Sirolimus-itraconazole interaction in a hematopoietic stem cell transplant recipient

Interactions between azole antifungal agents and immunosuppressants that are metabolized by cytochrome P450 3A4 (chiefly calcineurin inhibitors) are well documented. Interactions between itraconazole and sirolimus are known to occur in patients after solid organ transplantation, but interactions in hematopoietic stem cell transplant (HSCT) recipients have yet to be reported in the literature. We describe an allogeneic HSCT recipient who experienced supratherapeutic trough levels of sirolimus as a result of its coadministration with itraconazole. This patient was a 20-year-old African-American man who underwent HSCT for treatment of myelodysplastic syndrome with severe aplastic anemia. After several regimen changes, the patient received oral itraconazole 200 mg every 12 hours and sirolimus at a dosage of 7 mg/day on days 76-80 and 5 mg/day on days 81 and 82. His sirolimus whole blood trough levels were 17.5 and 35.6 ng/ml on days 80 and 82, respectively (therapeutic range 5-15 ng/ml). An interaction between itraconazole and sirolimus was suspected, and sirolimus was withheld on days 83-90. On day 90, the patient's sirolimus trough level had normalized to 4.4 ng/ml. Sirolimus was resumed at 1-2 mg/day, with adjustments as needed to maintain trough levels of 10-15 ng/ml. Both the itraconazole and sirolimus were eventually were discontinued. The patient died, however, from a disseminated adenovirus infection leading to end-organ failure. Sirolimus is extremely sensitive to the inhibitory potential of azole antifungals. We propose that itraconazole also has a potent effect on sirolimus metabolism. Preemptive sirolimus dosage reduction and close monitoring of its whole blood trough levels are required whenever this combination is considered to avoid immunosuppressant toxicity in already critically ill patients.