Payload-dependent photostability of antibody-drug conjugates

Antibody-Drug Conjugates (ADCs) are nowadays an important therapeutic modality. The (cytotoxic) payload, being the drug, is herewith conjugated to an antibody, which provides targeting. Whilst a lot of attention for technical characterization and stress testing often focuses on the antibody piece, there has been little attention for the payload. Hence, the present study compared the photostability of model ADCs with different payloads, chosen to be representative for the main payload classes used in current clinical and commercial ADC products (Auristatins (MMAE), Maytansinoids (DM1) and Camptothecins (Exatecan)). After photodegradation, the stability of ADCs was evaluated for sizing and charge differences, as well as presence of reactive oxygen species (ROS). Out of the three tested ADCs and the native mAb, the Exatecan-conjugated ADC exhibited the strongest photodegradation, demonstrated by strong coloration, increase in aggregate levels up to ∼ 8.5 % in size-exclusion HPLC, and a change in charge profile leading to a decrease in ion-exchange chromatography peak height to 27 % compared to unstressed samples. Additionally, strong presence of ROS was demonstrated only in samples containing Exatecan-ADC and Exatecan drug-linker. As the analyzed ADCs only vary in the conjugated payload, and the unconjugated mAb remained stable during forced photodegradation, our results indicate that Exatecan acts as a strong photosensitizer. This finding is different from photodegradation of monoclonal antibodies, in which proteinogenic photosensitizers such as tryptophan are typically responsible for photodegradation. Therefore, companies developing ADCs should -apart from the antibody framework- also focus their attention on the payload and how it plays a role in stress-related degradation pathways.