CD44v9 as a therapeutic target for antibody-drug conjugate in advanced breast cancer

Antibody-drug conjugates (ADCs) have revolutionized breast cancer therapy. HER2 is the only validated biomarker guiding ADC therapy in breast cancer. However, their clinical benefit remains confined to HER2- and TROP2-targeted therapies. Tumor heterogeneity and acquired resistance often limit durable responses, underscoring the need for novel, tumor-specific ADC targets. CD44 variant isoform 9 (CD44v9), a splice variant of the CD44 family, is largely absent in normal tissues but enriched in aggressive breast cancers, where it contributes to stemness, redox regulation, and therapy resistance. In this study, we developed a chimeric monoclonal antibody against CD44v9 (clone SUM24.1, IgG1) and conjugated it to the microtubule inhibitor monomethyl auristatin F (MMAF). CD44v9 expression was analyzed by immunohistochemistry in breast cancer and normal tissues. Binding, internalization, and cytotoxicity of anti-CD44v9-MMAF were assessed in triple-negative, inflammatory, and ADC-resistant breast cancer models. CD44v9 was abundantly expressed in breast cancer but absent in most normal tissues, including peripheral blood mononuclear cells. The anti-CD44v9 antibody exhibited strong and specific binding with efficient internalization, supporting selective payload delivery. Anti-CD44v9-MMAF demonstrated potent, antigen-dependent cytotoxicity in vitro and dose-dependent tumor inhibition in vivo, with minimal systemic toxicity. Notably, CD44v9 expression persisted in trastuzumab deruxtecan-, sacituzumab govitecan-, and T-DM1-resistant models, which remained sensitive to CD44v9-targeted therapy. Collectively, these findings establish CD44v9 as a tumor-specific and clinically actionable ADC target in advanced breast cancer and provide a strong preclinical rationale for the development of CD44v9-directed ADCs for patients with treatment-refractory disease.