The Molecular Mobility of Supercooled Amorphous Indomethacin as a Function of Temperature and Relative Humidity

Purpose. To determine the relaxation times of supercooled indomethacin as a function of temperature and relative humidity above Tg, and to analyze the results in the context of being able to predict such behavior at various storage conditions. Methods. Dielectric relaxation times were measured in the frequency domain (12 to 10⁵ Hz) for amorphous indomethacin equilibrated at 0, 56, and 83% relative humidity. The heating rate dependence of Tg for dry supercooled indomethacin was measured with differential scanning calorimetry and used to determine relaxation times. The results were compared with previously published shear relaxation times and enthalpy recovery data. Results. Very good agreement was observed between dielectric and shear relaxation times, and those obtained from the heating rate dependence of the Tg, for dry indomethacin as a function of temperature above Tg. The introduction of water lowered the dielectric relaxation times of supercooled indomethacin without significantly affecting its fragility. The relaxation times below Tg, found to be lower than those predicted by extrapolation of the data obtained above Tg, were analyzed in the context of the Adam-Gibbs-Vogel equation. Conclusions. The relaxation times of amorphous indomethacin obtained from the heating rate dependence of Tg were in good agreement with those obtained from shear and dielectric measurements, thus validating a relatively simple approach of assessing molecular mobility. The significant molecular mobility of amorphous indomethacin observed below Tg, and the significant plasticizing effects of sorbed water, help to explain why amorphous indomethacin crystallizes well below Tg over relatively short time scales.     

Molecular Mobility of Amorphous Pharmaceutical Solids Below Their Glass Transition Temperatures

Purpose. To measure the molecular mobility of amorphous pharmaceutical solids below their glass transition temperatures (Tg), using indomethacin, poly (vinyl pyrrolidone) (PVP) and sucrose as model compounds. Methods. Differential scanning calorimetry (DSC) was used to measure enthalpic relaxation of the amorphous samples after storage at temperatures 16-47 K below Tg for various time periods. The measured enthalpy changes were used to calculate molecular relaxation time parameters. Analogous changes in specimen dimensions were measured for PVP films using thermomechanical analysis. Results. For all the model materials it was necessary to cool to at least 50 K below the experimental Tg before the molecular motions detected by DSC could be considered to be negligible over the lifetime of a typical pharmaceutical product. In each case the temperature dependence of the molecular motions below Tg was less than that typically reported above Tg and was rapidly changing. Conclusions. In the temperature range studied the model amorphous solids were in a transition zone between regions of very high molecular mobility above Tg and very low molecular mobility much further below Tg. In general glassy pharmaceutical solids should be expected to experience significant molecular mobility at temperatures up to fifty degrees below their glass transition temperature.     

Stability and Solubility of Celecoxib-PVP Amorphous Dispersions: A Molecular Perspective

PURPOSE: The purpose of the current study is to evaluate the solubility advantage offered by celecoxib (CEL) amorphous systems and to characterize and correlate the physical and thermodynamic properties of CEL and its amorphous molecular dispersions containing poly(vinylpyrrolidone) (PVP). METHODS: The measurement of crystalline content, glass transition temperatures, and enthalpy relaxation was performed using differential scanning calorimetry. Solubility and dissolutions studies were conducted at 37 degrees C to elucidate release mechanisms. Further, the amorphous systems were characterized by polarized light microscopy and X-ray powder diffraction studies. RESULTS: The PVP content has a prominent effect on the stability and solubility profiles of amorphous systems. A dispersion of 20% w/w PVP with CEL resulted in a maxima in terms of solubility enhancement and lowering of relaxation enthalpy. The release of drug from amorphous molecular dispersions was found to be drug-dependent and independent of the carrier. CONCLUSIONS: The solubility enhancement and enthalpy relaxation studies with respect to PVP concentration helped in a better prediction of role of carrier and optimization of concentration in the use of solid dispersions or amorphous systems. The drug release mechanism is drug-controlled rather than carrier-controlled.     

