Prior studies from these laboratories demonstrated 3.2-fold potentiation of 5-fluorouracil (FUra) cytotoxicity by recombinant human interferon-alpha 2a (rIFN-alpha 2a) in GC3/cl colon adenocarcinoma cells that was significantly enhanced to 14-fold when FUra was combined with rIFN-alpha 2a + a mixture of the diasteroisomers of the biologically active (6S) and inactive (6R) leucovorin or 5-formyl-H4PteGlu (LV), events that were reversible by thymidine (dThd). In GC3/clTS-c3/c3 cells, deficient in thymidylate synthase, rIFN-alpha 2a cytotoxicity was not influenced by the concentration of dThd, indicating no direct effect at the level of dThd-less stress. Direct assays of thymidylate synthase indicated no significant difference between FUra-induced accumulation of total thymidylate synthase or free or unbound thymidylate synthase in cells receiving FUra + modulators. In addition, the cytotoxic activity of CB3717, a specific quinazoline-based inhibitor of thymidylate synthase, was not potentiated by rIFN-alpha 2a. These studies suggested that thymidylate synthase was not the primary target site for rIFN-alpha 2a activity. Since data indicated that a 5-fluoropyrimidine was required in the interaction among FUra, LV, and rIFN-alpha 2a, attention was focused at the level of DNA. Both DNA single-strand breaks (SSBs) and DNA double-strand breaks (DSBs) induced by FUra were significantly elevated by rIFN-alpha 2a and LV administered as single modulators and were influenced by the concentrations of both FUra and rIFN-alpha 2a. However; when FUra was combined with LV, rIFN-alpha 2a further potentiated the frequency of DNA SSBs, and data correlated with the relative cytotoxic activity of FUra-LV-rIFN-alpha 2a combinations. No effect on CB3717-induced DNA SSBs or DSBs by rIFN-alpha 2a was demonstrated. Drug exposure for 48 h was required to detect measurable differences in DNA SSB frequency among FUra-LV-rIFN-alpha 2a treatment groups and correlated with decreased clonogenic survival under these conditions. Continuous exposure to FUra (72 h) allowed shorter exposures to LV and/or rIFN-alpha 2a (48 h) to maintain maximal cytotoxicity. Shorter exposure times for FUra during continuous exposure to the modulators were less cytotoxic. Data suggest that the primary locus of the interaction among FUra, LV, and rIFN-alpha 2a lies at the level of DNA. rIFN-alpha 2a may exert its effects via enhancement of FUra base excision or incorporation into DNA, events that subsequently become influenced by thymidylate synthase inhibition and dThd-less stress and are further potentiated by LV.(ABSTRACT TRUNCATED AT 400 WORDS)