Impact of St John's wort treatment on the pharmacokinetics of tacrolimus and mycophenolic acid in renal transplant patients

Background. This study investigated the effect of St John's wort (SJW) extract on the pharmacokinetics of the immunosuppressants tacrolimus (TAC) and mycophenolic acid (MPA). Methods. Ten stable renal transplant patients received 600 mg SJW extract for 14 days in addition to their regular regimen of TAC and mycophenolate mofetil. Results. Dose-corrected AUC((0-12)) of TAC decreased significantly from 180 ng/ml/h at baseline to 75.9 ng/ml/h after 2 weeks of SJW treatment. To maintain therapeutic TAC concentrations, dose adjustments from a median 4.5 mg/day at baseline to 8.0 mg/day under SJW treatment were required. Two weeks after discontinuation of SJW, TAC doses were reduced to a median of 6.5 mg/day. MPA pharmacokinetics remained unaffected by comedication with hypericum extract. Conclusions. Administration of SJW extract to patients receiving TAC treatment can result in a serious drug interaction leading to markedly reduced TAC blood concentrations associated with the risk of organ rejection.