Supramolecular Organization of Nonstoichiometric Drug Hydrates: Dapsone

Investigation of the State of Hydration of a Non-stoichiometric Hydrate in a Low Dose Formulation Using 19F Solid-State NMR

A variable or non-stoichiometric hydrate of GDC-4379 was developed into a formulated capsule with a 1% drug loading. The water content of this hydrate varied from 0-0.7 moles over the relative humidity (RH) range of 0-98% (25°C). Since a variable state of hydration coupled with rapid equilibration of lattice water with the environmental RH can lead to challenges in formulation development, an analytical method to directly and accurately determine the state of hydration of the active in such a low dose formulation was deemed necessary. Owing to its high selectivity and fast acquisition times, 19F solid-state NMR was effectively utilized to directly determine the lattice water content of the active in the formulated capsule. By correlatingΔδ, the chemical shift difference between the isotropic peaks, with the relative humidity and ultimately the lattice water content, the state of hydration of GDC-4379 in the formulated capsule was experimentally determined as 0.63 moles of water/mole of anhydrate.

Flavone Cocrystals: A Comprehensive Approach Integrating Experimental and Virtual Methods

The dapsone/flavone cocrystal system served as a benchmark for both experimental and virtual screening methods. Expanding beyond this, two additional active pharmaceutical ingredients (APIs), sulfanilamide and sulfaguanidine, structurally related to dapsone were chosen to investigate the impact of substituents on cocrystal formation. The experimental screening involved mechanochemical methods, slurry experiments, hot-melt extrusion, and the contact preparation method. The virtual screening focused on crystal structure prediction (CSP), molecular complementarity, hydrogen-bond propensity, and molecular electrostatic potentials. The CSP studies not only indicated that each of the three APIs should form cocrystals with flavone but also reproduced the known single- and multicomponent phases. Experimentally, dapsone/flavone cocrystals ACC, BCC, CCC, and DCC were reproduced, CCC was identified as a nonstoichiometric hydrate, and a fifth cocrystal (ECC), a t-butanol solvate, was discovered. The cocrystal polymorphs ACC and BCC are enantiotripically related, and DCC, exhibiting a different stoichiometric ratio, is enthalpically stabilized over the other cocrystals. For the sulfaguanidine/flavone system, two novel, enantiotripically related cocrystals were identified. The crystal structures of two cocrystals and a flavone polymorph were solved from powder X-ray diffraction data, and the stability of all cocrystals was assessed through differential scanning calorimetry and lattice energy calculations. Despite computational indications, a diverse array of cocrystallization techniques did not result in a sulfanilamide/flavone cocrystal. The driving force behind dapsone's tendency to cocrystallize with flavone can be attributed to the overall strength of flavone interactions in the cocrystals. For sulfaguanidine, the potential to form strong API···API and API···coformer interactions in the cocrystal is a contributing factor. Furthermore, flavone was found to be trimorphic.

Mechanosynthesis of Stable Salt Hydrates of Allopurinol with Enhanced Dissolution, Diffusion, and Pharmacokinetics

Allopurinol (ALO) is a medication that treats gout and kidney stones by lowering uric acid synthesis in the blood. The biopharmaceutics classification system (BCS) IV drug exhibits poor aqueous solubility, permeability, and bioavailability. To overcome the bottlenecks of ALO, salts with maleic acid (MLE) and oxalic acid (OXA) were synthesized using the solvent-assisted grinding method. The novel multicomponent solids were characterized by PXRD, DSC, TGA, FT-IR, and SEM images. The crystal structures of these salts with variable stoichiometry were obtained using Rietveld refinement from the high-resolution PXRD data. The proton from the dicarboxylic acid is transferred to the most basic pyrimidine "N" of ALO. The N-H···N hydrogen-bonded ALO homodimer is replaced by the N+-H···O- ionic interactions in ALO-OXA (2:1:0.4) and ALO-MLE (1:1:1) salt hydrates. The organic salts improved solubility and dissolution up to 5-fold and the diffusion permeability up to 12 times compared to the native drug in a luminal pH 6.8 phosphate buffer medium. The salt hydrates were exceptionally stable during storage at 30 ± 5 °C and 75 ± 5% relative humidity. Superior dissolution and diffusion permeability of the ALO-MLE salt resulted in improved pharmacokinetics (peak plasma concentration) that offers a promising solid dosage form with enhanced bioavailability and lower dosage formulation.

