Surface Engineering of the Mechanical Properties of Molecular Crystals via an Atomistic Model for Computing the Facet Stress Response of Solids.

Invited for the cover of this issue are Mubarak Almehairbi, Vikram C. Joshi, Changquan Calvin Sun and Sharmarke Mohamed. The image depicts the digital exploration of the mechanical properties of crystals on specific facets that may be of interest for materials applications by "dialing-in" their stress response. Read the full text of the article at 10.1002/chem.202400779.

Advancing the Harmonization of Biopredictive Methodologies through the Product Quality Research Institute (PQRI) Consortium: Biopredictive Dissolution of Dipyridamole Tablets.

Biorelevant dissolution and its concept have been widely accepted and further developed to meaningfully predict the bioperformance of oral drug products. Biorelevant methodologies have been applied to design and optimize oral formulations, to facilitate formulation bridging, and to predict the outcome of bioperformance by coupling the results with modeling. Yet, those methodologies have often been independently customized to align with specific aspects of the oral drug products being developed. Therefore, the evolution of biorelevant dissolution methodologies has taken slightly diverse pathways rather than being standardized like compendial quality control (QC) methodologies. This manuscript presents an effort through the Product Quality Research Institute (PQRI, https://pqri.org) consortium entitled: the standardization of "in vivo predictive dissolution methodologies and in silico bioequivalent study working group" to find the key parameters for biorelevant dissolution, to identify the best practices, and to move toward standardization of biorelevant dissolution methodologies. This working group is composed of members from 10 pharmaceutical companies and academic institutes. The consortium project will be accomplished in five phases, whereby the first two phases have already been completed and published. In this paper, the next two phases are addressed by reporting the biorelevant dissolution profiles of dipyridamole, a weak base model drug, then incorporating the dissolution results into physiologically based biopharmaceutics modeling (PBBM) to determine whether they would lead to bioequivalence (BE) or non-BE.

Mechanical properties and peculiarities of molecular crystals.

In the last century, molecular crystals functioned predominantly as a means for determining the molecular structures via X-ray diffraction, albeit as the century came to a close the response of molecular crystals to electric, magnetic, and light fields revealed that the physical properties of molecular crystals were as rich as the diversity of molecules themselves. In this century, the mechanical properties of molecular crystals have continued to enhance our understanding of the colligative responses of weakly bound molecules to internal frustration and applied forces. Here, the authors review the main themes of research that have developed in recent decades, prefaced by an overview of the particular considerations that distinguish molecular crystals from traditional materials such as metals and ceramics. Many molecular crystals will deform themselves as they grow under some conditions. Whether they respond to intrinsic stress or external forces or interactions among the fields of growing crystals remains an open question. Photoreactivity in single crystals has been a leading theme in organic solid-state chemistry; however, the focus of research has been traditionally on reaction stereo- and regio-specificity. However, as light-induced chemistry builds stress in crystals anisotropically, all types of motions can be actuated. The correlation between photochemistry and the responses of single crystals-jumping, twisting, fracturing, delaminating, rocking, and rolling-has become a well-defined field of research in its own right: photomechanics. The advancement of our understanding requires theoretical and high-performance computations. Computational crystallography not only supports interpretations of mechanical responses, but predicts the responses itself. This requires the engagement of classical force-field based molecular dynamics simulations, density functional theory-based approaches, and the use of machine learning to divine patterns to which algorithms can be better suited than people. The integration of mechanics with the transport of electrons and photons is considered for practical applications in flexible organic electronics and photonics. Dynamic crystals that respond rapidly and reversibly to heat and light can function as switches and actuators. Progress in identifying efficient shape-shifting crystals is also discussed. Finally, the importance of mechanical properties to milling and tableting of pharmaceuticals in an industry still dominated by active ingredients composed of small molecule crystals is reviewed. A dearth of data on the strength, hardness, Young's modulus, and fracture toughness of molecular crystals underscores the need for refinement of measurement techniques and conceptual tools. The need for benchmark data is emphasized throughout.

Modulating Pharmaceutical Properties of Berberine Chloride through Cocrystallization with Benzendiol Isomers.

