Wetting Kinetics: an Alternative Approach Towards Understanding the Enhanced Dissolution Rate for Amorphous Solid Dispersion of a Poorly Soluble Drug

Developing amorphous solid dispersions of water-insoluble molecules using polymeric materials is a well-defined approach to improve the dissolution rate and bioavailability. While the selected polymer plays a vital role in stabilizing the amorphous solid dispersion physically, it is equally important to improve the dissolution profile by inhibiting crystallization from the supersaturated solution generated by dissolution of the amorphous material. Furthermore, understanding the mechanism of dissolution rate enhancement is of vital importance. In this work, wetting kinetics was taken up as an alternative approach for understanding the enhanced dissolution rate for amorphous solid dispersion of a poorly soluble drug. While cilostazol (CIL) was selected as the model drug, povidone (PVP), copovidone, and hypromellose (HPMC) were the polymers of choice. The concentrations against time profiles were evaluated for the supersaturated solutions of CIL in the presence and absence of the selected polymers. The degree of supersaturation increased significantly with increase in polymer content within the solid dispersion. While povidone was found to maintain the highest level of supersaturation for the greatest length of time both in dissolution and solution crystallization experiments, copovidone and hypromellose were found to be the less effective as crystallization inhibitor. The ability of polymers to generate and maintain supersaturated drug solutions was assessed by dissolution studies. The wetting kinetics was compared against the solid dispersion composition to establish a correlation with enhanced dissolution rate.KEY WORDS: Cilostazol, Crystallization inhibition, Solid dispersions, Supersaturated solutions, Wetting kinetics     

Physicochemical Investigations and Stability Studies of Amorphous Gliclazide

Gliclazide (GLI), a poorly water-soluble antidiabetic, was transformed into a glassy state by melt quench technique in order to improve its physicochemical properties. Chemical stability of GLI during formation of glass was assessed by monitoring thin-layer chromatography, and an existence of amorphous form was confirmed by differential scanning calorimetry and X-ray powder diffractometry. The glass transition occurred at 67.5°C. The amorphous material thus generated was examined for its in vitro dissolution performance in phosphate buffer (pH 6.8). Surprisingly, amorphous GLI did not perform well and was unable to improve the dissolution characteristics compared to pure drug over entire period of dissolution studies. These unexpected results might be due to the formation of a cohesive supercooled liquid state and structural relaxation of amorphous form toward the supercooled liquid region which indicated functional inability of amorphous GLI from stability point of view. Hence, stabilization of amorphous GLI was attempted by elevation of T_g via formation of solid dispersion systems involving comprehensive antiplasticizing as well as surface adsorption mechanisms. The binary and ternary amorphous dispersions prepared with polyvinylpyrrolidone K30 (as antiplasticizer for elevation of T_g) and Aerosil 200® and/or Sylysia® 350 (as adsorbent) in the ratio of 1:1:1 (w/w) using kneading and spray-drying techniques demonstrated significant enhancement in rate and extent of dissolution of drug initially. During accelerated stability studies, ternary systems showed no significant reduction in drug dissolution performance over a period of 3 months indicating excellent stabilization of amorphous GLI.Key words: amorphous, gliclazide, solid dispersion, stability studies, T_g     

Construction and Validation of Binary Phase Diagram for Amorphous Solid Dispersion Using Flory–Huggins Theory

Drug–polymer miscibility is one of the fundamental prerequisite for the successful design and development of amorphous solid dispersion formulation. The purpose of the present work is to provide an example of the theoretical estimation of drug–polymer miscibility and solubility on the basis of Flory–Huggins (F–H) theory and experimental validation of the phase diagram. The F–H interaction parameter, χ _d-p, of model system, aceclofenac and Soluplus, was estimated by two methods: by melting point depression of drug in presence of different polymer fractions and by Hildebrand and Scott solubility parameter calculations. The simplified relationship between the F–H interaction parameter and temperature was established. This enabled us to generate free energy of mixing (ΔG _mix) curves for varying drug–polymer compositions at different temperatures and finally the spinodal curve. The predicted behavior of the binary system was evaluated through X-ray diffraction, differential scanning calorimetry, and in vitro dissolution studies. The results suggest possibility of employing interaction parameter as preliminary tool for the estimation of drug–polymer miscibility.     

