R and S-Warfarins

The enantioselective reduction of unsaturated alcohol derivatives has been applied to the synthesis of several biologically active compounds (Scheme 24.12). Warfarin (123, R=H) is an important anticoagulant that is normally prescribed as the racemate, despite the enantiomers having dissimilar pharmacological profiles. One of the earliest reported uses of DuPhos was in the development of a chiral switch for this bioactive molecule, facilitating the preparation of (R)- and (S)-warfarin [184]. Although attempted reduction of the parent hydroxycoumarin 122 (R=H) led to formation of an unreactive cyclic hemiketal, hydrogenation of the sodium salt proceeded smoothly with Rh-Et-DuPhos in 86-89% ee. 

The Ki for HSA binding to racemic warfarin has been reported for 3-6 pM by various techniques, including frontal analysis and equilibrium dialysis, and is temperature- and pH-dependent. See Loun, B., Hage, D.S. Chiral separation mechanisms in protein-based HPLC columns. 1. Thermodynamic studies of (R)- and (S)-warfarin binding to immobilized human serum albumin. Anal. Chem. 1994, 66, 3814-3822. 

R)- and (S)-warfarin are in their bound forms. This takes place within the protein human serum albumin. (Adapted from Clarke et ah, 2001)... 

Table 5.24 Influence of Cytochrome P-450 Induction on the In Vitro Metabolism of R- and S-Warfarin...

M. Aycard, S. Letellier, B. Maupas, and F. Guyon, Determination of (R) and (S) warfarin in plasma by HPLC using precolumn derivation, J. Liquid Chromatogr., 75 2175 (1992). 

To assess the potential for an interaction between raloxifene and warfarin, 15 healthy postmenopausal women each received single doses of warfarin 20 mg before and during 2 weeks of dosing with raloxifene 120 mg/day (20). Raloxifene reduced the oral clearance of R- and S-warfarin respectively by 7.1 and 14% and the oral volume of distribution by 7.4 and 9.8%. Raloxifene reduced the maximum prothrombin time by 10% and the area under the prothrombin versus time curve from 0-120 hours by an average of 8%. The authors concluded that raloxifene may produce a small increase in systemic warfarin exposure but a reduced pharmacodynamic effect. Since the effects are slight this interaction is unlikely to have clinical consequences. 

TABLE 5.22 Influence of cytochrome P-450 induction on the in vitro metabolism of R- and S-warfarin... 

Warfarin enantiomers are extensively metabolized by liver, possess a low hepatic extraction ratio, and are extensively bound (> 99%) to plasma proteins (Table 3). Therefore any change in the protein binding of warfarin enantiomers may alter the clearance and plasma concentrations of R- and S-warfarin [54]. Yacobi and Levy [54] studied the plasma protein binding of racemic and individual enantiomers of warfarin in human blood. The free fraction of R-warfarin was significantly (32%) larger than that of S-warfarin (Table 3). The authors concluded that the difference in the potency of warfarin enantiomers could not be solely explained by the observed differences in the protein binding of the individual enantiomers but rather by the intrinsic ability of R- and S-warfarin for interactions with extravascular receptors. 

Interestingly, microsomal 7-hydroxylation of racemic warfarin appears to be lower than that of S-warfarin, indicating the possibility of a metabolic interaction between warfarin enantiomers [61]. These in vitro studies showed that R-warfarin affected the catalytic activity of CYP2C9 by a noncompetitive mechanism [61]. However, in an earlier in vivo study a lack of enantiomeric interaction between R- and S-warfarin was suggested after single 1.5mg/kg doses of the individual enantiomers and racemic warfarin [62]. The enantiospecific results obtained were successfully used to predict the pharmacokinetics and pharmacodynamics of racemic warfarin. The apparently contradictory findings between the two studies may be due to a number of experimental factors. One study used a supraclinical dose of warfarin in vivo, whereas the other involved human microsomes in vitro, the relevant hepatic concentrations of which are difficult to determine. 

The 4-hydroxycomnarins undergo biotransformation similar to warfarin, forming hydroxylated metabolites at the 6 and 7 positions. These hydroxylated metabolites accoimt for 63 to 99% of the metabolic clearance of 4-hydroxycoumarins, including acenocoumarol. The metabolic clearances of R- and S-acenocoumarol are 6 and 66 times higher than those of R- and S-warfarin, respectively [68]. The metabolism of acenocoumarol is also stereoselective in favor of the S enantiomer. Sulphaphenazole competitively inhibits the 7-hydroxylation of R- and S-acenocoumarol and the 6-hydroxylation of S-acenocoumarol. Omeprazole acts as a partial inhibitor of 6- and 7-hydroxylation of acenocoumarol enantiomers [68]. 

Yamazaki, H. Shimada, T. Human liver cytochrome P450 enzymes involved in the 7-hydroxylation of R- and S-warfarin enantiomers. Biochem. Pharmacol. 1997, 54, 1195-1203. 

In an open-label, erossover study, 12 healthy men were given donepezil 10 mg daily for 19 days with a single 25-mg dose of warfarin on day 14. The pharmaeokineties of R- and S-warfarin and the prothrombin times were unehanged by the presenee of the donepezil, and vital signs, ECG and laboratory tests were unaltered. ... 

In a randomised, crossover study in 12 healthy subjects, one tablet of St John s wort three times daily for 3 weeks modestly decreased the AUC of both R- and S -warfarin by about 25% after a single 25-mg dose of warfarin taken on day 14. In this study, the brand of St John s wort used was Biog-lan tablets, each tablet containing an extract equivalent to 1 g of Hypericum perforatum flowering herb top containing 825 micrograms of hypericin and 12.5 mg of hyperforin. ... 

Suttle AB, Vargo DL, WilkinsonLA, Birmingham BK, Lasseter K. Effect of zafirlukast on the pharmacokinetics of R- and S-warfarin in heStlty men. Clin Pharmacol The r 991) 61, 186. 

A significant increase in the plasma levels of (R)- and (S)-warfarin was observed in rats stabilized on warfarin (2 mg/kg daily) and then intraperitoneally administered 5 g/ kg of a Chinese salvia extract twice daily for 3 days. A corresponding significant increase in prothrombin time was observed (Chan et al. 1995). 

Loun B, Hage DS. Chiral separation mechanisms in protein-based HPLC columns, n. Kinetic studies of R- and S-warfarin binding to immobilized human serum albumin. Anal Chem 1996 68 1218-25. 

Robison A, Li HY. The first practical asymmetric synthesis of R and S-warfarin. Tetrahedron Lett. 1996 37(46) 8321-8324. 

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