Review article: immunogenicity of anti-TNF biologics in IBD - the role of patient, product and prescriber factors

BACKGROUND

Anti-drug antibodies (ADAs) to biologic therapies contribute to the loss of response and infusion reactions to anti-TNF drugs in patients with inflammatory bowel disease (IBD). The reasons behind this immunogenicity are complex, and have not been the focus of a dedicated review for prescribers.

AIM

To provide an overview of the patient, product and prescriber factors, which have been associated with the immunogenicity of anti-TNF therapy, and draw conclusions for clinical practice.

METHODS

Review of representative observational studies and clinical trials from the IBD and other literature, which report associations with ADA development, with a focus on infliximab and adalimumab.

RESULTS

ADAs develop in 10-20% of patients receiving anti-TNF maintenance therapy, and these patients are three times more likely to lose response as ADA-negative patients. Patient genotype plays a role in ADA risk in a minority of patients, but age or disease type is not a major factor. Drug mishandling, such as agitation or freeze-thaw cycles, can induce protein aggregates, which are known to be immunogenic. Prescription of maintenance therapy with concomitant immunomodulators, and achieving suitable trough drug levels, reduces the risk of ADAs in patients with IBD.

CONCLUSIONS

Patients and prescribers can take several steps to reduce the risk of development of anti-drug antibodies to anti-TNF antibodies. Further research is required to determine if immunogenic factors identified in other situations apply to use of anti-TNFs in IBD.

pH-Induced Metabolic Transitions in Artemia Embryos Mediated by a Novel Hysteretic Trehalase

Gastrula-stage embryos of the brine shrimp Artemia undergo reversible transitions between metabolically active and dormant states that are promoted by changes in intracellular pH. A macromolecular mechanism for this suppression of energy metabolism that involves regulation of the enzyme trehalase is reported here. Isolated trehalase from these embryos existed in two active forms that interconverted when exposed to physiological transitions in pH. This hysteretic interconversion was reversible, required minutes for completion, and involved a change in enzyme polymerization. The two states differed twofold in molecular size and were distinguishable electrophoretically. Compared to the smaller species, the polymerized form was strongly inhibited by acidic pH, adenosine 5'-triphosphate, and the substrate trehalose. Thus, the shift in assembly equilibrium toward the aggregated enzyme caused by pH values less than or equal to 7.4 may mediate the arrest of trehalose-fueled metabolism and respiration during dormancy in this cryptobiotic organism.

High pressure refolding of recombinant human growth hormone from insoluble aggregates. Structural transformations, kinetic barriers, and energetics

Two different types of insoluble, non-native aggregates of recombinant human growth hormone were formed by agitation in buffer or buffer containing 0.75 m guanidine HCl (GdnHCl) and characterized by infrared and second derivative UV spectroscopies. The degree of secondary structural perturbation was greater in the aggregates formed in 0.75 m GdnHCl. Both aggregate types were dissolved and refolded using high hydrostatic pressures in combination with either elevated temperature or non-denaturing levels of guanidine HCl or urea. The effects of a range of temperature, pressure, and chaotrope concentrations were tested and led to optimal conditions that approached 100% yield of native protein. The aggregates formed in 0.75 m GdnHCl required higher concentrations of urea or GdnHCl, or higher temperature or pressure for a yield equivalent to that for aggregates formed in buffer alone. Investigation of the effects of pressure, temperature, and chaotrope on unfolding of rhGH documented that under conditions used for optimal high pressure disaggregation and refolding, the native state is greatly favored thermodynamically (e.g. 25 kJ/mol at 2000 bar and 0.75 m GdnHCl). Dissolution of aggregates under pressure is a kinetically limited process. Comparison of refolding yields in GdnHCl and urea solutions suggest that pressure effects on electrostatic interactions do not dominate pressure effects on disaggregation. We suggest that non-native hydrogen bonds between protein molecules within aggregates of recombinant human growth hormone are responsible for the rate-limiting kinetic barrier in pressure-induced disaggregation.

Controlling deamidation rates in a model peptide: effects of temperature, peptide concentration, and additives

The rate of deamidation of the Asn residue in Val-Tyr-Pro-Asn-Gly-Ala (VYPNGA), a model peptide, was determined at pH 9 (400 mM Tris buffer) as a function of temperature and peptide concentration. Over the temperature range 5-65 degrees C, deamidation followed Arrhenius behavior, with an apparent activation energy of 13.3 kcal/mol. Furthermore, increasing the peptide concentration slows the rate of deamidation. Self-stabilization with respect to deamidation has not been reported previously. The rate of deamidation was also determined in the presence of sucrose and poloxamer 407 (Pluronic F127). In both cases, the rate of deamidation was retarded by up to 40% at 35 degrees C. In aqueous solutions containing poloxamer 407, the degree of stabilization is independent of formation of a reversible thermosetting gel. With sucrose, maximum reduction in the deamidation rate was attained with as little as 5% (w/v). Addition of sucrose results in a greater conformational preference for a type II beta-turn structure, which presumably is less prone to intramolecular cyclization and subsequent deamidation.

The effects of Tween 20 and sucrose on the stability of anti-L-selectin during lyophilization and reconstitution

We have chosen an anti-L-selectin antibody as a model protein to investigate the effects of sucrose and/or Tween 20 on protein stability during lyophilization and reconstitution. Native anti-L-selectin secondary structure is substantially retained during lyophilization in the presence of sucrose (1 or 0.125%). However, aggregation of the protein during reconstitution of lyophilized protein powders prepared without sucrose is not reduced by the presence of sucrose in the reconstitution medium. Aggregate formation upon reconstitution is completely inhibited by freeze drying the protein with sucrose and reconstituting with a 0.1% Tween 20 solution. Tween 20 (0.1%) also partially inhibits loss of native anti-L-selectin secondary structure during lyophilization. However, upon reconstitution the formulations lyophilized with Tween 20 contain the highest levels of aggregates. The presence of Tween in only the reconstitution solution appears to inhibit the transition from dimers to higher order oligomers. Potential mechanism(s) for the Tween 20 effects were investigated. However, no evidence of thermodynamic stabilization of anti-L-selectin conformation (e.g., by Tween 20 binding) could be detected.

Lyophilization-induced protein denaturation in phosphate buffer systems: monomeric and tetrameric beta-galactosidase

During freezing in phosphate buffers, selective precipitation of a less soluble buffer component and subsequent pH shifts may induce protein denaturation. Previous reports indicate significantly more inactivation and secondary structural perturbation of monomeric and tetrameric beta-galactosidase (beta-gal) during freeze-thawing in sodium phosphate (NaP) buffer as compared with potassium phosphate (KP) buffer. This observation was attributed to the significant pH shifts (from 7.0 to as low as 3.8) observed during freezing in the NaP buffer (1). In the current study, we investigated the impact of the additional stress of dehydration after freezing on the recovery of active protein on reconstitution and the retention of the native structure in the dried state. Freeze-drying monomeric and tetrameric beta-gal in either NaP or KP buffer resulted in significant secondary structural perturbations, which were greatest for the NaP samples. However, similar recoveries of active monomeric protein were observed after freeze-thawing and freeze-drying, indicating that most dehydration-induced unfolding was reversible on reconstitution of the freeze-dried protein. In contrast, the tetrameric protein was more susceptible to dehydration-induced denaturation as seen by the greater loss in activity after reconstitution of the freeze-dried samples relative to that measured after freeze-thawing. To ensure optimal protein stability during freeze-drying, the protein must be protected from both freezing and dehydration stresses. Although poly(ethylene glycol) and dextran are preferentially excluded solutes and should confer protection during freezing, they were unable to prevent lyophilization-induced denaturation. In addition, Tween did not foster maintenance of native protein during freeze-drying. However, sucrose, which hydrogen bonds to dried protein in the place of lost water, greatly reduced freezing- and drying-induced denaturation, as observed by the high retention of native protein in the dried state as well as the complete recovery of active beta-gal on reconstitution. These results indicate that addition of an effective stabilizer, such as sucrose, may minimize protein denaturation during freeze-drying in phosphate buffers, even if there are large-scale changes in solution pH during freezing.

