Chemical stability of peptides in polymers. 1. Effect of water on peptide deamidation in poly(vinyl alcohol) and poly(vinyl pyrrolidone) matrixes

Characterisation of factors contributing to the performance of nonwoven fibrous matrices as substrates for adenovirus vectored vaccine stabilisation

Adenovirus vectors offer a platform technology for vaccine development. The value of the platform has been proven during the COVID-19 pandemic. Although good stability at 2-8 °C is an advantage of the platform, non-cold-chain distribution would have substantial advantages, in particular in low-income countries. We have previously reported a novel, potentially less expensive thermostabilisation approach using a combination of simple sugars and glass micro-fibrous matrix, achieving excellent recovery of adenovirus-vectored vaccines after storage at temperatures as high as 45 °C. This matrix is, however, prone to fragmentation and so not suitable for clinical translation. Here, we report an investigation of alternative fibrous matrices which might be suitable for clinical use. A number of commercially-available matrices permitted good protein recovery, quality of sugar glass and moisture content of the dried product but did not achieve the thermostabilisation performance of the original glass fibre matrix. We therefore further investigated physical and chemical characteristics of the glass fibre matrix and its components, finding that the polyvinyl alcohol present in the glass fibre matrix assists vaccine stability. This finding enabled us to identify a potentially biocompatible matrix with encouraging performance. We discuss remaining challenges for transfer of the technology into clinical use, including reliability of process performance.

Amorphization of Thiamine Mononitrate: A Study of Crystallization Inhibition and Chemical Stability of Thiamine in Thiamine Mononitrate Amorphous Solid Dispersions

This study investigated thiamine degradation in thiamine mononitrate (TMN):polymer solid dispersions, accounting for the physical state of the vitamin and the recrystallization tendency of TMN in these dispersions. Results were compared with those from solid dispersions containing a different salt form of thiamine (thiamine chloride hydrochloride (TClHCl)). TMN:polymer dispersions were prepared by lyophilizing solutions containing TMN and amorphous polymers (pectin and PVP (polyvinylpyrrolidone)). Samples were stored in controlled temperature and relative humidity (RH) environments for eight weeks and monitored periodically by X-ray diffraction and high performance liquid chromatography (HPLC). Moisture sorption, glass transition temperature (T_g), intermolecular interactions, and pH were also determined. Similar to the TClHCl:polymer dispersions, thiamine was more chemically labile in the amorphous state than the crystalline state, when present in lower proportions in amorphous TMN:polymer dispersions despite increasing T_g values, when environmental storage conditions exceeded the T_g of the dispersion, and when co-formulated with PVP compared to pectin. When thiamine remained as an amorphous solid, chemical stability of thiamine did not differ as a function of counterion present (TMN vs. TClHCl). However, storage at 75% RH led to hydration of thiamine:PVP dispersions, and the resulting pH of the solutions as a function of thiamine salt form led to a higher chemical stability in the acidic TClHCl samples than in the neutral TMN samples.

Amorphization of Thiamine Chloride Hydrochloride: Effects of Physical State and Polymer Type on the Chemical Stability of Thiamine in Solid Dispersions

Thiamine is an essential micronutrient, but delivery of the vitamin in supplements or foods is challenging because it is unstable under heat, alkaline pH, and processing/storage conditions. Although distributed as a crystalline ingredient, thiamine chloride hydrochloride (TClHCl) likely exists in the amorphous state, specifically in supplements. Amorphous solids are generally less chemically stable than their crystalline counterparts, which is an unexplored area related to thiamine delivery. The objective of this study was to document thiamine degradation in the amorphous state. TClHCl:polymer dispersions were prepared by lyophilizing solutions containing TClHCl and amorphous polymers (pectin and PVP (poly[vinylpyrrolidone])). Samples were stored in controlled temperature (30–60 °C) and relative humidity (11%) environments for 8 weeks and monitored periodically by X-ray diffraction (to document physical state) and HPLC (to quantify degradation). Moisture sorption, glass transition temperature (T_g), intermolecular interactions, and pH were also determined. Thiamine was more labile in the amorphous state than the crystalline state and when present in lower proportions in amorphous polymer dispersions, despite increasing T_g values. Thiamine was more stable in pectin dispersions than PVP dispersions, attributed to differences in presence and extent of intermolecular interactions between TClHCl and pectin. The results of this study can be used to control thiamine degradation in food products and supplements to improve thiamine delivery and decrease rate of deficiency.

Bistable Threshold Humidity Sensor Switch with Rectangular Bimorph Bending Plate

Energy-autonomous bistable threshold sensor switches have the potential to reduce costs because they do not need any electrical energy supply for monitoring physical quantities, such as relative humidity. In previous work, a bistable beam-like sensor switch with switching hysteresis was manufactured from sheet metal and a partially coated water vapor-sensitive hydrogel (poly(vinyl alcohol)/poly(acryl acid)). Based on the beam theory, a corresponding mechanical model was developed. However, bending plates should be used instead of bending beams to separate the humidity to be measured from the electrical contacts. For this reason, this work deals with the development and realization of a mechanical model based on the plate theory to describe the deflection of a silicon bimorph bending plate partially coated with hydrogel that swells with increasing humidity. For implementing a switching hysteresis a plasma-enhanced chemical vapor deposition silicon dioxide (SiO₂) layer is used, which was deposited and structured on top of the silicon plate. The hydrogel layer itself is patterned on the surface of the bending plate using a stamp technique. To validate the mechanical model, the switching hysteresis of the miniaturized sensor switch was measured optically by a camera measurement device.

Spontaneous formation of nanoparticles on electrospun nanofibres

We report the spontaneous formation of nanoparticles on smooth nanofibres in a single-step electrospinning process, as an inexpensive and scalable method for producing high-surface-area composites. Layers of nanofibres, containing the proton conducting electrolyte, caesium dihydrogen phosphate, are deposited uniformly over large area substrates from clear solutions of the electrolyte mixed with polymers. Under certain conditions, the normally smooth nanofibres develop caesium dihydrogen phosphate nanoparticles in large numbers on their external surface. The nanoparticles appear to originate from the electrolyte within the fibres, which is transported to the outer surface after the fibres are deposited, as evidenced by cross-sectional imaging of the electrospun fibres. The presence of nanoparticles on the fibre surface yields composites with increased surface area of exposed electrolyte, which ultimately enhances electrocatalytic performance. Indeed, solid acid fuel cells fabricated with electrodes from processed nanofibre-nanoparticle composites, produced higher cell voltage as compared to fuel cells fabricated with state-of-the-art electrodes.

