Is diclofenac a valuable CYP2C9 probe in humans?

Introduction: Besides the low therapeutic index drug tolbutamide, there is no validated in vivo probe to assess the genetically determined CYP2C9 acti vity in humans. The in vitro CYP2C9-specific substrate diclofenac might be a valuable, well-tolerated probe candidate. In order to validate diclofenac as an in vivo CYP2C9 probe, we planned to show that urinary 4'-hydroxydicl ofenac/diclofenac metabolic ratio (MR) would correlate to the apparent part ial metabolic clearance of diclofenac into 4'-hydroxydiclofenac (Clmet). Patients and methods: Eighteen healthy volunteers received a single oral do se of 50 mg diclofenac in its enteric-coated form. Blood and urinary pharma cokinetics of diclofenac were studied over 48 h. Identification of the CYP2 C9 alleles (CYP2C9*1, CYP2C9*2, and CYP2C9*3) was performed with genomic DN A sequencing. Results; We observed a dramatic inter-individual variability in the delay o f diclofenac intestinal absorption since its first detectable blood concent ration ranged from 0.5 h to more than 12 h after drug intake. The Clmet of diclofenac could not be determined in two subjects who started to absorb th e drug after 12 h. No correlation could be observed between Clmet of diclof enac and the different MRs calculated at 0-4 h, 0-8 h, 0-12 h, 0-24 h and 0 -48 h urinary collections. The Clmet of diclofenac in heterozygous subjects tended to be lower than among wild-type homozygous subjects, but this diff erence did not reach statistical significance due to an insufficient number of subjects studied. Conclusion: Diclofenac, in its enteric-coated form, is not a useful in vivo CYP2C9 probe probably because of its highly variable intestinal absorption rate. However, since we found a lower metabolic clearance of diclofenac in heterozygous CYP2C9 subjects, as observed with other CYP2C9 substrates, di clofenac, in another galenic form, might be a potential probe to quantify C YP2C9 activity in humans.