Abstract PS3-13-21: The RXR agonist IRX4204 promotes cytotoxicity of Her2+ breast cancer cells by Her2-targeted CAR-T cells

Abstract Research Objectives and Rationale: IRX4204 is a potent and highly RXR nuclear receptor- selective agonist compound. It is 100-fold more potent as an RXR agonist than bexarotene, the only FDA-approved RXR agonist (approved for treatment of cutaneous T-cell lymphoma). IRX4204 is far more RXR selective than bexarotene, being devoid of RAR, LXR, PPAR gamma nuclear receptor agonism. With our collaborators we have previously reported that: (1) IRX4204 has synergistic cytotoxic effects with Her2-targeted antibodies, Her2-tyrosine kinase inhibitors, and paclitaxel on Her2+ breast cancer cell lines; (2) IRX4204 promotes apoptosis of Her2+ breast cancer cells; (3) IRX4204 promotes in vivo CD8 T-cell infiltration into murine Brca1 mutant breast cancer tumors. (Moyer et. al., Clinical Cancer Research, June 2024; and Moyer et. al., poster presented at SABCS 2023). CAR-T cells targeted against various types of hematologic malignancies have been highly clinically effective. However, progress in developing effective CAR-T cells against solid tumors, such as breast cancers, has been slow. Based on the previously reported combination effects of IRX4204 with anti-Her2-targeted agents on Her2+ breast cancers, we hypothesized that IRX4204 may have combination effects with Her2-targeted CAR-T cells on killing of Her2+ breast cancer. We now report that IRX4204 promotes infiltration and cytotoxicity of cultured spheroids of the Her2+ human breast cancer cell line SKBR3 by Her2-targeted CAR-T cells. Experimental Methods: Initial experiments examined effects of IRX4204 on expression of cell surface Her2 and various adhesion molecules on cultured SKBR3 cells by flow cytometry. To evaluate combination treatment effects with IRX4204 and Her2-targeted CAR-T cells, we obtained Her2-targeted human CAR-T cells from a commercial source (Promab: PM-CAR1024-1M; Her2 scf-v-CD28CD3zeta). SKBR3 cells were stained with cell tracker deep red and cultured till forming spheroids. Her2-targeted CAR-T cells were stained with cell trace violet and added to the SKBR3 cells at effector to target ratios of 0, 2.5, 5, and 10 to 1. Following 72 hours of co-incubation of SKBR3 cells with CAR-T cells, IRX4204 was added to the cultures at 10, 100, and 1000 nM concentrations, for 24 hours. Sytox Green Ready reagent was added to the cultures to quantitate cell death. Images were analyzed using automated systems to quantitate T-cell infiltration and cytotoxicity. Results: IRX4204 did not affect expression of cell surface Her2 on SKBR3 cells. IRX4204 substantially increased expression of the adhesion molecule ICAM-1 on SKBR3 cells, an effect which should promote T-cell adhesion, infiltration, and cytotoxicity by CAR-T cells. IRX4204 increased infiltration of CAR-T cells into SKBR3 spheroids in a dose dependent manner. IRX4204 alone had minimal cytotoxic effect on SKBR3 cells under these test conditions. However, IRX4204 synergistically promoted Her-2 targeted CAR-T cytotoxicity of SKBR3 Her2+ breast cancer cells. Conclusions: Combination of the RXR agonist IRX4204 with Her2-targeted CAR-T cells showed a dose dependent increase in CAR-T infiltration and death of spheroids of Her2+ SKBR3 breast cancer cells in vitro. These promising results support further development of Her2-targeted CAR-T cells, and combination of such cells with the RXR agonist IRX4204 as a therapeutic option for patients with advanced Her2+ breast cancer. Citation Format: M. E. Sanders, V. Vuligonda. The RXR agonist IRX4204 promotes cytotoxicity of Her2+ breast cancer cells by Her2-targeted CAR-T cells [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-13-21.