Chemical degradation of proteins in the solid state with a focus on photochemical reactions

Protein pharmaceuticals comprise an increasing fraction of marketed products but the limited solution stability of proteins requires considerable research effort to prepare stable formulations. An alternative is solid formulation, as proteins in the solid state are thermodynamically less susceptible to degradation. Nevertheless, within the time of storage a large panel of kinetically controlled degradation reactions can occur such as, e.g., hydrolysis reactions, the formation of diketopiperazine, condensation and aggregation reactions. These mechanisms of degradation in protein solids are relatively well covered by the literature. Considerably less is known about oxidative and photochemical reactions of solid proteins. This review will provide an overview over photolytic and non-photolytic degradation reactions, and specially emphasize mechanistic details on how solid structure may affect the interaction of protein solids with light.

Molecular dynamics simulation of amorphous hydroxypropyl-methylcellulose acetate succinate (HPMCAS): polymer model development, water distribution, and plasticization

Molecular models for HPMCAS polymer have been developed for molecular dynamics (MD) simulation that attempt to mimic the complex substitution patterns in HPMCAS observed experimentally. These molecular models were utilized to create amorphous HPMCAS solids by cooling of the polymeric melts at different water contents to explore the influence of water on molecular mobility, which plays a critical role in stability and drug release from HPMCAS-based solid matrices. The densities found for the simulated amorphous HPMCAS were 1.295, 1.287, and 1.276 g/cm(3) at 0.7, 5.7, and 13.2% w/w water, indicating swelling of the polymer with increasing water content. These densities compare favorably with the experimental density of 1.285 g/cm(3) for commercial HPMCAS-(AQOAT AS-MF) supporting the present HPMCAS models as a realistic representation of amorphous HPMCAS solids. Water molecules were observed to be mostly isolated from each other at a low water content (0.7% w/w), while clusters or strands of water were pervasive and broadly distributed in size at 13.2% w/w water. The average number of first-shell water molecules (n(w)) increased from 0.17 to 3.5, though the latter is still far below that (8.9) expected for the onset of a separate water phase. Increasing water content from 0.7 to 13.2% w/w was found to reduce the T(g) by ~81 K, similar to experimental observations. Plasticization with increasing water content resulted in increasing polymer mobility and water diffusivity. From 0.7 to 13.2% w/w water, the apparent water diffusivity increased from 1.1 × 10(-9) to 7.0 × 10(-8) cm(2)/s, though non-Einsteinian behavior persisted at all water contents explored. This and the water trajectories in the polymers suggest that water diffusion at 0.7% w/w water follows a "hopping" mechanism. At a higher water content (13.2% w/w) water diffusion follows dual diffusive processes: (1) fast water motions within water clusters; and (2) slower diffusion through the more rigid polymer matrix.

A mechanism-based kinetic analysis of succinimide-mediated deamidation, racemization, and covalent adduct formation in a model peptide in amorphous lyophiles

The succinimide intermediate generated during deamidation of asparagine-containing peptides and proteins has been implicated as having a role in the formation of multiple types of degradants in addition to hydrolysis products, including racemization products and, more recently, amide-linked, nonreducible protein and peptide aggregates. The formation of alternative degradants may be particularly important in solid-state formulations. This study quantitatively examines the role of the succinimide intermediate in hydrolysis, racemization, and covalent, amide-linked adduct formation in amorphous lyophiles. The degradation of a model peptide, Gly-Phe-L-Asn-Gly, and its L- or D-succinimide intermediates were examined in lyophiles containing hydroxypropyl methylcellulose and varying amounts of excess Gly-Val. Disappearance of the starting reactants and formation of up to 10 degradants were monitored when lyophiles were exposed to either 27°C/40% relative humidity (RH) or 40°C/75 RH using a stability indicating high-performance liquid chromatography method. Terminal degradant profiles were the same when the starting reactant was either Gly-Phe-L-Asn-Gly or its succinimide intermediate. Nucleophilic attack occurred preferentially at the α-carbonyl of the succinimide intermediate at ratios of approximately 2:1 for both water and the N-terminus of Gly-Val as the attacking nucleophiles. A mechanism-based kinetic model analysis indicates that hydrolysis, racemization, and covalent, amide-linked adduct formation all proceed via the succinimide intermediate.

Kinetics and mechanisms of deamidation and covalent amide-linked adduct formation in amorphous lyophiles of a model asparagine-containing Peptide

PURPOSE

Asparagine containing peptides and proteins undergo deamidation via a succinimide intermediate. This study examines the role of the succinimide in the formation of covalent, amide-linked adducts in amorphous peptide formulations.

METHODS

Stability studies of a model peptide, Gly-Phe-L-Asn-Gly, were performed in lyophiles containing an excess of Gly-Val at 'pH' 9.5 and 40°C/40% RH. Reactant disappearance and the formation of ten different degradants were monitored by HPLC. Mechanism-based kinetic models were used to generate rate constants from the concentration vs. time profiles.

RESULTS

Deamidation of Gly-Phe-L-Asn-Gly in lyophiles resulted in L- and D-aspartyl and isoaspartyl-containing peptides and four amide-linked adducts between the succinimide and Gly-Val. The kinetic analysis demonstrated competition between water and terminal amino groups in Gly-Val for the succinimide. The extent of covalent adduct formation was dependent on dilution effects due to its second order rate law.

CONCLUSION

The cyclic imide formed during deamidation of asparagine containing peptides in lyophiles can also lead to covalent adducts due to reaction with other neighboring peptides. A reaction model assuming a central role for the succinimide in the formation both hydrolysis products and covalent adducts was quantitatively consistent with the kinetic data. This mechanism may contribute to the presence of covalent, non-reducible aggregates in lyophilized peptide formulations.

The role of thiols and disulfides on protein stability

There has been a tremendous increase in the number of approved drugs derived from recombinant proteins; however, their development as potential drugs has been hampered by their instability that causes difficulty to formulate them as therapeutic agents. It has been shown that the reactivity of thiol and disulfide functional groups could catalyze chemical (i.e., oxidation and beta-elimination reactions) and physical (i.e., aggregation and precipitation) degradations of proteins. Because most proteins contain a free Cys residue or/and a disulfide bond, this review is focused on their roles in the physical and chemical stability of proteins. The effect of introducing a disulfide bond to improve physical stability of proteins and the mechanisms of degradation of disulfide bond were discussed. The qualitative/quantitative methods to determine the presence of thiol in the Cys residue and various methods to derivatize thiol group for improving protein stability were also illustrated.

