title: The apparent inhibition of inosine monophosphate dehydrogenase by mycophenolic acid glucuronide is attributable to the presence of trace quantities of mycophenolic acid.
abstract: BACKGROUND
  Mycophenolic acid glucuronide, the primary metabolite of the immunosuppressive agent mycophenolic acid, affords weak inhibition of proliferating and resting lymphocytes and recombinant human inosine monophosphate dehydrogenase in comparison to the active drug. We evaluated the hypothesis that mycophenolic acid is a trace contaminant of the glucuronide metabolite preparation and that this accounts for the observed effects of mycophenolic acid glucuronide on human inosine monophosphate dehydrogenase catalytic activity both in lymphocytes and the pure enzyme.
  
  
  METHODS
  We used negative ion electrospray HPLC-mass spectrometry (HPLC-MS) and HPLC-tandem MS (HPLC-MS-MS) to identify mycophenolic acid as a contaminant of mycophenolic acid glucuronide. Quantification of the mycophenolic acid contaminant was achieved using a negative ion electrospray HPLC-MS method in the selected-ion monitoring mode.
  
  
  RESULTS
  Trace amounts of mycophenolic acid were detected and definitively identified in the mycophenolic acid glucuronide preparation by the HPLC-MS-MS analysis. In addition to having identical HPLC retention times, pure mycophenolic acid and the contaminant produced the following major fragments upon HPLC-MS-MS analysis: deprotonated molecular ion, m/z 319; and fragment ions, m/z 275, 243, 205, and 191 (the most abundant fragment ion). Using the negative ion electrospray HPLC-MS procedure in the selected-ion monitoring mode, the quantity of the contaminant mycophenolic acid was determined to be 0.312% +/- 0.0184% on a molar basis.
  
  
  CONCLUSIONS
  These data provide strong support for the proposal that the apparent inhibition of the target enzyme inosine monophosphate dehydrogenase by mycophenolic acid glucuronide is attributable to the presence of trace amounts of contaminant mycophenolic acid.
authors.affiliation: Departments of Pathology and Laboratory Medicine, University of Pennsylvania Medical Center, Philadelphia, PA 19104, USA.
authors.forename: M
authors.initials: M
authors.lastname: Korecka
authors.forename: D
authors.initials: D
authors.lastname: Nikolic
authors.forename: R B
authors.initials: RB
authors.lastname: van Breemen
authors.forename: L M
authors.initials: LM
authors.lastname: Shaw
mesh_terms: D002851:Chromatography, High Pressure Liquid; D004340:Drug Contamination; D005965:Glucuronates / Q000032:analysis* / Q000737:chemistry; D020719:Glucuronides; D006801:Humans; D007168:IMP Dehydrogenase / Q000037:antagonists & inhibitors*; D007166:Immunosuppressive Agents / Q000032:analysis* / Q000737:chemistry; D013058:Mass Spectrometry; D009173:Mycophenolic Acid / Q000031:analogs & derivatives* / Q000032:analysis / Q000737:chemistry
chemical_list: D005965:Glucuronates; D020719:Glucuronides; D007166:Immunosuppressive Agents; C113145:mycophenolic acid glucuronide; D007168:IMP Dehydrogenase; D009173:Mycophenolic Acid