Prognostic Impact of Mutant to Wild-Type Ratio and Insertion Site in Acute Myeloid Leukemia with FLT3 Internal Tandem Duplication

FLT3 internal tandem duplications (FLT3-ITD) occur in about 25% of acute myeloid leukemia (AML), are associated with cooperating gene mutations (NPM1, DNMT3A), and confer an adverse prognosis. Several studies have indicated that the unfavorable impact of FLT3-ITD is influenced by a number of factors, such as the mutant to wild-type ratio (allelic ratio), insertion site of FLT3-ITD in the beta1 sheet of the tyrosine kinase domain 1, and the molecular background of cooperating mutations.

To evaluate the relative impact of FLT3-ITD allelic ratio and insertion site, as well as cooperating genetic lesions on prognosis and treatment decision making in a large cohort of homogeneously treated younger adult patients.

The basis of the study were 2377 younger adults (median age, 48 years; range, 16–62 years) with newly diagnosed AML enrolled on three prospective treatment trials of the German-Austrian AML Study Group (AMLSG) between 1993 and 2008. Patients with acute promyelocytic leukemia (n=99), core-binding factor AML (n=279) and AML with adverse-risk cytogenetics (n=436) according to the European LeukemiaNet recommendations were excluded. Based on material availability, the presence of FLT3-ITD could be analyzed in 1414 patients; NPM1 and DNMT3A mutational status was available in 97% and 84% of the patients, respectively. In FLT3-ITD positive AML (n=394), the allelic ratio, determined by Genescan-based fragment-length analysis, was available in 86% and the insertion site in 72%. Allogeneic hematopoietic stem cell transplantation (HSCT) in first complete remission was performed in 41% and 29% of FLT3-ITD positive and negative patients, respectively.

We first evaluated the prognostic impact of the different FLT3-ITD characteristics within the subgroup of FLT3-ITD positive patients. The allelic ratio was categorized into quartiles ranging from low to high. For the endpoints event-free (EFS), relapse-free (RFS) and overall survival (OS), only the fourth quartile with the highest allelic ratio showed a prognostic impact for all endpoints, whereas no difference was identified between the other three quartiles. For further analyses, the allelic ratio was dichotomized comparing the fourth quartile versus the other three quartiles. FLT3-ITD insertion site in the beta1 sheet was significantly associated with an unfavorable outcome for all endpoints. Additionally, FLT3-ITD size was directly correlated with the insertion site: the more C-terminal the ITD inserted in the FLT3 gene the longer the FLT3-ITD size. There was no prognostic impact of FLT3-ITD size neither as continuous nor as quartile-categorized variable. Multiple FLT3-ITDs, present in 13% of AMLs, were associated with an unfavorable prognosis. The presence of either NPM1 and/or DNMT3A mutations in FLT3-ITD positive patients did not alter the original FLT3 prognosis. In multivariable models for the endpoint OS of the total cohort of intermediate-risk AML, an independent prognostic impact beyond the variable FLT3-ITD was shown for the allelic ratio (fourth quartile) [HR, 1.4; p=0.037] and in trend for insertion site in the beta1 sheet [HR, 1.33; p=0.06]. Survival of patients exhibiting a high allelic ratio (n=43) or insertion site in the beta1 sheet (n=60) was comparable, with a median of 10 and 13 months and 4-year survival of 19% and 24%, respectively. Of note, outcome of patients with both high allelic ratio and insertion site in the beta1 sheet (n=21) was very poor with a median OS of 10 months and 4-year OS of 5%, respectively. In patients with FLT3-ITD positive AML without these unfavorable factors (n=144), median and 4-year OS were 15 months and 42%, respectively. Of note, a clear benefit of allogeneic HSCT in first CR was only seen in FLT3-ITD positive patients without these two unfavorable factors, with a 4-year OS of 63%. In comparison, the 4-year OS of the same subgroup of patients achieving a CR after induction therapy without proceeding to allogeneic HSCT during first CR was 35%. In contrast, outcome in patients with high allelic ratio and/or insertion site in the beta1 sheet remained poor despite allogeneic HSCT in first CR.

High FLT3-ITD allelic ratio and ITD insertion site in the beta1 sheet presented as prognostic indicators for poor outcome in patients with the presence of a FLT3-ITD. Only patients without these unfavorable FLT3-ITD features significantly benefitted from allogeneic HSCT.

Schlenk:Roche: Research Funding; Pfizer: Research Funding; Amgen: Research Funding.