Technology development to evaluate the effectiveness of viscosity reducing excipients

Addition of pharmaceutical excipients is a commonly used approach to decrease the viscosity of highly concentrated protein formulations, which otherwise could not be subcutaneously injected or processed. The variety of protein-protein interactions, which are responsible for increased viscosities, makes a portfolio approach necessary. Screening of several excipients to develop such a portfolio is time and money consuming in industrial settings. Responsible protein-protein interactions were investigated using the interaction parameter k_D obtained from dynamic light scattering measurements in the studies presented herein. Together with in-silico calculated excipient parameter, k_D could be used as a screening tool accelerating screening and formulation development as k_D is suitable to high-throughput formats using small quantities of protein and low concentrations. A qualitative correlation between k_D and high-concentration viscosity behavior could be shown in our case.

Real-time shape approximation and fingerprinting of single proteins using a nanopore

Established methods for characterizing proteins typically require physical or chemical modification steps or cannot be used to examine individual molecules in solution. Ionic current measurements through electrolyte-filled nanopores can characterize single native proteins in an aqueous environment, but currently offer only limited capabilities. Here we show that the zeptolitre sensing volume of bilayer-coated solid-state nanopores can be used to determine the approximate shape, volume, charge, rotational diffusion coefficient and dipole moment of individual proteins. To do this, we developed a theory for the quantitative understanding of modulations in ionic current that arise from the rotational dynamics of single proteins as they move through the electric field inside the nanopore. The approach allows us to measure the five parameters simultaneously, and we show that they can be used to identify, characterize and quantify proteins and protein complexes with potential implications for structural biology, proteomics, biomarker detection and routine protein analysis.

Effect of Excipients on Liquid-Liquid Phase Separation and Aggregation in Dual Variable Domain Immunoglobulin Protein Solutions

Liquid-liquid phase separation (LLPS) and aggregation can reduce the physical stability of therapeutic protein formulations. On undergoing LLPS, the protein-rich phase can promote aggregation during storage due to high concentration of the protein. Effect of different excipients on aggregation in protein solution is well documented; however data on the effect of excipients on LLPS is scarce in the literature. In this study, the effect of four excipients (PEG 400, Tween 80, sucrose, and hydroxypropyl beta-cyclodextrin (HPβCD)) on liquid-liquid phase separation and aggregation in a dual variable domain immunoglobulin protein solution was investigated. Sucrose suppressed both LLPS and aggregation, Tween 80 had no effect on either, and PEG 400 increased LLPS and aggregation. Attractive protein-protein interactions and liquid-liquid phase separation decreased with increasing concentration of HPβCD, indicating its specific binding to the protein. However, HPβCD had no effect on the formation of soluble aggregates and fragments in this study. LLPS and aggregation are highly temperature dependent; at low temperature protein exhibits LLPS, at high temperature protein exhibits aggregation, and at an intermediate temperature both phenomena occur simultaneously depending on the solution conditions.

Opalescence in monoclonal antibody solutions and its correlation with intermolecular interactions in dilute and concentrated solutions

Opalescence indicates physical instability of a formulation because of the presence of aggregates or liquid-liquid phase separation in solution and has been reported for monoclonal antibody (mAb) formulations. Increased solution opalescence can be attributed to attractive protein-protein interactions (PPIs). Techniques including light scattering, AUC, or membrane osmometry are routinely employed to measure PPIs in dilute solutions, whereas opalescence is seen at relatively higher concentrations, where both long- and short-range forces contribute to overall PPIs. The mAb molecule studied here shows a unique property of high opalescence because of liquid-liquid phase separation. In this study, opalescence measurements are correlated to PPIs measured in diluted and concentrated solutions using light scattering (kD ) and high-frequency rheology (G'), respectively. Charges on the molecules were calculated using zeta potential measurements. Results indicate that high opalescence and phase separation are a result of the attractive interactions in solution; however, the presence of attractive interactions do not always imply phase separation. Temperature dependence of opalescence suggests that thermodynamic contribution to opalescence is significant and Tcloud can be utilized as a potential tool to assess attractive interactions in solution.

Determination of the dipole moments of RNAse SA wild type and a basic mutant

In this study, we report the effects of acidic to basic residue point mutations (5K) on the dipole moment of RNAse SA at different pHs. Dipole moments were determined by measuring solution capacitance of the wild type (WT) and the 5K mutant with an impedance analyzer. The dipole moments were then (1) compared with theoretically calculated dipole moments, (2) analyzed to determine the effect of the point mutations, and (3) analyzed for their contribution to overall protein-protein interactions (PPI) in solution as quantitated by experimentally derived second virial coefficients. We determined that experimental and calculated dipoles were in reasonable agreement. Differences are likely due to local motions of residue side chains, which are not accounted for by the calculated dipole. We observed that the proteins' dipole moments increase as the pH is shifted further from their isoelectric points and that the wild-type dipole moments were greater than those of the 5K. This is likely due to an increase in the proportion of one charge (either negative or positive) relative to the other. A greater charge disparity corresponded to a larger dipole moment. Finally, the larger dipole moments of the WT resulted in greater attractive overall PPI for that protein as compared to the 5K.