Glass transition behavior of octyl β-D-glucoside and octyl β-D-thioglucoside/water binary mixtures

The lyotropic behavior and glass-forming properties of octyl β-d-glucoside (C8Glu) and octyl β-d-thioglucoside (C8SGlu)/water binary mixtures were evaluated using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The results clearly indicate that the mixture forms a glass in the supercooling state of liquid crystalline phases such as cubic, lamellar, and smectic. The glass transition temperature (T_g) of the mixture was strongly dependent on solute concentration, with a higher concentration correlating with a higher T_g. The experimental T_g was consistent with the predicted value calculated using the Couchman-Karasz equation in both the C8Glu and C8SGlu/water mixtures. The change of heat capacity at T_g showed the two bending points under variation of concentrations. And the highest temperature of phase transition from lamellar to isotropic solution was observed at around 50% molar concentration. It was expected that non-percolated state of water existed in extremely higher concentration ranges.     

Liquid-liquid phase transition temperatures increase when lipid bilayers are supported on glass

Membranes made from certain ternary mixtures of lipids can display coexisting liquid phases. In giant unilamellar vesicles, these phases appear as liquid domains which diffuse and coalesce after the vesicle is cooled below its miscibility transition temperature (T_m). Converting vesicles to supported lipid bilayers alters the mobility of the lipids and domains in the bilayer. At the same time, the miscibility transition temperature of the lipid mixture is altered. Here we compare T_m in vesicles and in supported bilayers formed by rupturing the same vesicles onto glass. We determine transition temperatures using fluorescence microscopy, and identify an increase in T_m when it is measured in identical membranes in solution and on a glass surface. We systematically alter the lipid composition of our membranes in order to observe the correlation between membrane composition and variation in T_m.     

Practical Considerations for Determination of Glass Transition Temperature of a Maximally Freeze Concentrated Solution

Glass transition temperature is a unique thermal characteristic of amorphous systems and is associated with changes in physical properties such as heat capacity, viscosity, electrical resistance, and molecular mobility. Glass transition temperature for amorphous solids is referred as (T _g), whereas for maximally freeze concentrated solution, the notation is (T _g′). This article is focused on the factors affecting determination of T _g′ for application to lyophilization process design and frozen storage stability. Also, this review provides a perspective on use of various types of solutes in protein formulation and their effect on T _g′. Although various analytical techniques are used for determination of T _g′ based on the changes in physical properties associated with glass transition, the differential scanning calorimetry (DSC) is the most commonly used technique. In this article, an overview of DSC technique is provided along with brief discussion on the alternate analytical techniques for T _g′ determination. Additionally, challenges associated with T _g′ determination, using DSC for protein formulations, are discussed. The purpose of this review is to provide a practical industry perspective on determination of T _g′ for protein formulations as it relates to design and development of lyophilization process and/or for frozen storage; however, a comprehensive review of glass transition temperature (T _g, T _g′), in general, is outside the scope of this work.     

Adverse effect of cake collapse on the functional integrity of freeze-dried bull spermatozoa

Under optimal freeze-drying conditions, solutions exhibit a cake-like porous structure. However, if the solution temperature is higher than the glass transition temperature of the maximally freeze-concentrated phase (T_g′) during drying phase, the glassy matrix undergoes viscous flow, resulting in cake collapse. The purpose of the present study was to investigate the effect of cake collapse on the integrity of freeze-dried bull spermatozoa. In a preliminary experiment, factors affecting the T_g′ of conventional EGTA buffer (consisting of Tris–HCl, EGTA and NaCl) were investigated in order to establish the main experimental protocol because EGTA buffer T_g′ was too low (−45.0 °C) to suppress collapse. Modification of the EGTA buffer composition by complete removal of NaCl and addition of trehalose (mEGTA buffer) resulted in an increase of T_g′ up to −27.7 °C. In the main experiment, blastocyst yields after ooplasmic injection of freeze-dried sperm preserved in collapsed cakes (drying temperature: 0 or −15 °C) were significantly lower than those of sperm preserved in non-collapsed cake (drying temperature: −30 °C). In conclusion, freeze-dried cake collapse may be undesirable for maintaining sperm functions to support embryonic development, and can be inhibited by controlling both T_g′ of freeze-drying buffer and temperature during the drying phase.     

