Improved Posttransplant Outcomes in Recent Years for AML Patients with FLT3-ITD and Wild-type NPM1: A Report from the EBMT Acute Leukemia Working Party

Acute myeloid leukemia (AML) is a genetically heterogenous disease. FMS-like tyrosine kinase 3 (FLT3) is mutated in about 25% to 30% of newly diagnosed AML cases (1–3), either by internal tandem duplication (FLT3-ITD) occurring in or near the juxta-membrane domain of the receptor, or by a point mutation involving the tyrosine kinase domain (TKD) and resulting in single amino acid substitution within the activation loop (4–6). The presence of FLT3-ITD mutation is associated with early relapse and an overall poor prognosis (2–4, 6–8). In cytogenetically normal patients with AML with FLT3-ITD mutation, the presence of concomitant nucleophosmin-1 (NPM1) mutation was associated with a better outcome compared with patients with wild-type NPM1 (2, 3, 7–11).

The availability of active FLT3 inhibitors has improved the survival of FLT3-mutated patients with AML (12–15). For instance, the RATIFY trial demonstrated that midostaurin combined with standard induction chemotherapy significantly prolonged overall survival (OS) for AML with either FLT3-ITD or TKD mutations (16). Benefit from midostaurin was particularly visible in patients who received allogeneic hematopoietic cell transplantation (allo-HCT) in first complete remission (CR1). Promising data are also reported with other FLT3 inhibitors such as gilteritinib, crenolanib, and quizartinib (17, 18). Finally, because of its long-time availability, sorafenib has been tested, alone or in combination, in various settings in FLT3-ITD AML, such as first-line therapy or for the treatment of relapse, including after allo-HCT failure (19–25). As a result, in the 2022 European Leukemia Net (ELN) classification, patients with FLT3-ITD mutation are classified in the intermediate-risk category, regardless of NPM1 mutation (26).

Allo-HCT is recommended in CR1 in patients with AML harboring FLT3-ITD (27–29). However, long-term survival has been historically poor due to early relapse, particularly for patients with wild-type NPM1. Important progress has been made in recent years, including improvement of transplant techniques and the use of haploidentical donors in patients lacking a human leukocyte antigen (HLA) matched donor. Furthermore, the use of FLT3 inhibitors, particularly sorafenib, as posttransplant maintenance, has significantly reduced the risk of posttransplant relapse and has improved survival (12, 20, 27, 30–35). However, the overall impact of these therapeutic advances on the posttransplant outcomes of patients with AML with FLT3-ITD, and either mutated or wild-type NPM1, allografted in CR1, remains unknown.

In this report, we assessed real-world changes over time in transplant characteristics and posttransplant outcomes in young patients with AML with FLT3-ITD and either NPM1 mutated or wild-type, using a large dataset from the European Society for Blood and Marrow Transplantation (EBMT) registry.

This is a retrospective, registry-based, multicenter analysis. Data were provided and approved by the Acute Leukemia Working Party (ALWP) of the EBMT. The EBMT is a voluntary working group of more than 600 transplant centers which are required to report all consecutive HCTs and follow-ups once a year. Audits are routinely performed to determine the accuracy of the data. Since January 2003, all transplant centers have been required to obtain written informed consent prior to data registration with the EBMT, following the guidelines of the Declaration of Helsinki, 1975. Eligibility criteria for this analysis included age from 18 to 60 years, AML with FLT3-ITD and intermediate karyotype, first allo-HCT in CR1 between 2012 and June 2021, and reported NPM1 mutation status. Donor types included matched sibling donors (MSD), unrelated donors regardless of HLA mismatch, and haploidentical donors. Cord blood transplants were excluded. The stem cell source was bone marrow (BM) or G-CSF–mobilized peripheral blood (PB). Patients who received in vitro T-cell depletion (TCD) were excluded.

Variables collected included recipient age at transplant, recipient and donor gender, date of diagnosis, karyotype, and NPM1 status at diagnosis, de novo versus secondary AML, number of inductions to reach CR1, measurable residual disease (MRD) status at transplant, time from diagnosis to transplant, year of transplant, Karnofsky performance status (KPS) score, HCT comorbidity index (HCT-CI) at time of transplant, transplant-related factors including conditioning regimen, in vivo TCD, GVHD prophylaxis, donor type, stem cell source (BM or PB), and patient and donor cytomegalovirus (CMV) status.

