Abstract P5-08-02: PTK6 small molecule inhibitor enhances efficacies of chemotherapy in mesenchymal TNBC
Authors: Ito K , Lee E , Sato K , Byerly JH , Zhu J , Irie HY 
Published: May 2019

Abstract
Background: EMT in cancer promotes resistance to chemotherapy and radiotherapy, as well as immune suppression in the tumor microenvironment. EMT is also associated with enhanced tumor dissemination to other organs. EMT is a dynamic cell re-programming process whereby cancer cells lose epithelial markers and acquire mesenchymal markers, enhanced cell migration, and anoikis resistance. EMT is promoted by transcriptional and epigenetic regulators, as well as by signaling pathways. TNBCType and 101-gene model have identified distinct subsets of TNBC that exhibit mesenchymal gene signatures and phenotypes. This particular subset may be associated with chemotherapy resistance and metastatic recurrence in patients with TNBC. We identified protein tyrosine kinase 6 (PTK6) as a promoter of EMT in mesenchymal TNBC through its ability to prevent degradation of SNAIL, a key EMT transcriptional factor. A higher level of SNAIL expression is associated with poor TNBC patient prognosis. We investigated whether SNAIL suppression and EMT reversal by PTK6 small molecule inhibitor treatment enhance efficacy of chemotherapeutic agents that are part of standard of care treatment for patients with TNBC. Methods: Mesenchymal TNBC cell lines or organoids generated from TNBC PDX tumors were treated with varying concentrations of PTK6 small molecule inhibitor alone or in combination with chemotherapeutic agents in 3D cell cultures. The cell viability was assessed using 3D CellTiter-Glo or alamar blue. The combination Index was calculated to examine potential synergistic effects (CI&lt;1: synergism, CI=1: additive, CI&gt;1: antagonism). The in vivo combination effects of PTK6 inhibitor and paclitaxel were also assessed in two TNBC PDX models. Gene ontology analysis, targeted RT-PCR gene expression profiling and protein array were performed to identify potential mechanisms for chemosensitization effects of PTK6 inhibitor treatment. Results: Pre-treatment with PTK6 inhibitor increases sensitivity to paclitaxel or doxorubicin in 3D matrigel culture of TNBC cell lines, as well as in TNBC PDX organoids. The Combination Index suggested synergies between PTK6 inhibitor and chemotherapy treatment (paclitaxel or doxorubicin). While administration of PTK6 inhibitor or paclitaxel alone only modestly suppressed growth of PDX tumors in vivo, PTK6 inhibitor treatment sensitized tumors to paclitaxel treatment, as evidenced by the dramatic suppression of tumor volume and rate of growth. Gene ontology analysis identified gene sets that are significantly differentially expressed in PTK6 inhibitor-treated TNBC tumors, including extracellular matrix, cell migration, cell cycle and microtubule activity. A targeted RT-PCR profiling and protein array found that PTK6 inhibitor modulates expression of molecules that are associated with chemotherapy resistance and immune regulation; decrease in MMP3, PLEK2, osteopontin, IL-6 and CCL5 and increase in CD40. We are currently investigating whether these changes are dependent on Snail downregulation/EMT reversal caused by PTK6 inhibition. Conclusion: PTK6 inhibition may sensitize TNBC to chemotherapy treatment by suppressing Snail and reversing EMT. We will further validate these effects of PTK6 inhibitor treatment with other chemotherapies and immunotherapies.
Citation Format: Ito K, Lee E, Sato K, Byerly JH, Zhu J, Irie HY. PTK6 small molecule inhibitor enhances efficacies of chemotherapy in mesenchymal TNBC [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 P5-08-02.