A broad glass transition in hydrated proteins

We performed Raman and Brillouin scattering measurements to estimate glass transition temperature, T_g, of hydrated protein. The measurements reveal very broad glass transition in hydrated lysozyme with approximate T_g ∼ 180 ± 15 K. This result agrees with a broad range of T_g ∼ 160–200 K reported in literature for hydrated globular proteins and stresses the difference between behavior of hydrated biomolecules and simple glass-forming systems. Moreover, the main structural relaxation of the hydrated protein system that freezes at T_g ∼ 180 K remains unknown. We emphasize the difference between the “dynamic transition”, known as a sharp rise in mean-squared atomic displacement <r²> at temperatures around T_D ∼ 200–230 K, and the glass transition. They have different physical origin and should not be confused.     

Molecular mobility studies on the amorphous state of disaccharides. I—thermally stimulated currents and differential scanning calorimetry

The relaxational processes in amorphous solid gentiobiose and cellobiose are studied by thermally stimulated depolarization currents (TSDC) in the temperature region from 108 K up to 423 K. The slow molecular mobility was characterized in the crystal and in the glassy state. The features of different motional components of the secondary relaxation have been monitored as a function of time as the glass structurally relaxes on aging. It is concluded that some modes of motion of this mobility are aging independent, while others are affected by aging. The value of the steepness index or fragility (T_g-normalized temperature dependence of the relaxation time) was obtained by differential scanning calorimetry (DSC) from the analysis of the scanning rate dependency of T_g.     

Characterization of the hidden glass transition of amorphous cyclomaltoheptaose

An amorphous solid of cyclomaltoheptaose (β-cyclodextrin, β-CD) was formed by milling its crystalline form using a high-energy planetary mill at room temperature. The glass transition of this amorphous solid was found to occur above the thermal degradation point of the material preventing its direct observation and thus its full characterization. The corresponding glass transition temperature (T_g) and the ΔC_p at T_g have, however, been estimated by extrapolation of T_g and ΔC_p of closely related amorphous compounds. These compounds include methylated β-CD with different degrees of substitution and molecular alloys obtained by co-milling β-CD and methylated β-CD (DS 1.8) at different ratios. The physical characterization of the amorphous states have been performed by powder X-ray diffraction and differential scanning calorimetry, while the chemical integrity of β-CD upon milling was checked by NMR spectroscopy and mass spectrometry.     

Influence of an Acrylic Polymer Blend on the Physical Stability of Film-Coated Theophylline Pellets

