X-ray Absorption Near-Edge Spectroscopy Analysis of Indomethacin in Crystalline Forms and in Amorphous Solid Dispersions

Characterization of Bisphosphonate Hydrate Crystals by Phosphorus K-Edge X-Ray Absorption Near-Edge Structure Spectroscopy

X-ray absorption near-edge structure (XANES) spectroscopy is a new method for the characterization of active pharmaceutical ingredients. XANES spectra show unique features depending on the electronic states of the X-ray absorbing elements and provide information about the chemical environment that affects the electronic states. In this study, six bisphosphonate hydrate crystals were used to investigate, for the first time, how the phosphorus K-edge XANES spectra are affected by the interatomic interactions and charged states of phosphonate moieties. Phosphorus K-edge XANES spectra showed several differences among the bisphosphonates. In particular, the chlorine atoms covalently bonded near the phosphonate and the number of electric charges of the phosphonate moieties seemed to have large effects on peak shape in XANES spectra. Unique shapes of the XANES spectra demonstrated that differences in interactions at the oxygen atoms of the phosphonate moieties could change the shapes of the XANES spectrum peaks to the extent that each material was distinguished based on the spectra. Since slight differences in interatomic interactions and charged states lead to variations in the spectra, XANES spectroscopy could be widely applied as the fingerprint method to evaluate active pharmaceutical ingredients.

The applications of machine learning to predict the forming of chemically stable amorphous solid dispersions prepared by hot-melt extrusion

Amorphous solid dispersion (ASD) is one of the most important strategies to improve the solubility and dissolution rate of poorly water-soluble drugs. As a widely used technique to prepare ASDs, hot-melt extrusion (HME) provides various benefits, including a solvent-free process, continuous manufacturing, and efficient mixing compared to solvent-based methods, such as spray drying. Energy input, consisting of thermal and specific mechanical energy, should be carefully controlled during the HME process to prevent chemical degradation and residual crystallinity. However, a conventional ASD development process uses a trial-and-error approach, which is laborious and time-consuming. In this study, we have successfully built multiple machine learning (ML) models to predict the amorphization of crystalline drug formulations and the chemical stability of subsequent ASDs prepared by the HME process. We utilized 760 formulations containing 49 active pharmaceutical ingredients (APIs) and multiple types of excipients. By evaluating the built ML models, we found that ECFP-LightGBM was the best model to predict amorphization with an accuracy of 92.8%. Furthermore, ECFP-XGBoost was the best in estimating chemical stability with an accuracy of 96.0%. In addition, the feature importance analyses based on SHapley Additive exPlanations (SHAP) and information gain (IG) revealed that several processing parameters and material attributes (i.e., drug loading, polymer ratio, drug's Extended-connectivity fingerprints (ECFP) fingerprints, and polymer's properties) are critical for achieving accurate predictions for the selected models. Moreover, important API's substructures related to amorphization and chemical stability were determined, and the results are largely consistent with the literature. In conclusion, we established the ML models to predict formation of chemically stable ASDs and identify the critical attributes during HME processing. Importantly, the developed ML methodology has the potential to facilitate the product development of ASDs manufactured by HME with a much reduced human workload.

Comparison of the physical properties of disodium etidronate amorphous forms prepared by different manufacturing methods

Amorphous forms of disodium etidronate were prepared by three manufacturing methods, heat drying, freeze drying, and anti-solvent precipitation, and the effects of these methods on the physical properties of disodium etidronate amorphous forms were evaluated for the first time. Variable temperature X-ray powder diffraction and thermal analyses revealed that these amorphous forms had different physical properties such as glass transition point, water desorption, and crystallization temperatures. These differences can be explained by the molecular mobility and water content in amorphous forms. The differences in the structural characteristics related to the differences in these physical properties could not be detected clearly by the spectroscopic methods like Raman spectroscopy and X-ray absorption near-edge spectroscopy. Dynamic vapor sorption analyses demonstrated that all amorphous forms were hydrated to form I, a tetrahydrated form, at above 50% relative humidity, and the transition to form I was irreversible. These amorphous forms require strict humidity control to avoid crystallization. Among the three amorphous forms of disodium etidronate, the amorphous form prepared by heat drying was the most suitable for manufacturing the solid formulation, considering the low water content and low molecular mobility.

Analysis of Cimetidine Crystal Polymorphs by X-ray Absorption Near-Edge Spectroscopy

Sulfur K-edge X-ray absorption near-edge spectroscopy (XANES) measurements were performed to characterize the crystal polymorphs of the active pharmaceutical ingredients (APIs) containing sulfur atoms. Cimetidine (CIM) was used as a model API. Each crystal form of CIM has its own XANES spectrum, so we can discriminate the crystal form by its spectrum. The analysis of the crystal structure of CIM revealed that the difference in the shape of XANES spectra was ascribable to the difference in the C-S-C bond angle of CIM molecules and the intermolecular hydrogen bonds, such as C-H···S and N-H···S, and S-S interaction. It was found that the peak shape of the XANES spectrum is gentle when the C-S-C bond angle is large, while the peak shape can be steep when the C-S-C bond angle is small. Furthermore, it was found that the peak energy values varied depending on the hydrogen bonds and S-S interaction. By linear combination fitting using XANES spectra, it was possible to quantify the ratio of CIM form A crystal in mixed powders of form A and monohydrate crystals. These results indicate that XANES measurements can be a useful technique to evaluate the crystal polymorphism of APIs containing S atom in pharmaceutical formulation.

