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.     

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.     

Usefulness of the Kohlrausch-Williams-Watts Stretched Exponential Function to Describe Protein Aggregation in Lyophilized Formulations and the Temperature Dependence Near the Glass Transition Temperature

Purpose. We studied the feasibility of using the Kohlrausch-Williams-Watts stretched exponential function (KWW equation) to describe protein aggregation in lyophilized formulations during storage. Parameters representing mean aggregation time (_a) and stretched exponential constant (_a) were calculated according to the KWW equation by assuming that the time required for protein molecules to aggregate () varies because of the fact that protein aggregation occurs at a rate that depends on the degree of protein deformation resulting from stresses created during freeze-drying. The temperature dependence of the parameters near the glass transition temperature was examined to discuss the possibility of predicting protein aggregation by accelerated testing. Methods. Protein aggregation in lyophilized bovine serum -globulin (BGG) formulations containing dextran or methylcellulose, at temperatures ranging from 10 to 80°C, was followed by size-exclusion chromatography. Results. Non-exponential BGG aggregation in lyophilized formulations could be described by the KWW equation. The _a and _a parameters changed abruptly around the NMR relaxation-based critical mobility temperature for formulations containing dextran and methylcellulose. In the glassy state, in contrast, the _a parameter of these formulations exhibited continuous temperature dependence. The parameter , as calculated from _a and _a, reflected differences in values between the two excipients. Conclusions. The results indicate that the parameter _a is reflective of physical changes wihtin lyophilized formulations. Within the temperature range, during which no abrupt changes in _a were observed, knowledge regarding the _aand _a parameters allows the rate of protein aggregation to be predicted. The parameter was found to be useful in comparing the protein aggregation behavior of formulations having different _a and _a values.     

Temperature Dependence of Bimolecular Reactions Associated with Molecular Mobility in Lyophilized Formulations

Purpose. We studied the temperature dependence of acetyltransfer between aspirin and sulfadiazine, a bimolecular reaction, inlyophilized formulations at temperatures near the glass transitiontemperature (T_g) and NMR relaxation-based critical mobilitytemperature (T_mc), to further understand the effect of molecularmobility on chemical degradation rates in solid pharmaceutical formulations.The temperature dependence of the hydrolysis rates of aspirin andcephalothin in lyophilized formulations was also studied as a model ofbimolecular reactions in which water is a reactant. Methods. Degradation of lyophilized aspirin-sulfadiazineformulations containing dextran and various amounts of water at temperaturesranging from 1°C to 80°C was analyzed by HPLC. The degradation ofcephalothin in lyophilized formulations containing dextran andmethylcellulose was also analyzed at temperatures ranging from 10°C to70°C. Results. Acetyl transfer in lyophilizedasprin—sulfadiazine formulations containing dextran exhibited atemperature dependence with a distinct break around T_mc, whichmay be ascribed to a change in the translational mobility of aspirin andsulfadiazine molecules. The hydrolysis of aspirin and cephalothin inlyophilized formulations, which is also a bimolecular reaction, did not showa distinct break, suggesting that water diffusion is not rate-limiting. Conclusions. The diffusion barrier of water molecules inlyophilized formulations appears to be smaller than the activational barrierof the hydrolysis of aspirin and cephalothin based on the results of thisstudy that the temperature dependence of the hydrolysis rate is almostlinear regardless of T_mc and T_g. On the other hand,the diffusion barrier of aspirin and sulfadiazine molecules appears to becomparable to the activational barrier of the acetyl transfer reactionbetween these com pounds, resulting in nonlinear temperature dependence.     

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.     

The Relationship Between Protein Aggregation and Molecular Mobility Below the Glass Transition Temperature of Lyophilized Formulations Containing a Monoclonal Antibody

Purpose. To find out if the physical instability of a lyophilized dosage form is related to molecular mobility below the glass transition temperature. Further, to explore if the stability data generated at temperatures below the glass transition temperature can be used to predict the stability of a lyophilized solid under recommended storage conditions. Methods. The temperature dependence of relaxation time constant, , was obtained for sucrose and trehalose formulations of the monoclonal antibody (5 mg protein/vial) from enthalpy relaxation studies using differential scanning calorimetry. The non-exponentiality parameter, , in the relaxation behavior was also obtained using dielectric relaxation spectroscopy. Results. For both sucrose and trehalose formulations, the variation in with temperature could be fitted Vogel-Tammann-Fulcher (VTF) equation. The two formulations exhibited difference sensitivities to temperature. Sucrose formulation was more fragile and exhibited a stronger non-Arrhenius behavior compared to trehalose formulation below glass transition. Both formulations exhibited <2% aggregation at t values <10, where t is the time of storage. Conclusions. Since the relaxation times for sucrose and trehalose formulations at 5°C are on the order of 10⁸ and 10⁶ hrs, it is likely that both formulations would undergo very little (<2%) aggregation in a practical time scale under refrigerated conditions.     

