Limited interaction between tacrolimus and P-glycoprotein in the rat small intestine.

The significance of intestinal P-glycoprotein (P-gp) in determining the oral bioavailability of tacrolimus has been still controversial. In this study, we reevaluated the interaction of tacrolimus with P-gp in the rat small intestine, by evaluating its absorption from the rat small intestine and its modulating effect on the absorption of known P-gp substrates (digoxin, methylprednisolone, and vinblastine). Intestinal absorption of tacrolimus itself was as extensive as other P-gp modulators such as cyclosporine and verapamil. While cyclosporine and verapamil significantly increased the absorption of methylprednisolone and vinblastine through potent inhibition of intestinal P-gp, tacrolimus failed to achieve this. When cyclosporine and tacrolimus were intravenously administered to rats, digoxin absorption was significantly increased by cyclosporine but not by tacrolimus. When tacrolimus was coadministered with clotrimazole, a specific CYP3A inhibitor, into the rat small intestine, the area under the curve of tacrolimus blood concentrations increased more than seven-fold compared with that of tacrolimus alone. Our present results strongly suggest that the interaction between tacrolimus and P-gp is limited in the rat small intestine and that extensive metabolism by CYP3A enzymes is more responsible for the low oral bioavailability of tacrolimus. It was considered that the extensive absorption of cyclosporine and verapamil was closely associated with their potent ability to inhibit intestinal P-gp.

[Relationship between dose of mycophenolate mofetil and the occurrence of cytomegalovirus infection and diarrhea in renal transplant recipients].

To establish guidelines for avoiding the side effects of mycophenolate mofetil (MMF) in renal transplant recipients with tacrolimus (TAC)-based immunosuppression, the relationship between the daily dose of MMF and the occurrence of side effects was analyzed in this study. The frequency of side effects was investigated retrospectively in 28 renal transplant recipients treated with immunosuppression (men 14 : women 14, age: 33.0+/-12.4 years, weight: 50.9+/-10.7 kg). Cytomegalovirus (CMV) infection and diarrhea were the most frequent side effects in the early transplant phase (from transplantation to 3-month biopsy) in the recipients. In 18 recipients, excluding the recipients with risk factors for CMV infection (ABO-incompatible transplantation, donor (+)/recipient (-) CMV serostatus, etc.), no significant correlation was shown between the daily dose of MMF and the occurrence of CMV infection in the two-sample t-test. On the other hand, the daily dose in the diarrhea group (33.2+/-4.3 mg/kg/day, n = 5) was significantly higher than that in the no-diarrhea group at 30 days (28.4+/-3.7 mg/kg/day, n = 23, p < 0.05) and 90 days (25.7+/-4.4 mg/kg/day, n = 21, p < 0.005) after transplantation, respectively. The receiver-operating characteristic (ROC) curve also revealed that the risk of diarrhea increased with a daily MMF dose higher than 30 mg/kg/day. In conclusion, to decrease the risk of diarrhea in the early transplant phase in renal transplant recipients with TAC-based immunosuppression, the daily dose of MMF should not be more than 30 mg/kg/day.