Use of LC–MS/TOF, LC–MSn, NMR and LC–NMR in characterization of stress degradation products: Application to cilazapril

Abstract

Forced degradation studies on cilazapril were carried out according to ICH and WHO guidelines. Significant degradation of the drug was observed in acid and base conditions, resulting primarily in cilazaprilat. In neutral condition, five degradation products were formed, while under oxidative condition, two degradation products were generated. In total, seven degradation products were formed, which were separated on an Inertsil C-18 column using a stability-indicating HPLC method. Structure elucidation of the degradation products was done by using sophisticated and hyphenated tools like, LC–MS/TOF, LC–MSn, on-line H/D exchange, LC–NMR and NMR. Initially, comprehensive mass fragmentation pathway of the drug was laid down. Critical comparison of mass fragmentation pathways of the drug and its hydrolytic degradation products allowed structure characterization of the latter. 1D and 2D proton LC–NMR studies further confirmed the proposed structures of hydrolytic degradation products. The oxidative degradation products could not be characterized using LC–MS and LC–NMR tools. Hence, these degradation products were isolated using preparative HPLC and extensive 1D (1H, 13C, DEPT) and 2D (COSY, TOCSY, HETCOR and HMBC) NMR studies were performed to ascertain their structures. Finally, degradation pathways and mechanisms of degradation of the drug were outlined.