Physical stability enhancement of theophylline via cocrystallization

Transformation pathways of cocrystal hydrates when coformer modulates water activity

An important attribute of cocrystals is that their properties can be tailored to meet required solubility and stability specifications. But before such practical uses can be realized, a better understanding of the factors that dictate co-crystal behavior is needed. This study attempts to explain the phase behavior of anhydrous/hydrated cocrystals when the coformer modulates both water activity and co-crystal solubility. Stability dependence on solution composition and water activity was studied for theophylline-citric acid (THP-CTA) anhydrous and hydrated cocrystals by both suspension and vapor equilibration methods. Eutectic points and associated water activities were measured by suspension equilibration methods to determine stability regions and phase diagrams. The critical water activity for the anhydrous-hydrate co-crystal was found to be 0.8. It is shown that (a) both water and coformer activities determine phase stability, and (b) excipients that alter water activity can profoundly affect the hydrate/anhydrous eutectic points and phase stability. Vapor phase stability studies demonstrate that cocrystals of highly water soluble coformers, such as citric acid, are predisposed to conversions due to moisture uptake and deliquescence of the coformer. The presence of such coformers as trace level impurities with co-crystal will alter hygroscopic behavior and stability.

Effect of position isomerism on the formation and physicochemical properties of pharmaceutical co-crystals

The effect of position isomerism on the co-crystals formation and physicochemical properties was evaluated. Piracetam was used as the model compound. Six position isomers, 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-dihydroxybenzoic acid (DHBA), were used as the co-crystal formers. Co-crystals were prepared on a 1:1 molar ratio by crystallization from acetonitrile. The solid-state properties of co-crystals were characterized using X-ray powder diffractometry (XRD), differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR). All co-crystal formers formed co-crystal with piracetam except 2,6-DHBA. This failure was possibly due to steric hindrance of two bulk hydroxyl groups and preference of intra-molecular hydrogen bonding formation between hydroxyl group and carboxylic acid group. The XRD patterns of resulting co-crystal indicated that they are highly crystalline and different than parental compounds. Based on the single crystal data, P_23DHBA is orthorhombic while P_24DHBA, P_34DHBA, and P_35DHB belong to monoclinlic system. The hydrogen bonding network patterns of the co-crystals are also different. DSC data showed that the melting temperatures of resulting co-crystals are all lower than that of the starting materials. The melting point rank order of the co-crystals is: P_24DHBA > P_34DHBA > P_23DHBA > P_25DHBA > P_35DHBA.

Benefits of cocrystallisation in pharmaceutical materials science: an update

Objectives

We provide a brief overview of recent applications of cocrystals for improving the physico-chemical and materials properties of active pharmaceutical ingredients, including solubility, humidity and thermal stability, dissolution rates and compressibility for tablet formation.

Key findings

This overview illustrates the pharmaceutical applications of cocrystals, with a selection of recent examples and also attempts to foresee future developments by proposing several directions not yet explored in the area of pharmaceutical cocrystallisation.

Summary

Reliable strategies for the synthesis and design of pharmaceutical cocrystals have now been established, and the potential of cocrystallisation for enhancing the solid-state properties of drugs is well recognised; the field is now moving towards the understanding of cocrystal structure–property relationships, for which systematic structural studies and computational approaches will play a key role.

Effect of Moisture on Powder Flow Properties of Theophylline

Powder flow is influenced by environmental factors, such as moisture and static electricity, as well as powder related factors, such as morphology, size, size distribution, density, and surface area. Pharmaceutical solids may be exposed to water during storage in an atmosphere containing water vapor, or in a dosage form consisting of materials (e.g., excipients) that contain water and are capable of transferring in to other ingredients. The effect of moisture on powder flowability depends on the amount of water and its distribution. The aim of this work was to examine the effect of humidity on the flow properties of theophylline using information derived from solid-state analysis of the systems investigated.

