Temperature- and Moisture-Induced Crystallization of Amorphous Lactose in Composite Particles with Sodium Alginate Prepared by Spray-Drying

The purpose of this study was to investigate the temperature- and moisture-induced crystallization of amorphous lactose in the composite particles prepared by spray-drying an aqueous solution of crystalline lactose and sodium alginate. The temperature-induced crystallization of amorphous lactose in the composite particles was suppressed by increasing the amount of sodium alginate in the particles. The stabilizing effect of sodium alginate on amorphous lactose in the composite particles was greater than that in physical mixtures having the same formulating ratios. The improved stability of amorphous lactose in the composite particles was attributed to an increase in the glass transition temperature (T_g) of the mixture. Moisture-induced crystallization of amorphous lactose was also retarded by increasing the amount of sodium alginate in composite particles. Although the T_g of the mixture was reduced by increasing the water content of the particles, the values were higher than that of 100% amorphous lactose when particles of the same water content were compared. The change in the T_g of the composite particles with increasing water content was interpreted as involving three components of the Gordon–Taylor equation. In the amorphous lactose–sodium alginate systems, the T_g values of the composite particles containing sodium alginate were higher than the theoretical line predicted by two components of the Gordon–Taylor equation. These results suggested that there was a specific interaction between the sodium alginate and lactose molecules. This specific interaction was suggested by the fact that only very little amorphous lactose was measured in the spray-dried composite particles stored under humid conditions using differential scanning calorimetry. This molecular interaction may also be partly responsible for the suppression of both the temperature- and moisture-induced crystallization of amorphous lactose in the composite particles.     

Prediction of Onset of Crystallization in Amorphous Pharmaceutical Systems: Phenobarbital,Nifedipine/PVP,and Phenobarbital/PVP

The aim of this work is to determine if a stability testing protocol based on the correlations between crystallization onset and relaxation time above the glass transition temperature (T_g) can be used to predict the crystallization onsets in amorphous pharmaceutical systems well below their T_g. This procedure assumes that the coupling between crystallization onset and molecular mobility is the same above and below T_g. The stability testing protocol has been applied to phenobarbital, phenobarbital/polyvinylpyrrolidone (PVP) (95/5, w/w), and nifedipine/PVP (95/5, w/w). Crystallization onsets have been detected by polarized light microscopy examination of amorphous films; molecular mobility has been determined by dielectric relaxation spectroscopy above T_g and by both isothermal calorimetry and modulated differential scanning calorimetry below T_g. We find that small amounts of PVP significantly retard re-crystallization. This dramatic effect of PVP is not related to mobility, so this approach applies, at best, to extrapolation of high temperature data on a given formulation to low temperatures. Variation in molecular mobility at these concentrations of PVP is not the dominant factor in determining variation in propensity for re-crystallization from glassy systems; we suggest surface interactions between PVP and nuclei and/or small crystals slowing growth control variation in crystallization kinetics between formulations. © 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:3887-3900, 2010     

The Effects of Absorbed Water on the Properties of Amorphous Mixtures Containing Sucrose

Purpose. To measure the water vapor absorption behavior of sucrose-poly(vinyl pyrrolidone) (PVP) and sucrose-poly(vinyl pyrrolidone co-vinyl acetate) (PVP/VA) mixtures, prepared as amorphous solid solutions and as physical mixtures, and the effect of absorbed water on the amorphous properties, i.e., crystallization and glass transition temperature, T_g, of these systems. Methods. Mixtures of sucrose and polymer were prepared by co-lyophilization of aqueous sucrose-polymer solutions and by physically mixing amorphous sucrose and polymer. Absorption isotherms for the individual components and their mixtures were determined gravimetrically at 30°C as a function of relative humidity. Following the absorption experiments, mixtures were analyzed for evidence of crystallization using X-ray powder diffraction. For co-lyophilized mixtures showing no evidence of crystalline sucrose, T_g was determined as a function of water content using differential scanning calorimetry. Results. The absorption of water vapor was the same for co-lyophilized and physically mixed samples under the same conditions and equal to the weighted sums of the individual isotherms where no sucrose crystallization was observed. The crystallization of sucrose in the mixtures was reduced relative to sucrose alone only when sucrose was molecularly dispersed (co-lyophilized) with the polymers. In particular, when co-lyophilized with sucrose at a concentration of 50%, PVP was able to maintain sucrose in the amorphous state for up to three months, even when the T_g was reduced well below the storage temperature by the absorbed water. Conclusions. The water vapor absorption isotherms for co-lyophilized and physically mixed amorphous sucrose-PVP and sucrose-PVP/VA mixtures at 30°C are similar despite interactions between sugar and polymer which are formed when the components are molecularly dispersed with one another. In the presence of absorbed water the crystallization of sucrose was reduced only by the formation of a solid-solution, with PVP having a much more pronounced effect than PVP/VA. The effectiveness of PVP in preventing sucrose crystallization when significant levels of absorbed water are present was attributed to the molecular interactions between sucrose, PVP and water.     

