Solution thermodynamics and solubility prediction of glibenclamide in Transcutol + water co-solvent mixtures at 298.15–333.15 K

Solution thermodynamics and solubility of glibenclamide (GBN) in binary co-solvent mixtures of Transcutol + water at temperature range of 298.15–333.15 K were investigated in present study. The modified Apelblat model was used to predict the solubility of GBN in co-solvent mixtures at various temperatures. The highest and lowest solubility of GBN were observed in pure Transcutol and pure water, respectively. Moreover, all co-solvent mixtures had highest solubility at 333.15 K. The experimental solubility data of GBN was correlated well with the modified Apelblat model at each temperature studied with relative absolute deviation in the range of 0.008–5.903 %. The correlation coefficients in co-solvent mixtures were observed in the range of 0.995–0.999 which indicated good fitting of experimental data with calculated one. The enthalpies and entropies for GBN dissolution were observed in the range of 2.012–38.215 kJ mol^?1 and 6.748–114.709 J mol^?1 K^?1, respectively indicating its dissolution is endothermic and an entropy-driven process. These results indicated that Transcutol can be used as a co-solvent in preformulation studies and formulation development of GBN.     

Disagreements Between Calorimetric and Van't Hoff Enthalpies of Adsorption: A New Langmuir-like Model to Account for the Effect of Solvent Displacement Stoichiometry

The reported inconsistencies between calorimetry and the van't Hoff equation hinder the utility of thermodynamics in pharmaceutical research. In ligand binding or adsorption assays, it is believed that the van't Hoff equation falls short because of the lack of stoichiometric treatment in the equilibrium constant. A new modified Langmuir-Like equation that accounts for the stoichiometry of solute adsorption and solvent displacement is proposed in this work. The performance of the model was evaluated by studying the adsorption of phenobarbital from aqueous solutions by commercial activated carbon. The amount of water occupying the adsorption sites was estimated by graphical analysis of the ‘knee point’ of water-vapor adsorption isotherms and was found to correlate well with the relative percentage of hydroxyl and carbonyl surface groups. It was found that one phenobarbital molecule displaces 2-6 water molecules from the adsorption site. It is shown that adsorption enthalpy was not affected by the adjustment for stoichiometry, supporting the notion that the van't Hoff enthalpy is intrinsic and is independent of the stoichiometry of solvent displacement in Langmuir-based binding. The widely reported disparities between the van't Hoff and calorimetric enthalpies are unlikely to be from a stoichiometric origin.     

截短侧耳素在醇-水复合溶剂体系溶解度的测定与关联

截短侧耳素是一种三环二萜类抗生素，因其抗菌机制独特，与一般抗菌药物不发生交叉耐药，而日益受到重视。本 研究测定了截短侧耳素在不同甲醇 ( 或乙醇 )- 水复合溶剂 ( 醇含量 0~50%) 中在 273.15~333.15K 溶解度，并采用多项式经验方程 和 Apelblat 方程进行关联。结果表明，随着温度的提高，随溶剂中甲醇或乙醇含量的增加，截短侧耳素的溶解度显著同步增加。 多项式经验方程和 Apelblat 方程都能很好拟合截短侧耳素溶解度数据，回归系数都在 0.993 以上。热力学分析表明，截短侧耳 素在甲醇 ( 或乙醇 )- 水复合溶剂中的溶解过程焓变都是正值，且随着溶剂中醇含量的增加而增加，是一吸热过程。溶解过程的 熵变在纯水中是负值，随着溶剂中醇含量的增加，熵变也随之增加并且成为溶解度大幅提升的主要驱动力。本论文的研究结果 可为截短侧耳素的提取工艺优化及制剂工艺开发提供重要的科学依据。     