Dry powders of stable protein formulations from aqueous solutions prepared using supercritical CO(2)-assisted aerosolization

We report on the use of a new supercritical carbon dioxide-assisted aerosolization coupled with bubble drying technology to prepare stabilized, dry, finely divided powders from aqueous protein formulations. In this study, the feasibility of this new technology was tested using two model proteins, lysozyme and lactate dehydrogenase (LDH). In the absence of excipients, lysozyme was observed to undergo perturbations of secondary structure observed by solid-state infrared spectroscopy. In the presence of sucrose, this unfolding was minimized. Lysozyme did not, however, undergo irreversible loss of activity, as all lysozyme powders generated by supercritical CO(2)-assisted aerosolization (with or without excipients) regained almost complete activity on reconstitution. The more labile LDH suffered irrecoverable loss of activity on reconstituting after supercritical CO(2)-assisted aerosolization and bubble drying in the absence of carbohydrate stabilizers. LDH could be stabilized throughout the nebulization, drying, and rehydration processes with the addition of sucrose, and almost complete preservation of activity was achieved with the further addition of a surface active agent, such as Tween 20, to the aqueous formulation prior to processing.

The ice nucleation temperature determines the primary drying rate of lyophilization for samples frozen on a temperature-controlled shelf

The objective of this study was to determine the influence of ice nucleation temperature on the primary drying rate during lyophilization for samples in vials that were frozen on a lyophilizer shelf. Aqueous solutions of 10% (w/v) hydroxyethyl starch were frozen in vials with externally mounted thermocouples and then partially lyophilized to determine the primary drying rate. Low- and high-particulate-containing samples, ice-nucleating additives silver iodide and Pseudomonas syringae, and other methods were used to obtain a wide range of nucleation temperatures. In cases where the supercooling exceeded 5 degrees C, freezing took place in the following three steps: (1) primary nucleation, (2) secondary nucleation encompassing the entire liquid volume, and (3) final solidification. The primary drying rate was dependent on the ice nucleation temperature, which is stochastic in nature but is affected by particulate content and the presence of ice nucleators. Sample cooling rates of 0.05 to 1 degrees C/min had no effect on nucleation temperatures and drying rate. We found that the ice nucleation temperature is the primary determinant of the primary drying rate. However, the nucleation temperature is not under direct control, and its stochastic nature and sensitivity to difficult-to-control parameters result in drying rate heterogeneity. Nucleation temperature heterogeneity may also result in variation in other morphology-related parameters such as surface area and secondary drying rate. Overall, these results document that factors such as particulate content and vial condition, which influence ice nucleation temperature, must be carefully controlled to avoid, for example, lot-to-lot variability during cGMP production. In addition, if these factors are not controlled and/or are inadvertently changed during process development and scaleup, a lyophilization cycle that was successful on the research scale may fail during large-scale production.

Annealing to optimize the primary drying rate, reduce freezing-induced drying rate heterogeneity, and determine T(g)' in pharmaceutical lyophilization

In a companion paper we show that the freezing of samples in vials by shelf-ramp freezing results in significant primary drying rate heterogeneity because of a dependence of the ice crystal size on the nucleation temperature during freezing.1 The purpose of this study was to test the hypothesis that post-freezing annealing, in which the product is held at a predetermined temperature for a specified duration, can reduce freezing-induced heterogeneity in sublimation rates. In addition, we test the impact of annealing on primary drying rates. Finally, we use the kinetics of relaxations during annealing to provide a simple measurement of T(g)', the glass transition temperature of the maximally freeze-concentrated amorphous phase, under conditions and time scales most appropriate for industrial lyophilization cycles. Aqueous solutions of hydroxyethyl starch (HES), sucrose, and HES:sucrose were either frozen by placement on a shelf while the temperature was reduced ("shelf-ramp frozen") or by immersion into liquid nitrogen. Samples were then annealed for various durations over a range of temperatures and partially lyophilized to determine the primary drying rate. The morphology of fully dried liquid nitrogen-frozen samples was examined using scanning electron microscopy. Annealing reduced primary drying rate heterogeneity for shelf-ramp frozen samples, and resulted in up to 3.5-fold increases in the primary drying rate. These effects were due to increased ice crystal sizes, simplified amorphous structures, and larger and more numerous holes on the cake surface of annealed samples. Annealed HES samples dissolved slightly faster than their unannealed counterparts. Annealing below T(g)' did not result in increased drying rates. We present a simple new annealing-lyophilization method of T(g)' determination that exploits this phenomenon. It can be carried out with a balance and a freeze-dryer, and has the additional advantage that a large number of candidate formulations can be evaluated simultaneously.

Partial molar volume, surface area, and hydration changes for equilibrium unfolding and formation of aggregation transition state: high-pressure and cosolute studies on recombinant human IFN-gamma

The equilibrium dissociation of recombinant human IFN-gamma was monitored as a function of pressure and sucrose concentration. The partial molar volume change for dissociation was -209 +/- 13 ml/mol of dimer. The specific molar surface area change for dissociation was 12.7 +/- 1.6 nm2/molecule of dimer. The first-order aggregation rate of recombinant human IFN-gamma in 0.45 M guanidine hydrochloride was studied as a function of sucrose concentration and pressure. Aggregation proceeded through a transition-state species, N*. Sucrose reduced aggregation rate by shifting the equilibrium between native state (N) and N* toward the more compact N. Pressure increased aggregation rate through increased solvation of the protein, which exposes more surface area, thus shifting the equilibrium away from N toward N*. The changes in partial molar volume and specific molar surface area between the N* and N were -41 +/- 9 ml/mol of dimer and 3.5 +/- 0.2 nm2/molecule, respectively. Thus, the structural change required for the formation of the transition state for aggregation is small relative to the difference between N and the dissociated state. Changes in waters of hydration were estimated from both specific molar surface area and partial molar volume data. From partial molar volume data, estimates were 25 and 128 mol H2O/mol dimer for formation of the aggregation transition state and for dissociation, respectively. From surface area data, estimates were 27 and 98 mol H2O/mol dimer. Osmotic stress theory yielded values approximately 4-fold larger for both transitions.

Maintenance of Quaternary Structure in the Frozen State Stabilizes Lactate Dehydrogenase during Freeze–Drying

Sugars inhibit protein unfolding during the drying step of lyophilization by replacing hydrogen bonds to the protein lost upon removal of water. In many cases, polymers fail to inhibit dehydration-induced damage to proteins because steric hindrance prevents effective hydrogen bonding of the polymer to the protein's surface. However, in certain cases, polymers have been shown to stabilize multimeric enzymes during lyophilization. Here we test the hypothesis that this protection is due to inhibition of dissociation into subunits during freezing. To test this hypothesis, as a model system we used mixtures of lactate dehydrogenase isozymes that form electrophoretically distinguishable hybrid tetramers during reversible dissociation. We examined hybridization and recovery of catalytic activity during freeze-thawing and freeze-drying in the presence of polymers (dextran, Ficoll, and polyethylene glycol), sugars (sucrose, trehalose, glucose), and surfactants (Tween 80, Brij 35, hydroxy-propyl beta-cyclodextrin). The surfactants did not protect LDH during freeze-thawing or freeze-drying. Rather, in the presence of Brij 35, enhanced damage was seen during both freeze-thawing and freeze-drying, and the presence of Tween 80 exacerbated loss of active protein during freeze-drying. Polymers and sugars prevented dissociation of LDH during the freezing step of lyophilization, resulting in greater recovery of enzyme activity after lyophilization and rehydration. This beneficial effect was observed even in systems that do not form glassy solids during freezing and drying. We suggest that stabilization during drying results in part from greater inherent stability of the assembled holoenzyme relative to that of the dissociated monomers. Polymers inhibit freezing-induced dissociation thermodynamically because they are preferentially excluded from the surface of proteins, which increases the free energy of dissociation and denaturation.

