New Solid Forms of Nitrofurantoin and 4-Aminopyridine Salt: Influence of Salt Hydration Level on Crystal Packing and Physicochemical Properties.

The crystallization of the poorly soluble drug nitrofurantoin (NFT) with 4-aminopyridine (4AmPy) resulted in three multicomponent solid forms with different hydration levels: anhydrous salt [NFT+4AmPy] (1:1), salt monohydrate [NFT+4AmPy+H2O] (1:1:1), and salt tetrahydrate [NFT+4AmPy+H2O] (1:1:4). Each salt was selectively prepared by liquid-assisted grinding in the presence of acetonitrile or ethanol/water mixture at a specific composition. The NFT hydrated salts were characterized using single crystal X-ray diffraction. The [NFT+4AmPy+H2O] salt (1:1:1) crystallized as an isolated site hydrate, while the [NFT+4AmPy+H2O] salt (1:1:4) crystallized as a channel hydrate. The dehydration processes of the NFT salt hydrates were investigated using differential scanning calorimetry and thermogravimetric analysis. A powder dissolution experiment was carried out for all NFT multicomponent solid forms in pH 7.4 phosphate buffer solution at 37 °C.

Formation Thermodynamics of Carbamazepine with Benzamide, Para-Hydroxybenzamide and Isonicotinamide Cocrystals: Experimental and Theoretical Study.

Formation thermodynamic parameters for three cocrystals of carbamazepine (CBZ) with structurally related coformers (benzamide (BZA), para-hydroxybenzamide (4-OH-BZA) and isonicotinamide (INAM)) were determined by experimental (cocrystal solubility and competitive reaction methods) and computational techniques. The experimental solubility values of cocrystal components at eutectic points and solubility product of cocrystals [CBZ + BZA], [CBZ + 4-OH-BZA], and [CBZ + INAM] in acetonitrile at 293.15 K, 298.15 K, 303.15 K, 308.15 K, and 313.15 K were measured. All the thermodynamic functions (Gibbs free energy, enthalpy, and entropy) of cocrystals formation were evaluated from the experimental data. The crystal structure of [CBZ + BZA] (1:1) cocrystal was solved and analyzed by the single crystal X-ray diffractometry. A correlation between the solubility products and pure coformers solubility values has been found for CBZ cocrystals. The relationship between the entropy term and the molecular volume of the cocrystal formation has been revealed. The effectiveness of the estimation of the cocrystal formation thermodynamic parameters, based on the knowledge of the melting temperatures of active pharmaceutical ingredients, coformers, cocrystals, as well as the sublimation Gibbs energies and enthalpies of the individual components, was proven. A new method for the comparative assessment of the cocrystal stability based on the H-bond propensity analysis was proposed. The experimental and theoretical results on the thermodynamic parameters of the cocrystal formation were shown to be in good agreement. According to the thermodynamic stability, the studied cocrystals can be arranged in the following order: [CBZ + 4-OH-BZA] > [CBZ + BZA] > [CBZ + INAM].

Simultaneous Improvement of Dissolution Behavior and Oral Bioavailability of Antifungal Miconazole via Cocrystal and Salt Formation.

Miconazole shows low oral bioavailability in humans due to poor aqueous solubility, although it has demonstrated various pharmacological activities such as antifungal, anti-tubercular and anti-tumor effects. Cocrystal/salt formation is one of the effective methods for solving this problem. In this study, different methods (liquid-assisted grinding, slurrying and lyophilization) were used to investigate their impact on the formation of the miconazole multicomponent crystals with succinic, maleic and dl-tartaric acids. The solid state of the prepared powder was characterized by differential scanning calorimetry, powder X-ray diffraction and scanning electron microscopy. It was found that lyophilization not only promotes partial amorphization of both salts but also allows obtaining a new polymorph of the miconazole salt with dl-tartaric acid. The lyophilized salts compared with the same samples prepared by two other methods showed better dissolution rates but low stability during the studies due to rapid recrystallization. Overall, it was determined that the preparation method of multicomponent crystals affects the solid-state characteristics and miconazole physicochemical properties significantly. The in vivo studies revealed that the miconazole multicomponent crystals indicated the higher peak blood concentration and area under the curve from 0 to 32 h values 2.4-, 2.9- and 4.6-fold higher than the pure drug. Therefore, this study demonstrated that multicomponent crystals are promising formulations for enhancing the oral bioavailability of poorly soluble compounds.

