Sweeping of Adsorbed Therapeutic Protein on Prefillable Syringes Promotes Micron Aggregate Generation

Impact of Post Manufacturing Handling of Protein-Based Biologic Drugs on Product Quality and User Centricity

This article evaluates the current gaps around the impact of post-manufacturing processes on the product qualities of protein-based biologics, with a focus on user centricity. It includes the evaluation of the regulatory guidance available, describes a collection of scientific literature and case studies to showcase the impact of post-manufacturing stresses on product and dosing solution quality. It also outlines the complexity of clinical handling and the need for communication, and alignment between drug providers, healthcare professionals, users, and patients. Regulatory agencies provide clear expectations for drug manufacturing processes, however, guidance supporting post-product manufacturing handling is less defined and often misaligned. This is problematic as the pharmaceutical products experience numerous stresses and processes which can potentially impact drug quality, safety and efficacy. This article aims to stimulate discussion amongst pharmaceutical developers, health care providers, device manufacturers, and public researchers to improve these processes. Patients and caregivers' awareness can be achieved by providing relevant educational material on pharmaceutical product handling.

Characterizing Silicone Oil-Induced Protein Aggregation with Stimulated Raman Scattering Imaging

Particles in biopharmaceutical products present high risks due to their detrimental impacts on product quality and safety. Identification and quantification of particles in drug products are important to understand particle formation mechanisms, which can help develop control strategies for particle formation during the formulation development and manufacturing process. However, existing analytical techniques such as microflow imaging and light obscuration measurement lack the sensitivity and resolution to detect particles with sizes smaller than 2 μm. More importantly, these techniques are not able to provide chemical information to determine particle composition. In this work, we overcome these challenges by applying the stimulated Raman scattering (SRS) microscopy technique to monitor the C-H Raman stretching modes of the proteinaceous particles and silicone oil droplets formed in the prefilled syringe barrel. By comparing the relative signal intensity and spectral features of each component, most particles can be classified as protein-silicone oil aggregates. We further show that morphological features are poor indicators of particle composition. Our method has the capability to quantify aggregation in protein therapeutics with chemical and spatial information in a label-free manner, potentially allowing high throughput screening or investigation of aggregation mechanisms.

Mechanism of Protein-PDMS Visible Particles Formation in Liquid Vial Monoclonal Antibody Formulation

Visible particles (VPs) formation in liquid monoclonal antibody formulations is a critical quality issue. Formulations that include poloxamer 188 (PX188) as a surfactant are prone to the formation of VPs comprising aggregated complexes of protein and polydimethylsiloxane (PDMS; silicone oil) derived from primary containers. However, the mechanisms through which these VPs form are complicated and remain to be fully elucidated. This study demonstrates for the first time the dominant spot and pathway of protein-PDMS VP formation in a particular liquid vial formulation. Specifically, when a vial sealed with a PDMS-coated stopper is stored in an upright position under conditions whereby the antibody solution has become well-adhered to the stopper and an air phase exists in the vicinity, protein-PDMS aggregates form on the stopper and are then desorbed into the drug solution to be detected as VPs. Here, we evaluated the effects of several factors on VP formation: adhesion of the drug solution to the stopper, storage orientation, silicone coating on the stopper, vial material, and hydrophobicity of PX188. Remarkably, we found that changing any one of the factors could significantly affect VP formation. Our findings are instructive for better understanding the mechanisms of VP formation in vial products and can provide strategies for VP mitigation in biotherapeutics.

Afraid of the wall of death? Considerations on monoclonal antibody characteristics that trigger aggregation during peristaltic pumping

Protein aggregation is of major concern in manufacturing of biopharmaceutics. Protein aggregation upon peristaltic pumping for filtration, transfer or filling is triggered by protein adsorption to the tubing surface and subsequent film rupture during roller movement. While the impact of tubing type and formulation has been studied in more detail, the contribution of the protein characteristics is not fully resolved. We studied the aggregation propensity of six monoclonal antibodies during peristaltic pumping and characterized their colloidal and conformational stability, hydrophobicity, and surface activity. A high affinity to the surface resulting in faster adsorption and film renewal was key for the formation of protein particles ≥ 1 µm. Film formation and renewal were influenced by the antibody hydrophobicity, potential for electrostatic self-interaction and conformational stability. The initial interfacial pressure increase within the first minute can serve as a good predictor for antibody adsorption and particle formation propensity. Our results highlight the complexity of protein adsorption and emphasize the importance of formulation development to reduce protein particle formation by avoidance of adsorption to interfaces.

