Green Mechanochemical Strategy for the Discovery and Selective Preparation of Polymorphs of Active Pharmaceutical Ingredient γ-Aminobutyric Acid (GABA)

Abstract

This study exploited a mechanochemical strategy to discover new polymorphs of a drug and meanwhile realize the selective preparation of various polymorphs without using bulk solvents. It is worth noting that in practice, the mechanochemical approach is a unique green and highly efficient method. On the one hand, about 20 years after the last discovery of the γ-aminobutyric acid (GABA) polymorph, we identified a new GABA polymorph (Form-III) by mechanochemical milling. Form-III is available exclusively by milling at present, and its crystal structure is also determined by crystal structure prediction (CSP) methods. On the other hand, through introducing a trace amount of solvents with different hydrogen bond donor/acceptor (α/β) abilities, we can achieve selective control of three GABA polymorphs during the milling process, which is not accessible by the traditional solution-based method. The mechanism of trace solvent-directed polymorphic outcomes was investigated from the change of the stability relationship between different polymorphs under milling conditions. As the milling proceeds, the crystal size decreases and the surface effect becomes significant so that the surface stability will dominate the overall stability. The adsorption of solvents with different α and β values on the crystal surface will affect the surface stability of various GABA polymorphs. Consequently, the polymorph with higher stability is able to survive under milling conditions.