Targeting Mcl-1 Enhances The Activity Of Tyrosine Kinase Inhibitor Gilteritinib In Flt3 Mutated Aml

Anti-apoptotic Bcl-2 proteins play critical roles in AML cell and AML stem/progenitor cell survival and drug resistance, hence are relevant therapeutic targets. Indeed, the combination of the selective Bcl-2 inhibitor venetoclax (VEN) with a hypomethylating agent elicits CR/CRi rates of > 65%, is well tolerated by elderly AML patients, and obtained FDA approval. However, despite of the major improvement in response rates, survival extension was limited and most patients ultimately relapsed largely due to the development of resistant disease. Molecular analysis of treated patients revealed that primary and adaptive resistance to VEN-based combinations was frequently characterized by acquisition or enrichment of clones activating signaling pathways such as FLT3 or RAS (DiNardo CD et al., Blood 2020). FLT3 is one of the most frequently mutated gene in AML, resulting in constitutive activation of FLT3 tyrosine kinase and its downstream signaling pathways such as RAS/MAPK, which can be targeted by FLT3 tyrosine kinase inhibitors (TKIs). However, patients treated with TKIs ultimately relapse and adapt to TKI therapy by reactivating the MAPK signaling pathway (Bruner JK et al., Cancer Res 2017), which is known to stabilize Mcl-1 levels. Furthermore, deregulated Mcl-1 expression was identified as a novel mechanism of primary TKI resistance in a subset of FLT3-ITD mutated AML patients (Breitenbuecher F et al., Blood 2009). Importantly, Mcl-1 expression can be induced by VEN treatment and is a major resistance factor to VEN (Pan R et al., Cancer Discover 2014; Carter BZ et al., ASH 2018). Hence, Mcl-1 inhibition may enhance the efficacy of TKIs in FLT3 mutated AML, targeting AML cells and stem/progenitor cells.