Cytochrome P450 inhibition through fruit supplement interactions often results in increased serum levels of calcineurin inhibitors, including tacrolimus. Cranberry extract is a supplement often used for the prevention of recurrent urinary tract infections (UTIs), which are common in renal allograft recipients. To our knowledge, a decrease in serum levels of tacrolimus as a result of cranberry extract interaction is unreported. A 40-year-old renal transplant patient taking cranberry extract capsules for her recurrent cystitis presented asymptomatically with low serum levels of tacrolimus. Dose increase had little effect on the level, and cessation of the cranberry extract returned levels to desired range. Cranberry extracts are an adjunctive therapy used in the management of recurrent UTIs. Tacrolimus, an immunosuppressive agent, is metabolized intestinally by isoenzymes of the P450 cytochrome. Cranberry extracts may alter this metabolism and lead to sub-therapeutic serum levels of tacrolimus. This interaction is heretofore unreported. Cranberry extracts should be carefully monitored in allograft recipients due to interactions with serum tacrolimus levels.
Keywords: cranberry juice; cytochrome p450; drug interactions; tacrolimus.