Phase I/II first-in-human CAR T–targeting MUC1 transmembrane cleavage product (MUC1*) in patients with metastatic breast cancer.

TPS1130 Background: Metastatic breast cancer (MBC) remains incurable and novel immunotherapy for durable response remains an unmet need. Chimeric antigen receptor (CAR) T-cell therapy, an innovative form of immunotherapy wherein autologous T-cells are genetically modified to target tumor specific cell-surface markers, has been developed for treatment of solid tumors. huMNC2-CAR44 recognizes the growth factor receptor form of MUC1, which is the transmembrane cleavage product called MUC1*. MUC1* is a Class I growth factor receptor that is activated by ligand-induced dimerization of its truncated extra cellular domain, which activates the MAP kinase signaling pathway as well as survival pathways. Onco-embryonic growth factor NME7 AB binds to an ectopic site on MUC1* that is only unmasked after MUC1 is cleaved and the tandem repeat domain is shed from the cell surface. The targeting head of the CAR, huMNC2, competes with NME7 AB for binding to this ectopic site. huMNC2 does not bind to full-length MUC1, hence highly tumor-selective. 70% of solid tumor cancers express a huMNC2 reactive MUC1* and huMNC2-scFv bound robustly to 93% of the breast cancers with minimal staining of normal tissues. huMNC2-CAR44 T cells completely obliterated a variety of MUC1* positive solid tumors in NSG mice in vivo. IND enabling animal studies demonstrated that huMNC2-CAR44 T potently inhibited MUC1* positive tumors xenografted into female NSG mice, whether the tumor cells were MUC1 negative cells stably transduced with MUC1* or breast cancer cells such as T47D that naturally express MUC1*. In one study, huMNC2-CAR44 T treated mice survived tumor-free for over 12 weeks, whereas control group had to be sacrificed at 3 weeks due to disease progression. Methods: This is a first-in human, phase I/II trial evaluating the safety and efficacy of huMNC2-CAR44 T in patients with MBC. Key inclusion criteria include age ≥18 years, ECOGPS 0-1, available FFPE tumor sample, tumor IHC ≥30% MUC1* and preserved organ function. Dose escalation is standard 3+3 design with dosing levels ranging from 3.3x10^5 to 1.0x10^7 CAR+ cells/kg, and fludarabine/cyclophosphamide lymphodepletion pre-treatment. Phase I accepts patients with MBC that has progressed through at least 3 previous lines of therapy. The primary objective of Phase I is to determine safety and determine a recommended Phase II dose (RPIID), with the exploratory objectives of assessing CAR T cell expansion, persistence, tumor penetration and potential tumor escape. Six (6) patients have been enrolled and five (5) patients have been treated to date. Phase II will be comprised of 3 cohorts of 15 patients in each arm of luminal, HER2+ and triple negative breast cancers for a total of 45 patients in Phase II. Clinical trial information: NCT04020575.