Using cocrystals to systematically modulate aqueous solubility and melting behavior of an anticancer drug

Five cocrystals of an anticancer compound have been assembled using a well-defined hydrogen-bond-based supramolecular approach that produced the necessary structural consistency in the resulting solids. These cocrystals contain aliphatic even-numbered dicarboxylic acids of increasing chain length, and as a result, the physical properties of the cocrystals can be related to the molecular structure of the acid. The melting points of the five cocrystals show an excellent correlation with the melting points of the individual acids, and it has also been shown that aqueous solubility can be increased by a factor of 2.5 relative to that of the individual drug. Consequently, cocrystals can offer a range of solid forms from which can be chosen an active ingredient where a particular physical property can be dialed in, provided that the cocrystals show considerable structural consistency and that systematic changes are made to the participating cocrystallizing agents.

Theophylline-gentisic acid (1/1)

In the title 1:1 cocrystal, C₇H₈N₄O₂·C₇H₆O₄, the anti-asthmatic drug theophylline (systematic name: 1,3-dimethyl-7H-purine-2,6-dione) and a non-steroidal anti-inflammatory drug, gentisic acid (systematic name: 2,5-dihydroxy­benzoic acid) crystallize together, forming two-dimensional hydrogen-bonded sheets involving N—H⋯O and O—H⋯N hydrogen bonds. The overall crystal packing features π–π stacking inter­actions [centroid–centroid distance = 3.348 (1) Å]. The cocrystal described herein belongs to the class of pharmaceutical cocrystals involving two active pharmaceutical ingredients which has been relatively unexplored to date.

Co-crystals: a novel approach to modify physicochemical properties of active pharmaceutical ingredients

Crystal form can be crucial to the performance of a dosage form. This is especially true for compounds that have intrinsic barriers to drug delivery, such as low aqueous solubility, slow dissolution in gastrointestinal media, low permeability and first-pass metabolism. The nature of the physical form and formulation tends to exhibit the greatest effect on bioavailability parameters of water insoluble compounds that need to be given orally in high doses. An alternative approach available for the enhancement of drug solubility, dissolution and bioavailability is through the application of crystal engineering of co-crystals. The physicochemical properties of the active pharmaceutical ingredients and the bulk material properties can be modified, whilst maintaining the intrinsic activity of the drug molecule. This article covers the advantages of co-crystals over salts, solvates (hydrates), solid dispersions and polymorphs, mechanism of formation of co-crystals, methods of preparation of co-crystals and application of co-crystals to modify physicochemical characteristics of active pharmaceutical ingredients along with the case studies. The intellectual property implications of creating co-crystals are also highly relevant.

Pharmaceutical Cocrystals and Their Physicochemical Properties

This review article will highlight and discuss the advances made over the last 10 years pertaining to physical and chemical property improvements through pharmaceutical cocrystals and, hopefully, draw closer the fields of crystal engineering and pharmaceutical sciences.

Trimorphs of a pharmaceutical cocrystal involving two active pharmaceutical ingredients: potential relevance to combination drugs

The first example of a trimorphic cocrystal involving two active pharmaceutical ingredients, ethenzamide and gentisic acid, is reported; metastable polymorphs convert to the stable form upon solid-state grinding; pharmaceutical cocrystals involving two or more APIs have potential relevance to combination drugs.

Dissolution improvement and the mechanism of the improvement from cocrystallization of poorly water-soluble compounds

PURPOSE

To demonstrate improvement in the dissolution of exemestane and megestrol acetate from cocrystallization using various particle sizes and to investigate the mechanism of the improved dissolution.

METHODS

Cocrystal screening was performed by slurry crystallization. The cocrystals were identified and characterized by powder X-ray diffraction, thermal analysis, and single crystal X-ray diffraction. Different particle sizes of each cocrystal were prepared from organic solutions. Solubility and dissolution rates were evaluated using dissolution tests. Transformation behavior of the cocrystals in suspension was analyzed by PXRD and polarization microscopy.

