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Phenytoin Metabolic Ratio, a Marker of CYP2C9 Activity, is Superior to the CYP2C9 Genotype as a Predictor of (S)-Warfarin Clearance

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Abstract

Background

CYP2C9 is a member of the cytochrome P450 (CYP) superfamily responsible for the metabolism of 16% of drugs that undergo oxidative metabolism. The activity of CYP2C9 exhibits marked inter-individual variability, which translates into prominent differences in the pharmacokinetics of CYP2C9 substrates, some of which are characterized by a narrow therapeutic window. Genetic polymorphisms in the gene encoding for CYP2C9 account for a fraction of the variability in CYP2C9 activity. The phenytoin metabolic ratio (PMR) is a marker of CYP2C9 activity in vivo, which correlates with CYP2C9 genetic polymorphisms.

Objective

The purpose of the current study was to evaluate the ability of the PMR to predict the oral clearance of (S)-warfarin (SWOCL) and its formation clearance towards its CYP2C9-mediated metabolites (SWCLf) [i.e., 6- and 7-hydroxy-(S)-warfarin].

Methods

The study was conducted in 150 healthy non-smoker subjects (segment 1) and 60 patients treated with warfarin (segment 2). In the first segment, the participants received on two separate occasions a single 300-mg dose of phenytoin and at least 7 days later a single dose of warfarin (5 or 10 mg). The same PMR procedure was performed in the second segment, except that it was performed either before warfarin initiation or after the patients had reached stable anticoagulation. The PMR was derived from the ratio of 5-(4-hydroxyphenyl)-5-phenyl-hydantoin content in a 24-hour urine collection to plasma phenytoin concentration 12- (PMR24/12) or 24- (PMR24/24) post-dosing. In segment 1, SWOCL was calculated from the ratio of (S)-warfarin dose to the warfarin area under the plasma concentration–time curve extrapolated to infinity and the SWCLf from the ratio of urine content of 6- and 7-hydroxy-(S)-warfarin to (S)-warfarin area under the (S)-warfarin plasma concentration–time curve until the last measured timepoint. In segment 2, estimated SWOCL was derived from the ratio of (S)-warfarin dose to the mid-interval plasma concentration of (S)-warfarin.

Results

The PMR, SWOCL, and SWCLf varied significantly between carriers of different CYP2C9 genotypes in both healthy subjects (p < 0.001) and patients (p < 0.005). However, PMR and SWOCL values exhibited substantial intra-genotypic variability. PMR24/12 and PMR24/24 were significantly correlated with SWOCL both in healthy subjects (r = 0.62 and r = 0.67, respectively, p < 0.001) and in patients (r = 0.57 and r = 0.61, respectively, p < 0.001). In a multiple regression model that included all variables that correlated with SWOCL, PMR was the strongest predictor, explaining 44% and 38% of the variability in SWOCL among healthy subjects and patients, respectively, and accounting for 95.7% (44%/46%) and 90.5% (38%/42%) of the total explained variability in SWOCL among healthy subjects and patients, respectively.

Conclusions

The PMR is the strongest predictor of SWOCL, and as such, it exhibits a significant advantage over the CYP2C9 genotype. The inclusion of PMR in future dosing algorithms of CYP2C9 substrates characterized by a narrow therapeutic window should be encouraged and further investigated.

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Acknowledgements

Dr. Chanan Shaul is a post-doctoral fellow at the Hebrew University of Jerusalem under the supervision of Prof. Yoseph Caraco and Prof. Meir Bialer. The technical support of Mrs. Ester Zadka and Mrs. Ilanit Linzer is greatly appreciated.

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Authors and Affiliations

  1. Clinical Pharmacology Unit, Division of Medicine, Hadassah-Hebrew University Medical Center, POB 12000, 91120, Jerusalem, Israel

    Chanan Shaul, Simcha Blotnick, Liat Adar, Mordechai Muszkat & Yoseph Caraco

  2. Institute of Drug Research, School of Pharmacy, Faculty of Medicine, Hebrew University, Jerusalem, Israel

    Chanan Shaul, Liat Adar & Meir Bialer

  3. David R. Bloom Center for Pharmacy, The Hebrew University of Jerusalem, Jerusalem, Israel

    Meir Bialer

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  1. Chanan Shaul
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  2. Simcha Blotnick
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  5. Meir Bialer
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  6. Yoseph Caraco
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Correspondence to Yoseph Caraco.

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Funding

This work was not supported by any funding source.

Conflicts of interest/competing interests

HS, SB, LA, MM, and YC do not have any conflicts of interest to disclose. MB received consultancy fees from Boehringer Ingelheim, Medison, Pharma 2B, Rekah-Vitamned, and Xenon Pharmaceuticals.

Ethics approval

The study protocols were approved by the Hadassah Institutional Review Board.

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Following a detailed explanation, all participants gave their written consent to participate in the study.

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Author contributions

CS, YC, and MB wrote the manuscript, LA, MB, and YC designed the research, CS and LA performed the research, and CS, SB, and LA analyzed the data.

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Shaul, C., Blotnick, S., Adar, L. et al. Phenytoin Metabolic Ratio, a Marker of CYP2C9 Activity, is Superior to the CYP2C9 Genotype as a Predictor of (S)-Warfarin Clearance. Clin Pharmacokinet 61, 1187–1198 (2022). https://doi.org/10.1007/s40262-022-01141-2

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