A specific molar ratio of stabilizer to protein is required for storage stability of a lyophilized monoclonal antibody

The selection of the appropriate excipient and the amount of excipient required to achieve a 2-year shelf-life is often done by using iso-osmotic concentrations of excipients such as sugars (e.g., 275 mM sucrose or trehalose) and salts. Excipients used for freeze-dried protein formulations are selected for their ability to prevent protein denaturation during the freeze-drying process as well as during storage. Using a model recombinant humanized monoclonal antibody (rhuMAb HER2), we assessed the impact of lyoprotectants, sucrose, and trehalose, alone or in combination with mannitol, on the storage stability at 40 degrees C. Molar ratios of sugar to protein were used, and the stability of the resulting lyophilized formulations was determined by measuring aggregation, deamidation, and oxidation of the reconstituted protein and by infrared (IR) spectroscopy (secondary structure) of the dried protein. A 360:1 molar ratio of lyoprotectant to protein was required for storage stability of the protein, and the sugar concentration was 3-4-fold below the iso-osmotic concentration typically used in formulations. Formulations with combinations of sucrose (20 mM) or trehalose (20 mM) and mannitol (40 mM) had comparable stability to those with sucrose or trehalose alone at 60 mM concentration. A formulation with 60 mM mannitol alone provided slightly less protection during storage than 60 mM sucrose or trehalose. The disaccharide/mannitol formulations also inhibited deamidation during storage to a greater extent than the lyoprotectant formulations alone. The reduction in aggregation and deamidation during storage correlated directly with inhibition of unfolding during lyophilization, as assessed by IR spectroscopy. Thus, it appears that the protein must be retained in its native-like state during freeze-drying to assure storage stability in the dried solid. Long-term studies (23-54 months) performed at 40 degrees C revealed that the appropriate molar ratio of sugar to protein stabilized against aggregation and deamidation for up to 33 months. Therefore, long-term storage at room temperature or above may be achieved by proper selection of the molar ratio and sugar mixture. Overall, a specific sugar/protein molar ratio was sufficient to provide storage stability of rhuMAb HER2.

Factors affecting short-term and long-term stabilities of proteins

Proteins are marginally stable and, hence, are readily denatured by various stresses encountered in solution, or in the frozen or dried states. Various additives are known to minimize damage and enhance the stability of proteins. This review discusses the current knowledge of the mechanisms by which these additives stabilize proteins against acute stresses, and also the various factors to be considered for long-term storage of proteins in solution.

Counteracting effects of renal solutes on amyloid fibril formation by immunoglobulin light chains

In primary (light chain-associated) amyloidosis, immunoglobulin light chains deposit as amyloid fibrils in vital organs, especially the kidney. Because the kidney contains high concentrations of urea that can destabilize light chains as well as solutes such as betaine and sorbitol that serve as protein stabilizers, we investigated the effects of these solutes on in vitro amyloid fibril formation and thermodynamic stability of light chains. Two recombinant light chain proteins, one amyloidogenic and the other nonamyloidogenic, were used as models. For both light chains, urea enhanced fibril formation by reducing the nucleation lag time and diminished protein thermodynamic stability. Conversely, betaine or sorbitol increased thermodynamic stability of the proteins and partially inhibited fibril formation. These solutes also counteracted urea-induced reduction in protein thermodynamic stability and accelerated fibril formation. Betaine was more effective than sorbitol. A model is presented to explain how the thermodynamic effects of the solutes on protein state equilibria can alter nucleation lag time and, hence, fibril formation kinetics. Our results provide evidence that renal solutes control thermodynamic and kinetic stability of light chains and thus may modulate amyloid fibril formation in the kidney.

Studies of the structure of insulin fibrils by Fourier transform infrared (FTIR) spectroscopy and electron microscopy

Fibril formation (aggregation) of insulin was investigated in acid media by visual inspection, transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy. Insulin fibrillated faster in hydrochloric acid than in acetic acid at elevated temperatures, whereas the fibrillation tendencies were reversed at ambient temperatures. Electron micrographs showed that bovine insulin fibrils consisted of long fibers with a diameter of 5 to 10 nm and lengths of several microns. The fibrils appeared either as helical filaments (in hydrochloric acid) or arranged laterally in bundles (in acetic acid, NaCl). Freeze-thawing cycles broke the fibrils into shorter segments. FTIR spectroscopy showed that the native secondary structure of insulin was identical in hydrochloric acid and acetic acid, whereas the secondary structure of fibrils formed in hydrochloric acid was different from that formed in acetic acid. Fibrils of bovine insulin prepared by heating or agitating an acid solution of insulin showed an increased content of beta-sheet (mostly intermolecular) and a decrease in the intensity of the alpha-helix band. In hydrochloric acid, the frequencies of the beta-sheet bands depended on whether the fibrillation was induced by heating or agitation. This difference was not seen in acetic acid. Freeze-thawing cycles of the fibrils in hydrochloric acid caused an increase in the intensity of the band at 1635 cm(-1) concomitant with reduction of the band at 1622 cm(-1). The results showed that the structure of insulin fibrils is highly dependent on the composition of the acid media and on the treatment.

Survival of water stress in annual fish embryos: dehydration avoidance and egg envelope amyloid fibers

Diapausing embryos of Austrofundulus limnaeus survive desiccating conditions by reducing evaporative water loss. Over 40% of diapause II embryos survive 113 days of exposure to 75.5% relative humidity. An early loss of water from the perivitelline space occurs during days 1-2, but thereafter, rates of water loss are reduced to near zero. No dehydration of the embryonic tissue is indicated based on microscopic observations and the retention of bulk (freezable) water in embryos as judged by differential scanning calorimetry. Such high resistance to desiccation is unprecedented among aquatic vertebrates. Infrared spectroscopy indicates frequent intermolecular contacts via beta-sheet (14%) in hydrated egg envelopes (chorions). These beta-sheet contacts increase to 36% on dehydration of the egg envelope. Interestingly, the egg envelope is composed of protein fibrils with characteristics of amyloid fibrils usually associated with human disease. These features include a high proportion of intermolecular beta-sheet, positive staining and green birefringence with Congo red, and detection of long, unbranched fibrils with a diameter of 4-6 nm. The high resistance of diapause II embryos to water stress is not correlated with ontogenetic changes in the egg envelope.

The excluding effects of sucrose on a protein chemical degradation pathway: methionine oxidation in subtilisin

The conformational stabilization of proteins by sucrose has been previously attributed to a preferential exclusion mechanism. The present study links this mechanism to stability against a chemical degradation pathway for subtilisin. Oxidation of a methionine residue adjacent to the active site to the sulfoxide form compromises subtilisin's enzymatic activity. In the presence of hydrogen peroxide and borate buffer, a borate-hydrogen peroxide complex binds to subtilisin's active site prior to the formation of methionine sulfoxide. Sucrose decreases the oxidation rate by limiting the accessibility of the complex to the methionine at the partially buried active site. The stabilization mechanism of sucrose is based on shifting the equilibrium of transiently expanding native conformations of subtilisin to favor the most compact states. Enzymatic parameter determination (kcat, KM) and hydrogen-deuterium exchange measurements confirm the limited conformational mobility of the enzyme in the presence of sucrose. Further support for limited mobility as the cause of oxidation inhibition by sucrose comes from the findings that neither viscosity nor possible interactions of sucrose with hydrogen peroxide, hydroxyl radicals, or borate can adequately explain the inhibition. The volume exclusion of sucrose from subtilisin is used to estimate the extent by which the native state of subtilisin must expand in solution to allow oxidation. The surface area of the oxidation-competent state is ca. 3.9% greater than that of the native state.

Protein Denaturation during Freezing and Thawing in Phosphate Buffer Systems: Monomeric and Tetrameric β-Galactosidase

During freezing in sodium and potassium phosphate (NaP and KP) buffer solutions, changes in pH may impact the stability of proteins. Since the degradation pathways for the model proteins, monomeric and tetrameric beta-galactosidase (beta-gal), chosen for this study are governed by conformational changes (i.e., physical instability) as opposed to chemical transformations, we explored how the stresses of freezing and thawing alter the protein's native structure and if preservation of the native conformation during freeze-thawing is a requisite for optimal recovery of activity. During freezing in NaP buffer, a significant pH decrease from 7.0 to as low as 3.8 was observed due to the selective precipitation of the disodium phosphate; however, the pH during freezing in KP buffer only increased by at most 0.3 pH units. pH-induced inactivation was evident as seen by the lower recovery of activity when freeze-thawing in NaP buffer as compared to KP buffer for both sources of beta-gal. In addition, we investigated the effects of cooling rate and warming rate on the recovery of activity for monomeric and tetrameric beta-gal. Optimal recovery of activity for the NaP samples was obtained when the processing protocol involved a fast cool/fast warm combination, which minimizes exposure to acidic conditions and concentrated solutes. Alterations in the native secondary structure of monomeric beta-gal as measured by infrared spectroscopy were more significant when freezing and thawing in NaP buffer as opposed to KP buffer. Conformational and activity analyses indicate that pH changes during freezing in NaP buffer contribute to denaturation of beta-gal. These results suggest that proteins formulated in NaP buffer should be frozen and thawed rapidly to minimize exposure to low pH and high buffer salts.

