Mechanism of insulin aggregation and stabilization in agitated aqueous solutions

Role of a novel excipient poly(ethylene glycol)-b-poly(L-histidine) in retention of physical stability of insulin at aqueous/organic interface

The aim of this study was to investigate whether a cationic polyelectrolyte, poly(ethylene glycol)-b-poly(L-histidine) diblock copolymer (PEG-polyHis), can stabilize insulin, at the aqueous/methylene chloride interface formed during the microencapsulation process. Insulin aggregation at this interface was monitored spectrophotometrically at 276 nm. The effects of protein concentration, pH of the aqueous medium, and the presence of poly(lactic-co-glycolic acid) (PLGA) in methylene chloride (MC) on insulin aggregation were observed. For the 2.0 mg/mL insulin solutions in phosphate buffer (PB), the effect of addition of Pluronic F-127 as a positive control and addition of PEG-polyHis as a novel excipient in PB was also evaluated at various insulin/polymeric excipient weight ratios. The conformation of insulin protected by PEG-polyHis and recovered after interfacial exposure was evaluated via circular dichroism (CD) spectroscopy. Greater loss in soluble insulin was observed with increasing insulin concentrations. pH 6.0 was selected for optimal ionic interactions between insulin and PEG-polyHis based on zeta potential and particle size studies. pH 4.5 and 7.4 (no ionic complexation between insulin and PEG-polyHis) were selected as controls to compare the stabilization effect of PEG-polyHis with that at pH 6.0. Incubation of PEG-polyHis with insulin at pH 6.0 drastically reduced protein aggregation, even in the presence of PLGA. PEG-polyHis and F-127 reduced insulin aggregation in noncomplexing pH conditions pointing to the role played by PEG in modulation of insulin adsorption at the interface. Far-UV (205-250 nm) CD study revealed negligible qualitative effects on soluble insulin's secondary structure after interfacial exposure. RP-HPLC and size-exclusion HPLC showed no deamidation of insulin or formation of soluble high molecular weight transformation products respectively. MALDI-TOF mass spectrometry confirmed the results from chromatographic procedures. Radioimmunoassay carried out on select samples showed that recovered soluble insulin had retained its immunoreactivity. An experimental method to simulate interfacial denaturation of proteins was designed for assessment of protein stability at the interface and screening for novel protein stabilizers. Understanding and manipulation of such polyelectrolyte-insulin complexation will likely play a role in insulin controlled delivery via microsphere formulation(s).

Role of a novel excipient poly(ethylene glycol)-b-poly(L-histidine) in retention of physical stability of insulin in aqueous solutions

PURPOSE

This study is to investigate whether poly(ethylene glycol) (PEG)-b-poly(L-histidine) [PEG-polyHis] can reduce aggregation of insulin in aqueous solutions on agitation by forming ionic complexes.

MATERIALS AND METHODS

Insulin aggregation on agitation was monitored spectrophotometrically and by fibrillation studies with a dye Thioflavin T. Pluronic F-127 as a control and PEG-polyHis as a novel multifunctional excipient were added to prevent destabilization of insulin. Conformation of insulin was evaluated in a circular dichroism (CD) study.

RESULTS

Ionic interactions between insulin and PEG-polyHis were induced in the pH range: 5.5-6.5. pH 5.5 was selected for further evaluation based on particle size/zeta potential studies. Ionic complexation with PEG-polyHis is more effective at pH 5.5 in stabilizing insulin (75% of insulin retained versus 0% with no excipient) than Pluronic F-127 (42% retained). PEG-polyHis guards against insulin aggregation in non-complexing pH conditions (pH 7.4), 64% insulin retained versus 58% with F-127 and 0% with no excipient) pointing to the potential role played by PEG in modulation of insulin surface adsorption. Rate of fibrillation was higher for plain insulin compared with addition of PEG-polyHis and Pluronic F-127 at both pH.

CONCLUSIONS

Understanding and manipulation of such polyelectrolyte-protein complexation will likely play a role in protein stabilization.

Aggregation of lysozyme and of poly(ethylene glycol)-modified lysozyme after adsorption to silica

Surface-induced aggregation is a common instability during protein storage, delivery and purification. This aggregation can lead to the formation of fibrils rich in intermolecular beta-sheet structure. Techniques to probe surface-clustering are limited. Here we use protein intrinsic fluorescence and thioflavin T probe fluorescence in a total internal reflection fluorescence (TIRF) sampling geometry to simultaneously monitor the kinetics of adsorption and aggregation for chicken egg lysozyme on a silica surface. We observe a slow surface-induced aggregation process that continues well after the lysozyme adsorption kinetics have plateaued. The rate of surface-induced aggregation is independent of the lysozyme concentration in solution. Consistent with the clustering observed via thioflavin T fluorescence, infrared amide I band spectra also show a 1.5-fold increase in intermolecular beta-sheet content upon lysozyme adsorption. Tryptophan emission spectra show no evidence for any tertiary structural change upon adsorption. Furthermore, we observe that the covalent modification of lysozyme with a single poly(ethylene glycol) (PEG) grafted chain does not inhibit aggregation on the surface, but a second PEG graft significantly inhibits the intermolecular beta-sheet formation.

