Sequencing asymmetry: the case for caution before first-line pirtobrutinib in chronic lymphocytic leukemia.

The phase III BRUIN CLL-313 and CLL-314 trials have established pirtobrutinib, a noncovalent Bruton tyrosine kinase (BTK) inhibitor, as a candidate for first-line therapy in chronic lymphocytic leukemia (CLL), and all-lines regulatory approval has now been granted in the European Union. We highlight an untested asymmetry: after failure of covalent BTK inhibitors, pirtobrutinib retains phase III-validated activity, whereas resistance to first-line pirtobrutinib can arise through kinase-domain mutations (e.g., A428D, L528W) that confer in vitro cross-resistance to covalent agents, the class that currently anchors relapse management. No clinical data exist on covalent BTK inhibitors after pirtobrutinib failure. We argue that, until reverse-sequence data emerge, a cautious approach that retains the clinically validated covalent-first sequence is reasonable for patients expected to need multiple lines of therapy, and we suggest mutation testing at progression, registry tracking of post-pirtobrutinib outcomes, and explicit discussion of sequencing in forthcoming guideline updates.