An intrinsically hydrophilic linker enables a stable, high-DAR exatecan-based HER2 ADC with potent antitumor activity

Antibody-drug conjugates (ADCs) combine the targeting specificity of monoclonal antibodies with the potent cytotoxicity of small-molecule drugs. However, ADC development using Exatecan, a potent Topoisomerase I inhibitor, has been challenged by its hydrophobicity, leading to aggregation, rapid clearance, and off-target toxicity. Herein, we report the design of an intrinsically hydrophilic drug-linker platform (SMP-70067-L) that enables the construction of a homogeneous Exatecan-based HER2-targeted ADC (SMP-70067-X) with a high drug-to-antibody ratio (DAR of 7.92). Similar to Trastuzumab deruxtecan (DS-8201a), which achieves high DAR without PEG or polysarcosine chains through a hydrophilic self-immolative spacer, SMP-70067-L integrates minimal hydrophilic elements, including a glutamic acid residue and a modified aromatic self-immolative spacer, to balance hydrophilicity, stability, and efficient payload release. The resulting ADC exhibits low aggregation (<1%), favorable plasma and thermal stability, and sustained exatecan release. SMP-70067-X demonstrates potent cytotoxicity in HER2-positive tumor cells (sub-nanomolar IC₅₀ values) and significantly enhanced antitumor efficacy compared to DS-8201a in HER2-moderate and HER2-low xenograft models. These results highlight the critical role of rational linker engineering in expanding the therapeutic window of hydrophobic Topoisomerase I inhibitor-based ADCs.