Solubility behaviour of griseofulvin in fatty acids

The solubility of griseofulvin in the straight-chain alkanoic acids from C₂ to C₂₂ and in the C₄ and C₅ alkanoic acids branched at C-2 was measured at various temperatures. The enthalpy of fusion of griseofulvin, measured by differential scanning calorimetry, was 39.39 kJ mol^?1 at the melting point (495.15 K) and 36.95 kJ mol^?1 at 373.15 K. The standard Gibbs free energy (ΔG°), standard enthalpy (ΔH°) and standard entropy (ΔS°) of solution were calculated at 373.15 K, from the van't Hoff plot of the temperature dependence of the mole fraction solubility in terms of the pure supercooled liquid solute as the standard state. With increasing chain length of the alkanoic acid solvents, ΔG° increased in parallel with the increase in pK_a of the acids in water, suggesting that the solubility behaviour involves specific solute-solvent proton interactions, while ΔH° and ΔS° fluctuated but tended to decrease in parallel with the corresponding decreases in their enthalpies and entropies of ionization, respectively. The fluctuations in ΔH° and ΔS° may be attributed to the different solid adducts containing griseofulvin and the solvent. An observed non-linear (logarithmic) decrease in solubility with decreasing molality of the carboxyl group in the liquid solvent on ascending the homologous series is attributed to the disturbing influence of the hydrocarbon chains on the specific solute-solvent hydrogen bonding. Chain-branching of the solvent at C-2 gave a reduced solubility of griseofulvin and higher ΔG°, ΔH° and ΔS° values compared with the corresponding straight chain acid.     

Excess free energy approach to the estimation of solubility in mixed solvent systems I: Theory

An approach is developed by which the solubility of an organic compound in mixed solvents may be estimated. In this approach, an expression for the excess Gibbs free energy of mixing for multicomponent solvent systems was used to obtain parameters characteristic of the interaction between the solvents. A fairly simple equation which predicts the solubility of a solute in a binary solvent system over the entire solvent composition range was then derived. The equation may be partitioned into terms that contain (a) pure solvent solubilities, (b) solvent-solvent interaction contributions, and (c) contributions from the solute-mixed solvent interactions. The required data are the molar volume of the solute, the pure solvent solubilities, and, theoretically, one experimentally determined solubility in a solvent mixture. The equation can be easily extended for systems with three or more solvents.     

Studies on the adsorption and solubility of nalidixic acid

The adsorption of nalidixic acid on certain pharmaceutical additives was investigated and its extent estimated. Also, the effects of various surface-active agents and hydrophilic polymers on the solubility of the drug was determined. The adsorption isotherms have revealed that the degree of adsorption increased in the order; microcrystalline cellulose < ethyl cellulose < silicon dioxide < aluminium magnesium silicate. Studies on the solubility of nalidixic acid at 37°C using different concentrations of hydrophilic polymers have demonstrated that methyl cellulose, polvinylpyrrolidone and polyethylene glycol 6000 have comparable effects. Polyvinyl alcohol caused only a marginal increase in solubility. The surface-active agents, sodium lauryl sulphate and polysorbate 80, also increased solubility of the drug. The values of the heat of solution and the free energy changes were calculated from the Van't Hoff plots. The values of heat of solution for all systems were comparable, but the free energy changes indicate that sodium lauryl sulphate causes the most marked effect on solubility.     

Correlation and in vitro studies on radioactive and nonradioactive albendazole-beta-cyclodextrin complex tablets

The work aims to confirm the complexation of albendazole (ABZ) by beta-cyclodextrin (beta-CD), and to compare them with pure ABZ tablets using radioactive and nonradioactive dissolution studies. The complex tablets were prepared by kneading a binary mixture of ABZ and beta-CD and a direct compression method. Nuclear magnetic resonance (NMR) spectroscopy, scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy were examined to prove the formation of complexes in the final products. The radiolabelled tablets were labelled with 99mTc-DTPA. Dissolution studies were performed with radiolabelled and nonradiolabelled tablets in two dissolution media (pH 1.2 and pH 7.4). The tablets were added to an acidic solution (pH = 1.2) to quantify the concentration of the drug inside the beta-CD cavity. The other medium (pH = 7.4) was used to prove the existence of non-complexed drug in each powder, as the drug's solubility increases with pH. It was observed that complexation occurred in all tablets, and beta-cyclodextrin (beta-CD) could increase the aqueous solubility. Further, a correlation was shown between dissolution results for radiolabelled and nonradiolabelled tablets. This study shows that the characterization studies were a good indicator for the ABZ: beta-CD complex. According to the phase solubility studies, the solubility of ABZ increased when the amount of beta-CD increased, and drug release from tablets in pH 7.4 and pH 1.2 media was dramatically improved by the addition of beta-CD compared with the pure ABZ tablet.     

