Abstract 2323: First-in-human CAR T for solid tumors targets the MUC1 transmembrane cleavage product
Authors: Bamdad Cynthia C. , Glennie Nelson D. , Stewart Andrew K. , Huang Pengyu , Smagghe Benoit J. , Swanson Tyler E. , Hanahoe Erin K. , Riley Gregory L. 
Published: August 2019

Purpose: To develop a CAR T for solid tumors that hits a wide range of cancers, is effective and has little or no effect on normal tissues.

We developed a MUC1* targeting CAR T that is scheduled for a 1st-in-human clinical trial for metastatic breast cancers at the Fred Hutchinson Cancer Research Center in Q1 2019.

Unlike previous attempts at making an anti-MUC1 cancer therapeutic, huMNC2-CAR44 targets MUC1*, which is the transmembrane cleavage product. MUC1* is a growth factor receptor that is activated when onco-embryonic growth factor NME7AB dimerizes its truncated extra cellular domain. The binding site for NME7AB is masked in full-length MUC1; anti-MUC1* antibody huMNC2 binds to the same site. huMNC2 does not bind to full-length MUC1 nor other cleaved forms of MUC1 that are on some healthy tissues that need to rapidly divide.

Monoclonal antibodies against the truncated MUC1* extra cellular domain were generated and screened by IHC for reactivity to cancerous tissue micro arrays (TMAs) and lack thereof on normal tissues. The best monoclonals were incorporated into CARs, transduced into human T cells and tested in vitro for specific killing of MUC1* positive but not MUC1* negative cells. We developed a novel cell line in which MUC1 is not cleaved. By adding specific cleavage enzymes, we identified antibodies that recognized conformational epitope that were created by specific cleavage enzymes. The final selection of an anti-MUC1* antibody for the targeting head of our CAR was based on its widespread binding to breast cancer tissues, low cross reactivity to normal tissues, and its recognition of a conformational epitope created when MUC1 is cleaved to MUC1* by a specific cleavage enzyme that is overexpressed in many cancers, especially breast cancers. In vivo we showed that injecting this cleavage enzyme near a MUC1/MUC1* breast tumor dramatically accelerated tumor growth, which was stopped by simultaneous injection of the cleavage enzyme and our CAR T cells.

IHC studies of thousands of normal vs. cancerous human tissue specimens show that huMNC2-scFv almost exclusively binds to tumor tissues, hitting over 90% of breast, 83% ovarian, 78% pancreatic and 71% of lung cancers. Recognition of breast cancer specimens appears not to be limited by cancer sub-type.

In vivo experiments of human tumors in NSG mice (n&gt;300), show that huMNC2-CAR44 T cells inhibited or completely obliterated a variety of MUC1* positive solid tumors. Dual tumor experiments showed that adequate MUC1* density is required for a CAR T response, further supporting the idea that huMNC2-CAR44 T cells will selectively kill MUC1* positive tumors, while sparing normal tissues.

Conclusion: MUC1* is the predominant form of MUC1 present on cancers. Antibodies that target conformational epitopes produced by specific subsets of cleavage enzymes make anti-MUC1* CAR T cells highly tumor selective. CARs could be patient specific based on which cleavage enzymes their tumors express.

Citation Format: Cynthia C. Bamdad, Nelson D. Glennie, Andrew K. Stewart, Pengyu Huang, Benoit J. Smagghe, Tyler E. Swanson, Erin K. Hanahoe, Gregory L. Riley. First-in-human CAR T for solid tumors targets the MUC1 transmembrane cleavage product [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2323.