Identification of FLT3-ITD as a biomarker for the treatment of AML by homoharringtonine

Abstract

Acute myeloid leukemia (AML) is a group of heterogeneous diseases characterized by an abnormal increase of myeloblasts in bone marrow (BM) and peripheral blood (PB). Conventional treatment with standard induction chemotherapy and allogeneic hematopoietic stem cell transplantation (HSCT) has reached an impasse with a cure rate of only 30%. A gain-of-function internal tandem duplication (ITD) of fms-like tyrosine kinase 3 (FLT3) was found in 30% of AML and was associated with an inferior treatment response and prognosis. Despite extensive clinical trials with FLT3 inhibitors, the response was at best transient, limiting their clinical application. Homoharringtonine (HHT) is a natural alkaloid that inhibits protein translation. It is primarily used in Mainland China for the treatment of AML. However, biomarkers predictive of treatment response to HHT are currently lacking. In this study, I have evaluated the anti-leukemia effects of HHT in FLT3-ITD^+AML and examined the mechanistic basis and potential synergism with FLT3 inhibitors.

The specific anti-leukemia effect of HHT in FLT3-ITD^+ AML was first demonstrated by its preferential effect in human AML cell lines carrying FLT3-ITD and murine Ba/F3 cell line transduced with human FLT3-ITD. Synergism between HHT and FLT3 inhibitors could be demonstrated in AML cell lines and primary AML samples carrying FLT3-ITD based on excess over bliss additivism (EOBA) and combination index (CI). Mechanistically, HHT suppressed expression of total and phosphorylated FLT3 protein and downstream signals of FLT3 including pSTAT5, pAKT and pERK protein. The effects were more prominent in HHT and sorafenib combination. There was no cross-resistance between HHT and sorafenib as sorafenib resistant cell line was sensitive to HHT and HHT resistant cell line was sensitive to sorafenib. In xenotransplantation model, in vivo treatment of HHT and sorafenib combination synergistically suppressed leukemic growth of FLT3-ITD^+ cell lines in NSG mice and prolonged their survival. Combination treatment was also effective in suppressing leukemic growth of primary AML xenografts. In conclusion, HHT exhibited preferential anti-leukemia effects in FLT3-ITD^+AML through down regulation of protein expression of FLT3 and its downstream signals. Significant synergism was observed between HHT and FLT3 inhibitors in vitro and in vivo. The study has provided the scientific basis for combination treatment in FLT3-ITD^+ AML patients.