Abstract 1143: Development of a CDK2-selective small molecule inhibitor INCB123667 for the treatment of CCNE1hi breast cancers
Authors: Chand Saswati , Hansbury Michael , Lo Yvonne , Feldman Pat , Carl Justine , Timmers Cynthia , Hummel Joshua , Wee Susan , Kim Sunkyu 
Published: June 2023

Abstract
Cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors have been approved for treatment of metastatic ER+ HER2− breast cancer as monotherapy or in combination with endocrine therapy. However, many patients become resistant to CDK4/6 therapy due to various mechanisms such as loss of tumor suppressor Rb, hyperactivation of CDK4/6, and/or CDK2/Cyclin E1 expression and activity, resulting in a highly unmet clinical need. Triple-negative breast cancers (TNBC), owing to a lack of hormone receptor (HR) expression and extensive heterogeneity, remain notoriously aggressive and unresponsive to current therapies. CCNE1 overexpression or amplification has been shown to be an oncogenic driver associated with poor clinical outcomes in ovarian, gastric, endometrial, and HR+ breast cancers. CCNE1hi cancers are dependent on CDK2 for growth and survival. CDK2 in complex with CCNE1 phosphorylates Rb, resulting in E2F target gene expression and G1 to S-phase cell cycle progression. Using a potent and selective CDK2 inhibitor, INCB123667, we demonstrate the efficacy of CDK2 inhibition in CCNE1hi breast cancer models. Analysis of genome-wide CRISPR screen data identified a subset of CCNE1hi breast cancer models dependent on CDK2 comprised of basal and luminal breast cancer subtypes, including TNBC, ER+ HER2−, and HR− HER2+ models. Two CCNE1hi cell lines, HCC1569 and MDAMB157, were evaluated in cell assays monitoring pRb, S-phase, and growth inhibition, and found to be sensitive to treatment with INCB123667 and insensitive to multiple CDK4/6 inhibitors. Through extensive biochemical studies, INCB123667 was observed to be potently active against CDK2 at subnanomolar range and inactive against other CDKs (CDK1, CDK4, CDK6, CDK7, and CDK9). INCB123667 also selectively induced senescence in CCNE1hi HCC1569 cells. In contrast, CCNE1 nonamplified cell lines, MCF7 and T47D, were inactive in pRb, S-phase, and growth inhibition assays when treated with INCB123667. Significant tumor growth inhibition was observed in CCNE1hi PDX and xenograft models with minimal body weight loss. We also observed dose-dependent inhibition of CDK2 substrates, including pRb S780, both in vitro and in vivo. To further characterize downstream signaling and transcriptional changes with CDK2 inhibition, we identified a discrete gene signature associated with cell cycle regulation, which was notably distinct from CDK4/6 responsive target genes. In conclusion, we have demonstrated CDK2 activity in CCNE1hi breast cancers using INCB123667, a potent and selective small molecule inhibitor of CDK2 currently in clinical development. Treatment resulted in cell growth inhibition and antitumor activity in vitro and in vivo. Our data demonstrate the therapeutic potential of CDK2 inhibition in CCNE1hi HR+ HER2− and TNBC breast cancer models.
Acknowledgments: R&D services provided by Pharmaron, Crown Bioscience, and Champions Oncology
Citation Format: Saswati Chand, Michael Hansbury, Yvonne Lo, Pat Feldman, Justine Carl, Cynthia Timmers, Joshua Hummel, Susan Wee, Sunkyu Kim. Development of a CDK2-selective small molecule inhibitor INCB123667 for the treatment of CCNE1hi breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1143.