Etoposide (ETO) has been used in the treatment of various tumors for many years but its metabolism is still not completely understood. In vitro studies in human liver microsomes revealed O-demethylation catalyzed by cytochrome P450 3A4 as metabolic pathway of ETO in man. The formed catechol has been shown to interact directly with DNA leading to DNA damage and inactivation. So far, this cytotoxic metabolite has never been detected and quantified in plasma of cancer patients.We investigated 10 patients with germ cell cancer treated with 2400 mg/m 2 ETO given as four daily iv infusions over 1 hour, 1500 mg/m 2 carboplatin/3 days and 10g/m 2 /4 days ifosfamide. Serial plasma samples were drawn and analyzed for ETO catechol by reversed-phase HPLC and electrochemical detection. The metabolite was synthesized by demethylation of ETO. Its identity was verified by NMR and mass spectroscopy. Pharmacokinetic parameters were calculated using non-compartmental methods.ETO catechol could be quantified in plasma of all patients up to 36 hours following the last infusion of etoposide. Plasma concentrations increased during therapy probably due to enzyme induction by concomitantly administered ifosfamide. Peak concentrations of 1.3+/-0.4 μg/ml were observed on the 4th day of treatment 3.4+/-1.3 hours after the start of ETO infusion. The apparent metabolite half-life (t 1 2 ) of 8.9+/-3.0 hours was slightly longer than the corresponding t 1 2 of ETO (6.5+/-1.3 h). AUC of the metabolite calculated on the 4th day of treatment was found to be 2.1+/-0.7% of the respective ETO AUC.In conclusion, cancer patients receiving high-dose ETO are exposed to considerable plasma levels of its cytotoxic catechol metabolite. Interindividual variability in its formation and interactions with other drugs may therefore have clinical implications. Future investigations should reveal its contribution to tumor response as well as to treatment-induced toxicity.