Aggregate Elasticity, Crystal Structure, and Tableting Performance for p-Aminobenzoic Acid and a Series of Its Benzoate Esters

Recent Advances in Nanomechanical Measurements and Their Application for Pharmaceutical Crystals

Mechanical behavior of pharmaceutical crystals directly impacts the formulation development and manufacturing of drug products. The understanding of crystal structure-mechanical behavior of pharmaceutical and molecular crystals has recently gained substantial attention among pharmaceutical and materials scientists with the advent of advanced nanomechanical testing instruments like nanoindentation. For the past few decades, instrumented nanoindentation was a popular technique for measuring the mechanical properties of thin films and small-length scale materials. More recently it is being implemented to investigate the mechanical properties of pharmaceutical crystals. Integration of correlative microscopy techniques and environmental control opened the door for advanced structure-property correlation under processing conditions. Preventing the degradation of active pharmaceutical ingredients from external factors such as humidity, temperature, or pressure is important during processing. This review deals with the recent developments in the synchronized nanomechanical measurements of pharmaceutical crystals toward the fast and effective development of high-quality pharmaceutical drug products. This review also summarizes some recent reports to intensify how one can design and control the nanomechanical properties of pharmaceutical solids. Measurement challenges and the scope for studying nanomechanical properties of pharmaceutical crystals using nanoindentation as a function of crystal structure and in turn to develop fundamental knowledge in the structure-property relationship with the implications for drug manufacturing and development are discussed in this review. This review further highlights recently developed capabilities in nanoindentation, for example, variable temperature nanoindentation testing, in situ imaging of the indented volume, and nanoindentation coupled Raman spectroscopy that can offer new quantitative details on nanomechanical behavior of crystals and will play a decisive role in the development of coherent theories for nanomechanical study of pharmaceutical crystal.

Discriminating the Interaction Anisotropy in Polymorphs Using Powder Brillouin Light Scattering

Drug product performance is polymorph specific, and it is imperative that solid phase stability be monitored throughout the manufacturing process to ensure final product quality and performance. PXRD remains the gold standard for polymorph identification, but due to a growing interest in continuous manufacturing, a need has emerged for alternative process analytical technologies (PATs) that can provide fast, reliable, and non-destructive polymorph discrimination amenable to in situ process monitoring. Herein we demonstrate an original application of powder Brillouin light scattering (p-BLS) for the discrimination of polymorphic molecular solids. We hypothesize that the anisotropic sound velocities directly reflect the strength and orientation of the intermolecular forces in molecular solids. Redistributing these forces upon polymorphic conversion should thus clearly be reflected in the sound frequency distributions obtained by p-BLS. To test this hypothesis, three model compounds - resorcinol, sulfamerazine and furosemide - were selected. Distinct, polymorph-specific, acoustic frequency distributions were observed, and these p-BLS spectra were interpreted using a hydrogen-bond analysis and energy frameworks calculated from CrystalExplorer. In conclusion, this study clearly demonstrates that the sound frequencies measured in p-BLS are sensitive to the interaction forces in molecular solids, and p-BLS is a novel optical technique capable of reliably discriminating polymorphs. Extending this study further, we fully expect that many pharmaceutically relevant processes - e.g., hydrate formation, co-crystallization, or amorphous instability - could potentially be monitored using p-BLS.

Elastic anisotropy of mechanically responsive molecular solids

Unique mechanical properties in molecular solids arise from a specific combination of structure and interaction anisotropy. Powder Brillouin light scattering offers new insight into the latter contribution to test current models for mechanical design.

Structure-mechanics and improved tableting performance of the drug-drug cocrystal metformin:salicylic acid

Drug-drug cocrystals (DDC) represent a unique subset of pharmaceutical materials offering distinct advantages in combination therapies, pharmacokinetics, and patient compliance. However, their structure-function relationships are rarely reported despite its central importance in successful medicine. A material-sparing approach consisting of a molecular and structural perspective is reported to evaluate tabletability of a model DDC, metformin:salicylic acid, relative to its components: metformin HCl (MET) and sodium salicylate (SAL). MET alone displayed a very poor tabletability, which could be attributed to its isotropic and stiff interaction topology. SAL displayed a highly anisotropic interaction topology with layers of strongly hydrogen-bonded salicylate molecules promoting deformation and tabletability. This is also confirmed by its low moduli. DDC yielded intermediate stiffness and elastic anisotropy material with an improved plastic flow and overall better tabletability. Overall, DDC is a promising therapeutic class requiring the physical-mechanical evaluation to assure their processability to enjoy their therapeutic advantages.

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.

Brillouin Light Scattering: Development of a Near Century-Old Technique for Characterizing the Mechanical Properties of Materials

Brillouin light scattering (BLS), a technique theoretically described nearly a century back by the French physicist Léon Brillouin in 1922, is a light-scattering method for determining the mechanical properties of materials. This inelastic scattering method is described by the Bragg diffraction of light from a propagating fluctuation in the local dielectric. These fluctuations arise spontaneously from thermally populated sound waves intrinsic to all materials, and thus BLS may be broadly applied to transparent samples of any phase. This review begins with a brief historical overview of the development of BLS, from its theoretical prediction to the current state of the art, and notes specific technological advancements that enabled the development of BLS. Despite the broad utility of BLS, no commercial spectrometer is currently available for purchase, but rather individual components are assembled to suit a specific application. Central to any BLS spectrometer is the interferometer, and its performance characteristics-scanning or non-scanning, multi-passing, and stabilization-are critical considerations for spectrometer design. Consistent with any light-scattering method, the frequency shift is a key observable in BLS, and we summarize the connection of this measurement to evaluate the mechanical properties of materials. With emphasis toward pharmaceutical materials analysis, we introduce the traditional BLS approach for single-crystal elasticity, and this is followed by a discussion of more recent developments in powder BLS. We conclude our review with a perspective on future developments in BLS that may enable BLS as a novel addition to the current catalog of process analytical technologies.

Aggregate Elasticity and Tabletability of Molecular Solids: a Validation and Application of Powder Brillouin Light Scattering

Describing the elastic deformation of single-crystal molecular solids under stress requires a comprehensive determination of the fourth-rank stiffness tensor (C_ijkl). Single crystals are, however, rarely utilized in industrial applications, and thus averaging techniques (e.g., the Voigt or Reuss approach) are employed to reduce the C_ijkl (or its inverse S_ijkl) to polycrystalline aggregate mechanical moduli. With increasing elastic anisotropy, the Voigt and Reuss-averaged aggregate moduli can diverge dramatically and, provided that drug molecules almost exclusively crystallize into low-symmetry space groups, warrants a significant need for accurate aggregate mechanical moduli. This elasticity data, which currently is largely absent for pharmaceutical materials, is expected to aid understanding how materials respond to direct compression and tablet formation. Powder Brillouin light scattering (p-BLS) has recently demonstrated facile access to porosity-independent, aggregate mechanical moduli. In this study, we extend our previous p-BLS model for obtaining mechanical properties and validate our approach against a broad library of molecular solids with diverse intermolecular interaction topologies and with previously determined C_ijkl which permits benchmarking our results. Our Young's and shear moduli determined with p-BLS strongly correlate, with limited bias (i.e., a near 1:1 relation), with the Voigt-averaged Young's and shear moduli determined using the C_ijkl. Through follow-on tabletability studies, we introduce initial classifications of tabletability behavior based on the results of our p-BLS studies and the apparent elastic anisotropy. With further development, this approach represents a robust and novel method to potentially identify materials for optimum tabletability at early developmental stages.