A Combined Experimental and Theoretical Study of Nitrofuran Antibiotics: Crystal Structures, DFT Computations, Sublimation and Solution Thermodynamics.

Single crystal of furazolidone (FZL) has been successfully obtained, and its crystal structure has been determined. Common and distinctive features of furazolidone and nitrofurantoin (NFT) crystal packing have been discussed. Combined use of QTAIMC and Hirshfeld surface analysis allowed characterizing the non-covalent interactions in both crystals. Thermophysical characteristics and decomposition of NFT and FZL have been studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and mass-spectrometry. The saturated vapor pressures of the compounds have been measured using the transpiration method, and the standard thermodynamic functions of sublimation were calculated. It was revealed that the sublimation enthalpy and Gibbs energy of NFT are both higher than those for FZL, but a gain in the crystal lattice energy of NFT is leveled by an entropy increase. The solubility processes of the studied compounds in buffer solutions with pH 2.0, 7.4 and in 1-octanol was investigated at four temperatures from 298.15 to 313.15 K by the saturation shake-flask method. The thermodynamic functions of the dissolution and solvation processes of the studied compounds have been calculated based on the experimental data. Due to the fact that NFT is unstable in buffer solutions and undergoes a solution-mediated transformation from an anhydrate form to monohydrate in the solid state, the thermophysical characteristics and dissolution thermodynamics of the monohydrate were also investigated. It was demonstrated that a combination of experimental and theoretical methods allows performing an in-depth study of the relationships between the molecular and crystal structure and pharmaceutically relevant properties of nitrofuran antibiotics.

Identification of a previously unreported co-crystal form of acetazolamide: a combination of multiple experimental and virtual screening methods.

In the search for new co-crystal forms, many studies only consider one method of co-crystallisation which may lead to incorrect results. In this work, we demonstrate the efficiency of applying multiple experimental and virtual screening methods for a more comprehensive search for co-crystals of acetazolamide. A new co-crystal of acetazolamide with 4-aminobenzoic acid ([ACZ + PABA] (1 : 1)) was discovered, although previously, it had been found in the blind spot of the liquid-assisted grinding (LAG) screening method. The new co-crystal was investigated by different analytical techniques, including the powder and single crystal X-ray diffraction, differential scanning calorimetry, dissolution and solubility methods. The specific features of the mechanochemical formation process for [ACZ + PABA] (1 : 1) were studied. It was found that the appearance of the blind spot of the LAG screening method can be caused by a number of reasons; among those are the high sensitivity to the solvent choice and the low rate of the reagent conversion into the reaction product. A comparison of the ACZ co-crystals with 4-aminobenzoic and 4-hydroxybenzoic acids revealed their close resemblance in terms of the packing energy gain and the driving force of co-crystallization. Therefore, the experimental problems in the formation of the [ACZ + PABA] (1 : 1) co-crystal were associated with a number of kinetic reasons, e.g. the high energy barrier of the nucleation process and the low growth rate of the co-crystal. Using the co-crystal screening of acetazolamide as an example, the effectiveness of five different virtual methods for predicting co-crystal formation was assessed. In order to carry out the virtual screening based on the formation thermodynamics of a hypothetical co-crystal, for the first time ever we studied the ACZ sublimation process. Four out of the five virtual screening methods confirm the formation of the new [ACZ + PABA] (1 : 1) co-crystal.

Pharmaceutical salts of emoxypine with dicarboxylic acids.