Effect of zinc binding and precipitation on structures of recombinant human growth hormone and nerve growth factor

Metal-induced precipitation of protein therapeutics is being used and further developed as a processing step in protein formulation and may have utility in protein purification and bulk storage. In such processes, it is imperative that native protein structure is maintained and the metal complexation is reversible. In the current study, we investigated the effects of zinc-induced precipitation on recombinant human growth hormone (rhGH) and recombinant human nerve growth factor (rhNGF). On the addition of ethylenediaminetetraacetic acid (EDTA), the precipitates were dissolved, yielding complete recovery of native protein in both cases. Both proteins have specific metal binding sites and require specific molar ratios of zinc to protein to initiate precipitation (zinc:rhGH > 2:1; zinc:rhNGF > 18:1). Furthermore, the secondary structures of both proteins were unperturbed in soluble zinc complexes and zinc-induced precipitates, as measured by infrared and circular dichroism spectroscopies. The soluble zinc complex of rhGH had minor tertiary structural alterations, whereas zinc binding did not alter the tertiary structure of rhNGF. These studies indicated that metal-induced precipitation provides a method to maintain proteins in their native state in precipitates, which may be useful for purification, storage, and formulation.

Comparative fourier transform infrared and circular dichroism spectroscopic analysis of alpha1-proteinase inhibitor and ovalbumin in aqueous solution

Alpha1-proteinase inhibitor (alpha1Pi) and ovalbumin are both members of the serpin superfamily. They share about a 30% sequence identity and exhibit great similarity in their three-dimensional structures. However, no apparent functional relationship has been found between the two proteins. Unlike alpha1Pi, ovalbumin shows no inhibitory effect to serine proteases. To see whether or not a conformational factor(s) may contribute to the functional difference, we carried out comparative analysis of the two proteins' secondary structure, thermal stability, and H-D exchange using FT-IR and CD spectroscopy. FT-IR analysis reveals significant differences in the amide I spectral patterns of the two proteins. Upon thermal denaturation, both proteins exhibit a strong low-wavenumber beta-sheet band at 1624 cm(-1) and a weak high-wavenumber beta-sheet band at 1694 cm(-1), indicative of intermolecular aggregate formation. However, the midpoint of the thermal-induced transition of alpha1Pi (approximately 55 degrees C) is 18 degrees C lower than that of ovalbumin (approximately 73 degrees C). The thermal stability analysis provides new insight into the structural changes associated with denaturation. The result of H-D exchange explains some puzzling spectral differences between the two proteins in D2O reported previously.

Entrapping intermediates of thermal aggregation in alpha-helical proteins with low concentration of guanidine hydrochloride

Aggregation of proteins is a problem with serious medical implications and economic importance. To develop strategies for preventing aggregation, the mechanism(s) and pathways by which proteins aggregate must be characterized. In this study, the thermally induced aggregation processes of three alpha-helix proteins (myoglobin, cytochrome c, and lysozyme) in the presence and absence of 1.0 m guanidine hydrochloride (GdnHCl) were investigated by means of infrared spectroscopy. In the absence of GdnHCl, intensities of the alpha-helix bands (approximately 1656 cm(-1)) decrease as a function of temperature at above 50 degrees C. With myoglobin and cytochrome c, the loss of helix bands was accompanied by the appearance of two new bands at 1694 and 1623 cm(-1), indicative of the formation of intermolecular beta-sheet aggregates. For lysozyme, bands indicative of intermolecular beta-sheet aggregates did not appear in any significant intensity. In the presence of 1.0 m GdnHCl, two major intermediate states rich in 3(10)-helix (represented by the band at 1663 cm(-1)) and beta-turn structure (represented by the band at 1667 cm(-1)), respectively, were observed. These findings demonstrated that IR spectroscopic studies of protein aggregation using a combination of thermal and chemical denaturing factors could provide a means to populate and characterize aggregation intermediates.

Stability of subtilisin and lysozyme under high hydrostatic pressure

The stabilities of subtilisin and lysozyme under hydrostatic pressures up to 200 MPa were investigated for up to 7 days at 25 degrees C. Methods were chosen to assess changes in tertiary and secondary protein structure as well as aggregation state. Tertiary structure was monitored in situ with second derivative UV spectroscopy and after pressure treatment by dynamic light scattering and second derivative UV spectroscopy. Secondary structure and potential secondary structural changes were characterized by second derivative FTIR spectroscopy. Changes in aggregation state were assessed using dynamic light scattering. Additionally, protein concentration balances were carried out to detect any loss of protein as a function of pressure. For the conditions tested, neither protein shows measurable changes in tertiary or secondary structure or signs of aggregation. Lysozyme concentration balances show no dependence on pressure. Subtilisin concentration balances at high protein concentration (4 mg/mL and higher) do not show pressure dependence. However, the concentration balances carried out at 0.4 mg/mL show a clear sign of pressure dependence. These results may be explained by protein interaction with the vial surface and appear to be rate limited by the equilibrium between active and inactive protein on the surface. Pressure increases protein loss, and the estimated partial molar volume change between the two states is estimated to be -20 +/- 10 mL/mol.

Stability of human serum albumin during bioprocessing: denaturation and aggregation during processing of albumin paste

PURPOSE

To assess the impact of various bioprocessing steps on the stability of freshly precipitated human serum albumin (HSA) obtained from pooled human plasma.

METHODS

After initial precipitation of HSA from plasma, the resultant paste is either (a) lyophilized or (b) washed with acetone and then air-dried in order to obtain a dry powder. The structure of HSA was examined using Fourier transform infrared (IR) spectroscopy. The extent of aggregation of redissolved HSA was measured using both dynamic light scattering and SDS-polyacrylamide gel electrophoresis (SDS-PAGE).

RESULTS

Both lyophilization and air-drying perturb the secondary structural composition of HSA, as detected by infrared (IR) spectroscopy. Upon dissolution of dried paste, most of the protein refolds to a native-like conformation. However, a small fraction of the protein molecules form soluble aggregates that can be detected by both dynamic light scattering and SDS-PAGE. The level of aggregation is so low that it could not be detected in the bulk by either circular dichroism or IR spectroscopy. The lyophilized protein, which appears to be more unfolded in the solid state than the acetone washed/air-dried material, exhibits a higher level of aggregation upon dissolution.

CONCLUSIONS

There is a direct correlation between the extent of unfolding in the solid state and the amount of soluble aggregate present after dissolution. Moreover, the presence of the aggregates persists throughout the remainder of the purification process, which includes dissolution, chromatography, sterile filtration and viral inactivation steps. Analytical methods used to monitor the stability of biopharmaceuticals in the final product can be used to assess damage inflicted during processing of protein pharmaceuticals.

Optimization of storage stability of lyophilized actin using combinations of disaccharides and dextran

The storage stability of a dry protein depends on the structure of the dried protein, as well as on the storage temperature relative to the glass transition temperature of the dried preparation. Disaccharides are known to preserve the native conformation of a dried protein; however, the resulting T(g) of the sample may be too low ensure adequate storage stability. On the other hand, formulations dried with high molecular weight carbohydrates, such as dextran, have higher glass transition temperatures, but fail to preserve native protein conformation. We tested the hypothesis that optimizing both protein structure and T(g) by freeze-drying actin with mixtures of disaccharides and dextran would result in increased storage stability compared to actin dried with either disaccharide or dextran alone. Protein structure in the dried solid was analyzed immediately after lyophilization and after storage at elevated temperatures with infrared spectroscopy, and after rehydration by infrared and circular dichroism spectroscopy. Structural results were related to the polymerization activity recovered after rehydration. Degradation was noted with storage for formulations containing either sucrose, trehalose, or dextran alone. Slight increases in T(g) observed in trehalose formulations compared to sucrose formulations did not result in appreciable increases in storage stability. Addition of dextran to sucrose or trehalose increased formulation T(g) without affecting the capacity of the sugar to inhibit protein unfolding during lyophilization and resulted in improved storage stability. Also, dextran provides an excellent amorphous bulking agent, which can be lyophilized rapidly with formation of strong, elegant cake structure. These results suggest that the strategy of using a mixture of disaccharide and polymeric carbohydrates can optimize protein storage stability.

