Understanding stress-induced disorder and breakage in organic crystals: beyond crystal structure anisotropy

Efficient determination of critical water activity and classification of hydrate-anhydrate stability relationship

For a pair of hydrated and anhydrous crystals, the hydrate is more stable than the anhydrate when the water activity is above the critical water activity (awc). Conventional methods to determine awc are based on either hydrate-anhydrate competitive slurries at different aw or solubilities measured at different temperatures. However, these methods are typically resource-intensive and time-consuming. Here, we present simple and complementary solution- and solid-based methods and illustrate them using carbamazepine and theophylline. In the solution-based method, awc can be predicted using intrinsic dissolution rate (IDR) ratio or solubility ratio of the hydrate-anhydrate pair measured at a known water activity. In the solid-based method, awc is predicted as a function of temperature from the dehydration temperature and enthalpy obtained by differential scanning calorimetry (DSC) near a water activity of unity. For carbamazepine and theophylline, the methods yielded awc values in good agreement with those from the conventional methods. By incorporating awc as an additional variable, the hydrate-anhydrate relationship is categorized into four classes based on their dehydration temperature (Td) and enthalpy (ΔHd) in analogy with the monotropy/enantiotropy classification for crystal polymorphs. In Class 1 (ΔHd< 0 and Td ≥ 373 K), no awc exists. In Class 2 (ΔHd>0andTd≥373K), awc always exists under conventional crystallization conditions. In Class 3 (ΔHd<0andTd<373K), awc exists when T>Td. In Class 4 (ΔHd>0andTd<373K), awc exists only when T Collapse

Key Words

• DSC
• IDR
• and dehydration enthalpy
• classification
• critical water activity
• hydrates
• solubility
• stability relationship

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Affiliation(s)

Xin Yao Research & Development, AbbVie Inc., North Chicago, IL 60064, United States.
Tianyi Xiang Pharmaceutical Materials Science and Engineering Laboratory, Department of Pharmaceutics, College of Pharmacy, University of Minnesota, Minneapolis, Minnesota, MN 55455, United States
Shuang Chen Research & Development, AbbVie Inc., North Chicago, IL 60064, United States
Busayo D Alagbe Research & Development, AbbVie Inc., North Chicago, IL 60064, United States
Geoff G Z Zhang Research & Development, AbbVie Inc., North Chicago, IL 60064, United States; Department of Industrial and Molecular Pharmaceutics (IMPH) School of Pharmacy, Purdue University, West Lafayette, IN 47907, United States
Richard S Hong Research & Development, AbbVie Inc., North Chicago, IL 60064, United States
Changquan Calvin Sun Pharmaceutical Materials Science and Engineering Laboratory, Department of Pharmaceutics, College of Pharmacy, University of Minnesota, Minneapolis, Minnesota, MN 55455, United States
Lian Yu School of Pharmacy, University of Wisconsin-Madison, Madison, Wisconsin 53705, United States
Ahmad Y Sheikh Research & Development, AbbVie Inc., North Chicago, IL 60064, United States

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A data-driven and topological mapping approach for the a priori prediction of stable molecular crystalline hydrates

Predictions of the structures of stoichiometric, fractional, or nonstoichiometric hydrates of organic molecular crystals are immensely challenging due to the extensive search space of different water contents, host molecular placements throughout the crystal, and internal molecular conformations. However, the dry frameworks of these hydrates, especially for nonstoichiometric or isostructural dehydrates, can often be predicted from a standard anhydrous crystal structure prediction (CSP) protocol. Inspired by developments in the field of drug binding, we introduce an efficient data-driven and topologically aware approach for predicting organic molecular crystal hydrate structures through a mapping of water positions within the crystal structure. The method does not require a priori specification of water content and can, therefore, predict stoichiometric, fractional, and nonstoichiometric hydrate structures. This approach, which we term a mapping approach for crystal hydrates (MACH), establishes a set of rules for systematic determination of favorable positions for water insertion within predicted or experimental crystal structures based on considerations of the chemical features of local environments and void regions. The proposed approach is tested on hydrates of three pharmaceutically relevant compounds that exhibit diverse crystal packing motifs and void environments characteristic of hydrate structures. Overall, we show that our mapping approach introduces an advance in the efficient performance of hydrate CSP through generation of stable hydrate stoichiometries at low cost and should be considered an integral component for CSP workflows.

Breakage Assessment of Lath-Like Crystals in a Novel Laboratory-Scale Agitated Filter Bed Dryer

Agitated filter bed dryer is often the equipment of choice in the pharmaceutical industry for the isolation of potent active pharmaceutical ingredients (API) from the mother liquor and subsequent drying through intermittent agitation. The use of an impeller to promote homogeneous drying could lead to undesirable size reduction of the crystal product due to shear deformation induced by the impeller blades during agitation, potentially causing off-specification product and further downstream processing issues. An evaluation of the breakage propensity of crystals during the initial development stage is therefore critical. A new versatile scale-down agitated filter bed dryer (AFBD) has been developed for this purpose. Carbamazepine dihydrate crystals that are prone to breakage have been used as model particles. The extent of particle breakage as a function of impeller rotational speed, size of clearance between the impeller and containing walls and base, and solvent content has been evaluated. A transition of breakage behaviour is observed, where carbamazepine dihydrate crystals undergo fragmentation first along the crystallographic plane [00l]. As the crystals become smaller and more equant, the breakage pattern switches to chipping. Unbound solvent content has a strong influence on the breakage, as particles break more readily at high solvent contents. The laboratory-scale instrument developed here provides a tool for comparative assessment of the propensity of particle attrition under agitated filter bed drying conditions.

Crystal surface defects as possible origins of cocrystal dissociation

Atomic force microscopy is used as a characterisation tool to investigate cocrystal dissociation under high relative humidity. Caffeine–glutaric acid as a model system showed possible role of crystal surface defects in the process of cocrystal dissociation.