Photosensitizers Form in Histidine Buffer and Mediate the Photodegradation of a Monoclonal Antibody

Impact of photo-oxidation on long term storage of affinity chromatography media used in multi-specific antibody manufacturing processes

Large scale manufacture of complex biotherapeutic formats such as multi-specific antibodies can require development of custom biomanufacturing platforms, particularly for purification processes. Substantial advances in affinity chromatography media have allowed monoclonal antibody-like processes for these formats, and simplified process development to enable fast speed to the clinic. Thorough assessment of chromatography media performance and stability is critical to ensure robust operation and consistent product quality over repeated cycles throughout its lifetime. However, evaluation of repeated cycling and extended storage for chromatography media is resource consuming, which typically delays rigorous study to later development stages and often is acquired through increased operational experience. These areas can present quality risks if not properly understood. In this work, a class of affinity chromatography media employing camelid antibody-fragment ligands were evaluated for extended storage in benzyl alcohol solution as an alternative to ethanol storage. Initial laboratory studies revealed resin discoloration after 12 months of exposure to ambient light at room temperature. Resin photo-stress studies confirmed light exposure as the root cause, with benzyl alcohol storage conditions producing a substantially greater degree of discoloration compared to ethanol. Extreme photo-stress over the course of 7 days was also found to negatively impact resin dynamic binding capacities, with more severe declines observed with benzyl alcohol storage conditions. Binding capacity loss of 54 % was observed for photo-stressed CaptureSelect Kappa XP compared to control conditions. Addition of antioxidants reduced or eliminated resin discoloration during photo-stress, indicating that benzyl alcohol storage accelerates photo-oxidation of the affinity chromatography ligands. The addition of l-methionine to benzyl alcohol solution prevented resin discoloration and maintained a dynamic binding capacity of 41 g/L for CaptureSelect Kappa XP even after extreme photo-stress. Of practical importance, a study of LambdaFabSelect resin used and stored in benzyl alcohol solution, under recommended conditions (2-8 °C storage, protected from light) in an internal GMP facility over a period of three years, showed no impact to resin color, performance, or product quality.

Histidine as a versatile excipient in the protein-based biopharmaceutical formulations

Adequate stabilization is essential for marketed protein-based biopharmaceutical formulations to withstand the various stresses that can be exerted during the pre- and post-manufacturing processes. Therefore, a suitable choice of excipient is a significant step in the manufacturing of such delicate products. Histidine, an essential amino acid, has been extensively used in protein-based biopharmaceutical formulations. The physicochemical properties of histidine are unique among amino acids and could afford multifaceted benefits to protein-based biopharmaceutical formulations. With a pKa of approximately 6.0 at the side chain, histidine has been primarily used as a buffering agent, especially for pH 5.5-6.5. Additionally, histidine exhibited several affirmative properties similar to those of carbohydrates (e.g., sucrose and trehalose) and could therefore be considered to be an alternative approach to established protein-based formulation strategies. The current review describes the general physicochemical properties of histidine, lists all commercial histidine-containing protein-based biopharmaceutical products, and discusses a brief outline of the existing research focused on the versatile applications of histidine, which can act as a buffering agent, stabilizer, cryo-/lyo-protectant, antioxidant, viscosity reducer, and solubilizing agent. The interaction between histidine and proteins in protein-based biopharmaceutical formulations, such as the Donnan effect during diafiltration of monoclonal antibody solutions and the degradation of polysorbates in histidine buffer, has also been discussed. As the first review of histidine in protein biopharmaceuticals, it helps to deepen our understanding of the opportunities and challenges associated with histidine as an excipient for protein-based biopharmaceutical formulations.

Stability of Protein Pharmaceuticals: Recent Advances

There have been significant advances in the formulation and stabilization of proteins in the liquid state over the past years since our previous review. Our mechanistic understanding of protein-excipient interactions has increased, allowing one to develop formulations in a more rational fashion. The field has moved towards more complex and challenging formulations, such as high concentration formulations to allow for subcutaneous administration and co-formulation. While much of the published work has focused on mAbs, the principles appear to apply to any therapeutic protein, although mAbs clearly have some distinctive features. In this review, we first discuss chemical degradation reactions. This is followed by a section on physical instability issues. Then, more specific topics are addressed: instability induced by interactions with interfaces, predictive methods for physical stability and interplay between chemical and physical instability. The final parts are devoted to discussions how all the above impacts (co-)formulation strategies, in particular for high protein concentration solutions.'