Surface-Induced Protein Aggregation and Particle Formation in Biologics: Current Understanding of Mechanisms, Detection and Mitigation Strategies

Protein stability against aggregation is a major quality concern for the production of safe and effective biopharmaceuticals. Amongst the different drivers of protein aggregation, increasing evidence indicates that interactions between proteins and interfaces represent a major risk factor for the formation of protein aggregates in aqueous solutions. Potentially harmful surfaces relevant to biologics manufacturing and storage include air-water and silicone oil-water interfaces as well as materials from different processing units, storage containers, and delivery devices. The impact of some of these surfaces, for instance originating from impurities, can be difficult to predict and control. Moreover, aggregate formation may additionally be complicated by the simultaneous presence of interfacial, hydrodynamic and mechanical stresses, whose contributions may be difficult to deconvolute. As a consequence, it remains difficult to identify the key chemical and physical determinants and define appropriate analytical methods to monitor and predict protein instability at these interfaces. In this review, we first discuss the main mechanisms of surface-induced protein aggregation. We then review the types of contact materials identified as potentially harmful or detected as potential triggers of proteinaceous particle formation in formulations and discuss proposed mitigation strategies. Finally, we present current methods to probe surface-induced instabilities, which represent a starting point towards assays that can be implemented in early-stage screening and formulation development of biologics.

Utility of Three Flow Imaging Microscopy Instruments for Image Analysis in Evaluating four Types of Subvisible Particle in Biopharmaceuticals

Subvisible particles (SVPs) are a critical quality attribute of parenteral and ophthalmic products. United States Pharmacopeia recommends the characterizations of SVPs which are classified into intrinsic, extrinsic, and inherent particles. Flow imaging microscopy (FIM) is useful as an orthogonal method in both the quantification and classification of SVPs because FIM instruments provide particle images. In addition to the conventionally used FlowCam (Yokogawa Fluid Imaging Technologies) and Micro-Flow Imaging (Bio-Techne) instruments, the iSpect DIA-10 (Shimadzu) instrument has recently been released. The three instruments have similar detection principles but different optical settings and image processing, which may lead to different results of the quantification and classification of SVPs based on the information from particle images. The present study compares four types of SVP (protein aggregates, silicone oil droplets, and surrogates for solid free-fatty-acid particles, milled-lipid particles, and sprayed-lipid particles) to compare the results of size distributions and classification abilities obtained using morphological features and a deep-learning approach. Although the three FIM instruments were effective in classifying the four types of SVP through convolutional neural network analysis, there was no agreement on the size distribution for the same protein aggregate solution, suggesting that using the classifiers of the FIM instruments could result in different evaluations of SVPs in the field of biopharmaceuticals.

Proteins on the rack: Mechanistic studies on protein particle formation during peristaltic pumping

Peristaltic pumping can cause protein particle formation. The expected causes were unfolding by heat in the pump head, oxidative stress by cavitation generated during roller movement, interfacial adsorption to the tubing wall and mechanical stress by stretching of the tubing itself. The pump head reached 28°C during experiments stayed well below the onset of the melting point of the proteins. Thus, heat may only be a relevant root cause for proteins containing domains with very low unfolding temperature. Analysis by terephthalic acid dosimetry and protein oxidation via RP-HPLC ruled out major induction of reactive hydroxyl radicals by pumping, indicating that cavitation does not play a significant role in particle generation. Addition of surfactants suppresses protein adsorption to the tubing wall and drastically reduced protein particle formation. This indicates that interfacial protein adsorption is a key element. Repeated stretching of tubing filled with protein solution led to the formation of protein particles, demonstrating that expansion and compression of the protein film on the tubing surface is the second key component for particle formation. Thus, protein particle generation during peristaltic pumping originates from the formation of a protein film on the tubing surface which gets stretched and compressed, leading to film fragments entering the bulk solution. This interplay of protein film formation and its rupture has been also observed at liquid/liquid or liquid/air interfaces.