Temperature- and moisture-induced crystallization of amorphous lactose in composite particles with sodium alginate prepared by spray-drying

The purpose of this study was to investigate the temperature- and moisture-induced crystallization of amorphous lactose in the composite particles prepared by spray-drying an aqueous solution of crystalline lactose and sodium alginate. The temperature-induced crystallization of amorphous lactose in the composite particles was suppressed by increasing the amount of sodium alginate in the particles. The stabilizing effect of sodium alginate on amorphous lactose in the composite particles was greater than that in physical mixtures having the same formulating ratios. The improved stability of amorphous lactose in the composite particles was attributed to an increase in the glass transition temperature (Tg) of the mixture. Moisture-induced crystallization of amorphous lactose was also retarded by increasing the amount of sodium alginate in composite particles. Although the Tg of the mixture was reduced by increasing the water content of the particles, the values were higher than that of 100% amorphous lactose when particles of the same water content were compared. The change in the Tg of the composite particles with increasing water content was interpreted as involving three components of the Gordon-Taylor equation. In the amorphous lactose-sodium alginate systems, the Tg values of the composite particles containing sodium alginate were higher than the theoretical line predicted by two components of the Gordon-Taylor equation. These results suggested that there was a specific interaction between the sodium alginate and lactose molecules. This specific interaction was suggested by the fact that only very little amorphous lactose was measured in the spray-dried composite particles stored under humid conditions using differential scanning calorimetry. This molecular interaction may also be partly responsible for the suppression of both the temperature- and moisture-induced crystallization of amorphous lactose in the composite particles.     

Recrystallization of Nifedipine and Felodipine from Amorphous Molecular Level Solid Dispersions Containing Poly(vinylpyrrolidone) and Sorbed Water

Purpose To compare the physical stability of amorphous molecular level solid dispersions of nifedipine and felodipine, in the presence of poly(vinylpyrrolidone) (PVP) and small amounts of moisture. Methods Thin amorphous films of nifedipine and felodipine and amorphous molecular level solid dispersions with PVP were stored at various relative humidities (RH) and the nucleation rate was measured. The amount of water sorbed at each RH was measured using isothermal vapor sorption and glass transition temperatures (T _g) were determined using differential scanning calorimetry. The solubility of each compound in methyl pyrrolidone was measured as a function of water content. Results Nifedipine crystallizes more easily than felodipine at any given polymer concentration and in the presence of moisture. The glass transition temperatures of each compound, alone and in the presence of PVP, are statistically equivalent at any given water content. The nifedipine systems are significantly more hygroscopic than the corresponding felodipine systems. Conclusions Variations in the physical stability of the two compounds could not be explained by differences in T _g. However, the relative physical stability is consistent with differences in the degree of supersaturation of each drug in the solid dispersion, treating the polymer and water as a co-solvent system for each drug compound.     