RESULTS

Two novel cocrystals were obtained: exemestane (EX)/maleic acid (MAL) (cocrystal 1) and megestrol acetate (MA)/saccharin (SA) (cocrystal 2). Cocrystal 1 showed a high dissolution rate even with large particles. Cocrystal 2 showed supersaturation with fine particles. The transformation from cocrystal 1 to EX was observed within 1 min in suspension. Cocrystal 2 was transformed to MA within 2-4 h.

CONCLUSIONS

Cocrystallizations of EX and MA improved initial dissolution rates compared to the respective original crystals. The mechanism of dissolution enhancement varied. With cocrystal 1, fine particle formation resulted in enhancement, whereas with cocrystal 2, enhancement was due to the maintenance of the cocrystal form and rapid dissolution before transformation to the original crystal.

Conformational richness and multiple Z' in salt co-crystal of N-methylpiperidine betaine with N-methylpiperidine betaine hexafluorosilicate

The co-crystal structure of N-methylpiperidine betaine with N-methylpiperidine betaine hexafluorosilicate represents an unusual case of a salt co-crystal with a high Z' value (3), unexpected conformational variability, and with nearly 50% of its contents disordered. The betaine units from the salt and co-crystal formers are paired into several homoconjugated dimers via very short, linear O(-)...H(+)...O(-) bridges. These hydrogen bonds are the dominating interactions in the co-crystal structure, in variance with the simple hexafluorosilicate salt, which has a structure governed by COOH...F hydrogen bonds. The SiF(2-)(6) anion in the co-crystal structure has only C-H...F interactions with the betaine units. The zwitterion:cation:anion stoichiometry is 3:3:1.5. Some of the betaine units display disorder, but each case is different. One of the SiF(2-)(6) anions is ordered while possessing exact crystallographic symmetry. The other one is disordered in a general position. In addition, there are three water molecules in the crystal structure. One is fully ordered, one has an H atom disordered in two positions and the third one occupies two alternative positions with unequal populations.

Exploring the relationship between cocrystal stability and symmetry: is Wallach's rule applicable to multi-component solids?

Comparison of structure and hydration stability of pairs of chiral and racemic binary cocrystals indicates that the racemic solid is more stable than the chiral one; we illustrate that this difference might arise from intermolecular (crystal packing) factors in one case, while intramolecular (molecular conformation) factors are more significant in the other.

Supramolecular synthon polymorphism in 2 : 1 co-crystal of 4-hydroxybenzoic acid and 2,3,5,6-tetramethylpyrazine

Co-crystals of 4-hydroxybenzoic acid and 2,3,5,6-tetramethylpyrazine (2 : 1) exhibit the first supramolecular synthon polymorphism in a co-crystal; metastable anti-hierarchic polymorph I converts to stable hierarchic form II.

Indomethacin-saccharin cocrystal: design, synthesis and preliminary pharmaceutical characterization

PURPOSE

To design and prepare cocrystals of indomethacin using crystal engineering approaches, with the ultimate objective of improving the physical properties of indomethacin, especially solubility and dissolution rate.

MATERIALS AND METHODS

Various cocrystal formers, including saccharin, were used in endeavours to obtain indomethacin cocrystals by slow evaporation from a series of solvents. The melting point of crystalline phases was determined. The potential cocrystalline phase was characterized by DSC, IR, Raman and PXRD techniques. The indomethacin-saccharin cocrystal (hereafter IND-SAC cocrystal) structure was determined from single crystal X-ray diffraction data. Pharmaceutically relevant properties such as the dissolution rate and dynamic vapour sorption (DVS) of the IND-SAC cocrystal were evaluated. Solid state and liquid-assisted (solvent-drop) cogrinding methods were also applied to indomethacin and saccharin.

