Identification, Isolation, and Characterization of Five Potential Degradation Impurities in Candesartan Cilexetil Tablets

Five impurities were observed when candesartan cilexetil tablets were subjected to stability and forced degradation studies. These impurities were successfully isolated and characterized as desethyl candesartan cilexetil, 1N-ethyl candesartan cilexetil, 2N-ethyl candesartan cilexetil, 1N-ethyl oxo candesartan cilexetil, and 2N-ethyl oxo candesartan cilexetil. A gradient reverse phase liquid chromatography (LC) and an isocratic preparative LC method were used to detect and isolate all five degradation products impurities simultaneously. Mass spectrometry, ¹H/¹³C, DEPT and 2D NMR experiments were extensively utilized to characterize these impurities. Even though desethyl candesartan cilexetil, 1N-ethyl candesartan cilexetil were 2N-ethyl candesartan cilexetil were documented in the literature as known impurities, the regioisomers 1N-ethyl oxo candesartan cilexetil and 2N-ethyl oxo candesartan cilexetil were never noticed. Single-crystal diffraction data has been used to confirm their structure unambiguously and synthetic preparations of all known and unknown impurities were also presented.