Particles in Biopharmaceutical Formulations, Part 2: An Update on Analytical Techniques and Applications for Therapeutic Proteins, Viruses, Vaccines and Cells

Particles in biopharmaceutical formulations remain a hot topic in drug product development. With new product classes emerging it is crucial to discriminate particulate active pharmaceutical ingredients from particulate impurities. Technical improvements, new analytical developments and emerging tools (e.g., machine learning tools) increase the amount of information generated for particles. For a proper interpretation and judgment of the generated data a thorough understanding of the measurement principle, suitable application fields and potential limitations and pitfalls is required. Our review provides a comprehensive overview of novel particle analysis techniques emerging in the last decade for particulate impurities in therapeutic protein formulations (protein-related, excipient-related and primary packaging material-related), as well as particulate biopharmaceutical formulations (virus particles, virus-like particles, lipid nanoparticles and cell-based medicinal products). In addition, we review the literature on applications, describe specific analytical approaches and illustrate advantages and drawbacks of currently available techniques for particulate biopharmaceutical formulations.

Chemical-gas Sterilization of External Surface of Polymer-based Prefilled Syringes and Its Effect on Stability of Model Therapeutic Protein

To reduce the risk of infection during intravitreal injections, the external surface of prefilled syringes (PFSs) must be sterilized. Usually, ethylene oxide (EO) gas or vaporized hydrogen peroxide (VHP) is used for sterilization. More recently, nitrogen dioxide (NO₂) gas sterilization has been developed. It is known that gas permeability is approximately zero into glass-PFSs. However, polymer-PFSs (P-PFSs) have relatively high gas permeability. Therefore, there are concerns about the potential impact of external surface sterilization on drug solutions in P-PFSs. In this study, P-PFSs [filled with water for injection (WFI) or human serum albumin (HSA) solution] were externally sterilized using EO, VHP, and NO₂ gases. For the WFI-filled syringes, the concentration of each gas that ingressed into the WFI was measured. For the HSA solution-filled syringes, the physical and chemical degradation of HSA molecules by each sterilant gas was quantified. For the EO- or VHP-sterilized syringes, the ingressed EO or hydrogen peroxide (H₂O₂) molecules were detected in the filled WFI. Additionally, EO-adducted or oxidized HSA molecules were observed in the HSA-filled syringes. In contrast, the NO₂-sterilized WFI-filled syringes exhibited essentially immeasurable ingressed NO₂, and protein degradation was not detected in HSA-filled syringes.

Plate Reader-Based Analytical Method for the Size Distribution of Submicron-Sized Protein Aggregates Using Three-Dimensional Homodyne Light Detection

The assessment of aggregates is essential in biopharmaceutical development. Although submicron-sized aggregates are considered to have a potential immunogenicity risk, analytical techniques are limited. In this study, we present a new analytical technique using three-dimensional homodyne light detection (3D-HLD). In this system, submicron-sized particles are quantified by combining the reflected light detection of each particle by high-speed 3D scan and then enhancing the amplitude of the reflected light using HLD. The particle concentrations and size distributions of human tetanus immune globulin (TIG) aggregates generated by stirring were measured using 3D-HLD. Both concentrations and distributions were comparable to those obtained via resonant mass measurement (RMM), a technique commonly used for submicron-sized particle measurement. Aiming at feasibility assessment of 3D-HLD for the high-through-put formulation development, 30 formulations of TIG and rituximab under agitation stress were analyzed by 3D-HLD. The results showed that 3D-HLD can automatically and simultaneously assess the aggregate concentrations and size distributions of at least 90 samples. This study demonstrates that 3D-HLD can be used for submicron-sized aggregate analysis as an orthogonal method to RMM and also as a screening tool during formulation development.

Influence of Protein Adsorption on Aggregation in Prefilled Syringes

Protein aggregate formation in prefilled syringes (PFSs) can be influenced by protein adsorption and desorption at the solid-liquid interface. Although inhibition of protein adsorption on the PFS surface can lead to a decrease in the amount of aggregation, the mechanism underlying protein adsorption-mediated aggregation in PFSs is unclear. This study investigated protein aggregation caused by protein adsorption on silicone oil-free PFS surfaces [borosilicate glass (GLS) and cycloolefin polymer (COP)] and the factors affecting the protein adsorption on the PFS surfaces. The adsorbed proteins formed multilayered structures that consisted of two distinct types of layers: proteins adsorbed on the surface of the material and proteins adsorbed on top of the proteins on the surface. A pH-dependent electrostatic interaction was the dominant force for protein adsorption on the GLS surface, while hydrophobic effects were dominant for protein adsorption on the COP surface. When the repulsion force between proteins was weak, protein adsorption on the adsorbed protein layer was increased for both materials and as a result, protein aggregation increased. Therefore, a formulation with high colloidal stability can minimize protein adsorption on the COP surface, leading to reduced protein aggregation.

