Bruton’s tyrosine kinase (BTK), a key member of the B-cell receptor (BCR) signalling pathway and the first Tec family tyrosine kinase identified as a dual-function regulator of apoptosis, promotes radiation-induced apoptosis but inhibits Fas-activated apoptosis in B-cells. BTK is a cytoplasmic, non-receptor tyrosine kinase (nRTK) that transmits signals via a variety of cell-surface molecules and is expressed by all cells of the haematopoietic lineage except T, NK, and plasma cells. In 1952, American paediatrician Ogdon Bruton discovered X-linked agammaglobulinemia (XLA), an inherited disease characterized by the absence of antibodies, resulting in recurrent bacterial infections and sepsis early in childhood. In 1993, the gene underlying XLA was identified as agammaglobulinemia tyrosine kinase (ATK) and B-cell progenitor kinase (BPK). This gene was named BTK by Ogden Bruton. Studies have shown that BTK not only signals B-cell receptor (BCR) responses for antigen engagement but also stimulates CD40, Toll-like receptors (TLRs), Fc receptors (FCRs) and chemokine receptors. Moreover, BTK can modulate signalling and overexpression, which leads to autoimmunity, and decreased levels of BTK improve autoimmune disease outcomes.
BTK inhibitors: preclinical and clinical development
Since BTK was confirmed to play a crucial role in B-cell maturation as well as in mast cell activation through the high-affinity IgE receptor, studies of BTK as a target have attracted substantial attention from drug researchers, resulting in the development of a diverse array of BTK inhibitors. According to the chemical scaffold structures and mechanisms of action, BTK inhibitors can be classified into two types based on their mode of binding to BTK. One type of BTK inhibitors is irreversible inhibitors that form a covalent bond with the amino acid residue Cys481 in the ATP-binding site of BTK. The other type of BTK inhibitors is reversible inhibitors that access the specific SH3 pocket of BTK, binding to an inactive conformation of the kinase. Most of the reported BTK inhibitors are irreversible inhibitors, and their published molecular structures reveal basic parent scaffolds, including imidazopyrimidine, 2,4-diaminopyrimidines, and imidazoquinoxaline linked to phenyl-morpholine, phenyl-piperazine and other substituting groups. Among the launched BTK inhibitors, ibrutinib is an irreversible, first-in-class, highly potent, small-molecule BTK inhibitor with subnanomolar activity (IC50=0.5 nM) against BTK that is used for the treatment of MCL, CLL and WM. However, due to toxicities related to off-target effects, ibrutinib is not approved by the FDA for the treatment of RA, SLE and other autoimmune diseases. Therefore, many reversible inhibitors have been intensely investigated, and some have been developed for long-term drug administration exclusively for the treatment of autoimmune diseases. Substantial efforts have been made over the years to further develop reversible inhibitors, but none of these have yielded significant breakthroughs.
Current development of small-molecular BTK inhibitors
Launched BTK inhibitors:
Ibrutinib is an oral, covalent, irreversible BTK inhibitor that exhibits high selectivity, prolonged pharmacodynamics, and potency in overcoming endogenous ATP competition by binding to Cys481 in the ATP-binding domain of BTK. Ibrutinib was first launched in the U.S. in 2013 for the treatment of MCL, and it was further used as a monotherapy for the treatment of CLL in 2014.
Acalabrutinib is a novel experimental anti-cancer drug and a second-generation BTK inhibitor developed by Acerta Pharma. Acalabrutinib, which is structurally related to the first-in-class BTK inhibitor ibrutinib, binds covalently to Cys481 and shows higher selectivity and inhibitory activity towards BTK.
Representative BTK inhibitors with published structures
Olmutinib is a potent inhibitor of BTK and a third-generation, irreversible epidermal growth factor receptor (EGFR)-mutation-selective tyrosine kinase inhibitor. According to its molecular structure, olmutinib possesses a thieno-[3,2-d]-pyrimidine core and a typical terminal acrylamide, which serves as a Michael acceptor that covalently binds to Cys481 located in the BTK hinge segment.
Tirabrutinib, an analogue of the ibrutinib scaffold, is an irreversible BTK inhibitor in early clinical trials that was initially developed by Ono Biomedical for the treatment of B-cell lymphoma and CLL. Tirabrutinib is simultaneously undergoing Phase II clinical trials for central nervous system lymphoma.
Spebrutinib is an orally available, potent, selective and covalent BTK inhibitor with an IC50 below 0.5nM. Spebrutinib was originally developed by Avila Therapeutics and was acquired by Celgene in March 2012.
RN-486 is a novel BTK inhibitor distinct from previous BTK inhibitors such as PCI-32765, CGI-1746 and GDC-0834. In contrast to the irreversible, covalently binding PCI-32765 and RN486 and the two previous reversible BTK inhibitors, CGI-1746 and GDC-0834, RN-486 can block the signalling pathway of BCR, as demonstrated by a marked inhibition of the phosphorylation of both BTK and PLC2 in B cells.
Liang, C., Tian, D., Ren, X., Ding, S., Jia, M., Xin, M., & Thareja, S. (2018). The development of Bruton's tyrosine kinase (BTK) inhibitors from 2012 to 2017: A mini-review. European journal of medicinal chemistry.