New salt forms of the antioxidant drug emoxypine (EMX, 2-ethyl-6-methylpyridin-3-ol) with pharmaceutically acceptable maleic (Mlt), malonic (Mln) and adipic (Adp) acids were obtained {emoxypinium maleate, C8H12NO+·C4H3O4-, [EMX+Mlt], emoxypinium malonate, C8H12NO+·C3H3O4-, [EMX+Mln], and emoxypinium adipate, C8H12NO+·C6H9O4-, [EMX+Adp]} and their crystal structures determined. The molecular packing in the three EMX salts was studied by means of solid-state density functional theory (DFT), followed by QTAIMC (quantum theory of atoms in molecules and crystals) analysis. It was found that the major contribution to the packing energy comes from pyridine-carboxylate and hydroxy-carboxylate heterosynthons forming infinite one-dimensional ribbons, with [EMX+Adp] additionally stabilized by hydrogen-bonded C(9) chains of Adp- ions. The melting processes of the [EMX+Mlt] (1:1), [EMX+Mln] (1:1) and [EMX+Adp] (1:1) salts were studied and the fusion enthalpy was found to increase with the increase of the calculated lattice energy. The dissolution process of the EMX salts in buffer (pH 7.4) was also studied. It was found that the formation of binary crystals of EMX with dicarboxylic acids increases the EMX solubility by more than 30 times compared to its pure form.

Drug-drug cocrystals of antituberculous 4-aminosalicylic acid: Screening, crystal structures, thermochemical and solubility studies.

Experimental multistage cocrystal screening of the antituberculous drug 4-aminosalicylic acid (PASA) has been conducted with a number of coformers (pyrazinamide (PYR), nicotinamide (NAM), isonicotinamide (iNAM), isoniazid (INH), caffeine (CAF) and theophylline (TPH)). The crystal structures of 4-aminosalicylic acid cocrystals with isonicotinamide ([PASA+iNAM] (2:1)) and methanol solvate with caffeine ([PASA+CAF+MeOH] (1:1:1)) have been determined by single X-ray diffraction experiments. For the first time for PASA cocrystals it has been found that the structural unit of the [PASA+iNAM] cocrystal (2:1) is formed by 2 types of heterosynthons: acid-pyridine and acid-amide. The desolvation study of the [PASA+CAF+MeOH] cocrystal solvate (1:1:1) has been conducted. The correlation models linking the melting points of the cocrystals with the melting points of the coformers used in this paper have been developed. The thermochemical and solubility properties for all the obtained cocrystals have been studied. Cocrystallization has been shown to lead not only to PASA solubility improving but also to its higher stability against the chemical decomposition.

New Solid Forms of the Antiviral Drug Arbidol: Crystal Structures, Thermodynamic Stability, and Solubility.

Salts of the antiviral drug Arbidol (umifenovir) with pharmaceutically relevant benzoate and salicylate anions were obtained, and their crystal structures were described. For Arbidol salicylate, an unstable solvate with acetonitrile was also found and characterized. Analysis of the conformational preferences of the Arbidol molecule in the crystal structures showed that it adopts two types of conformations, namely "open" and "closed", both of which correspond to local conformational energy minima of the isolated molecule. Thermal stability of the Arbidol salicylate solvates with chloroform and acetonitrile was analyzed by means of differential scanning calorimetry and thermogravimetric analysis. The standard thermodynamic functions of the salt formation were determined. The Gibbs energy change of the process was found to be negative, indicating that the formation of the salts from individual components is a spontaneous process. The dissolution study of the Arbidol salts performed in aqueous buffer solutions with pH 1.2 and 6.8 showed that both salts dissolve incongruently to form an Arbidol hydrochloride monohydrate at pH 1.2 and an Arbidol base at pH 6.8, respectively.

Influence of Secondary Interactions on the Structure, Sublimation Thermodynamics, and Solubility of Salicylate:4-Hydroxybenzamide Cocrystals. Combined Experimental and Theoretical Study.

