Quantification of isavuconazole in dried blood spots by LC-MS/MS and clinical application for therapeutic drug monitoring.

Invasive fungal diseases pose a significant threat to human health. Therapeutic drug monitoring (TDM) of isavuconazole, a new-generation triazole antifungal agent, is recommended; however, conventional cubital venous blood collection is invasive and requires large blood volumes as well as sample transportation and storage. Quantitative dried blood spot (qDBS) sampling can overcome these limitations and may provide better analytical accuracy than conventional dried blood spot assays. This study developed and validated an ultra-performance liquid chromatography-tandem mass spectrometry method to determine isavuconazole in qDBS. Samples were extracted using an acetonitrile-water solution (70:30, v/v). Chromatographic separation was achieved on an Acquity UPLC® BEH C18 column (100 × 2.1 mm, 1.7 µm). The mobile phase consisted of water (0.02% formic acid) and acetonitrile at a flow rate of 0.6 mL/min, with a total runtime of 2.5 min. The method was linear over 0.1-50 µg/mL and demonstrated good precision (maximum coefficient of variation, 9.9%) and accuracy. Mean recovery was 96.9%. Hematocrit values ranging from 0.2 to 0.65 did not significantly affect the analysis results. qDBS samples were stable for 17 days at room temperature and for 22 days at -20 °C. Reconstituted samples in the autosampler (4 °C) were stable for at least 24 h. Plasma and fingerprick qDBS concentrations from 20 patients were compared using Passing-Bablok regression and Bland-Altman analyses, confirming good agreement. Overall, these findings support qDBS sampling for isavuconazole TDM and suggest that it may replace or supplement conventional sampling in hematology patients (HCT range 0.15-0.40).