Resistance mutations in CML and how we approach them.

Among the variety of resistance mechanisms that may underlie a non-optimal response to tyrosine kinase inhibitor (TKI) therapy in chronic myeloid leukemia patients, secondary point mutations in the BCR::ABL1 kinase domain (KD) represent the only actionable one. Each of the 5 ATP-competitive inhibitors (imatinib, dasatinib, nilotinib, bosutinib, ponatinib) has a well-defined spectrum of resistance mutations. Growing clinical experience will soon allow to also elucidate the full spectrum of mutations conferring resistance to asciminib (that appear not to be confined to the myristate binding pocket). Regular molecular response (MR) monitoring is fundamental for evaluating treatment efficacy, catching early signs of relapse, and intervening promptly in case of confirmed failure. Whenever MR is not deemed satisfactory according to the European LeukemiaNet or the National Comprehensive Cancer Network definitions, BCR::ABL1 KD mutations testing should be performed. When needed, prompt and informed TKI switch can improve response and outcome and prevent the accumulation of mutations, including highly challenging compound mutations. Novel technologies like next-generation sequencing and digital polymerase chain reaction have recently been explored for BCR::ABL1 KD mutation testing; they have both advantages and disadvantages that are discussed in this article. This review also provides suggestions for interpretation and clinical translation of mutation testing results, which may not always be straightforward, particularly in cases of low-level or unknown mutations.