Zanubrutinib

Bruton's tyrosine kinase (BTK) is a non-receptor kinase and a signalling molecule for the B cell receptors expressed on the peripheral B cell surface. The BCR signalling pathway plays a crucial role in normal B-cell development but also the proliferation and survival of malignant B cells in many B cell malignancies, including mantle-cell lymphoma (MCL). Once activated by upstream Src-family kinases, BTK phosphorylates phospholipase-Cγ (PLCγ), leading to Ca2+ mobilization and activation of NF-κB and MAP kinase pathways. These downstream cascades promote the expression of genes involved in B cell proliferation and survival. The BCR signalling pathway also induces the anti-apoptotic protein Bcl-xL and regulates the integrin α4β1 (VLA-4)-mediated adhesion of B cells to vascular cell adhesion molecule-1 (VCAM-1) and fibronectin via BTK. Apart from the direct downstream signal transduction pathway of B cells, BTK is also involved in chemokine receptor, Toll-like receptor (TLR) and Fc receptor signalling pathways.

Zanubrutinib inhibits BTK by forming a covalent bond with cysteine 481 residue in the adenosine triphosphate (ATP)–binding pocket of BTK, which is the enzyme's active site. This binding specificity is commonly seen with other BTK inhibitors. Due to this binding profile, zanubrutinib may also bind with varying affinities to related and unrelated ATP-binding kinases that possess a cysteine residue at this position. By blocking the BCR signalling pathway, zanubrutinib inhibits the proliferation, trafficking, chemotaxis, and adhesion of malignant B cells, ultimately leading to reduced tumour size. Zanubrutinib was also shown to downregulate programmed death-ligand 1 (PD-1) expression and cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) on CD4+ T cells.