Influence of crystal structure on the tableting properties of n‐alkyl 4‐hydroxybenzoate esters (parabens)

Prediction of Mefenamic Acid Crystal Shape by Random Forest Classification

OBJECTIVE

Particle shape can have a significant impact on the bulk properties of materials. This study describes the development and application of machine-learning models to predict the crystal shape of mefenamic acid recrystallized from organic solvents.

METHODS

Crystals were grown in 30 different solvents to establish a dataset comprising solvent molecular descriptors, process conditions and crystal shape. Random forest classification models were trained on this data and assessed for prediction accuracy.

RESULTS

The highest prediction accuracy of crystal shape was 93.5% assessed by fourfold cross-validation. When solvents were sequentially excluded from the training data, 32 out of 84 models predicted the shape of mefenamic acid crystals for the excluded solvent with 100% accuracy and a further 21 models had prediction accuracies from 50-100%. Reducing the feature set to only solvent physical property descriptors and supersaturations resulted in higher overall prediction accuracies than the models trained using all available or another selected subset of molecular descriptors. For the 8 solvents on which the models performed poorly (< 50% accuracy), further characterisation of crystals grown in these solvents resulted in the discovery of a new mefenamic acid solvate whereas all other crystals were the previously known form I.

CONCLUSIONS

Random forest classification models using solvent physical property descriptors can reliably predict crystal morphologies for mefenamic acid crystals grown in 20 out of the 28 solvents included in this work. Poor prediction accuracies for the remaining 8 solvents indicate that further factors will be required in the feature set to provide a more generalized predictive morphology model.

Mechanical properties of starch esters at particle and compact level - Comparisons and exploration of the applicability of Hiestand's equation to predict tablet strength

Hydrophobic starch esters have potential as tablet matrix formers in controlled drug delivery. The mechanical properties of native starch (SN), starch acetate (SA) and starch propionate (SP) were studied at particle and compact level. Particle microhardness and modulus of elasticity were evaluated by nanoindentation. Force-displacement data of compressed powder were analyzed using Heckel in conjunction with piecewise regression, Kuentz-Leuenberger, Kawakita and Adams models, and yield pressure parameters were derived. Starches were characterized for chemical structure by Raman spectroscopy, crystallinity from powder x-ray diffraction (PXRD) patterns and surface energy from apparent contact angle measurements. A-type starch reflections were absent in the PXRDs of esters indicating greater amorphicity. Consequently, the particle microhardness of starch esters decreased leading to greater deformation during compaction and lower values of yield pressure parameters. These parameters increased with microhardness and ranked the starches in the order: SP < SA < SN. Fitting the experimental data into Hiestand's bonding index equation, a linear correlation (R² = 0.902) was established between experimental and calculated tablet strength describing results of all starches, when Adams (τ_ο') yield pressure was used as the 'effective compression pressure' in the above equation.

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.

Relating the tableting behavior of piroxicam polytypes to their crystal structures using energy-vector models

Piroxicam crystallises into two polytypes, α₁ and α_2, with crystal structures that contain identical molecular layers but differ in the way that these layers are stacked. In spite of having close structural similarity, the polytypes have significantly different powder tabletting behaviour: α₂ forms only weak tablets at low pressures accompanied by extensive capping and lamination, which make it impossible to form intact tablets above 100 MPa, while α₁ exhibits superior tabletability over the investigated pressure range (up to 140 MPa). The potential structural origin of the different behaviour is sought using energy-vector models, produced from pairwise intermolecular interaction energies calculated using the PIXEL method. The analysis reveals that the most stabilising intermolecular interactions define columns in both crystal structures. In α₂, a strongly stabilising interaction between inversion-related molecules links these columns into a 2-D network, while no comparable interaction exists in α₁. The higher dimensionality of the energy-vector model in α₂ may be one contributor to its inferior tabletability. A consideration of probable slip planes in the structures identifies regions where the benzothiazine groups of the molecules meet. The energy-vector models in this region are geometrically similar for both structures, but the interactions are more stabilising in α₂ compared to α₁. This feature may also contribute to the inferior tabletability of α₂.

Predicting mechanical properties of crystalline materials through topological analysis

With the aim to develop simple, programmatically generated, topology-based descriptors of crystal structures for application to mechanical properties prediction methods, we have developed a new geometric analysis protocol using the CSD Python API.

