Small molecule epigenetic screen identifies novel EZH2 and HDAC inhibitors that target glioblastoma brain tumor-initiating cells

// Natalie Grinshtein 1 , Constanza C. Rioseco 1 , Richard Marcellus 2 , David Uehling 2 , Ahmed Aman 2 , Xueqing Lun 3 , Osamu Muto 1 , Lauren Podmore 1 , Jake Lever 4 , Yaoqing Shen 4 , Michael D. Blough 3 , Greg J. Cairncross 3 , Stephen M. Robbins 3 , Steven J. Jones 4, 5, 6 , Marco A. Marra 4, 5 , Rima Al-Awar 2 , Donna L. Senger 3 , David R. Kaplan 1, 7 1 Program in Neurosciences and Mental Health, The Hospital for Sick Children, Toronto, Canada 2 Drug Discovery Group, Ontario Institute for Cancer Research, Toronto, ON, Canada 3 Arnie Charbonneau Cancer Institute, Department of Oncology, Cumming School of Medicine, University of Calgary, Calgary, Alberta, Canada 4 Canada’s Michael Smith Genome Sciences Centre, BC Cancer Agency, Vancouver, Canada 5 Department of Medical Genetics, University of British Columbia, Vancouver, British Columbia, Canada 6 Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia 7 Department of Molecular Genetics, University of Toronto, Toronto, ON Canada Correspondence to: David R. Kaplan, email: dkaplan@sickkids.ca Keywords: glioblastoma, drug discovery, epigenetics, UNC1999, HDAC inhibitor Received: March 17, 2016      Accepted: July 07, 2016      Published: July 18, 2016 ABSTRACT Glioblastoma (GBM) is the most lethal and aggressive adult brain tumor, requiring the development of efficacious therapeutics. Towards this goal, we screened five genetically distinct patient-derived brain-tumor initiating cell lines (BTIC) with a unique collection of small molecule epigenetic modulators from the Structural Genomics Consortium (SGC). We identified multiple hits that inhibited the growth of BTICs in vitro , and further evaluated the therapeutic potential of EZH2 and HDAC inhibitors due to the high relevance of these targets for GBM. We found that the novel SAM-competitive EZH2 inhibitor UNC1999 exhibited low micromolar cytotoxicity in vitro on a diverse collection of BTIC lines, synergized with dexamethasone (DEX) and suppressed tumor growth in vivo in combination with DEX. In addition, a unique brain-penetrant class I HDAC inhibitor exhibited cytotoxicity in vitro on a panel of BTIC lines and extended survival in combination with TMZ in an orthotopic BTIC model in vivo . Finally, a combination of EZH2 and HDAC inhibitors demonstrated synergy in vitro by augmenting apoptosis and increasing DNA damage. Our findings identify key epigenetic modulators in GBM that regulate BTIC growth and survival and highlight promising combination therapies.