Evaluation of the Physical Stability of Freeze-Dried Sucrose-Containing Formulations by Differential Scanning Calorimetry

Freeze-dried samples of sucrose with buffer salts, amino acids, or dextran have been analyzed with differential scanning calorimetry (DSC) to evaluate the use of DSC thermograms in predicting the physical storage stability. The glass transition temperature, T _g, of the amorphous cake, crystallization, and melting of sucrose are observed with DSC. T _g appeared to be an important characteristic of the physical stability of the amorphous freeze-dried cake. A storage temperature above T _g results in collapse or shrinkage of the cake, which for a sucrose-based formulation, may be accompanied by crystallization of the sucrose. The T _g of the amorphous sucrose is influenced by other components present in the cake. Dextran-40 raised T _g, while the addition of glycine to the formulation lowered T _g. The residual moisture content strongly influences T _g, since water acts as a plasticizer of the system; the higher the moisture content, the lower the T _g and the less physically stable the freeze-dried cake. Crystallization of amorphous sucrose is shown to be inhibited by high molecular weight components or ionic compounds. DSC analysis of freeze-dried cakes proved to be a powerful tool in formulation studies.     

Microcalorimetric Measurement of the Interactions Between Water Vapor and Amorphous Pharmaceutical Solids

Purpose. Use a microcalorimetric technique to measure the interactions between water vapor and amorphous pharmaceutical solids and describe the relationship between long-term physical stability and the storage relative humidity (RH) at constant temperature. Methods. A thermal activity monitor was used to characterize interactions of water vapor with spray-dried amorphous sucrose, lactose, raffinose, and sodium indomethacin. Differential scanning calorimetry was used to measure glass transition temperature, T _g. X-ray powder diffraction was used to confirm that the spray-dried samples were amorphous. Scanning electron microscopy was used to examine particle morphology. Specific surface area was determined by BET analysis of nitrogen and krypton adsorption isotherms. Results. The moisture-induced thermal activity traces (MITATs) of the materials in this study exhibit general behavior that helps explain the effect of moisture content on the physical stability of the glassy phase at a given storage temperature. At some RH threshold, RH _m, the MITAT exhibits a dramatic increase in the energy of interaction between water vapor and the glass that cannot be explained by a phase or morphology change. Calorimetric data indicate that water vapor-solid interactions are reversible below RH _m; above RH _m, energetic hysteresis is observed and water-water interactions predominate. In addition, the MITAT was deconvoluted into sorptive and nonsorptive components, making it possible to assign the observed heat flow to unique thermal events. Samples stored at a RH just below RH _m for more than 2 months show no evidence of morphology or phase change. In addition, the MITAT can be deconvoluted into sorptive and nonsorptive components by using a twin-calorimeter arrangement. This analysis provides specificity to the microcalorimetric analysis and helps explain the nature of the physical changes that occur during the hydration glassy phase. Conclusions. The MITAT is a useful tool to determine the onset of moisture-induced physical instability of glassy pharmaceuticals and may find a broad application to determine appropriate storage conditions to ensure long-term physical stability.     

Effects of Sugars and Polymers on Crystallization of Poly(ethylene glycol) in Frozen Solutions: Phase Separation Between Incompatible Polymers

Purpose. This study examined the effect of third components (low-molecular-weight saccharides and polymers) on the crystallization of poly(ethylene) glycol (PEG) in frozen solutions, focusing on the relationship between their crystallization-inhibiting ability and molecular compatibility. Methods. Effects of sugars and polymers on the crystallization of PEG 3000 in frozen solution were monitored by differential scanning calorimetry (DSC). Pulsed-NMR was employed to monitor the molecular mobility of water and solutes in the frozen solutions. Miscibility between PEG and third components in aqueous solution was estimated from the lowering of cloud point of PEG 20,000. Thermal analysis of frozen solutions containing some non-crystallizing solutes was used to examine the possibility of phase separation in frozen solutions. Results. Some sugars and polymers inhibited the crystallization of PEG and formed practically stable amorphous phases among ice crystals. The mobility of solute molecules in the amorphous phase increased above the softening temperature of maximally concentrated solutions (T_s), whereas that of water molecules appeared at a lower temperature. Mono- and disaccharides that are relatively less miscible with PEG in solution inhibit PEG crystallization to a lesser degree. Two T_s regions were observed in frozen solutions containing both polyvinylpyrrolidone (PVP) and dextran, at much lower concentrations than those causing aqueous two-phase separation at ambient temperatures. Conclusions. Ice crystallization raises the concentration of solutes in the remaining solution, which can lead to phase separation in the amorphous phase. Molecular compatibility between components is an important factor determining their propensity to phase separate and crystallize.     