The purpose of this study was to investigate the physical stability of a coating system consisting of a blend of two sustained release acrylic polymers and its influence on the drug release rate of theophylline from coated pellets. The properties of both free films and theophylline pellets coated with the polymer blend were investigated, and the miscibility was determined via differential scanning calorimetry. Eudragit^® RS 30 D was plasticized by the addition of Eudragit^® NE 30 D, and the predicted glass transition temperature (T _g) of the blend was similar to the experimental values. Sprayed films composed of a blend of Eudragit^® NE 30 D/Eudragit^® RS 30 D (1:1) showed a water vapor permeability six times greater than films containing only Eudragit^® NE 30 D. The presence of quaternary ammonium functional groups from the RS 30 D polymer increased the swellability of the films. The films prepared from the blend exhibited stable permeability values when stored for 1 month at both 25°C and 40°C, while the films which were composed of only Eudragit^® NE 30 D showed a statistically significant decrease in this parameter when stored under the same conditions. Eudragit^® NE 30 D/Eudragit^® RS 30 D (1:1)-sprayed films decreased in elongation from 180% to 40% after storage at 40°C for 1 month, while those stored at 25°C showed no change in elongation. In coated pellets, the addition of Eudragit^® RS 30 D to the Eudragit^® NE 30 D increased the theophylline release rate, and the pellets were stable when stored at 25°C for a period of up to 3 months due to maintenance of the physico-mechanical properties of the film. Pellets stored at 40°C exhibited a decrease in drug release rate over time as a result of changes in film physico-mechanical properties which were attributed to further coalescence and densification of the polymer. When the storage temperature was above the T _g of the composite, instabilities in both drug release rate and physical properties were evident. Stabilization in drug release rate from coated pellets could be correlated with the physico-mechanical stability of the film formulation when stored at temperatures below the T _g of the polymer.     

The amorphous state of spray-dried maltodextrin: sub-sub-Tg enthalpy relaxation and impact of temperature and water annealing

The annealing behaviour of a spray-dried maltodextrin was investigated by differential scanning calorimetry. Special attention was paid to the effect of temperature and humidity on the annealing process. Comparison was also made with the glassy state of the same compound prepared by various cooling processes. The presence of a very pronounced sub-T_g peak upon ageing reveals the specificities of the glass and the complexity of the relaxation spectrum of the spray-dried material. This peak seems actually to correspond to a partial ergodicity recovery that may be attributed to onset of molecular mobility occurring below T_g. The position of the sub-T_g peak with regard to the conventional T_g was systematically studied. It clearly showed the difference between the effect of temperature and water plasticization on the relaxations occurring in the glassy state of materials prepared by spray-drying.     

Disintegration Mediated Controlled Release Supersaturating Solid Dispersion Formulation of an Insoluble Drug: Design,Development, Optimization,and In Vitro Evaluation

The objective of this study was to develop a solid dispersion based controlled release system for drug substances that are poorly soluble in water. A wax-based disintegration mediated controlled release system was designed based on the fact that an amorphous drug can crystallize out from hydrophilic matrices. For this study, cilostazol (CIL) was selected as the model drug, as it exhibits poor aqueous solubility. An amorphous solid dispersion was prepared to assist the drug to attain a supersaturated state. Povidone was used as carrier for solid dispersion (spray drying technique), hydrogenated vegetable oil (HVO) as wax matrix former, and sodium carboxymethyl cellulose (NaCMC) as a disintegrant. The extreme vertices mixture design (EVMD) was applied to optimize the designed and developed composition. The optimized formulation provided a dissolution pattern which was equivalent to the predicted curve, ascertaining that the optimal formulation could be accomplished with EVMD. The release profile of CIL was described by the Higuchi’s model better than zero-order, first-order, and Hixson-Crowell’s model, which indicated that the supersaturation state of CIL dominated to allow drug release by diffusion rather than disintegration regulated release as is generally observed by Hixson-Crowell’s model. The optimized composition was evaluated for disintegration, dissolution, XRD, and stability studies. It was found that the amorphous state as well as the dissolution profile of CIL was maintained under the accelerated conditions of 40°C/75% RH for 6 months.KEY WORDS: cilostazol, controlled release, disintegration-mediated controlled release (DMCR), extreme vertices mixture design (EVMD), solid dispersion     

Simultaneous Determination of Glass Transition Temperatures of Several Polymers

Aims

A simple and easy optical method is proposed for the determination of glass transition temperature (T_g) of polymers.

Methods & Results

T_g was determined using the technique of microsphere imaging to monitor the variation of the refractive index of polymer microsphere as a function of temperature. It was demonstrated that the method can eliminate most thermal lag and has sensitivity about six fold higher than the conventional method in T_g determination. So the determined T_g is more accurate and varies less with cooling/heating rate than that obtained by conventional methods. The most attractive character of the method is that it can simultaneously determine the T_g of several polymers in a single experiment, so it can greatly save experimental time and heating energy.

