An Industry Perspective on the use of Forced Degradation Studies to Assess Comparability of Biopharmaceuticals.

Forced degradation, also known as stress testing, is used throughout pharmaceutical development for many purposes including assessing the comparability of biopharmaceutical products according to ICH Guideline Q5E. These formal comparability studies, the results of which are submitted to health authorities, investigate potential impacts of manufacturing process changes on the quality, safety, and efficacy of the drug. Despite the wide use of forced degradation in comparability assessments, detailed guidance on the design and interpretation of such studies is scarce. The BioPhorum Development Group is an industry-wide consortium enabling networking and sharing of common practices for the development of biopharmaceuticals. The BioPhorum Development Group Forced Degradation Workstream recently conducted several group discussions and a benchmarking survey to understand current industry approaches for the use of forced degradation studies to assess comparability of protein-based biopharmaceuticals. The results provide insight into the design of forced degradation studies, analytical characterization and testing strategies, data evaluation criteria, as well as some considerations and differences for non-platform modalities (e.g., non-traditional mAbs). This article presents survey responses from several global companies of various sizes and provides an industry perspective and experience regarding the practicalities of using forced degradation to assess comparability.

A Detailed Protocol for Generation of Therapeutic Antibodies with Galactosylated Glycovariants at Laboratory Scale Using In-Vitro Glycoengineering Technology.

N-linked glycosylation is an important post translational modification that occurs on Asparagine 297 residue or a homologous position on the Fc portion of monoclonal antibodies (mAbs). mAb Fc glycans play important roles in antibody structure, stability, and function including effector function and pharmacokinetics. The Fc glycans are made up of a wide variety of sugars including galactose, mannose, and sialic acid. The role of galactose in mediating antibody effector functions is not well understood. Hence, there is widespread interest in the antibody research community to understand the role of galactose in antibody effector functions as galactose is a major constituent of antibody glycans. This requires generation of highly enriched galactosylated variants that has been very challenging via cell culture process. To tackle this challenge, we developed a laboratory scale biochemical process to produce highly enriched galactosylated variants. In this article, we report optimized lab-scale workflows and detailed protocols for generation of deglycosylated, hypo-galactosylated and hyper-galactosylated variants of IgG therapeutic antibodies using the in-vitro glycoengineering technology. The optimized workflows offer short turnaround time and produce highly enriched deglycosylated/hypo-galactosylated/hyper-galactosylated IgG glycovariants, with high purity & molecular integrity as demonstrated by data from an example IgG.

2D NMR Analysis of the Effect of Asparagine Deamidation Versus Methionine Oxidation on the Structure, Stability, Aggregation, and Function of a Therapeutic Protein.

Two of the most common forms of chemical modifications that compromise the efficacy of therapeutic proteins are the deamidation of asparagine residues and oxidation of methionine residues. We probed how deamidation affects the structure, stability, aggregation, and function of interferon alpha-2a (IFNA2a), and compared with our earlier results on methionine oxidation. Upon deamidation, no significant changes were observed in the global secondary structure of IFNA2a with minor changes in its tertiary structure. However, deamidation destabilized the protein, and increased its propensity to aggregate under accelerated stress conditions. Cytopathic inhibition and antiproliferation assays showed drastic decrease in the functionality of deamidated IFNA2a compared to the wild-type. 2D NMR measurements showed structural changes in local protein regions, with no effect on the overall global structure of IFNA2a. These local protein regions corresponded well with the aggregation hot-spots predicted by computational programs, and the functional hot-spots identified by site-directed mutagenesis. When compared to the effects of methionine oxidation, deamidation caused lesser aggregation, because of lesser structural unfolding observed in aggregation hot-spots by 2D NMR. In comparison to oxidation, deamidation showed larger decrease in function, because deamidation affected key amino acid residues in functional hot-spots as observed by 2D NMR and structural modeling. Such quantitative comparison between the effects of deamidation and oxidation on a pharmaceutical protein has not been done before, and the high-resolution structural information on local protein regions obtained by 2D NMR provided a better insight compared to low-resolution methods that probe global protein structure.

