Interleukin-1 receptor (IL-1R) liquid formulation development using differential scanning calorimetry

Poly(vinylpyrrolidone)-Enhanced CRISPR-Cas System for Robust Nucleic Acid Diagnostics

Clustered regularly interspaced short palindromic repeats (CRISPR) technology has opened a new path for molecular diagnostics based on RNA programmed trans-cleavage activity. However, their accessibility for highly sensitive clinical diagnostics remains insufficient. In this study, we systematically investigated the impact of various surfactants on the CRISPR-Cas12a system and found that poly(vinylpyrrolidone) (PVP), a nonionic surfactant, showed the highest enhancement effect among these tested surfactants. Additionally, the enhancement effects of PVP are compatible and versatile to CRISPR-Cas12b and Cas13a systems, improving the sensitivity of these CRISPR-Cas systems toward synthetic targets by 1-2 orders of magnitude. By integrating the PVP-enhanced CRISPR system with isothermal nucleic acid amplification, both the two- and one-step assays exhibited comparable sensitivity and specificity to gold-standard quantitative polymerase chain reaction (qPCR) in the assay of clinical human papillomavirus (HPV) samples, thereby holding significant promise for advancing clinical diagnostics and biomedical research.

Extrapolating differential scanning calorimetry data for monoclonal antibodies to low temperatures

For irreversible denaturation transitions such as those exhibited by monoclonal antibodies, differential scanning calorimetry provides the denaturation temperature, Tm, the rate of denaturation at Tm, and the activation energy at Tm. These three quantities are essential but not sufficient for an accurate extrapolation of the rate of denaturation to temperatures of 25 °C and below. We have observed that the activation energy is not constant but temperature dependent due to the existence of an activation heat capacity, Cp,a. It is shown in this paper that a model that incorporates Cp,a is able to account for previous observations like, for example, that increasing the Tm does not always improve the stability at low temperatures; that some antibodies exhibit lower stabilities at 5 °C than at 25 °C; or that low temperature stabilities do not follow the rank order derived from Tm values. Most importantly, the activation heat capacity model is able to reproduce time dependent stabilities measured by size exclusion chromatography at low temperatures.

Molecular Mechanism of Antimicrobial Excipient-Induced Aggregation in Parenteral Formulations of Peptide Therapeutics

Antimicrobial preservatives are used as functional excipients in multidose formulations of biological therapeutics to destroy or inhibit the growth of microbial contaminants, which may be introduced by repeatedly administering doses. Antimicrobial agents can also induce the biophysical instability of proteins and peptides, which presents a challenge in optimizing the drug product formulation. Elucidating the structural basis for aggregation aids in understanding the underlying mechanism and can offer valuable knowledge and rationale for designing drug substances and drug products; however, this remains largely unexplored due to the lack of high-resolution characterization. In this work, we utilize solution nuclear magnetic resonance (NMR) as an advanced biophysical tool to study an acylated 31-residue peptide, acyl-peptide A, and its interaction with commonly used antimicrobial agents, benzyl alcohol and m-cresol. Our results suggest that acyl-peptide A forms soluble octamers in the aqueous solution, which tumble slowly due to an increased molecular weight as measured by diffusion ordered spectroscopy and ¹H relaxation measurement. The addition of benzyl alcohol does not induce aggregation of acyl-peptide A and has no chemical shift perturbation in ¹H-¹H NOESY spectra, suggesting no detectable interaction with the peptide. In contrast, the addition of 1% (w/v) m-cresol results in insoluble aggregates composed of 25% (w/w) peptides after a 24-hour incubation at room temperature as quantified by ¹H NMR. Interestingly, ¹H-¹³C heteronuclear single-quantum coherence and ¹H-¹H total correlation experiment spectroscopy have identified m-cresol and peptide interactions at specific residues, including Met, Lys, Glu, and Gln, suggesting that there may be a combination of hydrophobic, hydrogen bonding, and electrostatic interactions with m-cresol driving this phenomenon. These site-specific interactions have promoted the formation of higher-order oligomerization such as dimers and trimers of octamers, eventually resulting in insoluble aggregates. Our study has elucidated a structural basis of m-cresol-induced self-association that can inform the optimized design of drug substances and products. Moreover, it has demonstrated solution NMR as a high-resolution tool to investigate the structure and dynamics of biological drug products and provide an understanding of excipient-induced peptide and protein aggregation.

