Abstract 2338: CDK4/6 and autophagy inhibitors synergize to induce senescence in cancers with an intact G1/S checkpoint

Deregulation of the cell cycle machinery is a hallmark of most cancers. The crucial role of the CDK4/6-CyclinD pathway in tumorigenesis has led to the successful development and FDA approval (palbociclib) of CDK4/6 inhibitors for the treatment of advanced estrogen receptor positive breast cancer. However, two major clinical challenges remain: i) lack of a reliable biomarker to predict treatment response and ii) adverse events leading to interruption or discontinuation of treatment which possibly thus curtailing therapeutic benefit. We found that treatment of ER+ breast cancer cell lines with the CDK4/6 inhibitor palbociclib resulted in a dose-dependent sustained growth inhibition and senescence. Interestingly, breast cancer cells activate autophagy in response to palbociclib, a stress response process that promotes cancer cell survival. Genetic ablation of crucial autophagic genes or pharmacological inhibition of autophagy increases the sensitivity of ER+ breast cancer cells to palbociclib and the other CDK4/6 inhibitors, ribociclib and abemaciclib. This was confirmed in vivo, where the combination of palbociclib and autophagy inhibitor, hydroxychloroquine (HCQ) resulted in a significantly improved and a sustained tumor shrinkage. To identify biomarkers that predict response to CDK4/6 inhibition, we examined the G1 checkpoint proteins and found that knockdown of Rb or overexpression of the oncogenic low molecular weight isoforms of Cyclin-E (LMW-E) mediates resistance to palbociclib and its combination with autophagy inhibitor. More significantly, immunohistochemically staining of pre-treatment biopsies from palbociclib treated patients strengthened the correlation between palbociclib efficacy and an intact G1/S checkpoint (Rb+ve /LMWE-ve), resulting in a significantly longer progression free survival compared to the other patient groups; thus solidifying Rb and LMW-E as reliable prognostic biomarkers for palbociclib treatment. Finally, we examined the biomarker driven synergy between CDK4/6 and autophagy inhibition in several other cancer cell lines. Several solid tumors (ovarian, lung, pancreatic, colon, prostate) and triple negative breast cancer (TNBC) cell lines exhibited a synergistic response to palbociclib/HCQ combination treatment dependent upon an intact G1/S transition (Rb+/LMWE-). This was also verified in a TNBC patient derived xenograft (PDX) model. Thus, this study addresses the aforementioned limitations and provides a novel and promising biomarker-driven combination therapeutic strategy to treat breast and other solid tumors. We predict that this combination of CDK4/6 and autophagy inhibitors would be more beneficial than standard dose palbociclib in patients, allowing us to lower the dose, minimize palbociclib mediated toxicities and potentially improve overall patient survival - a goal that has not yet been met with currently approved treatment combinations. Citation Format: Smruthi Vijayaraghavan, Cansu Karakas, Xian Chen, Iman Doostan, Akshara S. Raghavendra, Min Yi, Ravi Amaravadi, Kelly Hunt, Debu Tripathy, Khandan Keyomarsi. CDK4/6 and autophagy inhibitors synergize to induce senescence in cancers with an intact G1/S checkpoint [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2338. doi:10.1158/1538-7445.AM2017-2338