Conformational origins of polymorphism in two forms of flufenamic acid

First-Principles Models of Polymorphism of Pharmaceuticals: Maximizing the Accuracy-to-Cost Ratio

Accuracy and sophistication of in silico models of structure, internal dynamics, and cohesion of molecular materials at finite temperatures increase over time. Applicability limits of ab initio polymorph ranking that would be feasible at reasonable costs currently represent crystals of moderately sized molecules (less than 20 nonhydrogen atoms) and simple unit cells (containing rather only one symmetry-irreducible molecule). Extending the applicability range of the underlying first-principles methods to larger systems with a real-life significance, and enabling to perform such computations in a high-throughput regime represent additional challenges to be tackled in computational chemistry. This work presents a novel composite method that combines the computational efficiency of density-functional tight-binding (DFTB) methods with the accuracy of density-functional theory (DFT). Being rooted in the quasi-harmonic approximation, it uses a cheap method to perform all of the costly scans of how static and dynamic characteristics of the crystal vary with respect to its volume. Such data are subsequently corrected to agree with a higher-level model, which must be evaluated only at a single volume of the crystal. It thus enables predictions of structural, cohesive, and thermodynamic properties of complex molecular materials, such as pharmaceuticals or organic semiconductors, at a fraction of the original computational cost. As the composite model retains the solid physical background, it suffers from a minimum accuracy deterioration compared to the full treatment with the costly approach. The novel methodology is demonstrated to provide consistent results for the structural and thermodynamic properties of real-life molecular crystals and their polymorph ranking.

Polymorphic transitions in flufenamic acid-trehalose composites

The combination of poorly-soluble drugs with small molecule co-formers to generate amorphous solid dispersions (ASDs) has great potential to improve dissolution rate and kinetic solubility, and thus increase the bioavailability of these active ingredients. However, such ASDs are known to be unstable and to crystallise upon storage or heating. In this work, we explore the crystallisation of flufenamic acid (FFA) from ASDs prepared with trehalose. FFA-trehalose mixtures were prepared at a range of w/w composition ratios, heated to melting and crash cooled to form ASDs. They were then subject to a further heat/cool cycle, which was monitored by simultaneous differential scanning calorimetry - X-ray diffraction to observe the phase changes occurring. These varied with the composition of the blend. Upon short-term storage, formulations with low trehalose contents (FFA:trehalose 5:1 w/w) recrystallised into form I FFA, while higher trehalose contents crystallised to FFA form IV. When heated, all FFA trehalose combinations ultimately recrystallised into form I before melting. Upon a second cooling cycle, systems with low trehalose content (FFA:trehalose 5:1 w/w) recrystallised into form IV, while higher trehalose contents led to FFA form I. It is thus clear that even with a single excipient it is possible to control the crystallisation pathway through judicious choice of the formulation parameters.

Conformational State of Fenamates at the Membrane Interface: A MAS NOESY Study

The present work analyzes the ¹H NOESY MAS NMR spectra of three fenamates (mefenamic, tolfenamic, and flufenamic acids) localized in the lipid-water interface of phosphatidyloleoylphosphatidylcholine (POPC) membranes. The observed cross-peaks in the two-dimensional NMR spectra characterized intramolecular proximities between the hydrogen atoms of the fenamates as well as intermolecular interactions between the fenamates and POPC molecules. The peak amplitude normalization for an improved cross-relaxation (PANIC) approach, the isolated spin-pair approximation (ISPA) model, and the two-position exchange model were used to calculate the interproton distances indicative of specific conformations of the fenamates. The results showed that the proportions of the A+C and B+D conformer groups of mefenamic and tolfenamic acids in the presence of POPC were comparable within the experimental error and amounted to 47.8%/52.2% and 47.7%/52.3%, respectively. In contrast, these proportions for the flufenamic acid conformers differed and amounted to 56.6%/43.4%. This allowed us to conclude that when they bind to the POPC model lipid membrane, fenamate molecules change their conformational equilibria.

