Renal clearance of sulphinpyrazone in man

Six healthy young volunteers received a single dose of sulphinpyrazone 200 mg p.o. Plasma concentration and urinary excretion rate curves showed large intersubject variation for sulphinpyrazone and its metabolites. The sulphide metabolite could only be detected in plasma and not before 3-7 h after ingestion. The total recovery in urine of all compounds varied from 30-56% of the dose. In two subjects the mean residence time of sulphinpyrazone was twice as long as in the other subjects (10.4 h compared with 4.6 h), but the area under the plasma concentration-time curve was comparable to that in the others (mean: 3.0 mg X ml-1 X min), indicating that drug absorption was quantitatively similar but delayed. The renal clearance of sulphinpyrazone varied from 14-40 ml X min-1 (mean: 28 ml X min-1). In view of the very high plasma protein binding of sulphinpyrazone, active tubular secretion is the predominant mechanism in its renal clearance. The same holds for the sulphone metabolite, which has a mean renal clearance of 24 ml X min-1, and even more for the p-hydroxysulphinpyrazone metabolite, which has a renal clearance of 118 ml X min-1. No unambiguous evidence was found in favour of concentration-dependent renal clearance of sulphinpyrazone or its metabolites over the concentration range studied. The renal clearance, especially of sulphinpyrazone, appeared to be dependent on urine pH and not on urine flow rate.