Abstract 4637: Developing a first in class of drug to inhibit protein arginine methyltransferase 5 (PRMT5) enzyme dysregulation in glioblastoma multiforme.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Glioblastoma Multiforme (GBM) is associated with a dismal prognosis despite intensive multimodal therapy, highlighting the need for novel therapeutic approaches. Chromatin remodeling complexes and associated co-repressors such as histone deacetylases (HDAC), DNA methyltransferases (DNMT) and protein arginine methyltransferase 5 (PRMT5) are involved in silencing tumor suppressor and regulatory gene expression and may contribute to glial cell transformation. PRMT5 silences the transcription of key regulatory genes by symmetric di-methylation (S2Me) of arginine (R) residues on histone proteins (H4R3 and H3R8) and works more efficiently when associated with other co-repressor enzymes. We have previously identified PRMT5 over-expression in GBM cell lines, primary GBM tumors, and GBM that spontaneously develop in a pre-clinical mouse model. The degree of PRMT5 over-expression inversely correlated with survival of GBM patients (r=-0.57, p=0.0001) and correlated with proliferation of GBM cell lines (r=0.81, p<0.0001), PRMT5 works concertedly with HDAC2, methyl-CpG binding domain protein 2 (MBD2) and DNMT3a to silence genes with anti-cancer and immune modulatory activities. PRMT5 silending in GBM cell lines leads to growth arrest, apoptosis and loss of HDAC2, DNMT3a and PRMT5 co-repressor complex recruitment at chemokine gene promoters. These findings provide evidence that PRMT5 is a central repressive element required for maintenance of target gene silencing. Thus, we explored methods to inhibit PRMT5 activity as a novel experimental therapeutic strategy for GBM. A rational design of small molecule compounds to inhibit PRMT5 activity led us to construct an in silico model of the human PRMT5 catalytic domain based on available homologous crystal structures. We screened a library of 10,000 compounds and eight small molecules were identified based on binding energy in the PRMT5 catalytic site. Enzyme inhibition assays showed that a lead compound (BLL1) was capable of selectively inhibiting PRMT5 and not PRMT1 or PRMT7 activity (p<0.0001). BLL1 interfered with maintenance of S2Me-H4R3 and S2Me-H3R8 in GBM cell lines by western blot and confocal microscopy. Dose titration experiments with BLL1 showed a dose-dependent response of inhibition of cellular proliferation in GBM cell lines. Combination treatment of GBM cells with subtoxic doses of BLL1, hypomethylating agent 5-azacitidine, and HDAC inhibitor TSA showed synergistic induction of cell death, loss of the epigenetic mark S2Me-H4R3, and de-repression of the immune modulating chemokine CXCL10. Preclinical in vivo studies have shown favorable toxicity and pharmacokinetic profiles for BLL1. We have successfully developed a first in class drug to selectively target dysregulated PRMT5 enzymatic activity in GBM. We are currently developing drugs with improved selectivity and potency. Citation Format: Fengting Yan, Kate Gordon, Kiran Mahasenan, Mark Lustberg, Lapo Alinari, Christian T. Earl, Balveen Kaur, Chenglong Li, Robert A. Baiocchi. Developing a first in class of drug to inhibit protein arginine methyltransferase 5 (PRMT5) enzyme dysregulation in glioblastoma multiforme. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4637. doi:10.1158/1538-7445.AM2013-4637