Antibody Structure and Function: The Basis for Engineering Therapeutics

Antibodies and antibody-derived macromolecules have established themselves as the mainstay in protein-based therapeutic molecules (biologics). Our knowledge of the structure–function relationships of antibodies provides a platform for protein engineering that has been exploited to generate a wide range of biologics for a host of therapeutic indications. In this review, our basic understanding of the antibody structure is described along with how that knowledge has leveraged the engineering of antibody and antibody-related therapeutics having the appropriate antigen affinity, effector function, and biophysical properties. The platforms examined include the development of antibodies, antibody fragments, bispecific antibody, and antibody fusion products, whose efficacy and manufacturability can be improved via humanization, affinity modulation, and stability enhancement. We also review the design and selection of binding arms, and avidity modulation. Different strategies of preparing bispecific and multispecific molecules for an array of therapeutic applications are included.

Multi-step conformational transitions in heat-treated protein therapeutics can be monitored in real time with temperature-controlled electrospray ionization mass spectrometry

Heat-induced conformational transitions are frequently used to probe the free energy landscapes of proteins. However, the extraction of information from thermal denaturation profiles pertaining to non-native protein conformations remains challenging due to their transient nature and significant conformational heterogeneity. Previously we developed a temperature-controlled electrospray ionization (ESI) source that allowed unfolding and association of biopolymers to be monitored by mass spectrometry (MS) in real time as a function of temperature. The scope of this technique is now extended to systems that undergo multi-step denaturation upon heat stress, as well as relatively small-scale conformational changes that are precursors to protein aggregation. The behavior of two therapeutic proteins (human antithrombin and an IgG1 monoclonal antibody) under heat-stress conditions is monitored in real time, providing evidence that relatively small-scale conformational changes in each system lead to protein oligomerization, followed by aggregation. Temperature-controlled ESI MS is particularly useful for the studies of heat-stressed multi-domain proteins such as IgG, where it allows distinct transitions to be observed. The ability of native temperature-controlled ESI MS to monitor both the conformational changes and oligomerization/degradation with high selectivity complements the classic calorimetric methods, lending itself as a powerful experimental tool for the thermostability studies of protein therapeutics.

Impact of additives on the formation of protein aggregates and viscosity in concentrated protein solutions

In concentrated protein solutions attractive protein interactions may not only cause the formation of undesired aggregates but also of gel-like networks with elevated viscosity. To guarantee stable biopharmaceutical processes and safe formulations, both phenomenons have to be avoided as these may hinder regular processing steps. This work screens the impact of additives on both phase behavior and viscosity of concentrated protein solutions. For this purpose, additives known for stabilizing proteins in solution or modulating the dynamic viscosity were selected. These additives were PEG 300, PEG 1000, glycerol, glycine, NaCl and ArgHCl. Concentrated lysozyme and glucose oxidase solutions at pH 3 and 9 served as model systems. Fourier-transformed-infrared spectroscopy was chosen to determine the conformational stability of selected protein samples. Influencing protein interactions, the impact of additives was strongly dependent on pH. Of all additives investigated, glycine was the only one that maintained protein conformational and colloidal stability while decreasing the dynamic viscosity. Low concentrations of NaCl showed the same effect, but increasing concentrations resulted in visible protein aggregation.

