Abstract LB324: BB3008, a potent and selective small molecular HPK1 inhibitor effective in multiple syngeneic tumor models
Authors: Duan Gongping , Li Min , Zhang Xingmin 
Published: April 2023

Abstract
Introduction: Hematopoietic progenitor kinase 1 (HPK1) is a serine/threonine kinase in the MAP4K family predominantly expressed in immune cells, and has been identified as a negative regulator of T cell and B cell receptor signaling and dendritic cell function. Inhibition of the kinase activity of HPK1 results in the activation of exhausted T cell in tumor microenvironment and enhanced anti-tumor immune response. Observation of HPK1 overexpression in various tumor tissues further indicates the kinase as a novel target for immuno-oncology therapy. Here we report BB3008 as a small molecular HPK1 inhibitor.
Methods: BB3008 was developed through structure-based drug design, and optimized by SAR analysis and medicinal chemistry iteration. Biochemical assays and cell-base functional assays were applied in compound evaluation. Pharmacokinetic (PK) and preliminary toxicity studies were performed with mice and dogs. Syngeneic tumor models in mice were conducted to demonstrate the in vivo efficacy of BB3008 as a single agent and in combination with mouse PD-1 antibody.
Results: BB3008 shows sub-nanomolar HPK1 potency, with good selectivity against kinases in the MAP4K family and other TCR signaling-related kinases, such as FYN and ZAP70. Inhibition of HPK1 by BB3008 strongly suppresses the phosphorylation at Ser376 of the downstream biomarker protein SLP-76, with IC50 at 30 nM. With the stimulation of CD3/CD28, boost of IL-2 production was observed in Jurkat cells, purified human T cells, and human peripheral blood mononuclear cells upon treatment of BB3008 at sub-micromolar concentration. BB3008 also enhanced the killing activity of T cell against EL4 lymphoma cell in vitro in a concentration-dependent manner. PK profile of BB3008 showed good bioavailability (&gt;80%) with a nearly linear dose-dependent exposure. The compound had no significant suppression on various normal cell lines proliferation, and had no hERG liability. In preliminary safety evaluation, no abnormal clinical symptoms and serum/blood tests were observed in mice and dogs dosed up to 1000mpk and 150mpk for 14 days, respectively. BB3008 showed significant tumor-growth inhibition rates in CT26, Hepa 1-6 and 4T1 mouse models as single agent, and in MC-38 mouse model in combination with mouse PD-1 antibody.
Conclusion: As a potent, selective and oral available small molecule HPK1 inhibitor, BB3008 shows good safety tolerance in preclinical animals, and promising efficacy in multiple solid tumor models, both as single agent or in combination with immune checkpoint blockade. The IND-enabling study of BB3008 is in progress.
Citation Format: Gongping Duan, Min Li, Xingmin Zhang. BB3008, a potent and selective small molecular HPK1 inhibitor effective in multiple syngeneic tumor models [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 LB324.