Effect of hematocrit and albumin concentration on hepatic clearance of tacrolimus (FK506) during rabbit liver perfusion.

Tacrolimus is an immunosuppressive agent used for organ transplantation. Studies were performed to examine the influence of different perfusate hematocrits and albumin concentrations on hepatic extraction of tacrolimus. In vitro binding, efflux and influx between red blood cells (RBCs) and buffer or plasma, and rabbit liver perfusion with use of human erythrocytes were studied. In the range of hematocrits from 0.05 to 0.4, plasma concentrations of tacrolimus were not affected by increased albumin content. Increased hematocrit caused decreases in whole blood:plasma (buffer) concentration ratios. The binding capacity of drug with RBCs was independent of hematocrit, with a value of 440 ng/ml of RBCs; the binding affinity was 0.876 ng/ml using plasma or buffer. Diffusion of tacrolimus from RBCs to buffer was rapid with a clearance of 0.940 ml/min, and equilibration was achieved within 2 min. Diffusion in the opposite direction (buffer-RBCs) was slower with a clearance of 0.576 ml/min. In such diffusion studies, plasma produced a greater difference between efflux (1.70 ml/min) and influx (0.276 ml/min) clearances. During liver perfusion, the major factor regulating elimination of tacrolimus was hematocrit. Both well-stirred and parallel-tube models reflected a low extraction ratio drug with values of 0.15 and 0.17 for the 0.05 and 0.2 hematocrits. Intrinsic clearances were 8.43 and 17.44 ml/min for the well-stirred and parallel-tube models. Albumin had a negligible influence on liver extraction of drug. A model-building process of characterizing nonlinear RBC binding, RBC diffusion rates, and liver perfusion parameters allows the complexities of tacrolimus hepatic clearance to be dissected and shows that strong RBC binding can be artificially perceived as causing a high clearance of the drug.