Predicting the solubility of drugs in solvent mixtures: multiple solubility maxima and the chameleonic effect

Ensemble completeness in conformer sampling: the case of small macrocycles

In this study we compare the three algorithms for the generation of conformer ensembles Biovia BEST, Schrödinger Prime macrocycle sampling (PMM) and Conformator (CONF) form the University of Hamburg, with ensembles derived for exhaustive molecular dynamics simulations applied to a dataset of 7 small macrocycles in two charge states and three solvents. Ensemble completeness is a prerequisite to allow for the selection of relevant diverse conformers for many applications in computational chemistry. We apply conformation maps using principal component analysis based on ring torsions. Our major finding critical for all applications of conformer ensembles in any computational study is that maps derived from MD with explicit solvent are significantly distinct between macrocycles, charge states and solvents, whereas the maps for post-optimized conformers using implicit solvent models from all generator algorithms are very similar independent of the solvent. We apply three metrics for the quantification of the relative covered ensemble space, namely cluster overlap, variance statistics, and a novel metric, Mahalanobis distance, showing that post-optimized MD ensembles cover a significantly larger conformational space than the generator ensembles, with the ranking PMM > BEST >> CONF. Furthermore, we find that the distributions of 3D polar surface areas are very similar for all macrocycles independent of charge state and solvent, except for the smaller and more strained compound 7, and that there is also no obvious correlation between 3D PSA and intramolecular hydrogen bond count distributions.

Structure-Permeability Relationship of Semipeptidic Macrocycles-Understanding and Optimizing Passive Permeability and Efflux Ratio

We herein report the first thorough analysis of the structure-permeability relationship of semipeptidic macrocycles. In total, 47 macrocycles were synthesized using a hybrid solid-phase/solution strategy, and then their passive and cellular permeability was assessed using the parallel artificial membrane permeability assay (PAMPA) and Caco-2 assay, respectively. The results indicate that semipeptidic macrocycles generally possess high passive permeability based on the PAMPA, yet their cellular permeability is governed by efflux, as reported in the Caco-2 assay. Structural variations led to tractable structure-permeability and structure-efflux relationships, wherein the linker length, stereoinversion, N-methylation, and peptoids site-specifically impact the permeability and efflux. Extensive nuclear magnetic resonance, molecular dynamics, and ensemble-based three-dimensional polar surface area (3D-PSA) studies showed that ensemble-based 3D-PSA is a good predictor of passive permeability.

Solubility Determination of Hydroquinone in Dichloromethane, Trichloromethane and Carbon Tetrachloride by Using the Co-solvent Calibration Method

In this report, the solubility of hydroquinone (1,4-dihydroxybenzene) was determined in dichloromethane, trichloromethane and carbon tetrachloride at 298 K by using cyclic voltammetry. By the proposed cosolvent calibration method a mixed solvent was used for the determination of solubility where acetonitrile was the co-solvent. The molar ratios of the two solvents were uniform during the whole procedure both in the calibration solutions and in solutions used for solubility determinations. The solubility of hydroquinone decreased by the decrease of solvent permittivity, which were in dichloromethane 5.683 mmol·L−1, in trichloromethane 2.785 mmol·L−1 and in carbon tetrachloride 0.321 mmol·L−1.

Estimating the Aqueous Solubility of Pharmaceutical Hydrates

Estimation of crystalline solute solubility is well documented throughout the literature. However, the anhydrous crystal form is typically considered with these models, which is not always the most stable crystal form in water. In this study, an equation which predicts the aqueous solubility of a hydrate is presented. This research attempts to extend the utility of the ideal solubility equation by incorporating desolvation energetics of the hydrated crystal. Similar to the ideal solubility equation, which accounts for the energetics of melting, this model approximates the energy of dehydration to the entropy of vaporization for water. Aqueous solubilities, dehydration and melting temperatures, and log P values were collected experimentally and from the literature. The data set includes different hydrate types and a range of log P values. Three models are evaluated, the most accurate model approximates the entropy of dehydration (ΔSd) by the entropy of vaporization (ΔSvap) for water, and utilizes onset dehydration and melting temperatures in combination with log P. With this model, the average absolute error for the prediction of solubility of 14 compounds was 0.32 log units.

Quantifying the chameleonic properties of macrocycles and other high-molecular-weight drugs

Key to the pharmaceutical utility of certain macrocyclic drugs is a 'chameleonic' ability to change their conformation to expose polar groups in aqueous solution, but bury them when traversing lipid membranes. Based on analysis of the structures of 20 macrocyclic compounds that are approved oral drugs, we propose that good solubility requires a topological polar surface area (TPSA, in Å(2)) of ≥0.2×molecular weight (MW). Meanwhile, good passive membrane permeability requires a molecular (i.e., 3D) PSA in nonpolar environments of ≤140Å(2). We show that one or other of these limits is almost invariably violated for compounds with MW>600Da, suggesting that some degree of chameleonic behavior is required for most high MW oral drugs.

