Analysis of NFATc1 amplification in T cells for pharmacodynamic monitoring of tacrolimus in kidney transplant recipients

BACKGROUND: Therapeutic drug monitoring (TDM) of tacrolimus, based on blood concentrations, shows an imperfect correlation with the occurrence of rejection. Here, we tested whether measuring NFATc1 amplification, a member of the calcineurin pathway, is suitable for TDM of tacrolimus. MATERIALS AND METHODS: NFATc1 amplification was monitored in T cells of kidney transplant recipients who received either tacrolimus- (n = 11) or belatacept-based (n = 10) therapy. Individual drug effects on NFATc1 amplification were studied in vitro, after spiking blood samples of healthy volunteers with either tacrolimus, belatacept or mycophenolate mofetil. RESULTS: At day 30 after transplantation, in tacrolimus-treated patients, NFATc1 amplification was inhibited in CD4(+) T cells expressing the co-stimulation receptor CD28 (mean inhibition 37%; p = 0.01) and in CD8(+)CD28(+) T cells (29% inhibition; p = 0.02), while this was not observed in CD8(+)CD28(-) T cells or belatacept-treated patients. Tacrolimus pre-dose concentrations of these patients correlated inversely with NFATc1 amplification in CD28(+) T cells (r(s) = -0.46; p < 0.01). In vitro experiments revealed that 50 ng/ml tacrolimus affected NFATc1 amplification by 58% (mean; p = 0.02). CONCLUSION: In conclusion, measuring NFATc1 amplification is a direct tool for monitoring biological effects of tacrolimus on T cells in whole blood samples of kidney transplant recipients. This technique has potential that requires further development before it can be applied in daily practice.