TDM-Based Approach for Properly Managing Intravenous Isavuconazole Treatment in a Complex Case Mix of Critically Ill Patients

<b>Objectives:</b> To assess the role of a real-time therapeutic drug monitoring (TDM)-guided expert clinical pharmacological advice (ECPA) program of isavuconazole in preventing under- or overexposure with the intent of improving efficacy and safety outcomes in the critically ill patients. <b>Methods:</b> This retrospective study included critical patients receiving intravenous isavuconazole for prophylaxis or treatment of invasive fungal infections (IFI) and undergoing at least one TDM-guided ECPA in the period 1 March 2021-31 March 2025. Desired isavuconazole exposure was defined as trough concentrations (C_min) of 1.0-5.1 mg/L. Efficacy outcome was assessed by means of bronchoalveolar (BAL) galactomannan (GM) index, breakthrough IFI, and 30-day mortality rate, whereas safety was assessed by means of hepatic test disturbances (HTD). Univariate analysis was carried out for assessing potential variables associated with isavuconazole under- or overexposure and for comparing features of solid organ transplant (SOT) recipients vs. non-SOT patients. Proportions of isavuconazole C_min underexposure, desired exposure, and overexposure were assessed at different timepoints from starting therapy. Trends over time of HTD in relation to isavuconazole exposure were assessed separately in patients having HTD or not at baseline. <b>Results:</b> Overall, 32 critical patients were included. A total of 166 TDM-guided ECPAs were provided. Median (IQR) average isavuconazole C_min was 3.5 mg/L (2.1-4.6 mg/L). Proportions of ECPAs with isavuconazole C_min under- and overexposure were 4.2% (7/166) and 16.3% (27/166), respectively. Patients experiencing underexposure had higher body mass index (30.1 vs. 25.5 kg/m²; <i>p</i> < 0.001). Trends of isavuconazole C_min under- and overexposure changed over time, significantly decreasing the former (10.5% <7 days vs. 4.3% 7-28 days vs. 0.0% >28 days; <i>p</i> < 0.001) and increasing the latter (5.3% <7 days vs. 12.8% 7-28 days vs. 29.3% >28 days; <i>p</i> < 0.001). HTD occurred in 15/32 patients, most of whom (10/15) were affected just at baseline. Patients with transient or persistent overexposure trended toward a higher risk of HTD compared to those without (33.3% vs. 8.3%; <i>p</i> = 0.11). <b>Conclusions:</b> A real-time TDM-guided approach could be a valuable tool for optimizing isavuconazole exposure, especially whenever dealing with obese patients or with prolonged treatment.