Myeloablative conditioning (MAC) was defined as a regimen containing either total body irradiation (TBI) with a dose greater than 6 Gy, a total dose of oral busulfan (Bu) greater than 8 mg/kg, or a total dose of intravenous Bu greater than 6.4 mg/kg. All other regimens were defined as reduced intensity conditioning (RIC; ref. 36). The diagnosis and grading of acute (37) and chronic GVHD (38) were performed by transplant centers using standard criteria. Cytogenetic subgroups were classified according to the MRC classification (39).

Similar to what was previously described (20), endpoints included leukemia-free survival (LFS), OS, non-relapse mortality (NRM), relapse incidence (RI), acute and chronic GVHD, and GVHD- and relapse-free survival (GRFS). All outcomes were measured from the time of allo-HCT. LFS was defined as survival without leukemia relapse or progression; patients alive without leukemia relapse or progression were censored at the time of last contact. OS was defined as death from any cause. NRM was defined as death without previous leukemia relapse. GRFS was defined as survival without grade 3–4 acute GVHD, extensive chronic GVHD, relapse, or death. All events censored at 2 years post allo-HCT were to be considered for the difference in follow-up. The probabilities of OS and LFS were calculated by using the Kaplan–Meier method. Cumulative incidence functions were used to estimate RI and NRM in a competing risk setting. As planned in the synopsis of the study, all comparisons were stratified on NPM1 status. Univariate comparisons were performed using the log-rank test for LFS, OS, and GRFS, and the Gray test for cumulative incidences. A Cox proportional-hazards model was used for multivariate regression. The type-1 error rate was fixed at 0.05 for determination of factors associated with time-to-event outcomes. All analyses were performed using SPSS 27.0 (SPSS Inc., Chicago, Illinois) and R 4.1.1 (R Development Core Team, Vienna, Austria, URL:https://www.R-project.org/).

The data analyzed in this study were provided and approved by the ALWP of the EBMT. All relevant data are provided within the article and the appendix.The relevant working party of the EBMT will review requests from qualified external researchers for data from the EBMT studies in a responsible manner that includes protecting patient privacy, assurance of data security and integrity, and furthering scientific and medical innovation. Additional details on data sharing criteria and the process for requesting access should be sent to [email protected]. Individual patient data will not be shared.

We identified 1,827 adult patients with AML (54% female; median age 49 years, range 18–60) with FLT3-ITD, intermediate karyotype and available NPM1 mutation status, allografted between 2012 and 2021 in CR1 from a matched sibling (32%), unrelated (56%) or haploidentical donor (12%). NPM1 was mutated in 72% of patients. At transplant, 834 patients were MRD-positive, 464 were MRD-negative and 529 were not evaluated or MRD status was missing. HCT-CI was zero in 59% of patients with available data. Conditioning was MAC in 64% of patients; 87% received a PB stem cell harvest. A TCD graft was given to 62% of patients, posttransplant cyclophosphamide (PTCy) to 17%. Most patients (66%) and donors (54%) were CMV-positive. Median follow-up of alive patients was 30 months [interquartile range (IQR), 28–32]. Patient and transplant characteristics are summarized in Tables 1 and 2.

We compared changes in patient and transplant characteristics over time in 688 (38%) patients transplanted between 2012 and 2016, and 1,139 (62%) patients transplanted between 2017 and 2021. Patients transplanted more recently had a shorter follow-up, were more likely to have NPM1 mutation, to be MRD-positive, to have a higher HCT-CI, to receive a transplant from a haplo donor, a CMV-positive donor, and to receive PB and PTCy (Tables 1 and 2).

For patients with wild-type NPM1, acute GVHD grade II–IV and grade III–IV at day 100 were encountered in 29% and 11% of patients transplanted in 2012–2016, respectively, and 21% and 7% for patients transplanted in 2017–2021, respectively. The 2-year cumulative incidence of chronic and extensive chronic GVHD was 31% and 14%, respectively for 2012–2016, and 36% and 19%, respectively for 2017–2021. The 2-year RI was 34% and 28% respectively in the two time periods, and NRM was 12% and 8%, respectively (Fig. 1). Importantly, the 2-year LFS and OS improved over time from 54% to 64% and from 63% to 71%, respectively (Fig. 1), whereas GRFS improved from 39% to 46%. In the Cox multivariate analysis (MVA; Table 3), transplantation in recent years significantly reduced the RI (HR = 0.71; P = 0.048) and acute GVHD grade II–IV (HR = 0.6; P < 0.01) and significantly improved LFS (HR = 0.67; P = 0.011), OS (HR = 0.66; P = 0.021), and GRFS (HR = 0.77; P = 0.043). The use of unrelated donors increased the risk of acute GVHD whereas the use of haploidentical donors increased NRM compared with MSD. TCD reduced the risk of chronic GVHD whereas the female donor to male recipient combination increased this risk. A MAC regimen reduced the risk of relapse.