Assessing Photostability of mAb Formulations In Situ Using Light-Coupled NMR Spectroscopy

Biopharmaceuticals, such as monoclonal antibodies (mAbs), need to maintain their chemical and physical stability in formulations throughout their lifecycle. It is known that exposure of mAbs to light, particularly UV, triggers chemical and physical degradation, which can be exacerbated by trace amounts of photosensitizers in the formulation. Although routine assessments of degradation following defined UV dosages are performed, there is a fundamental lack of understanding regarding the intermediates, transient reactive species, and radicals formed during illumination, as well as their lifetimes and immediate impact post-illumination. In this study, we used light-coupled NMR spectroscopy to monitor in situ live spectral changes in sealed samples during and after UV-A illumination of different formulations of four mAbs without added photosensitizers. We observed a complex evolution of spectra, reflecting the appearance within minutes of transient radicals during illumination and persisting for minutes to tens of minutes after the light was switched off. Both mAb and excipient signals were strongly affected by illumination, with some exhibiting fast irreversible photodegradation and others exhibiting partial recovery in the dark. These effects varied depending on the mAb and the presence of excipients, such as polysorbate 80 (PS80) and methionine. Complementary ex situ high-performance size-exclusion chromatography analysis of the same formulations post-UV exposure in the chamber revealed significant loss of purity, confirming formulation-dependent degradation. Both approaches suggested the presence of degradation processes initiated by light but continuing in the dark. Further studies on photoreaction intermediates and transient reactive species may help mitigate the impact of light on biopharmaceutical degradation.

Near UV light photo-degradation of histidine buffer: Mechanisms and role of Fe(III)

Pharmaceutical formulations are sensitive to light-induced degradation. Recent studies have attributed some of the light sensitivity to the presence of Fe(III), the most prevalent metal leachable from pharmaceutical containers. Histidine (His) can promote Fe(III) leaching from stainless steel, especially at elevated storage temperatures. Since there is the chance that combinations of His and Fe(III) are present in pharmaceutical formulations, we investigated the photo-degradation mechanisms of Fe(III)-containing His buffer during expsoure to near UV light. Our results indicate the formation of carbon dioxide radical anion (•CO2-), a powerful reductant, and other photoproducts such as aldehydes and His-derived radicals. The generation of •CO2- can be promoted by increasing concentrations of Fe(III) and inhibited by the addition of the Fe(III) chelator EDTA. Mechanistically, product formation can be rationalized by photo-induced ligand-to-metal-charge-transfer (LMCT), followed by a series of radical transformations of reaction intermediates.

Endogenous Photosensitizers in Human Skin

Endogenous photosensitizers play a critical role in both beneficial and harmful light-induced transformations in biological systems. Understanding their mode of action is essential for advancing fields such as photomedicine, photoredox catalysis, environmental science, and the development of sun care products. This review offers a comprehensive analysis of endogenous photosensitizers in human skin, investigating the connections between their electronic excitation and the subsequent activation or damage of organic biomolecules. We gather the physicochemical and photochemical properties of key endogenous photosensitizers and examine the relationships between their chemical reactivity, location within the skin, and the primary biochemical events following solar radiation exposure, along with their influence on skin physiology and pathology. An important take-home message of this review is that photosensitization allows visible light and UV-A radiation to have large effects on skin. The analysis presented here unveils potential causes for the continuous increase in global skin cancer cases and emphasizes the limitations of current sun protection approaches.

Primary Processes of Free Radical Formation in Pharmaceutical Formulations of Therapeutic Proteins

Oxidation represents a major pathway for the chemical degradation of pharmaceutical formulations. Few specific details are available on the mechanisms that trigger oxidation reactions in these formulations, specifically with respect to the formation of free radicals. Hence, these mechanisms must be formulated based on information on impurities and stress factors resulting from manufacturing, transportation and storage. In more detail, this article focusses on autoxidation, metal-catalyzed oxidation, photo-degradation and radicals generated from cavitation as a result of mechanical stress. Emphasis is placed on probable rather than theoretically possible pathways.