Solubility behavior and prediction for antihelmintics at several temperatures in aqueous and nonaqueous mixtures

A model based on solubility parameters is proposed to predict the solubility curves of antihelmintic drugs at several temperatures, including aqueous and non-aqueous mixtures. The solubility of the drugs was measured in ethanol-water and ethanol-ethyl acetate mixtures at 15-35 degrees C (mebendazole) and at 25 degrees C (thiabendazole and metronidazole). The solid phases were analyzed by differential scanning calorimerty. The polymorphic form A of mebendazole was also characterized from infrared spectroscopy. Markedly different solubility profile shapes were obtained against the solubility parameter of the mixtures: two symmetrical peaks (metronidazole), two maxima of different height (mebendazole) and a single peak (thiabendazole). The solubility parameter of the drugs was related to the co-solvent action of both mixtures and to the solubility peaks. The single equation proposed was able to predict solubility profiles of different shape, including both mixtures and all temperatures, providing reasonable physical meaning for the regression coefficients. The model was successfully tested for its predictive capability using a limited number of experimental data. More than 100 solubilities were predicted at several temperatures using 20 data point for each drug.

Solubility and Thermoresponsiveness of PMMA in Alcohol-Water Solvent Mixtures

To reduce the environmental burden of polymer processing, the use of non-toxic solvents is desirable. In this regard, the improved solubility of poly(methyl methacrylate) (PMMA) in ethanol/water solvent mixtures is very appealing. In this contribution, detailed investigations on the solubility of PMMA in alcohol/water solvent mixtures are reported based on turbidimetry measurements. PMMA revealed upper critical solution temperature transitions in pure ethanol and ethanol/water mixtures. However, around 80 wt-% ethanol content a solubility maximum was observed for PMMA as indicated by a decrease in the transition temperature. Moreover, the transition temperatures increased with increasing PMMA molar mass as well as increasing polymer concentration. Careful analysis of both heating and cooling turbidity curves revealed a peculiar hysteresis behaviour with a higher precipitation temperature compared with dissolution with less than 60 wt-% or more than 90 wt-% ethanol in water and a reverse hysteresis behaviour at intermediate ethanol fractions. Finally, the transfer of poly(styrene)-block-poly(methyl methacrylate) block copolymer micelles from the optimal solvent, i.e. aqueous 80 wt-% ethanol, to almost pure water and ethanol is demonstrated.

Tuning solution polymer properties by binary -solvent mixtures

The solubility of polymers can be significantly altered by the use of solvent mixtures. The solvent composition also effects the self-assembly properties of amphiphilic copolymers. In addition, water-ethanol mixtures are known to exhibit abnormal physicochemical properties due to the presence of hydration shells around the ethanol molecules, while at the same time both solvents have very low toxicity. However, the solution properties of amphiphilic copolymers in water-ethanol mixtures have been scarcely studied. Here we show that the solution polymer properties of amphiphilic copoly(2-oxazoline)s can be significantly altered in binary water-ethanol mixtures resulting in increased solubility, tuneable lower critical solution temperatures as well as polymer-solvent combinations with both a LCST followed by an UCST and improved dispersion stability. Surprisingly, it was found that polymers insoluble in both ethanol and water could be dissolved in water-ethanol mixtures, opening the way to novel formulations for drug delivery or personal care applications. Our results represent a straightforward method for tuning solution polymer properties without the synthetic efforts that are generally required to change the copolymer composition and properties.

Solubility parameter of drugs for predicting the solubility profile type within a wide polarity range in solvent mixtures

The solubility enhancement produced by two binary mixtures with a common cosolvent (ethanol-water and ethyl acetate-ethanol) was studied against the solubility parameter of the mixtures (delta1) to characterize different types of solubility profiles. Benzocaine, salicylic acid and acetanilide show a single peak in the least polar mixture (ethanol-ethyl acetate) at delta1=22.59, 21.70 and 20.91 MPa1/2, respectively. Phenacetin displays two solubility maxima, at delta1=25.71 (ethanol-water) and at delta1=23.30 (ethyl acetate-ethanol). Acetanilide shows an inflexion point in ethanol-water instead of a peak, and the sign of the slope does not vary when changing the cosolvent. The solubility profiles were compared to those obtained in dioxane-water, having a solubility parameter range similar to that covered with the common cosolvent system. All the drugs reach a maximum at about 90% dioxane (delta1=23 MPa1/2). A modification of the extended Hildebrand method is applicable for curves with a single maximum whereas a model including the Hildebrand solubility parameter delta1 and the acidic partial solubility parameter delta1a is required to calculate more complex solubility profiles (with inflexion point or two maxima). A single equation was able to fit the solubility curves of all drugs in the common cosolvent system. The polarity of the drug is related to the shape of the solubility profile against the solubility parameter delta1 of the solvent mixtures. The drugs with solubility parameters below 24 MPa1/2 display a single peak in ethanol-ethyl acetate. The drugs with delta2 values above 25 MPa1/2 show two maxima, one in each solvent mixture (ethanol-water and ethanol-ethyl acetate). The position of the maximum in ethanol-ethyl acetate shifts to larger polarity values (higher delta1 values) as the solubility parameter of the drug delta2 increases.

