Identification, Characterization, and the Determination of Process-related Impurities in Minodronic Acid Bulk Drug

Background: Minodronic acid (MA) is a third-generation nitrogen-containing heterocyclic bisphosphonate used to treat osteoporosis. In the process of starting materials research and preparation, the key intermediate impurities and degradation impurities have a great impact on the quality control of the drug. Objective: A sensitive, reliable, high-performance liquid chromatography (HPLC) method was developed and validated for the quantitative determination of MA and its related impurities (a total of 6 compounds, including 2 new impurities). Methods: The separation was achieved on an InertSustain ODS-4 C18 (250 mm × 4.6 mm, 5 μm) column using the mixture of 0.01 mol/L sodium pyrophosphate and 1 mmol tetrabutylammonium phosphate (the mobile phase pH was adjusted to 7.80 by phosphonic acid). Results: The quantitative analytical method was fully validated with respect to linearity (r > 0.999), sensitivity (limit of detection < 35 ng/mL), precision, accuracy (the recovery was between 98.7% and 104.2%), and robustness. Six process-related impurities in Minodronic Acid (MA) bulk drug were determined by high-performance liquid chromatography (HPLC). Furthermore, except for two starting materials, other four impurities were identified and characterized as 2-(imidazo[1,2-a] pyridin-3-yl) ethyl acetate (Imp-C), 2-(imidazo [1,2-a] pyridin- 3- yl)acetic acid (Imp-D), 3-(2-hydroxy-2,2- diphosphonoethyl)-4H-imidazo [1,2-a] pyridine -4- oxide (Imp-E) and 2,5- Dihydroxy- 3,6-bis(imidazo[1,2-a] pyridine-3-yl methyl) -2,5-dioxo- 1,4,2,5- dioxoDiphosphonium-3,6-diyl) bisphosphonic acid (Imp-F) using liquid chromatograph- mass spectrometer (LC-MS), MS/MS, Infrared Radiation and Nuclear Magnetic Resonance spectroscopy (1H-NMR and 13C-NMR). To the best of our knowledge, two of them (Imp- E and Imp-F) are new compounds and have not been reported previously. Conclusion: The HPLC method was developed and optimized, which could be applied for quantitative detection of the impurities, and further quality evaluation of MA.