Development and application of a high-performance liquid chromatography-based assay for determination of the activity of inosine 5'-monophosphate dehydrogenase in whole blood and isolated mononuclear cells.

With the objective of pharmacodynamic monitoring of the immunosuppressive efficacy of mycophenolate mofetil (MMF) (CellCept, Hoffman-LaRoche, Grenzach-Wyhlen, Germany), a method for determination of the inosine monophosphate dehydrogenase (IMPDH) activity in whole blood cell (WBC) lysates and mononuclear cells (MNCs) was developed. The assay is based on the incubation of WBC lysates or lysed MNCs in the presence of supplemented inosine 5'-monophosphate (IMP) and nicotimamide adenine dinucleotide (NAD). The formation of xanthosine 5'-monophosphate (XMP) was determined by high-performance liquid chromatography (HPLC) with ultraviolet (UV) detection. The analytical method was validated, and the obtained data demonstrated that the amount of XMP in WBC and MNC lysates can be reliably determined by this method. Under assay conditions the rate of XMP formation remained constant within the incubation period of 60 minutes and a quantification of product formation at 30 and 60 minutes proved to be sufficient to reliably characterize the IMPDH activity. Applications of this assay with whole blood indicated extremely high IMPDH-activities in samples from patients with renal transplant receiving MMF. IMPDH monitoring within 10 hours after administration of the morning dose demonstrated a marked enzyme inhibition between 2 hours and 3 hours postdosing, but the activities returned to predose levels within one dose interval. The analysis of isolated cell fractions indicated that the IMPDH-activity is predominantly located in erythrocytes. The contribution of MNCs to the whole blood activity remained below 10%. In order to simulate the in vivo exposure of MNCs to mycophenolic acid, an "erythrocyte- and platelet-free" whole blood was reconstituted by resuspension of isolated MNCs with plasma. This strategy allowed for the reliable measurement of IMPDH activity in the target cells of immunosuppression.