Isolation and Structural Elucidation of Oxidative Degradation Products of Bosutinib by Preparative HPLC, LC–MS, and NMR Spectroscopy

Bosutinib (BST), a 4‐anilino‐3‐quinolinecarbonitrile derivative, is a tyrosine kinase inhibitor approved for the treatment of chronic myelogenous leukemia. To assess the chemical stability of BST and elucidate its degradation pathways, forced degradation studies were conducted in accordance with International Council for Harmonisation quality guidelines Q1A(R2). Chromatographic analysis under oxidative stress conditions revealed the formation of two major degradation products (DP's), eluting at relative retention times of approximately 1.09 and 1.14, with respective abundances of 15% and 76%. These were isolated using preparative high‐performance liquid chromatography and structurally characterized using liquid chromatography–mass spectrometry and nuclear magnetic resonance spectroscopy. LC–mass spectrometry (MS) analysis indicated mass increases of +16 and +32 Da, consistent with mono‐ and di ‐oxidation, respectively. Nuclear magnetic resonance (NMR) data ( 1 H, 13 C, and 2D experiments) localized the oxidative modifications to the piperazinyl‐propoxy side chain and the aromatic methoxy groups. The identification and structural elucidation of these oxidative DP's are critical for understanding the degradation pathways of BST. This work underscores the importance of comprehensive impurity profiling not only for regulatory compliance and formulation development but also for ensuring drug product safety, particularly in oncology therapies where patient risk tolerance is low.