Abstract LB318: SB-4826, a first-in-class oral, covalent inhibitor of SUMO E1 that induces IFN signaling and inhibits tumor growth as monotherapy and in combination with immune checkpoint blockade
Authors: Canon Jude R. , Callinan Ashley , Bradley Sarah , Wang Laurie , Kui Mackenzie , Wein Ariel , Sulli Gabriele , Buchowiecki Peter , Hong Fang-Tsao , Kubota Miles , Walton Mary , Cee Victor J. 
Published: April 2023

Abstract
Sumoylation is a reversible post-translational modification of proteins by small ubiquitin-like modifier (SUMO) that regulates protein function and contributes to cellular response to stress. Sumoylation is an essential pathway for all cells and is involved in diverse processes such as inflammation, DNA damage response, signaling and cell survival/apoptosis. Due to its involvement in such cellular functions, sumoylation intersects with the majority of the hallmarks of cancer. Many cancers display elevated expression of SUMO pathway components, a mechanism co-opted by tumors to survive under stressful conditions. In addition to its tumor intrinsic functions, sumoylation has emerged as a novel target for activating anti-tumor immunity due to its role in regulating type I interferon (IFN) signaling. Targeting sumoylation is a compelling anti-cancer strategy due to the multiple mechanisms of action that may drive anti-tumor activity for cancer patients. We report the discovery of SB-4826, a novel, orally active, covalent small molecule inhibitor of the Sumo Activating Enzyme (SUMO E1), the initiating enzyme of the sumoylation cascade. SB-4826 selectively and irreversibly binds an allosteric pocket of SUMO E1 and drives potent biochemical and cellular activity. SB-4826 inhibits global sumoylation in cells and blocks proliferation across a broad panel (100+) of cancer cell lines. Cysteine-proteome profiling by mass spectrometry indicated that Cys-30 of SUMO E1 was the only cysteine-containing peptide (out of 14,541 measured) that met the criteria for covalent engagement indicating exquisite selectivity across the cellular proteome. In an in vivo pharmacodynamic model measuring IFN signaling SB-4826 treatment led to dose- and time-dependent induction of IFN regulated cytokines. SB-4826 treatment resulted in significant tumor growth inhibition in multiple in vivo models. In a syngeneic A20 lymphoma model SB-4826 led to tumor stasis. In a human RAJI xenograft model, SB-4826 induced near-complete inhibition of tumor growth as a single agent and resulted in tumor regression when combined with rituximab. In a mouse CT-26 colorectal tumor model which is generally refractory to anti-PD-1 treatment, SB-4826 significantly inhibited tumor growth as monotherapy and led to 60% complete tumor responses when combined with anti-PD-1. SB-4826 caused increased CD8+ T cell infiltration in CT-26 tumors supporting the role of SB-4826 in inducing IFN signaling and leading to a tumor microenvironment that is more responsive to immune checkpoint inhibition. SUMO E1 is a clinically validated cancer target as evidenced by recent data showing encouraging responses to TAK-981, an IV-administered, ATP-competitive inhibitor. SB-4826 is a differentiated and optimized SUMO E1 inhibitor that is administered orally and demonstrated improved activity in preclinical models. SB-4826 has favorable properties for development, is a new and promising cancer therapy, and is entering phase 1 clinical studies in 2023.
Citation Format: Jude R. Canon, Ashley Callinan, Sarah Bradley, Laurie Wang, Mackenzie Kui, Ariel Wein, Gabriele Sulli, Peter Buchowiecki, Fang-Tsao Hong, Miles Kubota, Mary Walton, Victor J. Cee. SB-4826, a first-in-class oral, covalent inhibitor of SUMO E1 that induces IFN signaling and inhibits tumor growth as monotherapy and in combination with immune checkpoint blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB318.