PURPOSE: To synthesize and characterize new cocrystals of berberine chloride (BCl) for potential pharmaceutical tablet formulation. METHODS: Solutions of BCl with each of three selected cocrystal formers, catechol (CAT), resorcinol (RES), and hydroquinone (HYQ) were slowly evaporated at room temperature to obtain crystals. Crystal structures were solved using single crystal X-ray diffraction. Bulk powders were characterized by powder X-ray diffraction, thermogravimetric-differential scanning calorimetry, FTIR, dynamic moisture sorption, and dissolution (both intrinsic and powder). RESULTS: Single crystal structures confirmed the formation of cocrystals with all three coformers, which revealed various intermolecular interactions that stabilized crystal lattices, including O-H···Cl- hydrogen bonds. All three cocrystals exhibited better stability against high humidity (up to 95% relative humidity) at 25 ℃ and higher intrinsic and powder dissolution rates than BCl. CONCLUSION: The enhanced pharmaceutical properties of all three cocrystals, as compared to BCl, further contribute to the existing evidence that confirms the beneficial role of cocrystallization in facilitating drug development. These new cocrystals expand the structure landscape of BCl solid forms, which is important for future analysis to establish a reliable relationship between crystal structure and pharmaceutical properties.

Pharmaceutical Lauryl Sulfate Salts: Prevalence, Formation Rules, and Formulation Implications.

The anionic surfactant sodium lauryl sulfate (SLS) is known to deteriorate the dissolution of some drugs by forming poorly soluble lauryl sulfate (LS) salts. However, because of the perception of its infrequent occurrence, this phenomenon is usually not investigated in drug development until unexpected dissolution slowdown is encountered. This work demonstrates the prevalence of this phenomenon, where 14 out of 18 compounds with diverse chemical structures, including salts of basic drugs, a quaternary ammonium salt, organic bases, and zwitterionic molecules, precipitated from a solution when mixed with SLS. Although no precipitation was observed for the other 4 compounds, their FTIR spectra suggested 3 of them had intermolecular interactions with SLS when dried from a solution. These results, along with the 5 other examples reported in the literature, demonstrate the prevalence of this phenomenon. The occurrence of precipitation is thermodynamically driven by the relative difference between the ion product in solution (Q) and the solubility product of the lauryl sulfate salt (Ksp). SLS, as a surfactant, also affects precipitation kinetics by influencing the interfacial tension of nuclei of the insoluble salt. When a potential issue associated with the LS salt is identified, effective mitigation strategies should be proactively designed and implemented to alleviate its possible negative impact on drug dissolution.

Mechanisms of Crystal Plasticization by Lattice Water.

PURPOSE: Water of crystallization has been observed to increase plasticity, decrease crystal hardness, and improve powder compressibility and tabletability of organic crystals. This work is aimed at gaining a molecular level insight into this observation. METHOD: We systematically analyzed crystal structures of five stoichiometric hydrate systems, using several complementary techniques of analysis, including energy framework, water environment, overall packing change, hydrate stability, and slip plane identification. RESULTS: The plasticizing effect by lattice water is always accompanied by an introduction of more facile slip planes, lower packing efficiency, and lower density in all hydrate systems examined in this work. Three distinct mechanisms include 1) changing the distribution of intermolecular interactions without significantly changing the packing of molecules to introduce more facile slip planes; 2) changing packing feature into a flat layered structure so that more facile slip planes are introduced; 3) reducing the interlayer interaction energies and increasing the anisotropy. CONCLUSION: Although the specific mechanisms for these five systems differ, all five hydrates are featured with more facile slip planes, lower packing efficiency, and lower density.

Novel Salt-Cocrystals of Berberine Hydrochloride with Aliphatic Dicarboxylic Acids: Odd-Even Alternation in Physicochemical Properties.

In this study, various structurally similar aliphatic dicarboxylic acids, namely, succinic acid, glutaric acid, adipic acid, and pimelic acid, were employed as coformers to obtain phase pure cocrystals with berberine chloride (BCl) by a slow solvent evaporation method. The structures of the four novel salt-cocrystals of BCl were determined by single crystal X-ray diffraction analysis and their solid-state properties were characterized. Compared with BCl·2H2O, all the cocrystals showed a higher melting point, improved powder dissolution and intrinsic dissolution rate (IDR), and lower hygroscopicity. It is noteworthy that the melting points and IDRs of these cocrystals exhibit an odd-even alternation with the carbon chain length of the acids.

