Purpose: Over 75% of all solid tumor cancers are characterized by aberrant expression of MUC1. Yet to date, no effective MUC1 targeting therapeutic has succeeded. The purpose of this body of work was to develop an antibody therapeutic that would effectively treat MUC1 positive solid tumor cancers, without damaging healthy MUC1 positive tissues.

Experimental Procedures: MUC1 is a heavily glycosylated transmembrane protein whose expression is restricted to the apical border of healthy epithelial tissues but uniformly expressed over cancerous tissues. We discovered that the cancer-associated form of MUC1 is transmembrane cleavage product that remains after cleavage and release of the bulk of the extracellular domain. We named cleaved MUC1, MUC1* (muk 1 star), and showed that it is a growth factor receptor that is activated by ligand-induced dimerization of its small extracellular domain. We also showed that 100% of pluripotent human stem cells express MUC1* but cleavage stops with the onset of differentiation. By studying stem cells and cancer cells in parallel, we sought to develop antibodies that would only recognize MUC1* as it is expressed on cancerous tissues but would not recognize MUC1* as it exists on some healthy cells and tissues.

Summary New Data: We previously showed that we had developed antibodies that recognized the cleaved, growth factor receptor form, MUC1* but not full length MUC1. However, MUC1 is cleaved to a healthy form of MUC1* on, for example, pluripotent stem cells, hematopoietic stem cells and stem cells of intestinal crypts. We now report that we have developed monoclonal antibodies that only recognize the cancerous form of MUC1* and do not recognize these healthy forms of MUC1*. To demonstrate that we have succeeded in deciphering the differences between cancerous MUC1* and healthy MUC1*, we also developed a set of monoclonal antibodies that only recognize healthy MUC1*. Thousands of human cancerous versus normal tissue specimens attest to the specificity of these antibodies. Cancer-specific anti-MUC1* antibodies (Fabs) alone blocked the growth of MUC1* positive breast and hormone refractory prostate cancers in animals, with no detectable adverse effects. On the basis of tissue safety studies and efficacy studies, a clinical candidate antibody MNC2 has been identified, humanized and incorporated into more than 20 CAR constructs. huMNC2-scFv-CAR T cells display typical cytokine release only when co-cultured with MUC1* positive cancer cells. huMNC2-scFv-CAR T cells selectively kill MUC1* positive cancers, while stimulating T cell expansion.

Conclusions: The relevant target for anti-cancer drugs is the extracellular domain of MUC1*, not full length MUC1. MUC1* is a growth factor receptor that is activated by NME family growth factors. Subtle differences between MUC1* as expressed on healthy stem-like cells and MUC1* expressed on cancerous cells has allowed the development of cancer-specific MUC1* antibodies, especially suited for cancer immunotherapies.

Citation Format: Cynthia Carol Bamdad, Andrew K. Stewart, Benoit J. Smagghe, Luke T. Deary, Victoria L. Kohler, Jared L. Dietz. MUC1* targeting CAR T [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3330. doi:10.1158/1538-7445.AM2017-3330