Optimization of Isavuconazole Dosing in Patients with Invasive Fungal Infections Through Therapeutic Drug Monitoring: Real-World Clinical Practice Experience.

Therapeutic drug monitoring (TDM) is routinely recommended for most antifungal triazoles to ensure efficacy and safety. Isavuconazole, however, was initially approved without this recommendation due to its predictable pharmacokinetic profile. Later clinical data have raised concerns about subtherapeutic exposures in certain populations. This prospective, single-center study aimed to assess the need for TDM of isavuconazole in critically ill and hematologic patients with invasive fungal infections. Between March 2022 and November 2023, patients receiving standard dosing of isavuconazole were enrolled, and plasma concentrations were measured to determine the proportion of patients with values outside the therapeutic range (1-4 µg/mL), particularly focusing on subtherapeutic levels. A total of 65 isavuconazole plasma concentrations from 24 patients (9 critically ill and 15 hematologic) were analyzed. Critically ill patients had lower initial concentrations than hematologic patients (median [range]: 0.75 [not detectable (ND)-5.18] vs. 3.03 [1.03-6.65] µg/mL), with 66.7% showing levels outside the therapeutic range and 55.5% having subtherapeutic concentrations. The coefficient of variation (CV%) of concentrations values at the first TDM was 124.7% in critically ill patients and 57.3% in hematologic patients. After dose adjustment in critically ill patients, the proportion with levels outside the therapeutic range decreased to 28.6%. These findings suggest that, despite initial assumptions, isavuconazole exhibits considerable pharmacokinetic variability in specific populations, particularly in critically ill patients, and the findings support the implementation of TDM to optimize antifungal therapy and improve patient outcomes in real-world clinical settings.