Conclusion

The method is not only applicable for polymer microspheres, but also for the materials with arbitrary shapes. Therefore, it is expected to be broadly applied to different fundamental researches and practical applications of polymers.     

Application of the Kwei equation to model the Tg behavior of binary blends of sugars and salts

Vitrification of sugar-based solutions plays an important role in cryopreservation, lyophilization, and the emerging field of anhydrous preservation. An understanding of the glass transition characteristics of such formulations is essential for determining an appropriate storage temperature to ensure an extended shelf life of vitrified products. To better understand the effect of salts on the glass transition temperature (T_g) of glass-forming sugars, we investigated several data-fitting models (Fox, Gordon–Taylor and Kwei) for sugar–salt formulations using data from the literature, as well as new data generated on blends of trehalose and choline dihydrogen phosphate (CDHP). CDHP has recently been shown to have promise as a stabilizing agent for proteins and DNA. The Kwei equation, which has a specific parameter characterizing intermolecular interactions, provides good fits to the T_g data for sugar–salt blends, and complements other commonly used models that are frequently used to model T_g data.     

Anti-plasticization of cassava starch by complexing fatty acids

The effect of adding 1–8% amylose complexing fatty acids (CFA), such as linoleic and oleic acids, on the glass transition temperature (T_g) of cassava starch (CS) with moisture content varying from 5 to 35% (dry basis) was studied. The main relaxation temperature (T_α), associated with the glass transition temperature of the samples (T_g), was determined by dynamic-mechanical-thermal analysis. The plasticizing behavior of water in the blends was evidenced by a decrease of T_α values with moisture content. The effect of CFA on CS was found to be a function of moisture content. At low moisture (<11%) it caused an anti-plasticization effect, while at higher moisture contents it produced plasticization. The anti-plasticizing effect of CFA on CS was attributed to amylose–lipid complex formation.     

Influence of the lamellar phase unbinding energy on the relative stability of lamellar and inverted cubic phases

Based on curvature energy considerations, nonbilayer phase-forming phospholipids in excess water should form stable bicontinuous inverted cubic (Q_II) phases at temperatures between the lamellar (L_α) and inverted hexagonal (H_II) phase regions. However, the phosphatidylethanolamines (PEs), which are a common class of biomembrane phospholipids, typically display direct L_α/H_II phase transitions and may form intermediate Q_II phases only after the temperature is cycled repeatedly across the L_α/H_II phase transition temperature, T_H, or when the H_II phases are cooled from T > T_H. This raises the question of whether models of inverted phase stability, which are based on curvature energy alone, accurately predict the relative free energy of these phases. Here we demonstrate the important role of a noncurvature energy contribution, the unbinding energy of the L_α phase bilayers, g_u, that serves to stabilize the L_α phase relative to the nonlamellar phases. The planar L_α phase bilayers must separate for a Q_II phase to form and it turns out that the work of their unbinding can be larger than the curvature energy reduction on formation of Q_II phase from L_α at temperatures near the L_α/Q_II transition temperature (T_Q). Using g_u and elastic constant values typical of unsaturated PEs, we show that g_u is sufficient to make T_Q > T_H for the latter lipids. Such systems would display direct L_α → H_II transitions, and a Q_II phase might only form as a metastable phase upon cooling of the H_II phase. The g_u values for methylated PEs and PE/phosphatidylcholine mixtures are significantly smaller than those for PEs and increase T_Q by only a few degrees, consistent with observations of these systems. This influence of g_u also rationalizes the effect of some aqueous solutes to increase the rate of Q_II formation during temperature cycling of lipid dispersions. Finally, the results are relevant to protocols for determining the Gaussian curvature modulus, which substantially affects the energy of intermediates in membrane fusion and fission. Recently, two such methods were proposed based on measuring T_Q and on measuring Q_II phase unit cell dimensions, respectively. In view of the effect of g_u on T_Q that we describe here, the latter method, which does not depend on the value of g_u, is preferable.     