Antimicrobial preservatives induce aggregation of interferon alpha-2a: the order in which preservatives induce protein aggregation is independent of the protein

Antimicrobial preservatives (APs) are included in liquid multi-dose protein formulations to combat the growth of microbes and bacteria. These compounds have been shown to cause protein aggregation, which leads to serious immunogenic and toxic side-effects in patients. Our earlier work on a model protein cytochrome c (Cyt c) demonstrated that APs cause protein aggregation in a specific manner. The aim of this study is to validate the conclusions obtained from our model protein studies on a pharmaceutical protein. Interferon α-2a (IFNA2) is available as a therapeutic treatment for numerous immune-compromised disorders including leukemia and hepatitis C, and APs have been used in its multi-dose formulation. Similar to Cyt c, APs induced IFNA2 aggregation, demonstrated by the loss of soluble monomer and increase in solution turbidity. The extent of IFNA2 aggregation increased with the increase in AP concentration. IFNA2 aggregation also depended on the nature of AP, and followed the order m-cresol>phenol>benzyl alcohol>phenoxyethanol. This specific order exactly matched with that observed for the model protein Cyt c. These and previously published results on antibodies and other recombinant proteins suggest that the general mechanism by which APs induce protein aggregation may be independent of the protein.

Role of benzyl alcohol in the unfolding and aggregation of interferon α-2a

Benzyl alcohol (BA) is the most widely used antimicrobial preservative in multidose protein formulations, and has been shown to cause protein aggregation. Our previous work on a model protein cytochrome c demonstrated that this phenomenon occurs via partial unfolding. Here, we examine the validity of these results by investigating the effect of BA on a pharmaceutically relevant protein, interferon α-2a (IFNA2). IFNA2 therapeutic formulations available on the pharmaceutical market contain BA as a preservative. Isothermal aggregation kinetics and temperature scanning demonstrated that BA induced IFNA2 aggregation in a concentration-dependent manner. With increasing concentration of BA, the apparent aggregation temperature of IFNA2 linearly decreased. Denaturant melts measured using protein intrinsic fluorescence and that of the 1-anilinonaphthalene-8-sulfonic acid dye indicated that IFNA2 stability decreased with increasing BA concentration, populating a partially unfolded intermediate. Changes in nuclear magnetic resonance chemical shifts and hydrogen exchange rates identified the structural nature of this intermediate, which correlated with an aggregation "hot-spot" predicted by computational methods. These results indicate that BA induces IFNA2 aggregation by partial unfolding rather than global unfolding of the entire protein, and is consistent with our earlier conclusions from model protein studies.

Intermolecular interactions and conformation of antibody dimers present in IgG1 biopharmaceuticals

Intermolecular interactions and conformation in dimer species of Palivizumab, a monoclonal antibody (IgG1), were investigated to elucidate the physical and chemical properties of the dimerized antibody. Palivizumab solution contains ∼1% dimer and 99% monomer. The dimer species was isolated by size-exclusion chromatography and analysed by a number of methods including analytical ultracentrifugation-sedimantetion velocity (AUC-SV). AUC-SV in the presence of sodium dodecyl sulphate indicated that approximately half of the dimer fraction was non-covalently associated, whereas the other half was dimerized by covalent bond. Disulphide bond and dityrosine formation were likely to be involved in the covalent dimerization. Limited proteolysis of the isolated dimer by Lys-C and mass spectrometry for the resultant products indicated that the dimer species were formed by Fab-Fc or Fab-Fab interactions, whereas Fc-Fc interactions were not found. It is thus likely that the dimerization occurs mainly via the Fab region. With regard to the conformation of the dimer species, the secondary and tertiary structures were shown to be almost identical to those of the monomer. Furthermore, the thermal stability turned out also to be very similar between the dimer and monomer.