Viscosity behavior of high-concentration monoclonal antibody solutions: correlation with interaction parameter and electroviscous effects

The purpose of this work was to understand the viscosity behavior of high-concentration monoclonal antibody (mAb) solutions in a wide range of solution conditions and generate guidelines helpful to formulate products with manageable viscosity. The zeta potential and effective isoelectric point (pI) were determined from electrophoretic mobility measurements. High-frequency rheology studies characterized viscoelasticity at high concentrations. The interaction parameter (k(D) ) obtained from dynamic light scattering quantified intermolecular interactions. Circular dichroism characterized conformational stability upon change in solution pH. Except for mAb-1, all other mAb solutions were found to be more viscous at solution pHs closer to the molecular pI. For mAb-2, mAb-3, and mAb-10,the k(D) indicated intermolecular attractions at the pI, wherein the net molecular charge (Z) was zero, whereas repulsions dominated at pHs away from the pI. At the pI, Z and, hence, the charge-induced repulsions are minimal, whereas the charge distribution becomes most conspicuous. The resulting dominance of nonspecific attractive interactions at the pI increases the self-association or aggregation behavior of protein molecules, leading to a higher viscosity at the pI. mAb-1 is an exception to this general behavior. The k(D) could serve as a qualitative screening tool to predict high-concentration viscosity behavior, whereas the correlation with net charge was inconsistent. A higher negative k(D) generally resulted in a more viscous solution at high concentrations; however, direct quantitative assessment was not possible.

Establishing a link between amino acid sequences and self-associating and viscoelastic behavior of two closely related monoclonal antibodies

PURPOSE

To investigate the underlying cause for the observed differences in self-associating and viscoelastic behavior between two monoclonal antibodies, MAb1, and MAb2.

METHODS

Several mutants were designed by swapping charged residues in MAb1 with those present in MAb2 at their respective positions and vice versa. Rheological analysis was done at low and high shear rates. Dynamic light scattering quantified intermolecular interactions in dilute solutions; sedimentation equilibrium analysis determined the corrected weight average molecular weight (M (wc)) to assess the self-associating behavior in high concentration. The molecular charge was estimated from electrophoretic mobility measurements.

RESULTS

Replacing the charged residues in the CDR of MAb1 resulted in a lower M (wc) and solution viscosity. The corresponding changes in either just the variable light (VL) or variable heavy (VH) chain showed only a partial decrease in viscosity, whereas changes in both VL and VH chains resulted in a dramatic reduction in viscosity. The converse case where the VL and VH chains of MAb2 were made to look like MAb1 did not self-associate or show increased viscosity.

CONCLUSIONS

Exposed charged residues in the CDR of MAb1 are critical in determining the self-associating and highly viscous behavior observed at high concentrations.

Factors affecting the viscosity in high concentration solutions of different monoclonal antibodies

The viscosity profiles of four different IgG(1) molecules were studied as a function of concentration at pH 6.0. At high concentrations, MAb-H and -A showed significantly higher viscosities as compared to MAb-G and -E. Zeta Potential (ξ) measurements showed that all the IgG(1) molecules carried a net positive charge at this pH. MAb-G showed the highest positive zeta potential followed by MAb-E, -H, and -A. A consistent interpretation of the impact of net charge on viscosity for these MAbs is not possible, suggesting that electroviscous effects cannot explain the differences in viscosity. Values of k(D) (dynamic light scattering) indicated that the intermolecular interactions were repulsive for MAb-E and -G; and attractive for MAb-H and -A. Solution storage modulus (G') in high concentration solutions was consistent with attractive intermolecular interactions for MAb-H and -A, and repulsive interactions for MAb-G and -E. Effect of salt addition on solution G' and k(D) indicated that the interactions were primarily electrostatic in nature. The concentration dependent viscosity data were analyzed using a modified Ross and Minton equation. The analysis explicitly differentiates between the effect of molecular shape, size, self-crowding, and electrostatic intermolecular interactions in governing high concentration viscosity behavior.

Long- and short-range electrostatic interactions affect the rheology of highly concentrated antibody solutions

PURPOSE

To explain the differences in protein-protein interactions (PPI) of concentrated versus dilute formulations of a model antibody.

METHODS

High frequency rheological measurements from pH 3.0 to 12.0 quantitated viscoelasticity and PPI at high concentrations. Dynamic light scattering (DLS) characterized PPI in dilute solutions.

RESULTS

For concentrated solutions at low ionic strength, the storage modulus, a viscosity component and a measure of PPI, is highest at the isoelectric point (pH 9.0) and lowest at pH 5.4. This profile flattens at higher ionic strength but not completely, indicating PPI consist of long-range electrostatics and other short-range attractions. At low concentrations, PPI are near zero at pI but become repulsive as the pH is shifted. Higher salt concentrations completely flatten this profile to zero, indicating that these PPI are mainly electrostatic.

CONCLUSIONS

This discrepancy occurs because long-range interactions are significant at low concentrations, whereas both long- and short-range interactions are significant at higher concentrations. Computer modeling was used to calculate antibody properties responsible for long- and short-range interactions, i.e. net charge and dipole moment. Charge-charge interactions are repulsive while dipole-dipole interactions are attractive. Their net effect correlated with the storage modulus profile. However, only charge-charge repulsions correlated with PPI determined by DLS.

