Fc-Fusion proteins represent a successful class of biopharmaceutical products, with already 13 drugs approved in the European Union and United States as well as three biosimilar versions of etanercept. Fc-Fusion products combine tailored pharmacological properties of biological ligands, together with multiple functions of the fragment crystallizable domain of immunoglobulins. There is a great diversity in terms of possible biological ligands, including the extracellular domains of natural receptors, functionally active peptides, recombinant enzymes, and genetically engineered binding constructs acting as cytokine traps. Due to their highly diverse structures, the analytical characterization of Fc-Fusion proteins is far more complex than that of monoclonal antibodies and requires the use and development of additional product-specific methods over conventional generic/platform methods. This can be explained, for example, by the presence of numerous sialic acids, leading to high diversity in terms of iso-
Fc galactosylation is a critical quality attribute for anti-tumor recombinant immunoglobulin G (IgG)-based monoclonal antibody (mAb) therapeutics with complement-dependent cytotoxicity (CDC) as the mechanism of action. Although the correlation between galactosylation and CDC has been known, the underlying structure-function relationship is unclear. Heterogeneity of the Fc N-glycosylation produced by Chinese hamster ovary (CHO) cell culture biomanufacturing process leads to variable CDC potency. Here, we derived a kinetic model of galactose transfer reaction in the Golgi apparatus and used this model to determine the correlation between differently galactosylated species from CHO cell culture process. The model was validated by a retrospective data analysis of more than 800 historical samples from small-scale and large-scale CHO cell cultures. Furthermore, using various analytical technologies, we discovered the molecular basis for Fc glycan terminal galactosylation changing the three-dimensional conformation of the Fc, which facilitates the IgG1 hexamerization, thus enhancing C1q avidity and subsequent complement activation. Our study offers insight into the formation of galactosylated species, as well as a novel three-dimensional understanding of the structure-function relationship of terminal galactose to complement activation in mAb therapeutics.
For the first time, the human Chorionic Gonadotropin (hCG) hormone at the intact level was characterized by reversed phase liquid chromatography (RPLC) coupled with high resolution mass spectrometry (HRMS). This heterodimeric protein is specific to human pregnancy, consists in an α and a β subunit, so-called hCGα and hCGβ, respectively, and has 8 glycosylation sites leading to a high number of isoforms. First, the LC method was optimized to separate the largest number of isoforms and also to facilitate the MS ionization process and data treatment. The initial mobile phase composition, slope of the gradient, and column temperature were appropriately selected to maximize the number of separated isoforms. Moreover, the MS detection parameters were adjusted to i) promote the efficient transfer of the heaviest ions, ii) avoid or limit the fragmentation of the ions and iii) improve the sensitivity. The repeatability of the final method in terms of retention times and peak areas was assessed. The method was next used to characterize two hCG-based drugs: Ovitrelle (a recombinant hCG, r-hCG) and Pregnyl (hCG isolated from urine of pregnant women, u-hCG). After the deconvolution step, the analytical method did not allow to observe the isoforms of the hCGβ. This may be due to its dramatic higher heterogeneity induced by its 6 glycosylation sites and a lack of ionization in the MS source. Nevertheless, the results revealed the presence of more than 30 hCGα isoforms, which differ by their number and their nature in the two drugs. Then, the molecular weights of the N-glycans already described in the literature for hCG were compiled in a database to identify the hCGα glycoforms by mass matching. This strategy was successfully applied for the identification of five glycoforms for both r-hCG and u-hCG. This work demonstrates for the first time the potential of RPLC-HRMS for the identification of the intact hCGα glycoforms.
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