Culturing Chinese hamster ovary cells on cyclo olefin polymer triggers epithelial-mesenchymal transition and spheroid formation, which increases the foreign gene expression driven by the Moloney murine leukemia virus long terminal repeat promoter

Chinese hamster ovary (CHO) cells are frequently used for recombinant protein production (RPP) as a host. While the RPP has been proven successful, there is still a compelling need for further improvement. Cyclo olefin polymer (COP) is a plastic material widely utilized due to its properties including its low protein absorption. We applied this as a raw material for RPP cell culture to see if the COP is suitable. A recombinant CHO cell line expressing the human erythropoietin (hEPO) gene under the control of the Moloney murine leukemia virus-long terminal repeat (MMLV-LTR) was established. When the cells were cultured in a dish made from COP, the cells attached to the bottom, and then started to float and form spheroids. RNASeq data analysis suggested the epithelial-mesenchymal transition (EMT) was triggered with receptor tyrosine kinase activation shortly after cultivation. It coincided with the hEPO transcription increase. After the cell floating, though EMT marker gene expression subsided, a hEPO expression increase sustained. When fibronectin was applied to COP dish surface, the cell floating was suppressed and hEPO expression decreased. We then treated cells with MβCD, a drug that destroys the lipid raft, eliminating molecules in the raft. This facilitated cell floating and spheroid formation coincided with hEPO expression enhancement. These results suggest interactions between a cell and COP surface might trigger the EMT and the subsequent event, both of which activated the MMLV-LTR promoter. Thus, employing COP for culturing cells, a potent RPP system could be established with its advantage for efficient protein purification.

Characterization of Aggregated Antibody-Silicone Oil Complexes: From Perspectives of Morphology, 3D Image, and Fcγ Receptor Activation

Pre-filled syringes (PFS) have been in widespread use as an administration device for therapeutic antibodies in recent decades. Generally, the inner barrel and syringe of PFS are coated with silicone oil (SO) for lubrication. Multiple studies have focused on the fact that the SO adsorbs denatured antibody molecules, and induces antibody aggregation. Aggregated antibodies are recognized as a potential risk for evoking immunogenic responses in patients. The characteristics of the aggregated antibody-SO complexes, including their concentration, population, shape, three-dimensional (3D) image, and Fcγ Receptors (FcγRs) activation have been obscurely acknowledged so far. In the present work, we prepared aggregated antibody-SO complexes by agitation and analyzed using multifaceted techniques such as flow imaging, confocal fluorescence microscopy, and cell-based assays for FcγRs activation. The results emphasized that the SO accelerates the increase in sub-visible particles and antibody aggregation. The confocal fluorescence microscopy analysis revealed the high-resolution 3D images of aggregated antibody-SO complexes. The FcγRs reporter cell assay clarified that the pre-mixed and agitated Ab + SO have higher FcγRs activation capability compared to the agitated Ab. Overall, this study advances the view that SO has an effect to increase the risk of agitation-induced aggregated antibody particles.

Can Cross-Linked Siliconized PFS Come to the Rescue of the Biologics Drug Product?

Silicone can present a challenge during the development of a biologics drug product particularly in pre-filled syringe (PFS). Due to silicone related challenges, substantial changes in components and manufacturing of the PFS are being sought. Cross-linking of the silicone being one of them, can help reduce mobilization of the silicone into drug product whilst retaining its functionality of lubrication during injection. In this work, we systematically compare the stability of a fusion protein and monoclonal antibody formulation in conventionally siliconized and cross-linked siliconized PFS available from commercial manufacturers. The two types of syringes did not influence the aggregation profile of proteins as determined by HP-SEC. Compared to conventionally siliconized PFS, a cross-linked siliconized PFS can have a favorable or indifferent impact (depending on vendor) on the sub-visible particles profile as assessed by light obscuration and microflow imaging. The different PFS after 24 months of long-term storage showed comparable functionality attributes like break-loose/gliding force and silicone oil distribution. Cross-linked siliconized PFS can offer an incremental advantage over conventionally siliconized PFS for the moderately concentrated protein solutions, however the differences in the quality of these PFS amongst manufacturers is an important aspect that needs to be considered as shown in this study.

