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Summary What is known and objective Azole antifungal drugs are often co‐administered with tacrolimus after allogeneic hematopoietic stem cell transplantation (HSCT). However, the influence of azole antifungal drugs on variation in tacrolimus pharmacokinetics when switching from intravenous tacrolimus (Tac‐iv) to once‐daily modified release tacrolimus (Tac‐MR) remains to be elucidated. This study was performed to evaluate the effects of oral azole antifungal drugs on variation in tacrolimus pharmacokinetics after conversion to Tac‐MR in HSCT patients. Methods Patients concomitantly receiving fluconazole (FLCZ) or voriconazole (VRCZ) along with tacrolimus were evaluated retrospectively. Blood tacrolimus concentrations before and after changing to oral administration were compared between FLCZ and VRCZ groups. Results and discussion A total of 52 patients (34 FLCZ and 18 VRCZ) were included in the analysis. There were no significant differences in the most recent daily dose (Div) and blood level (Civ) of Tac‐iv, Civ/Div, and ratio of daily dose of tacrolimus on the first to second day after changing to Tac‐MR (Dpo1‐2) to Div between FLCZ and VRCZ groups (P > 0.2). The trough levels of tacrolimus on the first to second day after switching to Tac‐MR (Cpo1‐2) and on the third to fifth day after the switch (Cpo3‐5) were significantly higher in the VRCZ group than the FLCZ group (P < 0.05). The values of (Civ/Div)/(Cpo1‐2/Dpo1‐2) and (Civ/Div)/(Cpo3‐5/Dpo3‐5) in the VRCZ group were significantly lower compared with those in the FLCZ group (P < 0.05). Furthermore, individual values of (Civ/Div)/(Cpo3‐5/Dpo3‐5) in the FLCZ group varied widely. What is new and conclusion Voriconazole increased blood tacrolimus level more markedly than FLCZ after switching to Tac‐MR, whereas FLCZ caused a large variation in tacrolimus blood level. These results suggest that therapeutic monitoring of tacrolimus after the switch may need to be performed carefully considering that orally co‐administered VRCZ and FLCZ exhibit different change in blood tacrolimus level just after the switch.

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