Characteristics and Outcome of Patients with Core Binding Factor Acute Myeloid Leukemia and FLT3-ITD: Results from an International Collaboration

Background: Core binding factor acute myeloid leukemia (CBF-AML) is defined by the presence of either t(8;21)(q22;q22) or inv(16)(p13.1q22)/t(16;16)(p13.1;q22) and is associated with a favorable outcome, particularly if treated with repetitive cycles of high-dose cytarabine as post-remission therapy. Long-time 10-year overall survival (OS) rate was reported of 58% in FLT3-ITD negative patients (pts; Allen et al. Leukemia 2013). Nevertheless, 30-40% CBF-AML pts experience relapse. FLT3-ITD mutations occur in roughly 5-10% of adult CBF-AML. However, their prognostic relevance is still controversial.

Aims: To characterize CBF-AML with FLT3-ITD and compare outcomes according to their genetic background.

Methods: We retrospectively studied 65 AML pts with CBF-AML and FLT3-ITD (median age at diagnosis, 54 years; range, 22-81 years) diagnosed between 1996 and 2018 within seven study groups/institutions of the US and Europe.

Results: Thirty-two (49%) of the 65 pts harbored t(8;21). Median white blood cell and platelet counts at diagnosis of patients with t(8;21) and inv(16) were 18.3/nl (range, 1.8-202/nl) and 31/nl (range, 7-372/nl), respectively. AML diagnoses were de novo in 61 (94%) and therapy-related in 4 (6%) of the pts. Thirty (46%) pts were female. Cytogenetic analysis revealed additional abnormalities (abn) in 38 (58%) pts, most frequently loss of X or Y (n=13; n=12 associated with t(8;21)), complex karyotype (≥3 abn; n=12; n=7 occurring in t(8;21)), trisomy 22 (n=7, all associated with inv(16)) or trisomy 8 (overall n=6, n=5 occurring in inv(16)). Four pts were positive for both mutations, FLT3-ITD as well as FLT3-TKD. Median ITD allelic ratio were 0.44 (range, 0.003-50) and median ITD size 60 bp (range, 3-120 bp).

Three older pts (median age, 75.5 years) were treated with either azacitidine + sorafenib, azacitidine + venetoclax or with etoposide + tipifarnib. All three patients receiving non-intensive therapy died within one year and were excluded from further analysis.

Complete remission (CR) after anthracycline-based induction therapy was achieved in 98% (n=61/62) of patients fit for intensive treatment including two pts treated with 7+3 ± midostaurin within the RATIFY trial. One patient died during induction.

Fifteen (24%) pts underwent allogeneic hematopoietic cell transplantation. Of those, 10 pts were transplanted in 1^st and 5 pts in 2^nd CR. Median follow-up for the entire cohort was 4.43 years (95%-CI, 3.35-8.97 years). Median and 4-year relapse-free survival (RFS) rates were 3.41 years (95%-CI, 1.26 years - not reached) and 44.9% (95%-CI, 32.9-61.4%). Median and 4-year overall survival rates (OS) were 4.48 years (95%-CI, 2.26 years - not reached) and 51.8% (95%-CI, 39.6.2-67.9%). Neither type of CBF-AML (p=0.60), nor additional chromosomal abn (p=0.80), nor presence of a complex karyotype (p=0.50) had a prognostic impact on OS. Higher age (≥60 years) was an in trend negative prognostic factor on RFS and OS (p=0.07, each). High allelic ratio (≥0.5) had no impact on RFS (p=0.3), but in trend on OS (p=0.10).

Conclusions: Despite a high remission rate pts with FLT3-ITD had an inferior outcome as compared to previously published data on CBF-AML without FLT3-ITD. Thus, CBF-AML with FLT3-ITD should not be classified within the low-risk category. CBF pts with FLT3-ITD warrants further study and should be included in FLT3-inhibitor trials.

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