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.     

Effect of pH, Counter Ion, and Phosphate Concentration on the Glass Transition Temperature of Freeze-Dried Sugar-Phosphate Mixtures

PURPOSE: The aim of the present work is to study the interaction of phosphate salts with trehalose and sucrose in freeze-dried matrices, particularly the effect of the salts on the glass transition temperature (Tg) of the sugars. METHODS: Freeze-dried trehalose and sucrose systems containing different amounts of sodium or potassium phosphate were analyzed by differential scanning calorimetry to determine the Tg and by Fourier-transform infrared spectroscopy (FTIR) analysis to evaluate the strength of the interaction between sugars and phosphate ions. RESULTS: Sucrose-phosphate mixtures show an increase in Tg up to 40 degrees C in a broad pH range (4-9) compared to that of pure sucrose. Sucrose-phosphate mixtures exhibit a higher Tg than pure sucrose while retaining higher water contents. Trehalose-phosphate mixtures (having a Tg of 135 degrees C at a pH of 8.8) are a better option than pure trehalose for preservation of labile materials. The -OH stretching of the sugars in the presence of phosphates decreases with increase in pH, indicating an increase in the sugar-phosphate interaction. CONCLUSIONS: Sugar-phosphate mixtures exhibit several interesting features that make them useful for lyophilization of labile molecules; Tg values much higher than those observed for the pure sugars can be obtained upon the addition of phosphate.     

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.     

The Influence of Polymer Glass Transition Temperature and Molecular Weight on Drug Release from Tablets Containing Poly(DL-lactic Acid)

Five molecular weight grades of poly(DL-lactic acid) were characterized using gel permeation chro-matography, differential scanning calorimetry, and viscometry to determine the effect of molecular weight on the glass transition temperature and the intrinsic viscosity. In addition, dynamic mechanical thermal analysis was used to assess the dynamic storage modulus and the damping factor of the polymer samples by detecting motional and structural transitions over a wide temperature range. Significant relationships were found between the molecular weight and these polymer properties. The five grades of poly(DL-lactic acid) were also incorporated as binders into matrix tablet formulations containing the model drug theophylline and microcrystalline cellulose. Dissolution studies showed significant correlations between the properties of the polymer and the matrix release profiles of the tablets. The release of theophylline slowed down progressively as the polymer molecular weight increased. The differences in release became less significant and reached a limiting asymptotic value as the molecular weight increased to 138,000. Further, tablet index testing was utilized to determine the compaction properties of the polymer granulations. Although there was no correlation with the molecular weight of PL A, brittle fracture index testing indicated very low brittleness for all granulations tested. However, bonding index determinations correlated very well with both the physical-mechanical properties of the polymer and drug release profiles.     

Effect of Compaction Temperature on Consolidation of Amorphous Copolymers with Different Glass Transition Temperatures

Purpose. The purpose of this study was to relate the combination of glass transition temperature (T _g) and temperature of measurement with the mechanical and compaction properties of some test materials. Methods. Copolymers with different T _gs were synthesised by free radical copolymerisation of methyl methacrylate with lauryl methacrylate. Elastic moduli were measured by dynamic mechanical analysis at different strain rates and temperatures. Compaction experiments were performed at different compaction speeds and temperatures. Results. The difference between temperature of measurement and T _g appears to determine both elastic modulus and yield strength completely. They both decrease with decreasing difference between temperature of measurement and Tg and increase with strain rate. At temperatures of measurement higher than the T _g, the elastic modulus is extremely low because the materials behave as rubbers. Consequently, the amount of energy stored during compaction decreases when the compaction temperature approaches the T _g and increases with strain rate. When the compaction temperature is higher than the T _g, the amount of stored energy is extremely large. The compaction experiments show that the final tablet porosity is completely determined by stress relaxation phenomena. Consequently, the final tablet porosity follows exactly the same relation as that of stored energy. Conclusions. The final tablet porosity is unequivocally determined by the amount of stored energy. This implies that tablet production at a temperature of about 20 K under the glass transition temperature of the material yields tablets with minimum porosity.     

