Simultaneous Water Sorption and Crystallization in ASDs 1: Stability Studies Lasting for Two Years.

One way to increase the slow dissolution rate and the associated low bioavailability of newly developed active pharmaceutical ingredients (APIs) is to dissolve the API in a polymer, leading to a so-called amorphous solid dispersion (ASD). However, APIs are often supersaturated in ASDs and thus tend to crystallize during storage. The kinetics of the crystallization process is determined by the amount of water the ASD absorbs during storage at relative humidity (RH), storage temperature, polymer type, and the drug load of the ASD. Here, the crystallization kinetics and shelf life of spray-dried ASDs were investigated for ASDs consisting of nifedipine (NIF) or celecoxib (CCX) as the APIs and of poly(vinylpyrrolidone-co-vinyl acetate) or hydroxypropyl methylcellulose acetate succinate as polymers. Samples were stored over 2 years at different RHs covering conditions above and below the glass transition of the wet ASDs. Crystallization kinetics and onset time of the crystallization were qualitatively studied by using powder X-ray diffraction and microscopic inspection and were quantitatively determined by using differential scanning calorimetry. It was found that the NIF ASDs crystallize much faster than CCX ASDs at the same drug load and at the same storage conditions due to both higher supersaturation and higher molecular mobility in the NIF ASDs. Experimental data on crystallization kinetics were correlated using the Johnson-Mehl-Avrami-Kolmogorov equation. A detailed thermodynamic and kinetic modeling will be performed in Part 2 of this paper series.

Simultaneous Water Sorption and Crystallization in ASDs 2: Modeling Long-Term Stabilities.

The physical stability of amorphous solid dispersions (ASDs) is a major topic in the formulation research of oral dosage forms. To minimize the effort of investigating the long-term stability using cost- and time-consuming experiments, we developed a thermodynamic and kinetic modeling framework to predict and understand the crystallization kinetics of ASDs during long-term storage below the glass transition. Since crystallization of the active phrarmaceutical ingredients (APIs) in ASDs largely depends on the amount of water absorbed by the ASDs, water-sorption kinetics and API-crystallization kinetics were considered simultaneously. The developed modeling approach allows prediction of the time evolution of viscosity, supersaturation, and crystallinity as a function of drug load, relative humidity, and temperature. It was applied and evaluated against two-year-lasting crystallization experiments of ASDs containing nifedipine and copovidone or HPMCAS measured in part I of this work. We could show that the proposed modeling approach is able to describe the interplay between water sorption and API crystallization and to predict long-term stabilities of ASDs just based on short-term measurements. Most importantly, it enables explaining and understanding the reasons for different and sometimes even unexpected crystallization behaviors of ASDs.

Dissolution Mechanisms of Amorphous Solid Dispersions: Application of Ternary Phase Diagrams To Explain Release Behavior.

The use of amorphous solid dispersions (ASDs) in commercial drug products has increased in recent years due to the large number of poorly soluble drugs in the pharmaceutical pipeline. However, the release behavior of ASDs is complex and remains not well understood. Often, the drug release from ASDs is rapid and complete at lower drug loadings (DLs) but becomes slow and incomplete at higher DLs. The DL where release becomes hindered is termed the limit of congruency (LoC). Currently, there are no approaches to predict the LoC. However, recent findings show that one potential cause leading to the LoC is a change in phase morphology after water-induced phase separation at the ASD/solution interface. In this study, the phase behavior of ASDs in contact with aqueous solutions was described thermodynamically by constructing experimental and computational ternary phase diagrams, and these were used to predict morphology changes and ultimately the LoC. Experimental ternary phase diagrams were obtained by equilibrating ASD/water mixtures over time. Computational ternary phase diagrams were obtained by Perturbed Chain Statistical Associating Fluid Theory (PC-SAFT). The morphology of the hydrophobic phase was studied with fluorescence confocal microscopy. It was demonstrated that critical point (plait point) composition approximately corresponded to the ASD DL, where the hydrophobic phase, formed during phase separation, became interconnected and hindered ASD release. This work provides mechanistic insights into the ASD release behavior and highlights the potential of in silico ASD design using phase diagrams.

Influence of Hydrophobic and Hydrophilic Chain Length of CiEj Surfactants on the Solubilization of Active Pharmaceutical Ingredients.

