Plasma and synovial fluid concentrations of diclofenac sodium and its major hydroxylated metabolites during long-term treatment of rheumatoid arthritis

Abstract

Plasma and synovial fluid concentrations of diclofenac sodium and its principal hydroxylated metabolites have been measured in sixteen rheumatoid patients on chronic therapy to investigate possible reasons for the drug's extended duration of action despite its apparent short elimination half-life in plasma. Diclofenac was detected in synovial fluid 2 h after dosing but at a lower level than in plasma. Thereafter synovial fluid concentrations remained relatively constant through to 11 h post-dosing whereas plasma levels in the same period declined rapidly from an initially high peak to near the sensitivity limit of the assay. Hydroxylated metabolites (free + conjugated) were rapidly formed with measurable concentrations of the 4' and 5 mono and dihydroxy derivatives being detected in plasma 2 h after dosing; levels of the 3' hydroxy metabolite were negligible at this time. Initially plasma levels of all metabolites were higher than those in synovial fluid but after 4 h synovial fluid levels were equal to or slightly higher than those in plasma. The significance of these findings is discussed in relation to the drug's overall clinical effect.