Efficient mechanochemistry of beta blockers: neutralization, salification, and effect of liquid additives

Abstract

Beta blockers are a class of ubiquitous cardiovascular drugs that have collectively received little attention from a crystal engineering standpoint. Here, we describe the use of mechanochemistry in the salification of five beta blockers (propranolol, metoprolol, acebutolol, atenolol, and labetalol) with nicotinic and isonicotinic acid. Firstly, liquid assisted grinding (LAG) was used to neutralize the commercial beta blocker salts, enabling the efficient gram-scale formation of the free bases, which are essential for cocrystallization. Thereafter, 1 : 1 mechanochemical cocrystallizations were successful in all but one case and nine salts were characterized, eight of which are novel. Furthermore, the racemic free base crystal structure of acebutolol is reported for the first time, as well as the first multicomponent crystal of labetalol that is not a simple salt. Salification was enabled by the large pK_a differences between the components, which facilitated the protonation of the basic amine on the beta blockers' alkanolamine skeleton. Thereafter, charge-assisted hydrogen bonding promoted cocrystallization. We envisage salification to be applicable to any beta blocker, considering the current study encompasses approximately one quarter of this drug class. Lastly, the role of different liquid additives in the LAG process was assessed, and the solvent identity was found to play a substantial role in the mechanochemical outcome, although it did not strictly correlate with polarity. This study demonstrates that LAG screening with a wide selection of solvents provides a path to achieve full conversion to products, explore the crystal landscape of multicomponent crystals, and assist in identifying additional phases and/or late stage polymorphs in solid form development.