The new idea of drug solid form screening|Experimental & predictive drug crystal form screening

API's solid-state research should be carried out throughout the entire drug development phase, which can, on the one hand, avoid risks by selecting the superior crystalline form for the solid form of drugs at the early stage of drug development; for generic drug companies, by developing different drug crystalline forms and requesting patent protection, they can bypass the patent barrier of the original drug companies and find another way to win a place in the market in the competition with the original drug companies.

|  Crystallographic screening approaches:


|   Pure experimental crystalline form screening
The traditional screening process for solid forms of drugs is to set up a large number of screening tests to obtain different potential solid forms of small molecule drugs and then characterize the physicochemical properties separately [1]. Based on a large number of characterization data, including melting point, hygroscopicity, solubility, stability dissolution, process feasibility, etc., the superior crystalline forms are screened. The single crystal incubation conditions of the relevant crystalline forms are then optimized to obtain the crystallographic data of the corresponding crystalline forms to prepare for the declaration. From the characterization of the solid form to the declaration, the drug has to go through a long waiting period to explore the relevant properties of the drug and the single crystal culture of the dominant crystal form, which will undoubtedly slow down the progress of the whole drug development.

|  Experimental + Predictive Crystallographic Screening
As research has intensified in recent years, more and more cases have shown that different crystal structures exhibit different properties. This suggests that we may be able to make reasonable predictions on the physicochemical properties, relative stability, and process feasibility of the corresponding solid form by crystal structure at the early stage of screening, and thus speed up the drug development. For example, the α-crystalline form of indomethacin is easier to compress than the γ-crystalline form because the crystal structure of the γ-crystalline form contains a slip surface, which enhances the ability of the molecule to compress and deform to a certain extent. Whereas, the α-crystalline form shows superior compressibility because of the tighter molecular arrangement [2].

Figure 3. The molecular arrangement of α and γ crystalline forms of indomethacin and experimental results of pressure test [2].

Also, based on the crystal structures of different drug crystalline forms, the Gibbs free energy of the corresponding crystalline forms can be calculated, and the relative stability of the crystalline forms can be inferred from the Gibbs free energy, which is also of guidance for the selection of the dominant crystalline form [3]. For example, there are five polycrystalline forms (FI, FV, FIV, FII, and FIII) of sulfathiazole (a typical antibacterial drug), and the crystalline form with the lowest Gibbs free energy under specific conditions can be calculated based on the crystal structure data of each crystalline form so that the appropriate dominant crystalline form can be selected according to the application scenario of the drug.

Figure 4. (a) Crystal structure data of polycrystalline form of sulfathiazole, (b) crystal structure of the polycrystalline form of sulfathiazole, (c) relative crystal stability of polycrystalline form of sulfathiazole at 300 K.

| MicroED solves the key bottleneck of "experiment + prediction" crystal screening
However, at the early stage of drug development, it is rarely possible to obtain perfect crystals of each solid form to achieve single-crystal resolution. MicroED technology can directly analyze the structure of powder crystals at the early stage of drug development without the process of single-crystal culture, which is undoubtedly the strongest help for drug development companies and has far-reaching and broad application potential.

MicroED (Micro Electron Diffraction) is a technique that uses cryo-electron microscopy to resolve the structure of tiny crystals, which is difficult to be handled by X-ray crystallography because the electron beam is much stronger than X-ray.

In the field of solid-state studies of drug small molecules, MicroED can be used as a new tool to facilitate solid form screening. Due to the high efficiency and universality of the test, MicroED is able to resolve the structure of the tiny crystals obtained at the early stage of screening. The MicroED can predict the physicochemical properties and process feasibility of different drugs based on their solid-state structures, enabling the initial screening of superior crystalline forms and preparing for drug submissions. This will accelerate the solid-state research of small molecule drugs.

|  Conclusion| MicroED solves the key bottleneck of "experiment + prediction" crystal screening
Therefore, we can predict the differences between the physical and chemical properties of different drug solid forms, such as melting point, density, pressure resistance, etc., by resolving the crystal structure of microcrystals at an early stage of drug solid-state development by MicroED. This not only saves the resources consumed in culturing single crystals but also allows for the initial screening of the dominant crystalline form of the drug prior to stability and process feasibility assessment. This avoids excessive investment of large amounts of resources

|  Extended reading: Importance of solid form studies of drugs
The screening of solid forms of drugs is to obtain various types of possible solid forms of drugs by various experimental means, to characterize the physicochemical properties of various forms by various solid-state analysis techniques, and to evaluate the biopharmaceutical performance of the superior forms by a multidisciplinary approach in order to screen out the superior drug crystalline forms that are suitable for production, have high bioavailability, and are convenient for formulation.

