title: In vitro metabolism of etoposide (VP-16-213) by liver microsomes and irreversible binding of reactive intermediates to microsomal proteins.
abstract: 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: D000818:Animals; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004578:Electron Spin Resonance Spectroscopy; D005047:Etoposide / Q000031:analogs & derivatives / Q000378:metabolism*; D051379:Mice; D008862:Microsomes, Liver / Q000378:metabolism*; D009249:NADP / Q000378:metabolism; D010634:Phenobarbital / Q000494:pharmacology; D010882:Piperonyl Butoxide / Q000494:pharmacology; D011335:Proadifen / Q000494:pharmacology; D011506:Proteins / Q000378:metabolism*
chemical_list: D011506:Proteins; C051505:3',4'-dihydroxyetoposide; D009249:NADP; D005047:Etoposide; D003577:Cytochrome P-450 Enzyme System; D011335:Proadifen; D010882:Piperonyl Butoxide; D010634:Phenobarbital