<p>Camptothecin (CPT)-based antibody–drug conjugates (ADCs) have become popular oncology biotherapeutics that integrate the targeted specificity of monoclonal antibodies (mAbs) with camptothecin and its derivatives. Because mAbs are vulnerable to photodegradation and the light-absorbing characteristics of camptothecin, understanding the effects of light exposure on CPT-based ADCs is critical to ensure product quality, efficacy, and safety. Herein, we harnessed the capabilities of advanced liquid chromatography-mass spectrometry (LC–MS) to perform in-depth molecular-level characterization of photo-stressed antibodies conjugated with a camptothecin derivative payload, deruxtecan (DXd). Peptide mapping via LC–MS/MS revealed significant oxidative modifications in methionine, tryptophan, and histidine residues, and degradation of the linker payload in the photo-stressed ADC molecules. Native SEC-MS with post-column denaturation was conducted to further characterize the covalent nature of high molecular weight species formed under photo stress and provide insights into aggregation mechanisms. Additionally, coupling LC to high-resolution MS enabled the identification and quantitation of degradants from excipients and the linker payload, thus providing a comprehensive understanding of light exposure effects on DXd ADCs. Application of these LC–MS based assays to evaluate the effects of various formulations on photostability revealed that histidine and methionine act as oxidative scavengers that protect ADC molecules against light-induced degradation. This study highlights the utility of LC–MS methods in the comprehensive characterization of photo-stressed CPT-based ADCs, and provides valuable insights for process optimization, formulation development, and storage condition recommendations.</p> Graphical Abstract <p></p>