Effect of Chemical Oxidation on the Higher Order Structure, Stability, Aggregation, and Biological Function of Interferon Alpha-2a: Role of Local Structural Changes Detected by 2D NMR.

PURPOSE: Oxidized interferons have been shown to aggregate and cause immunogenicity. In this study, the structural mechanisms underlying oxidation-induced interferon alpha-2a (IFNA2a) aggregation and loss of function were examined. METHODS: IFNA2a was oxidized using 0.037% vol/vol hydrogen peroxide. Oxidized protein was probed using biophysical methods that include denaturant melts, particle counting, proteolysis-coupled mass spectrometry, and 2D NMR. RESULTS: Oxidized IFNA2a did not show major changes in its secondary structure, but showed minor changes in tertiary structure when compared to the unoxidized protein. In addition, a significant loss of conformational stability was observed upon oxidation. Correspondingly, increased protein aggregation was observed resulting in the formation of sub-visible particles. Oxidized protein showed decreased biological function in terms of its anti-viral potency and cytopathic inhibition efficacy. Proteolysis-coupled mass spectrometry identified five methionine residues that were oxidized with no correlation between the extent of oxidation and their accessible surface area. 2D 15N-1H HSQC NMR identified residue-level local structural changes in the protein upon oxidation, which were not detectable by global probes such as far-UV circular dichroism and fluorescence. CONCLUSIONS: Increased protein aggregation and decreased function of IFNA2a upon oxidation correlated with the site of modification identified by proteolysis-coupled mass spectrometry and local structural changes in the protein detected by 2D NMR.

Biophysical analysis of the effect of chemical modification by 4-oxononenal on the structure, stability, and function of binding immunoglobulin protein (BiP).

Binding immunoglobulin protein (BiP) is a molecular chaperone important for the folding of numerous proteins, which include millions of immunoglobulins in human body. It also plays a key role in the unfolded protein response (UPR) in the endoplasmic reticulum. Free radical generation is a common phenomenon that occurs in cells under healthy as well as under stress conditions such as ageing, inflammation, alcohol consumption, and smoking. These free radicals attack the cell membranes and generate highly reactive lipid peroxidation products such as 4-oxononenal (4-ONE). BiP is a key protein that is modified by 4-ONE. In this study, we probed how such chemical modification affects the biophysical properties of BiP. Upon modification, BiP shows significant tertiary structural changes with no changes in its secondary structure. The protein loses its thermodynamic stability, particularly, that of the nucleotide binding domain (NBD) where ATP binds. In terms of function, the modified BiP completely loses its ATPase activity with decreased ATP binding affinity. However, modified BiP retains its immunoglobulin binding function and its chaperone activity of suppressing non-specific protein aggregation. These results indicate that 4-ONE modification can significantly affect the structure-function of key proteins such as BiP involved in cellular pathways, and provide a molecular basis for how chemical modifications can result in the failure of quality control mechanisms inside the cell.

Effect of Polysorbate 20 and Polysorbate 80 on the Higher-Order Structure of a Monoclonal Antibody and Its Fab and Fc Fragments Probed Using 2D Nuclear Magnetic Resonance Spectroscopy.

We examined how polysorbate 20 (PS20; Tween 20) and polysorbate 80 (PS80; Tween 80) affect the higher-order structure of a monoclonal antibody (mAb) and its antigen-binding (Fab) and crystallizable (Fc) fragments, using near-UV circular dichroism and 2D nuclear magnetic resonance (NMR). Both polysorbates bind to the mAb with submillimolar affinity. Binding causes significant changes in the tertiary structure of mAb with no changes in its secondary structure. 2D 13C-1H methyl NMR indicates that with increasing concentration of polysorbates, the Fab region showed a decrease in crosspeak volumes. In addition to volume changes, PS20 caused significant changes in the chemical shifts compared to no changes in the case of PS80. No such changes in crosspeak volumes or chemical shifts were observed in the case of Fc region, indicating that polysorbates predominantly affect the Fab region compared to the Fc region. This differential effect of polysorbates on the Fab and Fc regions was because of the lesser thermodynamic stability of the Fab compared to the Fc. These results further indicate that PS80 is the preferred polysorbate for this mAb formulation, because it offers higher protection against aggregation, causes lesser structural perturbation, and has weaker binding affinity with fewer binding sites compared to PS20.

