• COX-1;
  • COX-2;
  • cytochrome P450;
  • CYP2C9;
  • diclofenac;
  • NSAID;
  • prostaglandin E2;
  • thromboxane B2

Aims The cytochrome P450 enzyme CYP2C9 catalyses the 4′-hydroxylation of the nonsteroidal analgesic drug diclofenac in humans. We studied the influences of the known amino acid variants, CYP2C9*2 (Arg144Cys) and CYP2C9*3 (Ile359Leu), on diclofenac pharmacokinetics after a 50-mg oral dose of diclofenac in healthy volunteers. As a surrogate marker of diclofenac activity, the ex vivo formation of prostaglandin E2 and thromboxane B2, which reflects COX-2 and COX-1 activity, was measured.

Methods Genotyping was performed in 516 healthy volunteers to obtain 20 participants with all allelic combinations of the two CYP2C9 variants Arg144Cys (*2) and Ile359Leu (*3). Diclofenac and 4′-hydroxydiclofenac were quantified in plasma by reversed phase h.p.l.c. after oral intake of 50 mg diclofenac. Concentrations of thromboxane B2 (TxB2) and prostaglandin E2 (PGE2) were measured by immunoassays.

Results There was no evidence of impaired metabolism of oral diclofenac in heterozygous and homozygous carriers of the CYP2C9 alleles *2 and *3 compared with the wild type (mean CL/F (95% CI) 20.5 (11, 30) l h−1 for *1/*1, 29.9 (19, 40) l h−1 for *1/*2, 30.0 (4, 56) l h−1 for *2/*2, 22.6 (12, 33) l h−1 for *1/*3, 23.5 (11, 37) l h−1 for *3/*3 and 37.3 (−15, 89) l h−1 in *2/*3). Furthermore, plasma concentrations of the metabolite 4′-hydroxydiclofenac were not lower in carriers of the CYP2C9 low-activity alleles *2 and *3 compared with carriers of the CYP2C9*1/*1 genotype. Marked diclofenac mediated inhibition of COX-1- and COX-2 activity was detected in all individuals independent of CYP2C9 genotype.

Conclusions Polymorphisms of the CYP2C9 gene had no discernible effect on the pharmacokinetics and pharmacodynamics of diclofenac. The question of whether enzymes other than CYP2C9 play a major role in diclofenac 4′-hydroxylation in vivo or whether 4′-hydroxylation is not a rate-limiting step in diclofenac elimination in vivo, or whether the effect of the CYP2C9 polymorphisms is substrate-dependent, needs further investigation.