Comparative Relaxation Dynamics of Glucose and Maltitol

Purpose To demonstrate the utility of differential scanning calorimetry (DSC) for determining activation energy landscape in amorphous pharmaceutical systems throughout the sub-T _g and T _g regions.Materials and Methods DSC was employed to determine the effective activation energies (E) of the relaxation in sub-T _g and T _g regions as well as the sizes of cooperatively rearranging regions in glassy maltitol and glucose.Results It has been found that in the sub-T _g region E decreases with decreasing T reaching the values ∼60 (glucose) and ∼70 (maltitol) kJ mol⁻¹ that are comparable to the literature values of the activation energies for the β-relaxation. In the T _g region E decreases (from ∼250 to ∼150 kJ mol⁻¹ in maltitol and from ∼220 to ∼170 kJ mol⁻¹ in glucose) with increasing T as typically found for the α-relaxation. From the heat capacity measurements the sizes of cooperatively rearranging regions have been determined as 3.1 (maltitol) and 3.3 (glucose) nm.Conclusions DSC can be used for evaluating the energy landscapes. The E values for maltitol are somewhat greater than for glucose due to the added impeding effect of the bulky substitute group in maltitol. The comparable sizes of the cooperatively rearranging regions suggest a similarity of the heterogeneous glassy structures of the two compounds.     

Predictions of Onset of Crystallization from Experimental Relaxation Times I-Correlation of Molecular Mobility from Temperatures Above the Glass Transition to Temperatures Below the Glass Transition

Purpose Predicting onsets of crystallization at temperatures below T _g, from data above T _g, would require that the correlation between crystallization onset and mobility is same above and below T _g, and the techniques being used to measure mobility above and below T _g are measuring essentially the same kind of mobility. The aim of this work is to determine if the relaxation times obtained using different techniques (DSC, TAM) below T _g correlate with relaxation time obtained above T _g using dielectric spectroscopy.Methods Model compounds for this work were chosen based on their varied ΔH _f, ΔC _p(T _g) and H-bonding in crystalline state vs. amorphous state. Relaxation times above T _g were determined by the simultaneous fit of real and imaginary permittivity to the Cole-Davidson model. Tau and beta below T _g were determined using isothermal microcalorimetry (TAM) or MDSC. MDSC was used to calculate Kauzmann temperature and strength of the glass using established relationships.Results Indomethacin, nifedipine and flopropione showed Arrhenius temperature dependence throughout the entire temperature range and extrapolation of τ ^β measured above T _g by dielectric relaxation agreed with τ ^β measured below T _g by TAM/MDSC. Ketoconazole, however, showed the expected VTF behavior. For at least two compounds compared (indomethacin and ketoconazole), relaxation times measured by TAM and MDSC did not agree, with TAM giving significantly lower values of τ ^β , but TAM and MDSC relaxation times appeared to extrapolate to a common value at T _g.Conclusions It was found that, for all cases studied, relaxation time constants determined above and below T _g did appear to extrapolate to the same value around T _g indicating that molecular mobility measured above and below T _g using different techniques is highly correlated.     

Implications of Global and Local Mobility in Amorphous Sucrose and Trehalose as Determined by Differential Scanning Calorimetry

Purpose  To investigate the local and global mobility in amorphous sucrose and trehalose and their potential implications on physical stability. Methods  Amorphous sucrose was prepared by lyophilization while amorphous trehalose was prepared by dehydration of trehalose dihydrate. The variation in the effective activation energy of α-relaxation through glass transition has been determined by applying an isoconversional method. β-Relaxations were detected as shallow peaks, at temperatures below the glass transition temperature, caused by annealing glassy samples at different temperatures and subsequently heating at different rates in a differential scanning calorimeter. The effect of heating rate on the β-relaxation peak temperature formed the basis for the calculation of the activation energy. Results  α-Relaxations in glassy trehalose were characterized by larger activation energy barrier compared to sucrose, attributable to a more compact molecular structure of trehalose. The effect of temperature on viscous flow was greater in trehalose which can have implications on lyophile collapse. The size of the cooperatively rearranging regions was about the same for sucrose and trehalose suggesting similar dynamic heterogeneity at their respective glass transition temperatures. The activation energy of β-relaxations increased with annealing temperature due to increasing cooperative motions and the increase was larger in sucrose. The temperature at which β-relaxation was detected for a given annealing time was much less in sucrose implying that progression of local motions to cooperative motions occurred at lower temperatures in sucrose. Conclusions  Trehalose, having a lower free volume in the glassy state due to a more tightly packed molecular structure, is characterized by larger activation energies of α-relaxation and experiences a greater effect of temperature on the reduction in the activation energy barrier for viscous flow. The pronounced increase in cooperative motions in sucrose upon annealing at temperatures below (T _g −50) suggest that even a small excursion in temperature could result in a significant increase in mobility.     

