Pan-RAF inhibition induces apoptosis in acute myeloid leukemia cells and synergizes with BCL2 inhibition


Pan-RAF inhibitors have shown promise as antitumor agents in RAS and RAF mutated solid cancers. However, the efficacy of pan-RAF inhibitors in acute myeloid leukemia (AML) has not previously been explored. In AML, the RAS–RAF–MEK–ERK (MAPK) pathway is one of the most aberrantly activated oncogenic pathways, but previous targeting of this pathway by MEK inhibitors has not proven effective in clinical trials. Here we show that pan-RAF inhibition, but not MEK inhibition, induced cell death in 29% of AML samples while being nontoxic toward healthy bone marrow cells. Mechanistically, pan-RAF inhibition downregulated MCL1 protein synthesis and induced apoptosis in cells dependent on MCL1 for their survival. Furthermore, the combination of a pan-RAF and a BCL2 inhibitor overcame resistance to either compound alone in AML cell lines, as well as synergized and induced long-term responses ex vivo in AML patient samples relapsed or refractory to azacitidine + venetoclax treatment. Together, our results indicate that pan-RAF inhibition, alone or in combination with BCL2 inhibition, is a promising treatment strategy for AML.

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Fig. 1: Pan-RAF inhibition is cytotoxic to AML blasts ex vivo.
Fig. 2: Pan-RAF inhibition is more effective than MEK inhibition to induce apoptosis in AML cell lines.
Fig. 3: pan-RAF inhibition downregulates MCL1 protein expression.
Fig. 4: Pan-RAF inhibition downregulates MCL1 and induces death in cells dependent on MCL1 for their survival.
Fig. 5: Acquired resistance to MCL1 inhibition concomitantly confers resistance to pan-RAF inhibition.
Fig. 6: Pan-RAF inhibition downregulates MCL1 protein synthesis.
Fig. 7: Combined inhibition of pan-RAF and BCL2 overcomes resistance to either compound alone.
Fig. 8: Primary AML patient samples relapsed or refractory to azacitidine + venetoclax treatment are sensitive to combined inhibition of pan-RAF and BCL2.


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We thank, the patients and healthy donors for the samples, the technical staff for isolation of mononuclear cells from these samples, the FIMM High Throughput Biomedicine unit for drug screening of patient samples, and the FIMM Breeze team for data analysis. Aleksandr Ianevski for the analysis of drug synergy scores. Heikki Kuusanmäki for planning the experiments with patient samples clinically relapsed under azacitidine + venetoclax treatment. Professor Satu Mustjoki, Elina Parri, Heikki Kuusanmäki, and Amanda Ranta for help with the healthy peripheral blood mononuclear cell sample experiments. The project was supported by grants from Academy of Finland (KW; grant no. 277293), Novo Nordisk Foundation (KW Novo Nordisk Foundation Center for Stem Cell Biology, DanStem; grant no NNF17CC0027852), Cancer Foundation of Finland (KW), and Sigrid Juselius Foundation (KW). The Doctoral Programme in Biomedicine (DPBM) at the University of Helsinki (EK), University of Helsinki Early Career Grant (MK), and the Norwegian Cancer Society (BTG).

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MT, EK, and KW designed research; MT, EK, MV-K, and DB performed experiments and analyses. BTG, KP, MK, and CAH collected clinical samples and obtained ethical permits; KW supervised the study; MT, EK, and KW, wrote the manuscript; all the authors reviewed and approved the manuscript.

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Correspondence to Krister Wennerberg.

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KW has received research funding from Novartis unrelated to this study. CAH has received research funding from Celgene, Novartis, Oncopeptides, and Orion Pharma unrelated to this study. BTG has received honoraria from Pfizer, Astellas, Daiichi Sankyo, Seattle Genetics, and BerGenBio and owns shares in KinN Therapeutics AS and ACTII AS. MK has received research funding from AbbVie unrelated to this study.

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Tambe, M., Karjalainen, E., Vähä-Koskela, M. et al. Pan-RAF inhibition induces apoptosis in acute myeloid leukemia cells and synergizes with BCL2 inhibition. Leukemia (2020).

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