Evaluation of the Pharmacokinetic Interaction Between Letermovir and Tacrolimus in Allogeneic Hematopoietic Cell Transplantation Recipients.

Data describing the magnitude of the pharmacokinetic interaction between letermovir and tacrolimus in allogeneic hematopoietic cell transplantation (allo-HCT) recipients are limited, and varying outcomes have been reported. The need for empiric dose adjustment of tacrolimus on initiation of letermovir has not been established; instead, guidelines suggest closely monitoring the tacrolimus trough concentration and adjusting the dose as needed. A better understanding of this interaction is imperative to accurately manage the narrow therapeutic window of tacrolimus post-transplantation. The primary objective of this study was to determine the percent change in tacrolimus concentration-to-dose ratio over the 14-day period after initiation of letermovir. Secondary objectives were to describe the frequency of tacrolimus dose adjustments after initiation of letermovir, the percent change in daily tacrolimus dose over the 14-day period after initiation of letermovir, and the incidence of both subtherapeutic and supratherapeutic tacrolimus trough concentrations. This retrospective chart review included adult allo-HCT recipients at our institution who received tacrolimus in combination with oral letermovir and had been taking tacrolimus for at least 5 days before letermovir initiation. Patients receiving strong CYP3A4 inhibitors or i.v. tacrolimus were excluded. Thirty-five patients were included in the analysis. The median percent increase in tacrolimus concentration-to-dose ratio over the 14-day period after initiation of letermovir was 22% on days 2 to 4, 47% on days 5 to 7, 66% on days 8 to 11, and 81% on days 12 to 14. The mean frequency of tacrolimus dose adjustments was 0.66 on days 2 to 4, 0.69 on days 5 to 7, 1.06 on days 8 to 11, and 0.57 on days 12 to 14. The results of this study show that the pharmacokinetic interaction between tacrolimus and letermovir is substantial and continues to affect tacrolimus concentration over the 14-day period after letermovir initiation. Close monitoring of tacrolimus trough concentration on initiation of letermovir should be considered.