Pharmacodynamic Monitoring of Mycophenolate Mofetil in Renal Allograft Recipients K. Budde, P. Glander, K.P. Braun, T. Bo ¨ hler, J. Waiser, L. Fritsche, I. Mai, and H. Neumayer M YCOPHENOLATE MOFETIL (MMF) has become standard immunosuppressant after solid organ transplantation in most centers. The immunosuppressive activity of MMF is based on the reversible inhibition of inosine 5'-monophosphate dehydrogenase (IMPDH) by mycophenolic acid (MPA). MMF acts more specificly be- cause lymphocytes depend on de novo purine synthesis in contrast to most other cell types, which can generate purine nucleotides via the salvage pathway. 1 Pharmacodynamic monitoring of MMF by measurement of IMPDH activity is a novel approach for individualising MMF therapy, which may better reflect the biologic response to the drug. 1 Although it is well known that MMF inhibits IMPDH activity, there are only limited data on IMPDH activity in MMF-treated subjects. Inhibition of IMPDH activity and inhibition of GTP-synthesis in vivo indicate that MPA is retained considerably longer than might be expected from pharmacokinetic profiles. 2 Determination of IMPDH activ- ity in 23 healthy volunteers suggested a substantial interin- dividual variation of IMPDH activity, which may account for pharmacodynamic differences. 2 A recent investigation suggests an induction of enzyme activity in long-term MMF-treated subjects. 3 A major problem is the difficult assay system, which does not allow one to perform stan- dardized IMPDH determination over time. 4 Recently, we developed a new IMPDH method allowing the highly reproducible determination of IMPDH activity in mononu- clear cells. 5 So far, no data are available about diurnal variation of IMPDH activity in healthy subjects. Data on larger populations with regard to individual kinetic param- eters (K m , V max , and K i ) are lacking because of the technical difficulties with the IMPDH assay used in these studies. The aim of the present study was to characterize IMPDH activity in a population study of healthy subjects and MMF-treated patients, determining the time course and variability of IMPDH activity. METHODS AND PATIENTS For measurement of IMPDH activity in peripheral mononuclear cells (MNCs) we established a modified nonradioactive procedure described by Montero et al 6 for erythrocytes. It is based on the chromatographic determination of produced XMP. 5 Briefly, MNCs were isolated from the peripheral blood by Ficoll-density centrifu- gation and subsequently lysed by the addition of water. Lysate was stored at -80°C. After thawing and centrifugation the supernatant was used for further incubation and protein determination. The amount of generated XMP was determined by isocratic ion-pair- reversed phase high performance liquid chromatography with UV-detection at 254 nm. For the determination of interindividual IMPDH variability we investigated 65 healthy Caucasian blood donors. Blood was drawn between 8 AM and 10 AM. In seven dialysis patients IMPDH activity and MPA levels were determined after their first dose of 1g MMF. The study was done on the day before living related kidney transplantation was performed. All patients were fasting overnight; only water was allowed until 1 hour postdose. Blood was drawn for IMPDH activity and MPA concentration at nine time points: predose, 30 minutes, 1 hour, 90 minutes, 2 hours, 4 hours, 6 hours, 8 hours, and 11 hours after dosing. For comparison, time course of IMPDH activity with six timepoints between 8 AM and 7 PM was analysed in seven healthy volunteers without MMF. The study was approved by the local ethics committee. All patients and volunteers had given their informed consent. For determination of MPA concentration, plasma samples from EDTA-containing tubes were analysed within 12 hours using a commercially available test system (EMIT 2000 MPA, Dade-Behring) according to the manufacturers guidelines. Unless specified, all results are expressed as the mean  standard deviation. Correlation between variables was assessed using univariate linear regression analysis. Statistical differences between normally distributed groups were tested by a paired or unpaired t test using SPSS statistical software. P  .05 was considered to be significant. RESULTS We observed a high variability of IMPDH activity in MNCs from 65 healthy blood donors ranging from 3.9 to 32.9 nmol/h per mg protein. Mean IMPDH activity was 17.7  6.4 nmol/h per mg protein. The blood donors were 37.4  12 years of age. We did not find any correlation between age (range: 19 to 63 years) and IMPDH activity (r = .18). We could not detect any differences in IMPDH activity between 50 males (17.2  6.5 nmol/h per mg protein) and 15 females (19.36.0 nmol/h per mg protein). Furthermore, From the Department of Internal Medicine-Nephrology (K.B., P.G., K.P.B., T.B., J.W., L.F., H-H.N.), and Department of Clinical Pharmacology (I.M.), Charite ´ Campus Mitte, Humboldt Univer- sity, Schumannstr, 20/21, 10098 Berlin, Germany. Supported by a grant from Hoffmann-La Roche AG, Germany. Address reprint requests to K. Budde, Universitatsklinikum Charite ´ , Med Klinik M.S. Nephrologie, Schumannstr, 20/21, 10098 Berlin, Germany. © 2001 by Elsevier Science Inc. 0041-1345/01/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0041-1345(01)02407-1 Transplantation Proceedings, 33, 3313–3315 (2001) 3313