Eve Fishinevich

Purpose:Malignant peripheral nerve sheath tumor (MPNST) is a highly aggressive subtype of soft-tissue sarcoma with a high propensity to metastasize and extremely limited treatment options. Loss of the RAS-GAP NF1 leads to sustained RAF/MEK/ERK signaling in MPNST. However, single-agent MEK inhibitors (MEKi) have failed to elicit a sustained inhibition of the MAPK signaling pathway in MPNST.Experimental Design:We used pharmacological, biochemical, and genetic perturbations of the receptor tyrosine kinase and MAPK signaling pathway regulators to investigate the mechanisms of MEKi resistance and evaluated combination therapeutic strategies in various preclinical MPNST models in vitro and in vivo.Results:Here, we report that MEKi treatment resistance in MPNST involves two adaptive pathways: direct transcriptional upregulation of the receptor tyrosine kinase PDGFRβ and MEKi-induced increase in RAF dimer formation and activation of downstream signaling. Although the pharmacologic combination of a MEKi with a PDGFRβ-specific inhibitor was more effective than treatment with the MEKi alone, the combination of the MEKi and RAF dimer inhibitors led to a robust inhibition of MAPK pathway signaling. This combination treatment was effective in vitro and in vivo, as demonstrated by the significant increase in drug synergism and its high effectiveness in decreasing MPNST viability.Conclusions:Our findings suggest that the combination of MEKis and PDGFR and/or RAF dimer inhibitors can overcome MEKi resistance and may serve as a novel targeted therapeutic strategy for patients with NF1-deficient MPNST, which in turn could impact future clinical investigations for this patient population.

Supplementary Figure S2. MPNST cell lines response to combination treatment of trametinib and PDGFRα/β inhibitors.

Supplementary Table S2. Small molecule inhibitors used.

Supplementary Table S1. Drugs, chemicals, and reagents used.

Supplementary Figure S1. MPNST cell lines response to MEKi treatment.

Supplementary Table S4. Primers used for RT-qPCR.

Supplementary Figure S6. Different MEKi synergize with RAFi in MPNST.

Supplementary Figure S5. Effect of BRAF and CRAF KO on MPNST cells sensitivity to ripretinib treatment.

Supplementary Figure S3. MPNST cell lines response to trametinib and ripretinib treatment.

Supplementary Table S5. sgRNAs used for CRISPR/Cas9 KOs.