Protein stability in the amorphous carbohydrate matrix: relevance to anhydrobiosis

The formation of intracellular glass is proposed to be relevant to protein stabilization and survival of anhydrobiotic organisms in the dry state. The stability of proteins in the amorphous carbohydrate matrix and its relevance to seed survival have been investigated in the present study. Glucose-6-phosphate dehydrogenase (G6PDH) was preserved in the amorphous glucose/sucrose (1:10, w/w) matrix by freeze-drying. The stability of freeze-dried G6PDH was examined at temperatures above and below the glass transition temperature (T_g). The rate of G6PDH inactivation in the amorphous carbohydrate matrix deviated significantly from the Arrhenius kinetics, and conformed to the Williams-Landel-Ferry (WLF) relationship. The temperature dependence of G6PDH inactivation in two sets of samples with different T_g values was compared. Identical temperature dependence of G6PDH inactivation was observed after temperature normalization by (T?T_g). Seed survival of Vigna radiata Wilczek (mung bean) showed a similar WLF kinetics at storage temperatures T≥T_g. In situ protein stability in mung bean embryonic axes was studied using differential scanning calorimetry (DSC). Thermal stability of seed proteins exhibited a strong dependence on the T_g of intracellular glass. These results indicate an important role of the glassy state in protein stabilization. Our data suggest an association between protein stability in intracellular glass and seed survival during storage.     

Interactions between soluble sugars and POPC (1-palmitoyl-2-oleoylphosphatidylcholine) during dehydration: vitrification of sugars alters the phase behavior of the phospholipid

The phase behavior of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) was characterized as a function of hydration in the presence of combinations of sugars representative of sugars found in seed embryos having differing degrees of desiccation tolerance. The tendency of the sugar mixes to vitrify was also monitored as a function of hydration. Using differential scanning calorimetry, it was found that all sugars diminished the increase in the gel-to-fluid phase transition temperature (T_m) of POPC that occurred upon dehydration of the pure lipid. These results are analyzed in terms of the osmotic and volumetric properties of sugars. Also, it was found that in those samples for which the glass transition temperature (T_g) was greater than the (T_m) of POPC, T_m was lowered by approx. 20 C° from the value for the fully hydrated lipid. X-ray diffraction data confirmed that acyl chain freezing was deferred to a lower temperature during cooling of vitrified samples. The significance of these results is discussed in terms of the ability of many organisms to tolerate desiccation.     

Estimation of the Basic Reproductive Number and Mean Serial Interval of a Novel Pathogen in a Small,Well-Observed Discrete Population

Background

Accurately assessing the transmissibility and serial interval of a novel human pathogen is public health priority so that the timing and required strength of interventions may be determined. Recent theoretical work has focused on making best use of data from the initial exponential phase of growth of incidence in large populations.

Methods

We measured generational transmissibility by the basic reproductive number R₀ and the serial interval by its mean T_g. First, we constructed a simulation algorithm for case data arising from a small population of known size with R₀ and T_g also known. We then developed an inferential model for the likelihood of these case data as a function of R₀ and T_g. The model was designed to capture a) any signal of the serial interval distribution in the initial stochastic phase b) the growth rate of the exponential phase and c) the unique combination of R₀ and T_g that generates a specific shape of peak incidence when the susceptible portion of a small population is depleted.

Findings

Extensive repeat simulation and parameter estimation revealed no bias in univariate estimates of either R₀ and T_g. We were also able to simultaneously estimate both R₀ and T_g. However, accurate final estimates could be obtained only much later in the outbreak. In particular, estimates of T_g were considerably less accurate in the bivariate case until the peak of incidence had passed.

Conclusions

The basic reproductive number and mean serial interval can be estimated simultaneously in real time during an outbreak of an emerging pathogen. Repeated application of these methods to small scale outbreaks at the start of an epidemic would permit accurate estimates of key parameters.