A Mechanistic Investigation of An Amorphous Pharmaceutical and Its Solid Dispersions,Part II: Molecular Mobility and Activation Thermodynamic Parameters

Purpose. The ability of TSDC to characterize further amorphous materials beyond that possible with DSC was presented in part I (16) of this work. The purpose of part II presented here is to detect and quantitatively characterize time-scales of molecular motions (relaxation times) in amorphous solids at and below the glass transition temperature, to determine distributions of relaxation times associated with different modes of molecular mobility and their temperature dependence, and to determine experimentally the impact upon these parameters of combining the drug with excipients (i.e., solid dispersions at different drug to polymer ratios). The knowledge gleaned may be applied toward a more realistic correlation with physical stability of an amorphous drug within a formulation during storage. Methods. Preparation of amorphous drug and its solid dispersions with PVPK-30 was described in part I (16). Molecular mobility and dynamics of glass transition for these systems were studied using TSDC in the thermal windowing mode. Results. Relaxation maps and thermodynamic activation parameters show the effect of formulating the drug in a solid dispersion on converting the system (drug alone) from one with a wide distribution of motional processes extending over a wide temperature range at and below Tg to one that is homogeneous with very few modes of motion (20% dispersion) that becomes increasingly less homogeneous as the drug load increases (40% dispersion). This is confirmed by the high activation enthalpy (due to extensive intra- and intermolecular interactions) as well as high activation entropy (due to higher extent of freedom) for the drug alone vs. a close to an ideal system (lower enthalpy), with less extent of freedom (low entropy) especially for the 20% dispersion. The polymer PVPK-30 exhibited two distinct modes of motion, one with higher values of activation enthalpies and entropy corresponding to -relaxations, the other with lower values corresponding to -relaxations characterized by local noncooperative motional processes. Conclusions. Using thermal windowing, a distribution of temperature-dependent relaxation times encountered in real systems was obtained as opposed to a single average value routinely acquired by other techniques. Relevant kinetic parameters were obtained and used in mechanistically delineating the effects on molecular mobility of temperature and incorporating the drug in a polymer. This allows for appropriate choices to be made regarding drug loading, storage temperature, and type of polymer that would realistically correlate to physical stability.     

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.     

Molecular Mobility of Supercooled Amorphous Indomethacin,Determined by Dynamic Mechanical Analysis

Purpose. To determine the viscosity and the frequency-dependent shear modulus of supercooled indomethacin as a function of temperature near and above its glass transition temperature and from these data to obtain a quantitative measure of its molecular mobility in the amorphous state. Methods. Viscoelastic measurements were carried with a controlled strain rheometer in the frequency domain, at 9 temperatures from 44° to 90°C. Results. The viscosity of supercooled indomethacin shows a strong non-Arrhenius temperature dependence over the temperature range studied, indicative of a fragile amorphous material. Application of the viscosity data to the VTF equation indicates a viscosity of 4.5 × 10¹⁰ Pa.s at the calorimetric Tg of 41°C, and a T₀ of –17°C. From the complex shear modulus and the Cole-Davidson equation the shear relaxation behaviour is found to be non-exponential, and the shear relaxation time at Tg is found to be approximately 100 sec. Conclusions. Supercooled indomethacin near and above its Tg exhibits significant molecular mobility, with relaxation times similar to the timescales covered in the handling and storage of pharmaceutical products.     

Physical Properties of Solid Molecular Dispersions of Indomethacin with Poly(vinylpyrrolidone) and Poly(vinylpyrrolidone-co-vinyl-acetate) in Relation to Indomethacin Crystallization

Purpose. To measure solid-state features of amorphous molecular dispersions of indomethacin and various molecular weight grades of poly(vinylpyrrolidone), PVP, and poly(vinylpyrrolidone-co-vinylacetate), PVP/VA, in relation to isothermal crystallization of indomethacin at 30°C Methods. The glass transition temperatures (Tg) of molecular dispersions were measured using differential scanning calorimetry (DSC). FT-IR spectroscopy was used to investigate possible differences in interactions between indomethacin and polymer in the various dispersions. The enthalpy relaxation of 5% w/w and 30% w/w polymer dispersions was determined following various aging times. Quantitative isothermal crystallization studies were carried out with pure indomethacin and 5% w/w polymers in drug as physical mixtures and molecular dispersions. Results. All coprecipitated mixtures exhibited a single glass transition temperature. All polymers interacted with indomethacin in the solid state through hydrogen bonding and in the process eliminated the hydrogen bonding associated with the carboxylic acid dimers of indomethacin. Molecular mobility at 16.5°C below Tg was reduced relative to indomethacin alone, at the 5% w/w and 30% w/w polymer level. No crystallization of indomethacin at 30°C was observed in any of the 5% w/w polymer molecular dispersions over a period of 20 weeks. Indomethacin alone and in physical mixtures with various polymers completely crystallized to the form at this level within 2 weeks. Conclusions. The major basis for crystal inhibition of indomethacin at 30°C at the 5% w/w polymer level in molecular dispersions is not related to polymer molecular weight and to the glass transition temperature, and is more likely related to the ability to hydrogen bond with indomethacin and to inhibit the formation of carboxylic acid dimers that are required for nucleation and growth to the crystal form of indomethacin.     

