The transplanted child: New immunosuppressive agents and the need for pharmacokinetic monitoring.

BACKGROUND

Pharmacokinetic monitoring has been insufficiently studied in paediatric solid organ transplantation, especially because some agents are relatively new to paediatric use, are of new formulation modification or are being used in combinations not previously well studied. The choice of immunosuppressive drugs after paediatric renal transplantation is increasing. Cyclosporine A (CyA), tacrolimus and mycophenolate mofetil (MMF) use has become routine. While pharmacokinetic monitoring of CyA and tacrolimus is routine, few paediatric data on tacrolimus pharmacokinetics exist, and, for MMF, pharmacokinetic monitoring is performed in only a few Canadian centres. The aim of the present article is to provide guidelines for the use of these three drugs by using a large number of full pharmacokinetic profiles in children.

METHODS

One hundred forty-nine full pharmacokinetic 10-point profiles on cyclosporine microemulsion, 103 on the classic cyclosporine, 118 on tacrolimus and 114 on MMF were retrospectively analyzed. All pharmacokinetic profiles were obtained from paediatric renal transplant patients in steady state.

RESULTS

For pharmacokinetic monitoring of the classic cyclosporine formulation, evaluation of the trough levels suffices to estimate the area under the curve (AUC). For microemulsified cyclosporine, the trough levels do not provide a useful tool, and blood concentrations at 2 or 3 h (C2 or C3) after intake should be measured instead. Tacrolimus trough levels sufficiently estimate the AUC, but measuring the C4 yields the best prediction of the AUC. Nonetheless, C2 also provides a superior tool than the trough levels. Tacrolimus and CyA AUCs change substantially over time after renal transplantation. There is only a poor correlation between the trough level and the AUC for mycophenolic acid (MPA). No single time point provides a surrogate marker of the AUC. At least three time points are required to accurately estimate the AUC, and C1, C2 and C6 serve as the best markers. The article also describes the interaction between MPA and differing concomitant immunosuppression, as well as the variation of the MPA AUC with differing concentrations of cyclosporine.

CONCLUSIONS

Pharmacokinetic monitoring of these three drugs is mandatory in paediatric renal transplantation because it is impossible to predict the drug interactions and blood levels from a given dose. Target AUCs for a given time point after transplantation remain to be established.