Probing Beta Relaxation in Pharmaceutically Relevant Glasses by Using DSC

Purpose This study was conducted to demonstrate the use of differential scanning calorimetry (DSC) in detecting and measuring β-relaxation processes in amorphous pharmaceutical systems. Methods DSC was employed to study amorphous samples of poly(vinylpyrrolidone) (PVP), indomethacin (IM), and ursodeoxycholic acid (UDA) that were annealed at temperatures (T_a) around 0.8 of their glass transition temperatures (T_g). Dynamic mechanical analysis (DMA) was used to measure β-relaxation in PVP. Results Reheating the annealed samples gives rise to annealing peaks that occur below T_g. The peaks cannot be generated when annealing below the low temperature limit of β-relaxation. These limits are around 50°C for PVP, −20°C for IM, and 30°C for UDA. The effective activation energy (E) of the sub-T_g relaxation has been estimated for each T_a and found to increase with T_a, reflecting increasing contribution of the α-process. Estimates of E for β-relaxation have been obtained from the lowest T_a data, and are as follows: 68 (PVP), 56 (IM), 67 (UDA) kJ mol⁻¹. Conclusions DSC can be used for detecting β-relaxation processes and estimating its low temperature limit, i.e., the temperature below which amorphous drugs would remain stable. It can also provide comparative estimates of low temperature stability of amorphous drugs in terms of the activation energies of the β-relaxation.     

The Effect of Excipients on the Molecular Mobility of Lyophilized Formulations,as Measured by Glass Transition Temperature and NMR Relaxation-Based Critical Mobility Temperature

Purpose. The dependence of the molecular mobility of lyophilized formulations on pharmaceutical polymer excipients was studied. Molecular mobility as determined by NMR relaxation-based critical temperature of molecular mobility (T_mc) and glass transition temperature (T_g) is discussed in relation to the plasticizing effect of water in formulations. Methods. The T_mc and T_g of lyophilized -globulin formulations containing 6 different polymer excipients such as dextran, polyvinylpyrrolidone (PVP) and methylcellulose (MC) was determined by NMR and DSC. The molecular mobility of water in the formulations was determined by proton NMR and dielectric relaxation spectrometry (DRS). Results. T_mc varied with polymer excipients. T_mc increased as the ratio of bound water to mobile water increased and as the molecular mobility of mobile water decreased. The formulation containing MC exhibited a lower T_mc than the formulation containing dextran because of the smaller ratio of bound water and the higher molecular mobility of mobile water. The T_mc of the formulation containing PVP was higher than that expected from the higher T₂ values of water because of the lower molecular mobility of mobile water regardless of the higher ratio of mobile water. The T_mc of these lyophilized formulations was higher than their T_g by 23°C to 34°C, indicating that the formulations became a NMR-detected microscopically liquidized state below their T_g. Conclusions. The quantity and the molecular mobility of mobile water in lyophilized formulations can be considered to affect the T_mc of lyophilized formulations, which in turn governs their stability.     

Phase Behavior of Binary and Ternary Amorphous Mixtures Containing Indomethacin, Citric Acid, and PVP

Purpose. To better understand the nature of drug-excipient interactions we have studied the phase behavior of amorphous binary and ternary mixtures of citric acid, indomethacin and PVP, as model systems. Methods. We have prepared amorphous mixtures by co-melting or coprecipitation from solvents, and have measured glass transition temperatures with differential scanning calorimetry. Results. Citric acid and indomethacin in the amorphous state are miscible up to 0.25 weight fraction of citric acid, equivalent to about 2 moles of citric acid and 3 moles of indomethacin. Phase separation, as reflected by two Tg values, occurs without crystallization leading to a saturated citric acid-indomethacin amorphous phase and one essentially containing only citric acid. PVP-citric acid and PVP-indomethacin form non-ideal miscible systems at all compositions. A ternary system containing 0.3 weight fraction of PVP produces a completely miscible system at all citric acid-indomethacin compositions. The use of 0.2 weight fraction of PVP, however, only produces miscibility up to a weight fraction of 0.4 citric acid relative to indomethacin. The two phases above this point appear to contain citric acid in PVP and citric acid in indomethacin, respectively. Conclusions. Two components of an amorphous solid mixture containing citric acid and indomethacin with limited solid state miscibility can be solublized as an amorphous solid phase by the addition of moderate levels of PVP.     

