Estimation of impurity profiles of drugs and related materials Part 15. Identification of minor impurities in cimetidine

Abstract Besides several known impurities in cimetidine, two additional compounds at levels below 0.1% were detected by ion-pair reversed-phase high-performance liquid chromatography (HPLC). The impurities were isolated from crude cimetidine using normal-phase preparative HPLC. 1 H and 13 C NMR and mass spectrometric investigations revealed the structures of the impurities to be 2,5-bis[( N ′-cyano- N ″-methyl)guanidinoethylthiomethyl]-4-methylimidazole (VII) and 1,8-bis[( N ′-cyano- N ″-methyl)guanidino]-3,6-dithiaoctane (VIII). These structures were verified by synthesis of the impurities and comparison of the spectra and chromatographic (HPLC and TLC) retention data of the isolated and synthesized materials.