Solid dispersions of the penta-ethyl ester prodrug of diethylenetriaminepentaacetic acid (DTPA): formulation design and optimization studies

Abstract

The penta-ethyl ester prodrug of diethylenetriaminepentaacetic acid (DTPA), which exists as an oily liquid, was incorporated into a solid dispersion for oral administration by the solvent evaporation method using blends of polyvinylpyrrolidone (PVP), Eudragit® RL PO and α-tocopherol. D-optimal mixture design was used to optimize the formulation. Formulations that had a high concentration of both Eudragit® RL PO and α-tocopherol exhibited low water absorption and enhanced stability of the DTPA prodrug. Physicochemical properties of the optimal formulation were evaluated using Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). In vitro release of the prodrug was evaluated using the USP Type II apparatus dissolution method. DSC studies indicated that the matrix had an amorphous structure, while FTIR spectrometry showed that DTPA penta-ethyl ester and excipients did not react with each other during formation of the solid dispersion. Dissolution testing showed that the optimized solid dispersion exhibited a prolonged release profile, which could potentially result in a sustained delivery of DTPA penta-ethyl to enhance bioavailability. In conclusion, DTPA penta-ethyl ester was successfully incorporated into a solid matrix with high drug loading and improved stability compared to prodrug alone.     

Effects of a Citrate Buffer System on the Solid-State Chemical Stability of Lyophilized Quinapril Preparations

Purpose. The objective of this study was to examine the effect of a citric acid-citrate buffer system on the chemical instability of lyophilized amorphous samples of quinapril hydrochloride (QHCl). Methods. Molecular dispersions of QHCl and citric acid were prepared by colyophilization from their corresponding aqueous solutions with a molar ratio of QHCl to citric acid from 1:1 to 6:1 and solution pH from 2.49 to 3.05. Solid samples were subjected to a temperature of 80°C and were analyzed for degradation using high-performance liquid chromatography. The glass transition temperature, Tg, of all samples was measured by differential scanning calorimetry. Results. Samples were first examined by varying the Tg and maintaining the initial solution pH constant. At pH 2.49 the rate of reaction was found to be less dependent on the sample Tg, whereas at pH 2.75 the rate decreased with an increase in Tg. In a second set of experiments at a constant Tg of 70°C, the reaction rate increased as the pH increased. Conclusion. The overall solid-state chemical reactivity of amorphous quinapril depends on the relative amount of QHCl and Q^+–, the zwitterionic form of quinapril. At high proportions of Q^+– (higher pH values) the reaction rate seems to be strongly influenced by the Tg of the mixture, and hence the molecular mobility, whereas at higher proportions of QHCl (lower pH) the reaction rate is less sensitive to Tg, presumably because of different mechanistic rate determining steps for the two sets of conditions.     