Impact of sucrose level on storage stability of proteins in freeze-dried solids: II. Correlation of aggregation rate with protein structure and molecular mobility

The purpose of this study is to investigate the impact of sucrose level on storage stability of dried proteins and thus better understand the mechanism of protein stabilization by disaccharides in lyophilized protein products. Five proteins were freeze dried with different amounts of sucrose, and protein aggregation was quantified using Size Exclusion Chromatography. Protein secondary structure was monitored by FTIR. The global mobility was studied using Thermal Activity Monitor (TAM), and fast local dynamics with a timescale of nanoseconds was characterized by neutron backscattering. The density of the protein formulations was measured with a gas pycnometer. The physical stability of the proteins increased monotonically with an increasing content of sucrose over the entire range of compositions studied. Both FTIR structure and structural relaxation time from TAM achieved maxima at about 1:1 mass ratio for most proteins studied. Therefore, protein stabilization by sugar cannot be completely explained by global dynamics and FTIR structure throughout the whole range of compositions. On the other hand, both the fast local mobility and free volume obtained from density decreased monotonically with an increased level of sucrose in the formulations, and thus the local dynamics and free volume correlate well with protein storage stability.

Characterization of physical and viscoelastic properties of polymer films for coating applications under different temperature of drying and storage

The increasing tendency to enhance consumer products with added functionality is leading to ever more complex products. Nowadays more and more particulate products are coated to give the product specific functionalities. An appropriate approach is needed to be able to satisfy customer's requirements. In this work, three reference well-known coating agents, namely two grades of hydroxypropyl methylcellulose (HPMC) and one polyvinyl alcohol (PVA) were selected and investigated. Aqueous solutions of such polymers were obtained and viscosity and shear stress were measured function of shear rate, temperature and polymer concentration. The viscosities of the solutions appear to be mainly shear rate independent, they clearly show Newtonian behaviour. Drying and storage conditions influence on morphology and structure of the cast films were evaluated using scanning electron microscope (SEM). Dynamic mechanical thermal analysis (DMTA) experiments were carried out on HPMC and PVA cast films to assess the viscoelastic properties over wide temperature-frequency range. The time-temperature superposition principle was used to determine the shift factor, aT, and to compose a master curve. Magnitudes and profiles of storage modulus, E', loss modulus, E'', and tan delta master curves are discussed with relation to drying and storage conditions. No impact of drying temperature on the polymer properties was observed whereas the effect of storage temperature resulted to be relevant in terms of shifts in glass transition temperature and, only partially, changes in the magnitudes of E' and E''.

Inhibition of peptide acylation in PLGA microspheres with water-soluble divalent cationic salts

PURPOSE

To test the potential of water-soluble divalent cationic salts to inhibit acylation of octreotide encapsulated in poly(D,L-lactic-co-glycolic acid)-star (PLGA) microspheres.

METHODS

The divalent cationic salts, calcium chloride and manganese chloride, previously shown to disrupt peptide sorption, were introduced in PLGA microspheres prepared by the double emulsion-solvent evaporation method. Peptide stability was monitored by reversed-phase high performance liquid chromatography (RP-HPLC) and identified by liquid chromatography coupled with mass spectrometry (LC-MS) during microsphere degradation under physiological conditions for 4 weeks. Microsphere morphology and salt content were examined by scanning electron microscopy (SEM) and inductively coupled plasma-optical emission spectroscopy (ICP-OES), respectively.

RESULTS

Addition of divalent cationic salts solely to the organic phase did not provide acylation inhibition. However, addition of the salt inhibitors to both the primary emulsion and the outer water phase resulted in improved drug and salt encapsulation efficiency as well as significantly decreased salt leaching and octreotide acylation. After 28 days, the extent of acylation inhibition afforded by divalent cations was > 58% relative to 13% for the NaCl control group.

CONCLUSIONS

Water-soluble divalent cationic salts represent a suitable class of stabilizer of peptide acylation in PLGA microspheres and this study provides an important formulation approach to maximize stabilizer potency.

Effects of glycosylation on the stability of protein pharmaceuticals

In recent decades, protein-based therapeutics have substantially expanded the field of molecular pharmacology due to their outstanding potential for the treatment of disease. Unfortunately, protein pharmaceuticals display a series of intrinsic physical and chemical instability problems during their production, purification, storage, and delivery that can adversely impact their final therapeutic efficacies. This has prompted an intense search for generalized strategies to engineer the long-term stability of proteins during their pharmaceutical employment. Due to the well known effect that glycans have in increasing the overall stability of glycoproteins, rational manipulation of the glycosylation parameters through glycoengineering could become a promising approach to improve both the in vitro and in vivo stability of protein pharmaceuticals. The intent of this review is therefore to further the field of protein glycoengineering by increasing the general understanding of the mechanisms by which glycosylation improves the molecular stability of protein pharmaceuticals. This is achieved by presenting a survey of the different instabilities displayed by protein pharmaceuticals, by addressing which of these instabilities can be improved by glycosylation, and by discussing the possible mechanisms by which glycans induce these stabilization effects.

Role of deliquescence lowering in enhancing chemical reactivity in physical mixtures

Mixtures of deliquescent solids are susceptible to deliquescence lowering, where water vapor condensation occurs in mixtures at a lower critical relative humidity (RH(0mix)) than individual component critical relative humidities (RH(0)s). The purpose of this study was to evaluate the effect of deliquescence lowering on chemical reactivity. Sucrose, citric acid and their physical mixtures were characterized using vapor sorption analysis to determine RH(0) and RH(0mix). Acid-catalyzed sucrose hydrolysis kinetics was determined using polarimetric analysis. Physical mixtures of sucrose and citric acid crystals were prepared and stored at various relative humidities at 22 degrees C. For these physical mixtures, sucrose hydrolysis was found to occur only when the environmental RH exceeded RH(0mix). Degradation kinetics correlated with the storage RH, being fastest at higher RH. In addition, a lag period was initially observed, which was most prominent for samples stored close to RH(0mix). With exposure to RHs below RH(0mix), no sucrose degradation was detected over the experimental time period. In conclusion, mixtures of deliquescent solids showed increased water sorption at lower RHs, which caused solid dissolution and subsequently led to an increase in the chemical reactivity.

Correlations between molecular mobility and chemical stability during storage of amorphous pharmaceuticals

Recent studies have demonstrated that molecular mobility is an important factor affecting the chemical stability of amorphous pharmaceuticals, including small-molecular-weight drugs, peptides and proteins. However, quantitative correlations between molecular mobility and chemical stability have not yet been elucidated. The purpose of this article is to review literature describing the effect of molecular mobility on chemical stability during storage of amorphous pharmaceuticals, and to seek a better understanding of the relative significance of molecular mobility and other factors for chemical reactivity. We first consider the feature of chemical stability often observed for amorphous pharmaceuticals; changes in temperature dependence of chemical stability around matrix glass transition temperature (Tg), and greater stability associated with higher Tg. Secondly, we review papers which quantitatively studied the effects of the global mobility (often referred to as structural relaxation or -relaxation) of amorphous pharmaceuticals on chemical stability, and discuss correlations between chemical stability and global mobility using various equations that have thus far been proposed. Thirdly, the significance of local mobility of drug and excipient molecules in chemical reactivity is discussed in comparison with that of global mobility. Furthermore, we review literature reports which show no relationship between chemical stability and molecular mobility. The lack of apparent relationship is discussed in terms of the effects of the contribution of excipient molecules as reactants, the specific effects of water molecules, the heterogeneity of the matrix, and so on. The following summary has been obtained; the chemical stability of amorphous pharmaceuticals is affected by global mobility and/or local mobility, depending on the length scale of molecular mobility responsible for the chemical reactivity. In some cases, when activation energy for degradation processes is high and when other factors such as the specific effects of water and/or excipients contribute the degradation rate, stability seems to be largely independent of molecular mobility.

