Liquid chromatographic/electrospray ionization mass spectrometric identification of the oxidation end‐products of metformin in aqueous solutions

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Abstract
Metformin is an antihyperglycemic drug that exhibits some antioxidant properties. HO•-induced oxidation of metformin was studied in aqueous solution, in both aerated and deaerated conditions. Gamma radiolysis of water was used to generate HO• free radicals, capable of initiating one-electron oxidation of metformin. Oxidation end-products were identified by direct infusion mass spectrometry (MS) and high-performance liquid chromatography/mass spectrometry (HPLC/MSn): for every product, structure elucidation was based on its mass (simple mass spectra confirmed by HPLC/MS). In addition, fragmentation spectra (MS2, MS3 and MS4) and the determination of deuterium–hydrogen exchange sites provided valuable information allowing the complete identification of some of the end-products. At low radiation dose, four products were identified as primary ones, since they result from the direct attack of HO• radicals on metformin. These primary oxidation end-products were identified respectively as hydroperoxide of metformin, covalent dimer of metformin, methylbiguanide and 2-amino-4-imino-5-methyl-1,3,5-triazine. At high radiation dose, seven other products were identified as secondary ones, resulting from the HO•-induced oxidation of the primary end-products. A reaction scheme was postulated for the interpretation of the results. Copyright © 2004 John Wiley & Sons, Ltd.