Abstract A040: Remarkable in vivo responses to Ras targeting drug Daraxonrasib in a chemotherapy-resistant <b> <i>MMTV-Kras</i> </b> G12D syngeneic mammary tumor model

Abstract While most breast cancers are treated successfully by targeting hormone receptor signaling or HER2/Neu, the remaining tumors, known as Triple Negative Breast Cancers (TNBC), are typically managed with chemotherapy regimens that are often unsuccessful. TNBC do not share a common alteration, and thus diverse precision medicine strategies targeting rare drivers will have to be employed to improve treatments. The RAS pathway is altered in almost 12% of breast cancers. Activation occurs by gene amplification, activating point mutations in HRAS, NRAS, KRAS or the downstream target BRAF, or by inactivating mutations in the RAS inhibitor and tumor suppressor NF1. Thus, testing RAS inhibition in TNBC is warranted. To develop a model for RAS-driven mammary tumors, we generated MMTV-KrasG12D (murine gene) transgenic mice. Female mice developed Trp53 wild-type spontaneous mammary tumors at median latency of 13.5 months in virgin females, and 11 months in mice that had 1 litter. Spontaneous tumors were successfully transplanted orthotopically into syngeneic C57Bl/6j mice and were used as a model for treatment studies. We first examined response to the commonly used breast cancer chemotherapeutic Doxorubicin. As is known for p53 wild-type breast tumors, we observed a poor response, extensive residual disease, and induction of cellular senescence post-treatment. RNA-seq showed induction of p53 targets and senescence-associated immune modulatory genes and cytokines, while cell cycle genes were repressed. Treated tumors stained highly positive for the senescence marker SA-beta-Gal, but stained negative for the incorporation of S-phase marker BrdU. Investigating a precision medicine based therapy as an alternative, we found that ex-vivo plated cells from a MMTV-KrasG12D tumor were sensitive to drugs targeting mutant Kras (Zoldonrasib RMC-9805) or activated Ras (Daraxonrasib, RMC-6236). Phospho-ERK (pERK) and phospho-S6 (pS6) levels were markedly reduced after just 4h at 100nM concentrations. In vivo, a single dose of Zoldonrasib (100mg/kg) or Daraxonrasib (25mg/kg) in mice with orthotopic mammary transplants of MMTV-KrasG12D tumors resulted in near-complete inhibition of pERK by immunoblot, persisting up to 24h after treatment. Daraxonrasib (25mg/kg) treatment of MMTV-KrasG12D tumor-bearing mice for 7 days resulted in striking tumor responses, reducing the volume of large tumors by ∼80% (n=9) with no observable toxicity, though tumors relapsed after cessation of treatment. In sum, using an orthotopic, syngeneic model of Ras-driven mammary tumors, we show Ras targeting therapies overcome the inherent resistance and poor response of p53 wild-type tumors to chemotherapies. Ongoing in vivo studies will examine tumor autonomous transcriptome and cellular responses to Ras therapeutics and how the immune system contributes to regression and eventual relapse. Citation Format: Mehrnoosh Arabi, Wesley Frey, Zeinab Mheidly, Raegan Kvadas, Ashkan Shahbandi, Nathan Ungerleider, James G. Jackson. Remarkable in vivo responses to Ras targeting drug Daraxonrasib in a chemotherapy-resistant MMTV-Kras G12D syngeneic mammary tumor model [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: RAS Oncogenesis and Therapeutics; 2026 Mar 5-8; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(5_Suppl_1):Abstract nr A040.