MEK inhibitors lead to PDGFR pathway upregulation and sensitize tumors to RAF dimer inhibitors in NF1-deficient malignant peripheral nerve sheath tumor (MPNST)

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 employed pharmacological, biochemical, and genetic perturbations of the receptor tyrosine kinase (RTK) 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 (RTK) PDGFRβ, and MEKi-induced increase in RAF dimer formation and activation of downstream signaling. While the pharmacological combination of MEKi with a PDGFRβ specific inhibitor was more effective than treatment with MEKi alone, the combination of MEKi and RAF-dimer inhibitors led to a robust inhibition of the 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 MEKi and PDGFR and/or RAF dimer inhibitors can overcome MEKi resistance and may serve as a novel targeted therapeutic strategy for NF1-deficient MPNST patients, which in turn could impact future clinical investigations for this patient population.