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In vitro synergism between 5-fluorouracil and natural beta interferon in human colon carcinoma cells
The combination of 5-fluorouracil (FUra) plus IFN-beta was studied in vitro using a human colon carcinoma cell line, HCT-8. Continuous exposure to high concentrations of IFN-beta is cytotoxic to these cells (ED50, 600 +/- 50 IU/ml). A strong synergism (P < 0.002) was observed when a short-term (1...
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Published in: | Clinical cancer research 1995-11, Vol.1 (11), p.1337-1344 |
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Main Authors: | , , , , , , , , , , |
Format: | Article |
Language: | English |
Subjects: | |
Online Access: | Get full text |
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Summary: | The combination of 5-fluorouracil (FUra) plus IFN-beta was studied in vitro using a human colon carcinoma cell line, HCT-8.
Continuous exposure to high concentrations of IFN-beta is cytotoxic to these cells (ED50, 600 +/- 50 IU/ml). A strong synergism
(P < 0.002) was observed when a short-term (1-h), high-concentration exposure to fluoropyrimidine (300 or 1000 microM) was
followed by IFN-beta given continuously. In fact, the mean ratio between the expected (product of the survival of each agent
alone) and the observed clonogenic cell survival rates of the combination was 3.4 (range, 2.4-4.9). Longer exposures to the
fluoropyrimidine (24 h, 7 days) produced less than additive effects with IFN-beta, indicating strong schedule dependency for
this synergism. The mechanism of interaction was studied at four levels. First, thymidylate synthase (TS) activity, inhibition,
and recovery were measured by an in situ assay in cells treated with FUra, IFN-beta, and their combination. When the prolonged
infusion of IFN-beta followed a 1-h exposure to FUra, the observed TS inhibition and recovery, at each time point, were very
similar to the expected values, indicating that the interactions between these drugs at the level of TS are not the determinant
of the synergism. Second, cell cycle analysis was done. During the continuous exposure to IFN-beta, a significant accumulation
of HCT-8 cells in S-phase was observed at 24, 48, and 72 h compared to untreated controls (P = 0.003); however, under the
same experimental conditions producing synergy in the clonogenic assay, no significant cell cycle perturbations were produced
by the combination of FUra followed by IFN-beta compared to those caused by each agent alone. Third, using the alkaline elution
test, we also demonstrated that the synergism is not due to enhanced FUra-induced DNA single-strand break frequency in high
molecular weight DNA. Finally, nucleic acid incorporation experiments, using tritiated FUra, showed that the cytokine, given
continuously (300 IU/ml), enhanced the amount of FUra incorporated into nucleic acids 24 h after a 1-h exposure to 300 and
1000 microM of FUra. The median percentage of enhancement values were 31.6 +/- 11.5%,m for the lower drug concentration and
18. 4 +/- 8.1% for the higher drug concentration tested. These results suggest that the mechanism of this synergism may be
related to the ability of IFN-beta to promote the incorporation of intracellular FUra metabolites into nucleic acids and/ |
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ISSN: | 1078-0432 1557-3265 |