Cobimetinib impairs tumor survival and immune escape by altering my-CAFs in BRAFV600E-mutated anaplastic thyroid carcinoma.

Anaplastic thyroid carcinoma is one of the most aggressive thyroid cancers characterized by rapid progression and high resistance to conventional therapies. Although mutations in the BRAF gene are common in thyroid cancer, targeted therapies against this mutation show limited clinical benefit. This study investigated whether inhibition of downstream signaling pathways using the selected MEK inhibitor cobimetinib provides advantages over direct BRAF inhibition. Genomic and expression analyses of thyroid cancer datasets confirmed frequent BRAF alterations and activation of MAPK-pathway-associated signaling. Functional studies using thyroid cancer cell lines demonstrated that cobimetinib more effectively reduced MAPK pathway activity specifically reducing ERK1/2 phosphorylation compared to direct BRAF inhibition. This pathway suppression was associated with decreased tumor cell invasion, migration, and expression of epithelial-mesenchymal transition markers, alongside increased induction of late apoptosis. Furthermore, cobimetinib altered immune-related features of tumor cells by reducing expression of immune inhibitory molecules and cytokine secretion, while simultaneously enhancing expression of molecules involved in antigen presentation, such as HLA-ABC. Co-culture experiments with human fibroblasts showed that cobimetinib modulates tumor-fibroblast communication and fibroblast phenotype composition. Cobimetinib reduced the formation of specific immune-suppressive fibroblast subtypes, whereas exposure to interleukin-8 promoted their development and increased tumor cell migration. Secretome profiling confirmed that cobimetinib markedly depleted pro-tumorigenic cytokines, particularly reducing IL-8 secretion. While data provide preclinical evidence that downstream MEK inhibition alters tumor cell signaling and IL-8-associated tumor-fibroblast communication in ATC-centered models, they do not establish cobimetinib monotherapy as a replacement for clinically established BRAF/MEK combination therapies. Instead, these data support the broader therapeutic potential of MEK inhibitors as a strategy to target both tumor progression and immune evasion in aggressive endocrine malignancies.