Pharmacokinetics of warfarin enantiomers: A search for intrasubject correlations

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Abstract
The total body clearance of single doses of R(+)-, S(−)-, and racemic warfarin and the plasma concentration-anticoagulant effect curves for these drugs were determined in 10 healthy men, 21 to 51 yr old. This information was used to calculate the steady-state dose required to decrease prothrombin complex activity to about 30% of normal. This dose (mean ± SD in mg/70 kg/day) is 17.8 ± 9.3 for R(+)-warfarin, 6.63 ± 2.19 for S(−)-warfarin, and 9.4 ± 1.7 for racemic warfarin. There is a correlation between the maintenance doses of racemic and S(−)-warfarin (r = 0.89, p < 0.01) but not between those of racemic and R(+)-warfarin (r = 0.30, NS). According to these calculations, the maintenance dose of racemic warfarin required by an individual patient may be a useful predictor of the maintenance dose of S(−)-warfarin which will produce a comparable degree of anticoagulation. No such predictability is evident with respect to R(+)-warfarin. The slopes of the log plasma concentration-anticoagulant effect curves for R(+)-and S(−)-warfarin differ significantly. Therefore, the potency ratio of these enantiomers is dose-and concentration-dependent.