Investigation of the Spatial Structure of Flufenamic Acid in Supercritical Carbon Dioxide Media via 2D NOESY

The search for new forms of already known drug compounds is an urgent problem of high relevance as more potent drugs with fewer side effects are needed. The trifluoromethyl group in flufenamic acid renders its chemical structure differently from other fenamates. This modification is responsible for a large number of conformational polymorphs. Therefore, flufenamic acid is a promising structural modification of well-known drug molecules. An effective approach in this field is micronization, employing "green" supercritical fluid technologies. This research raises some key questions to be answered on how to control polymorphic forms during the micronization of drug compounds. The results presented in this work demonstrate the ability of two-dimensional nuclear Overhauser effect spectroscopy to determine conformational preferences of small molecular weight drug compounds in solutions and fluids, which can be used to predict the polymorphic form during the micronization. Quantitative analysis was carried out to identify the conformational preferences of flufenamic acid molecules in dimethyl sulfoxide-d6 medium at 25 °C and 0.1 MPa, and in mixed solvent medium containing supercritical carbon dioxide at 45 °C and 9 MPa. The data presented allows predictions of the flufenamic acid conformational preferences of poorly soluble drug compounds to obtain new micronized forms.

Selected Applications of Terahertz Pulses in Medicine and Industry

This article contains a brief summary of areas where terahertz technology is making an impact in research and industrial applications. We cover some of its uses in the pharmaceutical setting, where both imaging and spectroscopy play important roles. Medical applications are also being pursued in many research laboratories, primarily for imaging purposes and following on from the first results just over 20 years ago. The three-dimensional imaging capability of pulsed terahertz allows for the observation of tumours below the surface of tissue, such as basal cell carcinoma of skin. The recent use of the technology in studies of cultural heritage has shown to increase our understanding of the past. The power of terahertz is exemplified by the discussion on its importance in different industries, such as semiconductor circuit manufacturing and automotive assembly.

Multicomponent Solids of Niflumic and Mefenamic Acids Based on Acid-Pyridine Synthon

The present study discusses comparative structural features of fourteen multicomponent solids of two non-steroidal anti-inflammatory drugs, Niflumic and Mefenamic acids, with amine and pyridine-based coformers. All the solids were structurally characterized through PXRD, SCXRD, DSC, and the monophasic nature of some of the solids was established through Rietveld refinement. The solid forms include salt, cocrystal, hydrate, and solvate. Except for two, all the solids reported here showed relatively higher solubility compared to the acids. The difference in pKa and similarity in structural features of both the molecules enabled us to study the effect of ΔpKa on crystallization outcome systematically. The structures of all the solids are described through acid-pyridine synthon perspective.

Monitoring Polymorphic Phase Transitions in Flufenamic Acid Amorphous Solid Dispersions Using Hyphenated X-ray Diffraction-Differential Scanning Calorimetry

Flufenamic acid (FFA) is a highly polymorphic drug molecule with nine crystal structures reported in the Cambridge Structural Database. This study explores the use of synchrotron X-ray powder diffraction combined with differential scanning calorimetry to study crystallization and polymorphic phase transitions upon heating FFA–polymer amorphous solid dispersions (ASDs). Ethyl cellulose (EC, 4 cp) and hydroxypropylmethylcellulose (HPMC) grades with different viscosities and substitution patterns were used to prepare dispersions with FFA at 5:1, 2:1, 1:1, and 1:5 w/w drug/polymer ratios by quench cooling. We employed a 6 cp HPMC 2910 material and two HPMC 2208 samples at 4000 and 100 000 cp. Hyphenated X-ray diffraction (XRD)–differential scanning calorimetry (DSC) studies show that the 6 and 100 000 cp HPMCs and 4 cp EC polymers can stabilize FFA form IV by inhibiting the transition to form I during heating. It appears that the polymers stabilize FFA in both amorphous and metastable forms via a combination of intermolecular interactions and viscosity effects. Increasing the polymer content of the ASD also inhibits polymorphic transitions, with drug/polymer ratios of 1:5 w/w resulting in FFA remaining amorphous during heating. The comparison of FFA ASDs prepared with different samples of HPMCs and ECs suggests that the chemical substitution of the polymer (HPMC 2208 has 19–24% methoxy groups and 4–12% hydroxypropyl groups, while HPMC 2910 has 28–30% methoxy groups and 7–12% hydroxypropyl groups) plays a more significant role in directing polymorphic transitions than the viscosity. A previously unreported polymorph of FFA was also noted during heating but its structure could not be determined.

Visualising early-stage liquid phase organic crystal growth via liquid cell electron microscopy

Here, we show that the development of nuclei and subsequent growth of a molecular organic crystal system can be induced by electron beam irradiation by exploiting the radiation chemistry of the carrier solvent. The technique of Liquid Cell Electron Microscopy was used to probe the crystal growth of flufenamic acid; a current commercialised active pharmaceutical ingredient. This work demonstrates liquid phase electron microscopy analysis as an essential tool for assessing pharmaceutical crystal growth in their native environment while giving insight into polymorph identification of nano-crystals at their very inception. Possible mechanisms of crystal nucleation due to the electron beam with a focus on radiolysis are discussed along with the innovations this technique offers to the study of pharmaceutical crystals and other low contrast materials.