The N-Terminal Flanking Region Modulates the Actin Binding Affinity of the Utrophin Tandem Calponin-Homology Domain.

Despite sharing a high degree of sequence similarity, the tandem calponin-homology (CH) domain of utrophin binds to actin 30 times stronger than that of dystrophin. We have previously shown that this difference in actin binding affinity could not be ascribed to the differences in inter-CH-domain linkers [Bandi, S., et al. (2015) Biochemistry 54, 5480-5488]. Here, we examined the role of the N-terminal flanking region. The utrophin tandem CH domain contains a 27-residue flanking region before its CH1 domain. We examined its effect by comparing the structure and function of full-length utrophin tandem CH domain Utr(1-261) and its truncated Utr(28-261) construct. Both full-length and truncated constructs are monomers in solution, with no significant differences in their secondary or tertiary structures. Truncated construct Utr(28-261) binds to actin 30 times weaker than that of the full-length Utr(1-261), similar to that of the dystrophin tandem CH domain with a much shorter flanking region. Deletion of the N-terminal flanking region stabilizes the CH1 domain. The magnitude of the change in binding free energy upon truncation is similar to that of the change in thermodynamic stability. The isolated N-terminal peptide by itself is significantly random coil and does not bind to F-actin in the affinity range of Utr(1-261) and Utr(28-261). These results indicate that the N-terminal flanking region significantly affects the actin binding affinity of tandem CH domains. This observation further stresses that protein regions other than the three actin-binding surfaces identified earlier, irrespective of whether they directly bind to actin, also contribute to the actin binding affinity of tandem CH domains.

The N- and C-Terminal Domains Differentially Contribute to the Structure and Function of Dystrophin and Utrophin Tandem Calponin-Homology Domains.

Dystrophin and utrophin are two muscle proteins involved in Duchenne/Becker muscular dystrophy. Both proteins use tandem calponin-homology (CH) domains to bind to F-actin. We probed the role of N-terminal CH1 and C-terminal CH2 domains in the structure and function of dystrophin tandem CH domain and compared with our earlier results on utrophin to understand the unifying principles of how tandem CH domains work. Actin cosedimentation assays indicate that the isolated CH2 domain of dystrophin weakly binds to F-actin compared to the full-length tandem CH domain. In contrast, the isolated CH1 domain binds to F-actin with an affinity similar to that of the full-length tandem CH domain. Thus, the obvious question is why the dystrophin tandem CH domain requires CH2, when its actin binding is determined primarily by CH1. To answer, we probed the structural stabilities of CH domains. The isolated CH1 domain is very unstable and is prone to serious aggregation. The isolated CH2 domain is very stable, similar to the full-length tandem CH domain. These results indicate that the main role of CH2 is to stabilize the tandem CH domain structure. These conclusions from dystrophin agree with our earlier results on utrophin, indicating that this phenomenon of differential contribution of CH domains to the structure and function of tandem CH domains may be quite general. The N-terminal CH1 domains primarily determine the actin binding function whereas the C-terminal CH2 domains primarily determine the structural stability of tandem CH domains, and the extent of stabilization depends on the strength of inter-CH domain interactions.

Interdomain Linker Determines Primarily the Structural Stability of Dystrophin and Utrophin Tandem Calponin-Homology Domains Rather than Their Actin-Binding Affinity.

