Metoprolol: solid forms of a top selling antihypertensive†
From an investigation concerning the polymorphism of a metoprolol base, a top selling β-blocker antihypertensive drug, a new polymorph, form II, was identified and characterized by differential scanning calorimetry, X-ray powder diffraction and infrared spectroscopy. Form II, Tfus = 36.3 ± 0.2 °C, has a monotropic relationship with the usual commercial form I, Tfus = 51.1 ± 0.2 °C, but was recurrently obtained by melt-crystallization. These results point out that, due to the low melting temperatures involved, if the correct storage of form I is not ensured, it is possible that solid samples have their physicochemical properties modified due to polymorphic changes. Additionally, as a result of screening of metoprolol multicomponent solid forms, a 1 : 1 metoprolol : nicotinic acid (B3 vitamin) salt was synthetized and its crystalline structure was solved. In this structure, the cation conformation is similar to those in the succinate and fumarate salts, and clusters of four hydrogen bonded ions pack along the b axis. This salt is a promising candidate for formulations due to both its adequate thermal behavior and vitamin benefits.