Abstract 6770: Combining RAS inhibitors with the clinical RNR inhibitor BBI-825 to prevent the emergence of extrachromosomal DNA (ecDNA)-driven resistance to RAS-targeted therapies in colorectal cancer

Abstract Introduction: The advent of new RAS inhibitors provides an exciting chance to extend targeted therapy to the half of colorectal cancers (CRC) with KRAS mutations. However, the rapid development of resistance limits the clinical benefit of this approach. Focal and ecDNA-driven amplifications are a common mechanism of resistance; in the phase 1 trial of daraxonrasib (RAS multi ON), acquired mutant KRAS amplifications occurred in 30% of patients. In silico, ecDNA+ tumor cells show a strong survival dependence on ribonucleotide reductase (RNR), the rate-limiting enzyme in the de novo synthesis of deoxyribonucleotide triphosphates (dNTPs). BBI-825, an oral and selective small molecule inhibitor of RNR, has entered clinical trials. We investigated whether BBI-825, in combination with different RAS inhibitors, can delay or overcome the development of acquired resistance in CRC. Based on the vulnerability of ecDNA+ cells to BBI-825, we anticipated response may vary by tumor microsatellite status and evaluated microsatellite stable (MSS) and instable (MSI) CRC models. Methods: Human cancer models were treated with combined RAS and epidermal growth factor receptor (EGFR) antibodies. We used parental and resistant models generated from murine and human cancer cell lines, cell derived xenografts (CDX) and patient derived xenografts (PDX) to compare the effect of RAS inhibition alone and in combination with BBI-825 on tumor growth, signaling and acquired resistance. RAS inhibitors used were MRTX1133 and zoldonrasib (KRASG12D), sotorasib and adagrasib (KRASG12C) and daraxonrasib. Metaphase fluorescence in-situ hybridization (FISH) was used to evaluate ecDNA presence. Results: The addition of BBI-825 to RAS inhibitor (plus anti-EGFR antibody in human cells) delayed or prevented the emergence of resistance in the MSS parental cell lines, CDX and PDX. This combination suppressed the emergence of ecDNA-driven KRAS, RAF1 and MYC amplifications. In MSS CRC models that had already acquired resistance to RAS inhibitors, the addition of BBI-825 to RAS plus EGFR inhibition slowed tumor growth and reduced the copies of putative resistance gene amplifications. In contrast, in the MSI model, acquired resistance was likely mediated by new mutations in KRAS, and addition of BBI-825 did not significantly impact acquired resistance or tumor growth. Conclusions: We show in multiple models of MSS CRC that the selective RNR inhibitor BBI-825 combined with RAS inhibitors delayed or prevented RAS targeted therapy-acquired resistance in a novel approach that subverts the emergence of resistance. The selective sensitivity of MSS CRC models, particularly those that harbor ecDNA, further supports the distinct mechanism of action of BBI-825 and this potential new and important strategy to improve RAS inhibitor efficacy. Citation Format: Silvia Palladino, Joshua Lange, Nicholas Catalano, Salvador Garcia, Ryan Hansen, Huiyong Zhao, Prajakta Paresh Kokate, Elisa De Stanchina, Christian Hassig, Rona Yaeger. Combining RAS inhibitors with the clinical RNR inhibitor BBI-825 to prevent the emergence of extrachromosomal DNA (ecDNA)-driven resistance to RAS-targeted therapies in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6770.