In vitro activity: LLY-283 is the first potent and selective SAM-competitive chemical probe for PRMT5 (protein arginine methyltransferase 5). In a biochemical assay, LLY-283 inhibits the PRMT5 enzyme activity with an IC50 of 20 nM for methylation of an H4R3 derived peptide substrate, it exhibits more than 100-fold selectivity for PRMT5 than other histone methyltransferases and non-epigenetic targets. PRMTs binds to the lysine or arginine residues of substrate proteins via the substrate binding groove and SAM via the cofactor binding site. These two binding sites are linked by a narrow hydrophobic channel that brings the substrate and cofactor in close proximity to allow the transfer of the methyl group from the cofactor SAM to a lysine or arginine residue via an SN2 transition state.

Kinase Assay: LLY-283 inhibits the methylation of SmBB′ with an IC50 of 25 nM in MCF7 cells and also affects MDM4 splicing with an IC50 of 40 nM in A375 cells

Cell Assay: LLY-283, the first potent and selective SAM-competitive chemical probe for PRMT5 was recently discovered. LLY-283 potently inhibited PRMT5 with an IC50 of 20 nM in a biochemical assay. It was>100-fold selective for PRMT5 over other methyltransferases and nonepigenetic targets. In cellular assays, it inhibited the methylation of SmBB′ with an IC50 of 25 nM in MCF7 cells and also affected MDM4 splicing with an IC50 of 40 nM in A375 cells.