The impact of drying method and formulation on the physical properties and stability of methionyl human growth hormone in the amorphous solid state

The objective of this work was to investigate the impact of drying method and formulation on the physical stability (aggregation) and selected important physical properties of dried methionyl human growth hormone (Met-hGH) formulations. Solutions of Met-hGH with different stabilizers were dried by different methods (freeze drying, spray drying, and film drying), with and without surfactant. Properties of the dried powders included powder morphology, specific surface area (SSA), protein surface coverage, thermal analysis, and protein secondary structure. Storage stability of Met-hGH in different formulations was also studied at 50 degrees C and at 60 degrees C for 3 months. The dried powders displayed different morphologies, depending mainly on the method of drying and on the presence or absence of surfactant. Film dried powders had the lowest SSA (approximately 0.03 m(2)/g) and the lowest total protein surface accumulation (approximately 0.003%). Surfactant caused a reduction in the SSA of both spray dried and freeze dried powders. Spray dried powders showed greater protein surface coverage and SSA relative to the same formulations dried by other means. Greater in-process perturbations of protein secondary structure were observed with polymer excipients. Formulation impacted physical stability. In general, low molecular weight stabilizers provided better stability. For example, the aggregation rate at 50 degrees C of Met-hGH in a freeze dried trehalose-based formulation was approximately four times smaller than the corresponding Ficoll-70-based formulation. Drying method also influenced physical stability. In general, the film dried preparations studied showed superior stability to preparations dried by other methods, especially those formulations employing low molecular weight stabilizers.

The effect of neighboring amino acid residues and solution environment on the oxidative stability of tyrosine in small peptides

The effects of neighboring residues and formulation variables on tyrosine oxidation were investigated in model dipeptides (glysyl tyrosine, N-acetyl tyrosine, glutamyl tyrosine, and tyrosyl arginine) and tripeptide (lysyl tyrosyl lysine). The tyrosyl peptides were oxidized by light under alkaline conditions by a zero-order reaction. The rate of the photoreaction was dependent on tyrosyl pK(a), which was perturbed by the presence of neighboring charged amino acid residues. The strength of light exposure, oxygen headspace, and the presence of cationic surfactant, cetyltrimethylammonia chloride had a significant effect on the kinetics of tyrosyl photo-oxidation. Tyrosine and model tyrosyl peptides were also oxidized by hydrogen peroxide/metal ions at neutral pH. Metal-catalyzed oxidation followed first-order kinetics. Adjacent negatively charged amino acids accelerated tyrosine oxidation owing to affinity of the negative charges to metal-ions, whereas positively charged amino acid residues disfavored the reaction. The oxidation of tyrosine in peptides was greatly affected by the presence of adjacent charged residues, and the extent of the effect depended on the solution environment.

Ultrasonic rheology of a monoclonal antibody (IgG2) solution: Implications for physical stability of proteins in high concentration formulations

The purpose of this work was to investigate if physical stability of a model monoclonal antibody (IgG(2)), as determined by extent of aggregation, was related to rheology of its solutions. Storage stability of the model protein was assessed at 25 degrees C and 37 degrees C for three months in solutions ranging from pH 4.0 to 9.0 and ionic strengths of 4 mM and 300 mM. The rheology of IgG(2) solutions has been characterized at 25 degrees C in our previous work and correlation of solution storage modulus (G') with protein-protein interactions established. The extent of aggregation was consistent with solution rheology as understood in terms of changes in G' with protein concentration. Thermodynamic stability of native IgG(2) conformation increased with increasing pH. The correlation between rheology and aggregation was also assessed at increased ionic strengths. The decrease in aggregation was consistent with change in solution rheology profile at pH 7.4 and 9.0. The results provide evidence of a relationship between solution rheology and extent of aggregation for the model protein studied. The implications of this relationship for formulation and physical stability assessment in high concentration protein solutions are discussed.

Effect of hydration on the secondary structure of lyophilized proteins as measured by fourier transform infrared (FTIR) spectroscopy

The impact of hydration on the secondary structure of proteins using FTIR spectroscopy was investigated. Alternative sampling techniques were investigated since KBr pelletization of hydrated proteins is not recommended. Spectra of lyophilized dry proteins were collected in transmission mode by palletizing, mulling, and in ATR mode. Spectra for hydrated proteins were collected in mulls and in ATR mode. Spectra for reconstituted solutions were collected in transmission mode. Spectra of Protein-sucrose colyophilized mixtures were collected in KBr pellets and in ATR mode. Pure proteins underwent significant change in structure upon lyophilization, reforming upon reconstitution. ATR spectra differed from transmission spectra in peak intensity and position, suggesting a more nativelike structure even after correction for refractive index dispersion. No significant differences were found between KBr pellet and mull spectra. Colyophilization with sucrose led to protection of structure. The effect of hydration on the structure was protein dependent, ranging from loss of native structure (IgG) to partial reformation of native structure (BSA). It is concluded that spectra collected in different modes are not directly comparable and caution must be exercised in interpreting the data. Contrary to general view, the secondary structure of proteins in a hydrated state was not equivalent to that in solution.

The challenge of drying method selection for protein pharmaceuticals: product quality implications

Numerous drying methods are used to dry solutions of proteins in the laboratory and/or in pharmaceutical manufacturing. In this review article, we will discuss many of these drying methods. We will briefly introduce and compare the unit operations involved in the drying methods to give an insight on thermal history, and the different stresses that a drying method can present to an active ingredient, particularly for protein molecules. We will review and compare some important physico-chemical properties of the dried powder that result from using different drying methods such as specific surface area, molecular dynamics, secondary structure (for protein molecules), and composition heterogeneity. We will discuss some factors that might lead to differences in the physico-chemical properties of different powders of the same formulation prepared by different techniques. We will examine through a literature review how differences in some of these properties can affect storage stability. Also, we will review process modifications of the basic drying methods and how these modifications might impact physico-chemical properties, in-process stability and/or storage stability of the dried powders.