Determination of the Viscosity of an Amorphous Drug Using Thermomechanical Analysis (TMA)

Purpose. To evaluate thermomechanical analysis (TMA) as a technique for determining the viscosity of amorphous pharmaceutical materials. This property of amorphous drugs and excipients is related to their average rate of molecular mobility and thus to their physical and chemical stability. Methods. Indomethacin was selected as a model amorphous drug whose viscosity has previously been reported in the literature. A Seiko TMA 120C thermomechanical analyzer was utilized in isothermal penetration mode to determine the viscosity of the amorphous drug over the maximum possible range of temperatures. Results. Using a cylindrical penetration geometry it was possible to accurately determine the viscosity of amorphous indomethacin samples by TMA over the temperature range from 35 to 75°C. The results were consistent with those reported in the literature using a controlled strain rheometer over the range 44–75°C. The limiting lower experimental temperature for the TMA technique was extended to significantly below the calorimetric glass transition temperature (T_g 42°C), thus allowing a direct experimental determination of the viscosity at T_g to be made. Conclusions. Thermomechanical analysis can be used to accurately determine the viscosity of amorphous pharmaceutical materials at temperatures near and above their calorimetric glass transition temperatures.     

Effect of Temperature and Moisture on the Physical Stability of Binary and Ternary Amorphous Solid Dispersions of Celecoxib

The effectiveness of different polymers, alone or in combination, in inhibiting the crystallization of celecoxib (CEX) from amorphous solid dispersions (ASDs) exposed to different temperatures and relative humidities was evaluated. It was found that polyvinylpyrrolidone (PVP) and PVP-vinyl acetate formed stronger or more extensive hydrogen bonding with CEX than cellulose-based polymers. This, combined with their better effectiveness in raising the glass transition temperature (T_g) of the dispersions, provided better physical stabilization of amorphous CEX against crystallization in the absence of moisture when compared with dispersions formed with cellulose derivatives. In ternary dispersions containing 2 polymers, the physical stability was minimally impaired by the presence of a cellulose-based polymer when the major polymer present was PVP. On exposure to moisture, stability of the CEX ASDs was strongly affected by both the dispersion hygroscopicity and the strength of the intermolecular interactions. Binary and ternary ASDs containing PVP appeared to undergo partial amorphous–amorphous phase separation when exposed 94% relative humidity, followed by crystallization, whereas other binary ASDs crystallized directly without amorphous–amorphous phase separation.     

Comparison of Molecular Mobility in the Glassy State Between Amorphous Indomethacin and Salicin Based on Spin-Lattice Relaxation Times

Purpose. The purpose of the current study was to evaluate the molecular mobility of amorphous indomethacin and salicin in the relaxed glassy state based on spin-lattice relaxation times (T_1c) and to clarify the effects of molecular mobility on their physical stability.Methods. Pulverized glassy amorphous indomethacin and salicin samples were completely relaxed, and the T_1c values were investigated using solid-state ¹³C-nuclear magnetic resonance (NMR) at temperatures below the glass transition temperature (T_g). All NMR spectra were obtained using the T_1c measurement method combined with variable-amplitude cross-polarization, the Torchia method, and total sideband suppression method.Results. The T_1c value of amorphous indomethacin indicated that 73% of carbons were in a state of monodispersive relaxation, suggesting that the amorphous state was relatively homogeneous and restricted, particularly in backbone carbons. On the other hand, 92% of carbons of amorphous salicin exhibited both fast and slow biphasic relaxation. Individual structures of the salicin molecules behaved heterogeneously, and thus the entire molecule showed relatively fast local as well as slow mobility.Conclusions. At temperatures below T_g, amorphous salicin had relatively greater molecular mobility than amorphous indomethacin. This difference in the molecular mobility of the two compounds is correlated with their crystallization behavior. Solid-state ¹³C NMR provides valuable information on the physical stability of amorphous pharmaceuticals.     

