ERK1/2 pro-survival signalling is suppressed by pirtobrutinib in ibrutinib-resistant MYD88-mutated lymphoma cells

Covalent Bruton's tyrosine kinase-inhibitors (cBTK-i) are highly active in MYD88-mutated (MYD88^Mut) Waldenstrom's macroglobulinaemia and suppress nuclear factor kappa-light-chain-enhancer of activated B cells and extracellular signal-regulated kinases-1/2 (ERK1/2)-related signalling. BTK^Cys481 mutations are associated with cBTK-i acquired resistance and are accompanied by reactivation of ERK1/2 that promotes inflammatory cytokine secretion and paracrine-mediated resistance of BTK wild-type (BTK^WT) tumour cells. Pirtobrutinib is a non-covalent BTK-inhibitor that binds at non-BTK^Cys481 sites. We show that pirtobrutinib blocked p-ERK1/2, ERK1/2-driven inflammatory cytokines, and overcame paracrine-mediated resistance in MYD88^Mut lymphoma cells expressing mutated BTK^Cys481. Our results provide important mechanistic insights for the activity of pirtobrutinib in MYD88^Mut lymphomas carrying BTK^Cys481 mutations.