Physical stability and solubility advantage from amorphous celecoxib: the role of thermodynamic quantities and molecular mobility

Glassy pharmaceuticals, characterized by excess thermodynamic properties, are theoretically more soluble than their crystalline counterparts. The practical solubility advantage of the amorphous form of celecoxib (CEL) is lost due to its proclivity to lose energy and undergo solvent-mediated devitrification. Theoretical assessment of solubility advantage using differences in isobaric heat capacities (Cp) revealed a 7-21-fold enhancement in the solubility of the amorphous form over that of the crystalline state of CEL. The present study attempts to unveil these differences between experimental and theoretical solubility using thermodynamic parameters such as free energy, enthalpy, and entropy. Amorphous CEL exhibited 1.3-1.5 times enhancement in Cp over that for the crystalline form. The zero and critical molecular mobility regions, represented by Kauzmann temperature (TK) and glass transition temperature (Tg), were found to lie near 246 and 323 K, respectively, for amorphous CEL. The fictive temperature (Tf), an indicator of the configurational entropy of glass, was determined for glassy CEL, signifying the retention of considerable molecular mobility in the glassy phase that may favor nucleation even below Tg. Further, the estimation of various thermodynamic quantities and strength/fragility parameters (D = 11.5 and m = 67.0) postulated the classification of glassy CEL into moderately fragile liquid, as per Angell's classification. A comprehensive understanding of such thermodynamic facets of amorphous form would help in rationalizing the approaches toward development of stable glassy pharmaceuticals with adequate solubility advantage.     

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.     

Direct Observation of the Enthalpy Relaxation and the Recovery Processes of Maltose-Based Amorphous Formulation by Isothermal Microcalorimetry

Purpose. The applicability of isothermal microcalorimetry (IMC) for evaluating enthalpy relaxation and recovery processes of amorphous material was assessed. Methods. A maltose-based formulation was prepared by freeze-dry method. Differential scanning calorimetry (DSC) was used to investigate its glass transition and relaxation behaviors. IMC was applied to quantitatively analyze the relaxation and the recovery processes. The IMC data were analyzed using a derivative of the Kohlrausch-Williams-Watts equation. Results. The glass transition temperature of the formulation and its fictive temperature stored at 15°C for 1 year were 62 and 32°C, respectively. DSC study showed that annealing below the fictive temperature increased the enthalpy recovery, but it was decreased by annealing at higher temperatures. IMC enabled direct observation of the heat flow during both the relaxation and the recovery processes. The decay constant for the recovery process (recovery time) was much smaller and less sensitive to the temperature than that for the relaxation process (relaxation time). Conclusions. IMC was successfully used to obtain quantitative information on both relaxation and recovery processes of amorphous material. The relaxation parameters obtained by this method could explain the thermodynamic behavior of the formulation.     

Solid-State Characteristics of Amorphous Sodium Indomethacin Relative to Its Free Acid

Purpose. Having previously studied the amorphous properties of indomethacin (IN) as a model compound for drugs rendered amorphous during processing, we report on the formation and characterization of its sodium salt in the amorphous state and a comparison between the two systems. Methods. Sodium indomethacin (SI) was subjected to lyophilization from aqueous solution, rapid precipitation from methanol solution, and dehydration followed by grinding to produce, in each case, a completely amorphous form. The amorphous form of SI was analyzed using DSC, XRD, thermomicroscopy and FTIR. The method of scanning rate dependence of the glass transition temperature, Tg, was used to estimate the fragility of the SI system. Enthalpy relaxation experiments were carried out to probe the molecular mobility of the SI system below Tg. Results. The amorphous form of SI formed by different methods had a Tg equal to 121°C at a scanning rate of 20°C/min. This compares with a Tgfor indomethacin of 45°C. Estimation of fragility by the scanning rate dependence of Tg indicates no significant differences in fragility between ionized and unionized forms. Enthalpy relaxation measurements reveal very similar relaxation patterns between the two systems at the same degree of supercooling relative to their respective Tg values. Conclusions. The amorphous form of SI made by various methods has a Tg that is about 75°C greater than that of IN, most likely because of the greater density and hence lower free volume of SI. Yet, the change of molecular mobility as a function of temperature relative to Tgis not very different between the ionized and unionized systems.     

Elucidation of Compression-Induced Surface Crystallization in Amorphous Tablets Using Sum Frequency Generation (SFG) Microscopy

Purpose

To investigate the effect of compression on the crystallization behavior in amorphous tablets using sum frequency generation (SFG) microscopy imaging and more established analytical methods.

Method

Tablets containing neat amorphous griseofulvin with/without excipients (silica, hydroxypropyl methylcellulose acetate succinate (HPMCAS), microcrystalline cellulose (MCC) and polyethylene glycol (PEG)) were prepared. They were analyzed upon preparation and storage using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy (SEM) and SFG microscopy.