Kinetic measurements of protein conformation in a microchip

This paper presents a microchip-based system for collecting kinetic time-based information on protein refolding and unfolding. Dynamic protein conformational change pathways were studied in microchannel flow using a microfluidic device. We present a protein-conserving approach for quantifying refolding by dynamically varying the concentration of the chemical denaturants, guanidine hydrochloride and urea. Short diffusion distances in the microchannel result in rapid equilibrium between protein and titrating solutions. Dilutions on the chip were tightly regulated using pressure controls rather than syringe-based flow, as verified with extensive on-chip tracer dye controls. To validate this protein assay method, folding transition experiments were performed using two well-characterized proteins, human serum albumin (HSA) and bovine carbonic anhydrase (BCA). Transition events were monitored through fluorescence intensity shifts of the protein dye 8-anilino-1-naphthalenesulfonic acid (ANS) during dilutions of protein from urea or guanidine hydrochloride solutions. The enzymatic activity of refolded BCA was measured by UV absorption through the conversion of p-nitrophenyl acetate (p-NPA). The microchip protein refolding transitions using ANS were well-correlated with conventional plate-based experiments. The microfluidic platform enables refolding studies to identify rapidly the optimal folding strategy for a protein using small quantities of material.

Adsorption of human insulin and AspB28 insulin on a PTFE-like surface

The interactions of human insulin, Zn-free human insulin, and AspB28 insulin with a hydrophobic surface were studied by ellipsometry. All three insulin types investigated adsorbed with high affinity onto the hydrophobic surface, as the plateau of the adsorption isotherm, represented by the irreversible bound fraction, was reached at concentrations >10(-3) mg/ml. The plateau values for human insulin and Zn-free human insulin could not be distinguished with statistical significance, whereas the plateau value for AspB28 insulin was lower than those for the two others, with an adsorbed amount corresponding to a monolayer of insulin monomers. The results observed may be explained by differences in self-association patterns of the insulin types or by enhanced charge repulsion between the AspB28 analog and the negatively charged surface.

Insulin particle formation in supersaturated aqueous solutions of poly(ethylene glycol)

Protein microspheres are of particular utility in the field of drug delivery. A novel, completely aqueous, process of microsphere fabrication has been devised based on controlled phase separation of protein from water-soluble polymers such as polyethylene glycols. The fabrication process results in the formation of spherical microparticles with narrow particle size distributions. Cooling of preheated human insulin-poly(ethylene glycol)-water solutions results in the facile formation of insulin particles. To map out the supersaturation conditions conducive to particle nucleation and growth, we determined the temperature- and concentration-dependent boundaries of an equilibrium liquid-solid phase separation. The kinetics of formation of microspheres were followed by dynamic and continuous-angle static light scattering techniques. The presence of PEG at a pH that was close to the protein's isoelectric point resulted in rapid nucleation and growth. The time elapsed from the moment of creation of a supersaturated solution and the detection of a solid phase in the system (the induction period, t(ind)) ranged from tens to several hundreds of seconds. The dependence of t(ind) on supersaturation could be described within the framework of classical nucleation theory, with the time needed for the formation of a critical nucleus (size <10 nm) being much longer than the time of the onset of particle growth. The growth was limited by cluster diffusion kinetics. The interfacial energies of the insulin particles were determined to be 3.2-3.4 and 2.2 mJ/m(2) at equilibrium temperatures of 25 and 37 degrees C, respectively. The insulin particles formed as a result of the process were monodisperse and uniformly spherical, in clear distinction to previously reported processes of microcrystalline insulin particle formation.

Interfacial adsorption of insulin

The adsorption of human insulin to Teflon particles was studied with respect to conformational changes and the reversibility of adsorption was examined by total internal reflection fluorescence (TIRF). Adsorption isotherms for the adsorption of human insulin indicated high affinity adsorption, even at electrostatic repulsive conditions. The plateau value for adsorption was in accordance with a protein layer consisting primarily of insulin monomers. Conformational changes of the insulin upon adsorption, was investigated by circular dicroism (CD) and fluorescence spectroscopy. The results suggested unfolding of adsorbed insulin, as observed by a decrease in alpha-helix and increase in random coil conformation. The changes in protein structure was not only related to the adsorbed species being monomeric, since CD and fluorescence results were different for adsorbed insulin compared to a monomeric analog of human insulin. Furthermore, the thermal stability in the adsorbed state was changed compared to insulin in solution. On the basis of the TIRF studies with FITC-labelled insulin it was not possible to firmly conclude whether exchange between human insulin in the adsorbed state and in solution takes place, due to the limited time range investigated. However, the desorption mechanism appeared to be different with unlabelled insulin in the bulk solution compared to phosphate buffer.

Displacement of adsorbed insulin by Tween 80 monitored using total internal reflection fluorescence and ellipsometry

PURPOSE

This study was conducted to investigate the mechanism of action in the displacement of adsorbed insulin from a hydrophobic surface by Tween 80 and of the competitive adsorption of the two species.

METHODS

Total internal reflection fluorescence (TIRF) and ellipsometry were used as in situ methods to examine the processes taking place at hydrophobic model surfaces in the presence of insulin and Tween 80.

RESULTS

TIRF studies showed that the displacement of insulin by Tween 80 could be fitted to a sigmoidal function, indicating a nucleation-dependent process. Furthermore, a linear dependence between the apparent rate constant and the logarithm of the Tween 80 concentration was found. Competitive adsorption from solution mixtures of insulin and Tween 80 indicated that insulin was adsorbed first, but subsequently displaced by the surfactant. This displacement proved also to be dependent on the concentration of Tween 80 in the mixture.

CONCLUSIONS

The results indicate that Tween 80 at concentrations above critical micelle concentration can be used to protect insulin against surface adsorption. The presence of a lag phase in the displacement at low surfactant concentration indicates that the mechanism of action for Tween 80 to reduce adsorption of insulin may be by competing for sites at the surface.

