Abstract LB003: Fragment based discovery of MRTX9768, a synthetic lethal-based inhibitor designed to bind the PRMT5-MTA complex and selectively target MTAP/CDKN2A-deleted tumors

The MTAP gene is proximal to and co-deleted in nearly all CDKN2A-deleted cancers. This genetic alteration is present in an estimated 9% of all cancers and is especially prevalent in cancers with high unmet medical need (e.g. mesothelioma (32%), pancreatic (22%), lung squamous (20%)). Multiple independent research teams have demonstrated that tumor cell lines with homozygous MTAP deletions are hypersensitive to shRNA-mediated knock down of PRMT5. MTAP is required for the methionine salvage pathway and MTAP-del cells accumulate MTA, an inhibitory co-factor which competes for binding to the co-factor binding site of PRMT5 with the activating co-factor SAM. PRMT5 is a methyltransferase that adds symmetric dimethylarginine (SDMA) modification to proteins and is essential for mammalian cell survival. A small molecule that selectively binds and stabilizes the catalytically inactive PRMT5•MTA complex may represent a synthetic lethal-based precision medicine for the treatment of MTAP/CDKN2A—del tumors. Notably, 1st generation PRMT5 small molecule inhibitors do not target MTA-complexed PRMT5 and do not exhibit selective inhibition of MTAP-del cancer cells. Here we report a new series of compounds discovered via a fragment-based approach that selectively bind to the PRMT5•MTA complex. A fragment hit was identified in an SPR binding assay with PRMT5•MTA (KD 18 μM). The binding mode was determined by X-ray crystallography and revealed that the fragment makes productive interactions with K333, F327, E435, E444, E435, and W579 of PRMT5 as well as with the co-liganded MTA. Fragment growing aided by structure-based design identified a key interaction with the L312 backbone N-H that enhances binding to PRMT5•MTA (MRTX4646, SPR KD 57 nM). Further exploration highlighted an interaction with the F580 backbone N-H as important for cellular activity and selectivity. This interaction with F580 was illustrated by MRTX7512 which exhibits an IC50 value of 633 nM for inhibition of SDMA in engineered HCT116 MTAP-del cells and demonstrates 15-fold selectivity compared with HCT116 MTAP-WT cells (IC50 9763 nM). Further optimization of cellular potency and pharmacokinetic properties identified MRTX9768, a potent inhibitor of SDMA and cell proliferation in HCT116 MTAP-del cells (SDMA IC50 3 nM; prolif. IC50 11 nM) with marked selectivity over HCT116 MTAP-WT cells (SDMA IC50 544 nM; prolif. IC50 861 nM). In xenograft studies, oral administration of MRTX9768 demonstrates dose-dependent inhibition of SDMA in MTAP-del tumors, with less SDMA modulation observed in bone marrow. In summary, we have used a fragment-based approach to discover a new class of orally active PRMT5•MTA inhibitors that demonstrate selective antitumor activity in MTAP-del tumor cells while sparing MTAP-WT cells. Citation Format: Christopher R. Smith, Svitlana Kulyk, J. D. Lawson, Lars D. Engstrom, Ruth Aranda, David M. Briere, Robin Gunn, Krystal Moya, Lisa Rahbaek, Laura Waters, Anthony Ivetac, James G. Christensen, Peter Olson, Matthew A. Marx. Fragment based discovery of MRTX9768, a synthetic lethal-based inhibitor designed to bind the PRMT5-MTA complex and selectively target MTAP/CDKN2A-deleted tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB003.