Therapeutic benefits of maintaining CDK4/6 inhibitors and incorporating CDK2 inhibitors beyond progression in breast cancer

The combination of CDK4/6 inhibitors (CDK4/6i) and endocrine therapy (ET) has significantly improved outcomes for patients with hormone receptor-positive (HR+) metastatic breast cancer. However, most patients eventually develop resistance, leading to treatment discontinuation. The therapeutic benefits of maintaining CDK4/6i or transitioning to CDK2 inhibitors (CDK2i) beyond disease progression remain unclear. Here, we show that maintaining CDK4/6i and ET or combining them with CDK2i effectively suppresses the growth of drug-resistant HR+ breast cancer cells by prolonging G1-phase progression. CDK4/6i maintenance triggers a non-canonical pathway for Rb inactivation through post-translational degradation, resulting in attenuated E2F activity and slowed G1 progression. ET further augments this effect by inhibiting c-Myc-mediated E2F amplification. Although the maintenance of CDK4/6i and ET outperforms CDK2i with ET, the triple combination of CDK4/6i, CDK2i, and ET most effectively suppresses both E2F activity and tumor growth. Moreover, while overexpression of both cyclin E and A can promote resistance to the CDK4/6i and CDK2i combination, cyclin E plays a more pivotal role in developing resistance. These findings highlight the potential of sustained CDK4/6i therapy and the incorporation of CDK2i to mitigate resistance in HR+ breast cancer. To gain insight into differences in gene expression upon introduction of CDK2i after dual resistance, palbociclib (CDK4/6i) +fulvestrant (ET) resistant MCF-7 cells were either discontinued from treatment, maintained in palbociclib + fulvestrant, switched to INX-315 (CDK2i) + fulvestrant, or switched to palbociclib + INX-315 + fulvestrant for 20 days then submitted to bulk RNA sequencing.