Tandem calponin-homology (CH) domains are the most common actin-binding domains in proteins. However, structural principles underlying their function are poorly understood. These tandem domains exist in multiple conformations with varying degrees of inter-CH-domain interactions. Dystrophin and utrophin tandem CH domains share high sequence similarity (∼82%), yet differ in their structural stability and actin-binding affinity. We examined whether the conformational differences between the two tandem CH domains can explain differences in their stability and actin binding. Dystrophin tandem CH domain is more stable by ∼4 kcal/mol than that of utrophin. Individual CH domains of dystrophin and utrophin have identical structures but differ in their relative orientation around the interdomain linker. We swapped the linkers between dystrophin and utrophin tandem CH domains. Dystrophin tandem CH domain with utrophin linker (DUL) has similar stability as that of utrophin tandem CH domain. Utrophin tandem CH domain with dystrophin linker (UDL) has similar stability as that of dystrophin tandem CH domain. Dystrophin tandem CH domain binds to F-actin ∼30 times weaker than that of utrophin. After linker swapping, DUL has twice the binding affinity as that of dystrophin tandem CH domain. Similarly, UDL has half the binding affinity as that of utrophin tandem CH domain. However, changes in binding free energies due to linker swapping are much lower by an order of magnitude compared to the corresponding changes in unfolding free energies. These results indicate that the linker region determines primarily the structural stability of tandem CH domains rather than their actin-binding affinity.

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.

Missense mutation Lys18Asn in dystrophin that triggers X-linked dilated cardiomyopathy decreases protein stability, increases protein unfolding, and perturbs protein structure, but does not affect protein function.

Genetic mutations in a vital muscle protein dystrophin trigger X-linked dilated cardiomyopathy (XLDCM). However, disease mechanisms at the fundamental protein level are not understood. Such molecular knowledge is essential for developing therapies for XLDCM. Our main objective is to understand the effect of disease-causing mutations on the structure and function of dystrophin. This study is on a missense mutation K18N. The K18N mutation occurs in the N-terminal actin binding domain (N-ABD). We created and expressed the wild-type (WT) N-ABD and its K18N mutant, and purified to homogeneity. Reversible folding experiments demonstrated that both mutant and WT did not aggregate upon refolding. Mutation did not affect the protein's overall secondary structure, as indicated by no changes in circular dichroism of the protein. However, the mutant is thermodynamically less stable than the WT (denaturant melts), and unfolds faster than the WT (stopped-flow kinetics). Despite having global secondary structure similar to that of the WT, mutant showed significant local structural changes at many amino acids when compared with the WT (heteronuclear NMR experiments). These structural changes indicate that the effect of mutation is propagated over long distances in the protein structure. Contrary to these structural and stability changes, the mutant had no significant effect on the actin-binding function as evident from co-sedimentation and depolymerization assays. These results summarize that the K18N mutation decreases thermodynamic stability, accelerates unfolding, perturbs protein structure, but does not affect the function. Therefore, K18N is a stability defect rather than a functional defect. Decrease in stability and increase in unfolding decrease the net population of dystrophin molecules available for function, which might trigger XLDCM. Consistently, XLDCM patients have decreased levels of dystrophin in cardiac muscle.

High yield soluble bacterial expression and streamlined purification of recombinant human interferon α-2a.

Interferon α-2a (IFNA2) is a member of the Type I interferon cytokine family, known for its antiviral and anti-proliferative functions. The role of this family in the innate immune response makes it an attractive candidate for the treatment of many viral and chronic immune-compromised diseases. Recombinant IFNA2 is clinically used to modulate hairy cell leukemia as well as hepatitis c. Historically, IFNA2 has been purified from human leukocytes as well as bacterial expression systems. In most cases, bacterial expression of IFNA2 resulted in inclusion body formation, or required numerous purification steps that decreased the protein yield. Here, we describe an expression and purification scheme for IFNA2 using a pET-SUMO bacterial expression system and a single purification step. Using the SUMO protein as the fusion tag achieved high soluble protein expression. The SUMO tag was cleaved with the Ulp1 protease leaving no additional amino acids on the fusion terminus following cleavage. Mass spectrometry, circular dichroism, 2D heteronuclear NMR, and analytical ultracentrifugation confirmed the amino acid sequence identity, secondary and tertiary protein structures, and the solution behavior of the purified IFNA2. The purified protein also had antiviral and anti-proliferative activities comparable to the WHO International Standard, NIBSC 95/650, and the IFNA2 standard available from PBL Assay Science. Combining the expression and purification protocols developed here to produce IFNA2 on a laboratory scale with the commercial fermenter technology commonly used in pharmaceutical industry may further enhance IFNA2 yields, which will promote the development of interferon-based protein drugs to treat various disorders.