The Effect of Low Concentrations of Molecularly Dispersed Poly(Vinylpyrrolidone) on Indomethacin Crystallization from the Amorphous State

Purpose. To investigate the effect of low concentrations of molecularly dispersed poly(vinylpyrrolidone) (PVP) on indomethacin (IMC) crystallization from the amorphous state using particle size effects to identify possible mechanisms of crystallization inhibition. Methods. Different particle sizes of amorphous IMC and 1, 2, and 5% PVP were stored dry at 30°C for 84 days. PXRD was used to calculate the rate and extent of crystallization and the polymorph formed. Results. Crystallization from amorphous IMC and IMC/PVP molecular dispersions yielded the polymorph of IMC. Crystallization rates were reduced at larger particle size and in the presence of 1, 2, and 5%PVP. Crystallization did not reach completion in some IMC/PVP samples, with the quantity of uncrystallized amorphous phase proportional to particle size. Conclusions. Low concentrations of molecularly dispersed PVP affected IMC crystallization from the amorphous state. Formation of -IMC at rates dependent on particle size indicated that surface nucleation predominated in both the absence and presence of PVP. Excellent correlation was seen between the extent of crystallization and simulated depths of crystal penetration, supporting the hypothesis that increasing local PVP concentration inhibits crystal growth from surface nuclei into the amorphous particle.     

Effect of Glass Transition Temperature on the Stability of Lyophilized Formulations Containing a Chimeric Therapeutic Monoclonal Antibody

Purpose. The purpose of this study is to highlight the importance of knowing the glass transition temperature, T_g, of a lyophilized amorphous solid composed primarily of a sugar and a protein in the interpretation of accelerated stability data. Methods. Glass transition temperatures were measured using DSC and dielectric relaxation spectroscopy. Aggregation of protein in the solid state was monitored using size-exclusion chromatography. Results. Sucrose formulation (T_g ~ 59°C) when stored at 60°C was found to undergo significant aggregation, while the trehalose formulation (T_g ~ 80°C) was stable at 60°C. The instability observed with sucrose formulation at 60°C can be attributed to its T_g (~59°C) being close to the testing temperature. Increase in the protein/sugar ratio was found to increase the T_gs of the formulations containing sucrose or trehalose, but to different degrees. Conclusions. Since the formulations exist in glassy state during their shelf-life, accelerated stability data generated in the glassy state (40°C) is perhaps a better predictor of the relative stability of formulations than the data generated at a higher temperature (60°C) where one formulation is in the glassy state while the other is near or above its T_g.     

Effect of temperature and moisture on the miscibility of amorphous dispersions of felodipine and poly(vinyl pyrrolidone)

The physical stability of amorphous molecular level solid dispersions will be influenced by the miscibility of the components. The goal of this work was to understand the effects of temperature and relative humidity on the miscibility of a model amorphous solid dispersion. Infrared spectroscopy was used to evaluate drug–polymer hydrogen bonding interactions in amorphous solid dispersions of felodipine and poly(vinyl pyrrolidone) (PVP). Samples were analyzed under stressed conditions: high temperature and high relative humidity. The glass transition temperature (T_g) of select systems was studied using differential scanning calorimetry (DSC). Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to further investigate moisture-induced changes in solid dispersions. Felodipine-PVP solid dispersions showed evidence of adhesive hydrogen bonding interactions at all compositions studied. The drug–polymer intermolecular interactions were weakened and/or less numerous on increasing the temperature, but persisted up to the melting temperature of the drug. Changes in the hydrogen bonding interactions were found to be reversible with changes in temperature. In contrast, the introduction of water into amorphous molecular level solid dispersions at room temperature irreversibly disrupted interactions between the drug and the polymer resulting in amorphous-amorphous phase separation followed by crystallization. DSC, AFM, and TEM results provided further evidence for the occurrence of moisture induced immiscibility. In conclusion, it appears that felodipine-PVP solid dispersions are susceptible to moisture-induced immiscibility when stored at a relative humidity ≥75%. In contrast, the solid dispersions remained miscible on heating. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:169–185, 2010     

Freeze-Concentration Separates Proteins and Polymer Excipients Into Different Amorphous Phases

