Abstract 2357: CDK4/6 inhibition in early stage triple negative breast cancer

Abstract

Abstract Palbociclib (PD), a CDK4/6 inhibitor, has recently been approved for use in advanced ER positive breast cancer, demonstrating significant benefit in combination with endocrine therapies. Ductal carcinoma in situ (DCIS) is an inobligate precursor to invasive ductal carcinoma with anywhere from 14-53% of cases progressing. Progression is unpredictable and driven by poorly understood mechanisms. In this study, we assess the efficacy of PD in the context of this early stage disease using a triple negative DCIS model(MCF10DCIS), not only in inhibiting cell cycle but also in hindering the progression from in situ to invasive lesion. We demonstrate similar efficacy of PD in 2D on MCF10DCIS and immortal mammary epithelial cells (MCF10As) in preventing proliferation, inhibiting phosphorylation of RB and stopping cell cycle progression. Yet in 3D, MCF10DCIS cells form smaller, more organized spheres in the presence of drug while MCF10As appear unaffected, highlighting potential disease selectivity in vivo. Interestingly, when a cDNA array is performed on MCF10DCIS cells grown in 3D +/- PD, the most regulated genes are not well characterized cell cycle genes as one might predict based on previously published data and 2D response to treatment, suggesting alternative CDK4/6 targets may be responsible for the phenotype observed in 3D. We also demonstrate in a nude mouse xenograft model of MCF10DCIS that PD is able to significantly reduce overall tumor burden while simultaneously delaying the progression of lesions to invasive disease. Though the growth of PD treated MCF10DCIS lesions is delayed we demonstrate that proliferation is unaffected in vivo by the presence of the drug, based on equivalent Ki67 signal within the lesions. This reiterates the possibility that in 3D and in vivo contexts PD targets mechanisms outside of cell cycle regulation in order to slow tumor growth and delay invasive transition. We believe that investigation into alternative targets of CDK4/6 altered by PD in 3D and in vivo may provide insights on important mechanisms of DCIS progression in DCIS models and human patients. Citation Format: William B. Kietzman, Virginie Ory, Fransisco Saenz, Ghada Sharif, Anton Wellstein, Anna T. Riegel. CDK4/6 inhibition in early stage triple negative breast cancer [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 2357. doi:10.1158/1538-7445.AM2017-2357