The Challenging Search for BTK Inhibitors

Bruton’s Tyrosine Kinase (BTK) is a 76kDa non-receptor Tec kinase that plays numerous diverse roles in B-cell development, immunity, autoimmunity, infection, and cancer. From stem to stern, it consists of an N-terminal plekstrin homology (PH) domain, a TEC homology (TH) domain, two SRC homology (SH2 and SH3) domains, and a C-terminal kinase domain. Unlike SRC, BTK is a cytoplasmic protein and only present at interior cell membranes upon recruitment.¹

Structure & Corresponding Function

BTK’s PH domain binds phosphatidylinositol lipids within biological membranes, phospholipase C gamma 2 (PLCg2), and phosphatidylinositol (3,4,5)-triphosphate (PIP3). In it’s inactive state, the TH domain binds the activation loop face of the kinase domain to sterically hinder the lipid-binding pocket of the PH domain. Likewise, under conditions of inactivation, the SH2 and SH3 domains bind the distal side of the kinase domain to restrain access to its active site. Once at the cell membrane, BTK can become activated by the interaction with SRC kinases and phosphorylation occurs at position 551 of the kinase. This can also occur through interaction with a SYK kinase. The SH2 domain and kinase domain together form an allosteric interface, critical for BTK’s activation.¹

When BTK’s PH domain binds the membrane, subsequent PIP3 interaction induces BTK to phosphorylate PLCg2. Phosphorylated PLCg2 hydrolyzes PIP2 into two second messengers, inositol triphosphate (IP3) and diacylglycerol (DAG), that modulate the activity of downstream protein signaling.¹

BTK’s Role in Immunity

BTK mediates receptor signaling in several crucial areas. Antigen stimulated B-cell receptors utilize BTK to transmit their activation, leading to B-cell differentiation and proliferation. Additionally, BTK interacts with Toll-like receptors to link adaptive and innate immunity. SDF-1 activates BTK in response to chemokine signalling.²

In immune complex-driven activation, BTK affects immunity by increasing Treg/CD4+ ratios, thereby, having an immunosuppressive effect on T-cells. BTK is also involved in Mac-1 activation, recruiting neutrophils to sites of inflammation. In addition, BTK is a positive regulator of the NLRP3 inflammasome due to its pivotal role in NLRP3 tyrosine phosphorylation and IL-1beta release.²

In the area of pathogen defense, BTK activates calcineurin-NFAT in macrophages to repel fungal invaders. In response to bacterial infection, it mediates SKAP-2 dependent ROS activation in neutrophils to enhance pathogen clearance. When macrophage TLRs recognize single-stranded viral RNA, they initiate signaling through BTK-dependent activation of NF-κB. This triggers the production of multiple inflammatory cytokines and chemokines, as well as phagocytosis.²

Therapeutic Complexities

Unfortunately, increased levels of BTK support autoantibody production, driving autoimmune disease. Systemic lupus erythematosus (SLE) is characterized by the secretion of autoantibodies. SLE’s autoreactive B-cells exhibit increased levels of BTK expression. Additionally, rheumatoid arthritis (RA) is characterized by autoantibodies that cause chronic inflammation and joint pain. B-cell dysregulation via BCR signaling drives the production of autoantibodies and inflammatory cytokines that provoke and perpetuate RA. In multiple sclerosis, inhibition of BTK has prevented microglia from engulfing and demyelinating myelin sheaths.²

Due to its key role in B-cell proliferation, aberrantly upregulated BTK is a prime driver of several lymphoproliferative disorders: Chronic Lymphocytic Leukemia, Mantle Cell Lymphoma, and Diffuse Large B-cell Lymphoma.²

Mutations in BTK’s domains are the source of many disease manifestations and, of late, the target of many therapeutics. Due to the multifunctional, multisystem interactions of BTK, domain specific inhibitors have their advantages as well as their off-target effects on other Tec kinases. Specific first- and second-generation inhibitors have impacted native and adaptive immune functions. Additionally, cross inhibition of other proteins bearing Tec domains has led to increased bleeding risk. Resistance has also become an issue.³ Recent work even suggests that different isoforms may be key drivers in epithelial cancers.⁴ 

The search for effective, low side effect inhibitors remains a challenge.

References
1. Kueffer LE, Joseph RE and Andreotti AH (2021) Reining in BTK: Interdomain Interactions and Their Importance in the Regulatory Control of BTK. Front. Cell Dev. Biol. 9:655489. https://doi.org/10.3389/fcell.2021.655489
2. McDonald, C, Xanthopoulos, C, Kostareli, E. The role of Bruton’s tyrosine kinase in the immune system and disease. Immunology. 2021; 164: 722– 736. https://doi.org/10.1111/imm.13416
3. Arneson LC, Carroll KJ, Ruderman EM. Bruton’s Tyrosine Kinase Inhibition for the Treatment of Rheumatoid Arthritis. Immunotargets Ther. 2021 Aug 28;10:333-342. https://doi.org/10.2147/itt.s288550.
4. Wang X, Kokabee L, Kokabee M and Conklin DS (2021) Bruton’s Tyrosine Kinase and Its Isoforms in Cancer. Front. Cell Dev. Biol. 9:668996. https://doi.org/10.3389/fcell.2021.668996