Thermal stability and molecular microstructure of heat-induced cereal grains, revealed with Raman molecular microspectroscopy and differential scanning calorimetry

The objectives of the present study were to use Raman molecular microspectroscopy and differential scanning calorimetry (DSC) to reveal molecular thermal stability and thermal degradation behavior of heat-induced cereal grains and reveal the molecular chemistry of the protein structures of cereal grain tissues affected by heat processing and to quantify the protein secondary structures using multicomponent peak modeling Gaussian and Lorentzian methods. Hierarchical cluster analysis (CLA) and principal components analysis (PCA) were also conducted to identify molecular differences in the Raman spectra. Three cereal grain seeds, wheat, triticale, and corn, were used as the model for feed protein in the experiment. The specimens were autoclaved (moist heating) and dry-heated (roasted) at 121 °C for 80 min, respectively. Raman spectroscopy results revealed that there are marked differences in the secondary structures of the proteins subjected to various heating treatments of different cereals. The sensitivity of cereals to moist heating was much higher than the sensitivity to dry heating. The multivariate analyses (CLA and PCA) showed that heat treatment was significantly isolated between the different Raman raw spectra. The DSC study revealed that the thermal degradation behavior of cereals was significantly changed after moist- and dry-heat treatments. The position of the major endothermic peak of dry-heated cereals shifted toward a higher temperature, from 131.7 to 134.0 °C, suggesting the high thermal stability of dry-heated cereals. In contrast, the endothermic peak position was slightly decreased to 132.1 °C in the case of moist autoclaved heating. The digestive behavior and nutritive value of rumen-undegradable protein in animals may be related to the changes of the protein secondary molecular structure and thermal stability of the cereal grain materials, which is attributed by Raman microspectroscopy and DSC endotherm profiles.

Rational design of a fibroblast growth factor 21-based clinical candidate, LY2405319

Fibroblast growth factor 21 is a novel hormonal regulator with the potential to treat a broad variety of metabolic abnormalities, such as type 2 diabetes, obesity, hepatic steatosis, and cardiovascular disease. Human recombinant wild type FGF21 (FGF21) has been shown to ameliorate metabolic disorders in rodents and non-human primates. However, development of FGF21 as a drug is challenging and requires re-engineering of its amino acid sequence to improve protein expression and formulation stability. Here we report the design and characterization of a novel FGF21 variant, LY2405319. To enable the development of a potential drug product with a once-daily dosing profile, in a preserved, multi-use formulation, an additional disulfide bond was introduced in FGF21 through Leu118Cys and Ala134Cys mutations. FGF21 was further optimized by deleting the four N-terminal amino acids, His-Pro-Ile-Pro (HPIP), which was subject to proteolytic cleavage. In addition, to eliminate an O-linked glycosylation site in yeast a Ser167Ala mutation was introduced, thus allowing large-scale, homogenous protein production in Pichia pastoris. Altogether re-engineering of FGF21 led to significant improvements in its biopharmaceutical properties. The impact of these changes was assessed in a panel of in vitro and in vivo assays, which confirmed that biological properties of LY2405319 were essentially identical to FGF21. Specifically, subcutaneous administration of LY2405319 in ob/ob and diet-induced obese (DIO) mice over 7–14 days resulted in a 25–50% lowering of plasma glucose coupled with a 10–30% reduction in body weight. Thus, LY2405319 exhibited all the biopharmaceutical and biological properties required for initiation of a clinical program designed to test the hypothesis that administration of exogenous FGF21 would result in effects on disease-related metabolic parameters in humans.

Stabilizing additives added during cell lysis aid in the solubilization of recombinant proteins

Insoluble recombinant proteins are a major issue for both structural genomics and enzymology research. Greater than 30% of recombinant proteins expressed in Escherichia coli (E. coli) appear to be insoluble. The prevailing view is that insolubly expressed proteins cannot be easily solubilized, and are usually sequestered into inclusion bodies. However, we hypothesize that small molecules added during the cell lysis stage can yield soluble protein from insoluble protein previously screened without additives or ligands. We present a novel screening method that utilized 144 additive conditions to increase the solubility of recombinant proteins expressed in E. coli. These selected additives are natural ligands, detergents, salts, buffers, and chemicals that have been shown to increase the stability of proteins in vivo. We present the methods used for this additive solubility screen and detailed results for 41 potential drug target recombinant proteins from infectious organisms. Increased solubility was observed for 80% of the recombinant proteins during the primary and secondary screening of lysis with the additives; that is 33 of 41 target proteins had increased solubility compared with no additive controls. Eleven additives (trehalose, glycine betaine, mannitol, L-Arginine, potassium citrate, CuCl₂, proline, xylitol, NDSB 201, CTAB and K₂PO₄) solubilized more than one of the 41 proteins; these additives can be easily screened to increase protein solubility. Large-scale purifications were attempted for 15 of the proteins using the additives identified and eight (40%) were prepared for crystallization trials during the first purification attempt. Thus, this protocol allowed us to recover about a third of seemingly insoluble proteins for crystallography and structure determination. If recombinant proteins are required in smaller quantities or less purity, the final success rate may be even higher.