Adsorption of poly(ethylene glycol)-modified lysozyme to silica

Covalent grafting of poly(ethylene glycol) (PEG) to pharmaceutical proteins, "PEGylation", is becoming more commonplace due to improved therapeutic efficacy. As these conjugates encounter interfaces in manufacture, purification, and end use and adsorption to these interfaces may alter achievable production yields and in vivo efficacies, it is important to understand how PEGylation affects protein adsorption mechanisms. To this end, we have studied the adsorption of unmodified and PEGylated chicken egg lysozyme to silica, using optical reflectometry, total internal reflection fluorescence (TIRF) spectroscopy, and atomic force microscopy (AFM) under varying conditions of ionic strength and extent of PEG modification. PEGylation of lysozyme changes the shape of the adsorption isotherm and alters the preferred orientation of lysozyme on the surface. There is an abrupt transition in the isotherm from low to high surface excess concentrations that correlates with a change in orientation of mono-PEGylated conjugates lying with the long axis parallel to the silica surface to an orientation with the long axis oriented perpendicular to the surface. No sharp transition is observed in the adsorption isotherm for di-PEGylated lysozyme within the range of concentrations examined. The net effect of PEGylation is to decrease the number of protein molecules per unit area relative to the adsorption of unmodified lysozyme, even under conditions where the surface is densely packed with conjugates. This is due to the area sterically excluded by the PEG grafts. The other major effect of PEGylation is to make conjugate adsorption significantly less irreversible than unmodified lysozyme adsorption.

Guanidine hydrochloride can induce amyloid fibril formation from hen egg-white lysozyme

The formation of amyloid fibrils is an intractable problem in which normally soluble protein polymerizes and forms insoluble ordered aggregates. Such aggregates can range from being a nuisance in vitro to being toxic in vivo. The latter is true for lysozyme, which has been shown to form toxic deposits in humans. In the present study, the effects of partial denaturation of hen egg-white lysozyme via incubation in a concentrated solution of the denaturant guanidine hydrochloride are investigated. Results show that when lysozyme is incubated under moderate guanidine hydrochloride concentrations (i.e., 2-5 M), where lysozyme is partially unfolded, fibrils form rapidly. Thioflavin T, Congo red, X-ray diffraction, transmission electron microscopy, atomic force microscopy, and circular dichroism spectroscopy are all used to verify the production of fibrils under these conditions. Incubation at very low or very high guanidine hydrochloride concentrations fails to produce fibrils. At very low denaturant concentrations, the structure of lysozyme is fully native and very stable. On the other hand, at very high denaturant concentrations, guanidine hydrochloride is capable of dissolving and dis-aggregating fibrils that are formed. Raising the temperature and/or concentration of lysozyme accelerates fibril formation by further adding to the concentration of partially unfolded species. The addition of preformed fibrils also accelerates fibril formation but only under partially unfolding conditions. The results presented here provide further evidence that partial unfolding is a prerequisite to fibril formation. Partial denaturation can accelerate fibril formation in much the same way that mutations have been shown to accelerate fibril formation.

Protein aggregation and its inhibition in biopharmaceutics

Protein aggregation is arguably the most common and troubling manifestation of protein instability, encountered in almost all stages of protein drug development. Protein aggregation, along with other physical and/or chemical instabilities of proteins, remains to be one of the major road barriers hindering rapid commercialization of potential protein drug candidates. Although a variety of methods have been used/designed to prevent/inhibit protein aggregation, the end results are often unsatisfactory for many proteins. The limited success is partly due to our lack of a clear understanding of the protein aggregation process. This article intends to discuss protein aggregation and its related mechanisms, methods characterizing protein aggregation, factors affecting protein aggregation, and possible venues in aggregation prevention/inhibition in various stages of protein drug development.

Stimulation of Insulin Fibrillation by Urea-induced Intermediates

Fibrillar deposits of insulin cause serious problems in implantable insulin pumps, commercial production of insulin, and for some diabetics. We performed a systematic investigation of the effect of urea-induced structural perturbations on the mechanism of fibrillation of insulin. The addition of as little as 0.5 m urea to zinc-bound hexameric insulin led to dissociation into dimers. Moderate concentrations of urea led to accumulation of a partially unfolded dimer state, which dissociates into an expanded, partially folded monomeric state. Very high concentrations of urea resulted in an unfolded monomer with some residual structure. The addition of even very low concentrations of urea resulted in increased fibrillation. Accelerated fibrillation correlated with population of the partially folded intermediates, which existed at up to 8 m urea, accounting for the formation of substantial amounts of fibrils under such conditions. Under monomeric conditions the addition of low concentrations of urea slowed down the rate of fibrillation, e.g. 5-fold at 0.75 m urea. The decreased fibrillation of the monomer was due to an induced non-native conformation with significantly increased alpha-helical content compared with the native conformation. The data indicate a close-knit relationship between insulin conformation and propensity to fibrillate. The correlation between fibrillation and the partially unfolded monomer indicates that the latter is a critical amyloidogenic intermediate in insulin fibrillation.

Effect of high shear rate on stability of proteins: kinetic study

Size-exclusion chromatography (SEC) was used to monitor the time-course of protein degradation induced by high shear rates during the formulation and manufacture of controlled-release pharmaceutical dosage forms. SEC with multi-angle laser light-scattering (MALLS) detection was used to characterize the aggregation products, determining their absolute molecular weight. A stability-indicating method was developed and validated to obtain reliable drug degradation data. The results obtained according to the ICH guidelines confirm that the system and methods proposed are suitable for their intended use. The degradation kinetics are influenced by the type of protein and the effect of the shear rate on their stability. Reversible pseudo-first order degradation kinetics were observed for bovine beta-lactoglobulin, whereas for human (HSA) and bovine serum albumin (BSA), a monomer-dimer transition was observed, independently of the rate of shear. However, trimer formation was also observed for HSA, especially at high shear rates. The kinetic model may thus be described as a two-step process: a monomer-dimer, and dimer-trimer transition.