Drying-induced variations in physico-chemical properties of amorphous pharmaceuticals and their impact on stability (I): stability of a monoclonal antibody

The present study was conducted to investigate the impact of drying method and formulation on the storage stability of IgG1. Formulations of IgG1 with varying levels of sucrose with and without surfactant were dried by different methods, namely freeze drying, spray drying, and foam drying. Dried powders were characterized by thermal analysis, scanning electron microscopy, specific surface area (SSA) analysis, electron spectroscopy for chemical analysis (ESCA), solid state FTIR, and molecular mobility measurements by both isothermal calorimetry and incoherent elastic neutron scattering. Dried formulations were subjected to storage stability studies at 40 degrees C and 50 degrees C (aggregate levels were measured by size exclusion chromatography initially and at different time points). Both drying method and formulation had a significant impact on the properties of IgG1 powders, including storage stability. Among the drying methods, SSA was highest and perturbations in secondary structure were lowest with the spray-dried preparations. Sucrose-rich foams had the lowest SSA and the lowest protein surface accumulation. Also, sucrose-rich foams had the lowest molecular mobility (both fast dynamics and global motions). Stability studies showed a log-linear dependence of physical stability on composition. Preparations manufactured by "Foam Drying" were the most stable, regardless of the stabilizer level. In protein-rich formulations, freeze-dried powders showed the poorest storage stability and the stability differences were correlated to differences in secondary structure. In stabilizer-rich formulations, stability differences were best correlated to differences in molecular mobility (fast dynamics) and total protein surface accumulation.

Impact of critical process and formulation parameters affecting in‐process stability of lactate dehydrogenase during the secondary drying stage of lyophilization: A mini freeze dryer study

The stresses during the secondary-drying stage of lyophilization were investigated using a controlled humidity mini-freeze-dryer [Luthra S, Obert J-P, Kalonia DS, Pikal MJ. 2007. Investigation of drying stresses on proteins during lyophilization: Differentiation between primary and secondary-drying stresses on lactate dehydrogenase using a humidity controlled mini freeze-dryer. J Pharm Sci 96: 61-70.]. Lactate dehydrogenase (LDH), was formulated in: (1) Tween 80, (2) citrate buffer, and (3) both Tween 80 and citrate buffer. Protein activity recovery was measured as a function of relative humidity (RH), product temperature, and drying duration. Studies were also conducted with different concentrations of sucrose, sorbitol, and poly (vinyl pyrrolidone) (PVP). LDH stability was affected to a small extent by RH and significantly by drying temperature and duration. Complete stabilization of LDH was observed when lyophilized with sucrose and PVP but only a partial stabilization was observed with sorbitol. The mini-freeze-dryer enabled studying the process parameters independently, unlike a conventional study where these effects are generally convoluted. The results suggest that the stability of the protein is a function of the dynamics of the system during lyophilization. The origin of the stabilization effect of sucrose, which could, in principle, be attributed both to direct interaction with the protein or vitrification of the protein was elucidated using lyoprotectants that can either hydrogen bond well with the protein (sorbitol) or form a good glass (PVP). It appears both effects are required for complete stabilization of the protein.

Simultaneous Measurement of Water Desorption Isotherm and Heats of Water Desorption of Proteins Using Perfusion Isothermal Microcalorimetry

The purpose of this work was to study protein-water interactions using a perfusion isothermal calorimetry method by simultaneously measuring the water (de)sorption isotherm and heats of desorption (DeltaH(desorption)). Lysozyme, bovine serum albumin (BSA), and a monoclonal immunoglobulin (IgG) were studied. Desorption isotherms and DeltaH(desorption) were calculated using data from two perfusion systems, which measured heat flow resulting from interaction of water vapor with the protein sample and with pure water, respectively. The desorption isotherms calculated from the calorimetry were in good agreement with the gravimetric data. The average DeltaH(desorption) at high hydration was 54.6 kJ/mol and decreased (approaching heat of water evaporation) with desorption and passed through a minimum at protein specific water content, below which it increased again reaching 59.0 kJ/mol at the lowest hydration levels. The difference between the DeltaH(desorption) above the minimum and heat of water evaporation has been attributed to conformational changes in the protein. This conclusion is supported with data for lysozyme in which a dynamic glass like transition has been observed at the water content of the minimum in the calorimetric enthalpy data at 293 K. This work establishes perfusion calorimetry as a rapid and controlled method to study the thermodynamics of protein-water interaction.

A high-throughput method for detection of protein self-association and second virial coefficient using size-exclusion chromatography through simultaneous measurement of concentration and scattered light intensity

PURPOSE

To characterize protein self-association along with second virial coefficient (a measure of solution nonideality) using size-exclusion chromatography (SEC) utilizing a novel flow cell that is capable of simultaneously measuring protein concentration and scattered light intensity.

METHODS

beta-lactoglobulin A (beta Lg), known to exhibit NaCl-dependent monomer-dimer equilibrium at pH 3.0, was used as the model protein. A range of concentrations and corresponding scattered light intensities, obtained in the eluting peak from a single protein injection, in different solution conditions, were used to generate the Debye plots [Formula: see text]. The Debye light scattering equation was modified to include the monomer-dimer equilibrium model and the second virial coefficient to analyze the data obtained.

RESULTS

Debye plots of beta Lg, while linear at pH 2.3, 0 M NaCl (pure monomer) and at pH 3.0, 1 M NaCl (pure dimer), showed curvature at pH 3.0, for varying NaCl concentrations (0.02-0.5 M). The curvature was indicative of the association behavior of this protein. The modified Debye light scattering equation, when fit onto the nonlinear Debye plots, yielded apparent K (a) values ranging from 10(2) to 10(5) M(-1) under various solution conditions. The apparent K (a) values obtained from this method followed similar trend to those reported in literature.

