Identification of Clinically Relevant Predictive MRD Checkpoints in AML Patients with NPM1 Mutations: A Study of the AML Study Group (AMLSG).
Authors: Krönke Jan , Schlenk Richard , Jensen Kai-Ole , Eiwen Karina , Habdank Marianne , Späth Daniela , Krauter Jürgen , Ganser Arnold , Göhring Gudrun , Schlegelberger Brigitte , Döhner Hartmut , Döhner Konstanze 
Published: October 2019

Abstract
Abstract 1586
Poster Board I-612

Background
Mutations in the nucleophosmin 1 (NPM1) gene represent the most frequent gene mutations in acute myeloid leukemia (AML), with highest frequency (50-60%) in cytogenetically normal (CN)-AML. Several studies have shown the applicability and prognostic value of an NPM1 mutation (NPM1mut)-based assay for detection of minimal residual disease (MRD). So far, there are no studies evaluating the prognostic value of NPM1mut MRD levels in a large controlled cohort of AML patients (pts) enrolled on prospective clinical trials.

Aims
To evaluate the prognostic value of NPM1mut MRD levels in younger (16 to 60 years) AML pts harbouring NPM1 mutations type A, B or D, and to assess the influence of concurrent FLT3 internal tandem duplications (ITD).

Methods
All pts were enrolled in the prospective AMLSG 07-04 and AML HD98A treatment trials. Treatment comprised double induction therapy with ICE (idarubicin, cytarabine, etoposide) followed by high-dose cytarabine-based consolidation, autologous or allogeneic stem cell transplantation. Levels of NPM1mut expression ratios, defined as NPM1mut copies per 104ABL copies, were determined by RQ-PCR using TaqMan technology. Dilution series showed a maximum sensitivity of 10-6 and high specificity as no wildtype NPM1 could be detected.

Results
A total of 1079 samples, [bone marrow (BM), n=1062; peripheral blood, n= 17) from 212 pts were analyzed at diagnosis, after each treatment cycle, during follow-up and at relapse (median number of samples per pt, n=4; range, 1-16). NPM1mut expression ratios at diagnosis varied between 1.1×104 and 10.4×106 (median, 6.9×105). Pretreatment transcript levels were not associated with clinical characteristics (e.g., age, white cell counts, BM blasts) and did not impact on relapse-free (RFS) and overall survival (OS). Following the first induction cycle, the median decrease of the MRD level ratio normalized to pretreatment levels was 4.21×10-3, independent of the presence of concurrent FLT3-ITD (p=0.39). After the 2nd induction cycle, the median reduction of MRD levels was significantly stronger in the FLT3-ITDneg group (6.75×10-5) compared with the FLT3-ITDpos group (4.19×10-4) (p=0.003) and this differential effect was observed throughout consolidation therapy.
For evaluation of the prognostic impact of NPM1mut MRD levels, we compared patients achieving PCR-negativity with those with positive values at different checkpoints. The first reliable checkpoint was after double-induction therapy: the cumulative incidence of relapse (CIR) at 4 years of PCR-negative patients (n=27) was 0% compared with 48% (SE, 4.4%, p<0.00001) for PCR-positive patients (n=105). This translated into a significant better OS (p=0.0005). The second checkpoint was after completion of consolidation therapy (first measurement during follow-up period). Again, 4-year CIR was significantly (p<.00001) lower in the PCR-negative group with 11% (SE, 6.5%) compared with 51% (SE, 4.8%) in PCR-positive patients, again translating in a significantly better OS (p<.00001). In addition, the level of NPM1mut expression ratio at any time point examined after completion of therapy correlated with the risk of relapse, since 20 of 22 pts with a value above 1000 NPM1mut/104ABL copies relapsed after a median interval of 90 days (range, 11-709 days). The remaining 2 pts had increasing levels at last follow-up but are still in continuous complete remission (CR). In a few cases relapse prediction appeared to be limited due to inadequate increase of NPM1mut expression levels or to loss of NPM1 mutation at the time of relapse (n=5). On the other hand, we observed a number of pts (n=17) in continuous CR who had intermittent low (<1000 NPM1mut/104ABL copies) NPM1mut expression ratios.

Conclusions
The levels of NPM1mut expression at two distinct checkpoints, after double induction therapy and after completion of consolidation therapy, can be used as a prognostic factor in NPM1mut AML pts. The adverse outcome of pts carrying a concurrent FLT3-ITD is reflected by a significant lower reduction of tumor burden.

Disclosures
Göhring: Celgene Corp.:. Schlegelberger:Celgene Corp.:.