Abstract 4766: Transcriptomic profiling reveals a potential role for JAK/STAT inhibition in CDK4/6 inhibitor-resistant, ER+ breast cancers
Authors: Pesch Andrea M. , Gonzalez Thomas L. , Chandler Benjamin C. , Sun Siqi , Gersch Christina L. , Larios José M. , David Wadie S. , Speers Corey W. , Rae James M. 
Published: August 2019

Abstract
Purpose: Specific cyclin-dependent kinase (CDK) inhibitors are standard of care for patients with metastatic, estrogen receptor-positive (ER+) breast cancer. CDK4/6 inhibitors have improved rates of progression free survival among metastatic, ER+ patients, but resistance limits their clinical efficacy. Various mechanisms of resistance to CDK4/6 inhibitors have been reported, but a comprehensive understanding of this resistance remains elusive.
Methods: We generated in vitro models of acquired (AR) and intrinsic (IR) resistance to CDK4/6 inhibitors using ER+ breast cancer cell lines (MCF-7, T47D) cultured with either continuous high dose (500nM) or dose-escalated (50nM to 500nM) CDK4/6 inhibition over three months. RNA expression and gene set enrichment analysis (GSEA) was used to nominate potential pathways associated with AR and IR palbociclib resistance. Reverse phase protein array (RPPA) and western blots were used to measure protein and phosphoprotein levels in CDK4/6 inhibitor resistant cell lines to validate nominated pathways. Cellular proliferation assays were performed to calculate the half-maximal inhibitory concentration (IC50) with inhibitors for CDK4/6 and JAK/STAT.
Results: Proliferation assays confirmed that MCF-7 AR and IR cells are resistant to palbociclib (IC50 both &gt;1uM) compared to parental cells (60nM); similar results were observed in the T47D cell lines. Cells resistant to either palbociclib, ribociclib, or abemaciclib demonstrated cross resistance to all three inhibitors. GSEA of transcriptomic data identified 579 genes (from AR cells) and 936 genes (from IR cells) that were differentially expressed between palbociclib-resistant MCF-7s and parental controls. RPPA analyses identified several key pathways that regulate CDK4/6 inhibitor resistance in these models. From GSEA analysis, the interferon (JAK/STAT) signaling pathway was the most differentially expressed pathway identified between palbociclib-resistant and sensitive cells. Western blot analyses showed that baseline expression of phospho-STAT1 is significantly elevated in palbociclib-resistant cells. In cellular proliferation assays, palbociclib-resistant MCF-7s and T47Ds retained sensitivity to JAK/STAT inhibitors like the JAK2-selective compound AZ960.
Conclusions: Our data suggests that overactivation of JAK/STAT signaling may be directly involved in the development of CDK4/6 inhibitor resistance in ER-dependent tumors. CDK4/6 inhibitor-resistant cells retain sensitivity to single-agent JAK/STAT inhibition, suggesting that this may be a viable therapeutic option for patients with CDK4/6 inhibitor-resistant ER+ breast cancer. This work was supported in part by 5T32GM007767-40 (Pesch), the Breast Cancer Research Foundation (N003173 to JMR), the UM Rogel Cancer Center and the Taubman Emerging Scholar funds.
Citation Format: Andrea M. Pesch, Thomas L. Gonzalez, Benjamin C. Chandler, Siqi Sun, Christina L. Gersch, José M. Larios, Wadie S. David, Corey W. Speers, James M. Rae. Transcriptomic profiling reveals a potential role for JAK/STAT inhibition in CDK4/6 inhibitor-resistant, ER+ breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4766.