The Dual <scp>PIM</scp>/<scp>FLT3</scp> Inhibitor <scp>MEN1703</scp> Combines Synergistically With Gilteritinib in <scp>FLT3</scp>‐<scp>ITD</scp>‐Mutant Acute Myeloid Leukaemia

MEN1703 is a first-in-class, oral, Type I dual PIM/FMS-like tyrosine kinase 3 inhibitor (FLT3i) investigated in a Phase I/II DIAMOND-01 trial in patients with acute myeloid leukaemia (AML). Gilteritinib is a highly potent and selective oral FLT3i approved for the treatment of relapsed/refractory AML with FLT3 mutations. Although gilteritinib showed strong single-agent activity in FLT3-mutated AML, the development of gilteritinib resistance limits response durability, indicating the importance of novel combination strategies to improve disease outcome. PIM kinases govern FLT3-ITD signalling and increased PIM kinase expression is found in samples from AML patients relapsing on FLT3i. Here, we report that the simultaneous inhibition of PIM and FLT3, through the combination of MEN1703 and gilteritinib, can consistently improve the in vitro/in vivo antitumor activity over the single agents, demonstrating the benefit of this combination. Moreover, we demonstrate that resistance to gilteritinib can be circumvented by combining MEN1703 with gilteritinib. MEN1703 interferes with FLT3 upregulation, Mcl-1 overexpression and PIM kinase signalling, which are all involved in FLT3i resistance. We also show that MEN1703 downregulates stromal cytokines that promote cytokine-mediated resistance of AML blast cells to FLT3 inhibition. These results demonstrate the importance of the combination approach to overcome microenvironment-mediated resistance to FLT3 inhibitors.