Up to 90% of all newly developed active pharmaceutical ingredients (APIs) are poorly water soluble, most likely also showing a low oral bioavailability. In order to increase the aqueous solubility of these APIs, surfactants are promising excipients to increase both solubility and consequently bioavailability (e.g., in lipid- and surfactant-based drug delivery systems). In this work, we investigated the influence of hydrophobic and hydrophilic chain lengths of CiEj surfactants (C8E6, C10E6, and C10E8) toward the solubilization of fenofibrate, naproxen, and lidocaine. Furthermore, we investigated the partitioning of these APIs between the surfactant aggregates and the surrounding aqueous bulk phase. For all APIs considered, we determined the locus of API solubilization as well as the individual aggregation numbers (Nagg) of surfactants and API molecules in an API/surfactant aggregate. We further determined the hydrodynamic radius (Rh) of the API/surfactant aggregates in the absence and presence of the APIs. The size of the API/surfactant aggregates (Nagg, Rh) passes through a minimum upon lidocaine solubilization; it gradually increases upon naproxen solubilization and is almost constant upon fenofibrate solubilization. The results give valuable insights into the solubilization mechanisms of APIs in the CiEj surfactant aggregates. Our results reveal that fenofibrate is solely solubilized in the hydrophobic core of the CiEj surfactant aggregates, as only the hydrophobic chain length of the surfactant influences its solubilization. Naproxen is solubilized in the palisade layer of the surfactant aggregates, as both the hydrophobic and hydrophilic chain lengths are decisive for its solubilization. Lidocaine is mainly solubilized in the rather hydrophilic corona region of the surfactant aggregates, as the hydrophilic chain length of the surfactant governs its solubilization. The results further reveal that the hydrophilic/lipophilic balance is not an appropriate measure to estimate the solubilization capacity of surfactant aggregates.

Water Sorption in Rubbery and Glassy Polymers, Nifedipine, and Their ASDs.

Polymers like poly(vinylpyrrolidone-co-vinyl acetate) (PVPVA) or hydroxypropyl methylcellulose acetate succinate (HPMCAS) are commonly used as a matrix for amorphous solid dispersions (ASDs) to enhance the bioavailability of the active pharmaceutical ingredients (APIs). The stability of ASDs is strongly influenced by the water sorption in the ASD from the surrounding air. In this work, the water sorption in the neat polymers PVPVA and HPMCAS, in the neat API nifedipine (NIF), and in their ASDs of different drug loads was measured above and below the glass-transition temperature. The equilibrium water sorption was predicted using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) combined with the Non-Equilibrium Thermodynamics of Glassy Polymers (NET-GP).The water-sorption kinetics were modeled using the Maxwell-Stefan approach whereas the thermodynamic driving force was calculated using PC-SAFT and NET-GP. The water diffusion coefficients in the polymers, NIF, or ASDs were determined using the Free-Volume Theory. Using the water-sorption kinetics of the pure polymers and of NIF, the water-sorption kinetics of the ASDs were successfully predicted, thus providing the water diffusion coefficients in the ASD as a function of relative humidity and of the water concentration in polymers or ASDs.

Viscoelastic Behavior of Supercooled and Glassy ASDs at Humid Conditions Can Be Predicted.

Amorphous solid dispersions (ASDs) are commonly used to increase the dissolution rate of poorly soluble active pharmaceutical ingredients (APIs). Unfortunately, most ASDs are thermodynamically unstable and, even though kinetically stabilized, will thus eventually crystallize. The crystallization kinetics is determined by the thermodynamic driving force and by molecular mobility, which in turn depend on the drug load, temperature, and relative humidity (RH) at which the ASDs are stored. This work focuses on viscosity as an indicator for the molecular mobility in ASDs. The viscosity and shear moduli of ASDs consisting of the polymer poly(vinylpyrrolidone-co-vinyl acetate) or hydroxypropyl methylcellulose acetate succinate and the API nifedipine or celecoxib were studied using an oscillatory rheometer. The effects of temperature, drug load, and RH on the viscosity were investigated. With the knowledge of how much water is absorbed by the polymer or ASD and thereby also the knowledge of the glass-transition temperature of the wet polymer or ASD, the viscosity of dry and wet ASDs was predicted to be in very good agreement with experimental data just based on the viscosity of neat polymers and the glass-transition temperatures of wet ASDs.

Simultaneous Predictions of Chemical and Phase Equilibria in Systems with an Esterification Reaction Using PC-SAFT.

