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Abstract
Coumadin (rac-warfarin) is the most commonly used anticoagulant in the world; however, its clinical use is often challenging because of its narrow therapeutic range and interindividual variations in response. A critical contributor to the uncertainty is variability in warfarin metabolism, which includes mostly oxidative but also reductive pathways. Reduction of each warfarin enantiomer yields two warfarin alcohol isomers, and the corresponding four alcohols retain varying levels of anticoagulant activity. Studies on the kinetics of warfarin reduction have often lacked resolution of parent-drug enantiomers and have suffered from coelution of pairs of alcohol metabolites; thus, those studies have not established the importance of individual stereospecific reductive pathways. We report the first steady-state analysis of R- and S-warfarin reduction in vitro by pooled human liver cytosol. As determined by authentic standards, the major metabolites were 9R,11S-warfarin alcohol for R-warfarin and 9S,11S-warfarin alcohol for S-warfarin. R-warfarin (Vmax 150 pmol/mg per minute, Km 0.67 mM) was reduced more efficiently than S-warfarin (Vmax 27 pmol/mg per minute, Km 1.7 mM). Based on inhibitor phenotyping, carbonyl reductase-1 dominated R-and S-warfarin reduction, followed by aldo-keto reductase-1C3 and then other members of that family. Overall, the carbonyl at position 11 undergoes stereospecific reduction by multiple enzymes to form the S alcohol for both drug enantiomers, yet R-warfarin undergoes reduction preferentially. This knowledge will aid in assessing the relative importance of reductive pathways for R- and S-warfarin and factors influencing levels of pharmacologically active parent drugs and metabolites, thus impacting patient dose responses.
Footnotes
- Received March 12, 2017.
- Accepted June 6, 2017.
Financial support for undergraduate research was provided in part by the Summer Undergraduate Research Fellowship program sponsored by the Biochemistry and Molecular Biology Department at the University of Arkansas for Medical Sciences (UAMS) (to B.P.J. and D.L.P.), and the UAMS Summer Undergraduate Research Program to Increase Diversity in Research, National Institutes of Health (NIH) National Heart, Lung and Blood Institute award [Grant R25HL108825] to D.L.P. In addition, financial support was provided to GPM through a grant-in-aid from the SouthWest Affiliate of the American Heart Association [Grant 13GRNT16960043] and a pilot study grant provided by the NIH [Grant UL1 TR000039] awarded to the UAMS along with grants from the NIH National Library of Medicine [Grants R01LM012222 and R01LM012482]. The content of this manuscript is solely the responsibility of the authors and does not necessarily represent the official views of the UAMS, the American Heart Association, or NIH. Funding was provided by the U.S. National Science Foundation for the nuclear magnetic resonance spectrometer [Grant 1040470].
- Copyright © 2017 by The American Society for Pharmacology and Experimental Therapeutics
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