Is diclofenac a valuable CYP2C9 probe in humans?

Besides the low therapeutic index drug tolbutamide, there is no validated in vivo probe to assess the genetically determined CYP2C9 activity 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'-hydroxydiclofenac/diclofenac metabolic ratio (MR) would correlate to the apparent partial metabolic clearance of diclofenac into 4'-hydroxydiclofenac (Clmet). Eighteen healthy volunteers received a single oral dose of 50 mg diclofenac in its enteric-coated form. Blood and urinary pharmacokinetics of diclofenac were studied over 48 h. Identification of the CYP2C9 alleles (CYP2C9*1, CYP2C9*2, and CYP2C9*3) was performed with genomic DNA sequencing. We observed a dramatic inter-individual variability in the delay of diclofenac intestinal absorption since its first detectable blood concentration 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 the drug after 12 h. No correlation could be observed between Clmet of diclofenac 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 difference did not reach statistical significance due to an insufficient number of subjects studied. 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, diclofenac, in another galenic form, might be a potential probe to quantify CYP2C9 activity in humans.