Prediction of the association state of insulin using spectral parameters

Human insulin exists in different association states, from monomer to hexamer, depending on the conditions. In the presence of zinc the "normal" state is a hexamer. The structural properties of 20 variants of human insulin were studied by near-UV circular dichroism, fluorescence spectroscopy, and small-angle X-ray scattering (SAXS). The mutants showed different degrees of association (monomer, dimers, tetramers, and hexamers) at neutral pH. A correlation was shown between the accessibility of tyrosines to acrylamide quenching and the degree of association of the insulin mutants. The near-UV CD spectra of the insulins were affected by protein association and by mutation-induced structural perturbations. However, the shape and intensity of difference CD spectra, obtained by subtraction of the spectra measured in 20% acetic acid (where all insulin species were monomeric) from the corresponding spectra measured at neutral pH, correlate well with the degree of insulin association. In fact, the near-UV CD difference spectra for monomeric, dimeric, tetrameric, and hexameric insulin are very distinctive, both in terms of intensity and shape. The results show that the spectral properties of the insulins reflect their state of association, and can be used to predict their oligomeric state.

Micronization of insulin from halogenated alcohol solution using supercritical carbon dioxide as an antisolvent

Insulin was precipitated from solution in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) using supercritical carbon dioxide (CO2) as an antisolvent. Biosynthetic human insulin crystals were dissolved in HFIP and the solution was sprayed through an ultrasonic nozzle into supercritical CO2. The factors in the 2(3) factorial experimental design with a center point replicate included pressure (83.7 and 97.5 bar), solution concentration (15 and 30 mg/mL), and solution flow rate (2 and 4 mL/min). Temperature (37 degrees C), CO2 mass flow rate (137 g/min), and volume of solution sprayed (20 mL) were held constant. High-performance liquid chromatography, circular dichroism spectroscopy, infrared and Raman spectroscopy, scanning electron microscopy, dry powder size distribution analysis, thermogravimetric analysis, and atomic absorption spectroscopy were used to characterize the processed insulin powder. The processed insulin retained its potency, was slightly degraded chemically, and exhibited reversible structural changes. The precipitated powder consisted of physical aggregates of 50-nm spheres. Through deagglomeration of these aggregates, it may be possible to obtain discrete uniform particles (1-5 microm) suitable for pulmonary therapy. Over the ranges of operating variables studied, the factors chosen for the experimental design had little effect on the product characteristics.

Inverse relationship of protein concentration and aggregation

PURPOSE

To determine the effect of protein concentration on aggregation induced through quiescent shelf-life incubation or shipping-related agitation.

METHODS

All aggregation was measured by size-exclusion high-performance liquid chromatography. Aggregation was induced by time-dependent incubation under stationary conditions or by agitation caused by shaking, vortexing, or vibration using simulated shipping conditions.

RESULTS

Protein aggregation is commonly a second- or higher-order process that is expected to increase with higher protein concentration. As expected, for three proteins (PEG-GCSF, PEG-MGDF, and OPG-Fc) that were examined, the aggregation increased with higher protein concentration if incubated in a quiescent shelf-life setting. However, aggregation decreased with higher protein concentration if induced by an air/water interface as a result of agitation. This unexpected result may be explained by the rate-limiting effect on aggregation of the air/water interface and the critical nature of the air/ water interface to protein ratio that is greatest with decreased protein concentration. The non-ionic detergent polysorbate 20 enhanced the aggregation observed in the quiescently incubated sample but abrogated the aggregation induced by the air/water interface.

CONCLUSIONS

The effect of protein concentration was opposite for aggregation that resulted from quiescent shelf-life treatment compared to induction by agitation. For motionless shelf-life incubation, increased concentration of protein resulted in more aggregation. However, exposure to agitation resulted in more aggregation with decreased protein concentration. These results highlight an unexpected complexity of protein aggregation reactions.

A new mechanism for decreasing aggregation of recombinant human interferon-gamma by a surfactant: slowed dissolution of lyophilized formulations in a solution containing 0.03% polysorbate 20

To study the mechanisms by which Tween 20 (polysorbate 20) used in a reconstitution solution affects the aggregation of lyophilized recombinant human interferon-gamma (rhIFN-gamma), we used four types of buffered formulations containing 0.4-5 mg/mL rhIFN-gamma in either 10 mM potassium phosphate or phosphate buffered saline: (1) without excipients, (2) with 5% sucrose, (3) with 0.03% polysorbate 20, or (4) with the combination of 5% sucrose and 0.03% polysorbate 20. After lyophilization, infrared spectroscopy was used to analyze the secondary structure of the protein in the freeze-dried solid. Each solid showed structural perturbation of the protein. Each formulation was reconstituted with water or a 0.03% polysorbate 20 solution. Aggregation of rhIFN-gamma after reconstitution was measured by optical density at A(350), and recovery of soluble protein was determined by high-performance liquid chromatography and ultraviolet spectroscopy. After reconstitution with a 0.03% polysorbate 20 solution, aggregation levels in all formulations were either reduced or similar to those found after reconstitution with water. These results revealed the potential for recovery of native protein using the appropriate reconstitution conditions, even though the protein is non-native in the lyophilized state. Urea-induced unfolding with and without polysorbate 20 as measured by second-derivative ultraviolet spectroscopy indicated that a concentration of 0.03% polysorbate 20 lowered the free energy of unfolding for rhIFN-gamma (destabilizing). Polysorbate 20 also retarded refolding from urea solutions and increased aggregation. At a level of 0.03%, polysorbate 20 did not protect the protein against surface-induced aggregation during agitation. Dissolution times in water versus a 0.03% polysorbate 20 solution were measured using a rotating disk electrode for lyophilized formulations containing an electrochemically reactive species. The presence of 0.03% polysorbate 20 in the reconstitution solution nearly doubled the time required for dissolution of the phosphate buffered saline formulation, and the sucrose formulations dissolved 33-57% more slowly. Slowing the dissolution rates of lyophilized powders allows more time for the protein to refold while it decreases the maximum concentration of the protein at the dissolution interface, thus reducing the total amount of aggregation.