The role of solid state properties on the dissolution performance of flufenamic acid

Flufenamic acid is a nonsteroidal anti-inflammatory drug characterized by a low solubility and a variable oral bioavailability. Flufenamic acid is present in the commercial products in two polymorphic enantiotropic forms (Form I and III). Bioinequivalence was observed for commercial solid dosage forms due to the different dissolution rate of batches. Aim of this work is the full characterization of the solid state properties of flufenamic acid in order to evidence reasons of its variable dissolution properties. Two different batches of pure drug obtained by different suppliers were fully characterized. In order to evaluate the effect of the technological processes used for tablet production, the powders were submitted to grinding, kneading, and compression. Thermal analysis and X-ray diffraction studies proved that the drug was provided by both suppliers as Form I, Form III is obtained by recrystallization from ethanol or ethanol/water of both batches and no changes were observed after the different mechanical treatments. No difference was observed between the two forms in terms of equilibrium solubility values. Dissolution rate studies evidenced a difference between the two batches due to their different particle size, which disappeared after sieving. Interestingly, a significant difference in terms of intrinsic dissolution rate and surface wettability of the two compacted powders was observed, even after sieving, probably related to a different behavior of the two powder samples under compaction. These results should be taken into account, during a tablet formulation, in order to obtain a reproducible dissolution performance of the drug, regardless of its original supplier.

Structural investigation of a 2:1 co-crystal between diflunisal and isonicotinamide based on terahertz and Raman spectroscopy

In order to characterize molecular structure changes of drugs upon co-crystallization by means of spectroscopic techniques, vibrational spectra of solid-state diflunisal (DIF), isonicotinamide (ISO) and their 2:1 co-crystal have been investigated by using terahertz time-domain spectroscopy (THz-TDS) and Raman spectroscopy. A 2:1 DIF-ISO co-crystal between DIF and ISO has been synthesized by slow solution crystallization from ethanol. The experimental THz spectroscopy shows that the co-crystal has a few significantly different absorption peaks compared with raw parent materials within the frequency region from 0.2 to 1.6 THz. Likewise, some differences of vibrational spectra between the co-crystal and starting compounds could also be characterized by Raman spectral results. Density functional theory (DFT) was used to simulate optimized structures and vibrational modes of two kind of possible co-crystal theoretical forms (form I and II) between DIF and ISO. Theoretical co-crystal form I is shown with 2:1 theoretical binary-adduct formed by carboxylic acid-amide and carboxylic acid-pyridine under inter-molecular hydrogen bonding. Theoretical co-crystal form II has a similar structure as form I, meanwhile the only difference is that O63 atom simultaneously forms hydrogen bond with H33 and H64. Also the hydroxyl -OH and carboxyl group -COOH establish molecular heterocycle under intra-molecular hydrogen bonds in both forms. The theoretical results show that both THz and Raman spectra of co-crystal form II between DIF and ISO is more consistent with the experimental observations than those of co-crystal form I. These results provide us with a wealth of information and unique method for characterizing the composition of co-crystal structures and also inter-molecular hydrogen bonding interactions shown within pharmaceutical co-crystallization at the molecular level.

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.

Crystal anisotropy explains structure-mechanics impact on tableting performance of flufenamic acid polymorphs

Anisotropic features with other crystallographic properties like d-spacing, and attachment energy (E_att) can predict material performance during the secondary pharmaceutical processing. A newly developed state-of-the-art compression cell lodged in a powder X-ray diffractometer was used to measure anisotropic Young's moduli (YM) of flufenamic acid (FFA) polymorphs in this study. Methodology is based on the generation of a single crystal deformation in this cell, which reflects as a change in the d-spacing in the PXRD pattern. Anisotropic YM was calculated from such information gathered along different FFA planes. Measured FFA crystallographic molecular features were concatenated to understand macroscopic compaction (Heckel and Shapirao's parameters) and tableting performance. Block shaped crystals of FFA form I, and III after initial characterization with SEM, DSC, PXRD, and FTIR were compressed normal to X, Y, and Z-planes, identified from calculated PXRD pattern using the reported single crystal structure. YM of X and Y planes of form I was significantly higher than corresponding planes of form III. Z plane of form III showed significantly higher YM than that for form I. Low YM of form III can be attributed to its large d-spacing regardless of their high E_att than form I, as well as orientation of supramolecular acid dimer (OH⋯O) homosynthon chains in the FFA planes. FFA form I stiffness was further confirmed with lower densification and higher yield pressure of deformation than form III. Clearly, form III exhibited better compressibility, compactibility, and tableting performance than form I due to favorable molecular and macroscopic features. Thus, developed anisotropic measurement approach can be used to distinguish material performance in the early development stage of the pharmaceutical processes.

