Concomitant letermovir affects the optimal concentration-to-dose ratio of tacrolimus after switching from intravenous to oral tacrolimus administration in hematopoietic cell transplantation patients receiving fluconazole prophylaxis

Letermovir is often administered for cytomegalovirus prophylaxis after allogeneic hematopoietic cell transplantation (HCT). Concomitant use of letermovir and azole antifungals affects tacrolimus concentration. Therefore, in HCT recipients taking fluconazole, letermovir may affect the optimal tacrolimus conversion ratios when switching from continuous intravenous infusion to oral administration. In this study, we retrospectively evaluated tacrolimus conversion ratios in 104 HCT recipients taking fluconazole with and without concomitant letermovir. The median tacrolimus concentration-to-dose (C/D) ratios with and without letermovir were 18.2 and 20.6 (ng/mL)/(mg/day), respectively, before conversion from continuous infusion (C/Dciv) (p = 0.21) and 2.9 and 1.9 (ng/mL)/(mg/day), respectively, after conversion (p < 0.01). The median (C/Dpo)/(C/Dciv) ratios with and without letermovir were 0.15 and 0.10, respectively (p < 0.01). These results suggest that in HCT recipients taking fluconazole, the optimal conversion ratio when switching from continuous intravenous infusion to oral administration is 0.7-fold lower with concomitant letermovir than without it.