A Dual-Payload Antibody–Drug Conjugate Targeting CD276/B7-H3 Elicits Cytotoxicity and Immune Activation in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is a highly aggressive and heterogeneous disease that often relapses following treatment with standard radiotherapies and cytotoxic chemotherapies. Combination therapies have potential for treating refractory metastatic TNBC. In this study, we aimed to develop an antibody-drug conjugate with dual payloads (DualADC) as a chemoimmunotherapy for TNBC. The overexpression of an immune checkpoint transmembrane CD276 (also known as B7-H3) was associated with angiogenesis, metastasis, and immune tolerance in more than 60% of patients with TNBC. Development of a mAb capable of targeting the extracellular domain of surface CD276 enabled delivery of payloads to tumors, and a platform was established for concurrent conjugation of a traditional cytotoxic payload and an immunoregulating Toll-like receptor 7/8 agonist to the CD276 mAb. The DualADC effectively killed multiple TNBC subtypes, significantly enhanced immune functions in the tumor microenvironment, and reduced tumor burden by up to 90% to 100% in animal studies. Single-cell RNA sequencing, multiplex cytokine analysis, and histology elucidated the impact of treatment on tumor cells and the immune landscape. This study suggests that the developed DualADC could represent a promising targeted chemoimmunotherapy for TNBC. Significance: An anti-CD276 monoclonal antibody conjugated with both a cytotoxic drug and an immune boosting reagent effectively targets triple-negative breast cancer by inducing tumor cell death and stimulating immune cell infiltration.