RESULTS

The IND-SAC cocrystals were obtained from ethyl acetate. Physical characterization showed that the IND-SAC cocrystal is unique vis-à-vis thermal, spectroscopic and X-ray diffraction properties. The cocrystals were obtained in a 1:1 ratio with a carboxylic acid and imide dimer synthons. The dissolution rate of IND-SAC cocrystal system was considerably faster than that of the stable indomethacin gamma-form. DVS studies indicated that the cocrystals gained less than 0.05% in weight at 98%RH. IND-SAC cocrystal was also obtained by solid state and liquid-assisted cogrinding methods.

CONCLUSIONS

The IND-SAC cocrystal was formed with a unique and interesting carboxylic acid and imide dimer synthons interconnected by weak N-Hcdots, three dots, centeredO hydrogen bonds. The cocrystals were non-hygroscopic and were associated with a significantly faster dissolution rate than indomethacin (gamma-form).

Hierarchy of Supramolecular Synthons: Persistent Hydroxyl···Pyridine Hydrogen Bonds in Cocrystals That Contain a Cyano Acceptor

An analysis of the Cambridge Structural Database reveals >99% occurrence of the hydroxyl...pyridine supramolecular heterosynthon in crystal structures that contain hydroxyl and pyridine moieties in the absence of other hydrogen-bonding moieties. The occurrence of the hydroxyl...cyano supramolecular heterosynthon in crystal structures that contain hydroxyl and cyano moieties is ca. 77%. Such high frequencies indicate that these heterosynthons are strongly favored over the competing hydroxyl...hydroxyl supramolecular homosynthon. However, the CSD does not contain enough information to evaluate which supramolecular heterosynthon prevails when only OH, pyridine, and CN moieties are present in a crystal structure. We have addressed the competition between the hydroxyl...pyridine and the hydroxyl...cyano supramolecular heterosynthons by characterizing a series of 17 cocrystals that are composed of cocrystal formers which contain a permutation of OH, pyridine, and CN functional groups. Structural analysis reveals that all cocrystals are sustained by the hydroxyl...pyridine heterosynthon.

Celecoxib:nicotinamide dissociation: using excipients to capture the cocrystal's potential

The cocrystal of celecoxib and nicotinamide (Cel:Nic) was crystallized from chloroform in a 1:1 ratio, and the structure has been solved from powder X-ray diffraction data. The dissolution and solubility of Cel:Nic are medium dependent and can be attributed to differences in conversion of Cel:Nic to celecoxib polymorphs I and III (Cel-I and Cel-III). The presence of low concentrations of surfactants facilitates the rapid conversion of neat Cel:Nic to large aggregates of Cel-III that dissolve more slowly than commercial Cel-III into 1% SDS solution. In contrast, combinations of Cel:Nic with both 1-10% solid SDS and PVP wet rapidly and convert to a mixture of amorphous celecoxib and a micron-sized crystalline celecoxib form IV (Cel-IV), which has recently been shown to be up to 4-fold more bioavailable than marketed Cel-III. More than 90% of the suspended material dissolves within 2 min at 37 degrees C when transferred to 1% SDS solution. This example highlights the importance of exploring the form conversion of cocrystals in aqueous media prior to pharmacokinetic studies, and illustrates the potential of simple formulations to overcome the limitations caused by rapid dissociation of cocrystals and recrystallization of poorly soluble forms in aqueous media.

Screening for pharmaceutical cocrystal hydrates via neat and liquid-assisted grinding

The formation of cocrystal hydrates represents a potential route to achieve molecular materials with improved properties, particularly stability under conditions of high relative humidity. We describe the use of neat and liquid-assisted grinding for screening for hydrated forms of pharmaceutical cocrystals. In the case of liquid-assisted grinding, water is present in the reaction mixture as a liquid, whereas in the case of neat grinding, it is introduced by employing crystalline hydrates as reactants. The ability to form a cocrystal hydrate by either of the two methods appears to be variable, depending on the choice of cocrystal components. Theophylline readily forms a cocrystal hydrate with citric acid. This contrasts with the behavior of caffeine, which provides only an anhydrous cocrystal ("caffeine citrate") even when both reactants are crystalline hydrates. The preference of theophylline to form a cocrystal hydrate is qualitatively explained by similarity between crystal structures of the products and reactant hydrates. Overall, liquid-assisted grinding is less sensitive to the form of the reactant (i.e., hydrate or anhydrate) than neat grinding. For that reason liquid-assisted grinding appears to be a more efficient method of screening for cocrystal hydrates, and it is also applicable to screening for hydrates of APIs.

