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.

Directing Crystallization Outcomes of Conformationally Flexible Molecules: Polymorphs, Solvates, and Desolvation Pathways of Fluconazole

Control over polymorphism and solvatomorphism in API assisted by structural information, e.g., molecular conformation or associations via hydrogen bonds, is crucial for the industrial development of new drugs, as the crystallization products differ in solubility, dissolution profile, compressibility, or melting temperature. The stability of the final formulation and technological factors of the pharmaceutical powders further emphasize the importance of precise crystallization protocols. This is particularly important when working with highly flexible molecules with considerable conformational freedom and a large number of hydrogen bond donors or acceptors (e.g., fluconazole, FLU). Here, cooling and suspension crystallization were applied to access polymorphs and solvates of FLU, a widely used azole antifungal agent with high molecular flexibility and several reported polymorphs. Each of four polymorphic forms, FLU I, II, III, or IV, can be obtained from the same set of alcohols (MeOH, EtOH, isPrOH) and DMF via careful control of the crystallization conditions. For the first time, two types of isostructural channel solvates of FLU were obtained (nine new structures). Type I solvates were prepared by cooling crystallization in Tol, ACN, DMSO, BuOH, and BuON. Type II solvates formed in DCM, ACN, nPrOH, and BuOH during suspension experiments. We propose desolvation pathways for both types of solvates based on the structural analysis of the newly obtained solvates and their desolvation products. Type I solvates desolvate to FLU form I by hydrogen-bonded chain rearrangements. Type II solvates desolvation leads first to an isomorphic desolvate, followed by a phase transition to FLU form II through hydrogen-bonded dimer rearrangement. Combining solvent-mediated phase transformations with structural analysis and solid-state NMR, supported by periodic electronic structure calculations, allowed us to elucidate the interrelations and transformation pathways of FLU.

Understanding the thermal stability of apalutamide crystalline solvates through crystal structure analyses and computational studies

The conformational flexibility of APA molecules leads to different crystal packing. The stability of the solvent molecules in the crystal structures was established through their strong intermolecular interactions with the host molecule.

Mechanochemical Synthesis and Physicochemical Characterization of Previously Unreported Praziquantel Solvates with 2-Pyrrolidone and Acetic Acid

Two new solvates of the widely used anthelminthic Praziquantel (PZQ) were obtained through mechanochemical screening with different liquid additives. Specifically, 2-pyrrolidone and acetic acid gave solvates with 1:1 stoichiometry (PZQ-AA and PZQ-2P, respectively). A wide-ranging characterization of the new solid forms was carried out by means of powder X-ray diffraction, differential scanning calorimetry, FT-IR, solid-state NMR and biopharmaceutical analyses (solubility and intrinsic dissolution studies). Besides, the crystal structures of the two new solvates were solved from their Synchrotron-PXRD pattern: the solvates are isostructural, with equivalent triclinic packing. In both structures acetic acid and 2-pyrrolidone showed a strong interaction with the PZQ molecule via hydrogen bond. Even though previous studies have shown that PZQ is conformationally flexible, the same syn conformation as the PZQ Form A of the C=O groups of the piperazinone-cyclohexylcarbonyl segment is involved in these two new solid forms. In terms of biopharmaceutical properties, PZQ-AA and PZQ-2P exhibited water solubility and intrinsic dissolution rate much greater than those of anhydrous Form A.

Insight into amoxicillin sodium heterosolvates and non-solvated form: crystal structures, phase transformation behaviors, and desolvation mechanism

Methanol and methyl acetate molecules played different roles in the formation of S_M–M amoxicillin sodium heterosolvate. A two-step desolvation mechanism of novel heterosolvates was proposed.

Self-organization of 1,6-dialkyl-3a,6a-diphenylglycolurils in the crystalline state

Glycolurils with hydrophobic substituents readily form solids with low-energy surfaces and hydrophobic properties.

The Influence of Solvent on the Crystal Packing of Ethacridinium Phthalate Solvates

The synthesis, structural characterization and influence of solvents on the crystal packing of solvated complexes of ethacridine with phthalic acid: 6,9-diamino-2-ethoxyacridinium phthalate methanol solvate (1), 6,9-diamino-2-ethoxyacridinium phthalate ethanol solvate (2), 6,9-diamino-2-ethoxyacridinium phthalate isobutanol solvate (3), and 6,9-diamino-2-ethoxyacridinium phthalate tert-butanol solvate monohydrate (4) are described in this article. Single-crystal XRD measurements revealed that the compounds 1–4 crystallized in the triclinic P-1 space group, and the 6,9-diamino-2-ethoxyacridinium cations, phthalic acid anions and solvent molecules interact via strong N–H···O, O–H···O, C–H···O hydrogen bonds, and C–H···π and π–π interactions to form different types of basic structural motifs, such as: heterotetramer bis[···cation···anion···] in compound 1 and 2, heterohexamer bis[···cation···alcohol···anion···] in compound 3, and heterohexamer bis[···cation···water···anion···] in compound 4. Presence of solvents molecule(s) in the crystals causes different supramolecular synthons to be obtained and thus has an influence on the crystal packing of the compounds analyzed.

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.

Crystal structures of two furazidin polymorphs revealed by a joint effort of crystal structure prediction and NMR crystallography

This work presents the crystal structure determination of two elusive polymorphs of furazidin, an antibacterial agent, employing a combination of crystal structure prediction (CSP) calculations and an NMR crystallography approach. Two previously uncharacterized neat crystal forms, one of which has two symmetry-independent molecules (form I), whereas the other one is a Z' = 1 polymorph (form II), crystallize in P2₁/c and P1 space groups, respectively, and both are built by different conformers, displaying different intermolecular interactions. It is demonstrated that the usage of either CSP or NMR crystallography alone is insufficient to successfully elucidate the above-mentioned crystal structures, especially in the case of the Z' = 2 polymorph. In addition, cases of serendipitous agreement in terms of ¹H or ¹³C NMR data obtained for the CSP-generated crystal structures different from the ones observed in the laboratory (false-positive matches) are analyzed and described. While for the majority of analyzed crystal structures the obtained agreement with the NMR experiment is indicative of some structural features in common with the experimental structure, the mentioned serendipity observed in exceptional cases points to the necessity of caution when using an NMR crystallography approach in crystal structure determination.

Influence of crystal packing on the thermal properties of cocrystals and cocrystal solvates of olanzapine: insights from computations

A multicomponent supramolecular host with adaptive guest accommodation abilities is observed in the cocrystal solvates of the olanzapine–hydroquinone system.