Glycosylation Analysis for Multispecifics

Glycan modifications of recombinant proteins and antibodies can impact their function and stability and thus the safety and efficacy of drug products based on these molecules. Understanding these important critical quality attributes (CQAs) is essential, and therefore, regulatory bodies expect a comprehensive understanding of glycans for submission.

Comprehension necessitates overcoming inherent challenges such as positional isomers and heterogeneity, as well as method-induced artifacts that influence sensitivity and specificity, according to Paula Orens, senior global market development manager, Biopharma, at SCIEX.

Increased complexity for multispecifics

Multispecifics, including bi-, tri-, and tetraspecific antibodies, present additional challenges to glycan analysis beyond those of traditional monoclonal antibodies (mAbs) because the complexity of these molecules is much greater.

“Site-specific and intact mass analysis may become more challenging for increasingly complex molecules, particularly when the number of N-glycosylation sites, post translational modifications, and potential degradation products increases,” observes Oscar Potter, biological chemistries R&D manager in the Chemistries and Supplies Division at Agilent.

Structurally, multispecific antibodies are a combination of different antibody domains that allow the multispecific to bind at multiple sites. “The various domains will have their own glycosylation patterns (integral to efficacy, safety, and stability) and their own impurities, all of which need to be characterized comprehensively, effectively multiplying the glycosylation analysis complexity by the number of binding sites,” Orens explains. She adds that as the number of binding sites increases, so too does the risk of intra-protein cross talk.

Multispecifics can also include candidates in which an antibody-portion is combined with some other protein-based component. Chimeric proteins, where the domains are not only antibody in origin, are complicated to analyze due to the individual properties of each component, according to Orens. “For example,” she says, “fusion proteins are typically heavily glycosylated with N- and O-linked glycans to improve biological activity, properly stabilize the structure, or improve solubility.”

“Overall,” Orens concludes, “while the complexity of analysis is dependent on the types of domains used in a multispecific, it is certain that the analysis of these molecules will be more complicated than that of a mAb.”

Consequences of incorrect glycosylation analysis

The greater complexity of multispecifics raises the stakes for achieving correct glycan analysis. “Not only can the glycosylation patterns in multispecifics impact the stability, pharmacokinetics, off-target effects, and effector functions such as complement activation and antibody-dependent cellular cytotoxicity, conceivably the additional functions that multispecific antibodies are capable of might result in unique biological consequences from incorrect glycosylation,” Potter comments.

In addition, because glycosylation is a critical and highly regulated quality attribute of all antibodies, misunderstanding or incorrectly characterizing glycosylation patterns could lead to a variety of consequences, depending on the development stage of the candidate, notes Orens. She highlights a couple of examples. “At the hit-identification stage, a mis-characterized but suitable candidate could be eliminated due to poor efficacy or stability. At later development stages, meanwhile, a drug metabolism and pharmacokinetics study could exhibit faster-than-expected clearance or undesired concentration in the target tissue. An incorrect analysis could also result in promotion of a misunderstood candidate through to late-stage toxicology studies, causing unnecessary investment in time and resources for the project only to be shut down later than expected.”

Several analytical approaches

There are several different techniques used for the glycan analysis of antibody-based candidates. “Fundamentally, the same glycan analysis approaches are used for multispecific antibodies as mAbs, most commonly liquid chromatography (LC), capillary electrophoresis (CE), and mass spectrometry (MS). Each analytical approach offers a plethora of techniques which provide complementary or alternative details about the glycosylation profiles,” Orens observes. She does note, though, that the inherent complexity of the molecule will impart different challenges on each approach.

Typical analytical methods for glycosylation analysis of multispecifics are similar to those used for more traditional therapeutic mAbs, agrees Potter. “Released glycan analysis methods generate a glycan profile, usually based on separation by CE or LC. Intact LC/MS analysis can provide limited information about the major glycoforms, while LC/MS of protease-digested samples can give site-specific information for molecules with more than one glycosylation site,” he outlines.

Released glycan analysis relies on high-resolution separation of fluorescently labeled glycans by hydrophilic interaction liquid chromatography (HILIC) or CE. HILIC is used in conjunction with fluorescence or mass detection to separate and quantify individual glycan species over long gradients, according to Orens. Similarly, CE systems can offer comparable quantitation details with an alternative separation mode, often with faster separation. “These approaches are often used as monitoring techniques to ensure batch consistency during manufacturing,” she says.

Released glycan analysis once suffered from tedious sample prep, according to Potter, but protocols have been greatly simplified in recent years through rapid-release and instant fluorescent labeling sample prep kits. He observes that HILIC methods enable the most exact assignment of glycan structures, either using retention time reference standards or retention libraries, or by hyphenating with MS detection. “HILIC separations can often distinguish between isomeric structures, including potentially harmful alpha-galactose (α-Gal) structures masquerading as beneficial galactosylated complex glycans,” he says.

Measuring the intact mass of multispecific antibodies via LC/MS does not require any sample prep, which can be advantageous. A well-optimized method run on a high-resolution instrument such as a time-of-flight (TOF)-based system is needed, however, Potter observes.

Orens adds that TOF systems can be used as the detection method for several techniques to help identify and localize glycans, as well as quantify, resolve isomers, and structurally elucidate the different species.

Potter cautions, though, that while very convenient, measuring the intact mass may only reveal the sum of masses of the two or more glycans on each molecule, which may confound assignment of exact glycan structures. Distinguishing between isomers is also not possible.

The most information can be obtained by first digesting the multispecific with a protease such as IdeS (immunoglobulin G-degrading enzyme of Streptococcus pyogenes) or trypsin into smaller peptides, according to Potter. “Digestion allows for site-specific glycosylation analysis by LC/MS, which may be particularly useful when distinguishing between glycosylation in the fragment antigen-binding (Fab) and fragment crystallizable (Fc) regions, the latter of which greatly impacts the Fc effector functions of the antibody,” Potter contends.

Orens further notes that when peptide mapping is performed using a MS system with specialized fragmentation capabilities such as electron activated dissociation (EAD), it is possible to identify unknown modifications or impurities, then localize the species to a specific domain or amino acid of the protein. “EAD,” she says, “is integral to this process, as it differentiates between isomers and maps specific amino acids, and thus provides a comprehensive understanding of the protein with its modifications.”

Ultimately, Orens emphasizes, the appropriate analytical approach is entirely dependent on the purpose of the analysis.

An orthogonal approach is best

Given the importance of accurate glycan analysis for multispecifics, the best strategy for ensuring success according to both Orens and Potter is to use a combination of available methods.

“While using top down, sub-unit, or peptide mapping methods and state-of-the-art MS systems enables comprehensive glycan analysis of multispecifics since it provides the most information in a single step, including molecular weight, domain/site localization, species identity, and species quantity, most organizations employ multiple analytical approaches to support their characterization efforts and obtain secondary, confirmatory evidence,” says Orens.

“The best way to ensure accurate and comprehensive characterization,” agrees Potter, “is to use more than one orthogonal method. Released glycan analysis, intact mass LC/MS, and LC/MS after protease digestion needn’t be mutually exclusive—they can be complementary,” he concludes.

About the author

Cynthia A. Challener, PhD, is a contributing editor to BioPharm International®.

Article details

BioPharm International®
October 2024
Vol. 37, No. 9
Pages: 29-31

Citation

When referring to this article, please cite it as Challener, C.A. Glycosylation Analysis for Multispecifics. BioPharm International 2024 37 (9).