The modulation of 5-fluorouracil (FUra) metabolism by methotrexate (MTX) pretreatment in monolayer cultures of human colorectal adenocarcinoma, HCT-8, was examined and correlated to clonal growth of this cell line. There was a gradual and nearly linear total intracellular accumulation and incorporation into RNA of FUra for 30 hr in control cells. A 12-hr 10 µm MTX pretreatment before adding 100 µm FUra resulted in approximately a 3-fold increase in total FUra accumulation, 59% of which was fluorouridine triphosphate. Soluble fluorodeoxyuridine monophosphate was increased 5-fold following MTX pretreatment; however, [3H]deoxyuridine incorporation into the acid-precipitable fraction of cells pretreated with MTX was no more than that observed when FUra was given alone. There was also an increase in 5-phosphoribosyl 1-pyrophosphate pools following MTX which was associated with the enhanced FUra metabolism. The maximum synergistic inhibition of clonal growth occurred when FUra was given during the last 6 hr of a 24-hr MTX exposure period. Other antimetabolites associated with elevations of 5-phosphoribosyl 1-pyrophosphate also resulted in an enhanced total intracellular accumulation of FUra.

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Supported by Grant CH-145 from the American Cancer Society, a Swebilius Cancer Award from the Yale Comprehensive Cancer Center, and National Cancer Institutes Grants CA-27130 and CA-08341.

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