Effective extracellular payload release and immunomodulatory interactions govern the therapeutic effect of trastuzumab deruxtecan (T-DXd)

Trastuzumab deruxtecan (T-DXd) is an antibody-drug conjugate (ADC) targeting HER2, exhibiting significant clinical efficacy in breast cancer (BC) with varying HER2 expression, including HER2-low and HER2-ultralow. However, the precise mechanism underlying its efficacy and the contribution of immune activation in these settings remain unclear. Here, we demonstrate that T-DXd efficacy in HER2-low and HER2-negative BC is independent of HER2 engagement and ADC internalization. Instead, its activity relies on extracellular proteases, such as cathepsin L (CTSL), within the tumor microenvironment. Irrespective of their HER2 status, tumor and stromal compartments of invasive BC abundantly express CTSL, which efficiently cleaves the specialized linker of T-DXd, facilitating payload release and inducing cytotoxicity against HER2-low/negative tumors. In HER2-positive BC, the antibody backbone of T-DXd engages Fcγ-receptors and drives antibody-dependent cellular phagocytosis (ADCP). Concurrently, its cytotoxic payload (DXd) induces immunogenic cell death, further activating myeloid cells via TLR4 and STING pathways to enhance tumor antigen presentation to CD8+ T cells. Notably, T-DXd cytotoxicity also upregulates tumor CD47 expression, dampening immune activation. Combining T-DXd with CD47 checkpoint blockade significantly enhances anti-tumor immune responses in a HER2-transgenic BC mouse model, while also inducing durable CD8+ T cell memory to prevent tumor recurrence after therapy cessation. Trastuzumab deruxtecan (T-DXd) is an antibody-drug conjugate targeting HER2 but paradoxically efficient even in breast cancers expressing HER2 at very low levels. Here authors show that invasive breast cancers, even if their HER2 expression is negligible, secrete extracellular proteases, such as cathepsin L, which cleave the specialized linker of T-DXd, releasing the drug in the tumour microenvironment, while in HER2 positive breast cancers, T-DXd engages Fcγ receptors to promote phagocytosis of HER2-expressing cells and triggers payload-induced immunogenic cell death.