Abstract 5123: C018, a potent and selective CDK4&9 dual inhibitor

Abstract Background: CDK4&6 inhibitors have been approved for the treatment of metastatic ER+&HER2- breast cancer, either as monotherapy or in combination with endocrine therapy. However, many patients develop resistance to CDK4&6 inhibition through multiple mechanisms, including loss of the tumor suppressor Rb, hyperactivation of CDK4&6, and&or upregulation of CDK2&Cyclin E1 expression and activity, resulting in a significant unmet clinical need. CDK9 plays a critical role in RNA transcription as a catalytic subunit of positive transcription elongation factor b (P-TEFb), which phosphorylates the negative elongation factor (NELF), DRB sensitivity-inducing factor (DSIF), and the Ser2 residue within the C-terminal domain (CTD) of RNA polymerase II (Pol II), ultimately triggering transcriptional elongation. Accumulating evidence suggests that CDK9 activity may contribute to palbociclib resistance mechanisms, and targeting CDK9 represents a promising strategy to overcome resistance in breast cancer. Methods and Materials: Based on structure-activity relationship (SAR) studies and accelerated by the Convalife "AIDRUG.WORK" molecular design platform, we completed the early-stage screening of CDK4&9 inhibitors. The lead compound demonstrated >30-fold selectivity for CDK4 over CDK6 and >80-fold selectivity for CDK9 over CDK6. To explore the therapeutic potential of CDK4&9 dual inhibitors against palbociclib-resistant tumors, we evaluated the effects of CDK4&9 inhibition on cell viability using both palbociclib-sensitive and palbociclib-resistant MCF-7 cell lines. Results: The CDK4&9 inhibitor significantly impaired the viability of both palbociclib-sensitive and palbociclib-resistant MCF-7 cell lines through concurrent inhibition of CDK4 and CDK9 activity. Conclusions: Our findings demonstrate that simultaneous inhibition of CDK4 and CDK9 may exert synergistic effects in overcoming palbociclib resistance. CDK4&9 dual inhibition represents a promising therapeutic strategy to overcome both endocrine therapy and CDK4&6 inhibitor resistance in ER+ breast cancer, warranting further clinical development as a novel approach for treating resistant breast cancer. Citation Format: Jinwen Huang, Zhongyuan Li, Steve Shen. C018, a potent and selective CDK4&9 dual inhibitor [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 5123.