Effect of antimicrobial preservatives on partial protein unfolding and aggregation

One-third of protein formulations are multi-dose. These require antimicrobial preservatives (APs); however, some APs have been shown to cause protein aggregation. Our previous work on a model protein cytochrome c indicated that partial protein unfolding, rather than complete unfolding, triggers aggregation. Here, we examined the relative strength of five commonly used APs on such unfolding and aggregation, and explored whether stabilizing the aggregation 'hot-spot' reduces such aggregation. All APs induced protein aggregation in the order m-cresol > phenol > benzyl alcohol > phenoxyethanol > chlorobutanol. All these enhanced the partial protein unfolding that includes a local region which was predicted to be the aggregation 'hot-spot'. The extent of destabilization correlated with the extent of aggregation. Further, we show that stabilizing the 'hot-spot' reduces aggregation induced by all five APs. These results indicate that m-cresol causes the most protein aggregation, whereas chlorobutanol causes the least protein aggregation. The same protein region acts as the 'hot-spot' for aggregation induced by different APs, implying that developing strategies to prevent protein aggregation induced by one AP will also work for others.

Applications of differential scanning calorimetry for thermal stability analysis of proteins: qualification of DSC

Differential scanning calorimetry (DSC) has been used to characterize protein thermal stability, overall conformation, and domain folding integrity by the biopharmaceutical industry. Recently, there have been increased requests from regulatory agencies for the qualification of characterization methods including DSC. Understanding the method precision can help determine what differences between samples are significant and also establish the acceptance criteria for comparability and other characterization studies. In this study, we identify the parameters for the qualification of DSC for thermal stability analysis of proteins. We use these parameters to assess the precision and sensitivity of DSC and demonstrate that DSC is suitable for protein thermal stability analysis for these purposes. Several molecules from different structural families were studied. The experiments and data analyses were performed by different analysts using different instruments at different sites. The results show that the (apparent) thermal transition midpoint (T(m)) values obtained for the same protein by same and different instruments and/or analysts are quite reproducible, and the profile similarity values obtained for the same protein from the same instrument are also high. DSC is an appropriate method for assessing protein thermal stability and conformational changes.

Forced degradation of therapeutic proteins

The scope of this paper is to review approaches used for forced degradation (synonym, stress testing) of therapeutic proteins. Forced degradation studies play a central role in the development of therapeutic proteins, for example, for candidate selection, molecule characterization, formulation development, assay development, and comparability studies. Typical stress methods are addressed within this review, such as exposure to elevated temperatures, freeze-thawing, mechanical stress, oxidation, light, as well as various materials and devices used in the clinics during final administration. Stability testing is briefly described as far as relevant to the discussion of forced degradation studies. Whereas stability-testing requirements are defined in regulatory guidelines, standard procedures for forced degradation of therapeutic proteins are largely unavailable, except for photostability. Possible selection criteria to identify appropriate stress conditions and recommendations for setting up forced degradation studies for the different phases of development of therapeutic proteins are presented.

