Mechanochemical Transformations of Pharmaceutical Cocrystals: Polymorphs and Coformer Exchange

Transformations of solid samples under solvent-free or minimal solvent conditions set the future trend and define a modern strategy for the production of new materials. Of the various technologies tested in recent years, the mechanochemical approach seems to be the most promising for economic and ecological reasons. The aim of this review article is to present the current state of art in solid state research on binary systems, which have found numerous applications in the pharmaceutical and materials science industries. This article is divided into three sections. In the first part, we describe the new equipment improvements, which include the innovative application of thermo-mechanochemistry, sono-mechanochemistry, photo-mechanochemistry, electro-mechanochemistry, as well as resonant acoustic mixing (RAM), and transformation under high-speed sample spinning ("SpeedMixing"). A brief description of techniques dedicated to ex-situ and in-situ studies of progress and the mechanism of solid matter transformation (PXRD, FTIR, Raman and NMR spectroscopy) is presented. In the second section, we discuss the problem of cocrystal polymorphism highlighting the issue related with correlation between mechanochemical parameters (time, temperature, energy, molar ratio, solvent used as a liquid assistant, surface energy, crystal size, crystal shape) and preference for the formation of requested polymorph. The last part is devoted to the description of the processes of coformer exchange in binary systems forced by mechanical and/or thermal stimuli. The influence of the thermodynamic factor on the selection of the best-suited partner for the formation of a two-component stable structure is presented.

First-Order Derivative Spectrophotometry for Simultaneous Determination of Vitamin C and Nicotinamide: Application in Quantitative Analysis of Cocrystals

Vitamin C (l-ascorbic acid, ASC) and the amide form of vitamin B3 nicotinamide (NIC) can form cocrystals through hydrogen bonding. Currently, there is a lack of fast and reliable alternatives for precisely quantifying cocrystal components and their purity. Spectrophotometric analysis for quantifying such vitamin preparations is challenging due to overlapping absorbance bands in a narrow wavelength range in the ultraviolet (UV) region. Moreover, ASC undergoes progressive degradation if not diluted in a proper medium, requiring stability during quantitative analysis. This study adopted a fast, simple, and reliable two-component spectrophotometric assay for simultaneously determining ASC and NIC based on the first-order derivative spectrophotometry (FODS) method using sodium oxalate as a stabilizer for vitamin C. The FODS method showed linearity between 2 and 24 μg·mL-1 and good precision. The standard addition method was employed to validate FODS, with high recovery percentages (96.5 to 102.4% for ASC and 95.3 to 101.9% for NIC). The FODS method was successfully applied to quantify ASC and NIC in bulk powder produced by the gas antisolvent method. The proposed method could estimate cocrystal purity through mass balance regarding the expected 1:1 stoichiometry, confirmed by PXRD and DSC. Cocrystal purity determined by the FODS method (58-100%) aligned well with results from LC-MS (62-100%), with an accuracy exceeding 97%. The FODS method is as sensitive and accurate as high-performance liquid chromatography for simultaneously determining vitamin concentrations deriving from cocrystals. However, it is less costly, more efficient, and a suitable alternative to classical solid-state methods for estimating cocrystal purity.

Application of resonant acoustic mixing in the synthesis of vitamin C-nicotinamide variable stoichiometry cocrystals

The use of resonant acoustic mixing (RAM) to synthesize variable stoichiometry cocrystals of nicotinamide and vitamin C was investigated. Liquid assisted RAM (LA-RAM) was used to generate two polymorphs, Form I and II, of the 1 : 1 cocrystal of nicotinamide and vitamin C at a 700 mg scale using ethanol and methanol respectively as the liquid additives. LA-RAM was used to scale up polymorphs I and II of the 1 : 1 cocrystal to 20 grams. Finally, LA-RAM used was to produce a high purity 3 : 1 cocrystal of nicotinamide and vitamin C when either methanol or ethanol was used as the liquid additive. LA-RAM is demonstrated to be a scalable, environmentally friendly, ball-free method to make variable stoichiometry cocrystals.

Novel Ascorbic Acid Co-Crystal Formulations for Improved Stability

A series of co-crystals of ascorbic acid were prepared with equimolar amounts of co-crystal formers (CCFs), including isonicotinic acid, nicotinic acid, 3,4-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid and m-hydroxybenzoic acid, by slow solvent evaporation and solvent-assisted grinding. The co-crystals were characterized by single-crystal X-ray diffraction spectroscopy, powder X-ray diffraction, IR spectroscopy, differential scanning calorimetry and thermogravimetric analysis. Molecular dynamics (MD) simulations further validated the interaction energy and the possible intermolecular hydrogen bonds among VC and CCFs. The co-crystals showed improved stability when exposed to different wavelengths of light, pH and temperatures compared to the free analogue, especially at higher pH (~9) and lower temperature (~4 °C).

