Researchers in Canada report preclinical evidence that type II RAF inhibition may overcome both innate and acquired resistance to MAPK inhibitors in anaplastic thyroid cancer, a disease where responses to current BRAF-targeted therapy are often short-lived. The study findings were published May 18, 2026 in Cell Reports Medicine. The work was carried out by teams at at Western University and London Health Sciences Centre in London, Ontario, Canada, working with collaborators at the Ontario Institute for Cancer Research.
Anaplastic thyroid cancer remains one of the most lethal human malignancies, with few effective systemic options and a median survival measured in months. The MAPK pathway, hyperactivated in the majority of ATC cases through mutations including BRAF V600E, has been a primary therapeutic target, but durable responses to existing inhibitors have remained elusive. Even patients with BRAF V600E-driven tumors frequently fail to sustain benefit from the type I RAF inhibitor dabrafenib, which is approved in combination with trametinib for BRAF V600E-mutant ATC. The study from Anthony C. Nichols and colleagues addresses a core question in the field: why do these tumors resist MAPK pathway inhibition, and what alternative pharmacological strategy might circumvent that resistance.
The research team used a multi-platform approach anchored in a single index patient whose BRAF V600E ATC was profiled across five spatially distinct pre-treatment tumor regions and one post-treatment sample. This multi-region whole-genome sequencing strategy was designed to capture intratumoral heterogeneity and clonal evolution under therapeutic pressure. Complementary technologies included whole-exome sequencing, single-nucleus RNA sequencing of 6,367 high-quality cells, reverse-phase protein arrays for phosphoproteomic profiling, and patient-derived xenograft models built from two separate BRAF V600E ATC patients.
EphA2 as a tissue-specific driver of RAF inhibitor resistance
A central mechanistic finding of the study is the identification of EphA2, a receptor tyrosine kinase, as a tissue-specific mediator of RAF inhibitor resistance in thyroid cancer. EphA2 phosphorylation at serine 897 was found to be substantially elevated in thyroid cancers relative to melanoma and colorectal cancer, two tumor types where type I RAF inhibitors have established clinical utility. Because EphA2 activates both the MAPK and PI3K-AKT signaling axes simultaneously, its elevated activity in thyroid tissue creates a dual bypass mechanism that allows tumor cells to maintain downstream signaling even when BRAF is pharmacologically inhibited.
RAF inhibitors that perform well in BRAF V600E melanoma show considerably less durable activity in thyroid cancer. The study's phosphoproteomic data indicated that naporafenib, a type II RAF inhibitor, reduced EphA2 serine 897 phosphorylation by approximately sevenfold, an effect not observed with dabrafenib. Because naporafenib targets the inactive DFG-out conformation of RAF, it inhibits both BRAF and CRAF and suppresses RAF dimer activity — the mechanism by which elevated RAS signaling, common in thyroid cancers, causes paradoxical MAPK reactivation with type I inhibitors.
Across a panel of nine thyroid cancer cell lines, naporafenib demonstrated lower IC50 values than dabrafenib in BRAF V600E lines and maintained activity in lines with elevated RAS signaling, where dabrafenib caused paradoxical pathway activation.
Naporafenib-trametinib efficacy in PDX models
In vivo validation was conducted in two patient-derived xenograft models. In the primary PDX derived from the index patient, naporafenib monotherapy produced statistically significant tumor growth inhibition relative to vehicle control. The combination of naporafenib and trametinib produced superior tumor regression compared to naporafenib alone, with near-complete regression observed in multiple animals. These results were replicated in a second PDX model derived from an independent BRAF V600E ATC patient, confirming that the findings were not specific to a single tumor. Treated animals tolerated the combination without significant weight loss or overt toxicity signals at efficacious doses.