A method for visualization of the variation of noncovalent interactions in crystal structures of conformational polymorphs

A method for clear visualization of the variation of noncovalent interactions in crystal structures of conformational polymorphs is developed and introduced. The first stage of the method establishes the characteristics of all, without exception, noncovalent interactions in all crystal structures under discussion. This is possible using a strict and objective method of construction of Voronoi–Dirichlet polyhedra within the framework of the stereoatomic model of crystal structures. The second stage of the method then involves plotting of diagrams, showing the relation between parameters characterizing interatomic interactions and chosen geometric parameters of molecules. Application of the title method to highly polymorphic systems of ROY and flufenamic acid allows several imperceptible features of real crystal structures to be revealed and determines the value of different types of interactions in their conformational polymorphs. The method is universal as it can be readily adapted to any system of crystal structures in which noncovalent interactions change as a function of any parameters. Employment of the title method along with quantum chemical calculations offers opportunities for the correlation of potential energy of crystalline materials with noncovalent interactions in their structures, which is a giant step forward towards a more complete understanding of the relationship between the structure and properties of compounds.

Pharmaceutical cocrystals, salts and polymorphs: Advanced characterization techniques

The main goal of a novel drug development is to obtain it with optimal physiochemical, pharmaceutical and biological properties. Pharmaceutical companies and scientists modify active pharmaceutical ingredients (APIs), which often are cocrystals, salts or carefully selected polymorphs, to improve the properties of a parent drug. To find the best form of a drug, various advanced characterization methods should be used. In this review, we have described such analytical methods, dedicated to solid drug forms. Thus, diffraction, spectroscopic, thermal and also pharmaceutical characterization methods are discussed. They all are necessary to study a solid API in its intrinsic complexity from bulk down to the molecular level, gain information on its structure, properties, purity and possible transformations, and make the characterization efficient, comprehensive and complete. Furthermore, these methods can be used to monitor and investigate physical processes, involved in the drug development, in situ and in real time. The main aim of this paper is to gather information on the current advancements in the analytical methods and highlight their pharmaceutical relevance.

Pharmaceutical Co-Crystal of Flufenamic Acid: Synthesis and Characterization of Two Novel Drug-Drug Co-Crystal

Two novel pharmaceutical co-crystals of anti-inflammatory drug flufenamic acid (FFA) with 2-chloro-4-nitrobenzoic acid (CNB) and ethenzamide (ETZ) have been synthesized by solvent evaporation method as well as by solvent drop-assisted grinding method. The synthesized co-crystals were characterized thoroughly by various spectroscopic methods and crystal structures were determined by single-crystal x-ray diffraction technique. In FFA-CNB co-crystal, robust supramolecular acid-acid homosynthon was observed. FFA-ETZ co-crystal is formed via robust supramolecular acid-amide heterosynthon. In FTIR spectra, a significant shift in the carbonyl stretching frequency was observed for the co-crystals due to the presence of intermolecular hydrogen bond. ¹H nuclear magnetic resonance study suggests the presence of hydrogen bond in the solution state of FFA-ETZ co-crystal; however, it was absent for FFA-CNB co-crystal. Solubility study in Millipore water revealed that the solubility of FFA is increased by 2-fold when it is in the form of FFA-CNB co-crystal and no increment in the solubility of FFA was observed in FFA-ETZ co-crystal. About 5-fold increment in the solubility of FFA was observed in both the co-crystals in 0.1 N HCl (pH 1) solution. The synthesized co-crystals were found to be non-hygroscopic at ∼75% relative humidity and stable for a period of 6 months at ambient temperature (∼25°C).

Solid-state investigation on a new dimorphic substituted N-salicylidene compound: insights into its thermochromic behaviour

The solid-state polymorphism behaviour of a new thermochromic mechanosynthesized compound derived from ortho-vanillin is investigated through SCXRD and ab initio prediction methods.