Mechanisms by which moisture generates cocrystals

The purpose of this study is to determine the mechanisms by which moisture can generate cocrystals when solid particles of cocrystal reactants are exposed to deliquescent conditions (when moisture sorption forms an aqueous solution). It is based on the hypothesis that cocrystallization behavior during water uptake can be derived from solution chemistry using models that describe cocrystal solubility and reaction crystallization of molecular complexes. Cocrystal systems were selected with active pharmaceutical ingredients (APIs) that form hydrates and include carbamazepine, caffeine, and theophylline. Moisture uptake and crystallization behavior were studied by gravimetric vapor sorption, X-ray powder diffraction, and on-line Raman spectroscopy. Results indicate that moisture uptake generates cocrystals of carbamazepine-nicotinamide, carbamazepine-saccharin, and caffeine or theophylline with dicarboxylic acid ligands (oxalic acid, maleic acid, glutaric acid, and malonic acid) when solid mixtures with cocrystal reactants deliquesce. Microscopy studies revealed that the transformation mechanism to cocrystal involves (1) moisture uptake, (2) dissolution of reactants, and (3) cocrystal nucleation and growth. Studies of solid blends of reactants in a macro scale show that the rate and extent of cocrystal formation are a function of relative humidity, moisture uptake, deliquescent material, and dissolution rates of reactants. It is shown that the interplay between moisture uptake and dissolution determines the liquid phase composition, supersaturation, and cocrystal formation rates. Differences in the behavior of deliquescent additives (sucrose and fructose) are associated with moisture uptake and composition of the deliquesced solution. Our results show that deliquescence can transform API to cocrystal or reverse the reaction given the right conditions. Key indicators of cocrystal formation and stability are (1) moisture uptake, (2) cocrystal aqueous solubility, (3) solubility and dissolution of cocrystal reactants, and (4) transition concentration.

An Overview of Pharmaceutical Cocrystals as Intellectual Property

This review article focuses on the interaction among certain scientific, legal, and regulatory aspects of pharmaceutical crystal forms. The article offers an analysis of pharmaceutical cocrystals as patentable inventions by drawing upon recent scientific developments in the field. Several potential commercial advantages of pharmaceutical cocrystals are highlighted, and a number of recent court decisions involving salient issues are summarized. The article provides an outlook on how the developing field of cocrystallization may impact the pharmaceutical intellectual property landscape.

The salt-cocrystal continuum: the influence of crystal structure on ionization state

Salts and cocrystals are multicomponent crystals that can be distinguished by the location of the proton between an acid and a base. At the salt end of the spectrum proton transfer is complete, and on the opposite end proton transfer is absent in cocrystals. However, for acid-base complexes with similar pK(a) values, the extent of proton transfer in the solid state is not predictable and a continuum exists between the two extremes. For these systems, both the DeltapK(a) value (pK(a) of base - pK(a) of acid) and the crystalline environment determine the extent of proton transfer. A total of 20 complexes containing theophylline and guest molecules with DeltapK(a) values less than 3 have been prepared, resulting in 13 cocrystals, five salts, and two complexes with mixed ionization states based on IR spectroscopy and single-crystal diffraction data. We propose modifications to the DeltapK(a) rule for selecting salt screen counterions that focus on the discovery of solid forms with useful physical properties rather than an arbitrary cutoff value for DeltapK(a).