S-warfarin limited sampling strategy with a population pharmacokinetic approach to estimate exposure and cytochrome P450 (CYP) 2C9 activity in healthy adults

Eur J Clin Pharmacol. 2021 Sep;77(9):1349-1356. doi: 10.1007/s00228-021-03123-y. Epub 2021 Mar 23.


Purpose: S-warfarin is used to phenotype cytochrome P450 (CYP) 2C9 activity. This study evaluated S-warfarin limited sampling strategy with a population pharmacokinetic (PK) approach to estimate CYP2C9 activity in healthy adults.

Methods: In 6 previously published studies, a single oral dose of warfarin 10 mg was administered alone or with a CYP2C9 inducer to 100 healthy adults. S-warfarin concentrations were obtained from adults during conditions when subjects were not on any prescribed medications. A population PK model was developed using non-linear mixed effects modeling. Limited sampling models (LSMs) using single- or 2-timepoint concentrations were compared with full PK profiles from intense sampling using empiric Bayesian post hoc estimations of S-warfarin AUC derived from the population PK model. Preset criterion for LSM selection and validation were a correlation coefficient (R2) >0.9, relative percent mean prediction error (%MPE) >-5 to <5%, relative percent mean absolute error (%MAE) ≤ 10%, and relative percent root mean squared error (%RMSE) ≤ 15%.

Results: S-warfarin concentrations (n=2540) were well described with a two-compartment model. Mean apparent oral clearance was 0.56 L/hr and volume of distribution was 35.5 L. Clearance decreased 33% with the CYP2C9 *3 allele and increased 42% with lopinavir/ritonavir co-administration. During CYP2C9 constitutive conditions, LSMs at 48 hr and at 72 hr as well as 2-timepoint LSMs were within acceptable limits for R2, %MPE, %MAE, and %RMSE. During CYP2C9 induction, S-warfarin LSMs had unacceptable %MPE, %MAE, and %RMSE.

Conclusions: Phenotyping studies with S-warfarin in healthy subjects can utilize a single- and/or a 2-timepoint LSM with a population PK approach to estimate constitutive CYP2C9 activity.

Keywords: Cytochrome P450 2C9; Drug-drug interaction; Limited sampling; Phenotyping; Warfarin.

MeSH terms

  • Age Factors
  • Area Under Curve
  • Bayes Theorem
  • Cytochrome P-450 CYP2C9 / genetics
  • Cytochrome P-450 CYP2C9 / metabolism*
  • Cytochrome P-450 CYP2C9 Inducers / pharmacology*
  • Dose-Response Relationship, Drug
  • Drug Combinations
  • Female
  • Genotype
  • Healthy Volunteers
  • Humans
  • Lopinavir / pharmacology*
  • Male
  • Metabolic Clearance Rate
  • Models, Biological*
  • Phenotype
  • Ritonavir / pharmacology*
  • Sex Factors
  • Warfarin / administration & dosage
  • Warfarin / pharmacology*


  • Cytochrome P-450 CYP2C9 Inducers
  • Drug Combinations
  • lopinavir-ritonavir drug combination
  • Lopinavir
  • Warfarin
  • Cytochrome P-450 CYP2C9
  • Ritonavir