Abstract PS07-07: The RXR agonist, IRX4204, delays the formation of Brca1 mutant mammary tumors via modulation of the anti-tumor immune response

Abstract Background: Women with germline BRCA1 mutations are at an increased risk for developing breast cancer in their lifetime, often at a young age with more aggressive tumors. At present, prophylactic bilateral mastectomy is the most effective strategy for reducing breast cancer risk. However, this invasive procedure is irreversible and associated with potential complications. We and others have found that PARP inhibitors can delay Brca1-mutant tumor formation in mice and could be beneficial for the prevention of breast cancer. However, currently available PARP inhibitors are associated with modest toxicities that may not be acceptable to women without cancer. Thus, there remains an urgent need for the development of safe and effective therapies for the prevention of breast cancer. Here, we present data demonstrating the activity of IRX4204, a minimally toxic and highly specific agonist of the nuclear retinoid X receptor (RXR), to delay the formation of mammary tumors in a Brca1-deficient mouse model. This inhibitory effect on tumor growth is due, in part, to the role of IRX4204 in stimulating the anti-tumor immune response. Methods: We used the established MMTV-Cre, conditional Brca1 gene knockout, p53 heterozygous loss mouse model (BRCA1co/co; MMTV-Cre+/+; p53+/-) and selected for mutant female pups using PCR genotyping. At 16 weeks of age, all mice were separated into 4 treatment groups (n=10 per group): (1) sesame oil control; (2) the novel RXR agonist, IRX-4204 (10mg/kg); (3) high-dose IRX4204 (20 mg/kg); and (4) the RXR agonist, 9-cis-UAB-30 (5 mg/kg). All treatments were given by oral gavage five days per week, and mice were observed daily for tumor formation and toxicity. At the study endpoint, tumors and normal mammary glands were collected for additional analyses. Immunohistochemical staining was used to quantify CD8a, Ki-67 and cleaved caspase 3 expression in Brca1-deficient tumors. Oil Red O staining was used to measure changes in lipid accumulation in Brca1-deficient cell lines treated with IRX4204. qPCR was used to quantify the changes in gene expression of lipid metabolism-associated genes upon treatment with IRX4204 in vitro. Results: Vehicle-treated Brca1-deficient mice had a median time-to-tumor formation (TTF) of 211 days, with 100% developing tumors by 330 days. Mice treated with UAB 5 mg/kg had an improved median TTF of 261 days, whereas mice treated with IRX4204 10mg/kg or 20mg/kg had a median TTF of 347 and 304 days, respectively (p 0.01). In addition, 60% of mice treated with IRX4204 10 mg/kg remained tumor-free at 330 days. IRX4204-treated tumors showed an increased infiltration of CD8-positive T-cells over vehicle-treated tumors (p 0.05). Treatment of Brca1-deficient cell lines with IRX4204 in vitro resulted in a significant increase in lipid accumulation accompanied by a 2-fold increase of Srebf1 expression (a key transcription factor that regulates lipid homeostasis) within 24 hours of treatment (p 0.05). Conclusion: These data demonstrate a novel use of the RXR agonist, IRX4204, to delay the formation of Brca1-deficient mammary tumors. We have found that IRX4204 treatment modulates the tumor immune response through increased infiltration of cytotoxic CD8-positive T-cells in Brca1-deficient mammary tumors in vivo. We have also determined that IRX4204 modulates lipid metabolism in breast cancer cell lines in vitro. It is known that lipid-derived antigens can stimulate T-cell activity. Our findings suggest that RXR agonists may alter lipid antigen production to activate an anti-tumor response. Additional immune and lipidomic studies are on-going. This work was supported by NCI-PREVENT grant (to PB and AM HHSN26100008) and CFP Foundation (Odyssey Fellowship to CM). Citation Format: Cassandra Moyer, Darian Coleman, Jamal Hill, Lana A. Vornik, Michelle Savage, Martin Sanders, Sei Shizuko, Altaf Mohammed, Powel Brown, Abhijit Mazumdar. The RXR agonist, IRX4204, delays the formation of Brca1 mutant mammary tumors via modulation of the anti-tumor immune response 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 PS07-07.