Although teniposide (VM26) and etoposide (VP16) are eliminated mostly by nonrenal mechanisms, their cytochrome P450 metabolism in humans has not been reported. Our objective was to determine the affinity and capacity of P450 O-demethylation of VM26 and VP16 in a variety of human livers. Formation of catechols of VM26 and VP16 was detected in 24 and 26 of 26 liver microsomal preparations, respectively, with wide variability in maximum catechol formation rates from VM26 (41-fold) and VP16 (39-fold range), even among normal livers. Maximal activity measurements at 500 microM substrate were lower for VM26 catechol formation (mean = 1.5 nmol/mg/hr) than for VP16 catechol (mean = 3.2 nmol/mg/hr) in 26 livers (P less than .001). Maximal activities for VP16 and VM26 O-demethylation, and ethoxycoumarin O-deethylation were significantly higher in normal than in diseased livers. No differences were found in activities related to age, sex or race of the liver donor. In all five livers tested over a range of substrate concentrations, Km (19.7, 23.2, 43.5, 30.1 and 22.0 microM) and Vmax values (1.0, 1.2, 4.4, 8.2 and 4.0 nmol/mg/hr) for VM26 were lower compared to values for VP16 (Km = 60.2, 115.1, 87.3, 42.1 and 81.9 microM; Vmax = 1.4, 3.3, 10.3, 27.5 and 10.1 nmol/mg/hr). Despite higher VP16 catechol formation, VM26 underwent greater overall reduced nicotinamide adenine dinucleotide phosphate-dependent metabolism than VP16, consistent with greater nonrenal clearance of VM26 in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)