For patients with a FLT3-ITD mutation and NPM1 mutation, no significant changes over time in posttransplant outcomes were noted (Fig. 2). In MVA (Table 4), transplantation in recent years did not significantly affect any of the posttransplant outcomes. KPS < 90, recipient CMV-positive, and the use of unrelated or haploidentical donors increased NRM. OS was only affected by patient age whereas PTCy reduced the risk of chronic GVHD and improved GRFS. The use of unrelated donors increased the risk of acute GVHD whereas the use TCD reduced the risk of acute and chronic GVHD. A female donor to male recipient combination and the use of PB increased the risk of chronic GVHD.

This retrospective analysis of a homogenous cohort of 1,827 patients with AML, intermediate karyotype, and FLT3-ITD, allografted in CR1 analyzed trends in patients’ characteristics and outcomes over the last decade. Numbers of reported cases increased over the years, reflecting the general increase in numbers of allo-HCT and numbers of reporting centers. The most notable increase was in patients with MRD positivity before allo-HCT (from 121 in 2021–2016 to 343 patients in 2017–2021; Table 1).

Over time, we observed an improvement of posttransplant outcomes in patients with wild-type NPM1 but not for patients with NPM1 mutation. In the former group, the 2-year LFS increased from 54% to 64% (P < 0.02) and the 2-year OS increased from 63% to 71% (P < 0.05). These results were confirmed in MVA, which revealed a significant decrease over time in the RI and acute GVHD, translating to increased LFS, OS, and GRFS. Conversely, for patients with concomitant NPM1 mutation, the 2-year LFS remained stable over time at 64% for 2012–2016, and 65% for 2017–2021, with a similarly stable 2-year OS over time of 71% and 74%, respectively.

In 2015, the ALWP of the EBMT reported that in patients with AML with FLT3-ITD, the 2-year OS from allo-HCT was 66% in the presence of NPM1 mutation versus 54% in patients with wild-type NPM1 (P = 0.003; ref. 40). It is therefore remarkable to note here that in patients with AML with intermediate karyotype and FLT3-ITD, the prognostic value of concomitant NPM1 mutation on posttransplant outcomes is lost in recent years (2017–2021) with a 2-year LFS of 64% and 65% for patients with wild-type or mutated NPM1, respectively, and a 2-year OS of 71% and 74%, respectively. As such, our results are consistent with the ELN 2022 genetic risk classification that reclassified all patients with AML with intermediate karyotype and FLT3-ITD to the intermediate-risk group regardless of NPM1 mutation (26).

The improvement over time in posttransplant outcomes among patients with wild- type NPM1 can be explained by the combined effect of decreased RI (from 34% to 28%) and decreased NRM (from 12% to 8%) over time, although decreased RI but not NRM was significant in the MVA. The decreased NRM is likely explained by improvement in transplant techniques and supportive care. The decreased RI may be due to the additive value of FLT3 inhibitors before transplant as well as posttransplant maintenance (12, 16, 20, 30, 32–35, 41). Unfortunately, one of the main limitations of our study is the lack of available data on posttransplant maintenance therapy including FLT3 inhibitors and donor lymphocyte infusion.

In contrast, the lack of improvement in patients with mutated NPM1 may be due to a higher percentage of MRD-positive disease at transplant over time, as well as to the already excellent posttransplant outcomes for patients transplanted in 2012–2016. The addition of FLT3 inhibitors before and/or after allo-HCT have shown improvement in long-term outcomes (16, 33, 34). This beneficial effect of FLT3 inhibitors and allo-HCT is dependent on multiple factors including FLT3 diversity, cytogenetics, and co-occurring mutations, mainly NPM1 mutation (8, 42, 43). Despite the lack of data on the use of FLT3 inhibitors before and after transplant in our population, we can speculate that FLT3 inhibitors may have had a higher impact on patients with wild-type NPM1, who were historically at a higher risk of relapse, bringing them to the level of patients with mutated NPM1. Similarly, in a post hoc analysis of the RATIFY trial, most benefit from addition of midostaurin was shown in the NPM1 wild-type/FLT3-ITD high allelic ratio group (44). Thus, the outcomes of patients with adverse risk ELN who were treated with midostaurin in addition to chemotherapy followed by allo-HCT had similar outcomes as compared with patients with favorable ELN risk (44).