Impact of Surfactants on the Functionality of Prefilled Syringes

Previous studies revealed the impact of formulation factors (excipients and pH) on the functionality of prefilled syringes. Surfactant, a critical formulation component for therapeutic proteins and antibodies, aids in minimizing protein adsorption onto interfaces and reduces protein aggregation or particulate formation. This study evaluated the impact of different surfactants and protein concentration on the functionality of prefilled syringes. Syringes filled with solution formulations with different surfactants were stored at various temperatures and evaluated at selected time points. Upon thermal stress, polysorbate 80 and dodecyl-β-d-maltoside containing formulations showed significantly greater increase in glide force when compared with poloxamer 407 containing formulations. In contrast, syringes filled with poloxamer 188 containing formulations did not show any increase in glide force under the same conditions. Based on the results from this study, the increase in syringe glide force was inversely correlated with hydrophobic-lipophilic balance values and surface tension of different surfactants. The mechanism of increase in glide force was primarily the change of silicone oil coverage and lubricity in the barrel of syringes.

Stirring rate affects thermodynamics and unfolding kinetics in isothermal titration calorimetry

Isothermal titration calorimetry (ITC) directly provides thermodynamic parameters depicting the energetics of intermolecular interactions in solution. During ITC experiments, a titration syringe with a paddle is continuously rotating to promote a homogeneous mixing. Here, we clarified that the shape of the paddles (flat, corkscrew and small-pitched corkscrew) and the stirring rates influence on the thermodynamic parameters of protein-ligand interaction. Stirring with the flat paddle at lower and higher rate both yielded a lower exothermic heat due to different reasons. The complete reaction with no incompetent fractions was achieved only when the stirring was performed at 500 or 750 rpm using the small-pitched corkscrew paddle. The evaluation of the protein solution after 1,500 rpm stirring indicated that proteins in the soluble fraction decreased to 94% of the initial amount, among which 6% was at an unfolded state. In addition, a significant increase of micron aggregates was confirmed. Furthermore, a new approach for the determination of the unfolding kinetics based on the time dependence of the total reaction heat was developed. This study demonstrates that a proper stirring rate and paddle shape are essential for the reliable estimation of thermodynamic parameters in ITC experiments.

Immunogenicity of Bioproducts: Cellular Models to Evaluate the Impact of Therapeutic Antibody Aggregates

Patients treated with bioproducts (BPs) frequently develop anti-drug antibodies (ADAs) with potential neutralizing capacities leading to loss of clinical response or potential hypersensitivity reactions. Many factors can influence BP immunogenicity and could be related to the patient, the treatment, as well as to the product itself. Among these latter factors, it is now well accepted that BP aggregation is associated with an increased potential for immunogenicity, as aggregates seem to be correlated with ADA development. Moreover, the presence of high-affinity ADAs suggests a CD4 T-cell dependent adaptive immune response and therefore a pivotal role for antigen-presenting cells (APCs), such as dendritic cells (DCs). In this review, we address the in vitro methods developed to evaluate how monoclonal antibodies could trigger the immunization process by focusing on the role of aggregated antibodies in the establishment of this response. In particular, we will present the different cell-based assays that have been used to assess the potential of antibodies and their aggregates to modulate cellular mechanisms leading to activation and the biological parameters (cellular activation markers, proliferation and secreted molecules) that can be measured to evaluate the different cell activation stages and their consequences in the propagation of the immune response. Indeed, the use of such strategies could help evaluate the risk of BP immunogenicity and their role in mitigating this risk.

Critical analysis of techniques and materials used in devices, syringes, and needles used for intravitreal injections

Intravitreal injections have become the most commonly performed intraocular treatments worldwide. Because intravitreal injections may induce severe adverse events, such as infectious and noninfectious endophthalmitis, cataract, ocular hypertension, vitreous hemorrhage, or retinal detachment, appropriate awareness of the materials and techniques used are essential to reduce these sight-threatening complications. This review provides insights into the needles, syringes, silicone oil coating, sterilization methods, devices to assist intravitreal injections, scleral piercing techniques using needles, syringe handling, anesthesia, and safety issues related to materials and techniques. It is paramount that physicians be aware of every step involved in intravitreal injections and consider the roles and implications of all materials and techniques used. The ability to understand the theoretical and practical circumstances may definitely lead to state-of-the-art treatments delivered to patients. The most important practical recommendations are: choosing syringes with as little silicone oil as possible, or, preferably, none; avoiding agitation of syringes; awareness that most biologics (e.g., antiangiogenic proteins) are susceptible to changes in molecular properties under some conditions, such as agitation and temperature variation; understanding that improper materials and techniques may lead to complications after intravitreal injections, e.g., inflammation; and recognizing that some devices may contribute to an enhanced, safer, and faster intravitreal injection technique.