A Calorimetric Method to Estimate Molecular Mobility of Amorphous Solids at Relatively Low Temperatures

Purpose To present a calorimetry-based approach for estimating the initial (at the onset of annealing) relaxation time (τ ⁰) of organic amorphous solids at relatively low temperatures, and to assess the temperature where molecular mobility of the amorphous drug is reduced to a level comparable with the desired shelf-life of the product.Materials and Methods Values of τ ⁰ for six amorphous pharmaceutical compounds were estimated based on the nonlinear Adam–Gibbs equation. Fragility was determined from the scanning rate-dependence of the glass transition temperature (T _g). The initial enthalpic and entropic fictive temperatures were obtained from the T _g and the heat capacities (C _p) of the amorphous and crystalline forms.Results At a relatively low temperature (∼40°C or more below T _g), τ ⁰ for the different compounds varies by over an order of magnitude. For some materials, the practical storage temperature at T _g − 50 K was found to be still too high to ensure long-term stability. The estimated τ ⁰ is highly sensitive to the fragility of the material and the C _p of the crystalline and amorphous forms. Materials with high fragility or greater C _p differences between crystalline and amorphous forms tend to have longer τ ⁰.Conclusions The proposed method can be used to estimate molecular mobility at relatively low temperatures without having to conduct enthalpy recovery experiments. An accurate τ ⁰ determination from this method relies on faithful fragility measurements.     

β-Relaxation of Insulin Molecule in Lyophilized Formulations Containing Trehalose or Dextran as a Determinant of Chemical Reactivity

Purpose The purpose of this study was to elucidate whether the degradation rate of insulin in lyophilized formulations is determined by matrix mobility, as reflected in glass transition temperature (T_g), or by β-relaxation, as reflected in rotating-frame spin-lattice relaxation time . Methods The storage stability of insulin lyophilized with dextran was investigated at various relative humidities (RH; 12–60%) and temperatures (40–90°C) and was compared with previously reported data for insulin lyophilized with trehalose. Insulin degradation was monitored by reverse-phase high-performance liquid chromatography. Furthermore, the of the insulin carbonyl carbon in the lyophilized insulin–dextran and insulin–trehalose systems was measured at 25°C by ¹³C solid-state NMR, and the effect of trehalose and dextran on was compared at various humidities. Results The degradation rate of insulin lyophilized with dextran was not significantly affected by the T_g of the matrix, even at low humidity (12% RH), in contrast to that of insulin lyophilized with trehalose. The insulin–dextran system exhibited a substantially greater degradation rate than the insulin–trehalose system at a given temperature below the T_g. The difference in degradation rate between the insulin–dextran and insulin–trehalose systems observed at 12% RH was eliminated at 43% RH. In addition, the of the insulin carbonyl carbon at low humidity (12% RH) was prolonged by the addition of trehalose, but not by the addition of dextran. This difference was eliminated at 23% RH, at which point the solid remained in the glassy state. These findings suggest that the β-relaxation of insulin is inhibited by trehalose at low humidity, presumably as a result of insulin–trehalose interaction, and thus becomes a rate determinant. In contrast, dextran, whose ability to interact with insulin is thought to be less than that of trehalose, did not inhibit the β-relaxation of insulin, and thus, the chemical activational barrier (activation energy) rather than β-relaxation becomes the major rate determinant. Conclusions β-Relaxation rather than matrix mobility seems to be more important in determining the stability of insulin in the glassy state in lyophilized formulations containing trehalose and dextran.     

