Enhanced Payload Release Enables Disitamab Vedotin to Surpass Trastuzumab Emtansine and Retain Efficacy in Acquired Resistance to Clinical Anti-HER2 Therapies

<b>Background:</b> Resistance to HER2-targeted therapies remains a major limitation in the treatment of HER2-positive breast cancer, where disease progression inevitably occurs in advanced stages. Development of next-generation strategies that retain activity in resistant disease is therefore a critical priority. Disitamab vedotin (RC48) is a novel antibody-drug conjugate (ADC) targeting HER2 that couples a humanized anti-HER2 antibody to the potent microtubule-disrupting agent monomethyl auristatin E. <b>Methods:</b> We compared the activity and mechanism of action of RC48 with that of trastuzumab emtansine (T-DM1) across HER2-positive and HER2-low cellular models, including multiple sublines resistant to current HER2-targeted agents. <b>Results:</b> In HER2-overexpressing breast cancer cell lines, RC48 consistently demonstrated superior antiproliferative effect with respect to T-DM1. Treatment with RC48 induced G2/M arrest and apoptotic cell death, associated with increased pHistone-H3 and cyclin B1 and downregulation of Wee1, consistent with blockade of cell cycle progression in mitosis. Although RC48 and T-DM1 internalized similarly, RC48 displayed more efficient intracellular payload release, providing a mechanistic explanation for its enhanced efficacy. Notably, RC48 retained strong activity in BT474-derived sublines resistant to T-DM1, lapatinib, or neratinib, inducing cell cycle arrest, apoptosis, and caspase activation in all resistant models. In contrast, T-DM1 exhibited only partial effects in resistant cells and was completely ineffective in a T-DM1-refractory clone. <b>Conclusions:</b> Together, these findings identify disitamab vedotin as a potent next-generation HER2-targeting ADC with the unique capacity to overcome acquired resistance to HER2-directed therapies. RC48 represents a promising therapeutic strategy for patients with refractory HER2-positive breast cancer and warrants further clinical investigation.