The study of chemical reactions in multiple liquid phase systems is becoming more and more relevant in industry and academia. The ability to predict combined chemical and phase equilibria is interesting from a scientific point of view but is also crucial to design innovative separation processes. In this work, an algorithm to perform the combined chemical and liquid-liquid phase equilibrium calculation was implemented in the PC-SAFT framework in order to predict the thermodynamic equilibrium behavior of two multicomponent esterification systems. Esterification reactions involve hydrophobic reacting agents and water, which might cause liquid-liquid phase separation along the reaction coordinate, especially if long-chain alcoholic reactants are used. As test systems, the two quaternary esterification systems starting from the reactants acetic acid + 1-pentanol and from the reactants acetic acid + 1-hexanol were chosen. It is known that both quaternary systems exhibit composition regions of overlapped chemical and liquid-liquid equilibrium. To the best of our knowledge, this is the first time that PC-SAFT was used to calculate simultaneous chemical and liquid-liquid equilibria. All the binary subsystems were studied prior to evaluating the predictive capability of PC-SAFT toward the simultaneous chemical equilibria and phase equilibria. Overall, PC-SAFT proved its excellent capabilities toward predicting chemical equilibrium composition in the homogeneous composition range of the investigated systems as well as liquid-liquid phase behavior. This study highlights the potential of a physical sound model to perform thermodynamic-based modeling of chemical reacting systems undergoing liquid-liquid phase separation.

Carboxylation of Acetylene without Salt Waste: Green Synthesis of C4 Chemicals Enabled by a CO2 -Pressure Induced Acidity Switch.

The inherent formation of salt waste in C-H carboxylations is a key obstacle precluding the utilization of CO2 as C1 building block in the industrial synthesis of base chemicals. This challenge is addressed in a circular process for the production of the C4 base chemical dimethyl succinate from CO2 and acetylene. At moderate CO2 pressures, acetylene is doubly carboxylated in the presence of cesium carbonate. Hydrogenation of the C-C triple bond stabilizes the product against decarboxylation. By increasing the CO2 pressure to 70 bar, the medium is reversibly acidified, allowing an esterification of the succinate salt with methanol. The cesium base and the hydrogenation catalyst are regenerated and can be reused. This provides the proof of concept for a salt-free route to C4 chemicals from biogas (CH4 /CO2 ). The origin of this reversible acidity switch and the critical roles of the cesium base and the NMP/MeOH solvents were elucidated by thermodynamic modeling.

Measuring and Modeling Water Sorption in Amorphous Indomethacin and Ritonavir.

The amorphous state of an active pharmaceutical ingredient (API) enhances its water solubility compared to its crystalline state. However, it is well known that amorphous substances can absorb significant amounts of water therewith inducing API recrystallization. This work explores methods to obtain reliable information about water sorption in amorphous APIs and its modeling. Experimental water-sorption curves over a broad humidity range were obtained by measuring the mass increase of well-defined films of amorphous APIs. Water-sorption isotherms modeled using perturbed-chain statistical associating fluid theory (PC-SAFT) showed better accordance with the experimental data than those obtained using the Flory-Huggins model. Crank's diffusion equation was used to describe the water-sorption kinetics providing Fickian diffusion coefficients of water in indomethacin and in ritonavir. Stefan-Maxwell diffusion coefficients were obtained by converting Fickian diffusion coefficients using water activity coefficients obtained from PC-SAFT. Finally, the free-volume theory was applied to explain the persistent water concentration dependency of the Stefan-Maxwell diffusion coefficients.

The Shelf Life of ASDs: 1. Measuring the Crystallization Kinetics at Humid Conditions.

Amorphous solid dispersions (ASDs), where an active pharmaceutical ingredient (API) is dissolved in a polymer, are a favored formulation technique to achieve sufficient bioavailability of poorly water-soluble APIs. The shelf life of such ASDs is often limited by API crystallization. Crystallization depends strongly on the storage conditions (relative humidity and temperature) and the polymer selected for generating the ASD. Determining the crystallization kinetics of ASDs under various conditions requires suitable analytical methods. In this work, two different analytical methods were compared and cross-validated: The first builds on water-sorption measurements combined with thermodynamic predictions ( Eur. J. Pharm. Biopharm. 2018, 127, 183-193, DOI: 10.1016/j.toxrep.2018.11.002), whereas the second applies Raman spectroscopy. Using the two independent methods, factors influencing the crystallization kinetics of ASDs containing the API griseofulvin were investigated quantitatively. It was found that crystallization kinetics increases with increasing temperature and relative humidity. Additionally, the influence of different polymers (poly(vinylpyrrolidone-co-vinyl acetate) and Soluplus) on crystallization kinetics were investigated. The experimentally obtained crystallization kinetics were described using the Johnson-Mehl-Avrami-Kolmogorov model and are the basis for future shelf life predictions at desired storage conditions.

