Pharmacokinetic interaction between indomethacin and diflunisal

The effect of treatment with diflunisal on the steady-state pharmacokinetics of indomethacin has been studied in 16 healthy volunteers. The steady-state plasma concentration and AUC of indomethacin were significantly increased two- to threefold during treatment with diflunisal and its total clearance and total volume of distribution were significantly decreased. The urinary recovery of total indomethacin (unchanged + glucuronides) was significantly lower during administration of diflunisal, whereas excretion of the indomethacin metabolites desmethylindomethacin and desbenzoylindomethacin and their glucuronides was not significantly altered. The results can be explained by selective inhibition of glucuronidation of unchanged indomethacin by diflunisal. The interaction appears clinically relevant as potentially dangerous side effects of indomethacin are related to its plasma concentration.