Prognostic Impact of FLT3 ‐ ITD Microclones in AML : An EHA AML ‐Specialized Working Group Study

ABSTRACT The 2022 ELN recommendations reclassified fms‐like tyrosine kinase‐3 ( FLT3 )‐ITD mutated acute myeloid leukemia (AML) as intermediate‐risk, irrespective of allelic ratio (AR), and designated capillary electrophoresis (CE) as the preferred detection method. However, CE requires substantial expertise, particularly to identify FLT3 ‐ITD microclones ( FLT3 ‐ITD m ; AR < 0.05), whose clinical significance remains uncertain. Herein, we investigated diagnostic aspects and the prognostic impact of FLT3 ‐ITD m . An initial methodological study performed on 48 FLT3 ‐ITD+ samples, with AR 0.001–6.00, showed overall concordance between CE and next‐generation sequencing (NGS) results ( p < 0.001), with only three microclones (AR: 0.002, 0.001, and 0.008) identified by CE and not by NGS. We next analyzed 658 AML patients diagnosed with a FLT3 ‐ITD+ AML ( n = 446 with AR ≥ 0.05, and n = 212 with a microclone) between 2017 and 2022. Patients with FLT3 ‐ITD m , as compared to patients with AR ≥ 0.05, were older, more often had secondary‐type AML, and displayed a higher prevalence of myelodysplasia‐related gene mutations, while NPM1 ‐mutation frequency was similar in the two groups. Complete remission rates were lower in FLT3 ‐ITD m (74%) than in patients with AR ≥ 0.05 (88%) treated with intensive chemotherapy without FLT3‐inhibitors (FLT3i). In the intensively treated subgroup, multivariate analyses demonstrated that age, NPM1 mutation, and allogeneic stem cell transplantation, rather than FLT3 ‐ITD AR, were independent predictors of disease‐free survival. At relapse, 15 of 33 (45%) evaluable patients remained FLT3 ‐ITD+, with 10 expanding the original microclone. In conclusion, FLT3 ‐ITD m are associated with older age, secondary AML, and with expansion of the FLT3 ‐ITD clone upon relapse, thereby providing a rationale for the evaluation of FLT3i in randomized clinical trials.