Deactivation of anti-cancer drug letrozole to a carbinol metabolite by polymorphic cytochrome P450 2A6 in human liver microsomes

Xenobiotica. 2009 Nov;39(11):795-802. doi: 10.3109/00498250903171395.


Cytochromes P450 (P450) involved in letrozole metabolism were investigated. Among 13 recombinant P450 forms examined, only P450 2A6 and 3A4 showed activities in transforming letrozole to its carbinol metabolite with small K(m) and high Vmax values yielding apparent Vmax/K(m) values of 0.48 and 0.24 nl min(-1) nmol(-1) P450, respectively. The metabolic activities of individual human liver microsomes showed a significant correlation with coumarin 7-hydroxylase activities (P450 2A6 marker) at a letrozole concentration of 0.5 microM, while a good correlation was also seen with testosterone 6beta-hydroxylase activities (P450 3A4 marker) at 5 microM substrate concentration with different inhibition by 8-methoxypsolaren. Significantly low carbinol-forming activities were seen in human liver microsomes from individuals possessing CYP2A6*4/*4 (whole CYP2A6 gene deletion) at a letrozole concentration of 0.5 microM. A Vmax/K(m) value measured for CYP2A6.7 (amino acid substitution type) in human liver microsomes, in the presence of anti-P450 3A4 antibodies, was approximately seven-fold smaller than that for CYP2A6.1 (wild-type). These results demonstrate that P450 2A6 and 3A4 catalyse the conversion of letrozole to its carbinol metabolite in vitro at low and high concentrations of letrozole. Polymorphic variation of CYP2A6 is considered to be relevant to inter-subject variation in therapeutic exposure of letrozole.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antineoplastic Agents / chemistry
  • Antineoplastic Agents / pharmacokinetics*
  • Aryl Hydrocarbon Hydroxylases / genetics
  • Aryl Hydrocarbon Hydroxylases / metabolism*
  • Cross-Linking Reagents / pharmacology
  • Cytochrome P-450 CYP2A6
  • Cytochrome P-450 CYP3A / metabolism
  • Cytochrome P-450 CYP3A Inhibitors
  • Enzyme Inhibitors / pharmacology
  • Genotype
  • Humans
  • Ketoconazole / pharmacology
  • Letrozole
  • Methoxsalen / pharmacology
  • Microsomes, Liver / drug effects
  • Microsomes, Liver / enzymology*
  • Nitriles / chemistry
  • Nitriles / pharmacokinetics*
  • Oxidoreductases / metabolism
  • Steroid Hydroxylases / metabolism
  • Triazoles / chemistry
  • Triazoles / pharmacokinetics*


  • Antineoplastic Agents
  • Cross-Linking Reagents
  • Cytochrome P-450 CYP3A Inhibitors
  • Enzyme Inhibitors
  • Nitriles
  • Triazoles
  • Letrozole
  • Oxidoreductases
  • Steroid Hydroxylases
  • Aryl Hydrocarbon Hydroxylases
  • CYP2A6 protein, human
  • Cytochrome P-450 CYP2A6
  • Cytochrome P-450 CYP3A
  • steroid hormone 6-beta-hydroxylase
  • CYP3A4 protein, human
  • nicotine oxidase
  • Ketoconazole
  • Methoxsalen