Exploring the Supramolecular Interactions and Thermal Stability of Dapsone:Bipyridine Cocrystals by Combining Computational Chemistry with Experimentation

The application of computational screening methodologies based on H-bond propensity scores, molecular complementarity, molecular electrostatic potentials, and crystal structure prediction has guided the discovery of novel cocrystals of dapsone and bipyridine (DDS:BIPY). The experimental screen, which included mechanochemical and slurry experiments as well as the contact preparation, resulted in four cocrystals, including the previously known DDS:4,4'-BIPY (2:1, CC₄₄-B) cocrystal. To understand the factors governing the formation of the DDS:2,2'-BIPY polymorphs (1:1, CC₂₂-A and CC₂₂-B) and the two DDS:4,4'-BIPY cocrystal stoichiometries (1:1 and 2:1), different experimental conditions (such as the influence of solvent, grinding/stirring time, etc.) were tested and compared with the virtual screening results. The computationally generated (1:1) crystal energy landscapes had the experimental cocrystals as the lowest energy structures, although distinct cocrystal packings were observed for the similar coformers. H-bonding scores and molecular electrostatic potential maps correctly indicated cocrystallization of DDS and the BIPY isomers, with a higher likelihood for 4,4'-BIPY. The molecular conformation influenced the molecular complementarity results, predicting no cocrystallization for 2,2'-BIPY with DDS. The crystal structures of CC₂₂-A and CC₄₄-A were solved from powder X-ray diffraction data. All four cocrystals were fully characterized by a range of analytical techniques, including powder X-ray diffraction, infrared spectroscopy, hot-stage microscopy, thermogravimetric analysis, and differential scanning calorimetry. The two DDS:2,2'-BIPY polymorphs are enantiotropically related, with form B being the stable polymorph at room temperature (RT) and form A being the higher temperature form. Form B is metastable but kinetically stable at RT. The two DDS:4,4'-BIPY cocrystals are stable at room conditions; however, at higher temperatures, CC₄₄-A transforms to CC₄₄-B. The cocrystal formation enthalpy order, derived from the lattice energies, was calculated as follows: CC₄₄-B > CC₄₄-A > CC₂₂-A.

Effect of Vehicle Composition on the Preparation of Different Types of Dapsone Crystals for Topical Drug Delivery

Topical formulations composed of API-pure crystals have been increasingly studied, especially in regards to the impact of particle size in penetration efficiency. Less attention, however, has been devoted to the solid-state properties of drugs delivered to the skin. In this study, we address the effect of formulation composition on the crystal form existing in topical products. Dapsone (DAP) gel formulations were prepared by mixing an organic solution containing DAP with an aqueous solution containing polymers and preservatives. The organic solvent was chosen as ethoxydiglycol (DEGEE), polyethylene glycol (PEG), or 1-methyl-2-pirrolidone (MPR) to assess the impact of composition on DAP crystal form. Such solvent variations resulted in different particulate matter. In terms of crystalline nature, the presence of DEGEE in formulations induced the crystallization of DAP hydrate, while PEG cocrystal and a mixture of hydrate and MPR solvate crystallized from the same amounts of PEG and MPR, respectively. Microscopic analysis of the gels showed heterogeneous particles with different characteristics. The behavior of gels after application to the skin was also tested. Interestingly, the different formulations seemed to accumulate in different regions of the skin. This could be the result of the effect of vehicle composition/excipients on the characteristics of the skin, such as hydration. The site-specific accumulation, however, was more pronounced in crystal-loaded gels as opposed to blank formulations. These results indicate that future studies should consider the effect of formulation composition on the API crystal form landscape as part of the strategies used to successfully target drug delivery to the skin.

Along the road to Crystal Structure Prediction (CSP) of pharmaceutical-like molecules

Computational methods used for predicting crystal structures of organic compounds are mature enough to be routinely used with many rigid and semi-rigid organic molecules. The usefulness of Crystal Structure Prediction...

Pharmaceutical Hydrates Analysis-Overview of Methods and Recent Advances

This review discusses a set of instrumental and computational methods that are used to characterize hydrated forms of APIs (active pharmaceutical ingredients). The focus has been put on highlighting advantages as well as on presenting some limitations of the selected analytical approaches. This has been performed in order to facilitate the choice of an appropriate method depending on the type of the structural feature that is to be analyzed, that is, degree of hydration, crystal structure and dynamics, and (de)hydration kinetics. The presented techniques include X-ray diffraction (single crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD)), spectroscopic (solid state nuclear magnetic resonance spectroscopy (ssNMR), Fourier-transformed infrared spectroscopy (FT-IR), Raman spectroscopy), thermal (differential scanning calorimetry (DSC), thermogravimetric analysis (TGA)), gravimetric (dynamic vapour sorption (DVS)), and computational (molecular mechanics (MM), Quantum Mechanics (QM), molecular dynamics (MD)) methods. Further, the successful applications of the presented methods in the studies of hydrated APIs as well as studies on the excipients' influence on these processes have been described in many examples.

