Abstract P5-03-03: Polo-like Kinase 1 (PLK1) inhibition synergizes with docetaxel in basal breast cancer cells
Authors: Giordano A , Liu Y , Armeson KE , Britten CD , Kappler CS , Ethier SP 
Published: May 2019

Abstract
Background. Triple negative breast cancer (TNBC) is a heterogeneous disease entity and gene expression analyses have identified molecular subtypes that are refining our understanding of breast cancer biology and enabling development of targeted therapy. The basal-like 2 (BL2) subtype, as described by Lehmann et al, is characterized by overexpression of EGFR, loss of PTEN, and mutations in the TP53 gene and those patients had a 0% pathologic complete response rate following neoadjuvant chemotherapy. Thus, BL2 breast cancers are intrinsically resistant to chemotherapy and patients with this type of breast cancer have a poor overall survival rate. In a recent genome-scale short hairpin RNA (shRNA) screen of breast cancer cells, PLK1 was a frequent and strong hit in the basal-like subtype, inflammatory breast cancer SUM149, indicating its importance for growth and survival of these breast cancer cells. PLK1 regulates progression of cells through the G2 phase of the cell cycle by phosphorylating FOXM1, which then regulates the expression of cyclins and other genes necessary for cells to progress through cell cycle. However, most of the drug companies discontinued clinical development of PLK inhibitors (PLK1i) (e.g. GSK461364 GlaxoSmithKline, Volasertib Boehringer Ingelheim) in solid tumors due to elevate toxicity shown in phase I-II studies.
Methods. We selected GSK461364 as our PLKi for the in vitro experiments. GSK461364 was combined with docetaxel and cisplatin in a set of TNBC cells including SUM149, SUM159, SUM1315, SUM229, DU4475 and the control MCF10A. We performed cell proliferation assays in order to determine the IC25 (Inhibition Concentration of 25%) and IC50 concentration for each drug across the panel of cell lines. We then examined the influence of drugs on the clonogenic potential of the panel of cell lines, alone and in combination. Synergism or antagonism was determined using the Chou-Talalay method, by determining the combination index (CI) at Fa 0.5 (50% of cell death induced by drug treatment). We used FxCycle™ Stain (Invitrogen) for cell cycle analysis, and CD24/CD44 antibodies (BD Pharmingen™) for stem cell marker analysis. All experiments were carried in triplicate.
Results. GSK461364 caused growth inhibition with IC50 varying from 6.1 nmol/L (SUM229) to 56.5 nmol/L (DU4475). In our growth assay, GSK461364 showed synergism with docetaxel in SUM149 (CI 0.70) and SUM159 (CI 0.62), and with cisplatin in SUM149 (CI 0.58), SUM159 (CI 0.85), and SUM229 (CI 0.54). Only GSK461364 in combination with docetaxel decreased the clonogenic potential of SUM149 (p &lt; 0.001, Interaction Test, 0.001). In SUM149 cells, GSK461364 caused mitotic arrest in phase G2-M with the maximum effect 24 hours after administration of the drug. Moreover, when combined with docetaxel, GSK461364 reduced the relative percentage of CD44+/CD24-/dim population of SUM149.
Conclusion. PLKi in combination with chemotherapy shows promising results in a subset of TNBC intrinsically resistant to chemotherapy. GSK461364 significantly decreased the clonogenic potential and stem cell population of SUM149 when combined to docetaxel. In combination with docetaxel, we demonstrated that is possible to lower the dose of PLKi by 5 folds in order to obtain the same activity.
Citation Format: Giordano A, Liu Y, Armeson KE, Britten CD, Kappler CS, Ethier SP. Polo-like Kinase 1 (PLK1) inhibition synergizes with docetaxel in basal breast cancer cells [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-03-03.