DIFFERENTIAL-EFFECTS OF THE ANTIFOLATES METHOTREXATE, AMINOPTERIN ANDTRIMETREXATE ON MURINE HEMATOPOIETIC PROGENITOR CELLS

We investigated the effects of the antifolates methotrexate (MTX), ami nopterin (AMT) and trimetrexate (TMTX) on murine haemopoietic progenit or cells. Colony formation by late progenitors (CFU-C) was inhibited b y the antifolates and the addition of leucovorin (LV) to the cultures on day 0 or day 5 wholly or partially reversed the effects of MTX or A MT. The effect of TMTX was only effectively rescued by hypoxanthine pl us thymidine (HT). In the high proliferative potential colony forming cell (HPP-CFU-C) assay the antifolates induced formation of smaller co lonies, but not a reduction in absolute colony numbers. This effect wa s reversed by LV and indicated that the antifolates (except high TMTX concentrations) were not to?de to the HPP-CFU-C but reversibly inhibit ed proliferation of more mature progenitor cells. The direct effects o f the drugs on HPP-CFU-C were investigated in limiting dilution assays performed with fractionated bone marrow cells, Unheated cultures cont ained almost only large colonies, whereas the addition of antifolates induced a reversible shift towards smaller colonies, The present study indicates that MTX, AMT and low TMTX concentrations are not toxic to HPP-CFU-C but that the drugs induce a developmental arrest in more mat ure progenitor cell populations.