Interactions of formulation excipients with proteins in solution and in the dried state

A variety of excipients are used to stabilize proteins, suppress protein aggregation, reduce surface adsorption, or to simply provide physiological osmolality. The stabilizers encompass a wide variety of molecules including sugars, salts, polymers, surfactants, and amino acids, in particular arginine. The effects of these excipients on protein stability in solution are mainly caused by their interaction with the protein and the container surface, and most importantly with water. Some excipients stabilize proteins in solution by direct binding, while others use a number of fundamentally different mechanisms that involve indirect interactions. In the dry state, any effects that the excipients confer to proteins through their interactions with water are irrelevant, as water is no longer present. Rather, the excipients stabilize proteins through direct binding and their effects on the physical properties of the dried powder. This review will describe a number of mechanisms by which the excipients interact with proteins in solution and with various interfaces, and their effects on the physical properties of the dried protein structure, and explain how the various interaction forces are related to their observed effects on protein stability.

High-throughput measurement, correlation analysis, and machine-learning predictions for pH and thermal stabilities of Pfizer-generated antibodies

Generating stable antibodies is an important goal in the development of antibody-based drugs. Often, thermal stability is assumed predictive of overall stability. To test this, we used different internally created antibodies and first studied changes in antibody structure as a function of pH, using the dye ANS. Comparison of the pH(50) values, the midpoint of the transition from the high-pH to the low-pH conformation, allowed us for the first time to rank antibodies based on their pH stability. Next, thermal stability was probed by heating the protein in the presence of the dye Sypro Orange. A new data analysis method allowed extraction of all three antibody unfolding transitions and showed close correspondence to values obtained by differential scanning calorimetry. T(1%) , the temperature at which 1% of the protein is unfolded, was also determined. Importantly, no correlations could be found between thermal stability and pH(50) , suggesting that to accurately quantify antibody stability, different measures of protein stability are necessary. The experimental data were further analyzed using a machine-learning approach with a trained model that allowed the prediction of biophysical stability using primary sequence alone. The pH stability predictions proved most successful and were accurate to within pH ±0.2.

Effect of benzyl alcohol on recombinant human interleukin-1 receptor antagonist structure and hydrogen-deuterium exchange

Benzyl alcohol, a preservative commonly added to multidose therapeutic protein formulations, can accelerate aggregation of recombinant human interleukin-1 receptor antagonist (rhIL-1ra). To investigate the interactions between benzyl alcohol and rhIL-1ra, we used nuclear magnetic resonance to observe the effect of benzyl alcohol on the chemical shifts of amide resonances of rhIL-1ra and to measure hydrogen-deuterium exchange rates of individual rhIL-1ra residues. Addition of 0.9% benzyl alcohol caused significant chemical shifts of amide resonances for residues 90-97, suggesting that these solvent-exposed residues participate in the binding of benzyl alcohol. In contrast, little perturbation of exchange rates was observed in the presence of either sucrose or benzyl alcohol.

Microcalorimetry: a response to challenges in modern biotechnology

Almost any process in life is accompanied by heat changes which can be monitored by isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC). Both techniques are now established tools in fundamental research but over the last decade a clear tendency towards more problem-driven applications is noted. This review aims at summarizing these problem-oriented applications of microcalorimetry and the solutions both techniques can provide to problems in biotechnology. The biotechnological issues to which microcalorimetry has been successfully applied are as diverse as rational drug design, overcoming drug resistance, optimization of long-term stability of proteins, estimation of the bioavailability of drugs, control of complex pharmaceutical products or the optimization of gene delivery efficiency. The main limitation of microcalorimetry, which is the relatively large amounts of sample necessary for analysis, is less important in the biotechnology sector which frequently uses large-scale produced bulk products for analysis. The recently developed high-throughput DSC and ITC microcalorimeters will additionally reduce the labour intensity of these techniques. Due to the precision of microcalorimetric analyses and the versatility of processes which can be studied, it is expected that ITC and DSC will soon be key technologies in biotechnological research.