Thermodynamic modulation of light chain amyloid fibril formation

To obtain further insight into the pathogenesis of amyloidosis and develop therapeutic strategies to inhibit fibril formation we investigated: 1) the relationship between intrinsic physical properties (thermodynamic stability and hydrogen-deuterium (H-D) exchange rates) and the propensity of human immunoglobulin light chains to form amyloid fibrils in vitro; and 2) the effects of extrinsically modulating these properties on fibril formation. An amyloid-associated protein readily formed amyloid fibrils in vitro and had a lower free energy of unfolding than a homologous nonpathological protein, which did not form fibrils in vitro. H-D exchange was much faster for the pathological protein, suggesting it had a greater fraction of partially folded molecules. The thermodynamic stabilizer sucrose completely inhibited fibril formation by the pathological protein and shifted the values for its physical parameters to those measured for the nonpathological protein in buffer alone. Conversely, urea sufficiently destabilized the nonpathological protein such that its measured physical properties were equivalent to those of the pathological protein in buffer, and it formed fibrils. Thus, fibril formation by light chains is predominantly controlled by thermodynamic stability; and a rational strategy to inhibit amyloidosis is to design high affinity ligands that specifically increase the stability of the native protein.

High pressure fosters protein refolding from aggregates at high concentrations

High hydrostatic pressures (1-2 kbar), combined with low, nondenaturing concentrations of guanidine hydrochloride (GdmHCl) foster disaggregation and refolding of denatured and aggregated human growth hormone and lysozyme, and beta-lactamase inclusion bodies. One hundred percent recovery of properly folded protein can be obtained by applying pressures of 2 kbar to suspensions containing aggregates of recombinant human growth hormone (up to 8.7 mg/ml) and 0.75 M GdmHCl. Covalently crosslinked, insoluble aggregates of lysozyme could be refolded to native, functional protein at a 70% yield, independent of protein concentration up to 2 mg/ml. Inclusion bodies containing beta-lactamase could be refolded at high yields of active protein, even without added GdmHCl.

Hydrogen bonding between sugar and protein is responsible for inhibition of dehydration-induced protein unfolding

The nature of the interaction responsible for the inhibition of protein unfolding and subsequent damage by sugars during dehydration is unclear. The relationship between sample moisture content measured by coulometric Karl Fischer titration and the apparent moisture content predicted by the area of the protein side chain carboxylate band at approximately 1580 cm-1 in infrared spectra of dried protein-sugar samples was examined. For samples in which a high level of native protein structure was retained in the dried solid, the apparent moisture content predicted by the carboxylate band area was greater than the actual moisture content, indicating that protection results from direct sugar-protein hydrogen bonding and not entrapment of water at the protein surface. Further, we show that the degree of structural protection conferred by sucrose and trehalose apparent in second derivative, amide I infrared spectra, correlates with the extent of hydrogen bonding between sugar and protein. The failure of dextran to inhibit dehydration-induced lysozyme unfolding is shown to result from the inability of the polymer to hydrogen bond adequately to the protein. Therefore, formation of an amorphous phase alone is not sufficient to maintain protein structure during dehydration. Glucose hydrogen bonds to a high degree with dried lysozyme, but is incapable of inhibiting lyophilization-induced protein unfolding in the absence of an effective cryoprotectant. However, the addition of polyethylene glycol, which is known to protect proteins during freezing, but not drying, to glucose protected lysozyme structure during lyophilization. Together, these results show that hydrogen bonding between carbohydrate and protein is necessary to prevent dehydration-induced protein damage. However, hydrogen bonding alone is not sufficient to protect proteins during lyophilization in the absence of adequate freezing protection.

Protein formulation and lyophilization cycle design: prevention of damage due to freeze-concentration induced phase separation

Hemoglobin has been previously shown to unfold during freeze drying when lyophilized from formulations that undergo freeze-concentration induced phase separation (Heller et al. 1997. Biotechnol Prog 13:590-596). In this report, we show that such damage may be avoided using kinetic strategies to arrest the phase separation. By rapidly cooling samples during liquid nitrogen spray-freeze drying, the time that the formulation spends in temperature regimes (ca. -3 to -23 degrees C) in which phase separation is both thermodynamically favorable and kinetically realizable is minimized. Increased protein damage with decreasing cooling rates and/or longer annealing periods at -7 degrees C is observed by FTIR spectroscopy. Phase separation and concomitant protein damage may also be avoided by addition of mannitol at concentrations sufficient to cause crystallization. Mannitol crystals segregate the freeze concentrated solution into microscopic domains that block propagation and nucleation of phase separating events. Addition of noncrystallizing sugars, such as sucrose and trehalose, or nonionic surfactants, such as Tween 80 and Triton X-100, has little protective effect against phase separation induced damage during freezing drying.

Application of a thermodynamic model to the prediction of phase separations in freeze-concentrated formulations for protein lyophilization

Many of the compounds considered for use in pharmaceutical formulations demonstrate incompatibilities with other components at high enough concentrations, including pairs of polymers, polymers and salts, or even proteins in combination with polymers, salts, or other proteins. Freeze concentration can force solutions into a region where incompatibilities between solutes will manifest as the formation of multiple phases. Such phase separation complicates questions of the stability of the formulation as well as labile components, such as proteins. Yet, phase separation events are difficult to identify by common formulation screening methods. In this report, we use the osmotic virial expansion model of Edmond and Ogston (1) to describe phase-separating behavior of ternary aqueous polymer solutions. Second osmotic virial coefficients of polyethylene glycol 3350 (PEG) and dextran T500 were measured by light scattering. Assuming an equilibrium between ice and water in the freeze-concentrated solution, a degree of freeze concentration can be estimated, which, when combined with the phase separation spinodal, describes a "phase separation envelope" in which phase separation tendencies can be expected in the frozen solution. The phase separation envelope is bounded at low temperatures by the glass transition temperature of the freeze-concentrated solution. Scanning electron microscopic images and infrared spectroscopy of protein structure are provided as experimental evidence of the phase separation envelope in a freeze-dried system of PEG, dextran, and hemoglobin.

Effects of additives on the stability of Humicola lanuginosa lipase during freeze-drying and storage in the dried solid

The effects of various classes of additives on the stability of a protein with a relatively hydrophobic surface, Humicola lanuginosa lipase (HLL), during lyophilization and storage in the dried solid, were investigated. Prior to lyophilization, it was found that 1 M trehalose or 1% (wt/vol) Tween 20 caused the protein to precipitate. Infrared spectroscopy indicated that trehalose "salted-out" native HLL, whereas Tween 20 induced non-native aggregates. Optimal recovery of native protein in the initial dried solid was obtained in the presence of additives which formed an amorphous phase and which had the capacity to hydrogen bond to the dried protein (e.g., trehalose and sucrose). Additives which crystallized during lyophilization (e.g., mannitol) or which remained amorphous, but were unable to hydrogen bond to the dried protein (e.g., dextran), afforded less stabilization relative to that seen in the absence of additives. Optimal storage stability in the dried solid required that both protein unfolding during lyophilization was minimized and that the formulation was stored at a temperature below its Tg value. Crystallization of sucrose during storage greatly reduced the storage stability of HLL. This was attributed to the increased moisture content and the reduced Tg value in the remaining amorphous phase containing the protein. Sucrose crystallization and the resulting damage to the protein were inhibited by decreasing the mass ratio of sucrose:protein.

Use of infrared spectroscopy to assess secondary structure of human growth hormone within biodegradable microspheres

The purpose of this study was to test the utility of infrared (IR) spectroscopy to determine protein secondary structure in biodegradable microspheres. Encapsulation of proteins within biodegradable polymers, [e.g. poly(lactic-co-glycolic acid) (PLGA)] for controlled drug release has recently been the subject of intense research effort. The ability to assess protein integrity after microsphere production is necessary to successfully produce microspheres that release native proteins. We used IR spectroscopy, a noninvasive method-as opposed to conventional organic solvent extraction or in vitro release at elevated temperature-to assess the secondary structure of recombinant human growth hormone (rhGH) within dry and rehydrated microspheres. PLGA microspheres containing rhGH with different excipients were prepared by a conventional double-emulsion method. The protein IR spectra indicated that the encapsulation process could perturb the structure of rhGH and that excipients could inhibit this damage to varying degrees. A strong positive correlation was found between intensity of the dominant alpha-helical band in the spectra of rhGH in rehydrated microspheres and the percent monomer released from microspheres during incubation in buffer. We also studied microspheres prepared with zinc-precipitated rhGH. The addition of Zn2+ during microsphere processing partially inhibited protein unfolding and fostered complete refolding of rhGH upon rehydration. In conclusion, IR spectroscopy can serve as a valuable tool to assess protein structure within both dried and rehydrated microspheres.

