Pharmacokinetic investigations of isavuconazole in paediatric cancer patients show reduced exposure of isavuconazole after opening capsules for administration via a nasogastric tube

Objectives: To study the isavuconazole pharmacokinetics in a real-life paediatric cohort and confirm whether the isavuconazole exposures are within the adult exposure range. Furthermore, we are the first to describe unbound isavuconazole pharmacokinetics. 

Methods: In this prospective, observational study, the isavuconazole dosing regimen was as follows (IV/oral/ nasogastric tube): 5.4 mg/kg isavuconazole (maximum 200 mg/dose) three times daily on Days 1 and 2, followed by 5.4 mg/kg isavuconazole (maximum 200 mg/dose) once daily. At least one pharmacokinetic curve was assessed. Non-linear mixed effects modelling was used for analysis. Monte Carlo simulations were performed with the above mentioned maintenance dose for IV administrations and a weight band dosing regimen for oral/nasogastric tube administrations: I) <18 kg (100 mg daily); II) 18–37 kg (150 mg daily); III) >37 kg (200 mg daily). 

Results: Seventeen paediatric patients with a median age of 9 years (range 1–17) and median weight of 26.0 kg (range 8.4–78.5) were evaluated. A two-compartment model describing linear pharmacokinetics of the unbound concentrations and saturable protein binding fitted the isavuconazole concentrations best. The absolute bioavailability of isavuconazole was 41.0% (95% CI: 32.4%–50.8%). The median (IQR) simulated exposures (AUC_{0–24h, SS}) of the total isavuconazole concentrations after IV and oral/nasogastric tube administration were 87.7 mg·h/L (70.5–105.1) and 50.3 mg·h/L (39.0–62.4), respectively. The unbound isavuconazole fraction (unbound/total) ranged from 0.5% to 2.3%. 

Conclusions: This study revealed low bioavailability after nasogastric tube administration with opened capsules. Isavuconazole exposures were in the expected range following IV administration. Total and unbound isavuconazole pharmacokinetics were reported with a 5-fold range in the unbound fraction.