Improved protocol and data analysis for accelerated shelf-life estimation of solid dosage forms

PURPOSE

To propose and test a new accelerated aging protocol for solid-state, small molecule pharmaceuticals which provides faster predictions for drug substance and drug product shelf-life.

MATERIALS AND METHODS

The concept of an isoconversion paradigm, where times in different temperature and humidity-controlled stability chambers are set to provide a critical degradant level, is introduced for solid-state pharmaceuticals. Reliable estimates for temperature and relative humidity effects are handled using a humidity-corrected Arrhenius equation, where temperature and relative humidity are assumed to be orthogonal. Imprecision is incorporated into a Monte-Carlo simulation to propagate the variations inherent in the experiment. In early development phases, greater imprecision in predictions is tolerated to allow faster screening with reduced sampling. Early development data are then used to design appropriate test conditions for more reliable later stability estimations.

RESULTS

Examples are reported showing that predicted shelf-life values for lower temperatures and different relative humidities are consistent with the measured shelf-life values at those conditions.

CONCLUSIONS

The new protocols and analyses provide accurate and precise shelf-life estimations in a reduced time from current state of the art.

Effects of gamma radiation on β-lactoglobulin: Oligomerization and aggregation

The conformational changes and aggregation process of beta-lactoglobulin (beta-LG) subjected to gamma irradiation are presented. Beta-LG in solutions of different protein concentrations (3 and 10 mg/ml) and in solid state with different water activities (a(w)) (0.22; 0.53; 0.74) was irradiated using a Cobalt-60 radiation source at dose level of 1-50 kGy. Small-angle X-ray scattering (SAXS) was used to study the conformational changes of beta-LG due to the irradiation treatment. The irradiated protein was also examined by high performance size exclusion chromatography (HPSEC) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under nonreducing and reducing conditions and fluorescence. SAXS analysis showed that the structural conformation of irradiated beta-LG in solid state at different a(w) and dose level was essentially the same as the nonirradiated beta-LG. The scattering data also showed that the irradiation of beta-LG in solution promoted the formation of oligomers. Interestingly, from the data analysis and model building, it could be shown that the formed oligomers are linear molecules, built by linear combinations of beta-LG dimers (tetramers, hexamers, etc). The formation of oligomers was also evidenced by SDS-PAGE analysis and HPSEC chromatograms, in which products with higher molecular mass than that of the dimeric beta-LG were detected. Formation of intermolecular cross-linking between tyrosyl radicals are proposed to be at least partially responsible for this occurrence. From the results it could be shown that the samples irradiated in solution presented some conformational changes under gamma irradiation, resulting in well ordered oligomers and aggregates formed by cross-linking of beta-LG dimers subunits, while the samples irradiated in the solid state were not modified.

Formulation considerations for proteins susceptible to asparagine deamidation and aspartate isomerization

The asparagine (Asn) deamidation and aspartate (Asp) isomerization reactions are nonenzymatic intra-molecular reactions occurring in peptides and proteins that are a source of major stability concern in the formulation of these biomolecules. The mechanisms for the deamidation and isomerization reactions are similar since they both proceed through an intra-molecular cyclic imide (Asu) intermediate. The formation of the Asu intermediate, which involves the attack by nitrogen of the peptide backbone on the carbonyl carbon of the Asn or the Asp side chain, is the rate-limiting step in both the deamidation and the isomerization reactions at physiological pH. In this article, the influence of factors such as formulation conditions, protein primary sequence, and protein structure on the reactivity of Asn and Asp residues in proteins are reviewed. The importance of formulation conditions such as pH and solvent dielectric in influencing deamidation and isomerization reaction rates is addressed. Formulation strategies that could improve the stability of proteins to deamidation and isomerization reactions are described. The review is intended to provide information to formulation scientists, based on protein sequence and structure, to predict potential degradative sites on a protein molecule and to enable formulation scientists to set appropriate formulation conditions to minimize reactivity of Asn and Asp residues in protein therapeutics.

Coupling Between Chemical Reactivity and Structural Relaxation in Pharmaceutical Glasses

PURPOSE

To test the hypothesis that the molecular motions associated with chemical degradation in glassy amorphous systems are governed by the molecular motions associated with structural relaxation. The extent to which a chemical process is linked to the motions associated with structural relaxation will depend on the nature of the chemical process and molecular motion requirements (e.g., translation of a complete molecule, rotational diffusion of a chemical functional group). In this study the chemical degradation and molecular mobility were measured in model systems to assess the degree of coupling between chemical reactivity and structural relaxation. The model systems included pure amorphous cephalosporin drugs, and amorphous molecular mixtures containing a chemically labile drug and an additive expected to moderate molecular mobility.

METHODS

Amorphous drugs and mixtures with additives were prepared by lyophilization from aqueous solution. The physical properties of the model systems were characterized using optical microscopy and differential scanning calorimetry. The chemical degradation of the drugs alone and in mixtures with additives was measured using high-performance liquid chromatography (HPLC). Molecular mobility was measured using isothermal microcalorimetry to measure enthalpy changes associated with structural relaxation below T (g).

RESULTS

A weak correlation between the rates of degradation and structural relaxation times in pure amorphous cephalosporins suggests that reactivity in these systems is coupled to molecular motions in the glassy state. However, when sucrose was added to one of the cephalosporin drugs stability improved even though this addition reduced T (g) and the relaxation time constant, tau(D)(beta), suggesting that there was no correlation between reactivity and structural relaxation in the cephalosporin mixtures. In contrast, the rate of ethacrynate sodium dimer formation in mixtures was more strongly coupled to the relaxation time constant, tau(D)(beta).

CONCLUSIONS

These studies suggest that the extent to which chemical degradation is coupled to structural relaxation in glasses motions is determined by how closely the motions of the rate controlling step in chemical degradation are associated with structural relaxation. Moderate coupling between the rate of dimer formation for ethacrynate sodium in mixtures with sucrose, trehalose and PVP and structural relaxation constants suggests that chemical changes that require more significant molecular motion, and includes at least some translational diffusion, are more strongly coupled to the molecular motions associated with structural relaxation. The observation that sucrose stabilizes cefoxitin sodium even though it lowers T (g) and reduces the relaxation time constant, tau(D)(beta) is perhaps a result of the importance of other kinds of molecular motions in determining the chemical reactivity in glasses.