Structural variety of heterosynthons in linezolid cocrystals with modified thermal properties

In a search for new crystalline forms of linezolid with modified thermal properties five cocrystals of this wide range antibiotic with aromatic acids were obtained via mechanochemical grinding and analyzed with single crystal X-ray diffraction, solid-state NMR spectroscopy, powder X-ray diffraction and DSC measurements. The coformers used in this study were benzoic acid, p-hydroxybenzoic acid, protocatechuic acid, γ-resorcylic acid and gallic acid. In each of the cocrystals distinct structural features have been found, including a variable amount of water and different heterosynthons, indicating that there is more than one type of intermolecular interaction preferred by the linezolid molecule. Basing on the frequency of the observed supramolecular synthons, the proposed hierarchy of the hydrogen-bond acceptor sites of linezolid (LIN) is C=O_amide > C=O_oxazolidone > C-O-C_morpholine > C-N-C_morpholine > C-O-C_oxazolidone. In addition, aromatic-aromatic interactions were found to be important in the stabilization of the analyzed structures. The obtained cocrystals show modified thermal properties, with four of them having melting points lower than the temperature of the phase transition from linezolid form II to linezolid form III. Such a change in this physicochemical property allows for the future application of melting-based techniques of introducing linezolid into drug delivery systems. In addition a change in water solubility of linezolid upon cocrystalization was evaluated, but only in the case of the cocrystal with protocatechuic acid was there a significant (43%) improvement in solubility in comparison with linezolid.

Mechanochemical Formation of Racemic Praziquantel Hemihydrate with Improved Biopharmaceutical Properties

Praziquantel (PZQ) is the first-line drug used against schistosomiasis, one of the most common parasitic diseases in the world. A series of crystalline structures including two new polymorphs of the pure drug and a series of cocrystals of PZQ have been discovered and deposited in the Cambridge Structural Database (CSD). This work adds to the list of multicomponent forms of PZQ a relevant example of a racemic hemihydrate (PZQ-HH), obtainable from commercial PZQ (polymorphic Form A) through mechanochemistry. Noteworthy, the formation of the new hemihydrate strongly depends on the initial polymorphic form of PZQ and on the experimental conditions used. The new PZQ-HH has been fully characterized by means of HPLC, Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Hot-Stage Microscopy (SEM), Powder X-Ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), FT-IR, polarimetry, solid-state NMR (SS-NMR), solubility and intrinsic dissolution rate (IDR), and in vitro tests on Schistosoma mansoni adults. The crystal structure was solved from the powder X-ray diffraction pattern and validated by periodic-DFT calculations. The new bioactive hemihydrate was physically stable for three months and showed peculiar biopharmaceutical features including enhanced solubility and a double intrinsic dissolution rate in water in comparison to the commercially available PZQ Form A.

Supramolecular organisation of sulphate salt hydrates exemplified with brucine sulphate

The frequency of hydrate formation among organic sulphate salts is unravelled. Interconversion of the hydrates of brucine sulphate occurs with small changes in the relative humidity.

Intermolecular Interactions in Functional Crystalline Materials: From Data to Knowledge

Intermolecular interactions of organic, inorganic, and organometallic compounds are the key to many composition–structure and structure–property networks. In this review, some of these relations and the tools developed by the Cambridge Crystallographic Data Center (CCDC) to analyze them and design solid forms with desired properties are described. The potential of studies supported by the Cambridge Structural Database (CSD)-Materials tools for investigation of dynamic processes in crystals, for analysis of biologically active, high energy, optical, (electro)conductive, and other functional crystalline materials, and for the prediction of novel solid forms (polymorphs, co-crystals, solvates) are discussed. Besides, some unusual applications, the potential for further development and limitations of the CCDC software are reported.

Experimental and computational approaches to rationalise multicomponent supramolecular assemblies: dapsone monosolvates

The monosolvate crystal energy landscapes of dapsone (DDS) including the solvents carbon tetrachloride, acetone, cyclohexanone, dimethyl formamide, tetrahydrofuran, methyl ethyl ketone, 1,2-dichloroethane, 1,4-dioxane, dichloromethane and chloroform were established using experimental and computational approaches. To rationalise and understand solvate formation, solvate stability and desolvation reactions a careful control of the experimental crystallisation and storage conditions, a range of thermoanalytical methods and crystal structure prediction were required. Six of the eight DDS monosolvates are reported and characterised for the first time. Structural similarity and diversity of the at ambient conditions unstable monosolvates were apparent from the computed crystal energy landscapes, which had the experimental packings as lowest energy structures. The computed structures were used as input for Rietveld refinements and isostructurality of four of the monosolvates was confirmed. Packing comparisons of the solvate structures and molecular properties of the solvent molecules indicated that both size/shape of the solvent molecule and the possible DDSsolvent interactions are the important factors for DDS solvate formation. Through the combination of experiment and theory solvate stability and structural features have been rationalised.

