With standard cytotoxic chemotherapy the majority of patients with Acute Myeloid Leukemia (AML) fail to achieve long term remission. Protein Arginine Methyltransferase 5 (PRMT5) can catalyze methylation of arginine residues in target proteins like histones and P53. The epigenetic marks of PRMT5 on histones H3 (S2Me-H3R8) and H4 (S2Me-H4R3), directly lead to transcriptional silencing of genes. Overexpression of PRMT5 has been reported in hematologic and solid malignancies and represents a promising therapeutic target. Thus, we hypothesized that inhibition of PRMT5 enzymatic activity using a first in class, selective small molecule inhibitor “BLL1” would result in antileukemic activity and novel methods to treat AML.

PRMT5 was upregulated in AML-derived cell lines and primary patient samples. PRMT5 in complex with co-repressors like histone deacetylase (HDAC)s and DNA methyltrasferase (DNMT)s is involved in epigenetic silencing of tumor suppressor genes. Using Co-IP assay we captured PRMT5 in association with DNMT3a and HDAC2 in AML cells, in which treatment with BLL1 (25μM) resulted in disruption of this interaction. We confirmed the ability and specificity of BLL1 by detecting the loss of unique epigenetic mark of PRMT5, S2Me-H3R8. We previously reported that pharmacologic inhibition of DNMTs (by decitabine) and HDACs (by AR42) or the disruption of their repressor complexes (by bortezomib) can inhibit critical oncogenes in AML, KIT and FLT3, by upregulating miR-29b and consequently lead to better clinical response in AML. Similar to these epigenetic-targeting compounds, BLL1 upregulated miR-29b (>7 fold; MV411, FLT3 ITD+ cells) and resulted in downregulation of FLT3 RNA (>4 fold) and protein (3.8 fold). We examined the in vitro antileukemic activity of BLL1 in AML and demonstrated a dose-dependent inhibition of cell proliferation (MTS assay) in AML cell lines (MV411; IC50=6μM & Kasumi1, KITmut, IC50=11μM) and primary patient blasts (#1, FLT3 ITD+, NPM1+, CD34; IC50=8μM & #2, FLT3 ITD+, CD34+; IC50=7.5μM). Colony forming ability was significantly decreased (p<0.001) in BLL1 (2μM) treated AML cells compared to controls. We observed a BLL1-dependent increase in apoptosis measured by AnnexinV staining (25 & 50μM, MV411; 8.6 fold & Kasumi1; 4 fold) within 48 hrs. BLL1 treatment promoted cell maturation measured by increased monocyte differentiation marker CD11b in AML cells (50μM, MV411; 11.5 fold & Kasumi1; 6.8 fold) and primary patient blasts (25μM, #1; 4 fold).

In conclusion, our data suggest that inhibiting PRMT5 activity interferes with the production or activity of oncoproteins and, subsequently, disrupts leukemogenic and proliferation pathways in AML. Given the substantial antileukemic potential of PRMT5 inhibitor compound on AML cells and primary patient blasts, an assessment of therapeutic implications of this novel drug in vivo is highly warranted.

Citation Format: Somayeh S. Tarighat, Kiran Mahasenan, Danilo Perrotti, Ramiro Garzon, Michael Caligiuri, Chenglong Li, William Blum, Guido Marcucci, Robert A. Baiocchi. Preclinical and pharmacological activities of protein arginine methyltransferase 5(PRMT5) enzyme inhibition in acute myeloid leukemia . [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 1128. doi:10.1158/1538-7445.AM2013-1128