Identification of Etoposide Glucuronide as a Major Metabolite of Etoposide in the Rat and Rabbit¹

Isolated livers from male Sprague-Dawley rats were perfused at 20 ml/min for 3 h at 37°C with 100 ml of an oxygenated, recirculating solution of 20% rat blood in Krebs bicarbonate buffer containing 20 µg/ml [³H]etoposide. Ninety % of administered radioactivity was eliminated in bile over a 3-h collection period. The clearance of etoposide was 3.56 ml/min indicating that, in the rat, it is not highly extracted. Its clearance is, therefore, independent of hepatic blood flow. Etoposide was both excreted into the bile and metabolized by the liver. Perfusate and bile samples analyzed by reverse-phase high-performance liquid chromatography techniques were found to contain three peaks of radioactivity. Positive and negative ion fast atom bombardment mass spectrometry identified the first two peaks as etoposide glucuronides and the third peak as parent drug. Following the i.v. administration of etoposide to rabbits, etoposide glucuronide was also identified in rabbit urine. The recovery of etoposide both from rabbit urine and rat bile was increased by preincubation with glucuronidase. However, the glucuronides were relatively resistant to the action of glucuronidase and showed varying sensitivity to the type of glucuronidase and the reaction conditions used. These studies document the presence of etoposide glucuronide as an etoposide metabolite in two mammalian species and suggest that previous clinical studies using β-glucuronidase to quantitate glucuronide formation may have underestimated this metabolite due to its relative resistance to some glucuronidase preparations.