Abstract 3709: Resistance mechanisms to targeted molecular therapy in thyroid cancer

Thyroid cancer is increasingly common in the United States. Although prognosis is generally very good, thyroid cancer has a high recurrence rate and there are limited treatment options available for advanced metastatic disease. Since genetic mutations causing over-activation of the mitogen-activated protein kinase (MAPK) signaling pathway are found in over 70% of papillary thyroid cancers, molecular inhibitors targeting these kinases are promising therapeutics. A multikinase inhibitor, Sorafenib, was recently FDA-approved for treatment of advanced thyroid cancer as it significantly increased progression-free survival time. To build upon this success, improvement in efficacy and reduction in common side effects may be possible through more targeted approaches. The most common specific mutation to target is BRAFV600E which occurs in 50% of papillary thyroid cancers and 25% of anaplastic thyroid cancers. BRAFV600E is correlated with advanced stage, lymph node metastasis, extrathyroidal extension, resistance to traditional therapy, and cancer-related mortality in papillary thyroid cancer. PLX4032 (Vemurafenib) is a specific kinase inhibitor developed to halt BRAFV600E-mediated excessive activation of the MAPK pathway. BRAFV600E-positive malignant melanoma patients treated with PLX4032 often experience astounding tumor regression followed by development of drug resistance, which is thought to be acquired through signaling via alternative molecules, bypassing the need for BRAFV600E kinase activity. We hypothesized that treatment with PLX4032 would inhibit MAPK signaling and promote an anti-proliferative phenotype in thyroid cancer cell lines harboring BRAFV600E, without having these effects on BRAF-wildtype cells. In BRAF-mutated cells, we also predicted that certain signaling molecules could become activated to circumvent the PLX4032-mediated inhibition, as seen in melanoma. Results demonstrate that PLX4032 inhibits proliferation, causes apoptosis, and arrests cell cycle selectively in BRAFV600E-positive thyroid cells. These phenotypic responses corresponded to diminished phosphorylation of central signaling proteins downstream of BRAF, including MEK, ERK, and mTOR. The mutated papillary thyroid cancer cells express both MAP3K8 (COT) and CRAF, kinases capable of growth-promoting signaling independent of BRAFV600E, in the face of PLX4032 treatment over time. These results indicate that targeted inhibition of BRAFV600E creates powerful anti-tumor effects in thyroid cancer cells, although mechanisms of resistance similar to those seen in melanoma patients may also occur. Detailed investigation into cell signaling pathways that contribute to thyroid cancer progression will allow for more effective combinational targeted molecular therapy and limit development of drug resistance to specific inhibitors such as PLX4032. Citation Format: Elyse K. Hanly, Neha Y. Tuli, Robert Suriano, Robert Bednarczyk, Zbigniew Darzynkiewicz, Augustine L. Moscatello, Edward J. Shin, Jan Geliebter, Raj K. Tiwari. Resistance mechanisms to targeted molecular therapy in thyroid cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3709. doi:10.1158/1538-7445.AM2014-3709