Crystallographic and Energetic Insights into Reduced Dissolution and Physical Stability of a Drug-Surfactant Salt: The Case of Norfloxacin Lauryl Sulfate.

A commonly used pharmaceutical surfactant, sodium lauryl sulfate (SLS), has been reported to reduce the dissolution rate of drugs due to the formation of a less soluble drug-lauryl sulfate salt. In this study, we provide direct crystallographic evidence of the formation of salt between SLS and norfloxacin (NOR), [NORH+][LS-]·1.5 H2O. The available crystal structure also enables the use of the energy framework to gain an understanding of the structure-property relationship. Results show that the hydrophobic methyl groups in SLS dominate the surfaces of the [NORH+][LS-]·1.5 H2O crystals, resulting in the increased hydrophobicity and reduced wettability by aqueous media. Moreover, an analysis of molecular environments and energy calculations of water molecules provides insight into the stability of [NORH+][LS-]·1.5 H2O with variations in the relative humidity and temperature. In summary, important pharmaceutical properties, such as solubility, dissolution, and thermal stability, of the drug-surfactant salt [NORH+][LS-]·1.5 H2O have been characterized and understood based on crystallographic and energetic analyses of the crystal structure.

Molecular Interpretation of the Compaction Performance and Mechanical Properties of Caffeine Cocrystals: A Polymorphic Study.

The 1:1 caffeine (CAF) and 3-nitrobenzoic acid (NBA) cocrystal (CAF:NBA) displays polymorphism. Each polymorph shares the same docking synthon that connects individual CAF and NBA molecules within the asymmetric unit; however, the extended intermolecular interactions are significantly different between the two polymorphic modifications. These alternative interaction topologies translate to distinct structural motifs, mechanical properties, and compaction performance. To assist our molecular interpretation of the structure-mechanics-performance relationships for these cocrystal polymorphs, we combine powder Brillouin light scattering (p-BLS) to determine the mechanical properties with energy frameworks calculations to identify potentially available slip systems that may facilitate plastic deformation. The previously reported Form 1 for CAF:NBA adopts a 2D-layered crystal structure with a conventional 3.4 Å layer-to-layer separation distance. For Form 2, a columnar structure of 1D-tapes is displayed with CAF:NBA dimers running parallel to the (110) crystallographic direction. Consistent with the layered crystal structure, the shear modulus for Form 1 is significantly reduced relative to Form 2, and moreover, our p-BLS spectra for Form 1 clearly display the presence of low-velocity shear modes, which support the expectation of a low-energy slip system available for facile plastic deformation. Our energy frameworks calculations confirm that Form 1 displays a favorable slip system for plastic deformation. Combining our experimental and computational data indicates that the structural organization in Form 1 of CAF:NBA improves the compressibility and plasticity of the material, and from our tabletability studies, each of these contributions confers superior tableting performance to that of Form 1. Overall, mechanical and energy framework data permit a clear interpretation of the functional performance of polymorphic solids. This could serve as a robust screening approach for early pharmaceutical solid form selection and development.

Reduction of Punch-Sticking Propensity of Celecoxib by Spherical Crystallization via Polymer Assisted Quasi-Emulsion Solvent Diffusion.

Punch-sticking during tablet compression is a common problem for many active pharmaceutical ingredients (APIs), which renders tablet formulation development challenging. Herein, we demonstrate that the punch-sticking propensity of a highly sticky API, celecoxib (CEL), can be effectively reduced by spherical crystallization enabled by a polymer assisted quasi-emulsion solvent diffusion (QESD) process. Among three commonly used pharmaceutical polymers, poly(vinylpyrrolidone) (PVP), hydroxypropyl cellulose (HPC), and hydroxypropyl methylcellulose (HPMC), HPMC was the most effective in stabilizing the transient emulsion during QESD and retarding the coalescence of emulsion droplets and the initiation of CEL crystallization. These observations may arise from stronger intermolecular interactions between HPMC and CEL, consistent with solution 1H NMR analyses. SEM and X-ray photoelectron spectroscopy confirmed the presence of a thin layer of HPMC on the surfaces of spherical particles. Thus, the sticking propensity was significantly reduced because the HPMC coating prevents direct contact between CEL and the punch tip during tablet compression.

