Identification, Synthesis, and Characterization of Potential Oxidative Impurities of Venetoclax: Application of Meisenheimer Rearrangement

[Image: see text] Venetoclax is a potent BCL-2 inhibitor that is used for the treatment of several blood cancers. During the oxidative stress degradation of venetoclax, we observed the formation of two potential impurities at levels of about 8–10%, which have similar molecular weights. The two impurities were isolated and identified as 4-(3-((1H-pyrrolo[2,3-b]pyridin-5-yl)oxy)-4-(((3-nitro-4-(((tetrahydro-2H-pyran-4-yl)methyl)amino)phenyl)sulfonyl)carbamoyl)phenyl)-1-((4′-chloro-5,5-dimethyl-3,4,5,6-tetrahydro-[1,1′-biphenyl]-2-yl)methyl)piperazine 1-oxide (venetoclax N-oxide, VNO) and 2-((1H-pyrrolo[2,3-b]pyridin-5-yl)oxy)-4-(4-((4′-chloro-5,5-dimethyl-3,4,5,6-tetrahydro-[1,1′-biphenyl]-2-yl)methoxy)piperazin-1-yl)-N-((3-nitro-4-(((tetrahydro-2H-pyran-4-yl)methyl)amino)phenyl)sulfonyl)benzamide (venetoclax hydroxylamine impurity, VHA). To confirm these two compounds, we have synthesized each impurity individually and analyzed it by high-performance liquid chromatography, mass spectrometry, (1)H NMR, (13)C NMR, and 2D NMR. VNO was synthesized by the oxidation of venetoclax using m-CPBA in dichloromethane to get the required N-oxide impurity. After the confirmation of the VNO impurity, the VNO impurity was heated with water at reflux in a sealed tube for 36 h to get the VHA impurity of about 6–8% after 36 h. After thorough analysis, it was confirmed that venetoclax N-oxide undergoes [1,2] Meisenheimer rearrangement to form the venetoclax hydroxylamine impurity. These two impurities may be relevant reference standards in manufacturing venetoclax Active Pharmaceutical Ingredient (API) (or) tablets.