CONCLUSIONS

SEC combined with simultaneous detection of scattered light intensity and concentration provides a rapid means of detection of protein self-association. The short duration of sample detection and analysis combined with SEC makes this method a useful tool for high-throughput characterization of protein association during early stages of protein formulation.

Drying-Induced Variations in Physico-Chemical Properties of Amorphous Pharmaceuticals and Their Impact on Stability II: Stability of a Vaccine

OBJECTIVES

To investigate the impact of drying method on the storage stability of dried vaccine formulations.

MATERIALS AND METHODS

A sucrose-based formulation of a live attenuated virus vaccine of a parainfluenza strain, with and without surfactant, was dried from by different methods; freeze drying, spray drying and foam drying. Dried powders were characterized by differential scanning calorimetry, specific surface area (SSA) analysis and by electron spectroscopy for chemical analysis (ESCA) to evaluate vaccine surface coverage in the dried formulations. Dried formulations were subjected to storage stability studies at 4, 25 and 37 degrees C. The vaccine was assayed initially and at different time points to measure virus-cell infectivity, and the degradation rate constant of the vaccine in different dried preparations was determined.

RESULTS

SSA was highest with the spray dried preparation without surfactant (approximately 2.8 m(2)/g) and lowest in the foam dried preparations (with or without surfactant) (approximately 0.1 m(2)/g). Vaccine surface coverage was estimated based on ESCA measurements of nitrogen content. It was predicted to be highest in the spray dried preparation without surfactant and lowest in the foam with surfactant. Stability studies conducted at 25 degrees C and 37 degrees C showed that the vaccine was most stable in the foam dried preparation with surfactant and least stable in spray dried preparations without surfactant and in all freeze dried preparations regardless of the presence of surfactant. Addition of surfactant did lower the SSA and vaccine surface coverage in freeze dried preparations but still did not improve storage stability.

CONCLUSIONS

In drying methods that did not involve a freezing step, good storage stability of Medi 534 vaccine in the dried form was found with low SSA and low vaccine surface accumulation, both of which integrate into low fraction of vaccine at the surface. Ice appears to be a major destabilizing influence.

Investigation of Drying Stresses on Proteins during Lyophilization: Differentiation between Primary and Secondary-Drying Stresses on Lactate Dehydrogenase Using a Humidity Controlled Mini Freeze-Dryer

This article describes the design, performance testing, and application of a controlled humidity mini-freeze-dryer in studying the physical stability of lactate dehydrogenase during lyophilization. Performance evaluation of the mini-freeze-dryer was conducted with tests, namely water sublimation, radiation heat exchange, lowest achievable temperature, and leak testing. Protein stability studies were conducted by comparing protein activity at various stages of lyophilization with the initial activity. The shelf and condenser temperature were stable at <-40 degrees C, wall temperature was within 2 degrees C of the shelf temperature, and the leak rate was small. The chamber pressure was controlled by the ice on the condenser and the product temperature during sublimation was equal to the shelf temperature. Addition of Tween 80 prevented activity loss in solution and after freeze-thaw. No activity loss was observed after primary-drying even in absence of lyoprotectants and with collapse of cake structure. Five percent (w/w) sucrose concentration was required to achieve full stabilization. In conclusion, performance testing established that the mini-freeze-dryer was suitable for mechanistic freeze-drying studies. Secondary-drying was the critical step for protein stability. The concentration of sucrose required to stabilize the protein completely was several orders of magnitude higher than that required to satisfy the direct interaction requirement of the protein.

Ultrasonic storage modulus as a novel parameter for analyzing protein-protein interactions in high protein concentration solutions: correlation with static and dynamic light scattering measurements

The purpose of this work was to establish ultrasonic storage modulus (G') as a novel parameter for characterizing protein-protein interactions (PPI) in high concentration protein solutions. Using an indigenously developed ultrasonic shear rheometer, G' for 20-120 mg/ml solutions of a monoclonal antibody (IgG(2)), between pH 3.0 and 9.0 at 4 mM ionic strength, was measured at frequency of 10 MHz. Our understanding of ultrasonic rheology indicated decrease in repulsive and increase in attractive PPI with increasing solution pH. To confirm this behavior, dynamic (DLS) and static (SLS) light scattering measurements were conducted in dilute solutions. Due to technical limitations, light scattering measurements could not be conducted in concentrated solutions. Mutual-diffusion coefficient, measured by DLS, increased with IgG(2) concentration at pH 4.0 and this trend reversed as pH was increased to 9.0. Second virial coefficient, measured by SLS, decreased with increasing pH. These observations were consistent with the nature of PPI understood from G' measurements. Ultrasonic rheology, DLS, and SLS measurements were also conducted under conditions of increased ionic strength. The consistency between rheology and light scattering analysis under various solution conditions established the utility of ultrasonic G' measurements as a novel tool for analyzing PPI in high protein concentration systems.