Purpose. To study the miscibility of proteins and polymer excipients in frozen solutions and freeze-dried solids as protein formulation models. Methods. Thermal profiles of frozen solutions and freeze-dried solids containing various proteins (lysozyme, ovalbumin, BSA), nonionic polymers (Ficoll, polyvinylpyrrolidone [PVP]), and salts were analyzed by differential scanning calorimetry (DSC). The polymer miscibility was determined from the glass transition temperature of maximally freeze-concentrated solute (T_g) and the glass transition temperature of freeze-dried solid (T_g). Results. Frozen Ficoll or PVP 40k solutions showed T_g at –22°C, while protein solutions did not show an apparent T_g. All the protein and nonionic polymer combinations (5% w/w, each) were miscible in frozen solutions and presented single T_gs that rose with increases in the protein ratio. Various salts concentration-dependently lowered the single T_gs of the proteins and Ficoll combinations maintaining the mixed amorphous phase. In contrast, some salts induced the separation of the proteins and PVP combinations into protein-rich and PVP-rich phases among ice crystals. The T_gs of these polymer combinations were jump-shifted to PVP's intrinsic T_g at certain salt concentrations. Freeze-dried solids showed varied polymer miscibilities identical to those in frozen solutions. Conclusions. Freeze-concentration separates some combinations of proteins and nonionic polymers into different amorphous phases in a frozen solution. Controlling the polymer miscibility is important in designing protein formulations.     

Evaluation of Crystallization Behavior on the Surface of Nifedipine Solid Dispersion Powder Using Inverse Gas Chromatography

Purpose

To investigate crystallization behavior on the surface of amorphous solid dispersion powder using inverse gas chromatography (IGC) and to predict the physical stability at temperatures below the glass transition temperature (T _g).

Methods

Amorphous solid dispersion powder was prepared by melt-quenching of a mixture of crystalline nifedipine and polyvinylpyrrolidon (PVP) K-30. IGC was conducted by injecting undecane (probe gas) and methane (reference gas) repeatedly to the solid dispersion at temperatures below T _g. Surface crystallization was evaluated by the retention volume change of undecane based on the observation that the surface of the solid dispersion with crystallized nifedipine gives an increased retention volume.

Results

On applying the retention volume change to the Hancock-Sharp equation, surface crystallization was found to follow a two-dimensional growth of nuclei mechanism. Estimation of the crystallization rates at temperatures far below T _g using the Avrami-Erofeev equation and Arrhenius equation showed that, to maintain its quality for at least three years, the solid dispersion should be stored at ?20°C (T _g???65°C).

Conclusions

IGC can be used to evaluate crystallization behavior on the surface of a solid dispersion powder, and, unlike traditional techniques, can also predict the stability of the solid dispersion based on the surface crystallization behavior.     

Enthalpy Relaxation in Binary Amorphous Mixtures Containing Sucrose

Purpose. To compare the enthalpy relaxation of amorphous sucrose and co-lyophilized sucrose-additive mixtures near the calorimetric glass transition temperature, so as to measure the effects of additives on the molecular mobility of sucrose. Methods. Amorphous sucrose and sucrose-additive mixtures, containing poly(vinylpyrrolidone) (PVP), poly(vinylpyrrolidone-co-vinyl-acetate) (PVP/VA) dextran or trehalose, were prepared by lyophilization. Differential scanning calorimetry (DSC) was used to determine the area of the enthalpy recovery endotherm following aging times of up to 750 hours for the various systems. This technique was also used to compare the enthalpy relaxation of a physical mixture of amorphous sucrose and PVP. Results. Relative to sucrose alone, the enthalpy relaxation of co-lyophilized sucrose-additive mixtures was reduced when aged for the same length of time at a comparable degree of undercooling in the order: dextran PVP > PVP/VA > trehalose. Calculated estimates of the total enthalpy change required for sucrose and the mixtures to relax to an equilibrium supercooled liquid state (H) were essentially the same and were in agreement with enthalpy changes measured at longer aging times (750 hours). Conclusions. The observed decrease in the enthalpy relaxation of the mixtures relative to sucrose alone indicates that the mobility of sucrose is reduced by the presence of additives having a T_g that is greater than that of sucrose. Comparison with a physically mixed amorphous system revealed no such effects on sucrose. The formation of a molecular dispersion of sucrose with a second component, present at a level as low as 10%, thus reduces the mobility of sucrose below T_g, most likely due to the coupling of the molecular motions of sucrose to those of the additive through molecular interactions.