A chronic dose-ranging study of the pharmacokinetics of phenylbutazone in rheumatoid arthritic patients.

Phenylbutazone in doses of 200, 300 and 400 mg/day was administered chronically to six rheumatoid arthritic patients. At each steady-state the plasma levels of phenylbutazone, oxyphenbutazone and gamma-hydroxyphenylbutazone as well as the extents of binding of phenylbutazone and oxyphenbutazone to plasma proteins were measured. 2 Plasma concentrations of phenylbutazone did not increase proportionally with dose but when corrected for protein binding unbound concentrations of phenylbutazone did show a proportional increase with dose. 3 Plasma concentrations of oxyphenbutazone decreased with an increase in phenylbutazone dose suggesting either that the elimination of oxyphenbutazone is stimulated or its formation inhibited after chronic administration of phenylbutazone. 4 Binding studies with human serum albumin demonstrated the ability of phenylbutazone and oxyphenbutazone to mutually displace one another. Neither saturation of the protein binding sites nor displacement interactions could account for the changes in binding shown by phenylbutazone with increased dose. 5 gamma-hydroxyphenylbutazone concentrations increased proportionally with phenylbutazone dose reaching 68% of the phenylbutazone concentration in one patient. There was a large inter-subject variation in the gamma-hydroxyphenylbutazone concentrations.