Design of Ascorbic Acid Eutectic Mixtures With Sugars to Inhibit Oxidative Degradation

L-Ascorbic acid (ASC), commonly known as vitamin C, acts as an anti-oxidant in the biological system. It is extensively used as an excipient in pharmaceutical industry, food supplements in fruit juices, and food materials due to its free radicals scavenging activity. Main drawback of ASC is its poor aqueous stability owing to the presence of lactone moiety that is easily oxidized to dehydroascorbic acid and further degraded. To improve aqueous stability and inhibit oxidative degradation, ASC was co-crystallized to constitute binary eutectic compositions with mono and di-saccharides such as glucose, sucrose, lactose, and mannitol. The eutectics were confirmed by their (single) lower melting endotherm compared to ASC and sugars, although Powder X-ray diffraction (PXRD) and Fourier transform Infrared spectroscopy (FT-IR) data confirmed the characteristics of their physical mixture. Scanning electron microscope (SEM) images of the binary eutectics confirmed their irregular morphology. The ASC eutectics exhibited improved shelf-life by 2–5-fold in weakly acidic (pH 5) and neutral (pH 7) aqueous buffer medium, whereas the eutectic with glucose enhanced shelf-life only by 1.1–1.2-fold in acidic medium (pH 3.3 and 4). Notably, stabilizing effect of the sugar eutectics decreased with increasing acidity of the medium. In addition, higher binding energy of the disaccharide eutectics partially supports the aqueous stability order of ASC in the neutral pH medium due to more number of non-bonded interactions than that of monosaccharides.

Variable stoichiometry cocrystals: occurrence and significance

Stoichiometric variation in organic cocrystals, their synthesis, structure elucidation and properties are discussed. Accountable reasons for the occurrence of such cocrystals are emphasised.

Cocrystal engineering of pharmaceutical solids: therapeutic potential and challenges

This highlight presents an overview of pharmaceutical cocrystal production and its potential in reviving problematic properties of drugs in different dosage forms. The challenges and future outlook of its translational development are discussed.

A cocrystal of L-ascorbic acid with picolinic acid: the role of O-H...O, N-H...O and C-H...O hydrogen bonds and L-ascorbic acid conformation in structure stabilization

A new 1:1 cocrystal (L-Asc-Pic) of L-ascorbic acid (vitamin C) with picolinic acid was prepared as a powder and as single crystals. The crystal structure was solved and refined from single-crystal X-ray diffraction (SCXRD) data collected at 293 (2) and 100 (2) K. The samples of the L-Asc-Pic cocrystal were characterized by elemental (HCNS) analysis and titrimetric methods, TG/DTG/DSC, and IR and Raman spectroscopy. The asymmetric unit comprises a picolinic acid zwitterion and an L-ascorbic acid molecule. The stabilization energy of intermolecular interactions involving hydrogen bonds, the vibrational spectrum and the energies of the frontier molecular orbitals were calculated using the GAUSSIAN09 and the CrystalExplorer17 programs. The charge distribution on the atoms of the L-Asc-Pic cocrystal, L-ascorbic acid itself and its 12 known cocrystals (structures from Version 5.40 of the Cambridge Structural Database) were calculated by the methods of Mulliken, Voronoi and Hirshfeld charge analyses (ADF) at the bp86/TZ2P+ level of theory. The total effective charges and conformations of the L-ascorbic acid molecules in the new and previously reported cocrystals were compared with those of the two symmetry-independent molecules in the crystals of L-ascorbic acid. A correlation between molecular conformation and its effective charge is discussed.

Anisotropic elasticity and plasticity of an organic crystal

Organic crystals are generally considered to be brittle, inelastic materials, which pose challenges for application in flexible devices. Inspired by α helical proteins for their key structural role in flexible hair, here, we describe the construction of a spring-like hydrogen bonded network through the self-assembly of a-OH/e-OH cyclohexanol derivatives.

Supramolecular Gel as the Template for Catalysis, Inorganic Superstructure, and Pharmaceutical Crystallization

A supramolecular gel is a fascinating combination of flexibility and orderliness. While the supramolecular nature of crosslinking contributes towards the adaptivity and the reversibility of the system, orderliness at the molecular level amplifies the functional output and induces extraordinary selectivity into the system. Therefore, use of supramolecular gels as the soft template is an emerging area of research, which includes but not limited to catalysis of a chemical or a photochemical process, transcription of gel property to a substrate, or even controlling the nucleation of drug molecules. This review aims to highlight the template effect of supramolecular gels in the above-mentioned areas relevant to novel fundamental chemistry, technology, and healthcare.

Supramolecular Metal-Phenolic Gels for the Crystallization of Active Pharmaceutical Ingredients

The use of supramolecular gel media for the crystallization of active pharmaceutical ingredients (APIs) is of interest for controlling crystal size, morphology, and polymorphism, as these features determine the performance of pharmaceutical formulations. In contrast to supramolecular systems prepared from synthetic gelators, herein, supramolecular metallogels based on a natural polyphenol (tannic acid) are used for the crystallization of APIs. The gel-grown API crystals exhibit considerable differences in size, morphology, and polymorphism when compared with those formed in solutions. These physical features can also be tailored by varying the gel composition and additives, suggesting an influence of the gel medium on the crystallization outcomes. Furthermore, these gel-API crystal composites can be used for sustained drug release, indicating their potential as drug delivery systems. The facile preparation of these supramolecular gels and the use of naturally abundant components in their synthesis provide a generic platform for studying gel-mediated crystallization of diverse APIs.

Taming photo-induced oxidation degradation of dihydropyridine drugs through cocrystallization

Cocrystallization of nifedipine (NFD) and isonicotinamide (INA) resulted in two monotropic cocrystal polymorphs. The thermodynamically more stable polymorph presents superior photostability compared to the other. Cocrystallization can be an efficient approach to address the stability deficit of dihydropyridine drugs.

Cocrystallization in vitamin B9 gels to construct stoichiometry-controlled isostructural materials

The potential application of structurally equivalent Cl, Br, and I in the design of isostructural materials in VB₉ gels is illustrated by constructing pairs of solids that exhibit different fluorescence properties.