Protein photodegradation in the visible range? Insights into protein photooxidation with respect to protein concentration

Visible light (400-800 nm) can lead to photooxidation of protein formulations, which might impair protein integrity. However, the relevant mechanism of photooxidation upon visible light exposure is still unclear for therapeutic proteins, since proteinogenic structures do not absorb light in the visible range. Here, we show that exposure of monoclonal antibody formulations to visible light, lead to the formation of reactive oxygen species (ROS), which subsequently induce specific protein degradations. The formation of ROS and singlet oxygen upon visible light exposure is investigated using electron paramagnetic resonance (EPR) spectroscopy. We describe the initial formation of ROS, most likely after direct reaction of molecular oxygen with a triplet state photosensitizer, generated from intersystem crossing of the excited singlet state. Since these radicals affect the oxygen content in the headspace of the vial, we monitored photooxidation of these mAb formulations. With increasing protein concentrations, we found (i) a decreasing headspace oxygen content in the sample, (ii) a higher relative number of radicals in solution and (iii) a higher protein degradation. Thus, the protein concentration dependence indicates the presence of higher concentration of a currently unknown photosensitizer.

Photo-Oxidation Mechanisms in Liquid Pharmaceutical Formulations: The Overlooked Role of Singlet Oxygen Presented as a Case Study

PURPOSE

Oxidation is one of the most common degradation pathways for active pharmaceutical ingredients (APIs) in pharmaceutical formulations, mostly involving 1-electron processes via peroxy radicals and 2-electron processes by peroxides. In liquid pharmaceutical formulations, several factors can impact oxidative instabilities including pH, excipient impurities, headspace oxygen, and the potential for photo-oxidation. Photo-oxidation can be particularly challenging to characterize given the number of oxidative mechanisms which can occur. This was observed during formulation development of a new chemical entity, MK-1454, where a degradation peak was observed during photostability studies which was not previously observed during peroxide and peroxyradical forced stress studies.

METHODS

To gain a fundamental understanding of reactive oxygen species generation and its role in degradation of MK-1454, experiments were performed with materials which either generate or measure reactive oxygen species including organic hydroperoxides, singlet oxygen, and superoxide to fundamentally understand a photodegradation mechanism which was observed in the original formulation. LC-MS experiments further elucidated the structure and mechanism of this observed degradation pathway.

RESULTS

A clear relationship between the decrease in dissolved oxygen after light exposure and the loss of MK-1454 was established. The data indicate that singlet oxygen is the most likely contributor of a particular photodegradation product. The singlet oxygen was generated by the inactive ingredients in the formulation, and LC-MS confirm this as the most likely pathway.

CONCLUSION

This work highlights the importance of understanding photochemical degradation of APIs in solution formulations and provides approaches which can better elucidate those mechanisms and thereby control strategies.

Photo-Oxidation of Therapeutic Protein Formulations: From Radical Formation to Analytical Techniques

UV and ambient light-induced modifications and related degradation of therapeutic proteins are observed during manufacturing and storage. Therefore, to ensure product quality, protein formulations need to be analyzed with respect to photo-degradation processes and eventually protected from light exposure. This task usually demands the application and combination of various analytical methods. This review addresses analytical aspects of investigating photo-oxidation products and related mediators such as reactive oxygen species generated via UV and ambient light with well-established and novel techniques.

Visible Light Degradation of a Monoclonal Antibody in a High-Concentration Formulation: Characterization of a Tryptophan-Derived Chromophoric Photo-product by Comparison to Photo-degradation of N-Acetyl-l-tryptophan Amide

We investigated the discoloration of a highly concentrated monoclonal antibody (mAbZ) in sodium acetate (NaAc) and histidine/lysine (His/Lys) buffer after exposure to visible light. The color change of the mAbZ formulation was significantly more intense in NaAc buffer and developed a characteristic absorbance with a λ_max of ca. 450 nm. We characterized this photo-chemically generated chromophore by comparison with visible light photo-degradation of a concentrated solution of a model compound for protein Trp residues, N-acetyl-l-tryptophan amide (NATA). The photo-degradation of NATA generated a chromophoric product with a λ_max of ca. 450 nm and UV-vis spectroscopic properties identical to those of the product generated from mAbZ. This product was isolated and analyzed by high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) and ¹H, ¹³C, and ¹H-¹³C heteronuclear single-quantum correlation NMR spectroscopy. MS/MS analysis reveals a product characterized by the loss of 33 Da from NATA, referred to as NATA-33. Together, the NMR data suggest that this product may be N-(2,4-dihydrocyclopenta[b]indol-2-yl)acetamide (structure P3a) or a tautomer (P3b-d).