Solubility of drugs in aqueous solutions

This paper is devoted to the verification of the quality of experimental data regarding the solubility of sparingly soluble solids, such as drugs, environmentally important substances, etc. in mixed solvents. A thermodynamic consistency test based on the Gibbs-Duhem equation for ternary mixtures is suggested. This test has the form of an equation, which connects the solubilities of the solid, and the activity coefficients of the constituents of the solute-free mixed solvent in two mixed solvents of close compositions. The experimental data regarding the solubility of sparingly soluble substances can be verified with the suggested test if accurate data for the activity coefficients of the constituents of the solute-free mixed solvent are available. The test was applied to a number of systems representing the solubilities of sparingly soluble substances in mixed solvents. First, the test was scrutinized for four nonaqueous systems for which accurate solubility data were available. Second, the suggested test was applied to a number of systems representing experimental data regarding the solubility of sparingly soluble substances in aqueous mixed solvents.

Influence of solvent composition on the solid phase at equilibrium with saturated solutions of quinolones in different solvent mixtures

The dissolution profiles and solubilities of three quinolonic drugs (oxolinic, pipemidic, and nalidixic acids) in different solvent mixtures were studied. The behavior of the solid phase, during solubility experiments was in-depth investigated with the aim of detecting possible crystalline modifications, such as polymorphic transitions or solvate formations, that might modify drug stability and/or solubility properties. In order to test the influence of both the nature and polarity of the co-solvents, aqueous and non-aqueous binary mixtures have been prepared by using Lewis base (dioxane and ethyl acetate) and amphiprotic co-solvents (ethanol and water). Differential scanning calorimetry (DSC), hot stage microscopy, IR spectroscopy and X-ray powder diffraction were used in combination with solubility and dissolution studies to characterize and investigate the solid state properties of the original powders and the corresponding ones at equilibrium with the different pure solvents and solvent mixtures examined. The solid phases of nalidixic and oxolinic acids did not show any change after equilibration with the various pure solvents or binary solvent mixtures, regardless the chemical nature of the examined solvents. On the contrary, in the case of pipemidic acid, the different analytical techniques used to characterize the drug solid state enabled identification of a solvated form at equilibrium with pure dioxane and a trihydrated form in aqueous mixtures of water with both ethanol (amphiprotic) or dioxane (Lewis base) in a concentration range from 10 to 100% water.

Thermodynamic origin of the solubility profile of drugs showing one or two maxima against the polarity of aqueous and nonaqueous mixtures: niflumic acid and caffeine

The purpose of this work was to investigate the origin of the different solubility profiles of drugs against the polarity of solvent mixtures with a common cosolvent. Niflumic acid and caffeine where chosen as model drugs. The solubilities were measured at five or six temperatures in aqueous (ethanol-water) and nonaqueous (ethyl acetate-ethanol) mixtures. The enthalpies of solution were obtained at the harmonic mean of the experimental temperature. Solid phase changes were analyzed using differential scanning calorimetry and thermomicroscopy. A single solubility maximum was obtained for niflumic acid against the solubility parameter of both mixtures that is not related to solid phase changes. In contrast, caffeine displays two maxima and anhydrous-hydrate transition occurs at the solubility peak in the amphiprotic mixture. The apparent enthalpies of solution of both drugs show endothermic maxima against solvent composition that are related to hydrophobic hydration. A general explanation for the cosolvent action in aqueous mixtures is proposed. The dominant mechanism shifts from entropy to enthalpy at a certain cosolvent ratio dependent on the hydrophobicity and the solubility parameter of the drug. Niflumic acid and caffeine show enthalpy-entropy compensation in ethanol-water, and this relationship is demonstrated for the first time in nonaqueous mixtures. The results support that enthalpy-entropy compensation is a general effect for the solubility of drugs in solvent mixtures. The shape of the solubility curves is correlated with the compensation plots. The solubility peaks separate different enthalpy-entropy relationships that also differentiate the solubility behavior of the hydrate and the anhydrous forms of caffeine.