Deamidation, acylation and proteolysis of a model peptide in PLGA films

The relative rates of deamidation, acylation and proteolysis (i.e. amide bond cleavage) were determined for a model peptide (VYPNGA) in poly (dl-lactide-co-glycolide) films. Films were stored at 70 degrees C and either 95%, 75%, 60%, 45%, 28%, or approximately 0% relative humidity and at 37 degrees C and 95% relative humidity. Peptide degradation products were identified by ESI+MS/MS and quantitated by LC/MS/MS. Extensive overlap of degradation mechanisms occurred, producing a complex mixture of products. Acylation was the dominant peptide degradation reaction (10-20% of total peptide) at early stages of PLGA hydrolysis and at intermediate relative humidity (60-45% RH). Deamidation and proteolysis were dominant (25-50% and 20-40% of total peptide, respectively) at later stages and at high relative humidity (95-75% RH). Understanding the relative rates of each peptide degradation reaction will allow for improved design of PLGA formulations that preserve the stability of peptide and protein drugs.

Effect of N-1 and N-2 residues on peptide deamidation rate in solution and solid state

The deamidation kinetics of 7 model peptides (VYPNGA, VYGNGA, VFGNGA, VIGNGA, VGGNGA, VGPNGA, and VGYNGA) were studied at 70 degrees C in pH 10 buffer solutions and at 70 degrees C and 50% relative humidity in lyophilized solid formulations containing polyvinyl pyrrolidone (PVP). The disappearance of the model peptides from solution and solid-state formulations followed apparent first-order kinetics, proceeding to completion in solution. In the solid state, the reactions showed plateaus with approximately 10% to 30% of the model peptides remaining; this was thought to be due to reversible complexation of the peptides and the PVP followed by slow dissociation of the complexes. The residues immediately N-terminal to asparagine (N-1, N-2) influenced the rate of deamidation significantly in the solid state but had minimal effect in solution. Increases in the volume and hydrophobicity of the N-1 and N-2 residues decreased the rate of deamidation in the solid state, but neither parameter alone adequately accounted for the observed effects. An empirical model using a linear combination of volume and hydrophobicity was developed; it showed that the influences of the volume and the hydrophobicity of the residues in the N-1 and N-2 positions are approximately equally important for the N-1 and N-2 residues.

Water-soluble amphotericin B-polyvinylpyrrolidone complexes with maintained antifungal activity against Candida spp. and Aspergillus spp. and reduced haemolytic and cytotoxic effects

OBJECTIVES

Poor solubility and toxicity severely hinder the clinical use of amphotericin B (AmB), in spite of its attractive chemotherapeutic properties. Water-soluble complexes of AmB and polyvinylpyrrolidone (AmB-PVP) could display lower cytotoxicity while maintaining antifungal activity.

METHODS

AmB-PVP [with PVP of 10, 24 and 40 kDa (AC1, AC2 and AC4)] were compared with free AmB for (i) activity against Candida spp. (five albicans; nine non-albicans) and Aspergillus spp. (four strains), (ii) haemolysis of sheep red blood cells, and (iii) release of lactate dehydrogenase from J774 macrophages [with further comparison with free PVP and a liposomal formulation of amphotericin (AmBisome)].

RESULTS

MICs and MFCs of AC1, AC2 and AC4 against Candida spp. and of AC2 and AC4 against Aspergillus spp. were similar to those of AmB (and even lower for some Candida strains). Killing kinetics (24 h) were also similar. Haemolytic activity of AC2 and AC4 was 2-fold lower than that of free AmB. Cytotoxicity of AC2 towards J774 macrophages was 8-fold lower, and that of AC4 5-fold lower than that of AmB and not significantly different from that of AmBisome. The lower cytotoxicity of AC2, AC4 was correlated with a lower cellular accumulation of amphotericin. Spectroscopic analysis shows that the lower toxicity of AmB-PVP was not owing to significant change in the monomeric/polymeric forms ratio of the drug.

CONCLUSIONS

AmB-PVP complexes compared favourably with AmB for antifungal activity, were less haemolytic and cytotoxic than AmB, and show a similar cytotoxicity profile to AmBisome.

Deamidation of model β-turn cyclic peptides in the solid state

To investigate the importance of secondary structure on peptide deamidation in the solid state, two cyclic beta-turn peptides and their linear analogs were used as models of Asn residues in structured and unstructured domains, and incorporated into poly(vinyl pyrrolidone) (PVP)-based lyophilized solids. The secondary structure of the model peptides was determined in solution and the solid state using a combination of nuclear magnetic resonance (NMR) spectroscopy, circular dichroism (CD), and Fourier transform infrared (FTIR) spectroscopy. The model beta-turn cyclic peptides were found to be type II beta-turns while the linear analogs were determined to be predominantly unstructured. Quantitatively, the cyclic peptides consisted of approximately 80% beta-turn while the linear analogs contained only 30%-35% beta-turn. To characterize the solid environment, T(g), and moisture content of the solid-state formulations were determined. Accelerated stability studies were conducted in the solid state at 37 degrees C using formulations lyophilized from solutions at pH 8.8 (0.1 M borate buffer). The effect of matrix mobility on solid-state deamidation was investigated by altering the moisture content through variation of relative humidity or the addition of a plasticizer. Cyclic peptides degraded 1.2-8 times slower than the linear analogs under all of the conditions studied. The observed rate constants, however, for all of the peptides decreased dramatically (four orders of magnitude) in the glassy solids. This suggests the greater importance of matrix mobility in solid-state degradation. Molecular dynamics (MD) simulations were also performed to explore the low energy, preferred state of the peptides, and determine the structure around the beta-turn.

Impurity formation studies with peptide-loaded polymeric microspheres Part II. In vitro evaluation

Since acylated peptide impurities were isolated from octreotide microspheres following incubation in an in vivo environment, the present investigation was undertaken to determine the dosage form dynamics responsible for facilitating acylation. In particular, microsphere batches made with poly(L-lactide) (PLA) and poly(lactide-co-glycolide) (PLGA) 85:15 were studied for in vitro drug release, mass balance relationships, mass loss behavior, hydration uptake, and solid-state stability. Furthermore, native octreotide was incubated in a varying pH stability model (heat treated lactic acid solutions 42.5%, w/w) to determine the effects of acidity on impurity formation. From a review of the experimental results, the appearance of octreotide impurities or related substances occurred with the onset of polymeric mass loss. In fact, the significant formation of acylated peptide did not appear until >90% mass loss, which was observed at 14 days. It was surmised that because of water uptake, the hydrolytic cleavage of the polymeric backbone created an acidic microenvironment to facilitate the covalent coupling of peptide with polymer. The lactic acid solution stability model corroborated with greater evidence of acylation at pH 2.25 where the presence lactoyl (+72 m/z) derivatives of octreotide were confirmed by MALDI-TOF mass spectrometry.

