Poor Prognosis With Different Induction Rate Was Observed In Children With Acute Myeloid Leukemia and FLT3-ITD According To The ITD/WT Allelic Ratio: A Result From The Japanese Pediatric Leukemia/Lymphoma Study Group
Authors: Shimada Akira , Yamashita Yuka , Tomizawa Daisuke , Tawa Akio , Watanabe Tomoyuki , Yokozawa Toshiya , Kudo Kazuko , Taga Takashi , Iwamoto Shotaro , Terui Kiminori , Moritake Hiroshi , Kinoshita Akitoshi , Takahashi Hiroyuki , Nakayama Hideki , Koh Katsuyoshi , Goto Hiroaki , Kosaka Yoshiyuki , Saito Akiko Moriya , Fujimoto Junichiro , Horibe Keizo , Oki Kentaro , Hayashi Yasuhide , Adachi Souichi 
Published: October 2019

Abstract

Background
Acute myeloid leukemia harboring internal tandem duplication of fms-like tyrosine kinase 3 (AMLFLT3-ITD) is associated with poor prognosis, but the previous studies have reported that the inferior outcome is only confined to those with high allelic ratio (AR) of ITD/wild type (WT). In our previous AML99 study (2000-2002), AMLFLT3-ITD showed a poor outcome compared to the WT cases (5-year OS; 35% vs. 84%, P<0.0001). We, therefore, assigned all the patients with AMLFLT3-ITD to receive hematopoietic stem cell transplantation (HSCT) in first remission (1CR) in the JPLSG AML-05 study.

Patients & Methods
AML-05 study, registered at http://www.umin.ac.jp/ctr/ as UMIN000000511, is a Japanese nation-wide multi-institutional study for children (age<18 years) with de novo AML and enrolled 443 eligible patients from Nov. 2006 to Dec. 2010. Cases with acute promyelocytic leukemia or Down syndrome were excluded. FLT3-ITD was examined centrally for all the patients. After the 2 consecutive induction chemotherapies [(ECM: etoposide, Ara-C, and mitoxantrone) and (HCEI: HD Ara-C, etoposide, and idarubicin)], all the AMLFLT3-ITD patients were allocated to the high risk group and further received intensification therapy including HD Ara-C followed by HSCT in 1CR. All DNA samples were extracted from the first diagnostic bone marrow or peripheral blood and subjected to PCR and direct sequencing. AR of FLT3-ITD/WT was examined by GeneScan, and defined AR >0.4 as high and AR ≤ 0.4 as low as previously reported (Meshinchi S. Blood2006).

Results
We found 47 patients (10.6%) with AMLFLT3-ITD in this study (30 males, 17 females, and median age of 11 years at diagnosis). The median WBC count was 65,300/ml (3,690 - 522,050/mL). FAB classification included M1 (n=10), M2 (n=9), M4 (n=9), and M5 (n=11), and AML with normal karyotype was dominant (19/47, 40.4%).
Of the 29 patients (61.7%) who achieved CR, twenty-seven received HSCT in 1CR and 19 patients survived (19/27, 70.4%). On the other hand, 14/16 non-CR patients received HSCT, but only 4 survived. The only demographic difference between the 29 CR and 16 non-CR cases was the median WBC count at diagnosis (19,000 vs. 124,000/μL, P<0.001), and rapid clearance of bone marrow blasts after single induction course was observed in the CR group (median blast percentage dropped from 73% to 1.1% in the CR group, while that was 85% to 30.6% in the non-CR group). Finally, five-year OS, DFS and EFS for all 47 AMLFLT3-ITD patients were 41.3%, 58.4% and 36.1%, respectively.
AR was analyzed in 44 patients with median ratio of 0.68 (range, 0.11 to 4.47). Median AR was not different between CR vs. non-CR cases (0.53 vs. 0.72). There were no difference in 5-year OS (52.8% vs. 42.5%, P=0.302), DFS (54.5% vs. 64.5%, P=0.524), and EFS (50.0% vs. 34.4%, P=0.283) between patients with low (n=12) and high AR (n=32), however, induction rate was significantly higher in the low AR patients (91.7% vs. 53.1%, P=0.018).
It was rather surprising that all FLT3-ITDs were found only in JM domain and not in TKI domain in the current trial. In addition, six of 47 (12.8%) AMLFLT3-ITD patients had NPM1mutation simultaneously, and all received HSCT at 1CR and survived.

Discussion and Conclusion
We observed a different induction rate between AMLFLT3-ITD patients with low and high AR, but poor final outcomes in both. Regardless of the level of AR, patients with AMLFLT3-ITD, especially who fail to achieve remission, have dismal outcome and effective therapy combined with novel FLT3 inhibitor is urgently needed to overcome the disease.

Disclosures:
No relevant conflicts of interest to declare.