Abstract 1904: Dual-targeting CDK4 and CDK2 overcomes resistance to CDK4/6 inhibitors in HR+ breast cancer.

Abstract Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i)have proven clinical efficacy in hormone receptor-positive (HR+) breast cancer (BC) patients. However,current clinical challenges are frequent hematologic toxicities mediated by CDK6 inhibition and acquired drug resistance driven in part by cyclin E/CDK2 activation. The next generation of CDK inhibitors such as atirmociclib targeting CDK4 and tagtociclib targeting CDK2 have demonstrated encouraging early efficacy, and the combination of a selective CDK4 inhibitor and a selective CDK2 inhibitor has entered clinical trials. We propose that simultaneous targeting CDK4 and CDK2 with a small-molecule may provide a new therapeutic option. To address the efficacy of dual-targeting CDK4 and CDK2, compound-1308 (Cpd-1308) were synthesized. In NanoBRET assay measuring cellular target engagement, Cpd-1308 had an IC50 against CDK4 of 24nM, an IC50 against CDK2 of 123nM, and an IC50 against CDK6 of 521nM, suggesting a CDK4/2 inhibitor over CDK6. Antiproliferative potency and inhibition of retinoblastoma (RB) phosphorylation were tested in multiple cancer cell lines. Human primary hematopoietic stem and precursor CD34+ cells (CD34+ HSPC) were used to estimate impact on the inhibition of hematopoietic cells proliferation. Cpd-1308 and palbociclib are nearly equipotent in MCF7 cells, whereas Cpd-1308 showed a 7-fold reduced impact on CD34+HSPC cells when compared to palbociclib. These results suggested that CDK4:CDK6 selectivity of Cpd-1308 was greater than that of palbociclib. Further kinetic characterization using surface plasmon resonance (SPR) spectroscopy revealed that Cpd-1308 has long residence time on CDK2 in comparison with tagtociclib. Long residence time on CDK2 may potentially lead to improved efficacy against drug resistance driven in part by cyclin E/CDK2 activation. Cpd-1308 indeed showed superior potency on antiproliferation of derived palbociclib resistant MCF7 cells (MCF7-palbo-r) and derived abemaciclib resistant MCF7 cells (MCF7-abema-r). We tested the in vivo efficacy of Cpd-1308 in CDK4/6i-resistant HR+ breast cancer patient-derived xenografts tumors (PDX). Tumor growth was inhibited in a dose-dependent manner and significant growth inhibition was observed. Cpd-1308 demonstrated significant resistance-overcoming activity. Taken together, we demonstrate activity of Cpd-1308 in HR+ breast cancers and CDK4/6i-resistant breast cancers. Our research results indicate that co-targeting CDK4 and CDK2 with a small-molecule may grant new therapeutic opportunities for patients. Citation Format: Wenqing Cai, Siwen Liu, Lu Su, Quang Fu, Chen Liao. Dual-targeting CDK4 and CDK2 overcomes resistance to CDK4/6 inhibitors in HR+ breast 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 1904.