Conformational stability of lyophilized PEGylated proteins in a phase-separating system

PEGylation of proteins is of great interest to the pharmaceutical industry as covalent attachment of poly(ethylene glycol) (PEG) molecules can increase protein sera half-lives and reduce antigenicity. Not surprisingly, PEGylation significantly alters the surface characteristics of a protein, and consequently, its conformational stability during freezing and drying. Freeze concentration-induced phase separation between excipients has been previously shown to cause degradation of the secondary structure in lyophilized hemoglobin. In this report we show how PEGylation of two proteins, hemoglobin- and brain-derived neurotrophic factor (BDNF), influences partitioning and protein secondary structure as determined by FTIR spectroscopy in a system prone to freezing-induced phase separation. PEGylation of hemoglobin reduces the loss of structure induced by lyophilization in a PEG/dextran system that phase separates during freezing, perhaps due to altered partitioning. The partition coefficient for native hemoglobin favors the dextran-rich phase (PEG/dextran partition coefficient = 0.3), while PEGylated hemoglobin favors the PEG phase (partition coefficient = 3.1). In addition, we demonstrate that PEGylation alters hemoglobin's stability during lyophilization in the absence of other excipients. In contrast, because native BDNF already partitions into the PEG-rich phase, PEGylation of BDNF has a less dramatic effect on both partition coefficients and conformational stability during lyophilization. This is the first report on the effects of PEGylation on protein structural stability during lyophilization and points out the need to consider modification of formulations in response to changing protein surface characteristics.

Effects of additives on the stability of recombinant human factor XIII during freeze-drying and storage in the dried solid

Freeze-drying is often used to improve storage stability of therapeutic proteins. In order to obtain a product with optimal storage stability it is important to understand the mechanisms by which solutes protect the protein against freeze-drying-induced stresses and also against damage induced during subsequent storage. The objective of the current study was to examine the importance of various mechanisms proposed to account for acute and long-term storage stability using recombinant human Factor XIII (rFXIII)4 as a model protein. Initially, for acute stability during freeze-drying, it was found that solutes which formed an amorphous phase stabilized rFXIII to a greater degree than solutes which crystallized during freeze-drying. However, only amorphous solutes which were able to hydrogen bond to the protein, and thus preserve the native protein structure in the dried solid, provided optimal acute stability. Thus, in addition to forming an amorphous phase, it was also important to possess the ability to hydrogen bond to the protein. Long-term storage stability was found to be optimal in the presence of solutes which formed and maintained amorphous phases with Tg values above the storage temperature and which also preserved the native protein structure during freeze-drying. Solute crystallization during storage compromised storage stability.

Secondary structure of antifreeze proteins from overwintering larvae of the beetle Dendroides canadensis

Antifreeze proteins from overwintering larvae of the beetle Dendroides canadensis are among the most active antifreeze proteins known. The Dendroides AFPs (DAFPs) consist of 6 or 7, 12- or 13-mer repeat units with a consensus sequence of -C-T-X3-S-X5-X6-C-X8-X9-A-X11-T-X13-. Nearly all of the Cys residues are in internal disulfide bridges between positions 1 and 7 within the repeats. The study presented here identified the secondary structure of the DAFPs using infrared and circular dichroism (CD) spectroscopies. The eight disulfide bridges impose significant constraints on potential secondary structural features (i.e., a number of three-residue gamma-turns) which may lead to unusual infrared and CD spectra that require special interpretation. At 25 degreesC the DAFPs contain approximately 46% beta-sheet, 39% turn, 2% helix, and 13% random structure. In the presence of ice there is a slight increase in helix and beta-sheet structures and a decrease in both turn and especially random structures. This change in the presence of ice may reflect a certain amount of flexibility in the DAFP structure. These structural changes may permit an improved lattice match between the DAFPs and ice, a requisite for the noncolligative freezing-point-depressing activity of the DAFPs.

Tween protects recombinant human growth hormone against agitation-induced damage via hydrophobic interactions

In the absence of surfactants, recombinant human growth hormone (rhGH) rapidly forms insoluble aggregates during agitation. The nonionic surfactant Tween 20, when present at Tween:protein molar ratios >4, effectively inhibits this aggregation. Differential scanning calorimetry (DSC) of rhGH solutions showed melting transitions that decreased by ca. 2 degrees C in the presence of Tween. Circular dichroism (CD) studies of the same thermal transition showed that the decrease is specific to the relatively high protein concentrations required for DSC. CD studies showed melting transitions that decreased with lower protein concentrations. Tween has an insignificant effect on the melting transition of rhGH at lower protein concentrations (0.18 mg/mL). Injection titration microcalorimetry showed that the interaction of Tween with rhGH is characterized by a weak enthalpy of binding. For comparison, interferon-g, another protein which has been shown to bind Tween, also shows weak enthalpy of binding. Fluorescent probe binding studies and infrared spectroscopic investigations of rhGH secondary structure support suggestions in the literature (Bam, N. B.; Cleland, J. L., Randolph, T. W. Molten globule intermediate of recombinant human growth hormone: stabilization with surfactants. Biotechnol. Prog. 1996. 12, 801-809) that Tween binding is driven by hydrophobic interactions, with little perturbation of protein secondary structure.

Effect of Tween 20 on freeze-thawing- and agitation-induced aggregation of recombinant human factor XIII

Agitation- and freeze-thawing-induced aggregation of recombinant human factor XIII (rFXIII) is due to interfacial adsorption and denaturation at the air-liquid and ice-liquid interfaces. The aggregation pathway proceeds through soluble aggregates to formation of insoluble aggregates regardless of the denaturing stimuli. A nonionic surfactant, polyoxyethylene sorbitan monolaurate (Tween 20), greatly reduces the rate of formation of insoluble aggregates as a function of surfactant concentration, thereby stabilizing native rFXIII. Maximum protection occurs at concentrations close to the critical micelle concentration (cmc), independent of initial protein concentration. To study the mechanistic aspects of the surfactant-induced stabilization, a series of spectroscopic studies were conducted. Electron paramagnetic resonance spectroscopy indicates that binding is not occurring between Tween 20 and either the native state or a folding intermediate state of rFXIII. Further, circular dichroism spectroscopy suggests that Tween 20 does not prevent the secondary structural changes induced upon guanidinium hydrochloride-induced unfolding. Taken together, these results imply that Tween 20 protects rFXIII against freeze-thawing- and agitation-induced aggregation primarily by competing with stress-induced soluble aggregates for interfaces, inhibiting subsequent transition to insoluble aggregates.

A transient expansion of the native state precedes aggregation of recombinant human interferon-gamma

Aggregation of proteins, even under conditions favoring the native state, is a ubiquitous problem in biotechnology and biomedical engineering. Providing a mechanistic basis for the pathways that lead to aggregation should allow development of rational approaches for its prevention. We have chosen recombinant human interferon-gamma (rhIFN-gamma) as a model protein for a mechanistic study of aggregation. In the presence of 0.9 M guanidinium hydrochloride, rhIFN-gamma aggregates with first order kinetics, a process that is inhibited by addition of sucrose. We describe a pathway that accounts for both the observed first-order aggregation of rhIFN-gamma and the effect of sucrose. In this pathway, aggregation proceeds through a transient expansion of the native state. Sucrose shifts the equilibrium within the ensemble of rhIFN-gamma native conformations to favor the most compact native species over more expanded ones, thus stabilizing rhIFN-gamma against aggregation. This phenomenon is attributed to the preferential exclusion of sucrose from the protein surface. In addition, kinetic analysis combined with solution thermodynamics shows that only a small (9%) expansion surface area is needed to form the transient native state that precedes aggregation. The approaches used here link thermodynamics and aggregation kinetics to provide a powerful tool for understanding both the pathway of protein aggregation and the rational use of excipients to inhibit the process.

