Some observations and speculations on the factors influencing the concentration of phenylbutazone in synovial fluid.

Simultaneous plasma and synovial fluid samples have been obtained from patients, having reached a steady state on phenylbutazone, as a means of investigating factors influencing drug penetration into synovial fluid. Synovial fluid levels are lower than, but related to, plasma levels and are higher in rheumatoid disease (55-100% plasma levels) than in osteoarthritis (less than 50%). Patients with a more active form of the disease have higher synovial fluid drug concentrations. Results are discussed in the light of present knowledge and areas for further study are proposed.