First-in-human CAR T targets MUC1 transmembrane cleavage product

ABSTRACT

Purpose: To develop a MUC1-targeted CAR T that recognizes the growth factor receptor form, MUC1∗, does not bind full-length MUC1, hits a wide range of solid tumor cancers, binds to little or no normal tissues, and effectively kills tumor cells. Methods: Because MUC1 is expressed on normal epithelial tissues, we needed to develop an antibody that would only bind to the aberrant, cancerous form - MUC1∗. MUC1∗ (muk1 star) is the transmembrane portion that remains after MUC1 is enzymatically cleaved and the bulky tandem repeat domain is shed from the cell surface. MUC1∗ is a growth factor receptor that is activated by ligand-induced dimerization of its truncated extracellular domain. Via a novel screen we identified antibodies that bind to a specific conformation within the ectopic epitope that is created when MUC1 is cleaved to MUC1∗ by enzymes secreted by the tumor microenvironment. This set of antibodies competitively inhibit the binding of onco-embryonic growth factor NME7AB to the cancerous form of MUC1∗. We incorporated one of these cancer-specific antibodies into a CAR T. Results: huMNC2-CAR44 is in a 1st-in-human clinical trial [NCT04020575] for metastatic breast cancers, currently being performed at the Fred Hutchinson Cancer Research Center. Our IND-enabling studies showed that huMNC2-scFv bound robustly to 95% of breast cancer tissues, 83% ovarian cancers, 78% pancreatic cancers and 71% of lung cancer tissues, but showed little to no binding to normal tissues. In co-culture experiments, huMNC2-CAR44 T cells did not kill MUC1∗ negative cells, even if they expressed full-length MUC1, and the presence of MUC1∗ negative cells did not elicit a cytokine response from the CAR T cells. In vivo, huMNC2-CAR44 T cells inhibited or completely obliterated a variety of MUC1∗ positive solid tumors in NSG mice (n≥400). The human CD8+ huMNC2-CAR44 T cells expanded in animals as tumors shrunk, whereas the untransduced T cells did not. Clinical trial was slowed by COVID-19, as Seattle was the first hotbed of the virus. Thus far, there have been no serious adverse events attributed to the CAR T therapy. Even at the lowest dosage, patients have had robust CAR T cell expansion and have also had measurable signs of efficacy. Conclusions: MUC1∗ is the predominant form of MUC1 on cancerous tissues. Antibodies that bind to a specific conformation within the ectopic growth factor binding site in the MUC1∗ extra cellular domain are tumor selective. CAR T cells incorporating these antibodies are highly effective against solid tumors in animals. Robust staining of cancerous tissues and minimal to no staining of normal tissues predicts a large therapeutic window for huMNC2-CAR44 T cell dosing. Early patient responses appear to fulfill the predictions of the IND-enabling studies. We have now developed a cryopreservation formulation which enables shipping frozen product to additional clinical sites for bedside thaw and infusion. The trial is currently enrolling patients.