Inhibitory effect of 5-fluorouracil on cytochrome P450 2C9 activity in cancer patients

Basic Clin Pharmacol Toxicol. 2006 Feb;98(2):197-200. doi: 10.1111/j.1742-7843.2006.pto_304.x.


Drug interactions have been reported between 5-fluorouracil and cytochrome P450 2C9 (CYP2C9) substrates, S-warfarin and phenytoin. This study was performed to determine the influence of 5-fluorouracil on cytochrome P450 2C9 (CYP2C9) activity in colorectal cancer patients (n=17) receiving 5-fluorouracil. Losartan was used as a marker to assess CYP2C9 activity. Losartan and its CYP2C9 dependent metabolite, E-3174, were determined in urine. The ratios of urinary losartan/E-3174 before and after the 5-fluorouracil treatment were compared for each patient. Genotyping was performed to detect the CYP2C9*2 and CYP2C9*3. At the end of the first cycle of 5-fluorouracil, losartan/E-3174 ratio was increased by 28.0% compared to the pre-treatment values (P=0.15). In five patients recruited for phenotyping after three 5-fluorouracil cycles, the metabolic ratio was increased significantly by 5.3 times (P=0.03). The results suggest that in most patients 5-fluorouracil inhibited CYP2C9 activity. This inhibition was more pronounced when the total administered dose increased. This finding may help explain the mechanism of interaction between 5-fluorouracil and CYP2C9 substrates.

Publication types

  • Clinical Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aged
  • Aged, 80 and over
  • Antimetabolites, Antineoplastic / therapeutic use*
  • Aryl Hydrocarbon Hydroxylases / genetics
  • Aryl Hydrocarbon Hydroxylases / metabolism*
  • Colorectal Neoplasms / drug therapy
  • Colorectal Neoplasms / genetics
  • Colorectal Neoplasms / metabolism*
  • Cytochrome P-450 CYP2C9
  • Female
  • Fluorouracil / therapeutic use*
  • Genotype
  • Humans
  • Imidazoles / urine
  • Losartan / pharmacokinetics
  • Male
  • Middle Aged
  • Oxidation-Reduction
  • Tetrazoles / urine


  • Antimetabolites, Antineoplastic
  • Imidazoles
  • Tetrazoles
  • CYP2C9 protein, human
  • Cytochrome P-450 CYP2C9
  • Aryl Hydrocarbon Hydroxylases
  • losartan carboxylic acid
  • Losartan
  • Fluorouracil