Machine learning prediction of methionine and tryptophan photooxidation susceptibility

Photooxidation of methionine (Met) and tryptophan (Trp) residues is common and includes major degradation pathways that often pose a serious threat to the success of therapeutic proteins. Oxidation impacts all steps of protein production, manufacturing, and shelf life. Prediction of oxidation liability as early as possible in development is important because many more candidate drugs are discovered than can be tested experimentally. Undetected oxidation liabilities necessitate expensive and time-consuming remediation strategies in development and may lead to good drugs reaching patients slowly. Conversely, sites mischaracterized as oxidation liabilities could result in overengineering and lead to good drugs never reaching patients. To our knowledge, no predictive model for photooxidation of Met or Trp is currently available. We applied the random forest machine learning algorithm to in-house liquid chromatography-tandem mass spectrometry (LC-MS/MS) datasets (Met, n = 421; Trp, n = 342) of tryptic therapeutic protein peptides to create computational models for Met and Trp photooxidation. We show that our machine learning models predict Met and Trp photooxidation likelihood with 0.926 and 0.860 area under the curve (AUC), respectively, and Met photooxidation rate with a correlation coefficient (Q²) of 0.511 and root-mean-square error (RMSE) of 10.9%. We further identify important physical, chemical, and formulation parameters that influence photooxidation. Improvement of biopharmaceutical liability predictions will result in better, more stable drugs, increasing development throughput, product quality, and likelihood of clinical success.

Light-Induced Histidine Adducts to an IgG1 Molecule Via Oxidized Histidine Residue and the Potential Impact of Polysorbate-20 Concentration

PURPOSE

Histidine (His) undergoes light-induced reactions such as oxidation, crosslinking and addition. These reactions are initiated by singlet oxygen (¹O₂) to generate His photo-oxidation products, which are subject to nucleophilic attack by a non-oxidized His residue from another protein or by nucleophilic buffer components such as Tris and His. This report aims to identify light-induced His-adducts to a monoclonal antibody (mAb-1) due to the reaction of His molecules in the buffer with the photooxidized His residues under ICH light conditions. Since polysorbate-20 (PS-20) is a commonly used excipient in biotherapeutics formulation, it is also important to study the impact of PS-20 concentration on protein photostability.

RESULTS

We identified and characterized light-induced His-adducts of mAb-1 by LC-MS/MS. We showed that the levels of light-induced His-adducts generally correlate with the solvent accessibility of His residues in the protein. In addition, the presence of PS-20 at concentrations commonly used in protein drug formulations can significantly increase the levels of light-induced His-adducts.

CONCLUSIONS

Since His residues are present in a conserved region in the Fc domain, and may be present in the complementarity-determining region (CDR), the impact on the biological functions of the His-adducts observed here should be further studied to evaluate the risk of their presence.

Photoinduced cross-linking of formulation buffer amino acids to monoclonal antibodies

The correct choice of formulation buffer is a critical aspect of drug development and is chosen primarily to improve the stability of a protein therapeutic and protect against degradation. Amino acids are frequently incorporated into formulation buffers. In this study we have identified and characterized light induced cross-links between the side chain of histidine residues in an IgG4 monoclonal antibody and different amino acids commonly used in formulation buffers. These reactions have the potential to impact the overall product quality of the drug. The structure of each cross-link identified was elucidated using high performance liquid chromatography (HPLC) hyphenated to tandem mass spectrometry (MS/MS) with higher energy collisional dissociation (HCD). Furthermore, we speculate on the role of amino acids in formulation buffers and their influence on mAb stability. We theorize that whilst the adduction of formulation buffer amino acids could have a negative impact on product quality, it may protect against other pathways of photo-degradation.