Prevention of benzyl alcohol-induced aggregation of chymotrypsinogen by PEGylation

OBJECTIVES

Addition of the antimicrobial preservative benzyl alcohol to reconstitution buffer promotes the formation of undesirable aggregates in multidose protein formulations. Herein we investigated the efficiency of PEGylation (attachment of poly(ethylene glycol)) to prevent benzyl alcohol-induced aggregation of the model protein α-chymotrypsinogen A (aCTgn).

METHODS

Various PEG-aCTgn conjugates were prepared using PEG with a molecular weight of either 700 or 5000 Da by varying the PEG-to-protein ratio during synthesis and the formation of insoluble aggregates was studied. The effect of benzyl alcohol on the thermodynamic stability and tertiary structure of aCTgn was also examined.

KEY FINDINGS

When the model protein was reconstituted in buffer containing 0.9% benzyl alcohol, copious amounts of buffer-insoluble aggregates formed within 24 h (>10%). Benzyl alcohol-induced aggregation was completely prevented when two or five molecules of PEG with a molecular weight of 5000 Da were attached to the protein, whereas two or four molecules of bound 700 Da PEG were completely inefficient in preventing aggregation. Mechanistic investigations excluded prevention of structural perturbations or increased thermodynamic stability by PEGylation from being responsible for the prevention of aggregation. Simple addition of PEG to the buffer was also inefficient and PEG had to be covalently linked to the protein to be efficient.

CONCLUSIONS

The most likely explanation for the protective effect of the 5000 Da PEG is shielding of exposed hydrophobic protein surface area and prevention of protein-protein contacts (molecular spacer effect).

Predicting solution aggregation rates for therapeutic proteins: approaches and challenges

Non-native aggregation is a common concern during therapeutic protein product development and manufacturing, particularly for liquid dosage forms. Because aggregates are often net irreversible under the conditions that they form, controlling aggregate levels requires control of aggregation rates across a range of solution conditions. Rational design of product formulation(s) would therefore benefit greatly from methods to accurately predict aggregation rates. This article focuses on the principles underlying current rate-prediction approaches for non-native aggregation, the limitations and strengths of different approaches, and illustrative examples from the authors' laboratories. The analysis highlights a number of reasons why accurate prediction of aggregation rates remains an outstanding challenge, and suggests some of the important areas for research to ultimately enable improved predictive capabilities in the future.

Mechanisms of m-cresol-induced protein aggregation studied using a model protein cytochrome c

Multidose protein formulations require an effective antimicrobial preservative (AP) to inhibit microbial growth during long-term storage of unused formulations. m-cresol (CR) is one such AP, but it has been shown to cause protein aggregation. However, the fundamental physical mechanisms underlying such AP-induced protein aggregation are not understood. In this study, we used a model protein cytochrome c to identify the protein unfolding that triggers protein aggregation. CR induced cytochrome c aggregation at preservative concentrations that are commonly used to inhibit microbial growth. Addition of CR decreased the temperature at which the protein aggregated and increased the aggregation rate. However, CR did not perturb the tertiary or secondary structure of cytochrome c. Instead, it populated an "invisible" partially unfolded intermediate where a local protein region around the methionine residue at position 80 was unfolded. Stabilizing the Met80 region drastically decreased the protein aggregation, which conclusively shows that this local protein region acts as an aggregation "hotspot." On the basis of these results, we propose that APs induce protein aggregation by partial rather than global unfolding. Because of the availability of site-specific probes to monitor different levels of protein unfolding, cytochrome c provided a unique advantage in characterizing the partial protein unfolding that triggers protein aggregation.

Stability of protein pharmaceuticals: an update

In 1989, Manning, Patel, and Borchardt wrote a review of protein stability (Manning et al., Pharm. Res. 6:903-918, 1989), which has been widely referenced ever since. At the time, recombinant protein therapy was still in its infancy. This review summarizes the advances that have been made since then regarding protein stabilization and formulation. In addition to a discussion of the current understanding of chemical and physical instability, sections are included on stabilization in aqueous solution and the dried state, the use of chemical modification and mutagenesis to improve stability, and the interrelationship between chemical and physical instability.

