Thymidine enhancement of methotrexate and 5-fluorouracil toxicity in cultured human colon carcinoma

Immunomodulation and enhancement of antitumor activity by co-administration of 1,3-bis(2-chloroethyl)-1-nitrosourea and thymidine

The antitumor activity of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) has been shown previously to be enhanced markedly by the co-administration of pyrimidine deoxyribonucleosides (Lin and Prusoff, Cancer Res 47: 394-397, 1987). In the present study, we examined the cellular mechanisms underlying the augmentation effect of thymidine, one of the pyrimidine deoxyribonucleosides. It was found that thymidine did not increase the cytotoxicity of BCNU for B16/F10 melanoma cells in vitro. Instead, thymidine appeared to produce modulatory actions on the immune system of the tumor-bearing mice. More than 40% of the BCNU/ thymidine-cured mice specifically rejected secondary rechallenge with the B16/F10 tumor. Furthermore, these cured mice developed extensive depigmentation of their natural black hair, suggesting immune reactions to normal melanocytes. When spleen cells from normal mice were treated with BCNU alone, their response to T-cell mitogen phytohemagglutinin was suppressed markedly. This suppression was ablated by co-administration of BCNU with thymidine. Such BCNU/thymidine treatment also augmented the activity of tumor-specific cytotoxic T-cells in tumor-bearing mice. Taken together, these results suggest that the enhanced antitumor activity of combined BCNU and thymidine may result from the action of thymidine on the immune effector mechanisms, which facilitate the development of antitumor immune responses in the presence of immunosuppression induced by BCNU.

Biochemical pharmacology and analysis of fluoropyrimidines alone and in combination with modulators

After more than three decades since their introduction, fluoropyrimidines, especially FUra, are still a mainstay in the treatment of various solid malignancies. The antitumor effects of fluoropyrimidines are dependent upon metabolic activation. FdUMP, FUTP and FdUTP were identified as the key cytotoxic metabolites that interfere with the proper function of thymidylate synthase and nucleic acids. The relevance of these metabolites is cell-type specific. Recently, fluorouridine diphospho sugars have been detected, but the precise function of this class of metabolites is currently unknown. In mammalian systems fluoropyrimidines and their natural counterparts share the same metabolic pathways since the substrate properties in enzyme-catalyzed reactions are frequently comparable. Ongoing studies indicate that the metabolism and action of fluoropyrimidines exhibit circadian rhythms, which appear to be due to variations in the activity of metabolizing enzymes. Essential for the expanding knowledge of the pathways and effects of fluoropyrimidines has been the constant improvement of analytical methods. These include ligand binding techniques, numerous dedicated HPLC systems and 19F-NMR. Because the overall response rates achieved with fluoropyrimidines are modest, strategies based on biochemical modulation have been devised to enhance their therapeutic index. Biochemical modulators include a wide range of various compounds with different modes of action. In recently completed clinical trials, combinations of FUra with leucovorin, a precursor for 5,10-methylene tetrahydrofolate, or with levamisole, an anthelminthic with immunomodulatory activity, appeared to be superior to FUra alone. At the preclinical level combinations of fluoropyrimidines with, e.g. interferons or L-histidinol were demonstrated to be interesting candidates for further testing. The future therapeutic utility of fluoropyrimidines will depend on both the improvement of combination regimens currently used in the treatment of cancer patients and the judicious clinical implementation of promising experimental modulation strategies. Moreover, novel fluoropyrimidines with superior pharmacological properties may become important as part of or instead of modulation approaches.

Effect of 5-fluorouracil combination therapy on RNA processing in human colonic carcinoma cells

We have evaluated the RNA-directed cytotoxicity of 5-fluorouracil (5-FU) in human colonic carcinoma cells. The mode of action of 5-FU and its effects on human pre-rRNA processing were then examined. From these data, possible reasons why the disruption of pre-rRNA maturation could induce cytotoxic effects are considered. The results imply that inhibition of thymidylate synthase is not the sole primary cytotoxic lesion in this cell line. First, exogenous thymidine (dTHd) enchanced cytotoxicity. Second, addition of dThd to the cells was found to enhance incorporation of 5-FU into total cellular RNA. Third, 5-FU disrupted rRNA processing by a different mechanism from actinomycin D and methotrexate (MTX), suggesting that the inhibition was not just a consequence of cell death. Finally, the addition of dThd was found to enhance the disruption of rRNA processing consistent with an increase in concentration of 5-FU. These data are discussed in the light of literature reports and their potential for optimising 5-FU protocols.