CD44 splice variants as novel antibody-targets for advanced breast cancer

Few therapeutic options exist for triple-negative and heavily treated, hormone receptor-positive metastatic breast cancer (BC) patients. Antibody-drug conjugates (ADCs) improve overall survival for BC patients with low-HER2 expressing tumors by [Trastuzumab deruxtecan (T-DXd), Datopotamab deruxtecan (Dato-DXd), or Sacituzumab govitecan (SG)], but these strategies fail when HER2 targeting is not possible. Moreover, most patients who qualify and respond to T-DXd, Dato-DXd, or SG will relapse or progress, leaving even fewer therapeutic options available. Novel treatments that improve outcomes for patients who relapse after the current standard of care ADCs remain a critical clinical need. We propose a novel ADC-based therapeutic platform for aggressive and resistant BC to address two issues that continue to hamper the development and adoption of ADCs for clinical use: high treatment-associated toxicities due to on-target, off-tumor effects and poor tumor penetration. We will overcome these challenges with two specific technical advances: (1) development of conditionally-active antibodies with enhanced binding to antigens in the tumor microenvironment to minimize impacts on healthy tissues, and (2) drug conjugation via cyclic peptides, including an Arg-Gly-Asp tumor-penetrating peptide, and a neutrophil-elastase resistant linker (EGCit) to enhance drug penetration into tumors. To evaluate our ADC, we will target CD44 variant 9 (CD44v9), a marker associated with poor patient outcomes and overexpressed in the subgroup of BC patients who are resistant or unresponsive to HER2-targeted therapy; notably >50% of all BC are Her2-low and CD44v9+. We hypothesize that antibodies binding the novel CD44v9 target and engineered for optimized selectivity and payload penetration will effectively home to and kill aggressive breast cancer. We will achieve this objective by developing tumor-selective antibodies binding CD44v9 (Aim 1), creating anti-CD44v9 ADCs equipped with tumor-penetrating peptides (Aim 2), and evaluating the efficacy of the ADCs and CD44v9 antibodies in orthotopic and humanized mouse models of HER2-negative BC and BC that is resistance to FDA-approved ADCs (Aim 3). This project is technically innovative for its development of conditionally-active antibodies, the use of tumor- penetrating peptides, the introduction of a novel linker, the evaluation of CD44v9 as a novel target, and the combination of these elements into a single ADC. The results will provide proof of concept for a novel ADC platform for aggressive or drug-resistant BC and provide mechanistic insights into the biological role of CD44v9 in BC. When successful, the newly developed ADCs will offer a critical path forward for patients with aggressive BC who have limited treatment options. Project Number: 1R01CA310174-01 | Fiscal Year: 2026 | NIH Institute/Center: National Cancer Institute (NCI) | Principal Investigator: JENNIFER MAYNARD (+2 co-PIs) | Institution: UNIVERSITY OF TEXAS AT AUSTIN, AUSTIN, TX | Award Amount: $668,878 | Activity Code: R01 | Study Section: Translational Immuno-oncology Study Section[TIO] View on NIH RePORTER: https://reporter.nih.gov/project-details/11320455