Abstract 2625: Targeting prostate cancer stem cells (CSCs) with the novel BET bromodomain (BRD) protein inhibitor OTX015

In human neoplasias small subpopulations of cancer cells with stem cell-like properties are thought to play a role in progression, metastasis, disease recurrence and treatment failure. Effective targeting of CSCs may thus provide the basis for a paradigm shift in cancer therapy. Epigenetic and transcriptional regulators such as BRD proteins (BRD2/3/4) may contribute to the CSC phenotype and as such represent druggable targets for CSC-directed therapies. No data are currently available on the anti-CSC activity of BRD protein inhibitors. OTX015 is an orally bioavailable small molecule BRD protein inhibitor. It demonstrates broad anticancer activity in vitro and is currently under clinical evaluation in hematologic malignancies and solid tumor patients, including castrate-resistant prostate cancer. We investigated the effects of OTX015 on the phenotypes of bulk tumor cells and CSCs in a panel of human prostate cancer cell lines, including the androgen receptor positive (LNCaP, VCaP and 22RV1) and negative (DU145 and PC3) prostate cell lines. Similar levels of BRD proteins were present in the cell lines independent of androgen receptor (AR) status and the presence of ETS gene fusions. OTX015 strongly inhibited proliferation in all cell lines tested (IC50, 200-800 nM) by MTT assays after 72h-exposure. Soft agar clonogenic capability of prostate cancer cells was also inhibited by OTX015 (IC50, 20-100 nM). OTX015 also potently inhibited the CSC component in prostato-sphere forming assays (IC50, 1-100 nM) after 10 days of treatment. Of note, the CSC-directed activity of OTX015 was independent of the AR and ETS translocation state. Effective inhibition of CSCs by OTX015 was consistently associated with downregulation of critical CSC genes, including c-Myc and Nanog mRNA. In vivo experiments in 5 human tumor xenografts models (LNCaP, VCaP, 22RV1, DU145 and PC3) in nude mice are ongoing to evaluate antitumor and anti-CSC activity of OTX015. To our knowledge, this is the first evidence implicating BRD proteins in the expansion and maintenance of prostate CSCs independently of the specific biologic and genetic features of the bulk tumor cell population. These results suggest that the BRD protein inhibitor OTX015 could be effective for eradicating the CSC component in prostate cancer, providing the basis for novel treatment approaches. Citation Format: Gianluca Civenni, Silvia Pedrani, Sara Allegrini, Antonina Bruccoleri, Domenico Albino, Sandra Pinton, Ramon Garcia-Escudero, L9Houcine Ouafik, Esteban Cvitkovic, Giuseppina M. Carbone, Carlo V. Catapano. Targeting prostate cancer stem cells (CSCs) with the novel BET bromodomain (BRD) protein inhibitor OTX015. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2625. doi:10.1158/1538-7445.AM2015-2625