Solubilization of Aldehydes and Amines in Aqueous CiEj Surfactant Aggregates: Solubilization Capacity and Aggregate Properties.

Hydroformylation of olefins to aldehydes and subsequent reductive amination of aldehydes to amines takes place in an aqueous system using a water-soluble catalyst. It is limited to short-chain molecules due to an insufficient solubility of long-chain molecules in water. A promising approach to increase the solubility of long-chain aldehydes and amines is the addition of surfactants to the aqueous phase. In this work, we thus determined the solubilization capacity (SC) of different nonionic CiEj surfactants (C8E6, C10E6, and C10E8) toward long-chain aldehydes and amines. We used static and dynamic light scattering techniques to investigate the influence of both the surfactant and solute molecular structures on the SC as well as on the aggregation number (Nagg) and hydrodynamic radius (Rh) of mixed aggregates. Our data reveals that an optimum ratio of hydrophobic to hydrophilic chain length of CiEj surfactants exists where the SC toward long-chain aldehydes and amines possesses a maximum. Further, the size of the aggregates (Nagg, Rh) passes through a minimum upon amine solubilization, while upon aldehyde solubilization, the aggregate size increases gradually. The results shown in this work give valuable insights to the solubilization of aldehydes and n-amines into nonionic CiEj surfactants and facilitate the search of suitable surfactants for hydroformylation and reductive amination as "green" solvents based on the detailed knowledge about the aggregate structure.

Osmolyte effect on enzymatic stability and reaction equilibrium of formate dehydrogenase.

Osmolytes are well-known biocatalyst stabilisers as they promote the folded state of proteins, and a stabilised biocatalyst might also improve reaction kinetics. In this work, the influence of four osmolytes (betaine, glycerol, trehalose, and trimethylamine N-oxide) on the activity and stability of Candida bondinii formate dehydrogenase cbFDH was studied experimentally and theoretically. Scanning differential fluorimetric studies were performed to assess the thermal stability of cbFDH, while UV detection was used to reveal changes in cbFDH activity and reaction equilibrium at osmolyte concentrations between 0.25 and 1 mol kg-1. The thermodynamic model ePC-SAFT advanced allowed predicting the effects of osmolyte on the reaction equilibrium by accounting for interactions involving osmolyte, products, substrates, and water. The results show that osmolytes at low concentrations were beneficial for both, thermal stability and cbFDH activity, while keeping the equilibrium yield at high level. Molecular dynamics simulations were used to describe the solvation around the cbFDH surface and the volume exclusion effect, proofing the beneficial effect of the osmolytes on cbFDH activity, especially at low concentrations of trimethylamine N-oxide and betaine. Different mechanisms of stabilisation (dependent on the osmolyte) show the importance of studying solvent-protein dynamics towards the design of optimised biocatalytic processes.

Predicting Deliquescence Relative Humidities of Crystals and Crystal Mixtures.

The presence of water in the form of relative humidity (RH) may lead to deliquescence of crystalline components above a certain RH, the deliquescence RH (DRH). Knowing the DRH values is essential, e.g., for the agrochemical industry, food industry, and pharmaceutical industry to identify stability windows for their crystalline products. This work applies the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) to purely predict the DRH of single components (organic acids, sugars, artificial sweeteners, and amides) and multicomponent crystal mixtures thereof only based on aqueous solubility data of the pure components. The predicted DRH values very well agree with the experimental ones. In addition, the temperature influence on the DRH value could be successfully predicted with PC-SAFT. The DRH prediction also differentiates between formation of hydrates and anhydrates. PC-SAFT-predicted phase diagrams of hydrate-forming components illustrate the influence of additional components on the hydrate formation as a function of RH. The DRH prediction via PC-SAFT allows for the determining of the stability of crystals and crystal mixtures without the need for time-consuming experiments.

Solubility of Pharmaceutical Ingredients in Natural Edible Oils.

