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).