Molecular mobility in liquid and glassy states of Telmisartan (TEL) studied by Broadband Dielectric Spectroscopy

The molecular relaxation in liquid and glassy states of Telmisartan (TEL) has been studied by Broadband Dielectric Spectroscopy (BDS) covering wide temperature and frequency range. Multiple relaxation processes were observed. Besides the primary α-relaxation, two secondary relaxations β and γ (labelled in order of decreasing time scale) have been reported.Well-separated β-process observed above and below glass transition temperature T_g, has activation energy E_β = 81.8 kJ/mol and was identified as intermolecular Johari–Goldstein (JG) process. The γ-relaxation visible in dielectric loss spectra at very low temperatures is most likely non-JG relaxation. The temperature dependence of the relaxation times of α-process, measured over 11 orders of magnitude, cannot be described by a single Vogel–Fulcher–Tamman–Hesse (VFTH) equation. At temperature T_B = 475.8 K the change in relaxation dynamics occurred, consequently a new set of VFTH parameters was required. From low temperature VFTH fits the glass transition temperature T_g was estimated as T_g = 400.3 K and fragility index m = 87 was calculated. Of particular interest was the time scale of molecular motion below the glass transition temperature. Our observation clearly indicates that the α-relaxation times at room temperature most probably would exceed 3 years and amorphous TEL should maintain physically and chemically stable over prolonged storage time.     

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     

Significance of Local Mobility in Aggregation of β-Galactosidase Lyophilized with Trehalose,Sucrose or Stachyose

Purpose The purpose of this study is to compare the effects of global mobility, as reflected by glass transition temperature (T_g) and local mobility, as reflected by rotating-frame spin-lattice relaxation time (T_1ρ) on aggregation during storage of lyophilized β-galactosidase (β-GA). Materials and Methods The storage stability of β-GA lyophilized with sucrose, trehalose or stachyose was investigated at 12% relative humidity and various temperatures (40–90°C). β-GA aggregation was monitored by size exclusion chromatography (SEC). Furthermore, the T_1ρ of the β-GA carbonyl carbon was measured by ¹³C solid-state NMR, and T_g was measured by modulated temperature differential scanning calorimetry. Changes in protein structure during freeze drying were measured by solid-state FT-IR. Results The aggregation rate of β-GA in lyophilized formulations exhibited a change in slope at around T_g, indicating the effect of molecular mobility on the aggregation rate. Although the T_g rank order of β-GA formulations was sucrose < trehalose < stachyose, the rank order of β-GA aggregation rate at temperatures below and above T_g was also sucrose < trehalose < stachyose, thus suggesting that β-GA aggregation rate is not related to (T-T_g). The local mobility of β-GA, as determined by the T_1ρ of the β-GA carbonyl carbon, was more markedly decreased by the addition of sucrose than by the addition of stachyose. The effect of trehalose on T_1ρ was intermediate when compared to those for sucrose and stachyose. These findings suggest that β-GA aggregation rate is primarily related to local mobility. Significant differences in the second derivative FT-IR spectra were not observed between the excipients, and the differences in β-GA aggregation rate observed between the excipients could not be attributed to differences in protein secondary structure. Conclusions The aggregation rate of β-GA in lyophilized formulations unexpectedly correlated with the local mobility of β-GA, as indicated by T_1ρ, rather than with (T-T_g). Sucrose exhibited the most intense stabilizing effect due to the most intense ability to inhibit local protein mobility during storage.     

Physicochemical Characterization of Hot Melt Extruded Bicalutamide–Polyvinylpyrrolidone Solid Dispersions

