Warfarin, phenprocoumon and acenocoumarol are racemic mixtures of S- and R- enantiomers. The S-enantiomers of these coumarins have several times more anticoagulant activity than the R-enantiomers. The S-enantiomer of
warfarin is metabolised primarily by CYP2C9. The
metabolism of R-warfarin is more complex but this enantiomer is primarily metabolised by CYP1A2, CYP3A4, and CYP2C19. There is much more known about the
metabolism of
warfarin compared to other antico- agulants but it is established that S-phenprocoumon and S-acenocoumarol are also substrates for CYP2C9. Whilst the
metabolism of the coumarins, especially warfarin, is well known, the numerous interaction pathways and the variability in patient responses makes the
clinical consequences of alterations in
metabolism difficult to predict.