The C-terminal domain of the utrophin tandem calponin-homology domain appears to be thermodynamically and kinetically more stable than the full-length protein.

Domains are in general less stable than the corresponding full-length proteins. Human utrophin tandem calponin-homology (CH) domain seems to be an exception. Reversible, equilibrium denaturant melts indicate that the isolated C-terminal domain (CH2) is thermodynamically more stable than the tandem CH domain. Thermal melts show that CH2 unfolds at a temperature higher than that at which the full-length protein unfolds. Stopped-flow kinetics indicates that CH2 unfolds slower than the full-length protein, indicating its higher kinetic stability. Thus, the utrophin tandem CH domain may be one of the few proteins in which an isolated domain is more stable than the corresponding full-length protein.

The actin binding affinity of the utrophin tandem calponin-homology domain is primarily determined by its N-terminal domain.

The structural determinants of the actin binding function of tandem calponin-homology (CH) domains are poorly understood, particularly the role of individual domains. We determined the actin binding affinity of isolated CH domains from human utrophin and compared them with the affinity of the full-length tandem CH domain. Traditional cosedimentation assays indicate that the C-terminal CH2 domain binds to F-actin much weaker than the full-length tandem CH domain. The N-terminal CH1 domain is less stable and undergoes severe protein aggregation; therefore, traditional actin cosedimentation assays could not be used. To address this, we have developed a folding-upon-binding method. We refolded the CH1 domain from its unfolded state in the presence of F-actin. This results in a competition between actin binding and aggregation. A differential centrifugation technique was used to distinguish actin binding from aggregation. Low-speed centrifugation pelleted CH1 aggregates, but not F-actin or its bound protein. Subsequent high-speed centrifugation resulted in the cosedimentation of bound CH1 along with F-actin. The CH1 domain binds to F-actin with an affinity similar to that of the full-length tandem CH domain, unlike the CH2 domain. The actin binding cooperativity between the two domains was quantitatively calculated from the association constants of the full-length tandem CH domain and its CH domains, and found to be much smaller than the association constant of the CH1 domain alone. These results indicate that the actin binding affinity of the utrophin tandem CH domain is primarily determined by its CH1 domain, when compared to that of its CH2 domain or the cooperativity between the two CH domains.

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.

The N-terminal actin-binding tandem calponin-homology (CH) domain of dystrophin is in a closed conformation in solution and when bound to F-actin.

Deficiency of the vital muscle protein dystrophin triggers Duchenne/Becker muscular dystrophy, but the structure-function relationship of dystrophin is poorly understood. To date, molecular structures of three dystrophin domains have been determined, of which the N-terminal actin-binding domain (N-ABD or ABD1) is of particular interest. This domain is composed of two calponin-homology (CH) domains, which form an important class of ABDs in muscle proteins. A previously determined x-ray structure indicates that the dystrophin N-ABD is a domain-swapped dimer, with each monomer adopting an extended, open conformation in which the two CH domains do not interact. This structure is controversial because it contradicts functional studies and known structures of similar ABDs from other muscle proteins. Here, we investigated the solution conformation of the dystrophin N-ABD using a very simple and elegant technique of pyrene excimer fluorescence. Using the wild-type protein, which contains two cysteines, and the corresponding single-cysteine mutants, we show that the protein is a monomer in solution and is in a closed conformation in which the two CH domains seem to interact, as observed from the excimer fluorescence of pyrene-labeled wild-type protein. Excimer fluorescence was also observed in its actin-bound form, indicating that the dystrophin N-ABD binds to F-actin in a closed conformation. Comparison of the dystrophin N-ABD conformation with other ABDs indicates that the tandem CH domains in general may be monomeric in solution and predominantly occur in closed conformation, whereas their actin-bound conformations may differ.