Accurate and efficient representation of intramolecular energy in ab initio generation of crystal structures. II. Smoothed intramolecular potentials

The application of crystal structure prediction (CSP) to industrially relevant molecules requires the handling of increasingly large and flexible compounds. A revised model for the effect of molecular flexibility on the lattice energy that removes the discontinuities and non-differentiabilities present in earlier models (Sugden et al., 2016), with a view to improving the performance of CSP is presented. The approach is based on the concept of computing a weighted average of local models, and has been implemented within the CrystalPredictor code. Through the comparative investigation of several compounds studied in earlier literature, it is shown that this new model results in large reductions in computational effort (of up to 65%) and in significant increases in reliability. The approach is further applied to investigate, for the first time, the computational polymorphic landscape of flufenamic acid for Z' = 1 structures, resulting in the successful identification of all three experimentally resolved polymorphs within reasonable computational time.

Dapsone Form V: A Late Appearing Thermodynamic Polymorph of a Pharmaceutical

Five anhydrate polymorphs (forms I-V) and the isomorphic dehydrate (Hy_dehy) of dapsone (4,4'-diaminodiphenyl sulfone or DDS) were prepared and characterized in an interdisciplinary experimental and computational study, elucidating the kinetic and thermodynamic stabilities, solid form interrelationships, and structural features of the known forms I-IV, the novel polymorph form V, and Hy_dehy. Calorimetric measurements, solubility experiments, and lattice energy calculations revealed that form V is the thermodynamically stable polymorph from absolute zero to at least 90 °C. At higher temperatures, form II, and then form I, becomes the most stable DDS solid form. The computed 0 K stability order (lattice energy calculations) was confirmed with calorimetric measurements as follows, V (most stable) > III > Hy_dehy > II > I > IV (least stable). The discovery of form V was complicated by the fact that the metastable but kinetically stabilized form III shows a higher nucleation and growth rate. By combining laboratory powder X-ray diffraction data and ab initio calculations, the crystal structure of form V ( P2₁/ c, Z' = 4) was solved, with a high energy DDS conformation allowing a denser packing and more stable intermolecular interactions, rationalizing the formation of a high Z' structure. The structures of the forms I and IV, only observed from the melt and showing distinct packing features compared to the forms II, III, and V, were derived from the computed crystal energy landscapes. Dehydration modeling of the DDS hydrate led to the Hy_dehy structure. This study expands our understanding about the complex crystallization behavior of pharmaceuticals and highlights the big challenge in solid form screening, especially that there is no clear end point.

Imaging of dehydration in particulate matter using Raman line-focus microscopy

Crystalline solids can incorporate water molecules into their crystal lattice causing a dramatic impact on their properties. This explains the increasing interest in understanding the dehydration pathways of these solids. However, the classical thermal analytical techniques cannot spatially resolve the dehydration pathway of organic hydrates at the single particle level. We have developed a new method for imaging the dehydration of organic hydrates using Raman line-focus microscopy during heating of a particle. Based on this approach, we propose a new metastable intermediate of theophylline monohydrate during the three-step dehydration process of this system and further, we visualize the complex nature of the three-step dehydration pathway of nitrofurantoin monohydrate to its stable anhydrous form. A Raman line-focus mapping option was applied for fast simultaneous mapping of differently sized and shaped particles of nitrofurantoin monohydrate, revealing the appearance of multiple solid-state forms and the non-uniformity of this particle system during the complex dehydration process. This method provides an in-depth understanding of phase transformations and can be used to explain practical industrial challenges related to variations in the quality of particulate materials.

Explaining crystallization preferences of two polyphenolic diastereoisomers by crystal structure prediction

Crystal structure prediction is used to understand the differences in crystallization of catechin and epicatechin, and to explore the predictability of solvate formation.

Experimental and computational approaches to produce and characterise isostructural solvates

A combination of experiment and theory was applied to rationalise the formation, stability and phase transitions of isostructural dapsone hemisolvates. Critical solvent properties as well as structural and energetic features are discussed.

Computational and analytical approaches for investigating hydrates: the neat and hydrated solid-state forms of 3-(3-methylimidazolium-1-yl)propanoate

The interconversion pathways and stability ranges of OOCEMIM solid-state forms have been elucidated.