Abstract 6941: Bi- and tri-specific ADCs that target two distinct growth factor receptor forms of MUC1* and HER2 or alpha-v-beta-6 inhibit tumor recurrence and overcome acquired resistance

Abstract Purpose: Develop more effective cancer therapeutics by simultaneously targeting multiple drivers of growth, metastasis and resistance. Methods: MUC1 is aberrantly expressed on over 75% of solid tumor cancers. Although cloned 30 years ago, there is still no approved MUC1-targeted drug. Previous attempts, including those targeting trapped glycans, targeted the tandem repeat domain, which is shed after cleavage by cancer-associated enzymes. MUC1* with a 45-amino acid ecd (extracellular domain) is the only form that has been shown to function as a growth factor receptor via dimerization of ecd by embryonic growth factors. We show that where MUC1 is cleaved determines the conformation of the remaining ecd. Even within a truncated 45-amino acid ecd, there are distinct conformations that only exist on cancer cells and others that are only on progenitor cells. All these species have been mistakenly grouped together as “MUC1-C”. We developed monoclonal antibodies that specifically bind to each conformation and mapped their expression on cancer vs normal tissues vs progenitor cells. We then generated bi- and tri-specific ADCs that simultaneously bind to 2 or 3 cancer targets. Results: Data show there are 2 distinct MUC1* receptors that drive tumor growth and metastasis. Targeting one and not the other led to tumor recurrence. We developed bi- and tri-specific antibodies that simultaneously bind to both MUC1* growth factor receptors, and HER2 or alpha-v-beta-6. In antibody internalization experiments, multi-specific antibodies were internalized faster and more completely than mono-specifics. In ADC format, bi-specific antibodies that hit both MUC1* growth factor receptors inhibited tumor recurrence in animals. ADCs incorporating both a MUC1* antibody and Trastuzumab killed Trastuzumab-resistant cancer cells with an IC50 of 0.05 nM compared to 0.73 nM on Trastuzumab sensitive parent cells. Indeed, in Phase I huMNC2-CAR44 trial, patients who had acquired resistance to Trastuzumab or Fam-Trastuzumab Deruxtecan-nxki were best responders in our trial with 75% DCR, 11-month OS, 11.3 month increased survival in patients with high MUC1* expression. In animals, our novel linker payload out-performed industry gold standard Deruxtecan. Conclusions: It is critical that MUC1-targeted therapeutics hit the cancer-specific forms of MUC1*. Therapeutics that incorporate antibodies that bind to hematopoietic stem cells, for example, would be disastrous if given to cancer patients. Therapeutics that target full-length MUC1, whether aberrantly glycosylated or not, could give initial positive responses via ADC bystander killing. However, the hypothesis is that by killing cells expressing full-length MUC1, which plays no role in cell growth, the MUC1*-positive cell population would be enriched and would accelerate tumor growth and metastasis. Citation Format: Cynthia Carol Bamdad, Benoit J. Smagghe, Scott Moe, Mark G. Carter, Kevin R. Yi, Michael J. Nash, Robert McDermott, Trevor J. Grant, Salvatore Marchese, Daniel S. Miller, Natalie K. Miller, Andrew K. Stewart, . Bi- and tri-specific ADCs that target two distinct growth factor receptor forms of MUC1* and HER2 or alpha-v-beta-6 inhibit tumor recurrence and overcome acquired resistance 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 6941.