Modeling the Solubility of Pharmaceuticals in Pure Solvents and Solvent Mixtures for Drug Process Design

The knowledge of the solubility of pharmaceuticals in pure solvents and solvent mixtures is crucial for designing the crystallization process of drug substances. The first step in finding optimal crystallization conditions is usually a solvent screening. Since experiments are very time consuming, a model which allows for solubility predictions in pure solvents and solvent mixtures based only on a small amount of experimental data is required. In this work, we investigated the applicability of the thermodynamic model perturbed-chain statistical associating fluid theory (PC-SAFT) to correlate and to predict the solubility of exemplary five typical drug substances and intermediates (paracetamol, ibuprofen, sulfadiazine, p-hydroxyphenylacetic acid, and p-aminophenylacetic acid) in pure solvents and solvent mixtures. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:4205–4215, 2009     

Thermodynamics-based mathematical model for solubility prediction of glibenclamide in ethanol–water mixtures

Abstract

Temperature-dependent solubility data of glibenclamide (GBN) in various ethanol–water mixtures is not reported in literature so far. Therefore, the aim of this study was to determine the mole fraction solubility of GBN in various ethanol–water mixtures at the temperature range of 293.15 to 318.15?K. The solubility of GBN was determined by reported shake flask method and the experimental data was fitted in thermodynamics-based modified Apelblat model. The solubility of GBN was found to be increased with increase in temperature and mass fraction of ethanol in ethanol–water mixtures. The experimental data of GBN was well correlated with the modified Apelblat model at each temperature range with correlation coefficient of 0.9940–1.0000. The relative absolute deviation (AD) was found to be less than 0.1% except in pure ethanol and water. The positive values of enthalpies and entropies for GBN dissolution indicated that its dissolution is endothermic and an entropy-driven process.     

Solubility of drugs in aqueous solutions. Part 2: binary nonideal mixed solvent

As in a previous paper [Int. J. Pharm. 258 (2003) 193–201], the Kirkwood–Buff theory of solutions was employed to calculate the solubility of a solid in mixed solvents. Whereas in the former paper the binary solvent was assumed ideal, in the present one it was considered nonideal. A rigorous expression for the activity coefficient of a solute at infinite dilution in a mixed solvent [Int. J. Pharm. 258 (2003) 193–201] was used to obtain an equation for the solubility of a poorly soluble solid in a nonideal mixed solvent in terms of the solubilities of the solute in the individual solvents, the molar volumes of those solvents, and the activity coefficients of the components of the mixed solvent.

The Flory–Huggins and Wilson equations for the activity coefficients of the components of the mixed solvent were employed to correlate 32 experimental data sets regarding the solubility of drugs in aqueous mixed solvents. The results were compared with the models available in literature. It was found that the suggested equation can be used for an accurate and reliable correlation of the solubilities of drugs in aqueous mixed binary solvents. It provided slightly better results than the best literature models but has also the advantage of a theoretical basis.     

The influence of temperature on the sorption of benzocaine by nylon 6 from aqueous cosolvents

The influence of temperature on the sorption of benzocaine by nylon 6 powder from aqueous cosolvents has been assessed. At about 30° discontinuities appear in the Van't Hoff plots for sorption from water and aqueous PEG 400, which may be attributed to transitional changes in the polymer structure. No such discontinuities are apparent for sorption from aqueous ethanol over the temperature range 15–60°, which is indicative of plasticization by this cosolvent.     

