Drug-drug interaction between isavuconazole and tacrolimus: a case report indicating the need for tacrolimus drug-level monitoring

What is known and objectiveDespite the known significant drug-drug interaction between isavuconazole and tacrolimus, there are no recommendations on dose adjustment when these drugs are given concomitantly. We report on a patient with amediastinal Aspergillus fumigatus infection resistant to posaconazole and describe how she was successfully managed with tacrolimus therapeutic drug-level monitoring. Case summaryOur patient presented with a mediastial Aspergillus fumigatus infection, 2years after lung transplantation. A.fumigatus was resistant to posaconazole, and the patient had intolerance to voriconazole shown by elevated transaminases. The patient was given isavuconazole with drug-level monitoring. She was managed successfully with no adverse events. Tacrolimus concentration continued to increase after more than 2weeks of therapy and required a further reduction to 72% of the usual dose to maintain the target concentrations over a 8-week period. What is new and conclusionWhen isavuconazole is given to patients on tacrolimus, the dose of the latter will need considerable reduction. We would suggest an initial 50% reduction and recommend close weekly monitoring of tacrolimus concentration. Further dose decreases of 25-50% may be required.