Resistance to cyclin D-cdk4/6 inhibitors (CDK4/6i) represents an unmet clinical need and is frequently caused by compensatory CDK2 activity. Here we describe a novel strategy to prevent CDK4i resistance by using a therapeutic liposomal:peptide formulation, NP-ALT, to inhibit the tyrosine phosphorylation of p27Kip1(CDKN1B), which in turn inhibits both CDK4/6 and CDK2. We find that NP-ALT blocks proliferation in HR+ breast cancer (BC) cells, as well as CDK4i-resistant cell types, including Triple Negative (TN) BC. The peptide ALT is not as stable in primary mammary epithelium, suggesting that NP-ALT has little effect in non-tumor tissues. In HR+ BC cells specifically, NP-ALT treatment induces ROS and RIPK1-dependent necroptosis. Estrogen signaling and ERα appear required. Significantly, NP-ALT induces necroptosis in MCR7 ESRY537S cells, which contain an ER gain of function mutation frequently detected in metastatic patients, which renders them resistant to endocrine therapy. Here we show that NP-ALT causes necroptosis and tumor regression in treatment naïve, palbociclib-resistant and endocrine-resistant BC cells and xenograft models, demonstrating that p27 is a viable therapeutic target to combat drug resistance. Because the RAS/MAPK axis is the target of many therapies, NP-ALT, with its ability to target p27, can deal with the backend drug resistance seen in the presence of other inhibitors, extending the addressable market of this line of therapy.

Citation Format: Stacy Wister Blain, Irina Jilishitz, Jason L. Quinones, Priyank Patel, Grace Chen, Jared Pavetsky, Allison VanInwegen VanInwegen, Scott Schoninger, Manasi P. Jogalekar, Vladislav Tsiperson, Lingyue Yan, Yun Wu, Susan R. Gottesman, Jonathan Somma. Blocking p27Kip1phosphorylation with a liposomal:peptide drug induces Reactive Oxygen Species,necroptosis and tumor regression in breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB115.