Antirheumatic drug concentrations in human synovial fluid and synovial tissue. Observations on extravascular pharmacokinetics.

Antirheumatic drug concentrations have been measured in human synovial fluid and synovial tissue, and provide insights on: (1) extravascular pharmacokinetics; (2) articular pathophysiology; and (3) the factors which modify drug levels in inflamed tissues. Concentrations of free drug in synovial fluid and plasma are the same in all conditions except rheumatoid and infectious arthritis, where the most severely afflicted joints may contain lower synovial fluid drug concentrations. This finding may be relevant to the chronicity and intractability of chronic arthritis. After single-dose therapy and a characteristic 'equilibration time', higher concentrations are found in synovial fluid than in plasma - a phenomenon which results from relative drug sequestration across the trans-synovial diffusion barrier away from the organs of elimination. Studies of oral, parenteral, topical and intra-articular antirheumatic drug therapy are reviewed, and recommendations are made for the conduct of future studies.