The Relationship Between the Glass Transition Temperature and Water Vapor Absorption by Poly(vinylpyrrolidone)

Water associated with amorphous solids is known to affect significantly the physical and chemical properties of dosage form ingredients. An analysis of water vapor absorption isotherms of poly(vinylpyrrolidone) measured in this and other laboratories, over the range –40 to 60°C, along with the measurement of the glass transition temperature of poly(vinylpyrrolidone) as a function of water content is reported. It is observed that the amount of water vapor absorbed at a particular relative humidity increases with decreasing temperature, along with a significant change in the shape of the isotherm. It is also shown that at any temperature the state of the solid changes from a highly viscous glass to a much less viscous rubber in the region where absorbed water appears to enter into a solvent-like state. Further, the apparent tightly bound state, observed at low relative humidities, appears to exist when the polymer enters into a very viscous glassy state. It is concluded that the apparent states of water and polymer are interrelated in a dynamic manner and, therefore, that they cannot be uncoupled by simple thermodynamic analyses based only on a water-binding model.     

The Relationship Between the Glass Transition Temperature and the Water Content of Amorphous Pharmaceutical Solids

The glass transition temperature of an amorphous pharmaceutical solid is a critical physical property which can dramatically influence its chemical stability, physical stability, and viscoelastic properties. Water frequently acts as a potent plasticizer for such materials, and since many amorphous solids spontaneously absorb water from their surroundings the relationship between the glass transition temperature and the water content of these materials is important. For a wide range of amorphous and partially amorphous pharmaceutical solids, it was found that there is a rapid initial reduction in the glass transition temperature from the dry state as water is absorbed, followed by a gradual leveling off of the response at higher water contents. This plasticization effect could generally be described using a simplified form of the Gordon–Taylor/ Kelley–Bueche relationships derived from polymer free volume theory. Most of the systems considered showed a nearly ideal volume additivity and negligible tendency to interact. This is consistent with the hypothesis that such mixtures behave as concentrated polymer solutions and indicates that water acts as a plasticizer in a way similar to that of other small molecules and not through any specific or stoichiometric interaction process(es).     

Designing the Optimal Formulation for Biopharmaceuticals: A New Approach Combining Molecular Dynamics and Experiments

Biopharmaceuticals are often stored in a lyophilized form. However, stresses due to both the freezing and the drying steps of the lyophilization process can be harmful to protein stability, and appropriate excipients must be added to minimize detrimental effects. In this work, molecular dynamics was used to provide insight into the mechanisms of protein stabilization by different osmolytes, using lactate dehydrogenase as model protein. Our simulations indicate that good cryoprotectants are not always equally good as lyoprotectants, suggesting that synergistic effects may arise when different excipients are combined. This observation is in accordance with the experimental results. In fact, the enzymatic activity of lactate dehydrogenase after freeze-drying was investigated for various formulations, and the trend predicted by molecular dynamics was confirmed. More specifically, we found that the most effective stabilization of the protein structure is achieved when a good cryoprotectant is coupled with an efficient lyoprotectant. Ultimately, we propose a new approach to the design of formulations for protein-based therapeutics to be lyophilized, which combines simulations and experiments. In this new concept, the computational investigation allows a more knowledge-driven and targeted experimental campaign for the selection of the optimal excipients, making the whole process extremely time- and cost-effective.     

Effect of Cations and Anions on Glass Transition Temperatures in Excipient Solutions

