Abstract Background: Aggressive breast cancers, including triple-negative and inflammatory subtypes, urgently require novel therapeutic strategies. Antibody-drug conjugates (ADCs) have shown improved outcomes in HER2- and TROP2-positive breast tumors; however, the heterogeneous expression of target antigens and the development of acquired resistance often limit their long-term clinical benefit. Identifying tumor-selective antigens that remain expressed in advanced and ADC-refractory breast cancers is critical. CD44 variant isoform 9 (CD44v9), a splice variant of the CD44 family, is highly expressed in aggressive breast cancers but largely absent in normal tissues. Functionally, CD44v9 promotes cancer stemness, oxidative stress adaptation, and therapeutic resistance, making it a promising ADC target. Methods: CD44v9 expression was evaluated in human breast cancer cell lines, patient-derived xenografts, and tumor specimens by immunohistochemistry, with comparison to normal tissues. A chimeric anti-CD44v9 monoclonal antibody (clone SUM24.1) was conjugated to the microtubule inhibitor monomethyl auristatin F (MMAF). Binding, internalization, and cytotoxicity were assessed in triple-negative, inflammatory, and ADC-resistant breast cancer models. Antitumor efficacy was tested in TNBC xenografts and T-DXd-resistant breast cancer models. Results: CD44v9 was abundantly expressed across TNBC models and largely absent from a broad panel of normal human tissues, including peripheral blood mononuclear cells. The anti-CD44v9 antibody demonstrated strong binding to its target and efficient internalization, supporting selective MMAF delivery to cancer cells. Anti-CD44v9-MMAF induced potent, target-dependent cytotoxicity in vitro (p 0.0001) and significant tumor growth inhibition (TGI) in TNBC xenografts (SUM149: 50% TGI at 0.5 mg/kg, 90% TGI at 2 mg/kg; HCC1806: 50% TGI at 0.5 mg/kg, 95% TGI at 2 mg/kg; p 0.0001), accompanied by increased apoptosis, reduced proliferation, and minimal systemic toxicity. Notably, CD44v9 expression persisted in breast cancer cells resistant to trastuzumab deruxtecan (T-DXd) and sacituzumab govitecan, and these models remained sensitive to CD44v9-directed MMAF therapy in vitro (p 0.0001) and in SUM190-T-DXd-R xenografts (70% TGI at 2 mg/kg; p 0.0001), demonstrating CD44v9’s therapeutic independence from HER2 and TROP2 pathways. Conclusion: CD44v9 is a tumor-specific, biologically meaningful target for ADC development in advanced breast cancer. The preclinical efficacy and selectivity of anti-CD44v9-MMAF highlight its potential as a next-generation therapy for patients with CD44v9-positive, treatment-refractory breast cancers. Citation Format: Dileep R. Reddy, McKenna E. Flynn, Yasuaki Anami, Zhaoxuan Yang, Jayden Aleman, Minji Seo, Kyoji Tsuchikama, Jennifer A. Maynard, Naoto T. Ueno, Jangsoon Lee. CD44v9-targeted antibody-drug conjugate as a novel therapy for advanced breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2659.
Reddy et al. (Fri,) studied this question.