FLT3-ITD Positive AMLs Harbouring A Second MRD Molecular Marker May Benefit From a Minimal Residual Disease (MRD) Guided Transplant Decision

The FLT3 internal tandem duplication (FLT3-ITD) occurs in 15%-35% of all AMLs. FLT3-ITD-positive AMLs are associated with high relapse rates after reaching complete remission. Therefore these patients are considered for allogeneic stem cell transplantation (allo-SCT). Data shows that allo-SCT does not influence the overall survival (OS) of these patients. Minimal residual disease (MRD) monitoring in patients with acute myeloid leukemia (AML) can predict relapse clearly in advance and therefore allows early therapeutic intervention. Studies on MRD monitoring have shown a positive influence on OS in AML patients. Due to the high sequence variability of individual FLT3-ITD a universal PCR approach has a low sensitivity (approx. 1: 5x102) and therefore cannot be used for MRD monitoring.

We developed a novel cDNA-based, highly sensitive, quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) assay for the detection of the FLT3-ITD mutation level. On the basis of individual FLT3-ITD, mutation- specific forward or reverse primers were designed. The expression of FLT3-ITD was determined using complementary DNA samples at different points in time during diagnosis and subsequent treatment.

Retrospectively we studied 53FLT3-ITD-positive AML patients. 10 patients received palliative treatment, seven died during induction therapy; three patients are still in induction course of treatment. 32 patients achieved complete remission and 12 of them had undergone allo-SCT. For 47 patients we designed patient-specific qRT-PCR with mutation-specific forward and reverse primers. 41 (87%) assays were highly specific (1:104 - 1:106) and yielded similar results when compared to other high sensitive assays for molecular markers like NPM1 or PML-RARA.The median of follow-up time was 754 days (68-2546 days). MRD status was available for 23 patients after consolidation therapy. In 16 (81%) of these patients, FLT3-ITD negativity was demonstrated. MRD negativity predicted lasting remission independent of allo-SCT (N = 4) or non-allo-SCT (N=12). 3 patients relapsed after reaching MRD negativity. Only one patient relapsed without molecular relapse.

7 out of 32 patients stayed MRD positive after consolidation therapy. 5 of them underwent allo-SCT, nevertheless 3 of them stayed MRD positive (molecular non-responders) and finally relapsed. Furthermore we compared paired PB and BM samples at diagnosis and after induction therapy in 5 cases. The differences in FLT3-ITD expression were not statistically significant (p=0.8) which is in line with recent studies.

We conclude that highly sensitive detection of individual FLT3-ITD possesses equal prognostic power in AML like established molecular MRD markers. Using this approach MRD guided treatment decisions appear to be justified and should be incorporated in future studies.

Hallek:Janssen: Research Funding; Gilead: Research Funding; Roche: Research Funding. Kreuzer:Roche: Honoraria; Mundipharma: Honoraria.