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 micros by MD simulation.

RESULTS

Increasing water content from 0.5% to 10% w/w was found to reduce the Tg 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.

Effects of sucrose and mannitol on asparagine deamidation rates of model peptides in solution and in the solid state

Asparagine (Asn) degradation kinetics in two model peptides, Gly-Gln-Asn-Gly-Gly (GQNGG) and Val-Tyr-Pro-Asn-Gly-Ala (VYPNGA), were studied at 50 degrees C in pH 7 buffer solutions in the presence and absence of 5% (w/v) sucrose or mannitol and at 50 degrees C and 30% relative humidity in solid samples lyophilized from these solutions. Solid formulations were characterized using Karl Fischer coulometric titration, thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier-transform infrared spectrometry (FTIR), and solid-state nuclear magnetic resonance (NMR) spectroscopy. GQNGG and VYPNGA showed similar pseudo first-order deamidation rates in solution in the absence of sucrose and mannitol. Adding 5% sucrose or mannitol decreased the rates by no more than 17%. The model peptides degraded 2- to 80-fold more slowly in the solid formulations of sucrose and mannitol than in 5% solutions of these carbohydrates. Ratios of deamidation rates of the model peptides depended upon the solid matrix. In the mannitol solid, the ratio of deamidation rates of GQNGG and VYPNGA (GQNGG:VYPNGA) was approximately 8, while in the sucrose solid, the model peptides deamidated at similar rates (GQNGG:VYPNGA congruent with 1). DSC showed the mannitol formulations to be largely amorphous immediately after lyophilization with some ordered, crystalline-like structure; the extent of ordered structure increased during storage as shown by FTIR and ssNMR. In contrast, the sucrose formulation was largely amorphous after lyophilization and remained so during storage. Together, the results showed that 5% sucrose or mannitol in solution does not significantly change the rates of Asn deamidation of the model peptides, while sucrose stabilizes the model peptides against deamidation more than mannitol in the solid state.

Effects of acidic N + 1 residues on asparagine deamidation rates in solution and in the solid state

The deamidation kinetics of four model peptides (AcGQNGG, AcGQNDG, AcGQNEG, and AcGQNQG) were studied in solution (70 degrees C, pH 5-10) and in lyophilized solids [70 degrees C, 50% relative humidity, "effective pH" ('pH') 5-10] containing polyvinyl pyrrolidone. AcGQNGG, AcGQNEG, and AcGQNQG degraded exclusively through Asn deamidation, whereas AcGQNDG also displayed Asp isomerization, and Asp-Gly peptide bond cleavage. The pH/'pH'-rate profiles were consistent with a shift in the rate-determining step of Asn deamidation from carbonyl addition to expulsion of ammonia with increasing pH. In solution, AcGQNGG deamidated up to 38-fold faster than the other peptides, indicating the importance of steric effects of the N + 1 residue. AcGQNGG and AcGQNQG had up to 60 times slower rates of deamidation in the solid state than in solution. In contrast, the deamidation rates of AcGQNEG and AcGQNDG in the solid state were similar to those in solution. N + 1 Glu or Asp residue may enhance local hydration, so that the deamidation of Asn in the solid formulations actually proceeds in a solution-like environment.

In vivo release kinetics of octreotide acetate from experimental polymeric microsphere formulations using oil/water and oil/oil processes

The purpose of the present study was to characterize the in vivo release kinetics of octreotide acetate from microsphere formulations designed to minimize peptide acylation and improve drug stability. Microspheres were prepared by a conventional oil/water (o/w) method or an experimental oil/oil (o/o) dispersion technique. The dosage forms were administered subcutaneously to a rat animal model, and serum samples were analyzed by radioimmunoassay over a 2-month period. An averaged kinetic profile from each treatment group, as a result, was treated with fractional differential equations. The results indicated that poly(l-lactide) microspheres prepared by the o/o dispersion technique provided lower area under the curve (AUC) values during the initial diffusion-controlled release phase, 7.79 ngxd/mL, versus 75.8 ngxd/mL for the o/w batch. During the subsequent erosion-controlled release phase, on the other hand, the o/o technique yielded higher AUC values, 123 ngxd/mL, versus 42.2 ngxd/mL for the o/w batch. The differences observed between the 2 techniques were attributed to the site of drug incorporation during the manufacturing process, given that microspheres contain both porous hydrophilic channels and dense hydrophobic matrix regions. An o/o dispersion technique was therefore expected to produce microspheres with lower incorporation in the aqueous channels, which are responsible for diffusion-mediated drug release.

Prediction of the glass transition temperature of water solutions: comparison of different models

The glass transition temperature (Tg) of a sample is an important parameter that determines its stability during storage. While Tg can be measured by a variety of methods, it is a time-consuming procedure, especially if the sample is to be kept at subzero temperatures, in anhydrous conditions, or if sampling a portion of the specimen for analysis is cumbersome. Hence, predicting rather than directly measuring Tg as a function of the content of the constituents of a blend, mixture, or solution can be a powerful tool. Two main models for predicting Tg have been proposed: Couchman-Karasz (C-K) and Gordon-Taylor (G-T) formalisms. However, many aspects of both are theoretical/terminological in nature, and substantial controversy exists about the various experimental approaches to measuring Tg as well. Here, we compare C-K and G-T formalisms, as well as related problems that arise from the variety of definitions and methods of measuring Tg. Water-trehalose solutions are used as an example for application of the analysis. However, the same conclusions can be expanded to any other solutions so thermodynamical parameters (Tg, DeltaCp, and k) of 20 other commonly used solutes are provided. Practical pitfalls related to determining water content, including experimental data on thermal gravimetry, are also discussed.

N-Acyl substituted 7-amino-4-chloroisocoumarin: A peptide degradation model via an imide mechanism

During the coupling reaction between 3-alkoxy-7-amino-4-chloroisocoumarin and N-acyl alanine dipeptide, an unexpected deamidation reaction was observed. The proposed mechanism for this reaction involved the formation of an imide intermediate which after cleavage led to the release of amino acid moiety. The described deamidation reaction represents the first chemical model involving a non-peptidic moiety, which mimics biological and chemical deamidation processes occurring in proteins or peptides incorporating an asparagine or a glutamine residue.

