Drug displacement from protein binding: isolation of a redistributional drug interaction in vivo

The pharmacokinetic interaction between phenylbutazone and sulphadoxine has been studied in live sheep.2. Intravenous injection of phenylbutazone causes a rapid fall in the concentration of total sulphadoxine together with a simultaneous rise in the concentration of free sulphadoxine in plasma. Both processes are complete within 3 min of the injection of phenylbutazone.3. These reciprocal changes in the concentrations of total and free sulphadoxine in plasma do not depend on changes in the absorption, hepatic metabolism or renal excretion of sulphadoxine. By exclusion therefore, it is concluded that the effect of phenylbutazone in this interaction is solely to cause redistribution of the sulphadoxine.