Identification Of A First-In-Class Prmt5 Inhibitor With Potent In Vitro And In Vivo Activity In Preclinical Models Of Mantle Cell Lymphoma

Protein Arginine Methyltransferase-5 (PRMT5) has been reported to play a role in multiple diverse cellular processes including tumorigenesis. Overexpression of PRMT5 has been demonstrated in cell lines and primary patient samples derived from lymphomas, particularly Mantle Cell Lymphoma (MCL). Furthermore, knockdown of PRMT5 expression inhibits the proliferation of MCL cell lines. The mechanisms behind the oncogenic potential of PRMT5 are unclear, but the protein has been postulated to regulate processes such as cell death, cell cycle progression, and RNA processing through the dimethylation of arginine residues within a variety of cytoplasmic and nuclear target proteins. Epizyme developed small molecule inhibitors of PRMT5 enzyme activity in order to understand the role of PRMT5-mediated arginine methylation in tumorigenesis and to develop PRMT5-targeted cancer therapeutics. Here, we describe the identification and characterization of a potent and selective inhibitor of PRMT5 with anti-proliferative effects in both in vivo and in vitro models of MCL. A diverse compound library was screened for inhibitors of arginine methylation by purified recombinant PRMT5:MEP50 complex and multiple hits were identified. The inhibitors are SAM uncompetitive, peptide competitive and bind with the PRMT5:MEP50 complex in a unique binding mode not previously observed. Further optimization yielded YQ36286, an orally available inhibitor of PRMT5 with enzymatic activity in biochemical assays with an IC50 in the low nM range and broad selectivity against a panel of other histone methyltransferases. YQ36286 demonstrated potent cellular activity as measured by its ability to inhibit symmetric dimethylation of SmD3, a cytoplasmic PRMT5 substrate in a time- and concentration-dependent manner. Treatment of MCL cell lines with YQ36286 led to inhibition of SmD3 methylation and cell killing, with IC50s in the nM range. Oral dosing of YQ36286 demonstrated dose-dependent anti-tumor activity in multiple MCL xenograft models. In xenograft studies with the Z138 MCL cell line, near 95% tumor growth inhibition was observed after 21 days of dosing with a corresponding decrease in symmetrically dimethylated levels of PRMT5 substrates. In summary, we have developed the first potent and selective small molecule inhibitor of PRMT5 that has cellular activity and in vivo efficacy. MCL cells are dependent on PRMT5 activity for their survival as demonstrated with YQ36286. This small molecule represents a starting point for the development of PRMT5 inhibitors as potential cancer therapeutics.