Application of high‐frequency rheology measurements for analyzing protein–protein interactions in high protein concentration solutions using a model monoclonal antibody (IgG2)

The purpose of this work was to explore the utilization of high-frequency rheology analysis for assessing protein-protein interactions in high protein concentration solutions. Rheology analysis of a model monoclonal immunoglobulin G2 solutions was conducted on indigenously developed ultrasonic shear rheometer at frequency of 10 MHz. Solutions at pH 9.0 behaved as most viscous and viscoelastic whereas those at pH 4.0 and 5.4 exhibited lower viscosity and viscoelasticity, respectively. Intrinsic viscosity, hydrophobicity, and conformational analysis could not account for the rheological behavior of IgG2 solutions. Zeta potential and light scattering measurements showed the significance of electroviscous and specific protein-protein interactions in governing rheology of IgG2 solutions. Specific protein-protein interactions resulted in formation of reversible higher order species of monomer. Solution storage modulus (G'), and not loss modulus or complex viscosity, was the more reliable parameter for predicting protein-protein interactions. Predictions about the nature of protein-protein interactions made on the basis of solution G' were found to be consistent with observed effect of pH and ionic strength on zeta potential and scattered intensity of IgG2 solutions. Results demonstrated the potential of high-frequency storage modulus measurements for understanding behavior of proteins in solutions and predicting the nature of protein-protein interactions.

Removal of peroxides in polyethylene glycols by vacuum drying: implications in the stability of biotech and pharmaceutical formulations

The purpose of this study was to investigate the utility of vacuum drying for removing peroxides from polyethylene glycols (PEGs). PEG solutions (PEG 1450 and PEG 20000) containing varying levels of peroxides were prepared by storing under different light and temperature conditions. PEGs containing low and high levels of peroxides were vacuum dried from dilute and concentrated solutions (2.5%, 7.5%, 15%, and 50% wt/vol of PEG 1450 and 2.5%, 7.5%, 15%, and 25% wt/vol of PEG 20000). Ferrous ion oxidation in presence of ferric ion indicator xylenol orange (FOX) colorimetric assay was used to determine the concentration of peroxides. Peroxide content in PEGs increased upon storage. The increase was more pronounced when PEGs were stored at higher temperatures and exposed to light. Vacuum drying at 0.1 mm Hg for 48 hours at 25 degrees C resulted in greater than 90% decrease in the level of peroxides in all cases except when high peroxide containing 25% wt/vol solution of PEG 20000 or 50% wt/vol solution of PEG 1450 were dried. The reduction in the level of peroxides for PEGs dried from high peroxide containing 25% wt/vol solution of PEG 20000 and 50% wt/vol solution of PEG 1450 was found to be 88% and 52%, respectively. Oxidation of methionine in Met-Leu-Phe peptide was significantly reduced when vacuum-dried PEGs were used. Vacuum drying PEG solutions at low pressures is an effective method for the removal of the residual peroxides present in commercially available PEGs.

Protein Structural Conformation and Not Second Virial Coefficient Relates to Long-Term Irreversible Aggregation of a Monoclonal Antibody and Ovalbumin in Solution

PURPOSE

The purpose of the study was to investigate the relationship of the second virial coefficient, B22, to the extent of irreversible protein aggregation upon storage.

METHODS

A monoclonal antibody and ovalbumin were incubated at 37 degrees C (3 months) under various solution conditions to monitor the extent of aggregation. The B22 values of these proteins were determined under similar solution conditions by a modified method of flow-mode static light scattering. The conformation of these proteins was studied using circular dichroism (CD) spectroscopy and second-derivative Fourier transform infrared spectroscopy.

RESULTS

Both proteins readily aggregated at pH 4.0 (no aggregation observed at pH 7.4); the extent of aggregation varied with the ionic strength and the presence of cosolutes (sucrose, glycine, and Tween 80). Debye plots of the monoclonal antibody showed moderate attractive interactions at pH 7.4, whereas, at pH 4.0, nonlinear plots were obtained, indicating self-association. CD studies showed partially unfolded structure of antibody at pH 4.0 compared with that at pH 7.4. In the case of ovalbumin, similar B22 values were obtained in all solution conditions irrespective of whether the protein aggregated or not. CD studies of ovalbumin indicated the presence of a fraction of completely unfolded as well as partially unfolded species at pH 4.0 compared with that at pH 7.4.

CONCLUSIONS

The formation of a structurally altered state is a must for irreversible aggregation to proceed. Because this aggregation-prone species could be an unfolded species present in a small fraction compared with that of the native state or it could be a partially unfolded state whose net interactions are not significantly different compared with those of the native state, yet the structural changes are sufficient to lead to long-term aggregation, it is unlikely that B22 will correlate with long-term aggregation.

Application of Ultrasonic Shear Rheometer to Characterize Rheological Properties of High Protein Concentration Solutions at Microliter Volume

The purpose of this work was to conduct preliminary rheological analysis on high protein concentration solutions by using the technique of ultrasonic shear rheometry at megahertz frequencies. The work was aimed at establishing the viability of the technique for analyzing protein solution rheology as well as obtaining an initial understanding of the effect of solution conditions on solution rheology of a model protein. Bovine serum albumin (BSA) was used for this study, and rheological analysis was conducted at 20 microL sample volume between pH 2.0 and 9.0 at different ionic strengths at 25 degrees C using 5 and 10 MHz quartz crystals. Significant differences in storage modulus among solutions at pH 5.0, 7.0, and 9.0 could only be detected at 10 MHz, and the errors associated with measurements were smaller as compared to those at 5 MHz for all the solutions studied. Solutions at pH 2.0 and 3.0 showed a time-dependent change in solution rheology. For solutions at pH 5.0, 7.0, and 9.0, which did not show time dependence in solution rheology, loss modulus data at lower concentrations correlated well with the dilute solution data in the literature. At higher concentrations, pH 5.0 solutions exhibited a higher loss modulus than pH 7.0 and pH 9.0 solutions. Storage modulus decreased with increasing ionic strength, unlike loss modulus, which did not show any change, except at pI of protein when no effect was observed. The results show the potential of high frequency rheometry for analyzing subtle differences in rheology of pharmaceutically relevant protein solutions at microliter volume.

