Articular pharmacokinetics of protein-bound antirheumatic agents

By what mechanism do nonsteroidal anti-inflammatory drugs (NSAIDs) move from plasma into synovial fluid and back, and how does binding to plasma albumin in vitro relate to articular transport in vivo? To evaluate these issues, concurrent plasma and synovial fluid data of 8 different NSAIDs from 10 single-dose trials were analysed by a simple compartmental model incorporating intra-articular volume, synovial plasma flow rates and protein transport. All pharmacological and physiological data were taken from published studies of chronic knee effusions in patients with rheumatoid arthritis. The analysis shows that these protein-bound NSAIDs readily leave the vasculature and enter synovial fluid during each transit of synovial microvessels. The mean rate of transport, 0.23 min-1, is consistent with passive diffusion at rates far in excess of those attributable to movement of albumin-bound drug or of the small, free-drug fraction found by equilibrium dialysis. These findings are explained by association and dissociation of NSAIDs and albumin that occur far more rapidly than vascular transit. Ongoing dissociation makes bound drug available for transvascular exchange and thereby diminishes the pharmacokinetic significance of binding data obtained in vitro.