Characterization of an etoposide-glutathione conjugate derived from metabolic activation by human cytochrome P450

Naiyu Zheng, Shaokun Pang, Tomoyuki Oe, Carolyn A. Felix, Suzanne Wehrli, Ian A. Blair

Research output: Contribution to journalReview articlepeer-review

13 Citations (Scopus)


Etoposide (VP-16), a DNA topoisomerase II poison widely used as an antineoplastic agent is also known to cause leukemia. One of its major metabolic pathways involves O-demethylation to etoposide catechol (etoposide-OH) by cytochrome P450 3A4 (CYP3A4). The catechol metabolite can undergo sequential one- and two-electron oxidations to form etoposide semi-quinone (etoposide-SQ) and etoposide quinone (etoposide-Q), respectively, which have both been implicated as cytotoxic metabolites. However, etoposide-Q is known to react with glutathione (GSH), which can protect DNA from oxidative damage by this reactive metabolite. In this study, etoposide-Q was reacted with GSH and the two etoposide-GSH conjugates were characterized. The major conjugate was etoposide-OH-6′-SG and the minor product was etoposide-OH-2′-SG. Etoposide-OH-6′-SG, which arose from Michael addition of GSH to etoposide-Q, was characterized by mass spectrometry and 2-D NMR. It was identified as the sole product from in vitro metabolism experiments using recombinant human CYP3A4 or liver microsomes incubated with etoposide in the presence of GSH. Etoposide-OH6′-SG was also detected from incubations of etoposide-OH and GSH alone. Therefore, the presence of etoposide-OH, which can be formed from etoposide metabolism by CYP3A4, is essential for formation of the GSH conjugate. The oxidation of etoposide-OH to a quinone intermediate is likely the precursor in the formation of etoposide-OH-6′-SG.

Original languageEnglish
Pages (from-to)897-911
Number of pages15
JournalCurrent Drug Metabolism
Issue number8
Publication statusPublished - 2006 Dec


  • Cytochrome P450
  • Etoposide catechol
  • Etoposide metabolism
  • Etoposide quinone
  • Glutathione conjugate
  • Human liver microsomes
  • Mass spectrometry
  • NMR spectroscopy


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