Measuring tacrolimus (TAC) its whole blood concentrations (Cmin) is mandatory in liver and kidney transplant recipients (LTR and KTR). However, while having adequate Cmin, some patients still experience rejection/adverse events, highlighting the limitation of the actual monitoring. As TAC exerts its effect inside T cells, monitoring intracellular instead of whole blood concentrations is an appealing strategy. Some drug transporters expressed at the cells membrane could be major factors of TAC diffusion into peripheral blood mononuclear cells (PBMC).

The aim of the study was to explore the relationship between TAC diffusion into PBMC and cell membrane expression of four drug transporters (P-glycoprotein (P-gp), Multidrug resistance-associated protein 2 (MRP-2), Concentrative nucleoside transporter 3 (CNT-3) and Equilibrative nucleoside transporter 1(ENT-1)).

Sixty stable LTR and KTR were included in the study. TAC Blood and PBMC concentrations were determined by liquid chromatography coupled with tandem mass spectrometry. P-gp, MRP-2, CNT-3 and ENT-1 expression at the PBMC surface was evaluated using flux cytometry. Correlation between TAC PBMC/blood ratios and transporters expressions were explored using correlation tests.

TAC PBMC/blood ratios were not different between LTR and KTR and the results were then analyzed altogether. P-gp, CNT-3 and ENT-1 expressions show positive correlation with TAC PBMC/blood ratio. Correlation was stronger for ENT-1 (R2=0.31, p<0.0001) than for P-gp and CNT-3 (R2=0.17, p=0.001 and R2=0.14, p=0.003, respectively). MRP-2 expression was not correlated with PBMC/blood ratio (Fig. 1).

In this study, TAC diffusion into PBMC increased as a function of membrane expression of ENT-1, CNT-3 and P-gp. ENT-1 expression seems to be the strongest driver of TAC diffusion, which is consistent with its role as an influx transporter. Findings for P-gp might be explained by an induction of its expression when intracellular TAC concentration increases. ENT-1 and CNT-3 influx effects on TAC diffusion would then outweigh the P-gp efflux effect.