Introduction:The immunosuppressive effect of mycophenolic acid (MPA) is mediated by inhibition of the enzyme inosine monophosphate dehydrogenase (IMPDH). Measurement of IMPDH activity has been suggested as a pharmacodynamic approach for individualization and improvement of MPA therapy. Methods: The aim was to characterize the two IMPDH isoenzymes during exposure to MPA. Renal transplant patients (n=30 and n=7), liver transplant patients (n=11) and healthy volunteers (n=5) were included in 4 descriptive studies. The patients received standard triple/quadruple immunosuppressive regimens. The healthy volunteers received single doses of mycophenolate mofetil (MMF; 100-1000 mg). Samples were collected pretransplant and at repeated occasions up to 13 weeks posttransplant. The activity of IMPDH and expressions of IMPDH 1 and 2 were determined in CD4+ cells by HPLC-UV and real-time reverse-transcription PCR, respectively. Concentrations of MPA and purine nucleotides (healthy volunteers) were measured by HPLC-UV. Results: The results demonstrated a wide variability of IMPDH activity in CD4+ cells between individuals (up to 41-fold, pretransplant). Compared to patients without rejection (n=19), renal transplant patients with acute rejection (n=11) displayed higher IMPDH2 expression pretransplant (P=0.017) possibly reflecting a higher degree of cellular immune activation. A dose interval study in renal transplant recipients demonstrated higher MPA exposure (median 43%, P=0.031, n=6) at 13 vs. 2 weeks posttransplant. Furthermore, the same study revealed significant changes of MPA pharmacodynamics with time since transplantation. In contrast to the postdose reductions of IMPDH activity observed 1-2 weeks posttransplant, IMPDH activity was transiently increased within dosing intervals at week 13, reaching 7 times predose levels. Considerable IMPDH activity elevations were also observed in liver transplant recipients, demonstrating up to 4-fold increases after dosing. Two patients (1 renal, 1 liver) did not show any IMPDH inhibition postdose. Moreover, IMPDH1 gene expression was transiently upregulated postdose in renal recipients, and higher MPA exposure was associated with larger IMPDH1 elevations (r=0.81, P=0.023, n=7, MPA and IMPDH1 AUC 0−9h ). The maximum IMPDH1 expression was 52 (13−177)% higher at week 13 compared to week 1 (P=0.031, n=6). The study in healthy individuals revealed concentration-dependent MPA effects on IMPDH 1 and 2 expression, as well as an inverse relation between GTP and gene expression profiles. Following MMF doses of 250 and 500 mg, IMPDH1 expression was suppressed by median 29% (n=5, P=0.043) and 39% (n=5, P=0,043), while GTP concentrations increased (<54% above predose). Conclusions: The wide variability of IMPDH and the association between IMPDH2 expression and risk of acute rejection support the potential of IMPDH measurements to individualize MPA therapy. Furthermore, the significant influence of MPA on IMPDH1 and 2 expressions, possibly mediated through reduced guanine nucleotide levels, might...