A molecular dynamics simulation of reactant mobility in an amorphous formulation of a peptide in poly(vinylpyrrolidone)

The reaction pathways available for chemical decomposition in amorphous solids are determined in part by the relative mobilities of the potential reactants. In this study, molecular dynamics simulations of amorphous glasses of polyvinylpyrrolidone (PVP) containing small amounts of water, ammonia, and a small peptide, Phe-Asn-Gly, have been performed over periods of up to 100 ns to monitor the aging processes and associated structural and dynamic properties of the PVP segments and embedded solutes. Glass transition temperatures, Tg, were detected by changes in slopes of the volume-temperature profiles and the internal energy-temperature profiles for the inherent structures upon cooling at different rates. Analyses of the molecular trajectories below Tg reveal both temporal and spatial heterogeneity in polymer and solute mobility, with each molecule or part of a molecule displaying quite different relaxation behaviors for translational, rotational, and/or conformational motions. Rotations of individual polymer segments on the time scale up to 100 ns, though far from complete, are described by the Kohlrausch-Williams-Watts stretched exponential function with relaxation times tau on the order of 10-2.8 x 10(4) micros at an averaged stretching parameter beta of 0.39. The rotation rates are, on the average, faster for the side chains and for segments near the ends of the chains than for the backbones and segments near the middle of the chains. In contrast to their behavior in water, solute diffusive motions in the glassy polymer exhibit non-Einsteinian behavior over the time scale of the simulations characterized by two types of motion: (1) entrapments within relatively fluid microdomains surrounded by a matrix of relatively immobile polymer chains; and (2) jumps between microdomains with greater probability of hopping back to the solute's previous location. The average jump length and frequency are highly dependent on solute size, being much smaller for the tripeptide, Phe-Asn-Gly, than for water and ammonia. The diffusivities of water and ammonia, solutes capable of forming hydrogen bonds with the lactam residues within the polymer segments, are significantly reduced by strong electrostatic interactions. The conformational preferences of Phe-Asn-Gly were compared in the amorphous polymer and water to detect differences in the degree to which the tripeptide may be predisposed toward deamidation of the asparagine side chain in these environments. Although only minor differences are evident in peptide conformation, the conformational dynamics for the peptide embedded in the glassy polymer are characterized by a higher energy barrier between conformational states and 2.5-44-fold larger relaxation times for the dihedral angles of interest than in water. However, in the context of peptide deamidation, these differences may be of secondary importance in comparison to the more than two to three orders of magnitude reduction in the diffusivities of water, ammonia, and the tripeptide in PVP.

Dielectric study of the molecular mobility and the isothermal crystallization kinetics of an amorphous pharmaceutical drug substance

During the development of new pharmaceutical products based on drug substances in their amorphous form, the molecular mobility of an amorphous active ingredient was characterized in detail within a very broad time-temperature range. The relation between the isothermal crystallization kinetics and the dynamics of this amorphous substance was investigated. First, dynamic dielectric spectroscopy (DDS) and the thermostimulated current (TSC) techniques were used to analyze the molecular mobility of the amorphous drug substance over a wide frequency and temperature range (the drug substance is referred to as SSR in this text and was chosen as a model glass-forming system). Two relaxation processes, corresponding to different molecular motions, were identified. The beta(a)-relaxation process, associated with intramolecular oscillation of small dipolar groups, followed Arrhenius temperature behavior over the entire time-temperature domain that was studied. However, the main alpha(a)-relaxation process, assigned to the dielectric manifestation of the dynamic glass transition of the amorphous phase, was described by Vogel-Fulcher-Tammann (VFT) and Arrhenius behavior above and below the glass transition temperature (T(g)) respectively. The physical meaning of these complex dynamics is explained in the context of the Adam and Gibbs (AG) model, by the temperature dependence of the size of cooperatively rearranging regions (CRR) that govern the time scale of delocalized molecular motions. The distinction between the molecular mobility and the structural relaxation of amorphous systems below T(g) is discussed. This work shows that the complementary nature of both DDS and TSC techniques is essential to directly analyze the intramolecular and molecular motions of disordered phases over a wide time-temperature range above and below the T(g). Second, real-time dielectric measurements were carried out to determine the isothermal crystallization kinetics of the SSR amorphous drug. Whatever the crystalline form obtained over time in the crystallization process, the decrease of the dielectric response of amorphous phase, which is characteristic of the isothermal crystallization, was studied to monitor the time dependence of the degree of crystallinity. The characteristic crystallization time, derived from Kohlrausch-Williams-Watt (KWW)-Avrami analyses performed at different temperatures, followed an Arrhenius temperature dependence. Behaviors specific to the molecular mobility of the amorphous drug substance were compared with the characteristic crystallization time. It was concluded that the crystal growth process of the SSR drug seems to be controlled by the intramolecular motions involving the beta(a)-relaxation mode and not by the molecular motions responsible for the alpha(a)-relaxation mode in the range of temperatures >T(g). Subsequent studies will focus on the crystallization process of the SSR drug in the glassy state (T < T(g)).

Reaction of a peptide with polyvinylpyrrolidone in the solid state

During stability studies at high temperature (70 degrees C) and low relative humidity ( approximately 0%), the recovery of an asparagine containing hexapeptide (VYPNGA) and its known deamidation products from solid polyvinylpyrrolidone (PVP) matrices was incomplete. To determine the causes of this mass loss, formulations were prepared by lyophilizing solutions containing PVP, glycerol, and the Asn-hexapeptide in pH 7.5 phosphate buffer, followed by storage at 70 degrees C and 0% relative humidity. Asn-hexapeptide loss was mono-exponential and reached a plateau at about 30% remaining. Total recovery of the peptide and its known deamidation products was approximately 30% of peptide load. Size exclusion chromatography with fluorescence detection indicated the formation of a PVP-peptide adduct that was stable in the presence of 6 M guanidine hydrochloride. Similar stability studies using N-acetyl phenylalanine, phenylalanine ethyl ester, and N-acetyl tyrosine ethyl ester demonstrated that the reaction involves the peptide N-terminus. The adduct was disrupted in the presence of carboxypeptidase-A, suggesting the formation of an amide bond between the peptide and PVP. (15)N solid-state nuclear magnetic resonance spectroscopy using (15)N-labeled valine as a model of the peptide N-terminus showed different populations of (15)N, suggesting that noncovalent peptide-polymer interactions precede amide bond formation.