The purpose of this study was to investigate the effects of cations and anions of various electrolytes on the glass transition temperature (Tg') of frozen solutions of excipients commonly used in freeze-drying. The effect of electrolyte concentration on freezable water content was also investigated by measuring the enthalpy of melting (ΔH) using Differential Scanning Calorimetry (DSC). Cations and anions induce changes in Tg' of frozen solutions of commonly used parenteral excipients. These changes are dependent on the properties of the excipients used. Tg' values of 5% w/v solutions of maltose, trehalose, sucrose, dextran 40, and polyvinylpyrrolidone (PVP, 17K) were determined as a function of sodium chloride (NaCl) or potassium chloride (KCl) concentrations. In general, a significant decrease in Tg' was observed as a function of increasing the electrolyte concentration. For the disaccharide solutions, the decrease in Tg' due to the addition of NaCl or KCl was similar in magnitude, indicating that changing the cation from K⁺ to Na⁺ had no effect on Tg'. However, the decrease in Tg' for the PVP solution due to the addition of KCl was greater than that observed by the addition of NaCl. The differences in the electrolyte-induced changes on Tg' between the disaccharides and PVP may be potentially attributed to the formation of complexes between the cations and the properly oriented hydroxyl groups in the sugars leaving the anions (Cl^? ions) to exert their effect on Tg'. While zero cation effect would be consistent with these results for the disaccharides, these results do not mean that the cation effects are zero; they only mean that the cation effects are the same. For the PVP solution, K⁺ and Na⁺ ions are not engaged in complex formation with PVP due to the lack of hydroxyl groups. We hypothesize that the structure-breaking K⁺ ions increase the fluidity of water and exert a greater plasticizing effect on Tg', leading to a more significant decrease in Tg' than the structure-making Na⁺ ions, which increase the viscosity of water. The decrease in Tg' of frozen solutions of pharmaceutical excipients caused by the addition of electrolytes may be primarily attributed to an increase in the unfrozen plasticizing water surrounding the excipient molecules. Formulation scientists should evaluate the use of electrolytes in the formulation development of lyophilized products containing commonly used excipients. Electrolytes are often needed as stabilizers for protein formulations; however, their selection and use should be properly evaluated. Because electrolytes cause a decrease in Tg' as a function of electrolyte concentration, it is recommended that the minimum electrolyte concentration needed to maintain product stability should be used to minimize the effect of the electrolyte on lowering the Tg'.     

Molecular Mobility of Protein in Lyophilized Formulations Linked to the Molecular Mobility of Polymer Excipients,as Determined by High Resolution 13C Solid-State NMR

Purpose. The mobility of protein molecules in lyophilized protein formulations was compared with that of excipient molecules based on the spin-lattice relaxation time (T₁) of each molecule determined by high resolution ¹³C solid-state NMR. The relationship between molecular mobility and protein stability is discussed. Methods. Protein aggregation of lyophilized bovine serum --globulin (BGG) formulation containing dextran was measured by size exclusion chromatography. The T₁ of the BGG carbonyl carbon and dextran methin carbon in the formulation was determined by high resolution ¹³C NMR, and subsequently used to calculate the correlation time (_C) of each carbon. The spin-spin relaxation time (T₂) of BGG and dextran protons was measured by pulsed NMR spectrometry, and the critical temperature of appearance of Lorentzian relaxation due to liquid BGG and dextran protons (T_mc) was determined. Results. The _C of dextran methin carbon in BGG-dextran formulations exhibited a linear temperature dependence according to the Adam-Gibbs-Vogel equation at lower temperatures, and a nonlinear temperature dependence described by the Vogel-Tamman-Fulcher equation at higher temperatures. The temperature at which molecular motion of dextran changed was consistent with the T_mc. The _C of BGG carbonyl carbon exhibited a similar temperature dependence to the _C of the dextran methin carbon and substantially decreased at temperatures above T_mc in the presence of dextran. The temperature dependence of BGG aggregation could be described by the Williams-Landel-Ferry equation even at temperatures 20°C lower than T_mc. Conclusions. High resolution ¹³C solid-state NMR indicated that the molecular motion of BGG was enhanced above T_mc in association with the increased global segmental motion of dextran molecules.     

Prediction of the Human EP1 Receptor Binding Site by Homology Modeling and Molecular Dynamics Simulation

The prostanoid receptor EP1 is a G-protein-coupled receptor (GPCR) known to be involved in a variety of pathological disorders such as pain, fever and inflammation. These receptors are important drug targets, but design of subtype specific agonists and antagonists has been partially hampered by the absence of three-dimensional structures for these receptors. To understand the molecular interactions of the PGE2, an endogen ligand, with the EP1 receptor, a homology model of the human EP1 receptor (hEP1R) with all connecting loops was constructed from the 2.6 Å resolution crystal structure (PDB code: 1L9H) of bovine rhodopsin. The initial model generated by MODELLER was subjected to molecular dynamics simulation to assess quality of the model. Also, a step by step ligand-supported model refinement was performed, including initial docking of PGE2 and iloprost in the putative binding site, followed by several rounds of energy minimizations and molecular dynamics simulations. Docking studies were performed for PGE2 and some other related compounds in the active site of the final hEP1 receptor model. The docking enabled us to identify key molecular interactions supported by the mutagenesis data. Also, the correlation of r²=0.81 was observed between the Ki values and the docking scores of 15 prostanoid compounds. The results obtained in this study may provide new insights toward understanding the active site conformation of the hEP1 receptor and can be used for the structure-based design of novel specific ligands.     

