Rising Star in Immunotherapy: Development and Therapeutic Potential of Small-Molecule Inhibitors Targeting Casitas B Cell Lymphoma-b (Cbl-b)

Abstract

Casitas B cell lymphoma-b (Cbl-b) is a vital negative regulator of TCR and BCR signaling pathways, playing a significant role in setting an appropriate threshold for the activation of T cells and controlling the tolerance of peripheral T cells via a variety of mechanisms. Overexpression of Cbl-b leads to immune hyporesponsiveness of T cells. Conversely, the deficiency of Cbl-b in T cells results in markedly increased production of IL-2, even in the lack of CD28 costimulation in vitro. And Cbl-b −/− mice spontaneously reject multifarious cancers. Therefore, Cbl-b may be associated with immune-mediated diseases, and blocking Cbl-b could be considered as a new antitumor immunotherapy strategy. In this review, the possible regulatory mechanisms and biological potential of Cbl-b for antitumor immunotherapy are summarized. Besides, the potential roles of Cbl-b in immune-mediated diseases are comprehensively discussed, with emphasis on Cbl-b immune-oncology agents in the preclinical stage and clinical trials. ■ SIGNIFICANCE • Cbl-b is a novel intracellular immune checkpoint, closely related to the development of cancers. Therefore, developing agents targeting Cbl-b is of great significance. • The lead optimization, molecular docking, and related patents research of Cbl-b inhibitors are discussed in detail from the perspective of medicinal chemistry. • The regulatory mechanism, crystal structure, and clinical progress of Cbl-b are introduced in detail, providing guidance for developing Cbl-b inhibitors with better druggability in the future.

Optimization of a Novel DEL Hit That Binds in the Cbl-b SH2 Domain and Blocks Substrate Binding

“…Therefore, additional CI agents, especially those with a distinctive mechanism of action from checkpoint blockade, are urgently needed to improve the response rates and broaden indications to help more cancer patients . Toward this goal, we were interested in targeting Casitas B-lineage lymphoma proto-oncogene-b (Cbl-b), a RING E3 ligase, which acts as a negative regulator of T cell activation. − For example, it has been shown that in mouse, knockout of Cbl-b, as well as knock-in of a functionally inactive Cbl-b (C373A) mutant, leads to hyperactivation of T cells and spontaneous rejection of tumor. − A complicating factor is that in mammals, Cbl-b belongs to the Cbl family with two additional members: c-Cbl and Cbl-3. , The three members share a highly conserved N-terminus, including TKB and RING finger domains, while the C-terminus is more diverse, particularly for Cbl-3, which contains a significantly truncated C-terminus. In light of the high sequence and structural homology between Cbl-b and c-Cbl in the N-terminus, it was not readily clear to us how to achieve selectivity for Cbl-b over c-Cbl.…”

Deciphering the Selectivity of CBL-B Inhibitors Using All-Atom Molecular Dynamics and Machine Learning

“…Several studies hypothesized that selective CBL-B inhibition over C-CBL is the key in solid tumor treatment. , However, high similarities in the sequence and structure of CBL-B and C-CBL make the design of selective CBL-B inhibitors a daunting challenge. ,,, CBL-B and C-CBL share high sequence homology in tyrosine kinase binding (TKB), LHR, and RING domain. − ,, A study identified a few amino acids difference in the linker region between the N-terminal TKB domain and the C-terminal RING finger domain with 86% amino acid identity of CBL-B and C-CBL, respectively. , Therefore, how to exploit the little structural and sequence differences and their subtle dynamic and kinetic differences will be crucial to achieve selectivity. The binding affinities ( K d values) of known CBL-B inhibitors range from low-micromolar to nanomolar depending on assay conditions . There are a few k off values (dissociation rates) of CBL-B inhibitors in the literature, which are key kinetic properties of inhibitors. , Exploiting the structural difference, coupled with kinetics via their different dissociation behavior and their unique interaction along the dissociation pathways in CBL-B drug design, has not been well elaborated.…”