Only one crystal form of metoprolol succinate (Form I) was reported during previous researches and production. In this study, a new polymorph of metoprolol succinate (here named as Form II) was discovered and investigated by X-ray diffraction, thermoanalysis and infrared spectroscopy. The results show its crystal structure and thermal properties are significantly different with Form I. Compared with Form I, Form II exhibits specific diffraction pattern, lower melting temperature and weaker hydrogen bond effect. The thermostability testing suggests Form II is a metastable crystal form and will gradually transform into Form I undergoing treatment of high temperature and humidity. Metastable crystal form generally has better dissolubility than its corresponding stable form. Subsequent measurement also verified that Form II can dissolve in the water more quickly than Form I.
Keywords: Hydrogen bond; metoptolol succinate; polymorphic form; thermostability.