Thermodynamic stability, unfolding kinetics, and aggregation of the N-terminal actin-binding domains of utrophin and dystrophin.

Muscular dystrophy (MD) is the most common genetic lethal disorder in children. Mutations in dystrophin trigger the most common form of MD, Duchenne, and its allelic variant Becker MD. Utrophin is the closest homologue and has been shown to compensate for the loss of dystrophin in human disease animal models. However, the structural and functional similarities and differences between utrophin and dystrophin are less understood. Both proteins interact with actin through their N-terminal actin-binding domain (N-ABD). In this study, we examined the thermodynamic stability and aggregation of utrophin N-ABD and compared with that of dystrophin. Our results show that utrophin N-ABD has spectroscopic properties similar to dystrophin N-ABD. However, utrophin N-ABD has decreased denaturant and thermal stability, unfolds faster, and is correspondingly more susceptible to proteolysis, which might account for its decreased in vivo half-life compared to dystrophin. In addition, utrophin N-ABD aggregates to a lesser extent compared with dystrophin N-ABD, contrary to the general behavior of proteins in which decreased stability enhances protein aggregation. Despite these differences in stability and aggregation, both proteins exhibit deleterious effects of mutations. When utrophin N-ABD mutations analogous in position to the dystrophin disease-causing mutations were generated, they behaved similarly to dystrophin mutants in terms of decreased stability and the formation of cross-ß aggregates, indicating a possible role for utrophin mutations in disease mechanisms.

Solubilization of inclusion body proteins using n-propanol and its refolding into bioactive form.

Inclusion bodies of recombinant human growth hormone (r-hGH) were isolated from Escherichia coli, enriched and solubilized in 100mM Tris buffer containing 6M n-propanol and 2M urea. Around 4 mg/ml of r-hGH from inclusion bodies were solubilized in 6M n-propanol-based buffer containing 2M urea. Existence of native-like secondary structure of r-hGH in 6M n-propanol solution was confirmed by CD and fluorescence spectra. Solubilized r-hGH was subsequently refolded by pulsatile dilution, purified to homogeneity and found to be functionally active. Tris buffer containing 6M n-propanol and 2M urea also effectively solubilized a number of proteins expressed as inclusion bodies in E. coli. Mild solubilization of inclusion body proteins, chaotropic effect of n-propanol at high concentration and kosmotropic effect at lower concentration helped in improved refolding of the solubilized protein. Around 40% of the r-hGH in the form of inclusion body aggregates was refolded into bioactive form while using n-propanol as solubilization agent. Solubilization with 6M n-propanol solution thus can be a viable alternative for achieving high throughput recovery of bioactive protein from inclusion bodies of E. coli.

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.

Controlled release of bioactive recombinant human growth hormone from PLGA microparticles.

Controlled release formulation of recombinant human growth hormone (r-hGH) was achieved using poly lactide-co-glycolide (PLGA) polymer. Denaturation of r-hGH by dichloromethane during primary emulsification step of particle preparation was minimized by using human serum albumin whereas inclusion of sucrose and sodium bicarbonate helped in reducing protein denaturation during lyophilization and polymer particle degradation. Encapsulation efficiency of r-hGH entrapped in PLGA particles (size approximately 30 microm) was around 45% with protein load 20 microg of r-hGH/mg of polymer particles. Porous particles showed quick release of r-hGH in comparison to non-porous particles in vitro. More than 10 ng/mL of bioactive r-hGH was found in the serum of the experimental animals observed for a 30-day period after a single intramuscular injection of the polymeric formulation. Incorporation of optimal stabilizers is thus essential for the development of a stable, month long controlled release of polymer particle based r-hGH formulation.