ABSTRACT – PROJECT 1 The ability to overcome the development of chemotherapy resistance and maintain sensitivity to the most effective chemotherapy in ovarian cancer, platinum, is critical to improving outcomes in ovarian cancer. The ovarian cancer tumor microenvironment (TME) is an important, though understudied, mediator of treatment resistance. Specifically, the stromal composition of this TME dramatically influences how cancer responds to chemotherapy, yet no strategies exist to target the cancer-supportive properties of the stromal TME. Carcinoma-associated mesenchymal stem/stromal cells (CA-MSCs) are stromal progenitor cells that dictate the composition and function of the stromal TME. CA-MSCs strongly promote the development of chemotherapy resistance through the secretion of growth factors such as BMP2 and BMP4 and alteration of the extracellular matrix (ECM). CA-MSCs arise from cancer mediated epigenetic reprogramming of normal MSCs through altered genomic localization of the histone methyltransferase EZH2. WT1, a transcription factor highly upregulated in CA-MSCs, appears to drive this altered genomic localization of EZH2 necessary for CA- MSC formation. Importantly, the CA-MSC phenotype appears to be reversible and CA-MSC reversion or ‘normalization’ correlates with improved sensitivity to platinum and patient survival. Our preliminary data indicate pharmacologic inhibition of EZH2 blocks the reprograming of MSC into CA-MSC and increases CA- MSC normalization to prevent chemotherapy resistance. We thus hypothesize that inhibiting EZH2 will disrupt stromal support of ovarian cancer leading to improved treatment response, reduction in metastasis, and delayed disease recurrence with maintenance of platinum sensitivity. We propose to: Aim 1: Conduct a phase 1 dose-escalation study of the second generation EZH2 inhibitor, CPI-0209, in combination with Carboplatin in platinum sensitive, recurrent ovarian cancer. Aim 2: Determine the impact of EZH2 inhibition on the ovarian stromal TME. Aim 3: Determine the role of WT1 in mediating EZH2-driven CA-MSC reprogramming and chemotherapy resistance. Upon completion of this work, we will have determined the potential for using CPI-0209 in combination with platinum chemotherapy in recurrent ovarian cancer to prevent stromal-mediated chemotherapy resistance. We will also define the ability of CPI-0209 to induce CA-MSC normalization in patients. Further, we will have identified the mechanism of WT1 mediated EZH2 genomic localization during CA-MSC reprogramming enabling future, more focused targeting of the CA-MSC phenotype. Collectively, this work represents a novel approach to enhancing ovarian cancer chemotherapy sensitivity by targeting the cancer stromal TME.