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.

Influence of the co-encapsulation of different non-ionic surfactants on the properties of PLGA insulin-loaded microspheres

The aim of this work was to produce insulin-loaded microspheres allowing the preservation of peptide stability during both particle processing and insulin release. Our strategy was to combine the concepts of using surfactants to improve insulin stability while optimising overall microsphere characteristics such as size, morphology, peptide loading and release. Bovine insulin was encapsulated within poly(lactide-co-glycolide) (PLGA 50:50, Resomer RG504H) microspheres by the multiple emulsion-solvent evaporation technique. Microspheres were prepared by adding to the primary emulsion three non-ionic surfactants, poloxamer 188, polysorbate 20 and sorbitan monooleate 80, at different concentrations (1.5 and 3. 0% w/v). The presence of surfactants was found to decrease the mean diameter and to affect the morphology of the microspheres. Insulin encapsulation efficiency was reduced in the presence of surfactants and especially for sorbitan monooleate 80, in a concentration-dependent mode. The influence of the surfactants on the interactions between insulin and PLGA together with the primary emulsion stability were found to be the major determinants of insulin encapsulation. The release of insulin from microspheres was biphasic, showing an initial burst effect followed by a near zero-order release for all the batches prepared. The initial burst was related to the presence of insulin molecules located onto or near to the microsphere surface. In the presence of surfactants, a faster insulin release with respect to microspheres encapsulating insulin alone was observed. Insulin stability within microspheres after processing, storage and release was evaluated by reversed phase- and size-exclusion-HPLC. The analysis of microsphere content after processing and 6 months of storage showed that insulin did not undergo any chemical modification within microspheres. On the contrary, during the period of sustained release insulin was transformed in a high-molecular weight product, the amount of which was related to the surfactant used. In conclusion, polysorbate 20 at 3% w/v concentration was the most effective in giving regular shaped particles with both good insulin loading and slow release, and limiting insulin modification within microspheres.

Formation of insulin amyloid fibrils followed by FTIR simultaneously with CD and electron microscopy

Fourier transform infrared spectroscopy (FTIR), circular dichroism (CD), and electron microscopy (EM) have been used simultaneously to follow the temperature-induced formation of amyloid fibrils by bovine insulin at acidic pH. The FTIR and CD data confirm that, before heating, insulin molecules in solution at pH 2.3 have a predominantly native-like alpha-helical structure. On heating to 70 degrees C, partial unfolding occurs and results initially in aggregates that are shown by CD and FT-IR spectra to retain a predominantly helical structure. Following this step, changes in the CD and FTIR spectra occur that are indicative of the extensive conversion of the molecular conformation from alpha-helical to beta-sheet structure. At later stages, EM shows the development of fibrils with well-defined repetitive morphologies including structures with a periodic helical twist of approximately 450 A. The results indicate that formation of fibrils by insulin requires substantial unfolding of the native protein, and that the most highly ordered structures result from a slow evolution of the morphology of the initially formed fibrillar species.

Protein engineering as a strategy to avoid formation of amyloid fibrils

The activation domain of human procarboxypeptidase A2 (ADA2h) aggregates following thermal or chemical denaturation at acidic pH. The aggregated material contains well-defined ordered structures with all the characteristics of the fibrils associated with amyloidotic diseases. Variants of ADA2h containing a series of mutations designed to increase the local stability of each of the two helical regions of the protein have been found to have a substantially reduced propensity to form fibrils. This arises from a reduced tendency of the denatured species to aggregate rather than from a change in the overall stability of the native state. The reduction in aggregation propensity may result from an increase in the stability of local relative to longer range interactions within the polypeptide chain. These findings show that the intrinsic ability of a protein to form amyloid can be altered substantially by protein engineering methods without perturbing significantly its overall stability or activity. This suggests new strategies for combating diseases associated with the formation of aggregated proteins and for the design of novel protein or peptide therapeutics.

Characterization of glucose-sensitive insulin release systems in simulated in vivo conditions

We studied the glucose-responsive insulin controlled release system based on the hydrogel poly(2-hydroxyethyl methacrylate-co-N,N-dimethylaminoethyl methacrylate), also called poly(HEMA-co-DMAEMA), with entrapped glucose oxidase, catalase and insulin. When exposed to physiological fluids, glucose diffuses into the hydrogel, glucose oxidase catalyzes the glucose conversion to gluconic acid, causing swelling of the pH-sensitive hydrogel and subsequently increased insulin release. The higher the glucose concentration in the medium, the higher and faster the swelling and release rates. The effects of polymer morphology and oxygen availability on hydrogel swelling and on insulin release kinetics were tested. Polymer morphology was modified by changing the crosslinking agent (tetraethylene glycol dimethacrylate) concentration (0-0.95 vol%). Oxygen availability was modified by changing the immobilized catalase concentration (0-15 units catalase per unit glucose oxidase) and by bubbling oxygen through the medium. The results indicated that: (i) Hydrogels without crosslinking agent were found to be stable in water, and their sensitivity to pH and glucose was higher than the chemically crosslinked hydrogels. (ii) Immobilization of catalase in addition to glucose oxidase in hydrogels prepared without crosslinking agent, resulted in enhanced swelling kinetic. In addition, we carried out primary in vivo experiments on rats, which demonstrated that at least some of the entrapped insulin retains its active form and is effective in reducing blood glucose levels. Moreover, no tissue encapsulation was observed around matrices implanted in the peritoneum. In conclusion, the pH-sensitive hydrogel poly(HEMA-co-DMAEMA) can be manipulated to produce glucose-responsive insulin release system that is effective in reducing blood glucose levels.

