Combination of c-SRC and FLT3 Inhibitors Has An Additive Inhibitory Effect on FLT3 ITD but Not on FLT3 TKD Positive Cells

Abstract 2892

Activating mutations of FLT3 are frequent in patients with AML. Two types of mutations are most common: Internal tandem duplications (ITD) of the juxtamembrane domain in approximately 30% of patients and point mutations within the second tyrosine kinase domain (TKD) in about 7% of AML patients. Patients carrying the FLT3-ITD mutation have a significantly worse prognosis whereas FLT3-TKD mutations do not appear to influence the clinical outcome. Studies have shown that mice receiving a transplant of bone marrow expressing FLT3 ITD develop a myeloproliferative disease. In contrast, mice which were transplanted with FLT3 TKD infected bone marrow, suffer from a lymphoid disease. Thus, both FLT3 mutations seem to exert different biological functions. Interestingly, FLT3-ITD but not FLT3-TKD or FLT3-WT leads to a strong activation of the STAT5 signaling pathway.

Recently we have shown that c-SRC is the crucial signaling mediator of FLT3 ITD to activate STAT5. Based on these findings we investigated the effect of FLT3 inhibitors (Midostaurin, Sorafenib and Sunitinib) in combination with c-SRC inhibitors (Dasatinib and PD166-326) on FLT3 ITD and FLT3 TKD murine and human cell lines as well as on primary patient material.

In FLT3 ITD expressing murine myeloid 32D cells c-SRC inhibitors in combination with FLT3 inhibitors showed clear additive effects on growth inhibition, apoptosis and activation of STAT5. In contrast, c-SRC inhibitors had no additional effects in FLT3 TKD expressing cells. Accordingly, a strong additive effect of c-SRC and FLT3 inhibitors could also be demonstrated in the FLT3 ITD positive human AML cell line MV4-11. Finally FLT3 ITD and FLT3 TKD positive primary human AML cells were investigated. We were able to detect a significant additional growth inhibition of FLT3 ITD positive human cells by combining c-SRC and FLT3 inhibitors. In contrast, no further growth inhibition was observed by c-SRC inhibition in primary AML cells expressing the FLT3 TKD mutation.

Together our results confirm c-SRC as a crucial signaling mediator in FLT3-ITD but not FLT3-TKD positive AML. The combination of FLT3 and c-SRC inhibitors warrants further investigation in FLT3 ITD positive AML.