Second derivative tryptophan fluorescence spectroscopy as a tool to characterize partially unfolded intermediates of proteins

The application of second derivative tryptophan (Trp) fluorescence spectroscopy to characterize partially unfolded intermediates of proteins relevant to protein formulation was investigated. The second derivatives of the normalized emission scans of N-acetyl tryptophanamide (NATA), single-Trp containing proteins, somatostatin and human serum albumin (HSA), and two-Trp containing proteins previously shown to form partially unfolded intermediates, beta-lactoglobulin (beta Lg) and interferon alpha-2a (IFN alpha 2a), were studied in solution. The second derivative of NATA in water showed three bands at 340, 348 and 367 nm. The 340 nm band showed a blue shift, whereas the intensity of all three bands was affected by a decrease in solution polarity. Second derivative of single-Trp containing proteins, somatostatin and HSA, showed three negative bands, whereas, the second derivative of the two-Trp containing proteins, beta Lg and IFN alpha 2a, showed four bands, two of which lie in the 320-340 nm range. These two bands were attributed to the presence of the Trps in different microenvironments. The characteristic changes in the intensities of these two bands on addition of guanidine hydrochloride (beta Lg) and with a decrease in solution pH (IFN alpha 2a) were related to the presence of partially unfolded intermediates of these proteins. Thus, second derivative Trp fluorescence spectroscopy can be used as an important tool to identify partially unfolded states of proteins during formulation utilizing order of magnitude lower concentrations compared to such other technique as near UV CD.

Determination of second virial coefficient of proteins using a dual-detector cell for simultaneous measurement of scattered light intensity and concentration in SEC-HPLC

A method is proposed for the measurement of the B22 value of proteins in aqueous solutions in flow-mode that utilizes a novel fabricated dual-detector cell, which simultaneously measures protein concentration and the corresponding scattered light intensity at 90 degrees , after the protein elutes from a size-exclusion column. Each data point on the chromatograms obtained from the light scattering detector and the concentration (ultraviolet) detector is converted to Rayleigh's ratio, Rtheta, and concentration, c, respectively. The B22 value is calculated from the slope of the Debye plot (Kc/Rtheta versus c) generated from a range of concentrations obtained from these chromatograms for a single protein injection. It is shown that this method provides reliable determination of the B22 values for such proteins as lysozyme, chymotrypsinogen, and chymotrypsin in various solution conditions that agree well with those reported in literature.

Measurement of fluid viscosity at microliter volumes using quartz impedance analysis

The purpose of this work was to measure viscosity of fluids at low microliter volumes by means of quartz crystal impedance analysis. To achieve this, a novel setup was designed that allowed for measurement of viscosity at volumes of 8 to 10 microL. The technique was based on the principle of electromechanical coupling of piezoelectric quartz crystals. The arrangement was simple with measurement times ranging from 2 to 3 minutes. The crystal setup assembly did not impose any unwanted initial stress on the unloaded quartz crystal. Quartz crystals of 5- and 10-MHz fundamental frequency were calibrated with glycerol-water mixtures of known density and viscosity prior to viscosity measurements. True frequency shifts, for the purpose of this work, were determined followed by viscosity measurement of aqueous solutions of sucrose, urea, PEG-400, glucose, and ethylene glycol at 25 degrees C +/- 0.5 degrees C. The measured viscosities were found to be reproducible and consistent with the values reported in the literature. Minor inconsistencies in the measured resistance and frequency shifts did not affect the results significantly, and were found to be experimental in origin rather than due to electrode surface roughness. Besides, as expected for a viscoelastic fluid, PEG 8000 solutions, the calculated viscosities were found to be less than the reported values due to frequency dependence of storage and loss modulus components of complex viscosity. From the results, it can be concluded that the present setup can provide accurate assessment of viscosity of Newtonian fluids and also shows potential for analyzing non-Newtonian fluids at low microliter volumes.

Effect of vacuum drying on protein-mannitol interactions: the physical state of mannitol and protein structure in the dried state

The purpose of the present studies was to systematically investigate protein-mannitol interactions using vacuum drying, to obtain a better understanding of the effect of protein/mannitol wt/wt ratios on the physical state of mannitol and protein secondary structure in the dried state. Solutions containing beta-lactoglobulin (betaLg):mannitol (1:1-1:15 wt/wt) were vacuum dried at 5 degrees C under 3000 mTorr of pressure. The physical state of mannitol was studied using x-ray powder diffractometry (XRPD), polarized light microscopy (PLM), Fourier-transform infrared (FTIR) spectroscopy, and modulated differential scanning calorimetry (MDSC). XRPD studies indicated that mannitol remained amorphous up to 1:5 wt/wt betaLg:mannitol ratio, whereas PLM showed the presence of crystals of mannitol in all dried samples except for the 1:1 wt/wt betaLg:mannitol dried sample. FTIR studies indicated that a small proportion of crystalline mannitol was present along with the amorphous mannitol in dried samples at lower (less than 1:5 wt/wt) betaLg:mannitol ratios. The T(g) of the dried 1:1 wt/wt betaLg:mannitol sample was observed at 33.4 degrees C in MDSC studies, which indicated that at least a part of mannitol co-existed with protein in a single amorphous phase. Evaluation of the crystallization exotherms indicated that irrespective of the betaLg:protein wt/wt ratio in the initial sample, the protein to amorphous mannitol ratio was below 1:1 wt/wt in all dried samples. Second-derivative FTIR studies on dried betaLg and recombinant human interferon alpha-2a samples showed that mannitol affected protein secondary structure to a varying degree depending on the overall mannitol content in the dried sample and the type of protein.