Cocrystal screening of 4-hydroxybenzamide with a number of salicylates (salicylic acid, SA; 4-aminosalicylic acid, PASA; acetylsalicylic acid, ASA; and salicylsalicylic acid, SSA) was conducted to confirm the formation of two cocrystals, [SA+4-OHBZA] (1:1) and [PASA+4-OHBZA] (1:1). Their structures were determined using single-crystal X-ray diffraction, and the hydrogen-bond network topology was studied. Thermodynamic characteristics of salicylic acid cocrystal sublimation were obtained experimentally. It was proved that PASA cocrystallization with 4-OHBZA makes the drug more stable and prevents the irreversible process of decarboxylation of PASA resulting in formation of toxic 3-aminophenol. The pattern of non-covalent interactions in the cocrystals is described quantitatively using solid-state density functional theory followed by Bader analysis of the periodic electron density. It has been found that the total energy of secondary interactions between synthon atoms and the side hydroxyl group of the acid molecule in [SA+4-OHBZA] (1:1) and [PASA+4-OHBZA] (1:1) cocrystals is comparable to the energy of the primary acid-amide heterosynthon. The theoretical value of the sublimation enthalpy of [SA+4-OHBZA], 231 kJ/mol, agrees fairly well with the experimental one, 272 kJ/mol. The dissolution experiments with [SA+4-OHBZA] have proved that the relatively large cocrystal stability in relation to the stability of its components has a negative effect on the dissolution rate and equilibrium solubility. The [PASA+4-OHBZA] (1:1) cocrystal showed an enhancement of apparent solubility compared to that of the corresponding pure active pharmaceutical ingredient, while their intrinsic dissolution rates are comparable.

Pharmaceutical salts of ciprofloxacin with dicarboxylic acids.

New salts of antibiotic drug ciprofloxacin (CIP) with pharmaceutically acceptable maleic (Mlt), fumaric (Fum) and adipic (Adp) acids were obtained and their crystal structures were determined. The crystal lattices of the fumarate and adipate salts were found to accommodate the water molecules, while the maleate salt was anhydrous. The dehydration and melting processes were analyzed by means of differential scanning calorimetry and thermogravimetric analysis. Solubility and intrinsic dissolution rates of the salts were measured in pharmaceutically relevant buffer solutions with pH 1.2 and pH 6.8. Under acidic conditions, the salts were found to be less soluble than the parent form of drug, while the [CIP+Fum+H2O] and [CIP+Mlt] solids showed enhanced dissolution rate when compared to a commercially available ciprofloxacin hydrochloride hydrate. In the pH 6.8 solution, all the salts demonstrated solubility improvement and faster dissolution rate with respect to pure CIP.

Pharmaceutical cocrystals of diflunisal and diclofenac with theophylline.

Pharmaceutical cocrystals of nonsteroidal anti-inflammatory drugs diflunisal (DIF) and diclofenac (DIC) with theophylline (THP) were obtained, and their crystal structures were determined. In both of the crystal structures, molecules form a hydrogen bonded supramolecular unit consisting of a centrosymmetric dimer of THP and two molecules of active pharmaceutical ingredient (API). Crystal lattice energy calculations showed that the packing energy gain of the [DIC + THP] cocrystal is derived mainly from the dispersion energy, which dominates the structures of the cocrystals. The enthalpies of cocrystal formation were estimated by solution calorimetry, and their thermal stability was studied by differential scanning calorimetry. The cocrystals showed an enhancement of apparent solubility compared to the corresponding pure APIs, while the intrinsic dissolution rates are comparable. Both cocrystals demonstrated physical stability upon storing at different relative humidity.

Cocrystal screening of hydroxybenzamides with benzoic acid derivatives: a comparative study of thermal and solution-based methods.

The main problem occurring at the early stages of cocrystal search is the choice of an effective screening technique. Among the most popular techniques of obtaining cocrystals are crystallization from solution, crystallization from melt and solvent-drop grinding. This paper represents a comparative analysis of the following screening techniques: DSC cocrystal screening method, thermal microscopy and saturation temperature method. The efficiency of different techniques of cocrystal screening was checked in 18 systems. Benzamide and benzoic acid derivatives were chosen as model systems due to their ability to form acid-amide supramolecular heterosynthon. The screening has confirmed the formation of 6 new cocrystals. The screening by the saturation temperature method has the highest screen-out rate but the smallest range of application. DSC screening has a satisfactory accuracy and allows screening over a short time. Thermal microscopy is most efficient as an additional technique used to interpret ambiguous DSC screening results. The study also included an analysis of the influence of solvent type and component solubility on cocrystal formation.