Different molecular motions in lyophilized protein formulations as determined by laboratory and rotating frame spin-lattice relaxation times

The spin-lattice relaxation times in the laboratory and rotating frame (T(1) and T(1rho)) of protons and carbons in lyophilized bovine serum gamma-globulin formulation containing dextran were determined by (1)H solid-state pulsed nuclear magnetic resonance (NMR) and high-resolution (13)C solid-state NMR. The temperature dependence of T(1) and T(1rho) of dextran protons in the lyophilized formulation suggests that the correlation time, tau(c), of the methylene protons in dextran is approximately 10(-6) s at -100 degrees C and 60% relative humidity, and decreases to 10(-7) s at 0 degrees C. When temperature is increased from 0 degrees C, the increased motion of the methylene groups is reflected in T(1), but is too fast to be observed by changes in T(1rho). Thus, the motion of the methine groups rather than the methylene groups begins to be reflected in T(1rho). The correlation time of the methine protons as determined by T(1rho) was of the same order as that of the methine carbons as determined by T(1rho). As the temperature is increased past the glass/rubber transition temperature, both the methylene and methine motions are greatly enhanced, resulting in much shorter T(1) and T(1rho) relaxation times.

Effect of viscosity on the deamidation rate of a model Asn-hexapeptide

The effect of viscosity on the deamidation rate of a model Asn-containing hexapeptide (l-Val-l-Tyr-Pro-l-Asn-Gly-l-Ala) was assessed in aqueous solution and in solids containing varying amounts of poly(vinyl pyrrolidone) (PVP) and water. Stability studies were conducted at 0.1 mg/mL peptide and 0-50% PVP (w/w) in aqueous solution, and at 5% (w/w) peptide and different relative humidities (31.6, 53.1, 74.4 and 96%) in the solid state. The parent peptide and its deamidation products were analysed by reverse-phase high-performance liquid chromatography. Deamidation rates decreased with increasing solvent viscosity in a manner described by a semi-empirical mathematical model developed to describe this relationship. The results suggest that the motion of the Asn side-chain along the reaction coordinate is a function of the macroscopic solvent viscosity. However, the apparent energy barrier for the diffusive movement of the side-chain appears to be less than the energy barrier for that associated with macroscopic viscosity. The dependence of the deamidation rate on viscosity in both viscous solution and hydrated solids further demonstrates the importance of mobility in peptide deamidation.

Chemical reactivity in solid-state pharmaceuticals: formulation implications

Solid-state reactions that occur in drug substances and formulations include solid-state phase transformations, dehydration/desolvation, and chemical reactions. Chemical reactivity is the focus of this chapter. Of particular interest are cases where the drug-substance may be unstable or react with excipients in the formulation. Water absorption can enhance molecular mobility of solids and lead to solid-state reactivity. Mobility can be measured using various methods including glass transition (T(g)) measurements, solid-state NMR, and X-ray crystallography. Solid-state reactions of drug substances can include oxidation, cyclization, hydrolysis, and deamidation. Oxidation studies of vitamin A, peptides (DL-Ala-DL-Met, N-formyl-Met-Leu-Phe methyl ester, and Met-enkaphalin acetate salt), and steroids (hydrocortisone and prednisolone derivatives) are discussed. Cyclization reactions of crystalline and amorphous angiotensin-converting enzyme (ACE) inhibitors (spirapril hydrochloride, quinapril hydrochloride, and moexipril) are presented which investigate mobility and chemical reactivity. Examples of drug-excipient interactions, such as transacylation, the Maillard browning reaction, and acid base reactions are discussed for a variety of compounds including aspirin, fluoxitine, and ibuprofen. Once solid-state reactions are understood in a pharmaceutical system, the necessary steps can be taken to prevent reactivity and improve the stability of drug substances and products.

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" (taua) and stretched exponential constant (betaa) were calculated according to the KWW equation by assuming that the time required for protein molecules to aggregate (tau) 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 gamma-globulin (BGG) formulations containing dextran or methylcellulose, at temperatures ranging from 10 to 80 degrees C, was followed by size-exclusion chromatography.

RESULTS

Non-exponential BGG aggregation in lyophilized formulations could be described by the KWW equation. The taua and betaa, parameters changed abruptly around the NMR relaxation-based critical mobility temperature for formulations containing dextran and methylcellulose. In the glassy state, in contrast, the taua parameter of these formulations exhibited continuous temperature dependence. The parameter taur, as calculated from taua, and betaa, reflected differences in tau values between the two excipients.

CONCLUSIONS

The results indicate that the parameter betaa is reflective of physical changes wihtin lyophilized formulations. Within the temperature range, during which no abrupt changes in betaa were observed, knowledge regarding the taua and betaa parameters allows the rate of protein aggregation to be predicted. The parameter taur was found to be useful in comparing the protein aggregation behavior of formulations having different taua and betaa values.

Effect of 'pH' on the rate of asparagine deamidation in polymeric formulations: 'pH'-rate profile

The rate of Asn deamidation of a model hexapeptide (L-Val-L-Tyr-L-Pro-L-Asn-Gly-L-Ala) was measured as a function of effective pH ('pH') in glassy and rubbery polymeric solids containing poly(vinyl pyrrolidone) (PVP) and in solution controls at 70 degrees C. The reaction exhibited pseudo-first-order kinetics in all samples over a wide 'pH' range (0.5 < 'pH' < 12); the formation of similar products suggests that the reaction mechanism is unaffected by matrix type. Rates of deamidation were comparable for the polymeric and solution samples in the acidic range ('pH' < 4). Solution-state rates were faster than those in polymeric solids at neutral 'pH' (6 < 'pH' < 8), increasing to a > 10,000-fold difference in the basic range ('pH' > 8). Specific base catalysis was observed in solution and in the polymeric solids under neutral conditions (6 < 'pH' < 8). In solution, the reaction exhibited general base catalysis for 'pH' > 8, whereas the reaction was 'pH'-independent in the polymeric solids in this range. The 'pH'-rate profile and supporting buffer catalysis data are consistent with a change in the rate-determining step in the basic range from 'pH'-dependent attack of the deprotonated backbone amide nitrogen on the Asn side chain in solution to 'pH'-independent ammonia expulsion in the polymeric solids. The results suggest that polymer matrix incorporation not only affects the magnitude of the deamidation rate constant but also the 'pH' dependency of the reaction and the rate-determining step in the basic 'pH' range.

Diffusion of short chain alcohols from amorphous maltose-water mixtures above and below their glass transition temperature

The apparent diffusion coefficient for short chain alcohols in undercooled maltose-water mixtures close to the calorimetric glass transition temperature, Tg, was measured by following desorption using headspace gas chromatography. The plasticising effect of the alcohols on Tg was characterised using differential scanning calorimetry. The initial appearance of alcohol in the headspace showed a linear dependence on the square root of time, allowing it to be modelled as a Fickian diffusive process. The diffusion coefficient decreased with increasing molecular size of alcohol and proximity to Tg. Close to the glass transition the variation of diffusion coefficient with temperature and composition does not follow that of viscosity and, for ethanol, divergence was observed at Tg/T> 0.88.