Solid molecular dispersions of bicalutamide (BL) and polyvinylpyrrolidone (PVP) were prepared by hot melt extrusion technology at drug‐to‐polymer ratios of 1:10, 2:10, and 3:10 (w/w). The solid‐state properties of BL, physical mixtures of BL/PVP, and hot melt extrudates were characterized using differential scanning calorimetry (DSC), powder X‐ray diffractometry (PXRD), Raman, and Fourier transform infrared (FTIR) spectroscopy. Drug dissolution studies were subsequently conducted on hot melt extruded solid dispersions and physical mixtures. All hot melt extrudates had a single T_g between the T_g of amorphous BL and PVP indicating miscibility of BL with PVP and the formation of solid molecular dispersions. PXRD confirmed the presence of the amorphous form of BL within the extrudates. Conversely, PXRD patterns recorded for physical mixtures showed sharp bands characteristic of crystalline BL, whereas DSC traces had a distinct endotherm at 196°C corresponding to melting of crystalline BL. Further investigations using DSC confirmed solid‐state plasticization of PVP by amorphous BL and hence antiplasticization of amorphous BL by PVP. Experimentally observed T_g values of physical mixtures were shown to be significantly higher than those calculated using the Gordon–Taylor equation suggesting the formation of strong intermolecular interactions between BL and PVP. FTIR and Raman spectroscopy were used to investigate these interactions and strongly suggested the presence of secondary interaction between PVP and BL within the hot melt extrudates. The drug dissolution properties of hot melt extrudates were enhanced significantly in comparison to crystalline BL and physical mixtures. Moreover, the rate and extent of BL release were highly dependent on the amount of PVP present within the extrudate. Storage of the extrudates confirmed the stability of amorphous BL for up to 12 months at 20°C, 40% RH whereas stability was reduced under highly humid conditions (20°C, 65% RH). Interestingly, BL recrystallization after storage under these conditions had no effect on the dissolution properties of the extrudates. © 2009 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 99: 1322–1335, 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.     

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.     

Time-Dependence of Molecular Mobility during Structural Relaxation and its Impact on Organic Amorphous Solids: An Investigation Based on a Calorimetric Approach

Purpose To develop a calorimetry-based model for estimating the time-dependence of molecular mobility during the isothermal relaxation of amorphous organic compounds below their glass transition temperature (T _g).Methods The time-dependent enthalpy relaxation times of amorphous sorbitol, indomethacin, trehalose and sucrose were estimated based on the nonlinear Adam‐Gibbs equation. Fragility was determined from the scanning rate dependence of T _g. Time evolution of the fictive temperature was determined from T _g, the heat capacity of the amorphous and crystalline forms, and from the enthalpy relaxation data.Results Relaxation time changes significantly upon annealing for all compounds studied. The magnitude of the increase in relaxation time does not depend on any one parameter but on four parameters: T _g, fragility, and the crystal–liquid and glass–liquid heat capacity differences. The obtained mobility data for indomethacin and sucrose, both stored at T _g−16 K, correlated much better with their different crystallization tendencies than did the Kohlrausch‐Williams‐Watts (KWW) equation.Conclusions The observed changes in relaxation time help explain and address the limitations of the KWW approach. Due consideration of the time-dependence of molecular mobility upon storage is a key element for improving the understanding necessary for stabilizing amorphous formulations.     

Effect of Polymer Content and Molecular Weight on the Morphology and Heat- and Moisture-Induced Transformations of Spray-Dried Composite Particles of Amorphous Lactose and Poly(vinylpyrrolidone)

Purpose. The aim was to investigate the influence of polymer content and molecular weight on the morphology and heat- and moisture-induced transformations, as indicators of stability, of spray-dried composite particles of amorphous lactose and poly(vinylpyrrolidone) (PVP). Methods. Amorphous lactose and composite particles of amorphous lactose with different contents and molecular weights of PVP were prepared by spray drying. The nanostructure of the particles was analyzed by x-ray powder diffractometry, the morphology by light microscopy and SEM, the glass transition temperatures (T_g), crystallization temperatures (T_c), heats of crystallization and melting temperatures by differential scanning calorimetry, and moisture-induced crystallizations gravimetrically and by microcalorimetry. Results. All the types of particles prepared were amorphous. The T_g was unchanged or only marginally increased as a result of the inclusion of PVP. However, crystallization temperature, time to moisture-induced crystallization, and particle morphology were affected by both content and molecular weight of PVP. Conclusions. Increased content and molecular weight of PVP may have the potential to increase the physical stability of amorphous lactose. However, T_g seems not to be a relevant indicator for the stability of this type of amorphous composite materials.     