Effects of drying methods and additives on structure and function of actin: mechanisms of dehydration-induced damage and its inhibition

Limited stability impedes the development of industrial and pharmaceutical proteins. Dried formulations are theoretically more stable, but the drying process itself causes structural damage leading to loss of activity after rehydration. Lyophilization is the most common method used to dry proteins, but involves freezing and dehydration, which are both damaging to protein. We compared an air-drying method to freeze-drying to test the hypothesis that terminal dehydration is the critical stress leading to loss of activity. The secondary structure of air-dried and freeze-dried actin was analyzed by infrared spectroscopy and related to the level of activity recovered from the rehydrated samples. Actin dried by either method in the absence of stabilizers was highly unfolded and the capacity to polymerize was lost upon rehydration. The degree of unfolding was reduced by air-drying or freeze-drying actin with sucrose, and the level of activity recovered upon rehydration increased. The addition of dextran to sucrose improved the recovery of activity from freeze-dried, but not air-dried samples. Dextran alone failed to protect the structure and function of actin dried by either method, indicating that proteins are not protected from dehydration-induced damage by formation of a glassy matrix. In some cases, recovered activity did not correlate directly with the level of structural protection conferred by a particular additive. This result suggests that secondary structural protection during drying is a necessary but not sufficient condition for the recovery of activity from a dried protein after rehydration.

Aggregation of recombinant human interferon gamma: kinetics and structural transitions

Protein aggregation is a complex phenomenon that can occur in vitro and in vivo, usually resulting in the loss of the protein's biological activity. While many aggregation studies focus on a mechanism due to a specific stress, this study focuses on the general nature of aggregation. Recombinant human interferon-gamma (rhIFN-gamma) provides an ideal model for studying protein aggregation, as it has a tendency to aggregate under mild denaturing stresses (low denaturant concentration, temperature below the Tm, and below pH 5). All of the aggregates induced by these stresses have a similar structure (high in intermolecular beta-sheet content and a large loss of alpha-helix) as determined by infrared and circular dichroism spectroscopy. Thermally induced and denaturant-induced aggregation processes follow first-order kinetics under the conditions of this study. Spectroscopic and kinetic data suggest that rhIFN-gamma aggregates through an intermediate form possessing a large amount of residual secondary structure. In contrast to the aggregates formed under denaturing stresses, the salted-out protein has a remarkably nativelike secondary structure.

Stability of lipid/DNA complexes during agitation and freeze-thawing

It is well established that cationic liposomes facilitate the delivery of DNA and offer substantial advantages over viral-based delivery systems. However, these synthetic vectors readily aggregate in liquid formulations which in clinical trials requires preparation of lipid/DNA complexes at the bedside immediately before injection. This temporal requirement could be eliminated if complexes were formulated as stable preparations that could be shipped, stored, and administered as needed. To this end, our study investigates the stability of lipid/DNA complexes during physical stresses that might be encountered during shipping and storage, i.e., agitation and freeze-thawing. Our data show that agitation significantly reduces transfection rates in complexes prepared with three different commercially available lipid formulations. Additional experiments indicate that slow freezing is more damaging than rapid freezing, and that sucrose is able to preserve transfection and complex size during freeze-thawing. These results are consistent with previous reports and demonstrate that frozen formulations may be suitable for maintaining transfection rates of lipid/DNA complexes. Under certain conditions, we observe a reproducible 3-fold increase in transfection after freeze-thawing that is prevented by high concentrations of sucrose. Together, these data suggest that physical stresses can alter structural characteristics of lipid/DNA complexes that can markedly affect rates of DNA delivery.

Intermolecular beta-sheet results from trifluoroethanol-induced nonnative alpha-helical structure in beta-sheet predominant proteins: infrared and circular dichroism spectroscopic study

2,2,2-Trifluoroethanol (TFE)-induced nonnative alpha-helical structure in peptides and proteins has been extensively studied with circular dichroism (CD) spectroscopy. However, to date, complementary information from infrared (IR) spectroscopy has not been reported. Using both IR and CD spectroscopy, we demonstrate here that the TFE-induced nonnative alpha-helical structure in two beta-sheet-predominant proteins, beta-lactoglobulin and alpha-chymotrypsin, is unstable in comparison with those found in the alpha-helix-predominant proteins myoglobin and cytochrome c under identical conditions. IR spectra showed that, immediately after dissolution of the beta-sheet proteins in 50% (v/v) TFE, a strong amide I band component appears at 1654 cm-1 in H2O and at 1650 cm-1 in D2O, which is ascribed to alpha-helical structure. However, the intensities of the alpha-helical bands decrease as a function of time, concomitant with the appearance of two new band components near 1620 and 1695 cm-1 in H2O and 1612 and 1684 cm-1 in D2O, a typical IR spectral pattern for an intermolecular beta-sheet aggregate. Clear gels begin to develop within 30 min. No similar spectral changes were observed for the alpha-helical proteins. CD spectra suggested initially that the TFE-induced alpha-helix was retained in the gelled state. However, further analysis of the spectra, and Gaussian function modeling with basic spectra, indicated that the apparent alpha-helix signal was actually due to a combination of signals from intermolecular beta-sheet and residual alpha-helix. These results indicate that the TFE-induced nonnative alpha-helix structure in predominantly beta-sheet proteins is unstable and readily converts to an intermolecular beta-sheet aggregate.

The role of vitrification in anhydrobiosis

Numerous organisms are capable of surviving more or less complete dehydration. A common feature in their biochemistry is that they accumulate large amounts of disaccharides, the most common of which are sucrose and trehalose. Over the past 20 years, we have provided evidence that these sugars stabilize membranes and proteins in the dry state, most likely by hydrogen bonding to polar residues in the dry macromolecular assemblages. This direct interaction results in maintenance of dry proteins and membranes in a physical state similar to that seen in the presence of excess water. An alternative viewpoint has been proposed, based on the fact that both sucrose and trehalose form glasses in the dry state. It has been suggested that glass formation (vitrification) is in itself sufficient to stabilize dry biomaterials. In this review we present evidence that, although vitrification is indeed required, it is not in itself sufficient. Instead, both direct interaction and vitrification are required. Special properties have often been claimed for trehalose in this regard. In fact, trehalose has been shown by many workers to be remarkably (and sometimes uniquely) effective in stabilizing dry or frozen biomolecules, cells, and tissues. Others have not observed any such special properties. We review evidence here showing that trehalose has a remarkably high glass-transition temperature (Tg). It is not anomalous in this regard because it lies at the end of a continuum of sugars with increasing Tg. However, it is unusual in that addition of small amounts of water does not depress Tg, as in other sugars. Instead, a dihydrate crystal of trehalose forms, thereby shielding the remaining glassy trehalose from effects of the added water. Thus under less than ideal conditions such as high humidity and temperature, trehalose does indeed have special properties, which may explain the stability and longevity of anhydrobiotes that contain it. Further, it makes this sugar useful in stabilization of biomolecules of use in human welfare.

Inhibition of isoproterenol-induced tachycardia by azimilide in the isolated perfused guinea pig heart

The class III antiarrhythmic agent, azimilide, has been shown to inhibit dihydroalprenolol binding to the beta-adrenergic receptor of rat brain and heart in an in-vitro ligand-binding assay. Azimilide, was assessed for beta-adrenergic activity, either agonist or antagonist, in the isolated perfused guinea pig heart in comparison with class III reference agents and the class II agent, propranolol. Varying concentrations of compound (0.03-100 microM) were retrogradely perfused and the effects on corrected QT interval, baseline heart rate, and isoproterenol-stimulated heart rate were measured. Propranolol, dl-sotalol, azimilide, and d-sotalol inhibited isoproterenol-induced tachycardia with IC50 values (the concentration giving 50% inhibition of isoproterenol-stimulated heart rate) of 0.12, 1.4, 14.6, and 38.0 microM, respectively. Clofilium, dofetilide, and sematilide did not affect the action of isoproterenol. Dofetilide, clofilium, azimilide, sematilide, dl-sotalol, and d-sotalol increased the QTc interval approximately 20 ms at concentrations of 0.1, 0.3, 1.0, 3.0, 30.0, and 100.0 microM, respectively. The class III antiarrhythmic agents also slowed baseline heart rate and exhibited linear R-R and QT-interval relationships of similar slope. Azimilide's antagonism of isoproterenol in this isolated heart model may reflect a direct receptor interaction or a contribution from the bradycardic action of the compound, which distinguishes it from several other pure IKr-blocking class III antiarrhythmic agents.

