Abstract 3474: Identification of the selective CBL-b inhibitors which effectively prevent T cells from exhaustion and demonstrate synergistic anti-tumor activity in combination with an anti-PD1 antibody
Authors: Zhou Feng , Li Zhengtao , Liu Yajing , Yang Guimei , Mai Wansun , Yang Wenqing , Xue Liting , Chen Ping , Tang Renhong 
Published: April 2023

Abstract
The E3 ubiquitin ligase Casitas B-lineage lymphoma b (Cbl-b) is a member of the highly conserved family of Cbl (casitas b-lineage lymphoma) proteins. CBL-b is widely expressed in immune cells and plays as a “gate keeper” to negatively regulate the activation of immune cells. Engagement of TCR/BCR triggers the Cbl-b-mediated ubiquitination and degradation of the key components in TCR axis, such as Vav1 and PLCγ, and consequently attenuates TCR/BCR signal axis which finally results in T/B cells exhaustion. Besides, Cbl-b was reported to negatively regulate the activation of dendritic cells, NK, NKT, and mast cells. Cbl-b KO mice show resistance to tumor transplantation, and the anti-tumor activities depend on the T and NK cells. According to the above observation, Cbl-b emerges as a novel intracellular immune checkpoint target to prevent immune cells from exhaustion and improve the antitumor immunotherapy. Here we identified a series of highly potent Cbl-b inhibitors. The inhibition of Cbl-b protein was evaluated by an in vitro CBL-b/E2-ubiquitin assay. Results showed that our candidates strongly inhibited Cbl-b with IC50s at single-digital nanomolar level, and meanwhile, these compounds displayed over than 10-fold selectivity against c-CBL/E2-ubiquitin, a member of the Cbl family, which was reported to synergistically enhance toxicity caused by Cbl-b. Consistent with the Cbl-b inhibition, these compounds further enhanced activation of Jurkat T cells as well as human PBMCs with dramatical increase of IL-2 or IFNɣ secretion in presence of anti-CD3/CD28, and without any significant cytotoxicity even up to 10 µM concertation. Furthermore, these compounds exhibited the restoring function in exhausted T cells in presence with PEG2 or adenosine treatment. Favorable ADME and bioavailability supported the oral administration. In naïve mice, our inhibitors dose-dependently promoted the cytokine IL-2 and IFNɣ production in serum and spleens following the anti-CD3 challenge. In a syngeneic model, CT26, oral administration of our Cbl-b inhibitor effectively inhibited tumor growth, while completely inhibited tumor growth in combination with an anti-PD1 antibody. Further study demonstrated that the increased lymphocytes infiltration and cytokines release in tumor tissues were observed either in Cbl-b mono or in anti-PD1 combo groups, which correlated well with anti-tumor efficacy. In conclusion, our selective Cbl-b inhibitors demonstrated robust immune activation and anti-tumor activity in vitro and in vivo and showed synergistic activities with immune checkpoint inhibitors, such as anti-PD(L)1. Currently, one of our leading molecules is in IND enabling studies.
Citation Format: Feng Zhou, Zhengtao Li, Yajing Liu, Guimei Yang, Wansun Mai, Wenqing Yang, Liting Xue, Ping Chen, Renhong Tang. Identification of the selective CBL-b inhibitors which effectively prevent T cells from exhaustion and demonstrate synergistic anti-tumor activity in combination with an anti-PD1 antibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3474.