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

Biochem Pharmacol. 1999 Aug 1;58(3):477-86. doi: 10.1016/s0006-2952(99)00099-4.


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 (approximately 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.

MeSH terms

  • Antimetabolites, Antineoplastic / pharmacology*
  • Antineoplastic Agents, Phytogenic / antagonists & inhibitors
  • Antineoplastic Agents, Phytogenic / pharmacology*
  • Breast Neoplasms / drug therapy
  • Breast Neoplasms / pathology
  • Cell Cycle / drug effects
  • Cell Division / drug effects
  • Cyclin-Dependent Kinase Inhibitor p21
  • Cyclins / biosynthesis
  • DNA Damage / drug effects
  • DNA, Neoplasm / drug effects
  • Drug Administration Schedule
  • Drug Antagonism
  • Fluorouracil / antagonists & inhibitors
  • Fluorouracil / metabolism
  • Fluorouracil / pharmacology*
  • Humans
  • Paclitaxel / antagonists & inhibitors
  • Paclitaxel / pharmacology*
  • RNA, Neoplasm / metabolism
  • Thymidylate Synthase / antagonists & inhibitors
  • Tumor Cells, Cultured
  • Tumor Suppressor Protein p53 / biosynthesis


  • Antimetabolites, Antineoplastic
  • Antineoplastic Agents, Phytogenic
  • CDKN1A protein, human
  • Cyclin-Dependent Kinase Inhibitor p21
  • Cyclins
  • DNA, Neoplasm
  • RNA, Neoplasm
  • Tumor Suppressor Protein p53
  • Thymidylate Synthase
  • Paclitaxel
  • Fluorouracil