Zonisamide (1,2-benzisoxazole-3-methanesulphonamide), a new anticonvulsant, is mainly metabolized to 2-sulphamoylacetylphenol by reduction of the benzisoxazole ring. Recent studies have shown that mammalian liver enzymes are responsible for the reduction of zonisamide. Because intestinal bacteria can also mediate the reduction of xenobiotics, this study was designed to evaluate the role of intestinal bacteria in in-vivo reductive metabolism of zonisamide. Treatment of rats with antibiotics significantly reduced the urinary and faecal excretion of 2-sulphamoylacetylphenol after oral administration of zonisamide. Re-contamination of the antibiotic-treated rats with microflora restored the excretion of the metabolite. The caecal contents of the control rats had significant zonisamide reductase activity, whereas little or no zonisamide reductase activity was observed with the caecal contents of the antibiotic-treated rats. Eight pure strains of intestinal bacteria were tested for zonisamide reductase activity and the highest was observed in Clostridium sporogenes. We concluded that intestinal bacteria play a major role in the reductive metabolism of zonisamide to 2-sulphamoylacetylphenol in-vivo.

This study presents data showing individual differences in aldehyde oxidase activity in human and monkey liver cytosols. When assayed with benzaldehyde as a substrate, a significant inter‐subject variation in the activity was found in the human liver preparations. When assayed with N1‐methylnicotinamide as a substrate, the inter‐subject variation of the activity was also observed, but to a lesser extent compared with that of the activity with benzaldehyde. Similarly, variations in aldehyde oxidase activity were found in the monkey liver preparations when assayed with benzaldehyde or N1‐methylnicotinamide. The present study suggested that at least two isozymes of aldehyde oxidase exist in the human liver preparations.

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