Synthesis and characterization of mesoporous SBA-15 and SBA-16 as carriers to improve albendazole dissolution rate

Albendazole (ABZ, anti-parasitic active pharmaceutical ingredient) is a crystalline low water-soluble drug, thus the dissolution rate in gastrointestinal fluids is limited. Consequently, the improvement of the water solubility and dissolution rate of ABZ implies a great challenge for a more efficient treatment of hydatidosis. In this context, SBA-15 and SBA-16 ordered mesoporous silica materials were synthetized and loaded with ABZ. X-ray diffraction, FT-IR spectroscopy, nitrogen physisorption manometry, particle size distribution and scanning electronic microscopy were used to characterize unloaded and loaded materials (ABZ/SBA-15 and ABZ/SBA-16). The loaded ABZ amount in the carriers was estimated by elemental analysis. For the loaded materials, the drug solubility and release profile were evaluated. In addition, mathematical models were compared to explain the dissolution kinetics of ABZ from mesoporous solids. ABZ was successfully loaded into the mesopores. The amorphous state of the adsorbed ABZ was confirmed by differential scanning calorimetry that resulted in a notable increment in the dissolution rate compared to crystalline ABZ. Drug release behaviors were well simulated by the Weibull model for ABZ/SBA-15 and by the Gompertz function for pure ABZ and ABZ/SBA-16. The SBA-15 carrier exhibited the highest drug loading and dissolution rate becoming a promising material to improve ABZ bioavailability.     

Estimation of the solubility parameter of trimethoprim by current methods

Group contribution, the extended Hildebrand solubility (EHS) and the extended Hansen solubility approaches are utilised to estimate the solubility parameter value of trimethoprim. The solubility data in the binary solvent series showed peak solubility at the solubility parameter of solvent, 11.7 H. The regression expression on the lines of the EHS approach yielded 12.06 H. The data on the solubility of trimethoprim in individual solvents was processed as per the extended Hansen approach. The partial solubility parameters were obtained and are used to estimate the solubility parameter, 12.93 H. Extending the Flory-Huggins size correction to this approach gave a value of 10.95 H and improved the correlation coefficient by 3 percent. Partitioning of hydrogen bonding (four parameter approach) didn't improve the value.     

Thermal denaturation of tryptophan synthase α-subunit

Thermal denaturation for the wild-type of tryptophan synthase α-subunit from E. coli and one of its mutant proteins was followed by CD measurements at various pHs in the alkaline region and the results from van't Hoff analyses of the thermal denaturation curves were compared with those from calorimetry. Although the far-u.v. CD spectra of the thermally denatured proteins differed from those of the completely denatured states in 3.2 M guanidine hydrochloride, the titration curves by denaturants at higher temperatures were not sigmoidal but straight lines, indicating that the cooperative structure of the proteins has been completely destroyed by heating. The ratio of calorimetric enthalpy change to van't Hoff enthalpy change obtained from calorimetric study was unity, indicating that the thermal denaturation of the proteins was a two-state system. The unfolding heat capacity change (ΔCp) of the wild-type protein from van't Hoff analysis of the thermal denaturation curves by CD measurement was estimated to be 2.45 kcal/mol ± deg, which was similar to that from calorimetry. The values of unfolding enthalpy change at denaturation temperatures were lower by about 15 kcal/mol compared to those from calorimetry.     

Disagreements Between Calorimetric and Van't Hoff Enthalpies of Adsorption II: Effect of pH and pH Buffers on Phenobarbital Adsorption to Activated Carbon

The reported inconsistencies between the van't Hoff equation and calorimetry hinder the utility of thermodynamics in biochemical and pharmaceutical research. A novel thermodynamic approach is developed herein for ligand adsorption with a focus on the interpretation of calorimetric data in the presence of concurrent proton exchange reactions. Such exchange reactions typically result in a pH-dependence of calorimetric measurements that obscures intrinsic binding enthalpies. It is shown that for the adsorption of phenobarbital to activated carbon, the measured calorimetric enthalpy is a result of three linked acid/base equilibria. A model was established to predict the intrinsic binding enthalpy using 1) the adsorbate's pKa and 2) the adsorbate's enthalpy of protonation. The observed calorimetric enthalpy of binding exhibited both pH and buffer-dependence and was between -5 and -42 kJ/mol. Meanwhile, the predicted intrinsic enthalpy (-25.1 kJ/mol) of binding was in excellent agreement with the measured intrinsic enthalpy (-25.6 kJ/mol). Corrections to the observed calorimetric enthalpies allowed comparisons with enthalpies obtained from the van't Hoff method. It is shown that the predicted intrinsic calorimetric enthalpy agrees well with the van't Hoff enthalpies in instances where observed enthalpies significantly deviated. This treatment is general and is not specific to phenobarbital or activated carbon.     