Toward a Molecular Understanding of the Impact of Crystal Size and Shape on Punch Sticking.

Punch sticking during tablet manufacturing is a common problem facing the pharmaceutical industry. Using several model compounds, effects of crystal size and shape of active pharmaceutical ingredients (API) on punch sticking propensity were systematically investigated in this work to provide molecular insights into the punch-sticking phenomenon. In contrast to the common belief that smaller API particles aggravate punch sticking, results show that particle size reduction can either reduce or enhance API punch sticking, depending on the complex interplay among the particle surface area, plasticity, cohesive strength, and specific surface functional groups. Therefore, other factors, such as crystal mechanical properties, surface chemistry of crystal facets exposed to the punch face, and choice of excipients in a formulation, should be considered for a more reliable prediction of the initiation and progression of punch sticking. The exposure of strong electronegative groups to the punch face facilitates the onset of sticking, while higher plasticity and cohesive strength aggravate sticking.

Recent Advances in Co-processed APIs and Proposals for Enabling Commercialization of These Transformative Technologies.

Optimized physical properties (e.g., bulk, surface/interfacial, and mechanical properties) of active pharmaceutical ingredients (APIs) are key to the successful integration of drug substance and drug product manufacturing, robust drug product manufacturing operations, and ultimately to attaining consistent drug product critical quality attributes. However, an appreciable number of APIs have physical properties that cannot be managed via routes such as form selection, adjustments to the crystallization process parameters, or milling. Approaches to control physical properties in innovative ways offer the possibility of providing additional and unique opportunities to control API physical properties for both batch and continuous drug product manufacturing, ultimately resulting in simplified and more robust pharmaceutical manufacturing processes. Specifically, diverse opportunities to significantly enhance API physical properties are created if allowances are made for generating co-processed APIs by introducing nonactive components (e.g., excipients, additives, carriers) during drug substance manufacturing. The addition of a nonactive coformer during drug substance manufacturing is currently an accepted approach for cocrystals, and it would be beneficial if a similar allowance could be made for other nonactive components with the ability to modify the physical properties of the API. In many cases, co-processed APIs could enable continuous direct compression for small molecules, and longer term, this approach could be leveraged to simplify continuous end-to-end drug substance to drug product manufacturing processes for both small and large molecules. As with any novel technology, the regulatory expectations for co-processed APIs are not yet clearly defined, and this creates challenges for commercial implementation of these technologies by the pharmaceutical industry. The intent of this paper is to highlight the opportunities and growing interest in realizing the benefits of co-processed APIs, exemplified by a body of academic research and industrial examples. This work will highlight reasons why co-processed APIs would best be considered as drug substances from a regulatory perspective and emphasize the areas where regulatory strategies need to be established to allow for commercialization of innovative approaches in this area.

Material-Sparing and Expedited Development of a Tablet Formulation of Carbamazepine Glutaric Acid Cocrystal- a QbD Approach.

PURPOSE: To efficiently develop a tablet formulation of carbamazepine using a soluble cocrystal with excess coformer to maintain phase stability during dissolution. METHODS: The carbamazepine - glutaric acid cocrystal (CBZ-GLA, 1:1) and excess glutaric acid (GLA) were mixed with suitable tablet excipients, which were selected to address powder flowability and tabletability deficiencies specifically. Tablet friability and dissolution profiles were evaluated to guide formulation optimization. Dry granules were prepared by milling simulated ribbons. RESULTS: A binary blend of CBZ-GLA and GLA had poor flowability and marginal tabletability. Therefore, silica coated Avicel PH-102 (sMCC) was applied as a binder to improve the flow property and tabletability. A formulation consisting of sMCC, CBZ-GLA, and GLA exhibited good manufacturability but did not show improved dissolution because of rapid precipitation of CBZ dihydrate when CBZ-GLA came in contact with water. Dry granulation of CBZ-GLA and GLA dramatically improved dissolution profile due to the intimate contact between CBZ-GLA and GLA. Such cocrystal - coformer granules also led to much improved tablet manufacturability and dissolution. CONCLUSION: The successful tablet development of CBZ-GLA, using < 3 g of the cocrystal in <3 weeks, demonstrates an efficient workflow for tablet formulation development based on material-sparing and predictive powder characterization techniques. This workflow is useful for early tablet development using enabling solid form, such as cocrystal, when only a small amount of material is available.

