Objective: To determine the extent of diclofenac clearance due to hydroxylation to 4’-hydroxydiclofenac in 20 adults genotyped as CYP2C9 amino acid variants using individualized, coupled, physiology-based pharmacokinetic (PBPK) models.

Methods: In vivo experimental plasma concentration time curves for diclofenac and its CYP2C9-mediated metabolite, 4-OH diclofenac, were taken from Kirchheiner et al [1].   Plasma concentration time curves for each individual were simulated using the physiology-based pharmacokinetic software, PK-Sim^® (Bayer Technology Services GmbH, Leverkusen, Germany) based on the individuals age, height and weight and the physico-chemistry of the parent and metabolite drug. Diclofenac and 4-OH diclofenac models were linked such that the production of the 4-OH diclofenac from the diclofenac model was the input function to the liver intracellular space of the 4-OH diclofenac model.  Using the individual plasma concentration time curves for diclofenac and 4-OH diclofenac, three intrinsic clearances were optimized for each individual; CL1, clearance of the diclofenac to 4-OH diclofenac via CYP2C9, CL2, the remainder of diclofenac clearance and CL3, the total clearance of 4-OH diclofenac.

The intrinsic clearance of diclofenac mediated by CYP2C9 was determined in each individual and compared to the individual CYP2C9 genotype.

Results: Clearance of diclofenac due to CYP2C9 metabolism was low with, on average, less than 25 % being due to the production of 4-OH diclofenac. In comparison to the wildtype, CYP2C9*1, individuals homozygous for the less active variant CYP2C9*3 showed lower intrinsic clearance. There was no clear trend for individuals carrying either the CYP2C9*2 allele or for individuals heterozygous for CYP2C9*3.

Conclusion: Due to the low clearance of diclofenac through the CYP2C9 pathway, total clearance is not affected by genotype; a finding in accordance with clinical studies but not with in vitro data showing that CYP2C9 serves as a major elimination pathway.  The use of individualized coupled PBPK models allows for a separation of clearance pathways for an assessment of genotype effects.

References:
[1] Kirchheiner J, Ingolf M, Steinbach N et al. 2003. Pharmacokinetics of diclofenac and inhibition of cyclooxygenases 1 and 2: no relationship to the CYP2C9 genetic polymorphism in humans.  Br J Clin Pharmacol 55: 51-61.