Acquired Fms-Like Tyrosine Kinase 3 Internal Tandem Duplication (FLT3 ITD) During Leukaemic Transformation From Underlying Myelodysplasia: Successful Rescue with Sorafenib
Authors: Gill Harinder , Man Cheuk Him , Lam Karen , Liang Raymond , Ma Edmond , Kwong Yok Lam , Leung Anskar Y. H. 
Published: October 2019

Abstract
Abstract 4927
Leukemic transformation from antecedent myelodysplastic syndromes (MDS) has a dismal prognosis and is often resistant to conventional intensive chemotherapy. The leukemic process may involve stepwise acquisition of genetic mutations and emergence of subclones with proliferative and survival advantages. Identifying molecular targets during leukemic progression may provide new treatment options for this group of patients. In this study, the mutational status of a gene encoding for FMS-like tyrosine kinase 3 (FLT3) at MDS and leukemic transformation as well as the clinical responseto FLT3 inhibitor sorafenib were evaluated.
Eighty patients with MDS according to the World Health Organization 2008 classification were prospectively evaluated. Patients who subsequently developed secondary acute myeloid leukemia (sAML) were identified. Bone marrow (BM) samples at the diagnosis of MDS and sAML were collected and genomic DNA (gDNA) was extracted. Internal tandem duplication (ITD) of FLT3 at the juxtamembrane domain and tyrosin kinase 1 domain spanning exons 14 and 15were identified by polymerase chain reaction (PCR). To increase the sensitivity so that small MDS clones with FLT3-ITD could be detectable, an ITD-specific PCR was designed in which the PCR products from FLT3-ITD+ sAML samples were cloned into pGEMT-Easy vector (Promega, USA), amplified, extracted and sequenced by Sanger sequencing. PCR primers targeting specific ITD sequence were designed. Patients with FLT3-ITD+ sAML were treated with Sorafenib at 400mg twice daily. Bone marrow aspiration and biopsy was performed around 3 weeks following treatment.
Between August 2009 to August 2012, 12 patients with sAML from antecedent MDS were identified of whom 5 were FLT3 ITD+ sAML. ITD-specific PCR performed on the respective MDS samples were negative in four and weakly positive in one patients. Four patients (all with refractory anaemia with excess blasts – 2 with normal karyotype at initial diagnosis) received sorafenib 400mg twice daily of whom two achieved complete remission (CR) and one CR with insufficient hematological recovery (CRi) three weeks after treatment. There was no overt dysplasia at morphological remission. Two of these patients respectively underwent subsequent allogeneic hematopoietic stem cell transplantation (HSCT) at 5 and 6 months after treatment (sorafenib was stopped before transplantation) and remained in remission for 10 and 12 months after initial CRi/CR. The third patient who was not eligible for HSCT received maintenance azacitidine concurrence with continuous sorafenib treatment and was 2 months in remission after CR. The fourth patient has just received sorafenib, awaiting bone marrow reassessment. Despite the small number of patients recruited, our observation showed that acquisition of FLT3 ITD is associated with leukemic progression in MDS and most importantly the use of FLT3 inhibitor effectively induced remission that can be further consolidated with allogeneic HSCT or demethylating agents. The effective clearance of myeloblasts with sorafenib also provided an opportunity whereby MDS clonal heterogeneity and evolution during leukemic transformation can be biologically evaluated.

Disclosures:
No relevant conflicts of interest to declare.