Molecular Origin of the Distinct Tabletability of Loratadine and Desloratadine: Role of the Bonding Area - Bonding Strength Interplay.

PURPOSE: To explain the different tabletability of two structurally similar H1-receptor antihistamine drugs, loratadine (LOR) and desloratadine (DES), based on the molecular basis of bonding area and bonding strength. METHODS: LOR and DES were characterized by powder X-ray diffractometry, thermal analysis, and dynamic water sorption. The compressibility, tabletability, compactibility, and Heckel analysis of their bulk powders and formulations were evaluated. A combined energy framework and topological analysis was used to characterize the crystal structure - mechanical property relationship. Surface energy of bulk powder was assessed by contact angle measurement using the Owens/Wendt theory. RESULTS: Both LOR and DES bulk powders are phase pure and stable under compaction. The superior tabletability of LOR is attributed to both larger bonding area (BA) and higher interparticle bonding strength (BS). The larger BA of LOR results from its experimentally established higher plasticity, which is explained by the presence of more densely packed molecular layers with smooth surface topology. The higher BS of LOR corresponded to its significantly higher dispersive component of the surface energy. CONCLUSIONS: This work provides new insights into the molecular origins of BA and BS, which can be applied to improve mechanical properties and tableting performance of drugs through appropriate crystal engineering.

Effect of Hydroxypropyl Cellulose Level on Twin-Screw Melt Granulation of Acetaminophen.

This study investigated the effect of binder level on the physicochemical changes and tabletability of acetaminophen (APAP)-hydroxypropyl cellulose (HPC) granulated using twin-screw melt granulation. Even at 5% HPC level, the tablet tensile strength achieved up to 3.5 MPa. A minimum of 10% HPC was required for the process robustness. However, 20% HPC led to tabletability loss, attributable to the high mechanical strength of APAP granules. The over-granulated APAP granules had thick connected HPC scaffold and low porosity. Consequently, these granules were so strong that they underwent a lower degree of fracture under compression and higher elastic recovery during decompression. HPC was enriched on the surface of APAP extrudates at all HPC levels. Amorphous APAP was also observed on the extrudate surface at 20% HPC level, and it recrystallized within 24 h storage. To achieve a robust process and optimal improvement in APAP tabletability, the preferred HPC level was 10 to 15%.

Computational Techniques for Predicting Mechanical Properties of Organic Crystals: A Systematic Evaluation.

Understanding of the structure-mechanical properties relationship in organic crystals can potentially facilitate the design of crystals with desired mechanical properties through crystal engineering. To understand and predict crystal mechanical properties, including tableting behavior, a number of computational methods have been developed to analyze crystal structure. These include visualization, attachment energy calculation, topological analysis, energy framework, and elasticity tensor calculation. However, different methods often lead to conflicting predictions. There is a need for a computational tool kit for predicting crystal mechanical properties from crystal structures. Using α-oxalic acid anhydrous (OAA) and dihydrate (OAD) as a model system, we have systematically compared their predictive accuracy of the mechanical properties, experimentally determined using powder compaction and nanoindentation. We have found that crystal plasticity can be accurately predicted based on energy framework combined with topological analysis and DFT calculated elasticity tensor. Although very useful in characterizing crystal packing features, structure visualization, topology analysis, and attachment energy calculations alone are insufficient for accurately identifying the slip planes and predicting mechanical properties and tableting behavior of organic crystals.

Reduced Punch Sticking Propensity of Acesulfame by Salt Formation: Role of Crystal Mechanical Property and Surface Chemistry.