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

The tableting performance for p-aminobenzoic acid (PABA) and a series of its benzoate esters with increasing alkyl chain length (methyl-, ethyl-, and n-butyl) was determined over a broad range of compaction pressures. The crystalline structure of methyl benzoate (Me-PABA) exhibits no slip systems and does not form viable compacts under any compaction pressure. The ethyl (Et-PABA) and n-butyl (Bu-PABA) esters each have a similar, corrugated-layer structure that displays a prominent slip plane and improves material plasticity at low compaction pressure. The compact tensile strength for Et-PABA is superior to that for Bu-PABA; however, neither material achieved a tensile strength greater than 2 MPa over the compression range studied. Complementary studies with powder Brillouin light scattering (BLS) show the maxima of the shear wave, acoustic frequency distribution red shift in an order consistent with both the observed tabletability and attachment energy calculations. Moreover, zero-porosity aggregate elastic moduli are determined for each material using the average acoustic frequency obtained from specific characteristics of the powder BLS spectra. The Young's moduli for Et- and Bu-PABA is significantly reduced relative to PABA and Me-PABA, and this reduction is further evident in tablet compressibility plots. PABA, however, is distinct with high elastic isotropy as interpreted from the narrow and well-defined powder BLS frequency distributions for both the shear and compressional acoustic modes. The acoustic isotropy is consistent with the quasi-isotropic distribution of hydrogen bonding for PABA. At low compaction pressure, PABA tablets display the lowest tensile strength of the series; however, above a compaction pressure of ca. 70 MPa PABA tablet tensile strength continues to increase while that for Et- and Bu-PABA plateaus. PABA displays lower plasticity relative to either ester, and this is consistent with its crystalline structure and high yield pressure determined from in-die Heckel analysis. Overall the complementary approach of using both structural and the acoustic inputs uniquely provided from powder BLS is anticipated to expand our comprehension of the structure-mechanics relationship and its role in tableting performance.

Implication of microstructure on the mechanical behaviour of an aspirin–paracetamol eutectic mixture

The present work investigates the mechanical behaviour of an aspirin–paracetamol (ASP–PCM) eutectic mixture (EM).

Sandwich crystals of butyl paraben

Sandwich crystals: one polymorph. Opaque middle layer: 0.1 μm scale size pores. Parallel outer layer: translucent crystalline layers.

Impact of micromeritic properties of an active pharmaceutical ingredient on its compaction behavior

Physical characteristics of an active pharmaceutical ingredient (API) can have a significant impact on the processability of a high drug loading formulation. This paper provides an example where different micromeritic properties of an API were obtained by crystallization under different conditions, resulting in different tableting behavior. While the API form purity was maintained during the crystallization process change, significant changes were incurred in the surface geometry, porosity and surface area of the API. The batches consisting of particles with greater surface irregularity and porosity gave tablets of higher mechanical strength.

Comparative physical, mechanical and crystallographic properties of a series of gemfibrozil salts

Objectives

Understanding the impact of the counterion on the properties of an acidic or basic drug may influence the choice of salt form, especially for less potent drugs with a high drug load per unit dose. The aim of this work was to determine the influence of the hydrogen bonding potential of the counterion on the crystal structure of salts of the poorly soluble, poorly compressible, acidic drug gemfibrozil and to correlate these with mechanical properties.

Methods

Compacts of the parent drug and the salts were used to determine Young's modulus of elasticity using beam bending tests. Crystal structures were determined previously from X-ray powder diffraction data.

Key findings

The free acid, tert-butylamine, 2-amino-2-methylpropan-1-ol and 2-amino-2-methylpropan-1,3-diol salts had a common crystal packing motif of infinite hydrogen-bonded chains with cross-linking between pairs of adjacent chains. The tromethamine (trsi) salt, with different mechanical properties, had a two-dimensional sheet-like network of hydrogen bonds, with slip planes, forming a stiffer compact.

Conclusions

The type of counter ion is important in determining mechanical properties and could be selected to afford slip and plastic deformation.

On the identification of slip planes in organic crystals based on attachment energy calculation

Knowledge of slip or cleavage planes can facilitate the fundamental understanding of mechanical properties of organic crystals important to pharmaceutical operations, such as tableting and milling. Slip/cleavage planes were frequently assigned based on attachment energy calculation. These crystallographic planes can also be identified by visualization of crystals characterized by stacking layers of high molecular density and often strengthened by two-dimensional hydrogen bonding network. Using 14 organic crystals exhibiting such layered structures, predicted slip planes by attachment energy calculation employing three force fields, Dreiding, cvff, and COMPASS, were compared to those identified by crystal structure visualization. Overall, slip/cleavage planes in <50% crystals were successfully predicted by attachment energy calculation. Thus predicted slip/cleavage planes by attachment energy calculation may not be always accurate and should be treated with caution.