Disposition and oxidative metabolism of phenylbutazone in man

Summary

The absorption and elimination of orally administered¹⁴C-phenylbutazone and the role of oxidation in its metabolism have been studied. The main routes of excretion of¹⁴C-phenylbutazone and its metabolites were investigated in 3 patients with rheumatoid arthritis, and in 1 patient with a T-tube in the common bile duct. Up to 9 days after an oral dose of¹⁴C-phenylbutazone 600 mg (30 µCi) 63% of the radioactivity was found in the urine and 14% had appeared in the faeces. The cumulative excretion of radioactivity in bile amounted to 9.5% of the dose in 4 days. Only 1% of the radioactivity in the urine and bile was due to unchanged phenylbutazone. The role of oxidative metabolism of phenylbutazone in healthy human subjects was studied by gas chromatography. In 3 subjects given a single dose of phenylbutazone 600 mg, only 8.3% of the dose was excreted in urine as oxidized metabolites after 5 days. However, in 5 patients who had taken phenylbutazone for more than 5 weeks, these metabolites accounted for 23.4% of the dose. These results suggest that oxidative metabolism becomes more important after continued administration of the drug. After a single dose of phenylbutazone, the side-chain oxidized metabolite (II) was the major free derivative excreted in urine, but the ring oxidized metabolite, oxyphenbutazone (I), was much more important than the former in plasma. However, after prolonged treatment there was little difference between the concentration of the two metabolites in plasma. This finding suggests that side-chain oxidation is increased relative to ring oxidation on prolonged treatment with phenylbutazone. A third derivative containing hydroxyl groups both in the phenyl ring and in the side-chain (metabolite III) was found in urine in experiments with phenylbutazone, but in only one out of 3 volunteers given repeated doses of oxyphenbutazone.