Protein Sub-Visible Particle and Free Radical formation of a Freeze-Dried Monoclonal Antibody Formulation During Dropping

Dropping during shipping and handling of liquid biopharmaceutical formulations has long been known to cause protein degradation and aggregation. On the other hand, accidental dropping of freeze-dried protein formulations is generally considered not a major issue for biopharmaceutical quality. Reports of stability and especially the underling degradation mechanism(s) during shipping and handling of freeze-dried protein formulations were rarely seen in literature. In this manuscript, we report an interesting phenomenon in which repeated dropping of freeze-dried monoclonal antibody X (mAb-X) formulation powder resulted in significant protein sub-visible particles (SbVPs) in the reconstituted liquid as determined by the sensitive particle analyzing technique micro-flow imaging (MFI). Free radicals were observed after repeated dropping by electron paramagnetic resonance (EPR). Formation of SbVPs could be partially inhibited by the free radical scavengers methionine and 3-carbamoyl-2,2,5,5-tetramethyl-1-pyrrolidin-yloxy free radical (CTPO). The amount of free radicals and SbVPs was correlated to the sample temperature during dropping. Therefore we propose that the high temperature formed during dropping was probably the root cause for protein aggregation and free radical formation, which could further cause protein aggregation. Our observations suggest that similar to liquid protein formulations, dropping of freeze-dried protein formulations should also be avoided or mitigated.

Degradation of polysorbates 20 and 80 catalysed by histidine chloride buffer

Polysorbates are amphiphilic, non-ionic surfactants, and they represent one of the key components of biopharmaceuticals. They serve as stabilisers, and their degradation can cause particle formation, which has been an industry-wide issue over the past decade. To determine the influence of the buffers most frequently used in biopharmaceuticals on polysorbate degradation, an accelerated stability study was carried out using placebo formulations containing 0.02% polysorbates and 20 mM buffers (pH 5.5, 6.5). These included histidine chloride, sodium citrate, sodium succinate and sodium phosphate buffers. The rate of polysorbate degradation was highest in histidine chloride buffer, and therefore we further focused on the mechanism here. The predominant degradation pathway of polysorbates in this buffer was ester hydrolysis, catalysed by the imidazole moiety of the histidine. Interestingly, the presence of therapeutic proteins in the formulations slowed histidine-catalysed degradation of polysorbates in 50% of cases, with negligible degradation seen otherwise. This emphasises the complex nature of the interactions between the components of biopharmaceutical drug products. Nonetheless, there are disadvantages of using histidine chloride buffers in biopharmaceuticals that contain polysorbates. Careful consideration should be given to selection of excipients used in parenteral formulations, whereby compatibility between buffer and surfactant is of key importance.

Discovery of a Photoinduced Histidine-Histidine Cross-Link in an IgG4 Antibody

A novel histidine-histidine (His-His) photooxidative cross-link has been identified in an IgG4 antibody. It was formed between the side chain of a histidine residue of the antibody and histidine from the formulation buffer. The structure of the cross-link was elucidated using high performance liquid chromatography (HPLC) hyphenated to tandem mass spectrometry (MS/MS) with higher energy collisional dissociation (HCD). The cross-link was found in multiple conformations, as the location of the oxygen varied. Furthermore, the extent of cross-link formation was shown to correlate with the amount of light the antibody was exposed to as well as the solvent accessibility of each modification site.

Shifting Paradigms Revisited: Biotechnology and the Pharmaceutical Sciences

In 2003, Crommelin et al. published an article titled: "Shifting paradigms: biopharmaceuticals versus low molecular weight drugs" (https://doi.org/10.1016/S0378-5173(03)00376-4). In the present commentary, 16 years later, we discuss pharmaceutically relevant aspects of the evolution of biologics since then. First, we discuss the increasing repertoire of biologics, in particular, the rapidly growing monoclonal antibody family and the advent of advanced therapy medicinal products. Next, we discuss trends in formulation and characterization as well as summarize our current insights into immunogenicity of biologics. We spend a separate section on new product(ion) paradigms for biologics, such as cell-free production systems, production of advanced therapy medicinal products, and downscaled production approaches. Furthermore, we share our views on issues related to reaching the patient, including routes and techniques of administration, alternative development models for affordable biologics, biosimilars, and handling of biologics. In the concluding section, we outline outstanding issues and make some suggestions for resolving those.

