Identification and characterization of potential impurities of amlodipine maleate

Six impurities ranging from 0.43 to 1.42% in amlodipine maleate were detected by a simple isocratic reverse-phase high performance liquid chromatography (HPLC). LC–MS was performed to identify the mass of the impurities. Based on the spectral data (IR, NMR and MS), the structures of these impurities were characterized as 3-ethyl 5-methyl 4-(2-chlorophenyl)-2-[2-(1,3-dioxo-2,3-dihydro-1H-2-isoindolyl)ethoxymethyl]-6-methyl-1,4-dihydro-3,5-pyridinedicarboxylate (impurity I); 5-ethyl 3-methyl 4-(2-chlorophenyl)-2-methyl-6-[2-(2-methylcarbamoylphenyl-carboxamido)ethoxymethyl]-1,4-dihydro-3,5-pyridinedicarboxylate (impurity II); besylate salt of 3-ethyl 5-methyl 2-(2-aminoethoxymethyl)-4-(2-chlorophenyl)-6-methyl-3,5-pyridinedicarboxylate (impurity III); dimethyl 2-(2-aminoethoxymethyl)-4-(2-chlorophenyl)-6-methyl-1,4-dihydro-3,5-pyridinecarboxylate (impurity IV); 3-ethyl 5-methyl 2-(2-aminoethoxymethyl)-4-(4-chlorophenyl)-6-methyl-1,4-dihydro-3,5-pyridinedicarboxylate (impurity V); diethyl 2-(2-aminoethoxymethyl)-4-(2-chlorophenyl)-6-methyl-1,4-dihydro-3,5-pyridinedicarboxylate (impurity VI).