Anti-plasticizing Effect of Amorphous Indomethacin Induced by Specific Intermolecular Interactions with PVA Copolymer

The mechanism of how poly(vinyl alcohol-co-acrylic acid-co-methyl methacrylate) (PVA copolymer) stabilizes an amorphous drug was investigated. Solid dispersions of PVA copolymer, poly(vinyl pyrrolidone) (PVP), and poly(vinyl pyrrolidone-co-vinyl acetate) (PVPVA) with indomethacin (IMC) were prepared. The glass transition temperature (T_g)-proportion profiles were evaluated by differential scanning calorimetry (DSC). General T_g profiles decreasing with the IMC ratio were observed for IMC–PVP and IMC–PVPVA samples. An interesting antiplasticizing effect of IMC on PVA copolymer was observed; T_g increased up to 20% IMC ratio. Further addition of IMC caused moderate reduction with positive deviation from theoretical values. Specific hydrophilic and hydrophobic interactions between IMC and PVA copolymer were revealed by infrared spectra. The indole amide of IMC played an important role in hydrogen bonding with PVA copolymer, but not with PVP and PVPVA. X-ray diffraction findings and the endotherm on DSC profiles suggested that PVA copolymer could form a semicrystalline structure and a possibility of correlation of the crystallographic nature with its low hygroscopicity was suggested. PVA copolymer was able to prevent crystallization of amorphous IMC through both low hygroscopicity and the formation of a specific intermolecular interaction compared with that with PVP and PVPVA.     

Surface crystallization of indomethacin below Tg

Purpose To study the surface crystallization of indomethacin (IMC) below T _g and its effects on the kinetics of overall crystallization.Methods Crystal growth rates in liquid layers formed between microscope cover glasses were measured with the top cover glass in place and removed. Polymorphs were identified by powder X-ray diffraction, Raman microscopy, and melting-point determination by hot-stage microscopy. Surface crystals were identified by scratching the sample surface, by cutting the sample to expose its interior, and by analyzing the intensity of X-ray diffraction. Amorphous IMC particles of different sizes were stored at 40°C (T _g−2°C) and analyzed at different times by differential scanning calorimetry to obtain the kinetics of crystallization.Results Crystal growth of IMC below T _g at the free surface was approximately two orders of magnitude faster than that in the bulk, resulting in a surface layer of crystals around a slower-crystallizing interior. Surface crystallization yielded mainly the γ polymorph. Amorphous IMC powders showed rapid initial crystallization at 40°C, but the crystallization abruptly slowed down at “saturation levels” below 100%; the larger the particles, the lower the “saturation level.”Conclusion The faster surface crystallization of IMC than the bulk crystallization leads to unusual crystallization kinetics wherein a rapid initial increase of crystallinity is followed by an abrupt slowdown of crystallization. Surface crystallization should be distinguished from bulk crystallization in modeling and controlling the crystallization of amorphous solids.     

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.     

Effects of Solute Miscibility on the Micro- and Macroscopic Structural Integrity of Freeze-Dried Solids

The purpose of this study was to elucidate the effect of solute miscibility in frozen solutions on their micro- and macroscopic structural integrity during freeze-drying. Thermal analysis of frozen solutions containing poly(vinylpyrrolidone) (PVP) and dextran showed single or multiple thermal transitions (T′_g: glass transition temperature of maximally freeze-concentrated solutes) depending on their composition, which indicated varied miscibility of the concentrated noncrystalline polymers. Freeze-drying of the miscible solute systems (e.g., PVP 10,000 and dextran 1060, single T′_g) induced physical collapse during primary drying above the transition temperatures (> T′_g). Phase-separating PVP 29,000 and dextran 35,000 mixtures (two T′_gs) maintained their cylindrical structure following freeze-drying below both of the T′_gs (<?24°C). Primary drying of the dextran-rich systems at temperatures between the two T′_gs (? 20 to ? 14°C) resulted in microscopically disordered “microcollapsed” cake-structure solids. Freeze-drying microscopy (FDM) analysis of the microcollapsing polymer system showed locally disordered solid region at temperatures between the collapse onset (T_c1) and severe structural change (T_c2). The rigid dextran-rich matrix phase should allow microscopic structural change of the higher fluidity PVP-rich phase without loss of the macroscopic cake structure at the temperature range. The results indicated the relevance of physical characterization and process control for appropriate freeze-drying of multicomponent formulations.     