Results

Compression-induced crystallization occurred predominantly on the surface of the neat amorphous griseofulvin tablets, with minimal crystallinity being detected in the core of the tablets. The presence of various types of excipients was not able to mitigate the compression-induced surface crystallization of the amorphous griseofulvin tablets. However, the excipients affected the crystallization rate of amorphous griseofulvin in the core of the tablet upon compression and storage.

Conclusions

SFG microscopy can be used in combination with ATR-FTIR spectroscopy and SEM to understand the crystallization behaviour of amorphous tablets upon compression and storage. When selecting excipients for amorphous formulations, it is important to consider the effect of the excipients on the physical stability of the amorphous formulations.

     

Nuclear Magnetic Resonance and Infrared Spectroscopic Analysis of Nedocromil Hydrates

Purpose. Nedocromil sodium (NS), which is used in the treatment ofreversible obstructive airway diseases, such as asthma, has been foundto exist in the following solid phases: the heptahemihydrate, thetrihydrate, a monohydrate, an amorphous phase, which contains variableamounts of water, and a recently discovered methanol + water (MW)solvate. Our aim was to apply ¹³C solid-state nuclear magneticresonance (NMR) spectroscopy and solid-state Fourier transform infrared(FTIR) spectroscopy to the study of specific interactions in the varioussolid forms of NS. Methods. The ¹³ solid-state NMR and FTIR spectra of the varioussolid forms of NS were obtained and were related to the crystalstructures of NS, the conformations of the nedocromil anion, and theinteractions of the water molecules in these crystals. Results. The ¹³C solid-state NMR spectrum is sensitive to theconformation of the nedocromil anion, while the solid-state FTIR spectrumis sensitive to interactions of water molecules in the solid state. In NSmonohydrate, for which the crystal structure has not yet been solved,and in the amorphous phase, the information about the conformationsof the nedocromil anion and the interactions of the water moleculesare deduced from the ¹³C solid-state NMR spectra and solid-state FTIRspectra, respectively. Conclusions. ¹³C solid-state NMR spectroscopy and solid-state FTIRspectroscopy are shown to be powerful complementary tools forprobing the chemical environment of molecules in the solid state,specifically the conformation of the nedocromil anion and the interactions ofwater-molecules, respectively.     

Structural and Dynamic Properties of Crystalline and Amorphous Phases in Raffinose-Water Mixtures

Purpose. To obtain an improved characterisation of the raffinose-water solid-solid and solid-liquid state diagram, and to study the thermophysical behaviour of the solid amorphous phase. This information is expected to shed light on the potential of rafTinose as a pharmaceutical excipient, for stabilising labile preparations at high temperatures. Methods. X-ray diffraction, scanning electron microscopy, polarised-light microscopy, differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) were applied to study raffinose pentahydrate and its behaviour during progressive dehydration. Results. Isothermal dehydration of raffinose pentahydrate led to its gradual amorphisation, but also to minor changes in the diffractograms, suggesting the probability of lower stable hydrates. Their existence was confirmed by DSC. Anhydrous raffinose was found to be completely amorphous, and this was supported by the gradual disappearance of birefringence during dehydration. In contrast, electron micrographs, taken during the dehydration process, exhibited no changes in the original ultrastructural crystal morphology. The widths of the glass-to-fluid transitions and the absolute specific heats of crystalline and amorphous phases in the vitreous and fluid states were used to estimate some structural and relaxation characteristics of amorphous raffinose-water mixtures. Conclusions. Raffinose forms the most 'fragile' glass of those pharmaceutical excipients for which data are available. In its thermomechanical properties, it is superior to trehalose and should therefore be effective as a long-term stabiliser for dried biopharmaceutical preparations at temperatures up to 65°C.     

