UNEXPECTED AUGMENTATION OF MYCOPHENOLIC-ACID PHARMACOKINETICS IN RENAL-TRANSPLANT PATIENTS RECEIVING TACROLIMUS AND MYCOPHENOLATE MOFETIL IN COMBINATION THERAPY, AND ANALOGOUS IN-VITRO FINDINGS

Mycophenolate mofetil (MMF) a potent immunosuppressive agent, has rece ntly been approved for clinical use (CellCept(TM)) in renal transplant patients in combination with cyclosporine (CsA). With the expanded us e of tacrolimus (Prograf(TM)) as well in renal transplant patients, th ere is a lack of pharmacokinetic studies clarifying drug interactions between the three agents. A pharmacokinetic study was performed on 18 stable renal transplant patients receiving MMF and tacrolimus together , and four control groups, one receiving tacrolimus alone, two receivi ng CsA, in combination with MMF (1.0 or 1.5 g bid), and one receiving CsA microemulsion (Neoral(TM)). Area-under-the-curve values were calcu lated for each drug to assess if there was a reciprocal effect on the respective bioavailability of each. In vitro, the immunosuppressive ef fect of trough level plasma from each patient group was studied using mixed lymphocyte culture (MLC), as well as MLC reactions spiked with v arious combinations of each drug. There was a minimal effect of MMF on tacrolimus pharmacokinetics. However, patients receiving tacrolimus a nd MMF displayed significantly higher levels (C-min and area under the curve) of mycophenolic acid (MPA) than those receiving CsA (Sandimmun e(TM) or Neoral(TM)) and the same dose of MMF (50.2 +/- 16.5 vs 32.1 /- 16.7 mu g h/ml AUC, p < 0.02). Equivalent MPA levels could be attai ned in patients receiving CsA if the MMF dose was increased by 50% (1. 5 g bid). There were also significantly lower levels of the glucuronid e metabolite of MPA (MPAG) (755 +/- 280 vs 1230 +/- 250 mu g h/ml AUC, p = 0.02), suggesting a specific inhibition (either direct or indirec t) of the conversion of MPA to MPAG in tacrolimus patients, as opposed io those receiving CsA. For each drug combination, there was a positi ve correlation between the plasma immunosuppressive effect seen in MLC assays and the MMF dose. In addition, trough plasma from patients rec eiving tacrolimus and MMF was significantly more MLC inhibitory than f rom those receiving CsA or CsA microemulsion and equivalent-dose MMF. Culture media containing MPA and tacrolimus equal to clinical therapeu tic trough concentrations (10 ng/ml) were significantly more MLC inhib itory than CsA at equivalent clinical therapeutic trough concentration s (200 ng/ml) with equivalent MPA levels. These studies in renal trans plant patients suggest that tacrolimus in combination with MMF may res ult in a greater degree of immunosuppression than may be anticipated.