A Quantitative Assessment of the Significance of Molecular Mobility as a Determinant for the Stability of Lyophilized Insulin Formulations

Purpose The purpose was to explore a method for quantitatively assessing the contribution of molecular mobility to the chemical reactivity of amorphous solids. Degradation of insulin in lyophilized formulations containing trehalose and poly(vinylpyrrolidone)(PVP) was chosen as a model system, and the temperature- and glass transition temperature (T_g)-dependence of the degradation rate was analyzed to obtain the relative contributions of molecular mobility and that of the chemical activational barrier reflected in the energy of activation.Methods Insulin degradation and dimerization in lyophilized trehalose and PVP formulations were monitored at various relative humidities (6–60% RH) and temperatures (10–60°C) by reverse-phase high-performance liquid chromatography (HPLC) and high-performance size-exclusion chromatography (HP-SEC), respectively. The T_g and fragility parameter of the lyophilized insulin formulations were determined by differential scanning calorimetry (DSC).Results Insulin degradation in the initial stage was describable with first-order kinetics for both of the trehalose and PVP formulations. The temperature- and T_g-dependence of the degradation rate indicated that the reactivity of insulin in the trehalose formulation is affected by molecular mobility at low humidity (12% RH), such that the ratio of the observed rate constant (k′) to the rate constant governed only by the activational barrier (k) was 0.051 at the T_g. At higher humidities, in contrast, the value of k′/k was much higher (0.914, 0.978, and 0.994 for 23% RH, 33% RH, and 43% RH, respectively), indicating that insulin degradation rate is determined predominantly by the activational barrier. For insulin degradation in the PVP formulation at temperatures below T_g, the contribution of molecular mobility to the degradation rate appeared to be negligible, as the extrapolated value of t₉₀ at the T_g exhibited a large difference between the formulations with differing T_g values (because of differing water contents).Conclusions The reactivity of insulin in the trehalose and PVP formulations can be described by an equation including factors reflecting the activational barrier (activation energy and frequency coefficient) and factors reflecting the molecular mobility (T_g, fragility parameter and a constant representing the relationship between the molecular mobility and the reaction rate). Thus, analysis of temperature dependence based on the proposed equation allows quantitative assessment of the significance of molecular mobility as a factor affecting chemical reactivity.     

Improving the solubility and bioavailability of dihydroartemisinin by solid dispersions and inclusion complexes

Dihydroartemisinin (DHA) is a poorly water-soluble drug that displays low bioavailability after oral administration. Attempts have been made to improve the solubility of DHA. Yet, no information is available concerning improved bioavailability. This study aimed to improve the water solubility of DHA by two systems: solid dispersions with polyvinylpyrrolidone (PVPK30, PVPK25, PVPK15) and inclusion complexes with hydroxypropyl-β-cyclodextrin (HPβCD), as well as improving the bioavailability of both systems. The phase transition of DHA with hydrophilic polymers was evaluated by X-ray diffraction (XRD) and differential scanning calorimetery (DSC). DHA became amorphous in DHA-HPβCD complexes and showed more amorphous behavior in XRD analyses with rise in molecular weight of PVP. Melting onset temperature of DHA decreased, while DSC thermograms revealed the peak area and enhanced enthalpy change (DH) in solid dispersions as well as inclusion complexes. DHA solubility was enhanced 84-fold in DHA-HPβCD complexes and 50-times in DHA-PVPK30. The improved solubility using the four polymers was in the following order: HPβCD > PVPK30 > PVPK25 > PVPK15. Values of area under curve (AUC) and half life (t_1/2) of DHA-PVPK30 were highest followed by DHA-HPβCD, DHA-PVPK15 and DHA-PVPK25. V_d/f of DHA-PVPK30 was 7-fold. DHA-HPβCD, DHA-PVPK15 and DHA-PVPK25 showed significantly different pharmacokinetic parameters compared with DHA solutions. The 95% confidence interval was meaningful in AUC and t_1/2. Pharmacokinetic parameters revealed that all four-test preparations were significantly more bioavailable than DHA alone.     

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.