Solid state characterization on active pharmaceutical ingredients (APIs) is essential, as most drug products are developed, formulated and delivered in solid forms1, 2. APIs can exist in multiple solid forms, such as polymorphs, salts, cocrystals, and inclusion complexes, while these forms often display distinct physicochemical properties that directly affect their biopharmaceutical behaviours1. For decades, X-ray crystallography has been the primary technique in structural chemistry, including single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD) 3. However, for SCXRD, its applicability is limited by the requirement for large and well-diffracted crystals4, whereas PXRD pattern is frequently affected by peak overlap, crystal preferred orientation and crystalline phase purity, which hinder direct structure solution5, leaving the structures of many potential pharmaceuticals unresolved.
Recently, 3D electron diffraction (3D ED), also referred to microcrystal electron diffraction (MicroED), has emerged as a powerful technique for structure elucidation of micro- and nanocrystals, including macromolecules, zeolites, metal–organic frameworks (MOFs), and pharmaceuticals6. Benefiting from the strong interaction between electrons and matter, 3D ED enables crystal structure determination from extremely tiny crystals, up to 106 times smaller than those required for conventional SCXRD. In addition, 3D ED enables compositional analysis through high-throughput, automated workflows7, highlighting its broad applicability in pharmaceutical solid-form screening.
This work focuses on structure determination and phase analysis of those pharmaceuticals that crystallise as very thin crystals and complex spherulitic structures that are inaccessible to conventional X-ray crystallography. In the first example, the crystal structures of a non-steroidal anti-inflammatory drug indomethacin’s δ form were determined using 3D electron diffraction, revealing that the crystals obtained from melt crystallisation and that obtained from solution crystallisation are in fact two distinct polymorphs, a misconception that persisted for 47 years8.
In the second example, compact spherulitic polymorphs of anticancer drug vemurafenib were directly solved by 3D ED, without the need for single-crystal growth, significantly broadening the scope of pharmaceutical solids9.
In the third example, an automated 3D ED protocol was implemented to perform a phase analysis to a complicated multi-phasic drug-polymer system, antifungal drug griseofulvin (GSF) and polyethylene glycol (PEG). Batch data collection (over 230 datasets) using EPU-D (ThermoFisher Scientific), a widely available commercial software, combined with automated cluster-based data analysis10, enabled data acquisition and processing within three days, leading to the discovery of an elusive new GSF–PEG inclusion complex from an unstable multi-phasic system11.
Together, these advances prove 3D ED as a powerful technique for pharmaceutical solid-form screening and discovery. Future integration with high-throughput sample handling, automation and AI-assisted data analysis in 3D ED is expected to further accelerate solid-form discovery pipelines in drug development.