Discovery of Non-Nucleotide Small-Molecule STING Agonists via Chemotype Hybridization

The identification of agonists of the stimulator of interferon genes (STING) pathway has been an area of intense research due to their potential to enhance innate immune response and tumor immunogenicity in the context of immuno-oncology therapy. Initial efforts to identify STING agonists focused on the modification of 2',3'-cGAMP (1) (an endogenous STING activator ligand) and other closely related cyclic dinucleotides (CDNs). While these efforts have successfully identified novel CDNs that have progressed into the clinic, their utility is currently limited to patients with solid tumors that STING agonists can be delivered to intratumorally. Herein, we report the discovery of a unique class of non-nucleotide small-molecule STING agonists that demonstrate antitumor activity when dosed intratumorally in a syngeneic mouse model.

Abstract 1140: A novel MTA non-competitive PRMT5 inhibitor

The chromosome 9p21 (chr9p21) locus is deleted in almost 10% of all cancer types. This locus includes the CDKN2A gene that encodes the critical tumor suppressors p19-ARF and p16-INK4a. Methylthioadenosine phosphorylase (MTAP), a gene proximal to CDKN2A, is co-deleted in 80%-90% of tumors with CDKN2A deletion. MTAP plays a critical role in the methionine salvage pathway, and the deletion of MTAP results in the accumulation of its substrate methythioadenosine (MTA). Accumulation of MTA partially inhibits the activity of the arginine methyltransferase PRMT5, causing MTAP deficient cancer cells to be more sensitive to the genetic knockdown of PRMT5. In contrast to genetic knockdown, sensitivity to pharmacological inhibition of PRMT5 does not appear to stratify with MTAP status. However, currently known PRMT5 inhibitors all possess SAM competitive or uncompetitive MOIs, which generally require displacement of MTA from the active site for binding. We hypothesize that leveraging the high MTA state induced by MTAP deficiency will require an inhibitor that can bind PRMT5 without disrupting bound MTA. Here we report the discovery of a PRMT5 inhibitor with a novel binding mode that is compatible with MTA binding. While this compound possesses the desired MOI, it shows only modestly increased potency toward MTAP-null cells. Mathematical simulations of different inhibitor mechanisms indicate that the degree of selectivity that can be achieved depends on the difference in MTA levels between MTAP-null and WT cells. Our in vitro data suggest that the elevation in intracellular MTA concentrations that occurs with MTAP deletion is not sufficient to confer significantly increased sensitivity to PRMT5 inhibition. We anticipate that the therapeutic index that can be achieved between MTAP-null tumor cells and PRMT5-sensitive normal tissues will be similarly limited in vivo.

Citation Format: Rohit Malik, Peter K. Park, Christopher M. Barbieri, Yuval Blat, Steven Sheriff, Carolyn A. Weigelt, Lisa M. Kopcho, Muge Celiktas, Max Ruzanov, Joseph G. Naglich, Jennifer L. Price, Mary Harner, Kevin M. Omalley, JIngjing Deng, William Schmitz, Guo Li, Zheming Ruan, Lan-ying Qin, Gerald J. Duke, Iyoncy Rodrigo, Mark R. Witmer, David G. Harden, Shilpa Demes, Brian J. Arey, Matt Soars, Brian E. Fink, Ashvinikumar V. Gavai, Gregory D. Vite, Charles F. Voliva. A novel MTA non-competitive PRMT5 inhibitor [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 1140.