Adaptive, non-genetic reactivation of oncogenic signaling is a major barrier to durable responses in targeted cancer therapy. In Neurofibromatosis type 1 (NF1), germline NF1 loss drives constitutive RAS activation and chronic dependence on RAF/MEK/ERK (MAPK) signaling. Plexiform neurofibromas (PNs) arise in approximately half of NF1 patients and may progress to malignant peripheral nerve sheath tumors (MPNSTs), the leading cause of NF1-related mortality. Although MEK inhibitors provide clinical benefit in inoperable PNs, therapeutic responses are limited by adaptive ERK reactivation and rapid tumor regrowth upon dose reduction or treatment interruption. MPNSTs are also largely refractory to MEK inhibition and have no approved targeted therapeutic option.

PN and MPNST cell models were treated with MEK inhibitors alone or combined with inhibitors of different nodes of the MAPK pathway, including the clinical-stage multi-selective RAS(ON) inhibitor RMC-6236 (or its preclinical analogue RMC-7977). MAPK pathway dynamics were assessed by ERK phosphorylation kinetics and downstream signaling outputs, including apoptotic markers. Therapeutic efficacy, tolerability, and response durability were evaluated in patient-derived MPNST orthoxenograft models.

Of all combination partners tested, RAS(ON) inhibitors proved to be the most synergistic with MEK inhibition. Single-agent MEK or RAS(ON) blockade produced incomplete and transient ERK suppression, followed by rebound pathway activation consistent with adaptive signaling rewiring. In contrast, the double blockade achieved sustained ERK inactivation and effectively prevented pathway reactivation while inducing caspase-3 cleavage in vitro. In vivo, the combination produced significant tumor control without overt systemic toxicity. Importantly, while monotherapy-treated tumors rapidly resumed growth following treatment discontinuation, combination-treated tumors exhibited markedly prolonged disease control.

Adaptive ERK reactivation represents a central non-genetic resistance mechanism in NF1-associated tumors. Vertical MAPK pathway blockade with combined RAS(ON) and MEK inhibition suppresses signaling plasticity and enables durable tumor control. These findings provide strong mechanistic and translational rationale for clinical evaluation of this strategy in NF1-driven peripheral nerve tumors.

This work was supported by funding from the Children Cancer Free Foundation (KiKa; project number 524)