Effects of CYP induction by rifampicin on tamoxifen exposure

Clin Pharmacol Ther. 2012 Jul;92(1):62-7. doi: 10.1038/clpt.2011.372. Epub 2012 May 23.


Tamoxifen undergoes biotransformation into several metabolites, including endoxifen. Differences in metabolism contribute to the interindividual variability in endoxifen concentrations, potentially affecting treatment efficacy. We evaluated the effects of cytochrome P450 (CYP) induction by rifampicin on the exposure levels of tamoxifen and its metabolites and found that coadministration of rifampicin resulted in markedly reduced (up to 86%, P ≤ 0.040) concentrations of tamoxifen and its metabolites. Given the extensive metabolism undergone by tamoxifen, several factors may have contributed to this effect. Similar drug-drug interactions may exist between tamoxifen and other strong CYP inducers.

Publication types

  • Randomized Controlled Trial

MeSH terms

  • Adult
  • Antineoplastic Agents / pharmacokinetics
  • Biotransformation* / drug effects
  • Biotransformation* / physiology
  • Breast Neoplasms / drug therapy
  • Cytochrome P-450 Enzyme System / genetics
  • Cytochrome P-450 Enzyme System / metabolism
  • Drug Interactions
  • Drug Monitoring
  • Drug Therapy, Combination / adverse effects
  • Drug Therapy, Combination / methods
  • Female
  • Genotype
  • Humans
  • Middle Aged
  • Rifampin / pharmacokinetics*
  • Tamoxifen / pharmacokinetics*
  • Treatment Outcome


  • Antineoplastic Agents
  • Tamoxifen
  • Cytochrome P-450 Enzyme System
  • Rifampin