The Use of a 2,2'-Azobis (2-Amidinopropane) Dihydrochloride Stress Model as an Indicator of Oxidation Susceptibility for Monoclonal Antibodies

Protein oxidation is a major pathway for degradation of biologic drug products. Past literature reports have suggested that 2,2-azobis (2-amidinopropane) dihydrochloride (AAPH), a free radical generator that produces alkoxyl and alkyl peroxyl radicals, is a useful model reagent stress for assessing the oxidative susceptibility of proteins. Here, we expand the applications of the AAPH model by pairing it with a rapid peptide map method to enable site-specific studies of oxidative susceptibility of monoclonal antibodies and their derivatives for comparison between formats, the evaluation of formulation components, and comparisons across the stress models. Comparing the free radical-induced oxidation model by AAPH with a light-induced oxidation model suggests that light-sensitive residues represent a subset of AAPH-sensitive residues and therefore AAPH can be used as a preliminary screen to highlight molecules that need further assessment by light models. In sum, these studies demonstrate that AAPH stress can be used in multiple ways to evaluate labile residues and oxidation sensitivity as it pertains to developability and manufacturability.

Role of Buffers in Protein Formulations

Buffers comprise an integral component of protein formulations. Not only do they function to regulate shifts in pH, they also can stabilize proteins by a variety of mechanisms. The ability of buffers to stabilize therapeutic proteins whether in liquid formulations, frozen solutions, or the solid state is highlighted in this review. Addition of buffers can result in increased conformational stability of proteins, whether by ligand binding or by an excluded solute mechanism. In addition, they can alter the colloidal stability of proteins and modulate interfacial damage. Buffers can also lead to destabilization of proteins, and the stability of buffers themselves is presented. Furthermore, the potential safety and toxicity issues of buffers are discussed, with a special emphasis on the influence of buffers on the perceived pain upon injection. Finally, the interaction of buffers with other excipients is examined.

Investigation of a Degradant in a Biologics Formulation Buffer Containing L-Histidine

PURPOSE

An unknown UV 280 nm absorbing peak was observed by SEC for protein stability samples formulated in L-histidine during a stress stability study. Understanding the source would enhance the confidence in the SEC results. We identified the unknown peak, studied the cause, and evaluated ways to eliminate it.

METHODS

The unknown peak was fractionated by preparative size exclusion chromatography separations, and subsequently analyzed by Hydrophilic Interaction Chromatography (HILIC) coupled with Time-of-Flight (TOF) high resolution mass spectrometry. The possible degradation was also studied with the presence of different excipients, including metal cations, chelating agents, and amino acids.

RESULTS

The unknown peak was identified to be trans-urocanic acid, a degradant of histidine, based on evidences from HILIC retention time, UV profile, accurate mass measurement, trans-cis isomerization, and pI measurement. The degradation from histidine to urocanic acids was not affected by the presence of Fe(2+), but slightly activated by Mn(2+). The chelating agents, EDTA and DTPA, counteracted the Mn(2+) effects. This degradation was evidenced to be caused by contamination. Adding alanine or cysteine as an excipient was found to reduce this degradation by 97 and 98%, respectively.

CONCLUSIONS

L-histidine formulation buffer can be contaminated to induce histidine degradation to trans-urocanic acid, which shows a large UV 280 nm absorbing peak at the total permeation volume under SEC conditions. Amino acids alanine and cysteine effectively inhibit this histidine degradation.

Role of surface exposed tryptophan as substrate generators for the antibody catalyzed water oxidation pathway

The reaction of singlet oxygen with water to form hydrogen peroxide was catalyzed by antibodies and has been termed as the antibody catalyzed water oxidation pathway (ACWOP) (Nieva and Wentworth, Trends Biochem. Sci. 2004, 29, 274-278; Nieva et al. Immunol. Lett. 2006, 103, 33-38). While conserved and buried tryptophans in the antibody are thought to play a major role in this pathway, our studies with a monoclonal antibody, mAb-1 and its mutant W53A, clearly demonstrate the role of surface-exposed tryptophans in production of hydrogen peroxide, via the photo-oxidation pathway. Reactive oxygen species (ROS) such as singlet oxygen and superoxide were detected and site-specific tryptophan (Trp53) oxidation was observed under these conditions using RP-HPLC and mass spectrometry. The single mutant of the surface exposed Trp53 to Ala53 (W53A) results in a 50% reduction in hydrogen peroxide generated under these conditions, indicating that surface exposed tryptophans are highly efficient in transferring light energy to oxygen and contribute significantly to ROS generation. ACWOP potentially leads to the chemical instability of mAb-1 via the generation of ROS and is important to consider during clinical and pharmaceutical development of mAbs.