The increased cytotoxicity in colon adenocarcinoma of methotrexate-5-fluorouracil is not associated with increased induction of lesions in DNA by 5-fluorouracil.

Sequential methotrexate-5-fluorouracil (Mtx-5-Fu) reduces proliferation of human colon adenocarcinoma cells. The DNA of treated cells was examined by cell lysis in dilute alkali in order to detect if there is any release of single-stranded DNA fragments, which occurs in cells treated with 5-Fu. The results showed that in spite of increased cytotoxicity of Mtx-5-Fu there is reduced release of DNA fragments. The findings are paralleled by a reduced incorporation of 5-Fu into DNA. Hypoxanthine reduces the growth inhibitory effect of Mtx-5-Fu. In treated cells grown in the presence of hypoxanthine, alkaline lysis causes release of DNA fragments from bulk DNA, in contrast to cells grown without hypoxanthine. Hence, the increased cytotoxicity of Mtx-5-Fu in human colon adenocarcinoma is not associated with enhanced lesions in DNA by 5-Fu.