Antipyrine metabolite formation and excretion in patients with chronic renal failure

In the present study the influence of chronic renal insufficiency on antipyrine clearance, metabolite formation and excretion was investigated in 8 patients. After oral administration of antipyrine, the parent compound, its metabolites and their conjugates were assayed in plasma and urine. Besides the parent drug, 3-hydroxymethylantipyrine (HMA) was present in plasma in the free and conjugated forms, whereas 4-hydroxyantipyrine (OHA) and norantipyrine (NORA) were found only in the conjugated form. The same was true for urine. The plasma concentrations of these metabolites are too low to be measured in subjects with normal renal function. Plasma antipyrine clearance in the patients was in the same range as in healthy subjects. Investigation of metabolite kinetics, however, revealed that the rate of formation of NORA was preferentially decreased, whereas that of OHA and HMA were unaltered. Renal clearance of the metabolites of antipyrine was severely impaired in patients with renal insufficiency, and the resulting accumulation made it possible for the first-time to measure the antipyrine metabolites in plasma. Mean residence times of metabolites were longer than that of the parent compound. Renal clearances of the conjugates were correlated with the creatinine clearance, but were somewhat higher. Renal clearance of free HMA was lower and was also correlated with creatinine clearance. The mean clearance for glucuronidation of HMA was 93.1 ml/min. The results suggest that in healthy subjects Phase I metabolism is the rate-limiting step in the elimination of antipyrine, which is essential for its application as a model drug in metabolism studies.