Polyethylene glycol-induced precipitation of interferon alpha-2a followed by vacuum drying: development of a novel process for obtaining a dry, stable powder

Feasibility studies were performed on the development of a novel process based on polyethylene glycol (PEG)-induced precipitation of proteins followed by vacuum drying in the presence of sugars to obtain dry protein powders. Apparent solubility of interferon alpha-2a (IFNalpha2a) was determined in the presence of various PEGs and the effect of solution pH, ionic strength, and temperature was investigated. IFNalpha2a precipitate was dried at a shelf temperature of 25 degrees C at 100 mTorr either as it is or in the presence of mannitol and/or trehalose. The dried IFNalpha2a formulations were subjected to accelerated stability studies at 40 degrees C (3 months), and the stability was compared with that of a similar lyophilized formulation. The results indicated that more than 90% of the protein could be precipitated using 10% wt/vol PEG 1450 at pH 6.5 at a solution ionic strength of 71 mM. Vacuum drying of the precipitate only resulted in the formation of insoluble aggregates of IFNalpha2a; however, this was prevented by the addition of either mannitol or trehalose. The addition of excess mannitol resulted in low residual moisture content and better handling of the final dried product. Accelerated storage stability did not show any aggregation and showed less than 5% formation of oxidized IFNalpha2a in the dried formulation containing IFNalpha2a:trehalose:mannitol in a 1:10:100 wt/wt ratio upon storage at 40 degrees C for 3 months. The stability of this vacuum dried formulation was comparable with that of a similar lyophilized formulation.

Temperature- and pH-Induced Multiple Partially Unfolded States of Recombinant Human Interferon- 2a: Possible Implications in Protein Stability

PURPOSE

To study the effect of solution conditions on the structural conformation of recombinant human interferon-alpha2a (IFNalpha2a) to investigate its tendency to form partially unfolded intermediates.

METHODS

The structural properties of IFNalpha2a were studied at various pH values (2.0-7.4) and temperatures (5 degrees C-80 degrees C) using Trp fluorescence emission, fluorescence quenching, near- and far-UV circular dichroism (CD) spectroscopy, and DSC.

RESULTS

Fluorescence intensity measurements as a function of temperature indicated the onset of the thermal unfolding of IFNalpha2a, denoted by Td, around 60 degrees C above pH 4.0. Td was not observed at pH 3.5 and below. Acrylamide and iodide quenching studies indicated partial unfolding of protein with decrease in pH and with increase in temperature up to 50 degrees C. Near-UV CD studies indicated a significant loss in the tertiary structure of protein on increase in temperature from 15 degrees C to 50 degrees C at all solution pHs. DSC scans supported results obtained from fluorescence and CD studies at pH 4.0 and below. DSC, however, was insensitive to changes that occurred at moderate temperatures at pH 5.0 and 7.4.

CONCLUSIONS

IFNalpha2a has a tendency to acquire multiple partially unfolded states with structural conformations sensitive to solution pH and temperature. These states were formed at moderate temperatures, and it is speculated that these partially unfolded states could play an important role in the aggregation of proteins during the long-term storage of aqueous protein formulations.

Steady-state tryptophan fluorescence spectroscopy study to probe tertiary structure of proteins in solid powders

The purpose of this work was to obtain information about protein tertiary structure in solid state by using steady state tryptophan (Trp) fluorescence emission spectroscopy on protein powders. Beta-lactoglobulin (betaLg) and interferon alpha-2a (IFN) powder samples were studied by fluorescence spectroscopy using a front surface sample holder. Two different sets of dried betaLg samples were prepared by vacuum drying of solutions: one containing betaLg, and the other containing a mixture of betaLg and guanidine hydrochloride. Dried IFN samples were prepared by vacuum drying of IFN solutions and by vacuum drying of polyethylene glycol precipitated IFN. The results obtained from solid samples were compared with the emission scans of these proteins in solutions. The emission scans obtained from protein powders were slightly blue-shifted compared to the solution spectra due to the absence of water. The emission scans were red-shifted for betaLg samples dried from solutions containing GuHCl. The magnitude of the shifts in lambda(max) depended on the extent of drying of the samples, which was attributed to the crystallization of GuHCl during the drying process. The shifts in the lambda(max) of the Trp emission spectrum are associated with the changes in the tertiary structure of betaLg. In the case of IFN, the emission scans obtained from PEG-precipitated and dried sample were different compared to the emission scans obtained from IFN in solution and from vacuum dried IFN. The double peaks observed in this sample were attributed to the unfolding of the protein. In the presence of trehalose, the two peaks converged to form a single peak, which was similar to solution emission spectra, whereas no change was observed in the presence of mannitol. We conclude that Trp fluorescence spectroscopy provides a simple and reliable means to characterize Trp microenvironment in protein powders that is related to the tertiary conformation of proteins in the solid state. This study shows that the use of fluorescence spectroscopy of proteins can be extended from simple protein aqueous solutions to protein powders, precipitates, and semidried protein samples to gain understanding of protein tertiary structure in these physical states.