Sequence-dependent antagonism between fluorouracil and paclitaxel in human breast cancer cells

Jean L. Grem, Diana Nguyen, Brian P. Monahan, Vivian Kao, Francois J. Geoffroy

Research output: Contribution to journalArticlepeer-review

51 Scopus citations


The effects of 24-hr exposures to 5-fluorouracil (FUra) and paclitaxel in various sequences were studied in MCF-7 breast cancer cells to determine an optimal schedule for possible clinical use. In clonogenic assays, pre-exposure to FUra followed by paclitaxel resulted in marked antagonism, while sequential paclitaxel followed by FUra was optimal. Concurrent or pre-exposure to paclitaxel did not affect [3H]FUra metabolism, [3H]FUra-RNA incorporation, or the extent of FUra-mediated thymidylate synthase inhibition. Paclitaxel led to G2/M phase accumulation that persisted for up to 24 hr after drug exposure, while a 24-hr FUra exposure produced S-phase accumulation. FUra pre-exposure diminished paclitaxel-associated G2/M phase block, whereas subsequent exposure to FUra after paclitaxel did not. FUra exposure resulted in transient induction of p53 and p21, which returned to basal levels 24 hr after drug removal. p53 and p21 protein content also increased markedly during paclitaxel exposure, accompanied by phosphorylation of Bcl-2. Double-stranded DNA fragmentation (~ 50 kb) was seen at 48 hr when cells were exposed to paclitaxel for an initial 24-hr period. Paclitaxel-associated DNA fragmentation was not prevented by concurrent or subsequent exposure to FUra. Thus, paclitaxel-mediated G2/M phase arrest appeared to be a crucial step in induction of DNA fragmentation. Since an initial 24-hr paclitaxel exposure did not interfere with subsequent FUra metabolism or thymidylate synthase inhibition, and delayed exposure to FUra did not impede either paclitaxel-mediated induction of mitotic blockade or DNA fragmentation, the sequence of paclitaxel followed by FUra is recommended for clinical trials. Copyright (C) 1999 Elsevier Science Inc.

Original languageEnglish (US)
Pages (from-to)477-486
Number of pages10
JournalBiochemical Pharmacology
Issue number3
StatePublished - Aug 1 1999
Externally publishedYes


  • Bcl-2
  • DNA damage
  • Drug interactions
  • Fluorouracil
  • Taxanes
  • p21
  • p53

ASJC Scopus subject areas

  • Biochemistry
  • Pharmacology


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