Notes: 4-hydroxy Tolbutamide is a metabolite of the potassium channel antagonist tolbutamide (Item No. 19888).{14143} It is formed from tolbutamide by the cytochrome P450 (CYP) isoform CYP2C9 in human liver microsomes and COS-7 cell lysates expressing the human enzyme. 4-hydroxy Tolbutamide has been used as a marker of CYP2C9 activity in rat hepatocytes.{62997}