Differential effects of folinic acid and glycine, adenosine, and thymidine as rescue agents in methotrexate-treated human cells in relation to the accumulation of methotrexate polyglutamates.

By converting methotrexate (MTX) into poly-gamma-glutamyl derivatives, cultured human fibroblasts accumulated high intracellular levels of drug. Once polyglutamates had been formed, DNA synthesis and cell growth remained suppressed even after MTX had been removed from the culture medium. Co-cultivation of cells with MTX and folinic acid reversed the effect of MTX on polyglutamate formation, DNA synthesis, and cell growth. However, if folinic acid was added to the culture medium following a preincubation in methotrexate, DNA synthesis initially remained inhibited and cell growth was only gradually restored. Co-cultivation of cells with 0.67 mM glycine, 37.5 micrometers adenosine, and 41.3 micrometers thymidine (GAT) and MTX did not prevent polyglutamate formation but allowed cells to grow. If GAT was removed from the culture medium along with MTX, cell growth and DNA synthesis were inhibited. If GAT was added to the culture medium following growth in MTX, cell growth recovered. These studies differentiate the effects of GAT and folinic acid treatment. Folinic acid prevented MTX polyglutamate accumulation and reversed the effects of MTX on cell growth when present along with MTX in the cultures. Folinic acid was only partially effective in circumventing the MTX-induced block in folate metabolism when added after pretreatment with MTX. In contrast, GAT allowed growth of cells both in the presence of MTX and after a preincubation in MTX. In contrast, GAT allowed growth of cells both in the presence of MTX and after a preincubation in MTX. However, co-incubation in MTX plus GAT resulted in the accumulation of polyglutamates and a sustained inhibition of cell growth and DNA synthesis upon removal of both MTX and GAT from the culture medium.