Plasma and synovial fluid concentrations of nimesulide and its main metabolite after a single or repeated oral administration in patients with knee osteoarthritis.

The aim of this study was to evaluate plasma and synovial fluid concentrations of the non-steroidal anti-inflammatory drug nimesulide and its major metabolite (hydroxynimesulide, M1), after a single 100 mg dose of nimesulide and a repeated (14 day) administration, 100 mg twice a day, in patients with osteoarthritis of the knee and joint effusion. Nimesulide was rapidly absorbed in plasma and distributed in synovial fluid. On day 1, effective concentrations were present 30 min after the first dose and on day 14, the synovial fluid concentration of nimesulide was significantly higher than that measured on day 1; no accumulation was observed in plasma. After 14 days of treatment, both the plasma and synovial fluid concentrations of M1 were significantly higher than those measured on day 1. These data may help to explain the rapid onset of the analgesic effect of nimesulide demonstrated in several clinical conditions, including painful osteoarthritis.