Abstract PO3-26-06: A self-replicating RNA Precision Immunotherapeutic for Overcoming Resistance to Endocrine Therapy in Estrogen Receptor Positive Breast Cancer (ER+BC)

Abstract Advanced or metastatic ER+HER2- breast cancer (ER+BC) is an incurable disease. Standard 1L therapy (SOC) utilizes endocrine blockade with a nonsteroidal aromatase inhibitor (NSAI) combined with a CDK4/6 inhibitor (CDK4/6i). While most patients respond and disease remains controlled for over 2 years (mPFS ~25-28 months), virtually all patients progress over time, requiring more toxic therapies with decreasing periods of disease control. Prolonging the time that the tumor remains under endocrine control is therefore of significant medical value. The most common molecular drivers of endocrine resistance are well characterized and include mutations in ESR1 and bypass mechanisms in select signaling pathways. RBI-1000 is a self-replicating RNA (srRNA) precision immunotherapeutic to generate robust immunity directed against acquired resistance mutations that develop in ER+ BC in response to endocrine blockade by NSAI. RBI-1000 includes 6 on-target mutations within the estrogen receptor ligand binding domain, and bypass mutations either in the form of activating mutations in PI3K kinase or amplifications of HER2/HER3. Precision immunotherapy (PIO), combined with SOC targeted therapy, educates the patient’s immune system to eliminate tumor cells that express acquired resistance mutations. Thus, the tumor is in a lose-lose situation where it is either eliminated by the targeted therapy, or if it develops acquired resistance mutations to the targeted therapy, it is killed by the PIO. Specifically for endocrine resistance, RBI-1000 PIO offers an attractive therapeutic approach to prevent/delay endocrine resistance specifically, by generating an effective immune response against those tumor cells that express the resistance-associated molecules that arise while on NSAI+CDK4/6i. The ability to encode multiple targets in a single therapeutic srRNA molecule, alongside the prolonged expression of srRNA in host cells (>49 days) creates a versatile and powerful cancer drug development platform. srRNA precision immunotherapeutics are delivered intramuscularly (i.e. a jab in the non-dominant deltoid) and generate an effective immune response via the same pathways and mechanisms as a traditional vaccine. We have demonstrated that this srRNA encapsulated in a lipid nanoparticle primes polyfunctional CD4 and CD8 T cells and antibody responses, leading to tumor growth inhibition and improved survival in a mouse model expressing the targeted acquired resistance mutations. Priming of T cells against acquired mutations is also confirmed in human HLA-transgenic mice. The immune cell-mediated elimination of clones expressing the acquired resistance mutations is predicted to prolong endocrine control of ER+BC, in an analogous manner to small molecule or monoclonal antibody targeted therapies, but with a more favorable dosing and adverse event profile due to precise immunologic targeting and no DDI. RBI-1000 is anticipated to enter clinical studies in the first half of 2024. Citation Format: Zelanna Goldberg, Christian Maine, Gabrielle Dailey, Christine Domingo, Gaelle Picarda, Hunter Little, Annie Chou, Jessica Sparks, Darina Spasova, Shiegeki Miyake-Stoner, Zachary C Hartman, Christopher Rabiola, Erika Crosby, Herbert Lyerly, Nathaniel Wang, Parinaz Aliahmad. A self-replicating RNA Precision Immunotherapeutic for Overcoming Resistance to Endocrine Therapy in Estrogen Receptor Positive Breast Cancer (ER+BC) [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 PO3-26-06.