It is well known that the solid form of a drug affects the solubility, dissolution rate, melting point, stability, compressibility, and other properties of the Active Pharmaceutical Ingredient (API), which are critical for efficacy and consistency. Therefore, the solid forms of APIs in both APIs and formulations need to be monitored throughout the drug development process to ensure consistency in all properties and bioavailability [4-5]. The solid forms of drug molecules usually include polycrystalline, salt, eutectic, and amorphous forms.  

There is no luckier scenario for a drug development team than that the superior crystal form selected in early pharmacokinetic studies remains free of efficacy or consistency risks due to crystal switching after the product is launched. However, this utopian fantasy is often considered far from reality. In fact, if a change in the solid form of a drug is discovered late in the drug development process, new and more comprehensive poly-crystallization studies, optimization of the crystallization process, and development of new formulations are often required. Once faced with this situation, the drug may be at risk of losing its market. For example, in the case of ritonavir, a drug for the treatment of AIDS, the phenomenon of crystallization occurred during the production process, and more than a year after its launch, it was found that the drug changed from the crystalline form I to crystalline form II, which had a lower solubility than crystalline form I, thus making the effective dose of the drug lower and eventually forcing the drug to be withdrawn from the market [4].

On the other hand, the original pharmaceutical companies can obtain technical protection by applying for patents on drug crystalline forms, which can form a patent barrier to generic companies while extending the protection period of basic patents. For example, for Glaxo's anti-ulcer drug ranitidine (Zantac), the patent for crystal type I expired, and then a crystal type II, which is currently used as a drug, was discovered, and its protection was extended by applying for a new patent [6].

On the one hand, it can avoid the risk by selecting the superior crystalline form for the solid form of the drug at the early stage of drug development; for generic drug companies, by developing different crystalline forms of drugs and requesting patent protection, they can bypass the patent barrier of the original drug companies and find another way to win a place in the market in the competition with the original drug companies.

|  MicroED Technology Research Team
MicroED has been selected as one of the world's top 10 technological breakthroughs in Science 2018.
Currently, the global research team of MicroED technology includes Tamir Gonen from the U.S. and Xiaodong Zou from Stockholm University, Sweden, whose research interests have been in biomolecules, while Xiaodong Zou's team has obtained very fruitful research results in small molecules, such as the structural analysis of zeolites and MOFs. At present, we have also made very good progress in the structural analysis of biomolecules, such as the first structural analysis of unknown proteins by MicroED (DOI:10.1126/sciadv.aax4621). The founding team of ReadCrystal is from the MicroED technology invention group of Stockholm University in Sweden, which has the leading technology level in MicroED. The company has purchased the international advanced MicroED testing platform since then and has solved the structure of dozens of substances by MicroED, covering drug small molecules, proteins, etc. We provide professional and rapid commercial structure analysis services for pharmaceutical companies and research institutions.

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[2]Aitipamula, S et al. Polymorphism in Molecular Crystals and Cocrystals. Advancesin Organic Crystal Chemistry: Comprehensive Reviews 2015, 265-298.
[3]Xuan H; Jinfeng L et al. Crystal Structure Optimization and Gibbs Free EnergyComparison of Five Sulfathiazole Polymorphs by the Embedded Fragment QM Methodat the DFT Level. Crystals 2019, 9, 256-256
[4]Renu C; Anupam S et al. Crystal Structures and Physicochemical Properties ofFour New Lamotrigine Multicomponent Forms. Cryst. Growth Des. 2013, 13, 858−870
[5]Zhenguo G;Sohrab R et al. Recent Developments in theCrystallization Process: Toward the Pharmaceutical Industry. Engineering,2017,3,343-353.
[6]Armas H, Peeters O, et al. Solid state characterization and crystal structurefrom X-ray powder diffraction of two polymorphic forms of ranitidine base. JPharm Sci. 2009 Jan;98, 146-58.