Abstract PO1-26-05: Animal Study Compares CAR T cell Exhaustion & Ability to Kill Low Antigen expressing breast cancer cells among three CAR constructs including one with 1XX mutations

Abstract Background: CAR T cells for the treatment of solid tumor cancers has not yet achieved the same success as CAR T cells for treatment of blood cancers. Two of the hurdles that must be cleared for effective use of CAR T cells for solid tumor cancers are: 1) CAR T cell exhaustion; and 2) failure to recognize and kill low antigen expressing cancer cells. A promising approach to overcoming CAR T cell exhaustion, referred to as “1XX” was developed in the Sadelain Lab at MSKCC. Mutation of Tyrosines to Phenylalanine in ITAMs 2 and 3, of the CD3z signaling domain, restrict signaling to ITAM 1. This slowing down of signaling has been reported to be effective at eliminating or greatly reducing CAR T cell exhaustion in blood cancers [Feucht, et al 2019; Park et al 2022; Schoutrop, et al 2023]. Purpose: To evaluate the potential of the 1XX mutations to overcome CAR T cell exhaustion and inability to kill low antigen expressing cells in an animal model of breast cancer. Specifically, we tested the ability of three different CARs to eliminate human breast tumors that expressed variable levels of the target antigen, MUC1*, and their ability to suppress tumor recurrence over approximately 100 days. Methods: All three CARs were targeted to the tumor by the same antibody fragment, huMNC2, that recognizes MUC1*, which is the transmembrane cleavage product of MUC1 that functions as a potent growth factor receptor. The CARs are: 1) huMNC2-41BB-CD3z; 2) huMNC2-CD28-CD3z; and 3) huMNC2-CD28-1XX. Heterogeneous tumors expressing different levels of target antigen were made as follows. T47D breast cancer cells were engineered to express even more of the target, MUC1*, and were also engineered to fluoresce green. T47D wild type cells were engineered to fluoresce red. Heterogenous tumors consisting of 250,000 cells were implanted sub-cu into female NSG mice bearing 90-day estrogen pellets. The tumors comprised either 70% wild-type/30% overexpressing cells, or 85% wild-type/15% overexpressing cells or 92.5% wild-type/7.5% overexpressing cells. Tumor engraftment was verified by bioluminescence at Day 6 post implantation. There was a total of 150 animals, 5 animals per group. Animals were administered a single dose of CAR T cells into the tail vein on Day 7, wherein the Effector to Target ratio was either 10:1, 5:1 or 1:1. Tumor volume was measured weekly by Luciferase/Luciferin bioluminescence on an IVIS instrument. In addition, the red versus green fluorescence of the tumor was tracked periodically as an indicator of which cells, high or low antigen expressing, were being killed. Between Day 93 and Day 96, animals were sacrificed, cells were recovered from blood and spleen, recovered tumors were weighed, tumor cells dissociated and fluorescent images were captured to determine which cells escaped CAR T cell killing. Conclusions: The CAR with the 1XX mutations in CD3z, huMNC2-CD28-1XX, was much more effective at suppressing breast tumor recurrence than either CAR with wild-type CD3z. At sacrifice, significantly more CAR T cells were recovered from huMNC2-CD28-1XX than from huMNC2-41BB-CD3z or huMNC2-CD28-CD3z. At high dose, the CARs with wild-type CD3z effectively suppressed the high antigen expressing cells, but the recurrent tumors were essentially made up of the low antigen expressing cells that had escaped CAR T cell killing. At low CAR T cell dose, the CARs with wild-type CD3z appeared to become exhausted about 40 days post treatment, when tumors began to recur. Post sacrifice analysis of the recurrent tumors showed that they were made up of both high and low antigen expressing cells. huMNC2-CD28-1XX effectively killed both the high antigen and low antigen expressing cells as evidenced by the live fluorescent imaging and the post-sacrifice analysis of residual tumor. Citation Format: Cynthia Bamdad, Andrew Stewart, Benoit Smagghe, Mark Carter, Danica Walkley, Kevin Yi, Jac-Leen Nash, Michael Nash, Trevor Grant, Gregory Riley. Animal Study Compares CAR T cell Exhaustion & Ability to Kill Low Antigen expressing breast cancer cells among three CAR constructs including one with 1XX mutations [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-26-05.