The formulation of biopharmaceutical products

Biopharmaceutical products represent a diverse group of products that includes proteins, peptides, nucleic acids, whole cells, viral particles and vaccines. The conformation of the macromolecule or cell must be maintained to retain biological activity, and animal models for biological activity and characterization assays are often developed in tandem with initial formulation studies. This presents the formulation scientist with a unique set of challenges when compared to those for small molecules. This review focuses on approaches to the formulation of macromolecules into biopharmaceutical products, and provides examples of studies that have been undertaken within the authors' laboratories.

Instability, stabilization, and formulation of liquid protein pharmaceuticals

One of the most challenging tasks in the development of protein pharmaceuticals is to deal with physical and chemical instabilities of proteins. Protein instability is one of the major reasons why protein pharmaceuticals are administered traditionally through injection rather than taken orally like most small chemical drugs. Protein pharmaceuticals usually have to be stored under cold conditions or freeze-dried to achieve an acceptable shelf life. To understand and maximize the stability of protein pharmaceuticals or any other usable proteins such as catalytic enzymes, many studies have been conducted, especially in the past two decades. These studies have covered many areas such as protein folding and unfolding/denaturation, mechanisms of chemical and physical instabilities of proteins, and various means of stabilizing proteins in aqueous or solid state and under various processing conditions such as freeze-thawing and drying. This article reviews these investigations and achievements in recent years and discusses the basic behavior of proteins, their instabilities, and stabilization in aqueous state in relation to the development of liquid protein pharmaceuticals.

Site-specific insulin conjugates with enhanced stability and extended action profile

Two different hydrophilic moieties, carboxyl derivatives of monosaccharidic (Glc, Gal, Man, Fuc) glycosides and methoxypolyethylene glycols of varying MW, were covalently attached to the insulin GlyA1, PheB1 and/or LysB29 amino groups (seven possible derivatives), and resulting insulin conjugates purified to homogeneity. In vivo bioactivity in rats of most derivatives was preserved while disubstituted PEG-insulins showed decreased potency. Only site-specific modification of PheB1 amino group with either moiety resulted in pronouncedly increased resistance of insulin to fibrillation, indicating that the B-chain N-terminus of the insulin molecule is mechanistically involved in the fibrillation process. Immunogenicity in vivo and in vitro of monoglycosylated insulins was comparable to that of insulin, diglycosylated insulins showed immunogenicity enhancement. Immunogenicity of PEG-insulins was significantly suppressed. PheB1-glycosylated insulins administered subcutaneously in dogs displayed extended action profiles, the most effective being PheB1-galactosylated insulin, resembling the pharmacodynamic response of intermediate-acting insulin preparations. The pharmacokinetic parameters of these insulin derivatives were not significantly different from that of insulin even though absorption and residence time and clearance were increased, providing some explanation for prolonged action profile. Lectin-specific binding as a retardation basis is not likely involved. In support of this, subcutaneously administered PheB1-PEG(600)-insulin showed an even more protracted action profile, suggesting that the basis of retardation is physical and nonspecific. This implies that by increasing PEG chain MW, further delay/prolongation of action can be achieved to yield new soluble basal insulin substitutes with potential clinical applications.

Probing residue-level unfolding during lysozyme precipitation

We have employed nuclear magnetic resonance (NMR) measurements of hydrogen exchange to identify residue-level conformational changes in hen egg white lysozyme (HEWL) as induced by salt precipitation. Deuterated HEWL was dissolved into a phosphate (H2O) buffer and precipitated at pH 2.1 upon addition of solid KSCN or (ND4)2SO4, allowing isotope labeling of unfolded regions. After 1 h, each precipitate was then dissolved at pH 3.8 to initiate refolding and preserve labeling and subsequently purified for NMR analysis. HEWL precipitated by 1.0 M KSCN exhibited increased hydrogen exchange at 14 residues out of 42 normally well-protected in the native state. Of the affected residues, 9 were situated in the beta-sheet/loop domain. A similar, though less extensive, effect was observed at 0.2 M KSCN. Precipitation by 1.2 M (ND4)2SO4 resulted in none of the changes detected with KSCN. The popularity of ammonium sulfate as a precipitant is thus supported by this observed preservation of structural integrity. KSCN, in comparison, produced partial unfolding of specific regions in HEWL due most likely to known preferential interactions between -SCN and proteins. The severity of unfolding increased with KSCN concentration such that, at 1.0 M KSCN, almost the entire beta-sheet/loop domain of HEWL was disrupted. Even so, a portion of the HEWL core encompassed by three alpha-helices remained intact, possibly facilitating precipitate dissolution.

