Macrophages as Drivers of Resistance to Dabrafenib Plus Trametinib Therapy in Anaplastic Thyroid Cancer.

Anaplastic thyroid carcinoma (ATC) is an aggressive malignancy with poor response to standard therapies. While Dabrafenib plus Trametinib (DT) combination therapy has improved the survival of patients with BRAF-mutant ATC, resistance remains a challenge. Given the association of macrophages with poor prognosis, and that SPRY4 has emerged as a mediator in ATC-macrophage interactions, we investigated their role in DT resistance and evaluated the therapeutic potential of macrophage targeting. Transwell co-cultures of BRAF-mutant ATC cell lines (T235 and T238) with THP-1-derived macrophages were established, and viability, invasion, and macrophage phenotype were assessed. Macrophages were also characterised in tumour samples from eleven ATC patients treated with DT. In vitro, DT significantly reduced ATC cell viability and invasion. However, macrophage co-culture significantly restored these effects, also promoting cytoskeletal remodelling and increased vimentin expression, despite DT treatment. DT significantly downregulated SPRY4 and suppressed MAPK signalling and PD-L1. These effects were also significantly reversed by macrophages. Under DT, macrophages showed an increase in M2-like polarisation in co-culture. Patients' tumour samples were highly infiltrated with macrophages. In vitro targeting of macrophages with Edicotinib, in combination with DT, significantly enhanced the anti-tumour effects of DT and decreased the M2-like polarisation, bypassing the macrophage-mediated DT resistance. Overall, the in vitro findings suggest that macrophages modulate ATC response to DT by enhancing viability, invasion, MAPK signalling and PD-L1, supporting a pro-tumoural phenotype. Targeting macrophages overcomes this resistance, highlighting the CSF1/CSF1R signalling pathway within the ATC-macrophage axis as a promising therapeutic target to improve DT efficacy.