Furthermore, the use of sorafenib in the posttransplant setting resulted in an improvement in LFS in two prospective randomized trials (34, 45). Whether the improvement in long-term survival in our population was due to the addition of FLT3 inhibitors during induction chemotherapy in the pretransplant setting or was due to posttransplant maintenance is difficult to determine.

Some limitations of our study must be considered in this retrospective registry-based analysis, including the lack of information on MRD prior to allo-HCT for around one third of the patients. In addition, the percentage of patients who were MRD-positive patients was significantly higher than more recent studies, suggesting an apparent lack of benefit of FLT3 inhibitor use during induction on MRD. One potential explanation for that is a reporting bias (patients MRD-positive being reported in the registry more often than patients MRD-negative). Nevertheless, our results are comparable to those of a recently reported randomized trial that tested sorafenib as maintenance therapy for preventing relapse in FLT3-ITD AML undergoing allo-HCT (45). In this study, the 2-year incidence of relapse was 11.9% for patients who received posttransplant sorafenib and 31.6% for those who received placebo. Furthermore, we frequently missed information on molecular genetics other than NPM1 and FLT3-ITD mutations, also the FLT3-ITD allelic ratio at diagnosis that could have a differential impact on the role of NPM1 co-mutation. We are therefore unable to determine whether the FLT3-ITD allelic ratio had any predictive value for the beneficial effect of allo-HCT with time. As discussed above, the lack of detailed information on prophylactic posttransplant maintenance therapy is another limitation which unfortunately precluded defining precisely the role of different innovations in the observed improvement in outcome among patients with wild-type NPM1.

In summary, this study represents the largest analysis to date assessing trends over time and predictive factors for outcome of patients with AML with FLT3-ITD after allo-HCT. In these patients with wild-type NPM1, we noticed a significant decrease over time in the RI and a significant improvement in LFS, OS, and GRFS, likely reflecting the efficacy of FLT3 inhibitors in this high-risk setting. On the contrary, no significant change over time was noticed in posttransplant outcomes of patients harboring NPM1 mutation. These results can serve as a benchmark for future studies examining outcome in patients with AML with intermediate karyotype, FLT3-ITD and wild-type NPM1.

A. Bazarbachi reports grants and personal fees from Janssen, Pfizer, Roche, Novartis, and Biologix; and personal fees from Takeda, MSD, Amgen, and Pharmamed outside the submitted work. N. Kröger reports grants and personal fees from Neovii, Jazz Pharmaceuticals, Celgene/BMS, Riemser, and Novartis; and personal fees from Sanofi, Gilead/Kite, AOP Pharma, and Amgen during the conduct of the study and outside the submitted work. J. Versluis reports other support from ExCellThera and AbbVie outside the submitted work. G. Bug reports personal fees from BMS, grants from Jazz, and grants and personal fees from Novartis during the conduct of the study and grants and personal fees from Kite/Gilead, grants from Neovii, and personal fees from Affimed outside the submitted work. J. Esteve reports personal fees and nonfinancial support from AbbVie; personal fees from Astellas and BMS; grants and personal fees from Jazz Pharmaceuticals and Novartis; and grants from Pfizer outside the submitted work. M. Mohty reports personal fees from Jazz Pharmaceuticals, Amgen, Takeda, Pfizer, Adaptive, and Novartis and grants and personal fees from Janssen and Sanofi outside the submitted work. No disclosures were reported by the other authors.

A. Bazarbachi: Conceptualization, formal analysis, methodology, writing–original draft, project administration, writing–review and editing. M. Labopin: Data curation, formal analysis, investigation, methodology, writing–review and editing. T. Gedde-Dahl: Validation, investigation, writing–review and editing. P. Remenyi: Validation, investigation, writing–review and editing. E. Forcade: Validation, investigation, writing–review and editing. N. Kröger: Validation, investigation, writing–review and editing. G. Socié: Validation, investigation, writing–review and editing. C. Craddock: Validation, investigation, writing–review and editing. J.H. Bourhis: Validation, investigation, writing–review and editing. J. Versluis: Validation, investigation, writing–review and editing. I. Yakoub-Agha: Validation, investigation, writing–review and editing. U. Salmenniemi: Validation, investigation, writing–review and editing. J. El-Cheikh: Investigation, visualization, writing–review and editing. G. Bug: Validation, investigation, writing–review and editing. J. Esteve: Validation, investigation, writing–review and editing. A. Nagler: Validation, investigation, writing–review and editing. F. Ciceri: Supervision, validation, investigation, writing–review and editing. M. Mohty: Conceptualization, resources, supervision, validation, investigation, writing–review and editing.

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