Oxidation of the novel oxazolidinone antibiotic linezolid in human liver microsomes

In vitro studies were conducted to identify the hepatic enzyme(s) responsib le for the oxidative metabolism of linezolid. In human liver microsomes, li nezolid was oxidized to a single metabolite, hydroxylinezolid (M1). Formati on of M1 was determined to be dependent upon microsomal protein and NADPH. Over a concentration range of 2 to 700 mu M, the rate of M1 formation confo rmed to first-order (nonsaturable) kinetics. Application of conventional in vitro techniques were unable to identify the molecular origin of M1 based on the following experiments: a) inhibitor/substrates for various cytochrom e P-450 (CYP) enzymes were unable to inhibit M1 formation; b) formation of M1 did not correlate (r(2) < 0.23) with any of the measured catalytic activ ities across a population of human livers (n = 14); c) M1 formation was not detectable in incubations using microsomes prepared from a baculovirus ins ect cell line expressing CYPs 1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5, and 4A11. In addition, results obtained from an in vitro P-450 i nhibition screen revealed that linezolid was devoid of any inhibitory activ ity toward the following CYP enzymes (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2 E1, and CYP3A4). Additional in vitro studies excluded the possibility of fl avin-containing monooxygenase and monoamine oxidase as potential enzymes re sponsible for metabolite formation. However, metabolite formation was found to be optimal under basic (pH 9.0) conditions, which suggests the potentia l involvement of either an uncharacterized P-450 enzyme or an alternative m icrosomal mediated oxidative pathway.