Abstract 2434: The role of a more invasive phenotype in response to MAPK-directed therapies in thyroid cancer

Abstract Advanced papillary thyroid cancer (PTC) and anaplastic thyroid cancer (ATC) are the leading causes of endocrine cancer death. Mutations in the MAP kinase (MAPK) pathway are common in PTC and ATC, especially in BRAF, with a prevalence of 40-60%. Despite the importance of the MAPK pathway in thyroid cancer, therapies targeting this pathway are not approved for BRAF-mutant PTC patients. While the combination of BRAF and MEK inhibition is approved for patients with BRAF-mutant ATC, these patients often progress. An emerging mechanism of resistance to targeted therapies is an invasive phenotype switch in which cells transition from a proliferative, therapy sensitive population to an invasive, therapy resistant population. Here, we sought to determine whether increased invasion plays a role in resistance to BRAFi in BRAF-mutant PTC and ATC. In our panel of BRAF-mutant PTC and ATC cell lines with varying sensitivity/resistance to the BRAF inhibitor dabrafenib (BRAFi), we showed that cells resistant to BRAFi exhibit a 1.8 to 2.2 fold increase (p<0.04) in invasion while sensitive cells do not. Using Reverse Phase Protein Array, we identified a 2.0-fold increase in the extracellular matrix protein, fibronectin (FN1), in response to BRAFi treatment. We further identified a 1.6 to 3.2 fold increase (p<0.02) in FN1 secretion in resistant cell lines and found that conditioned media from BRAFi-treated resistant cells promotes invasion 3.8 to 5.7-fold (p<0.0048). Accordingly, treatment with FN1 phenocopies BRAFi-treatment by increasing invasion 1.9 to 2.1 fold (p<0.04), and depletion of FN1 blocks this increase in invasion. Resistant cells with depleted FN1 also fail to exhibit a BRAFi-induced increase in secreted FN1. MAPK pathway reactivation is a common mechanism of resistance to inhibitors of the MAPK pathway, which we have shown can be blocked by dual BRAF and ERK inhibition (Hicks, HM; McKenna, LR et al. Mol Carcinog. 60(3) 2021). ERK inhibition also mitigates the increase in invasion observed in response to single-agent BRAFi (p<0.0014) or FN1 (p<0.0271) in resistant cells. We further observed that dual inhibition of BRAF and ERK slows tumor growth in vivo in a BRAFi-resistant patient-derived xenograft model (p=0.02). These data indicate that thyroid cancer cells resistant to BRAF inhibition exhibit a more invasive phenotype characterized by an increase in FN1 and a pro-invasive secretome. Dual inhibition of BRAF and ERK ablates this BRAFi-driven increase in invasion and slows tumor growth in vivo, providing a potential therapeutic strategy for BRAF-mutant thyroid cancer patients. Citation Format: Hannah M. Hicks, Veronica L. Espinoza, Sharon B. Sams, Nikita Pozdeyev, Rebecca E. Schweppe. The role of a more invasive phenotype in response to MAPK-directed therapies in thyroid cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2434.