Fig. 5.

Comparison of folates or antifolates accumulated in the cytosolic and membrane fractions, and inhibitory effects of pemetrexed, (6S)-5-CHO-THF and folic acid on binding of [³H]folic acid to FR at the cell surface at pH 7.4 and 6.5. (A) Cells were exposed to 50 nM tritiated pemetrexed, 5-CHO-THF, folic acid, or methotrexate for 2 days in folate-free RPMI-medium in a 5% CO₂ incubator. GAT was added to cell cultures during exposure to pemetrexed and methotrexate to prevent cytotoxicity mediated by these agents. The data for PCFT-4 cells are indicated on the left side of the graph, and the values for M160-8 cells are shown on the right side of the graph. (B) [³H]Folic acid bound to the surface of M160-8 cells was assessed with 20 nM [³H]folic acid in the presence or absence of unlabeled pemetrexed, (6S)-5-CHO-THF, or folic acid. Data are the mean ± S.E.M. from three independent experiments in both panels. (C) Comparison of 20 nM [³H]folic acid bound to the surface of M160-8 cells at pH 7.4 and pH 6.5 in the presence or absence of various concentrations of unlabeled folic acid, pemetrexed, or (6S)-5-CHO-THF; *P < 0.05 for the comparison in A and C.