High‐Performance Liquid Chromatography–Ultraviolet Assay for the Determination of Pirtobrutinib Levels in a Patient With Mantle Cell Lymphoma

Pirtobrutinib is a Bruton's tyrosine kinase inhibitor used to treat mantle cell lymphoma and chronic lymphocytic leukemia. Pirtobrutinib has a steady-state trough concentration of > 825 ng/mL, corresponding to a 90% inhibitory concentration of Bruton's tyrosine kinase. Therefore, maintaining stable trough concentrations of pirtobrutinib is clinically important; however, no methods of monitoring pirtobrutinib levels have been developed. In this study, our aim was to develop a method to determine pirtobrutinib levels in human plasma and validate it for therapeutic drug monitoring. Pirtobrutinib and ibrutinib (internal standard) were separated on a reversed-phase column using a mobile phase comprising 0.5% KH₂PO₄ (pH 4.5) and acetonitrile (52:48, v/v) at a flow rate of 1.0 mL/min. Ultraviolet detection was performed at 234 nm. Calibration curves for pirtobrutinib were linear (r² = 0.9998) in the range of 0.25-10 μg/mL. The intraday and interday validation coefficients were 0.72%-2.86% and 1.29%-3.22%, respectively. This study is the first one to develop and validate a method for quantifying pirtobrutinib in human plasma. These findings may support the widespread application of therapeutic drug monitoring for pirtobrutinib.