The Activation Energy at T g and the Fragility Index of Indomethacin,Predicted from the Influence of the Heating Rate on the Temperature Position and on the Intensity of Thermally Stimulated Depolarization Current Peak

Purpose. The purpose of this study was to estimate the activation energy at the glass transition temperature (and the fragility index) of amorphous indomethacin from the influence of heating rate on the features of the relaxation peaks obtained by thermally stimulated depolarization currents (TSDC) and to compare the obtained results with those obtained by other procedures based on TSDC data. Methods. The glass transition temperature region of amorphous indomethacin was characterized at different heating rates by TSDC in a way similar to that used to determine the kinetics of the glass transition relaxation by differential scanning calorimetry. The features of a thermal sampled TSDC peak, namely the temperature location and the intensity, depend on the heating rate. Results. The activation energy for structural relaxation (directly related to glass fragility) was estimated from the heating rate dependence of the TSDC peak location, T _m, and of the maximum intensity of the TSDC peak, I(T _m). Conclusions. The methods for determining the activation energy for structural relaxation and fragility of indomethacin from TSDC data obtained with different heating rates were compared with other procedures previously proposed. TSDC, which is not a very familiar technique in the community of pharmaceutical scientists, proved to be a very convenient technique to study molecular mobility and to determine the fragility index in glass-forming systems. The value of 60 appears as a reasonable value of the fragility index of indomethacin.     

Rapid Assessment of the Structural Relaxation Behavior of Amorphous Pharmaceutical Solids: Effect of Residual Water on Molecular Mobility

Purpose Use RH-perfusion microcalorimetry and other analytical techniques to measure the interactions between water vapor and amorphous pharmaceutical solids; use these measurements and a mathematical model to provide a mechanistic understanding of observed calorimetric events.Materials Isothermal microcalorimetry was used to characterize interactions of water vapor with a model amorphous system, spray-dried raffinose. Differential scanning calorimetry was used to measure glass transition temperature, T _g. High-sensitivity differential scanning calorimetry was used to measure enthalpy relaxation. X-ray powder diffraction (XRPD) was used to confirm that the spray-dried samples were amorphous. Scanning electron microscopy (SEM) was used to examine particle morphology. Gravimetric vapor sorption was used to measure moisture sorption isotherms. Thermogravimetric analysis (TGA) was used to measure loss on drying.Results A moisture-induced thermal activity trace (MITAT) provides a rapid measure of the dependence of molecular mobility on moisture content at a given storage temperature. At some relative humidity threshold, RH_m, the MITAT exhibits a dramatic increase in the calorimetric rate of heat flux. Simulations using calorimetric data indicate that this thermal event is a consequence of enthalpy relaxation.Conclusions RH-perfusion microcalorimetry is a useful tool to determine the onset of moisture-induced physical instability of glassy pharmaceuticals and could find a broad application to determine appropriate storage conditions to ensure long-term physical stability. Remarkably, thermal events measured on practical laboratory timescales (hours to days) are relevant to the stability of amorphous materials on much longer, pharmaceutically relevant timescales (years). The mechanistic understanding of these observations in terms of enthalpy relaxation has added further value to the use of RH-perfusion calorimetry as a rapid means to characterize the molecular mobility of amorphous solids.     

Increased Stabilizing Effects of Amphiphilic Excipients on Freeze-Drying of Lactate Dehydrogenase (LDH) by Dispersion into Sugar Matrices

Purpose. The stabilizing effect of amphiphilic excipients and sugars against protein inactivation during freeze-drying was studied in relation to their physical states in freeze-dried cakes. Methods. Physical states of amphiphilic excipients were studied by powder X-ray diffractometry and differential scanning calorimetry (DSC). Stabilizing effects of excipients were studied using lactate dehydrogenase (LDH) as a model protein. Results. Although poly(ethylene glycols) (PEGs) 1000 to 20000 crystallized when freeze-dried alone, the addition of sugars decreased their crystallinity by dispersing PEG into sugar-dominant matrices. Sugars species, molecular weight of PEGs, and buffer concentration also affected the crystallinity of PEGs. Sugars also dispersed some of other amphiphilic excipients, which tended to crystallize or become microscopically liquid when freeze-dried without sugar. Only the amphiphilic excipients that remained amorphous solid state protected the enzyme during freeze-drying in the absence of sugars. However, combinations of sucrose and all the amphiphilic excipients studied increased the stabilizing effects markedly. The remaining activities were greater than the sum of their individual ones. Conclusions. Various amphiphilic excipients are good stabilizers for freeze-drying of proteins when dispersed into sugar-dominant matrices.     

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.