Development and characterisation of a method for determining N-terminal heterogeneity in the human recombinant...

Recombinant human follicle-stimulating hormone (rhFSH) consists of α- and β subunits linked by non-covalent bonds. Both subunits exhibit N-terminal heterogeneity, with the β-subunit exhibiting heterogeneity to a greater extent. In this study, we developed a comprehensive method for assessing β-subunit N-terminal heterogeneity based on a previously developed peptide mapping approach. The new method, which includes optimised sample preparation and reversed-phase high-performance liquid chromatography, facilitated the separation of integrated (NSCELTNITIAIEK) βL1 and truncated (CELTNITIAIEK) βL1 forms. The identity of the separated integrated and truncated forms was confirmed via tandem mass spectrometry. Optimised sample preparation streamlined the process by simultaneously performing trypsin digestion (with calcium chloride enhancement) and PNGase F deglycosylation for a total preparation time of just 1 day. The enzymatic digestion and sample preparation processes underwent several evaluations, including scaled-up sample preparation, sample preparation variability assessment and extended incubation stability studies. The sample preparation process demonstrated excellent robustness in all evaluations. Results consistently showed approximately 50 % N-terminal truncation of two amino acids in the β-subunit across different manufacturers' samples, highlighting the utility of the method in assessing the batch-to-batch consistency and comparability of biosimilar products. This integrated approach combines peptide mapping and N-terminal heterogeneity analysis into a single, efficient workflow, thereby significantly enhancing quality control processes and enhancing the safety and efficacy of recombinant FSH products.