Powder adhesion or sticking onto punches is one of the outstanding issues in pharmaceutical tablet manufacturing. We show in this work that, at comparable particle sizes, the acesulfame potassium exhibited pronouncedly reduced propensity to punch sticking than acesulfame. Detailed analyses revealed strong correlation between sticking propensity and crystal mechanical properties and surface chemistry. The free acid was highly plastic with high cohesive strength, while the salt form was brittle. During compaction, surfaces of acesulfame in contact with the punch face are abundant in electronegative functional groups, while those of the salt consist of mainly hydrophobic groups. Thus, acesulfame underwent stronger interactions with the electron-deficient punch. Consequently, the strikingly different onset and severity of sticking propensity between the two solid forms of acesulfame could be clearly explained based on their different crystal mechanical properties and surface characteristics. By providing molecular insight into the outstanding problem of punch sticking in tablet manufacturing, this work expands the list of pharmaceutical applications of crystal engineering.

Polymer Nanocoating of Amorphous Drugs for Improving Stability, Dissolution, Powder Flow, and Tabletability: The Case of Chitosan-Coated Indomethacin.

As a result of its higher molecular mobility, the surface of an amorphous drug can grow crystals much more rapidly than the bulk, causing poor stability and slow dissolution of drug products. We show that a nanocoating of chitosan (a pharmaceutically acceptable polymer) can be deposited on the surface of amorphous indomethacin by electrostatic deposition, leading to significant improvement of physical stability, wetting by aqueous media, dissolution rate, powder flow, and tabletability. The coating condition was chosen so that the positively charged polymer deposits on the negatively charged drug. Chitosan coating is superior to gelatin coating with respect to stability against crystallization and agglomeration of coated particles.

Developing Biologics Tablets: The Effects of Compression on the Structure and Stability of Bovine Serum Albumin and Lysozyme.

Oral administration is advantageous compared to the commonly used parenteral administration for local therapeutic uses of biologics or mucosal vaccines, since it can specifically target the gastrointestinal (GI) tract. It offers better patient compliance, even though the general use of such a delivery route is often limited by potential drug degradation in the GI tract and poor absorption. Using bovine serum albumin (BSA) and lysozyme as two model proteins, we studied their solid-state properties, mechanical properties, and tabletability as well as effects of compaction pressure, particle size, and humidity on protein degradation. It was found that BSA and lysozyme are highly hygroscopic, and their tablet manufacturability (powder caking, punch sticking, and tablet lamination) is sensitive to the humidity. BSA and lysozyme exhibited high plasticity and excellent tabletability and remained amorphous at high pressure and humidity. As for protein stability, lysozyme was resistant to high pressure (up to 300 MPa) and high humidity (up to 93%). In contrast, BSA underwent aggregation upon compression, an effect that was more pronounced for smaller BSA particles. High humidity accelerated the aggregation of BSA during incubation, but it did not further synergize with mechanical stress to induce protein degradation. Thus, compression can potentially induce protein aggregation, but this effect is protein-dependent. Therefore, strategies (e.g., the use of excipients, optimized manufacturing processes) to inhibit protein degradation should be explored before their tablet dosage form development.

Expedited Tablet Formulation Development of a Highly Soluble Carbamazepine Cocrystal Enabled by Precipitation Inhibition in Diffusion Layer.

PURPOSE: To address the problem of precipitation of a poorly soluble drug during dissolution of highly soluble cocrystals by preparing granules intimately mixed with a water-soluble polymer. METHODS: Effectiveness of polymers as precipitation inhibitors during the dissolution of carbamazepine-nicotinamide (CBZ-NCT) cocrystal was assessed based on induction time of crystallization from a supersaturated solution in presence of different polymers at two concentrations. Dissolution was evaluated by both intrinsic dissolution rate (IDR) and USP dissolution method. Powder manufacturability was assessed using a shear cell and compaction simulator to assess flowability and tabletability, respectively. RESULTS: Hydroxypropyl methylcellulose acetate succinate (HPMCAS) was the most effective polymer against precipitation of CBZ and the IDR of a 1:1 (w/w) CBZ-NCT/HPMCAS mixture was the highest. The final formulation of 1:1 CBZ-NCT/HPMCAS granule exhibited excellent flowability, good tabletability, and significantly improved drug release rate than cocrystal formulations without HPMCAS or the CBZ formulation. CONCLUSION: The particle engineering strategy of modifying the diffusion layer on the surface of highly soluble cocrystal with a polymer is effective for inhibiting premature precipitation of CBZ. Assisted with predictive tools for characterizing powder flowability and tabletability, the design of high quality tablet product with improved drug release rate and manufacturability can be achieved in an efficient manner.