Binding of piroxicam to synovial fluid and plasma proteins in patients with rheumatoid arthritis.

The protein binding of piroxicam in synovial fluid and plasma from patients with rheumatoid arthritis was studied in vitro by equilibrium dialysis. The binding parameters were calculated from the experimental data with the Scatchard model, assuming binding to two classes of sites. Each plasma sample was diluted to an albumin concentration equal to that of synovial fluid from the same patient. The association constants for primary and secondary binding sites in the concentration range of piroxicam 4.5-90 X 10(-5) mol/l were similar in synovial fluid and in plasma. For synovial fluid K1 = 2.38 X 10(5) l/mol and K2 = 2.29 X 10(3) l/mol; for plasma K1 = 1.93 X 10(5) l/mol and K2 = 2.08 X 10(3) l/mol. The number of binding sites was also the same in the two fluids. Although the concentration of piroxicam in synovial fluid was about half that in plasma, the binding of piroxicam to protein in synovial fluid was the same as in plasma.