Characterization of Ambroxol and Its Hydrochloride Salt Crystals by Bromine K-Edge X-Ray Absorption Near-Edge Structure Spectroscopy and X-Ray Crystal Structure Analysis

Polymorphic crystals of ambroxol, forms I and II, and form A ambroxol hydrochloride crystals were characterized with bromine K-edge X-ray absorption near-edge structure (XANES) spectroscopy and single-crystal X-ray structure analysis. The XANES spectra had unique shapes depending on the crystal forms. Refined single-crystal structures revealed different interatomic interactions around bromine atoms, such as C-H…Br and N-H…Br hydrogen bonds, Br…O halogen bonds, and N-H…π interactions. Differences in these weak interactions could affect the electronic states of the bromines, resulting in differences in the XANES spectra. The results demonstrated that weak non-conventional interatomic interactions could alter the shape of XANES spectra. Hence, the spectra could be used for evaluating polymorphs of active pharmaceutical ingredients.

C-H▪▪▪S Hydrogen Bonds in Ampicillin and Amoxicillin Crystals Investigated by Sulfur K-Edge X-Ray Absorption Near-Edge Structure Spectroscopy and Single-Crystal X-Ray Structure Analysis

Sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy was evaluated for its ability to detect non-conventional C-H▪▪▪S hydrogen bonds in crystals of the sulfur-containing penam antibiotics ampicillin and amoxicillin. The XANES spectra of the nearly isomorphous crystals of ampicillin trihydrate and amoxicillin trihydrate were very similar, whereas that of ampicillin anhydrate displayed unique features. Single-crystal X-ray structure analyses revealed that the C-H▪▪▪S hydrogen bond geometries and the chemical types of the hydrogen donors differed between the isomorphous trihydrate crystals and ampicillin anhydrate crystal. These observations demonstrate that the shapes of the sulfur K-edge XANES spectra are dependent on the nature of the C-H▪▪▪S hydrogen bonds. Sulfur K-edge XANES spectroscopy shows promise for use in the detection and analysis of non-covalent interactions, including hydrogen bonds to sulfur atoms, within active pharmaceutical ingredients.

Direct detection of the crystal form of an active pharmaceutical ingredient in tablets by X-ray absorption fine structure spectroscopy

Different crystal forms of active pharmaceutical ingredients (APIs) may display variations in physicochemical properties. During the drug development process, the definitive purpose is to maintain homogeneous quality in a single crystalline form. Hence, it is important to evaluate and understand the properties of each crystal form of APIs in pharmaceutics. In this study, forms 0, Ⅰ, Ⅱ, III of bromhexine hydrochloride, and form S of bromhexine were characterized by the commonly used methods X-ray powder diffraction, thermogravimetry-differential thermal analysis, and single crystal structure X-ray diffraction. Additionally, X-ray absorption fine structure spectroscopy (XAFS), a seldom used method in the pharmaceutics discipline was also applied to explore the chemical environment of bromine atoms in forms 0, Ⅰ, Ⅱ and S as well as chloride ions in forms 0 to Ⅱ. The XAFS spectra of each form were different from each of the other forms which indicated the chemical environment around target elements in the crystal polymorphs were distinct. Then, we measured the commercial bromhexine hydrochloride tablets with XAFS measurement and found that XAFS could distinguish the crystal form in the tablets. Hence, we demonstrated that XAFS measurements would be applicable as one of the methods for the direct detection of APIs in the tablets.

Bromine K-Edge X-Ray Absorption Near-Edge Structure Analysis on Hydrobromide-Salt Crystals and the Solid Dispersion of Active Pharmaceutical Ingredients

Bromine K-edge X-ray absorption near-edge structure (XANES) spectroscopy analyses were used to evaluate the crystals of the active pharmaceutical ingredients, eletriptan hydrobromide, dextromethorphan hydrobromide and scopolamine hydrobromide salts and the solid dispersion of eletriptan hydrobromide. The crystals and the solid dispersion of the active pharmaceutical ingredient (API) salts could be discriminated based on the shape of the XANES spectra. The differences in the shape of XANES spectra was ascribable to the differences in the interatomic interactions of the bromine ions based on the crystal structures. Ratio of the eletriptan hydrobromide α-form crystal in mixed powders of α-form and monohydrate crystals could be quantified by the linear-combination fitting using their XANES spectra. These results indicated that the XANES spectroscopy are a potent method for evaluating the APIs of pharmaceutical formulations even at the higher energy region around the bromine K-edge of 13470 eV.