Abstract 613: Oral CHK1 inhibitor BBI-355 allows flexibility of dose and schedule with demonstration of monotherapy and combinational antitumor activity in extrachromosomal DNA (ecDNA) driven preclinical models

Abstract Focal, high copy number amplifications of prevalent oncogenes (e. g. , MYC, EGFR) frequently occur on extrachromosomal DNA (ecDNA), highly transcribed units of circular, non-chromosomal DNA. While use of targeted therapies is associated with improved survival in patients with cancers that have activating oncogene mutations or fusions, they have demonstrated limited activity for patients whose tumors harbor oncogene amplifications, including those mediated by ecDNA. We have previously identified Checkpoint Kinase 1 (CHK1) as a target essential for ecDNA function in cancer, as tumor cells with oncogene amplification on ecDNA exhibit increased DNA replication stress (RS) and are hypersensitive to inactivation of CHK1, cells’ master regulator of RS. Thus, we have developed BBI-355, a novel, orally bioavailable, and selective small molecule inhibitor of CHK1, which is currently being studied in the first-in-human Phase 1/2 POTENTIATE clinical trial for patients with cancer harboring oncogene amplifications, including on ecDNA (NCT05827614). In vitro antiproliferation activity of BBI-355 was observed in a panel of ecDNA+ oncogene amplified tumor lines and non-amplified lines, with heightened sensitivity and induction of RS observed in ecDNA+ cells. Oral administration of BBI-355 resulted in dose dependent on-target activity, with induced p-CHK1-S345 protein expression, in agreement with measured plasma and tumor concentrations. Subsequent pharmacokinetic/pharmacodynamic (PK/PD) modeling guided continuous and intermittent dose/schedules of single agent BBI-355 that demonstrated durable antitumor activity, including tumor regressions, in multiple ecDNA+ tumor models. Single agent targeted therapy treatment of cancers with oncogene amplification on ecDNA can result in acquired resistance via ecDNA-based mechanisms, further increasing reliance on CHK1. Enhanced antitumor activity of BBI-355 in combination with agents targeting the protein products of amplified oncogenes was demonstrated in multiple in vivo tumor models; these included combination with a pan-FGFR inhibitor in FGFR2 ecDNA+ amplified gastric cancer and combination with CDK4/6 inhibitors in CDK4 ecDNA+ amplified sarcoma models. The first ecDNA-directed therapy (ecDTx) in development, BBI-355 demonstrated significant antitumor activity as a single agent and in combination with targeted therapies in multiple ecDNA+ oncogene amplified tumor models. Oral dosing of BBI-355 provides dose/schedule flexibility to attempt to optimize antitumor activity with a tolerable safety profile in patients with highly aggressive cancers and significant unmet need. The POTENTIATE clinical trial is open and currently enrolling. Citation Format: Ryan J. Hansen, Auzon Steffy, Joshua Plum, Ardalan Ardeshiri, Edison Tse, Salvador Garcia, Ben Norman, Jafar Moininazeri, AnneMarie Pferdekamper, Evan Holmes, Debbie Liao, Rachelle Elsdon, Joshua Lange, Anthony Pinkerton, Julius Apuy, Sudhir Chowdhry, Christian Hassig, Shailaja Kasibhatla. Oral CHK1 inhibitor BBI-355 allows flexibility of dose and schedule with demonstration of monotherapy and combinational antitumor activity in extrachromosomal DNA (ecDNA) driven preclinical models abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 613.