Abstract
In vitro studies were conducted to identify the hepatic enzyme(s) responsible for the oxidative metabolism of linezolid. In human liver microsomes, linezolid was oxidized to a single metabolite, hydroxylinezolid (M1). Formation of M1 was determined to be dependent upon microsomal protein and NADPH. Over a concentration range of 2 to 700 μM, the rate of M1 formation conformed 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 cytochrome P-450 (CYP) enzymes were unable to inhibit M1 formation; b) formation of M1 did not correlate (r2< 0.23) with any of the measured catalytic activities across a population of human livers (n = 14); c) M1 formation was not detectable in incubations using microsomes prepared from a baculovirus insect 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 inhibition screen revealed that linezolid was devoid of any inhibitory activity toward the following CYP enzymes (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4). Additional in vitro studies excluded the possibility of flavin-containing monooxygenase and monoamine oxidase as potential enzymes responsible for metabolite formation. However, metabolite formation was found to be optimal under basic (pH 9.0) conditions, which suggests the potential involvement of either an uncharacterized P-450 enzyme or an alternative microsomal mediated oxidative pathway.
Footnotes
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Send reprint requests to: Larry C. Wienkers, Ph.D., Drug Metabolism Research, Pharmacia Corporation, 7265-300-313, 301 Henrietta St., Kalamazoo, MI. E:mail: larry.c.wienkers{at}am.pnu.com
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↵2 Feenstra KL, Slatter JG, Stalker DJ, Welshman IR, Sams JP, Hauer MJ, Cathcart KS, Verburg MT, Johnson MG, Bothwell BE, Koets MD, Peng GW, Stryd RP and Fagerness PE (1998) Metabolism and excretion of the oxazolidinone antibiotic linezolid (PNU-100766) following oral administration of [14C]PNU-100766 to healthy human volunteers, 8th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC). Session 92-A, Poster A-53, 1998 Sept 24–27, San Diego, CA.
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↵3 Additional studies aimed to elucidate the mechanism of linezolid microsomal oxidation are currently underway.
- Abbreviations used are::
- linezolid (Zyvox)
- (S)-N-[[3-[3-fluoro-4-(4-morpholinyl)phenyl]- 2-oxo-5-oxazolidinyl]methyl]-acetamide
- CYP or P-450
- cytochrome P-450
- QUIN
- quinidine
- KETO
- ketoconazole
- MEPH
- (S)-mephenytoin
- ANF
- α-naphthoflavone
- LAUR
- lauric acid
- SULF
- sulfaphenazole
- COUM
- coumarin
- ORPH
- orphenadrine
- RETN
- retinoic acid
- pNITR
- para-nitrophenol
- LC
- liquid chromatography
- MS
- mass spectrometry
- ROS
- reactive oxygen species
- FMO
- flavin-containing monooxygenase
- APCI
- atmospheric pressure chemical ionization
- Received December 3, 1999.
- Accepted May 17, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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