Effect of mycophenolate acyl-glucuronide on human recombinant type 2 inosine monophosphate dehydrogenase.

BACKGROUND: The immunosuppressive effect of mycophenolic acid (MPA) is essentially attributed to IMPDH II inhibition, which leads to a reduction of lymphocyte proliferation. We investigated the action of the MPA metabolites MPA-phenyl-glucuronide (MPAG) and MPA-acyl-glucuronide (AcMPAG) on recombinant human IMPDH II (rhIMPDH II), as well as their passage into lymphocytes in vitro.
METHODS: We measured rhIMPDH II activity spectrophotometrically through the initial velocity of NADH formation, leading to the computation of the kinetic parameters K(m), IC(50), and K(i) (Michaelis constant, half-maximal inhibition concentration, and inhibition constant). We measured intracellular and extracellular concentrations of MPA, MPAG, and AcMPAG after incubation of Jurkat lymphoma cells with each compound separately, using liquid chromatography-tandem mass spectrometry.
RESULTS: MPA and AcMPAG showed an inhibition of rhIMPDH II (IC(50) 25.6 microg/L and 301.7 microg/L, respectively; the K(i) of MPA for NAD and IMP was 50.8 and 57.7 nmol/L, respectively; and that of AcMPAG for NAD and IMP was 382.0 and 511.0 nmol/L. MPAG had no significant effect on the enzyme. AcMPAG apparently acts by the same uncompetitive inhibition mechanism as MPA, with a 12-fold higher IC(50) and an 8-10 times higher K(i). When coincubated with MPA, AcMPAG activity was negligible at pharmacological concentrations. Furthermore, after 6-h incubation at their respective maximum concentration (C(max)), MPA was 10 times more concentrated in Jurkat cells than AcMPAG.
CONCLUSIONS: AcMPAG is a weaker inhibitor of rhIMPDH II than MPA and is less concentrated in lymphocytes in vitro, suggesting that it would not be pharmacologically active in vivo and might not need to be monitored in MPA-treated patients.