Characterization of the protein binding of non-steroidal anti-inflammatory drugs in synovial fluid

The clinical pharmacokinetic properties of NSAID's are markedly influenced by the affinity and extent of their protein binding properties in plasma and synovial fluid. Whilst the total level of NSAID is dependent upon concentration and affinity of local protein, it is the unbound / free fraction that is the species which readily diffuses between plasma and synovial fluid. The synovial fluid kinetics of the NSAlD's have received remarkably little attention until recent years, despite the increasing accumulation of evidence casting doubt on a consistent pharmacokinetic correlation between plasma concentrations of NSAID's and respective analgesic / anti - inflammatory activity. This study examined the effect of some physical and environmental factors on the protein binding of piroxicam and ibuprofen in synovial fluid. In this current work, the Amicon MPS-1 ultrafiltration device provided rapid and consistent separation of unbound NSAID in synovial fluids and HSA solutions. Radiolabelled piroxicam provided a convenient and relatively simple means of measuring the free fraction of this drug in synovial fluid using this device, with a coefficient of variation of 9.8%. The 2-arylpropionic acids include some marketed and commonly prescribed NSAID's. They possess a chiral centre, are stereospecific in action and may undergo metabolic inversion from the inactive R-(-) to the active S-(+) enantiomer. The prostaglandin inhibitory effect resides with the S-(+) enantiomer, yet with the exception of S-(+) naproxen, these agents are generally administered as racemic mixtures. The extent of metabolic chiral inversion in man is variable and may be one very important factor in explaining the variability in response to these agents.Despite the fact that the enantiomers of the 2-arylpropionic acid NSAID's differ in pharmacological properties, we are still witnessing articles in the literature that generate data of a non stereospecific nature. In order to characterize the protein binding of the enantiomers of the chiral NSAID's (ibuprofen and ketoprofen) in synovial fluid, a GC/MS assay procedure was developed that was capable of determining ibuprofen enantiomer concentrations of 2ng/mL with a coefficient of variation of 5_ 10.8% and 7.4% for s-ibuprofen and r-ibuprofen respectively. The entire procedure (complete in a matter of minutes), was in marked contrast to the more lengthy methods found in the literature. The procedure was readily adaptable to allow resolution of ketoprofen enantiomers with a similar degree of sensitivity. The free fractions of piroxicam and ibuprofen enantiomers were found to be markedly dependent upon their respective drug and associated albumin concentrations in synovial fluid according to pre-determined physical conditions. Binding of piroxicam was found to be independent of pH at concentrations of drug and albumin typically found in synovial fluid. There were marked differences in the extent of binding of ibuprofen enantiomers to human serum albumin (HSA) solutions, \fatty acid - stripped\" HSA solutions and synovial fluid. The current studies suggest that caution with interpretation of binding constants determined from experimental data (in these and similar studies) is essential as most of the commonly calculated values are based on saturation functions such as those described by Scatchard and Klotz. Consideration should also be given to alternative mathematical functions when interpreting the results of non-saturated binding studies. As synovial fluid is close to the target tissue where a NSAM's effect is exerted it is reasonable to expect its concentration in this fluid to be a better indicator of drug activity in rheumatic disease than in plasma. However before an attempt to correlate synovial fluid NSAID concentration with clinical response can be made consideration of the physical and environmental parameters affecting the protein binding of the whole class of these drugs in synovial fluid may be beneficial."