Maintenance of transfection rates and physical characterization of lipid/DNA complexes after freeze-drying and rehydration

It is well established that cationic liposomes form complexes with DNA and effectively transfect cells in vivo and ex vivo. Lipid/DNA complexes have proven safe and nonimmunogenic in clinical trials; however, they are known to aggregate readily in liquid formulations. This physical instability requires clinicians to prepare lipid/DNA complexes immediately prior to injection. In order to eliminate problems associated with this temporal requirement, we investigated the feasibility of preserving complexes as a dried preparation that could be tested, stored, and rehydrated as needed. To this end, our study evaluated the ability of different stabilizers to preserve transfection rates of complexes during acute freeze-drying stress. Our data show that complexes lyophilized in 0.5 M sucrose or trehalose possessed transfection rates similar to those of fresh preparations. In addition, dried complexes that exhibited full transfection activity upon rehydration had sizes comparable to nonlyophilized controls. Our work demonstrates that lipid/DNA complexes can be stabilized as dried powders that offer significant advantages over current liquid formulations. Furthermore, the correlation of transfection rates with maintenance of complex diameter suggests that size plays a critical role in lipid-based DNA delivery.

Spectroscopic study of secondary structure and thermal denaturation of recombinant human factor XIII in aqueous solution

The secondary structure and thermal denaturation (in H2O vs D2O) of recombinant human factor XIII in aqueous solutions were investigated using infrared and circular dichroism (CD) spectroscopies. The infrared amide I spectrum of the protein in H2O solution at 25 degrees C exhibited an absorbance maximum near 1642 cm-1, indicating the presence of a predominantly beta-sheet structure. Quantitative analysis revealed that the native protein contains 13-16% alpha-helix, 41-49% beta-sheet, 29% beta-turn, and 10-14% extended strand structures. The presence of a strong low-wavenumber beta-sheet band at 1641 cm-1 and a weak high-wavenumber beta-sheet band at 1689 cm-1 indicated that the beta-sheet structure of the protein is predominantly antiparallel. Quantitative analysis of the CD spectrum using the SELCON method indicated a secondary structural content of 10% alpha-helix, 40-50% beta-sheet, 20-35% beta-turns, and 20-35% unordered elements, which matches that determined by X-ray crystallography. The apparent discrepancy with the contents of unordered element determined by infrared spectroscopy is reconciled by considering that CD spectroscopy and X-ray crystallography assign extended loops and strands to unordered elements, whereas infrared spectroscopy recognizes these as distinct structured elements. During heating above 60 degrees C, a pair of new infrared bands appeared at 1626 and 1693 cm-1 for the protein in H2O and 1619 and 1683 cm-1 in D2O, indicating a formation of intermolecular beta-sheet aggregates. The intensities of the new bands increased as a function of temperature, concomitant with an intensity decrease in bands for the native protein structural elements. As expected, there was an increase in thermal stability in D2O relative to that in H2O, which was manifested as an increase of about 5 degrees C in the temperature for initial loss of infrared bands assigned to native structural elements and for appearance of bands due to intermolecular beta-sheet. In addition, the midpoint of the thermally induced transitions in infrared spectra were about 2.5 degrees C higher in D2O than in H2O. Based on the infrared analysis, the thermally denatured state of the protein in both H2O and D2O contains predominantly intermolecular beta-sheet. The broad, poorly resolved absorbance that spans the region between the intermolecular beta-sheet bands was assigned to an ensemble of heterogeneous structural elements (including unordered), none of which is populated to a high enough degree to result in a distinct infrared band. Results from CD spectroscopy support these conclusions about the structure of the denatured, aggregated protein.

Real-time in situ monitoring of lysozyme during lyophilization using infrared spectroscopy: dehydration stress in the presence of sucrose

PURPOSE

First, to investigate the role of sucrose in stabilizing protein structure (as measured by changes in the amide I band of lysozyme) caused by dehydration encountered during lyophilization. Second, to demonstrate the utility of internal reflection spectroscopy as a tool for conducting controlled lyophilization experiments.

METHODS

A custom-built internal reflection FTIR accessory was used to follow the entire freeze-drying process of solutions consisting of 49.4 mg/mL lysozyme in the presence and absence of 10% sucrose in real-time. Studies were carried out using D2O as a transparent medium in the infrared region of the protein amide bands. Potential self-association of the protein in the presence of sucrose was investigated using dynamic light scattering. Hydration levels were determined using a multiple regression equation. Differential scanning calorimetry (DSC) permitted characterization of the final lyophilized product. Moisture content was determined using Karl Fischer titration.

RESULTS

Throughout freezing and drying, minimal changes were observed both in frequency (1647 +/- 1 cm-1) and bandwidth (46 +/- 1 cm-1) of the amide I band in the presence of sucrose. In contrast, greater changes in frequency and band width were seen in the absence of sucrose. A successfully lyophilized cake was obtained which had properties of a glass as measured by DSC, with a Tg of 50 degree C. The lyophilized product containing sucrose had 4% moisture by weight. Three distinct rates of water desorption were discovered during drying under vacuum (50 mg/hr within the sample temperature range from -35 degrees to -25 degrees C; 30 mg/hr from -10 degrees to 25 degrees C; 1.2 mg/hr from 27 degrees to 38 degrees C).

CONCLUSIONS

The inclusion of sucrose served to minimize perturbations of protein structure caused by freezing and dehydration stresses encountered during lyophilization (compared to studies conducted in absence of sucrose). The results support the water replacement hypothesis and underscore the role of the sugar in preserving a native structure in the dried state. This investigation demonstrates the usefulness of infrared spectroscopy in evaluating lyophilization process parameters and formulation design.

Hydrophobic ion pairing as a method for enhancing structure and activity of lyophilized subtilisin BPN' suspended in isooctane

The use of enzymes in low water environments permits reactions to occur that are difficult or impossible in aqueous solution. In this manner, proteases can be used to form, rather than hydrolyze, ester and amide linkages. Presumably, the native-like structure of the enzyme must remain intact for catalysis to transpire. However, little is known regarding the integrity of the overall structure of lyophilized proteins suspended in organic media. In this study, the structural changes that occur during the freeze-drying process and those effected by suspension in the organic solvent were examined. Using Fourier-transform infrared spectroscopy, the secondary structure of lyophilized subtilisin BPN' was monitored and correlated to the level of enzymatic activity when suspended in isooctane. In addition, the ability of ionic detergents to stabilize subtilisin BPN' via ion pairing was evaluated. It was found that subtilisin unfolds to some degree during lyophilization, whether it is ion paired or not. Furthermore, there are structural changes observed when the enzyme is placed in isooctane, although the effects are less with ion-paired subtilisin. This higher level of retention of secondary structure results in increased enzymatic activity.

Preferential exclusion of sucrose from recombinant interleukin-1 receptor antagonist: role in restricted conformational mobility and compaction of native state

Understanding the mechanism for sucrose-induced protein stabilization is important in many diverse fields, ranging from biochemistry and environmental physiology to pharmaceutical science. Timasheff and Lee [Lee, J. C. & Timasheff, S. N. (1981) J. Biol. Chem. 256, 7193-7201] have established that thermodynamic stabilization of proteins by sucrose is due to preferential exclusion of the sugar from the protein's surface, which increases protein chemical potential. The current study measures the preferential exclusion of 1 M sucrose from a protein drug, recombinant interleukin 1 receptor antagonist (rhIL-1ra). It is proposed that the degree of preferential exclusion and increase in chemical potential are directly proportional to the protein surface area and that, hence, the system will favor the protein state with the smallest surface area. This mechanism explains the observed sucrose-induced restriction of rhIL-1ra conformational fluctuations, which were studied by hydrogen-deuterium exchange and cysteine reactivity measurements. Furthermore, infrared spectroscopy of rhlL-1ra suggested that a more ordered native conformation is induced by sucrose. Electron paramagnetic resonance spectroscopy demonstrated that in the presence of sucrose, spin-labeled cysteine 116 becomes more buried in the protein's interior and that the hydrodynamic diameter of the protein is reduced. The preferential exclusion of sucrose from the protein and the resulting shift in the equilibrium between protein states toward the most compact conformation account for sucrose-induced effects on rhIL-1ra.