New strategies for the microencapsulation of tetanus vaccine

The progress toward the development of a single dose tetanus vaccine has been limited by the poor stability of the protein antigen, tetanus toxoid (TT), during its encapsulation in, and release from, biodegradable polymer microspheres. To investigate alternative microencapsulation approaches that may improve the stability of TT under these conditions, a two-step microencapsulation method has been devised to form microcapsules which consist of: (a) forming microcores of TT in a hydrophilic support matrix by spray-congealing, followed by (b) coating the microcores with poly(lactide-co-glycolide) (PLGA) by an oil-in-oil solvent extraction method. Several protein stabilizers including gelatin (with or without poloxamer 188), dextran, sodium glutamate, and polyethylene glycol were examined as potential core-materials. Among them, gelatin was superior in its ability to impart stability to TT against heat and moisture-induced inactivation. Microcores of this latter stabilizer and TT were encapsulated in PLGA using the foregoing technique, which exposed the dry antigen to minimal water in order to prevent its irreversible inactivation during exposure to the organic solvent. The microencapsulation method resulted in minimal loss of antigenically active TT (approximately 10-20%). Microscopic analysis of the microcapsules following preparation showed the microcores to be fully encapsulated. However, microcapsules containing TT and gelatin released the active antigen nearly completely within one day. Fluorescence confocal microscopy revealed that the swelling of the hydrophilic core-material was responsible for the burst-release behaviour. Manipulation of the polymer coating could not slow down this 'explosion' of the microcapsules. TT-containing PLGA microcapsules have been prepared using a novel microencapsulation method, which retains an extremely high fraction of antigenically active TT. Hence, these mechanistic approaches may be useful in the development of effective single-dose vaccines.

Long-term and high-temperature storage of supercritically-processed microparticulate protein powders

PURPOSE

The long-term and high-temperature storage of dry, micron-sized particles of lysozyme, trypsin, and insulin was investigated. Subsequent to using supercritical carbon dioxide as an antisolvent to induce their precipitation from a dimethylsulfoxide solution, protein microparticles were stored in sealed containers at -25, -15, 0, 3, 20, 22, and 60 degrees C. The purpose of this study was to investigate the suitability of supercritical antisolvent precipitation as a finishing step in protein processing.

METHODS

Karl Fisher titrations were used to determine the residual moisture content of commercial and supercritically-processed protein powders. The secondary structure of the dry protein particles was determined periodically during storage using Raman spectroscopy. The proteins were also redissolved periodically in aqueous buffers and assayed spectrophotometrically for biological activity and by circular dichroism for structural conformation in solution.

RESULTS

Amide I band Raman spectra indicate that the secondary structure of the protein particles, while perturbed from that of the solution state, remained constant in time, regardless of the storage temperature. The recoverable biological activity upon reconstitution for the supercritically-processed lysozyme and trypsin microparticles was also preserved and found to be independent of storage temperature. Far UV circular dichroism spectra support the bioactivity assays and further suggest that adverse structural changes, with potential to hinder renaturation upon redissolution, do not take place during storage.

CONCLUSIONS

The present study suggests that protein precipitation using supercritical fluids may yield particles suitable for long-term storage at ambient conditions.

Toward understanding insulin fibrillation

Formation of insulin fibrils is a physical process by which partially unfolded insulin molecules interact with each other to form linear aggregates. Shielding of hydrophobic domains is the main driving force for this process, but formation of intermolecular beta-sheet may further stabilize the fibrillar structure. Conformational displacement of the B-chain C-terminal with exposure of nonpolar, aliphatic core residues, including A2, A3, B11, and B15, plays a crucial role in the fibrillation process. Recent crystal analyses and molecular modeling studies have suggested that when insulin fibrillates this exposed domain interacts with a hydrophobic surface domain formed by the aliphatic residues A13, B6, B14, B17, and B18, normally buried when three insulin dimers form a hexamer. In rabbit immunization experiments, insulin fibrils did not elicit an increased immune response with respect to formation of IgG insulin antibodies when compared with native insulin. In contrast, the IgE response increased with increasing content of insulin in fibrillar form. Strategies and practical approaches to prevent insulin from forming fibrils are reviewed. Stabilization of the insulin hexameric structure and blockage of hydrophobic interfaces by addition of surfactants are the most effective means of counteracting insulin fibrillation.

Host cell properties and external pH affect proinsulin production by Saccharomyces yeast

The expression of a hybrid gene encoding an alpha-factor prepro leader peptide-miniproinsulin (MPI) fusion [MPI is the same as the LysArg human insulin precursor described by Thim et al. (1986)] was tested in a series of isogenic yeast strains to investigate the influence of some genetic and physiological factors on heterologous production in yeast. We found that: (i) an MF alpha 1 gene disruption in haploid cells, as well as MF alpha 1 gene product expression in diploid cells, do not affect the MPI secretion level; (ii) under conditions of exogenous leucine availability, MPI production is hindered by leucine auxotrophy (a leu2 mutation); (iii) rho- mutations increase the per-cell MPI yield approximately three-fold; (iv) the MPI yield is apparently dependent on the pH of the culture medium: the higher the external pH, the larger the per-cell MPI yield.

Physical stabilization of insulin by glycosylation

The modification of human insulin by the covalent attachment of monosaccharide moieties to insulin amino group(s) alters the aggregation and self-association behavior, improving both the pharmaceutical stability and biological response. The synthesis of p-succinamidophenyl glucopyranoside-insulin conjugate(s) (SAPG-insulin) has resulted in seven possible glucosylated insulin derivatives (three monosubstituted, three disubstituted, and one trisubstituted). These derivatives were isolated and purified using ion exchange chromatography. Characterization of the derivatives includes determining the site and number of sugar groups attached for each individual derivative and an evaluation of biological activity. Nearly all the derivatives retained in vivo biological activity comparable to insulin. In addition, extensive physicochemical characterization of the glucosylated insulin derivatives was undertaken to determine association/aggregation properties using GPC, dynamic light scattering, UV/Vis, and CD spectroscopy. Protein self-association was most suppressed with the disubstituted derivatives, especially those substituted on PheB1, and the trisubstituted derivative. The same general pattern was observed for physical stability of glucosylated insulin derivatives. As the number of glucosyl moieties attached to insulin increased, solution physical stability dramatically improved. Yet, the most significant impact to stability was glycosylation at the PheB1 site.

