Decrease in voriconazole concentration-to-dose ratio after letermovir initiation: a retrospective, observational study.

Letermovir is used to prevent cytomegalovirus infection in hematopoietic stem cell transplantation (HSCT) recipients. Although this agent decreases voriconazole exposure in healthy individuals, the effect of coadministration of letermovir and voriconazole in HSCT recipients is unknown. This retrospective, observational, single-center study was conducted between January 2016 and July 2019 to examine the voriconazole concentration-to-dose ratio over three periods: (A) (days -7 to -1 [day 0: day of HCST]), (B) (days 4-10), and (C) (days 11-17). Forty-two HSCT recipients administered voriconazole were divided into the following two groups based on letermovir coadministration: letermovir (n = 15) and control (n = 27). The percent change (-33.2%, p < 0.05) in the voriconazole concentration-to-dose ratio from periods A to C in the letermovir group was significantly lower than that in the control group. Therefore, frequent therapeutic drug monitoring of voriconazole concentrations and subsequent dose adjustments should be performed regularly in HSCT recipients.