Comparison of various cosolvency models for calculating solute solubility in water-cosolvent mixtures

Previously published cosolvency models are critically evaluated in terms of their ability to mathematically correlate solute solubility in binary solvent mixtures as a function of solvent composition. Computational results show that the accuracy of the models is improved by increasing the number of curve-fit parameters. However, the curve-fit parameters of several models are limited. The combined nearly ideal binary solvent/Redlich-Kister, CNIBS/R-K, was found to be the best solution model in terms of its ability to describe the experimental solubility in mixed solvents. Also resented is an extension of the mixture response surface model. The extension was found to improve the correlational ability of the original model.     

A model to represent solvent effects on the chemical stability of solutes in mixed solvent systems

The applicability of the combined nearly ideal binary solvent/Redlich-Kister (CNIBS/R-K) equation for quantification of solvent effects on the stability of a solute is shown employing the experimental data of three solutes in different aqueous binary solvents. The proposed model provides a simple computational method to correlate/predict the instability rate constant of a drug in mixed solvent systems. The accuracy of the model is compared with that of a model proposed by Connors and co-workers employing various methods including mean percentage deviation (MPD) as comparison criteria. The obtained overall MPD values for the proposed model to correlate and predict the instability rate constants are 2.05 +/- 1.44 and 4.41 +/- 3.21%, respectively, where the corresponding values for Connors' model are 4.34 +/- 3.28 and 10.74 +/- 9.86%. The results suggest that by using only five experimental instability rate constants at different concentrations of the cosolvent in a binary mixture, it is possible to predict unmeasured values falling between data points within an acceptable error range.     

Solubility prediction of salicylic acid in water-ethanol-propylene glycol mixtures using the Jouyban-Acree model

To show the applicability of a solution model, i.e. the Jouyban-Acree model, for predicting the solubility of a solute in ternary solvent systems based on model constants computed using solubility data of the solute in binary solvent systems, the solubility of salicylic acid in water-ethanol, water-propylene glycol, ethanol-propylene glycol mixtures was determined. A minimum number of three data points from each binary system was used to calculate the binary interaction parameters of the model. Then the solubility in other binary solvent compositions and also in a number of ternary solvents was predicted, and the mean percentage deviation (MPD) was calculated as an accuracy criterion. The overall MPD (+/-SD) was 7.3 (+/-7.3)% and those of a similar predictive model was 15.7 (+/-11.5)%. The mean difference between the proposed and a previous model was statistically significant (paired t-test, p < 0.004).     

Development and oral bioavailability assessment of a supersaturated self‐microemulsifying drug delivery system (SMEDDS) of albendazole

Objectives Albendazole's (ABZ) poor aqueous solubility is a major determinant of its variable therapeutic response (20–50%). The purpose of this study was to develop and optimize the composition of a self‐microemulsifying drug delivery system (SMEDDS) of ABZ and assess its oral pharmacokinetics in rabbits. Methods A D‐optimal mixture design of experiments was used to select the levels of constraints of the formulation variables. The predicted composition was optimized using four responses: dispersion performance, droplet sizes, dissolution efficiency (DE) and time for 85% drug release (t_85%). Key findings The optimal composition of the ABZ‐SMEDDS formulation, with approximately 5 mg/g drug loading of ABZ, was predicted to be Cremophor EL (30% w/w), Tween 80 (15% w/w), Capmul PG‐8 (10% w/w) and acidified PEG 400 (45% w/w). An increase of 63% in the relative bioavailability compared with the commercial suspension was obtained with ABZ‐SMEDDS as measured by albendazole sulfoxide (ABZSO) plasma levels. The area under the curve (AUC_0→24h) and the peak plasma concentration (C_max) of ABZ‐SMEDDS was higher than those obtained with the commercial suspension by 56% and 52%, respectively. Conclusions This study demonstrates a strategy for the development of a supersaturated SMEDDS formulation of a drug with low aqueous solubility.