Abstract B082: A novel, potent and selective ribonucleotide reductase (RNR) inhibitor, BBI-825, blocks extrachromosomal DNA (ecDNA) amplification-mediated resistance to KRASG12C inhibitor in colorectal cancer (CRC)
Authors: Chowdhry Sudhir , Hansen Ryan , Mauger Jacques , Pferdekamper AnneMarie , Tse Edison , Norman Ben , Moininazeri Jafar , Garcia Salvador , Truong Yen , Liao Debbie , Lange Joshua , Holmes Evan , Celeste Anthony , Solis Dave , Steffy Auzon , Plum Joshua , Wiese Julie , Apuy Julius , Ardeshiri Ardalan , Milutinovic Snezana , Wilkinson Deepti , Pinkerton Anthony , Hassig Christian , Kasibhatla Shailaja 
Published: December 2023

Abstract
Introduction: KRASG12C inhibitors are a new therapeutic approach to treat KRAS mutant-driven cancers, but as with other oncogene directed therapies, adaptive resistance develops quickly and frequently. The predominant resistance mechanisms are 1) secondary mutations in KRAS and/or other mitogen-activated protein kinase (MAPK) signaling genes and 2) amplification of KRAS or MAPK genes. Extrachromosomal DNA (ecDNA) are the predominant site of focal high copy number oncogene or resistance gene amplifications in human cancers. Previous studies have demonstrated KRASG12C amplification on ecDNA to be an acquired resistance mechanism after treatment with approved KRASG12C inhibitors. Cancers enabled by ecDNA remain underserved by current therapeutic options, including targeted therapies, and have poor prognosis. An in silico “loss of function” screen revealed ribonucleotide reductase (RNR) as a critical target required for the survival of ecDNA amplified tumor cells. RNR is the rate-limiting enzyme in the biosynthesis of deoxyribonucleotide triphosphates (dNTP), the building blocks of DNA and ecDNA replication and repair. While non-selective drugs with RNR inhibitory activity (e.g., gemcitabine) indirectly validate RNR as a cancer target, no selective small molecules targeting RNR have been approved.
Methods: We have developed BBI-825, a potent, selective, and orally bioavailable RNR inhibitor (RNRi) for the treatment of ecDNA-enabled oncogene or resistance gene amplified cancers.
Results: BBI-825 inhibits RNR with an IC50 of 20 nM in vitro and reduces deoxyadenosine triphosphate (dATP) levels in ecDNA amplified cancer cells, with an EC50 of less than 100 nM. On target selectivity of BBI-825 was confirmed using SAFETYscan cellular screening assay investigating target selectivity over 78 targets spanning hormone receptors, kinases, calcium, GPCR, ion channel, and transporter signaling. Importantly, BBI-825 prevented resistance-conferring ecDNA-enabled amplification of KRASG12C in KRASG12C mutated colorectal cancer cells treated with the KRAS inhibitor adagrasib in vitro. Prevention of ecDNA amplification correlated with antiproliferation of BBI-825-treated tumor cells in long-term cultures. Treatment of KRASG12C mutated CRC tumor xenograft models with single agent adagrasib resulted in rapid acquired resistance through KRASG12C amplification on ecDNA. In concordance with in vitro observations, co-treatment with BBI-825 delayed the development of adagrasib resistance concurrent with an abrogation of ecDNA-enabled KRASG12C amplification (P=0.001). Furthermore, BBI-825 significantly inhibited the growth of pre-established adagrasib-resistant KRASG12C ecDNA-amplified tumors in vivo (tumor growth inhibition (TGI) = 92%, p&lt;0.002), reinforcing clinical actionability in the KRAS inhibitor-refractory setting.
Conclusions: These findings support the advancement of BBI-825 as a selective RNR inhibitor for the treatment of ecDNA-enabled oncogene or resistance gene amplified cancers, including KRASG12C mutant CRC.
Citation Format: Sudhir Chowdhry, Ryan Hansen, Jacques Mauger, AnneMarie Pferdekamper, Edison Tse, Ben Norman, Jafar Moininazeri, Salvador Garcia, Yen Truong, Debbie Liao, Joshua Lange, Evan Holmes, Anthony Celeste, Dave Solis, Auzon Steffy, Joshua Plum, Julie Wiese, Julius Apuy, Ardalan Ardeshiri, Snezana Milutinovic, Deepti Wilkinson, Anthony Pinkerton, Christian Hassig, Shailaja Kasibhatla. A novel, potent and selective ribonucleotide reductase (RNR) inhibitor, BBI-825, blocks extrachromosomal DNA (ecDNA) amplification-mediated resistance to KRASG12C inhibitor in colorectal cancer (CRC) [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B082.