hematological effects of MTX, and maneuvers were tested with the aim of improving the utility of MTX as an agent for in vivo selection. Progenitors were assayed as colony forming unit cells in culture (CFU-C) and in the spleens of irradiated mice (day 11 CFU-S). A single injection of MTX at doses up to 250 mg/kg (more than three times the LD10) failed to reduce CFU-C numbers significantly in the femur or spleen assayed 1-3 days later. However, consistent declines in the number of mononuclear cells per femur reflected a significant depletion of nonclonogenic precursor cells. Preceding administration of pegylated stem cell factor (SCF), 100 micrograms/kg per day, increased CFU-C killing by a single dose of 5-fluorouracil (5-FU) 15- to 65-fold in the femur, and 5- to 15-fold in the spleen, consistent with previous reports. In contrast, despite preceding SCF administration there was no significant progenitor killing by MTX. Similar results were obtained using a second folate analog, trimetrexate. These results suggest that the mechanism by which folate analogs exert their hematological toxicity is through the depletion of relatively mature, nonclonogenic precursor cells, and not by killing progenitors. This information is relevant to the use of DHFR in gene therapy protocols, and suggests that folate analogs are poorly suited agents for selection at the level of clonogenic progenitor cells in vivo.