LAUSR

Background: About 50% of MPNST arise in patients with neurofibromatosis type 1, and these tumors are among the most difficult types of soft tissue sarcoma to manage. NF1 is a tumor suppressor gene essential for negative regulation of RAS activity and is the most common genetic alteration in MPNST (86%). Despite a number of clinical trials, there has been little improvement in overall patient survival in patients with relapsed disease, and thus novel therapeutic approaches are needed. The concept of pharmacologic MEK inhibition has been applied to models of MPNST. However, the preclinical responses to single agent MEK inhibitor (MEKi) have been partial at best and suggest a need for a better understanding of the role of ERK and other effector pathways. A complex interplay of upstream signaling or parallel pathways may characterize NF1-driven tumorigenesis and inhibiting more than one RAS effector pathway may be necessary for complete anti-tumor effect. Methods: Using a panel of well-characterized MPNST cell lines, levels of NF1, select tumor suppressors and RTKs, and markers of activated ERK and PI3K/AKT signaling were detected by immunoblot. Isoform-specific RAS activity was evaluated using active-Ras immunoprecipitation assay. We determined the sensitivity of MPNST cells to MEKi and identified actionable alterations in signaling that underlie adaptive and acquired resistance by immunoblot analysis. We have developed and characterized novel in vitro models that are resistant to MEKi using proteomic, biochemical, and genetic approaches, in order to identify mechanisms of resistance, providing rationale for combination therapies. The anti-tumor efficacy of combination strategies was further examined in NF1-MPNST models, both in vitro and in vivo. Results: NF1-MPNST demonstrate variable responses to MEKi. Relief of negative feedback and signaling adaptation to MEKi results in compensatory activation of the PDGFRβ/PI3K/AKT axis. Combination of MEKi plus PI3K/mTORki effectively inhibits MPNST cell growth. Additionally, HGF/c-Met signaling is elevated in a MEKi-resistant model; recombinant HGF stimulation or inducible HGF overexpression confer resistance to MEK inhibition in parental cells originally sensitive to MEKi. In support of this observation, inducible shRNA-mediated HGF or MET knockdown restores the sensitivity of MEKi-resistant cells to MEKi; and combination MEKi+METi is active against MEKi resistance. Conclusion: Our ongoing work has begun to unravel the complicated networks which undergo adaptive modeling in response to MEKi. We have identified one mechanism of acquired resistance to MEKi, and found that combination strategies using MEKi+PI3K/mTORi or MEKi+METi may delay or prevent MEKi resistance, which may benefit patients with MPNST harboring NF1 genetic alterations. Citation Format: Jiawan Wang, Kai Pollard, Christine Pratilas. Adaptive and acquired signaling response to MEK inhibition in NF1-associated malignant peripheral nerve sheath tumor [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A121. doi:10.1158/1535-7163.TARG-19-A121