Natural edible oils (NEOs) are common excipients for lipid-based formulations. Many of them are complex mixtures comprising hundreds of different triglycerides (TGs). One major challenge in developing lipid-based formulations is the variety in NEO compositions affecting the solubility of active pharmaceutical ingredients. In this work, solubilities of indomethacin (IND), ibuprofen (IBU), and fenofibrate (FFB) in soybean oil and in coconut oil were measured via differential scanning calorimetry, high-performance liquid chromatography, and Raman spectroscopy. Furthermore, this work proposes an approach that mimics NEOs using one key TG and models the API solubilities in these NEOs based on perturbed-chain statistical associating fluid theory (PC-SAFT). Key TGs were determined using the 1,2,3-random hypothesis, and PC-SAFT parameters were estimated via a group-contribution method. Using the proposed approach, the solubility of IBU and FFB was modeled in soybean oil and coconut oil. Furthermore, the solubilities of five more APIs (IND, cinnarizine, naproxen, griseofulvin, and felodipine) were modeled in soybean oil. All modeling results were found in very good agreement with the experimental data. The influence of different NEO kinds on API solubility was examined by comparing FFB and IBU solubilities in soybean oil and refined coconut oil. PC-SAFT was thus found to allow assessing the batch-to-batch consistency of NEO batches in silico.

Thermodynamic Modeling of Solvent-Impact on Phase Separation in Amorphous Solid Dispersions during Drying.

Understanding and prevention of unwanted changes of a pharmaceutical formulation during the production process is part of the critical requirements for the successful approval of a new drug product. Polymer-based formulations, so-called amorphous solid dispersions (ASDs), are often produced via solvent-based processes. In such processes, active pharmaceutical ingredients (APIs) and polymers are first dissolved in a solvent or solvent mixture, then the solvent is evaporated, for example, via spray drying or rotary evaporation. During the drying step, unwanted liquid-liquid phase separation may occur, leading to polymer-rich and API-rich regions with crystallization potential, and thus, heterogeneities and a two-phasic system in the final ASD. Phase separation in ASDs may impact their bioperformance because of the locally higher degree of API supersaturation. Although it is known that the choice of the solvent plays an important role in the formation of heterogeneities, solvent-impact on ASD drying and eventual product quality is often neglected in the process design. This study aims to investigate for the first time the phase behavior and drying process of API/polymer/solvents systems from a thermodynamic perspective. Unwanted phase changes during the drying process of the ASD containing hydroxypropyl methylcellulose acetate succinate and naproxen prepared from acetone/water or ethanol/water solvent mixtures were predicted using the thermodynamic model PC-SAFT. The predicted phase behavior and drying curves were successfully validated by confocal Raman spectroscopy.

In-Silico Screening of Lipid-Based Drug Delivery Systems.

PURPOSE: This work proposes an in-silico screening method for identifying promising formulation candidates in complex lipid-based drug delivery systems (LBDDS). METHOD: The approach is based on a minimum amount of experimental data for API solubilites in single excipients. Intermolecular interactions between APIs and excipients as well as between different excipients were accounted for by the Perturbed-Chain Statistical Associating Fluid Theory. The approach was applied to the in-silico screening of lipid-based formulations for ten model APIs (fenofibrate, ibuprofen, praziquantel, carbamazepine, cinnarizine, felodipine, naproxen, indomethacin, griseofulvin and glibenclamide) in mixtures of up to three out of nine excipients (tricaprylin, Capmul MCM, caprylic acid, Capryol™ 90, Lauroglycol™ FCC, Kolliphor TPGS, polyethylene glycol, carbitol and ethanol). RESULTS: For eight out of the ten investigated model APIs, the solubilities in the final formulations could be enhanced by up to 100 times compared to the solubility in pure tricaprylin. Fenofibrate, ibuprofen, praziquantel, carbamazepine are recommended as type I formulations, whereas cinnarizine and felodipine showed a distinctive solubility gain in type II formulations. Increased solubility was found for naproxen and indomethacin in type IIIb and type IV formulations. The solubility of griseofulvin and glibenclamide could be slightly enhanced in type IIIb formulations. The experimental validation agreed very well with the screening results. CONCLUSION: The API solubility individually depends on the choice of excipients. The proposed in-silico-screening approach allows formulators to quickly determine most-appropriate types of lipid-based formulations for a given API with low experimental effort. Graphical abstract.

Selecting Excipients Forming Therapeutic Deep Eutectic Systems-A Mechanistic Approach.