Determination of Glass Transition Temperature and in Situ Study of the Plasticizing Effect of Water by Inverse Gas Chromatography

Purpose. To use an inverse gas chromatographic (IGC) method to determine the glass transition temperature (Tg) of some amorphous pharmaceuticals and to extend this technique for the in situ study of the plasticizing effect of water on these materials. Methods. Amorphous sucrose and colyophilized sucrose-PVP mixtures were the model compounds. Both IGC and differential scanning calorimetry (DSC) were used to determine their Tg. By controlling the water vapor pressure in the IGC sample column, it was possible to determine the Tg of plasticized amorphous phases. Under identical temperatures and vapor pressures, the water uptake was independently quantified in an automated water sorption apparatus. Results. The Tg of the dry phases, determined by IGC and by DSC, were in very good agreement. With an increase in the environmental relative humidity (RH), there was a progressive decrease in Tg as a result of the plasticizing effect of water. Because the water uptake was independently quantified, it was possible to use the Gordon-Taylor equation to predict the Tg values of the plasticized materials. The predicted values were in very good agreement with those determined experimentally using IGC. A unique advantage of this technique is that it provides complete control over the sample environment and is thus ideally suited for the characterization of highly reactive amorphous phases. Conclusions. An IGC method was used (a) to determine the glass transition temperature of amorphous pharmaceuticals and (b) to quantify the plasticizing effect of water on multicomponent systems.     

The Use of Inverse Phase Gas Chromatography to Study the Glass Transition Temperature of a Powder Surface

PURPOSE: To measure the glass transition temperature (Tg) at the surface of a hydrophobic particle at different temperatures and humidities, on the hypothesis that the surface may be plasticized to a different extent to the bulk due to slow water sorption giving a concentration gradient of water through the particles. METHODS: Amorphous indomethacin was exposed to a range of relative humidities (RH) and temperatures in an inverse gas chromatograph (IGC). The retention volumes of decane were calculated at all conditions using center of mass (Vcom) and peak height (Vmax) methods. The extent of water sorption was determined gravimetrically. RESULTS: The Vcom retention volumes were found to deviate from Vmax results at certain critical humidities at each temperature. This was taken as a novel method for determining the Tg of the sample surface at different experimental conditions. Extrapolating the critical RH to lower the Tg to experimental temperature to 0% RH yeilded a Tg similar to literature values. Water sorption data provided valuable information on changes in mobility of the amorphous form as a function of temperature and RH. CONCLUSIONS: It is possible to use IGC to determine the Tg of the surface of particles at defined conditions. This overcomes the problems of conventional methods of assessing Tg, relating to disruption of water sorption on heating. This helps in the understanding of the physical form of the surface of hydrophobic particles and how and when the surface will start to crystallize.     

Distribution and Effect of Water Content on Molecular Mobility in Poly(vinylpyrrolidone) Glasses: A Molecular Dynamics Simulation

Purpose This work explores the distribution of water and its effects on molecular mobilities in poly(vinylpyrrolidone) (PVP) glasses using molecular dynamics (MD) simulation technology.Methods PVP glasses containing 0.5% and 10% w/w water and a small amount of ammonia and Phe-Asn-Gly were generated. Physical aging processes and associated structural and dynamic properties were monitored vs. time for periods up to 0.1 μs by MD simulation.Results Increasing water content from 0.5% to 10% w/w was found to reduce the T_g by about 90 K and increase the rates of volume and enthalpy relaxation. At 0.5% w/w, water molecules are mostly isolated and uniformly distributed while at 10% w/w, water distribution is markedly heterogeneous, with strands of water molecules occupying channels between the polymer chains. At 10% w/w, each water molecule has an average of 2.0 neighboring water molecules. The plasticization effects of water were revealed in diffusion coefficient increases of 3.7-, 7.3-, and 7.6-fold for water, ammonia, and the individual polyvinylpyrrolidone segments, respectively, and in shorter relaxation times (37- to 47-fold) for rotation of polymer segments with an elevation in water content from 0.5% to 10% w/w. Water diffusivity was found to linearly correlate with the number of neighboring water molecules. Rotation of the PVP segments is comprised of a fast wobble motion within a highly restrained cavity and a slow rotation over a wider angular space. Only the slow rotation was shown to be significantly affected by water content.Conclusions Water distribution in the PVP glass is highly heterogeneous at 10% w/w water, reflecting the formation of water strands or small clusters rather than complete phase separation. Local enhancement of mobility with increasing water content has been demonstrated using MD simulations.