Stability of a trivalent recombinant protein vaccine formulation against botulinum neurotoxin during storage in aqueous solution

The adsorption of recombinant botulinum neurotoxin (BoNT) protein-derived vaccine antigens to aluminum salt adjuvants has been previously studied for the development of a trivalent vaccine against the neurotoxins (Vessely et al., in press, J Pharm Sci). The current paper describes an investigation of the stability of recombinant BoNT antigens adsorbed to aluminum salt adjuvants during storage in aqueous solution. Both chemical and physical changes occurred during storage. Phosphate groups in the buffer exchanged with hydroxyl groups on the adjuvant surface. The resulting changes in solution pH and adjuvant surface chemistry promoted more favorable electrostatic interaction between the BoNT proteins and the surface, possibly increasing the affinity of the proteins for the surface during storage. Fluorescence and UV spectroscopy suggested changes to protein structure during storage, whereas differential scanning calorimetry showed changes to thermal processes related to protein conformation and/or surface adsorption. The consequence of the chemical and physical changes to the proteins was a decrease in the ability to desorb protein from the adjuvant surface during storage. Overall, the results of this study emphasize the utility of a thorough characterization of the interactions between protein antigens and aluminum salt adjuvants.

Mapping of solution components, pH changes, protein stability and the elimination of protein precipitation during freeze-thawing of fibroblast growth factor 20

This study discusses the effect of key factors like containers, buffers and the freeze (controlled vs. flash freezing) and thawing processes on the stability of a therapeutic protein fibroblast growth factor 20 (FGF-20). The freezing profiles monitored by 15 temperature probes located at different regions in a 2-L bottle during freezing can be grouped into three categories. A rapid drop in temperature was observed at the bottom followed by the top and middle center of the bottle. The freeze-thawing behavior in a 50 ml tube is considerably uniform, as expected. Among phosphate, HEPES (4-(2-hydroxyethyl)-1-piperazine ethanesulfonic acid), citrate and histidine (each containing 0.5 M arginine-sulfate) buffer systems, a minimum pH change (0.4 pH unit vs. approximately 1.7 pH unit) was observed for the phosphate buffer system. Thawing in a 50 ml tube at room temperature standing resulted in a significant phase separation in citrate, histidine and HEPES buffers; however, phase separation was least in the phosphate buffer system. These phase separations were found to be temperature dependent. No effect of Polysorbate 80 on freeze-thawing of FGF-20 was observed. Significant concentration gradients in major buffer components and protein concentration were observed during freeze-thawing in a 2-L bottle. The segregation patterns of the various components were similar with the top and bottom layers containing lowest and highest concentrations, respectively. In the formulation buffer no pH gradient was formed, and the precipitation of FGF-20 during thawing at the top layer was related to an insufficient amount of arginine-sulfate and the precipitation at the bottom layer was due to a salting out effect. The precipitate generated during thawing goes into solution easily upon mixing whole solution of the bottle and the various gradient formations do not cause any irreversible change in structure, stability and isoform distribution of FGF-20. Comparison of slow freezing and flash freezing data suggests that the gradients in excipient and protein concentrations are mainly formed during thawing.

Use of Microcalorimetry and Its Correlation with Size Exclusion Chromatography for Rapid Screening of the Physical Stability of Large Pharmaceutical Proteins in Solution

The utility of microcalorimetry as a rapid screening tool for assessing the solution stability of high molecular weight pharmaceutical proteins was evaluated by using model recombinant antibodies, Protein I and Protein II. Changes in the transition midpoint, T(m), were monitored as a function of pH and/or in the presence of excipients, and results were compared with traditional accelerated stability data from samples that were analyzed by size exclusion chromatography (SEC). The data from microcalorimetry were well correlated with those from SEC for predicting both optimal solution pH as well as excipient effects on solution stability. These results indicate that microcalorimetry can be an efficient screening tool useful in identifying optimal pH conditions and excipients to stabilize pharmaceutical proteins in solution formulations.