Solubility parameters as predictors of miscibility in solid dispersions

This paper reports interactions and possible incompatibilities in solid dispersions of hydrophobic drugs with hydrophilic carriers, with solubility parameters employed as a means of interpreting results. Systems containing ibuprofen (IB) and xylitol (XYL) in varying proportions and systems of IB with other sugars and a sugar polymer were produced using solvent evaporation and fusion methods. Additionally, bridging agents were employed with IB/XYL systems to facilitate the production of a solid dispersion. Results show that IB formed no interactions with any of the sugar carriers but interacted with all the bridging agents studied. The bridging agents were immiscible with XYL in the liquid state. Results of other reported drug/carrier systems and those from the systems studied in this paper were interpreted using Hildebrand solubility parameters. A trend between differences in drug/carrier solubility parameters and immiscibility was identified with incompatibilities evidence when large solubility parameter differences exist between drug and carrier. It was concluded that Hildebrand parameters give an indication of possible incompatibilities between drugs and carriers in solid dispersions, but that the use of partial solubility parameters may provide a more accurate prediction of interactions in and between materials and could provide more accurate indications of potential incompatibilities.

Nonlinear enthalpy-entropy compensation for the solubility of phenacetin in dioxane-water solvent mixtures

The solubility of phenacetin was determined at five temperatures in solvent mixtures of aprotic-amphiprotic mixtures of dioxane and water. Enthalpy-entropy compensation analysis is used to study the effect of changing polarity of the medium on the solute. The apparent heats of solution and free energies of solution are nonlinear functions of the cosolvent (dioxane) ratio. The free energy curve goes through a minimum at 80-90% dioxane in water, whereas the apparent heat of solution displays a maximum at low cosolvent ratio (40% dioxane) and a minimum at high cosolvent ratio (90% dioxane). A plot of delta H against delta G shows a nonlinear compensation effect. Two different mechanisms (entropy and enthalpy) are suggested to be the driving forces to increase solubility. These two mechanisms can be related to the nonlinearity of the compensation effect. The slope changes from positive to negative at 40% dioxane. The overall nonlinear function can also be considered as two linear relationships that intersect at 40% dioxane. The results support the usefulness of enthalpy-entropy compensation analysis for a better understanding of the solubility of drugs in aqueous mixtures as related to the role of cosolvents.

Solvent effects on chemical processes. 10. Solubility of alpha-cyclodextrin in binary aqueous-organic solvents: relationship to solid phase composition

The equilibrium solubility at 25 degrees C of alpha-cyclodextrin was measured in several binary aqueous-organic solvent mixtures, the organic cosolvents being methanol, 2-propanol, ethylene glycol, and acetone. Solubility maxima were observed as the solvent composition was varied from pure water to pure cosolvent. The maximum in methanol systems was hardly detectable, but it was very pronounced in 2-propanol and acetone, and two maxima were seen in the ethylene glycol system. Karl Fischer analysis of the solid phase isolated from 2-propanol/water equilibrium systems showed that alpha-cyclodextrin hexahydrate is the stable form in water, whereas a solid phase containing three water molecules per molecule of alpha-cyclodextrin is the stable form in the presence of 2-propanol. The appearance of a solubility maximum in this system, and by extension presumably in the other cosolvent systems, is attributed to the existence of more than one stable solid phase of alpha-cyclodextrin.

Enthalpy and entropy contributions to the solubility of sulphamethoxypyridazine in solvent mixtures showing two solubility maxima

The solubility of sulphamethoxypyridazine was measured at several temperatures in mixtures of water:ethanol and ethanol:ethyl acetate. Sulphamethoxypyridazine was chosen as a model drug to compare the solvation effects of proton donor-proton acceptor (water and ethanol) and proton acceptor (ethyl acetate) solvents and mixtures of these solvents because this drug contains functional groups capable of Lewis acid-base interaction. A plot of the mole fraction solubility against the solubility parameter (delta 1 = 30.87 MPa1/2 (20:80 v/v water:ethanol) and another at delta 1 = 20.88 MPa1/2 (30:70 v/v ethanol:ethyl acetate) at all the temperatures under study. The enthalpies and entropies of mixing as well as the enthalpies and entropies of transfer of sulphamethoxypyridazine from ethanol of mixing as well as the enthalpies and entropies of transfer of sulphamethoxypyridazine from ethanol to water:ethanol and ethanol:ethyl acetate mixtures were calculated to compare solvation characteristics of the solvent mixtures toward the drug. As ethanol is added to water, the entropy increases and the structure of the solvent mixture became less ordered, favouring the interaction of the drug with the solvent mixture. On the other hand, in the case of the ethanol:ethyl acetate mixture, solubility is favoured by the more negative enthalpy values. This way, the same result, i.e. a solubility maximum, is obtained by different routes. In the ethanol:water mixtures, the dissolution process if entropy-controlled while enthalpy is the driving force in the case of ethanol:ethyl acetate mixtures. The two solvent systems show enthalpy-entropy compensation. Water deviates from the linear relationship due possibly to its hydrophobic effect.