Parameters Influencing Cavitation Within Vials Subjected to Drop Shock

The pharmaceutical industry has made improvements to mitigate protein degradation during the drug manufacturing process, storage, and transportation. However, there is less quality control after the manufacturer releases the drug. Previous research has shown that drop shock due to mishandling and accidental dropping of therapeutic vials may cause cavitation, aggregation, and particle formation. In this study, the cavitation behavior of Deionized (DI) water and 10mM L-Histidine buffer solution which were subjected to drop shock by hand dropping were investigated to study the effects of vial materials, solutions, fill volumes, drop heights, and internal vial geometries. A Phantom v7 high-speed camera was used to record images at a rate of 66,700 frames per second of the vials as they underwent drop shock. These videos were then reviewed to find the angle of impact, and to determine if there was cavitation. The results indicate that decreasing fill height by using a smaller fill volume or larger diameter vials were found to mitigate cavitation across drop heights. Secondly, results indicate there is a significant difference between the cavitation behavior of glass and plastic vials, and plastic had more cavitation cases. Lastly, there was not a significant difference in the occurrence of cavitation between DI water and L-Histidine buffer solution.

Science and art of protein formulation development

Protein pharmaceuticals have become a significant class of marketed drug products and are expected to grow steadily over the next decade. Development of a commercial protein product is, however, a rather complex process. A critical step in this process is formulation development, enabling the final product configuration. A number of challenges still exist in the formulation development process. This review is intended to discuss these challenges, to illustrate the basic formulation development processes, and to compare the options and strategies in practical formulation development.

[Toward the Proper Biophysical Characterization of Aggregates in Biopharmaceuticals]

Biopharmaceuticals are often formulated as liquid dosage forms. During manufacturing and storage, protein molecules and active pharmaceutical ingredients form aggregates due to various stresses, including shaking and agitation, as well as by contact with silicone oils coated on pre-fillable syringes. The diameter of protein aggregates ranges from 15-20 nm, and that of dimers comprising a large number of antibody molecules can be up to 100 μm. Among these aggregates, those with a diameter of <100 nm are called nanometer aggregates, while those ranging between 100 nm and 1 μm are called sub-micron aggregates, and those ranging between 1 and 100 μm are called micron aggregates. In the last ten years, aggregates have been studied to determine their physical characteristics and their impact on immunogenicity. As a result, novel analytical methods and instruments for such characterizations have been established for a majority of aggregates, including those that are difficult to evaluate. Here, the biophysical features of protein aggregates are explained, followed by an introduction to the different methods for aggregate characterization, including their advantages and actual results. Finally, future perspectives and expectations regarding the characterization of protein aggregates are proposed.

Steric Repulsion Forces Contributed by PEGylation of Interleukin-1 Receptor Antagonist Reduce Gelation and Aggregation at the Silicone Oil-Water Interface

Silicone oil, used as a lubricating coating in pharmaceutical containers, has been implicated as a cause of therapeutic protein aggregation. After adsorbing to silicone oil-water interfaces, proteins may form interfacial gels, which can be transported into solution as insoluble aggregates if the interfaces are perturbed. Mechanical interfacial perturbation of both monomeric recombinant human interleukin-1 receptor antagonist (rhIL-1ra) and PEGylated rhIL-1ra (PEG rhIL-1ra) in siliconized syringes resulted in losses of soluble monomeric protein. However, the loss of rhIL-1ra was twice that for PEG rhIL-1ra; even though in solution, PEG rhIL-1ra had a lower ΔG_unf and exhibited a more perturbed tertiary structure at the interface. Net protein-protein interactions in solution for rhIL-1ra were attractive but increased steric repulsion because of PEGylation led to net repulsive interactions for PEG rhIL-1ra. Attractive interactions for rhIL-1ra were associated with increases in intermolecular β-sheet content at the interface, whereas no intermolecular β-sheet structures were observed for adsorbed PEG rhIL-1ra. rhIL-1ra formed interfacial gels that were 5 times stronger than those formed by PEG rhIL-1ra. Thus, the steric repulsion contributed by the PEGylation resulted in decreased interfacial gelation and in the reduction of aggregation, in spite of the destabilizing effects of PEGylation on the protein's conformational stability.