Approaches to analysis of aggregates and demonstrating mass balance in pharmaceutical protein (basic fibroblast growth factor) formulations

Denaturation, aggregation, and precipitation, which are common events in protein aging, limit the development of pharmaceutical protein formulations. Using the example of basic fibroblast growth factor (bFGF), we describe a systematic approach for quantitative recovery of soluble and insoluble aggregates in aged samples to achieve mass balance in three analytical methods, UV spectroscopy, size exclusion HPLC (HP-SEC), and reverse phase HPLC. Soluble aggregates were evaluated by UV spectroscopy and HP-SEC; the latter method was modified to include 2 M guanidine hydrochloride (GnHCl) in the mobile phase in order to differentiate and simultaneously analyze native and denatured protein. Insoluble aggregates were first solubilized with GnHCl and then recovered quantitatively with the modified HP-SEC method. Chaotrope treatment did not affect the UV peak absorbance but altered the HPLC profiles. The changes were consistent with dissociation of disulfide-linked multimers to monomers with an intramolecular disulfide linkage. This phenomenon was used for estimation of the monomer-multimer distribution in the untreated aggregated sample. These methods established approaches for quantitative recovery and characterization of bFGF aggregates.

Secondary structure characterization of microparticulate insulin powders

The secondary structure content of microparticulate insulin powders produced by the supercritical antisolvent (SAS) precipitation technique was investigated via Raman spectroscopy. Precipitate samples were generated at 25 and 35 degrees C processing temperatures. Both precipitate samples gave amide I band spectra that were shifted roughly +10 cm-1 relative to the commercial powder. The corresponding secondary structure estimates had significantly increased beta-sheet contents with concomitant decreases in alpha-helix contents relative to the commercial protein; the sum of beta-turn and random coil content remained essentially unchanged. The magnitude of the perturbation was slightly greater for the 35 degrees C sample. Dissolution of the commercial powder and precipitates in 0.01 M HCl yielded solution structure estimates similar to that of the commercial powder. An analysis of insulin in dimethyl sulfoxide, the suspending solvent in the SAS process, indicated that some, but not all, of the structural change observed for the precipitate samples may be attributed to solvent exposure. These results are suggestive of extensive beta-sheet-mediated intermolecular interactions in precipitate states, consistent with analyses of irreversible protein aggregate/fibril states. Interestingly, unlike irreversible protein aggregates, the insulin powders recover essentially full biological activity on reconstitution.

Probing the conformation of protein (bFGF) precipitates by fluorescence spectroscopy

Aggregation and precipitation are major events in the handling and aging of most protein pharmaceuticals. We demonstrate the utility of fluorescence spectroscopy in determining protein conformation in precipitates using basic fibroblast growth factor (bFGF) as an example. Conversion of the native to the soluble denatured from by chaotropes was accompanied by an increase in tryptophan emission. The emission spectra of resuspended precipitates were as reproducible as the spectra of the soluble form. The sum of emission spectra of native soluble bFGF and denatured precipitated bFGF was superimposable on the spectrum of the unfractionated suspension, suggesting that quantitative analysis of denatured aggregates in turbid protein formulations is possible. The ratio of tryptophan to tyrosine emissions increased with increasing extent of denaturation both in solution and in suspension. For example, salting out by ammonium sulphate increased the fluorescence index (indicative of denaturation) which was reversible upon dissolution. In addition, aging (35 degrees C) of bFGF in the presence of sulphated ligands produced precipitates with native-like fluorescence index, in contrast to denatured precipitates formed without ligands.

Insulin structure and stability

Insulin is composed of 51 amino acids in two peptide chains (A and B) linked by two disulfide bonds. The three-dimensional structure of the insulin molecule (insulin monomer), essentially the same in solution and in solid phase, exists in two main conformations. These differ in the extent of helix in the B chain which is governed by the presence of phenol or its derivatives. In acid and neutral solutions, in concentrations relevant for pharmaceutical formulation, the insulin monomer assembles to dimers and at neutral pH, in the presence of zinc ions, further to hexamers. Many crystalline modifications of insulin have been identified but only those with the hexamer as the basic unit are utilized in preparations for therapy. The insulin hexamer forms a relatively stable unit but some flexibility remains within the individual molecules. The intrinsic flexibility at the ends of the B chain plays an important role in governing the physical and chemical stability of insulin. A variety of chemical changes of the primary structure (yielding insulin derivatives), and physical modifications of the secondary to quaternary structures (resulting in "denaturation," aggregation, and precipitation) are known to affect insulin and insulin preparations during storage and use (Fig. 8). The tendency of insulin to undergo structural transformation resulting in aggregation and formation of insoluble insulin fibrils has been one of the most intriguing and widely studied phenomena in relation to insulin stability. Although the exact mechanism of fibril formation is still obscure, it is now clear that the initial step is an exposure of certain hydrophobic residues, normally buried in the three-dimensional structure, to the surface of the insulin monomer. This requires displacement of the COOH-terminal B-chain residues from their normal position which can only be accomplished via monomerization of the insulin. Therefore, most methods stabilizing insulin against fibrillation share the property of being able to counteract associated insulin from being disassembled. Chemical deterioration of insulin during storage of pharmaceutical preparations is mainly due to two categories of chemical reactions, hydrolysis and intermolecular transformation reactions leading to insulin HMWT products. The predominant hydrolysis reaction is deamidation of Asn residues which in acid solution takes place at residue A21, in neutral medium at residue B3. An amazing hydrolytic cleavage of the backbone A chain, presumably autocatalyzed by an adjacent insulin molecule, has been identified in insulin preparations containing rhombohedral crystals in combination with free zinc ions.(ABSTRACT TRUNCATED AT 400 WORDS)