Mycophenolic acid β-D-glucuronide (MPAG) is the primary metabolite of the antibiotic mycophenolic acid, a potent inhibitor of inosine monophosphate dehydrogenase. Inhibition of this enzyme blocks de novo GTP generation as well as RNA and DNA synthesis in lymphocytes, thus preventing lymphocyte proliferation and activation. Mycophenolic acid, typically used in clinical settings as a chemotherapeutic or immunosuppressant agent, is rapidly conjugated on adsorption and circulates mostly in the form of its glucuronide conjugate. At 6 mg/mouse/day, MPAG has been reported to show significant antitumor activity against Ehrlich solid tumors. This glucuronide is typically used as an internal standard for therapeutic drug monitoring analyses of serum or whole blood by HPLC or LC-MS to confirm that patients remain within the drug’s therapeutic range.