BTK Inhibition — B-cell Malignancies

Bruton's tyrosine kinase (BTK) is a non-receptor cytoplasmic tyrosine kinase and critical component of the B-cell antigen receptor (BCR) signalling cascade. Following BCR engagement, BTK is recruited to the membrane and activated by upstream LYN and SYK kinases, subsequently phosphorylating phospholipase C-gamma-2 (PLCgamma2) to generate IP3 and DAG. This drives calcium mobilisation and PKC activation, with downstream NF-kappaB, MAPK, and PI3K signalling collectively promoting B-cell survival, proliferation, differentiation, and tissue migration. In B-cell malignancies including CLL, mantle cell lymphoma, and Waldenstrom's macroglobulinaemia, the BCR-BTK axis is constitutively active, sustaining malignant B-cell survival and proliferation.

First-generation covalent BTK inhibitors (ibrutinib) bind irreversibly to Cys481 in the BTK ATP-binding pocket, providing durable pathway suppression. However, ibrutinib's off-target inhibition of related kinases (ITK, EGFR, TEC) contributes to class-specific toxicities including atrial fibrillation, bleeding, and arthralgias. Second-generation covalent inhibitors (acalabrutinib, zanubrutinib) were designed with greater BTK selectivity, demonstrating improved tolerability profiles in head-to-head comparisons in CLL. Acquired resistance to covalent BTK inhibitors most commonly arises via the C481S BTK mutation, which prevents covalent bond formation.

Pirtobrutinib, a non-covalent (reversible) BTK inhibitor, was specifically engineered to retain potent BTK inhibition in the presence of C481S and other resistance mutations, establishing a treatment option in patients who have progressed on covalent BTK inhibitors. This non-covalent binding mechanism enables activity regardless of Cys481 mutation status and with a differentiated selectivity profile. BTK inhibition is also under investigation in autoimmune conditions including rheumatoid arthritis, SLE, and multiple sclerosis, given BTK's role in BCR-dependent B-cell activation and toll-like receptor signalling in myeloid cells.