In vitro metabolism of etoposide (VP-16-213) by liver microsomes and irreversible binding of reactive intermediates to microsomal proteins

Biochem Pharmacol. 1987 Feb 15;36(4):527-36. doi: 10.1016/0006-2952(87)90362-5.


We have studied the metabolism of VP-16-213 (etoposide, VP-16), an antitumor agent, by mouse liver microsomes to reactive intermediates and the subsequent covalent binding to microsomal proteins. This metabolism was shown to involve the O-demethylation of VP-16 and resulted in the formation of a 3',4'-dihydroxy derivative (DHVP-16) which was identified by both HPLC and mass spectrometry. The formation of DHVP-16 was cytochrome P-450-mediated as indicated by its dependence on NADPH, its increased production following treatment of mice with phenobarbital, and its marked inhibition by SKF-525A and piperonyl butoxide. Furthermore, DHVP-16 formation required oxygen. Microsomal incubation of VP-16 resulted in an irreversible binding of the drug to the proteins, which was also shown to be cytochrome P-450 dependent. The covalent binding of the VP-16 metabolite(s) was inhibited by DHVP-16 in a dose-dependent fashion, suggesting that the reactive intermediates that bound to proteins were derived from DHVP-16. Electron spin resonance studies indicated that the same semiquinone radical was formed during enzymatic (oxidation or reduction) metabolism of DHVP-16 and the o-quinone derivative of VP-16 (VP-16-Q). VP-16-Q and its semiquinone radical are suggested to be the bioalkylating species.

MeSH terms

  • Animals
  • Chromatography, High Pressure Liquid
  • Cytochrome P-450 Enzyme System / metabolism
  • Dose-Response Relationship, Drug
  • Electron Spin Resonance Spectroscopy
  • Etoposide / analogs & derivatives
  • Etoposide / metabolism*
  • Mice
  • Microsomes, Liver / metabolism*
  • NADP / metabolism
  • Phenobarbital / pharmacology
  • Piperonyl Butoxide / pharmacology
  • Proadifen / pharmacology
  • Proteins / metabolism*


  • Proteins
  • 3',4'-dihydroxyetoposide
  • NADP
  • Etoposide
  • Cytochrome P-450 Enzyme System
  • Proadifen
  • Piperonyl Butoxide
  • Phenobarbital