Structure-Guided In-Use Stability Assessment of Monoclonal Antibody Tislelizumab

<b>Background/Objectives</b>: Monoclonal antibody (mAb) stability is critical not only during manufacturing but also at the point of clinical administration. For therapies like tislelizumab (Tevimbra), a programmed death-1 (PD-1) targeting IgG mAb, delays in dosing often result in prepared infusions being discarded, contributing to substantial drug waste despite being engineered for improved stability. <b>Methods</b>: To evaluate the physicochemical in-use stability of tislelizumab in a ready-to-administer format, we mapped degradation pathways, including post-translational modifications (PTMs); peptide alterations; pH and solution characteristics-under 12-month storage (ultra-long), under 1-month storage (0, 7, 14, 21, 28 and 31 days), and under exposure-related forced degradation conditions including room temperature, elevated temperature, pH (acidic/basic), oxidation and UV exposure. Structural analysis was contextualised to the known PD-1 binding site, making stability assessment relevant to tislelizumab's mechanism-of-action in blocking PD-1. To assess solution stability, a validated size-exclusion chromatography (SEC) assay was applied to all conditions. <b>Results</b>: Aggregation was identified as the primary degradation pathway during ultra-long-term storage. SEC and chemical assessment revealed no measurable changes in protein quantity, aggregation, peptide integrity, or PTM profile over 31 days at 2-8 °C in polyolefin intravenous bags (1.6 mg/mL). <b>Conclusions</b>: These results support the structural and physicochemical stability of tislelizumab under refrigerated conditions.