Discovery of a First-in-Class Inhibitor of the PRMT5-Substrate Adaptor Interaction.

PRMT5 and its substrate adaptor proteins (SAPs), pICln and Riok1, are synthetic lethal dependencies in MTAP-deleted cancer cells. SAPs share a conserved PRMT5 binding motif (PBM) which mediates binding to a surface of PRMT5 distal to the catalytic site. This interaction is required for methylation of several PRMT5 substrates, including histone and spliceosome complexes. We screened for small molecule inhibitors of the PRMT5-PBM interaction and validated a compound series which binds to the PRMT5-PBM interface and directly inhibits binding of SAPs. Mode of action studies revealed the formation of a covalent bond between a halogenated pyridazinone group and cysteine 278 of PRMT5. Optimization of the starting hit produced a lead compound, BRD0639, which engages the target in cells, disrupts PRMT5-RIOK1 complexes, and reduces substrate methylation. BRD0639 is a first-in-class PBM-competitive inhibitor that can support studies of PBM-dependent PRMT5 activities and the development of novel PRMT5 inhibitors that selectively target these functions.

Targeted Covalent Inhibition of Plasmodium FK506 Binding Protein 35.

FK506-binding protein 35, FKBP35, has been implicated as an essential malarial enzyme. Rapamycin and FK506 exhibit antiplasmodium activity in cultured parasites. However, due to the highly conserved nature of the binding pockets of FKBPs and the immunosuppressive properties of these drugs, there is a need for compounds that selectively inhibit FKBP35 and lack the undesired side effects. In contrast to human FKBPs, FKBP35 contains a cysteine, C106, adjacent to the rapamycin binding pocket, providing an opportunity to develop targeted covalent inhibitors of Plasmodium FKBP35. Here, we synthesize inhibitors of FKBP35, show that they directly bind FKBP35 in a model cellular setting, selectively covalently modify C106, and exhibit antiplasmodium activity in blood-stage cultured parasites.