The majority of all newly identified active pharmaceutical ingredients (APIs) have a low solubility in water (partly smaller than marble). In order to enhance their solubility and bioavailability, the formulation of these APIs, as part of therapeutic deep eutectic systems (THEDES), has been recently shown to be a promising approach. By choosing the right excipient, the melting point of the API/excipient mixture can be lowered below body temperature or even room temperature, resulting in a liquid formulation. To date, because of a lack of mechanistic understanding of how THEDES are formed, the identification of suitable excipients for a given API is almost exclusively based on heuristic decisions and trial-and-error-based approaches. This is both very time-consuming and expensive. The purpose of this work is to reduce the experimental effort to identify suitable excipients for a given API solely based on the melting properties (melting temperature and melting enthalpy) of the API and excipient and accounting for intermolecular interactions via a predictive thermodynamic model [in this case, UNIFAC(Do)]. Lidocaine, ibuprofen, and phenylacetic acid were considered as model APIs, whereas thymol, vanillin, lauric acid, para-toluic acid, benzoic acid, and cinnamic acid were considered as model excipients. The formation of THEDES from these components was predicted and confirmed using differential scanning calorimetry. The results indicate that the experimental effort for the identification of suitable API/excipient combinations can be drastically reduced by thermodynamic modeling, leading to more efficient and tailor-made formulations in the future.

Combined co-solvent and pressure effect on kinetics of a peptide hydrolysis: an activity-based approach.

The application of co-solvents and high pressure has been reported to be an efficient means to tune the kinetics of enzyme-catalyzed reactions. Co-solvents and pressure can lead to increased reaction rates without sacrificing enzyme stability, while temperature and pH operation windows are generally very narrow. Quantitative prediction of co-solvent and pressure effects on enzymatic reactions has not been successfully addressed in the literature. Herein, we are introducing a thermodynamic approach that is based on molecular interactions in the form of activity coefficients of substrate and of enzyme in the multi-component solution. This allowed us to quantitatively predict the combined effect of co-solvent and pressure on the kinetic constants, i.e. the Michaelis constant KM and the catalytic constant kcat, of an α-CT-catalyzed peptide hydrolysis reaction. The reaction was studied in the presence of different types of co-solvents and at pressures up to 2 kbar, and quantitative predictions could be obtained for KM, kcat, and finally even primary Michaelis-Menten plots using activity coefficients provided by the thermodynamic model PC-SAFT.

Prediction and Experimental Validation of Co-Solvent Influence on Michaelis Constants: A Thermodynamic Activity-Based Approach.

Co-solvents are known to influence the Michaelis constant K M of enzyme-catalyzed reactions. In the literature, co-solvent effects on K M are usually explained by interactions between enzyme and co-solvent. Very recent works replaced substrate concentrations with thermodynamic activities to separate enzyme-co-solvent from substrate-co-solvent interactions This yields the thermodynamic-activity-based Michalis constant K M a . In this work, this approach was extended to alcohol dehydrogenase (ADH)-catalyzed reduction of acetophenone (ACP), a two-substrate reaction. It was experimentally found that polyethylene glycol (PEG) 6000 increased K M of ACP and decreased K M of nicotinamide adenine dinucleotide (NADH). To predict K M a values, non-covalent interactions between substrates and reaction media were taken into account by electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) modelling. In contrast to experimental K M values, their activity-based pendants K M a were independent of co-solvent. To further verify the approach, the reduction of 2-pentanone catalyzed by the same ADH was investigated. Interestingly, the addition of PEG caused a decrease of both K M of 2-pentanone and K M of NADH. Based on K M a values obtained from K M in co-solvent-free conditions and activity coefficients from ePC-SAFT, the influence of the co-solvent on K M was quantitatively predicted. Thus, the approach known for pseudo one-substrate reactions was successfully transferred to two-substrate reactions. Furthermore, the advantage of thermodynamic activities over concentrations in the field of enzyme kinetics is highlighted.

Choosing Appropriate Solvents for ASD Preparation.

Amorphous solid dispersions (ASDs) are often used for formulating poorly water-soluble active pharmaceutical ingredients (APIs). In an ASD, the amorphous API is embedded in a suitable matrix excipient in order to stabilize the amorphous state and control the dissolution performance. ASDs can be prepared by commonly dissolving the API and the polymer in a suitable organic solvent which is evaporated afterward (e.g., via spray drying) aiming at a homogeneous API distribution in the polymer matrix. Sometimes, unexpected solvent influences on the heterogeneity of the dry ASD are observed. Thermodynamic predictions using the Perturbed-Chain Statistical Associating Fluid Theory combined with experimental investigations via Raman spectroscopy, differential scanning calorimetry, and microscopy performed in this work revealed the amorphous phase separation (APS) between the solvent and the polymer as causing the ASD heterogeneities. It will be shown that thermodynamic modeling allows for identifying appropriate solvents that will neither show APS with the polymeric excipient nor at any time of the drying process of ASD formulations.