A new metabolite of
diflunisal has been identified in volunteers and patients after multiple dose administration. The metabolite was isolated from human urine by
silica gel chromatography and was further purified by reversed phase HPLC.
Arylsulfatase from Helix pomatia and from Aerobacter aerogenes completely hydrolyzed the isolated metabolite to
diflunisal, although hydrolysis by bacterial
arylsulfatase was extremely slow. Electron impact mass spectra for
diflunisal and its
sulfate conjugate were virtually identical. Negative ion fast atom bombardment mass spectra clearly showed the quasimolecular ion [M-H]- at m/z 329 (base peak) as well as a large fragment ion (90% relative intensity) at m/z 249 corresponding to the loss of the
sulfate moiety. Urinary excretion patterns in volunteers and
rheumatoid arthritis patients revealed that
sulfate conjugation of
diflunisal is a minor metabolic pathway after single 500-mg dose administration (less than 10% of the dose), whereas it becomes a major pathway (21.3-44.3% of the dose) following multiple doses (500 mg b.i.d.). In one volunteer, who ingested 500 mg
diflunisal b.i.d. for 5 weeks, it was shown that the percentage of the dose excreted as
diflunisal sulfate gradually increased during the first week to approximately 30% and stayed virtually unchanged for the remaining 4 weeks of
diflunisal intake. These preliminary observations are not compatible with the idea that
sulfate conjugation is capacity-limited at lower substrate concentrations than
glucuronide conjugation, nor do they suggest that sulfation of
diflunisal is rate-limited by depletion of inorganic
sulfate body stores.