Salicylamide cocrystals: screening, crystal structure, sublimation thermodynamics, dissolution, and solid-state DFT calculations.

A new cocrystal of 2-hydroxybenzamide (A) with 4-acetamidobenzoic acid (B) has been obtained by the DSC screening method. Thermophysical analysis of the aggregate [A:B] has been conducted and a fusion diagram has been plotted. Cocrystal formation from melts was studied by using thermomicroscopy. A cocrystal single-crystal was grown and its crystal structure was determined. The pattern of noncovalent interactions has been quantified using the solid-state DFT computations coupled with the Bader analysis of the periodic electron density. The sublimation processes of A-B cocrystal have been studied and its thermodynamic functions have been calculated. The classical method of substance transfer by inert gas-carrier was chosen to investigate sublimation processes experimentally. The lattice energy is found to be 143 ± 4 kJ/mol. It is lower than the sum of the corresponding values of the cocrystal pure components. The theoretical value of the lattice energy, 156 kJ/mol, is in reasonable agreement with the experimental one. A ternary phase diagram of solubility (A-B-ethanol) has been plotted and the areas with solutions for growing thermodynamically stable cocrystals have been determined.

Influence of position and size of substituents on the mechanism of partitioning: a thermodynamic study on acetaminophens, hydroxybenzoic acids, and parabens.

The objective of the present investigation was to study the influence of size, nature, and topology of substituents on the thermodynamic characteristics of sublimation, fusion, solubility, solvation, and partitioning processes of some drug and druglike molecules. Thermodynamic functions of sublimation process 2-acetaminophen and 3-acetaminophen were obtained on the basis of temperature dependencies of vapor pressure by the transpiration method. Thermodynamic characteristics of solubility processes in water, n-octanol, and n-hexane were calculated from the temperature dependencies of solubility using the solubility saturation method. For evaluation of fusion parameters, differential scanning calorimetry was used. A new approach to distinguishing specific and nonspecific energetic terms in the crystal lattice was developed. Specific and nonspecific solvation terms were distinguished using the transfer from the "inert" n-hexane to the other solvents. For the acetaminophen compounds and for some related drug molecules, the correlation between melting points and a parameter describing the ratio between specific and nonspecific interaction in the crystal lattices was obtained. A diagram enabling analysis of the mechanism of the partitioning process was applied. It was found that for isomers of benzoic acids and for acetaminophens, the position of substituents affects the mechanism of the partitioning process but not the extent of partitioning (DeltaG(tr)(0) values). In contrast to this, an increased size of substituents (parabens) leads to essential changes in DeltaG(tr)(0) values, but the mechanism of the partitioning process stays the same.

Extent and mechanism of solvation and partitioning of isomers of substituted benzoic acids: a thermodynamic study in the solid state and in solution.

Temperature dependency of saturated vapour pressure and thermochemical characteristics of fusion processes for 2-, 3- and 4-methoxybenzoic acids (anisic acids) were measured and thermodynamic functions of sublimation, fusion, and evaporation calculated. A new approach to split specific and nonspecific energetic terms in the crystal lattice was developed. For methoxybenzoic acid isomers as well as for a number of analogous molecules, a parameter describing molecular packing density by the ratio of free volume of the molecules in the crystal lattice and van der Waals molecular volume is defined. Its relationship to Gibbs energy of sublimation and to the respective melting points was analysed. Temperature dependencies of solubility in buffers with pH 2.0 and 7.4, n-octanol and n-hexane were measured. The thermodynamic functions of solubility, solvation and transfer processes were deduced. Concentration dependence of partition coefficients for the outlined isomers was measured. Specific and nonspecific solvation terms were distinguished using the transfer from the 'inert' n-hexane to the other solvents. Comparison analysis of specific and nonspecific interactions in the solid state and in solution was carried out. A diagram enabling analysis of the mechanism of the partitioning process was applied. It was found that position of substituents essentially affects the mechanism of partitioning in buffer pH 2.0, however, at pH 7.4, the mechanism is independent of the position of the substituent.