Dual Inhibition of Mitogen-Activated Protein Kinase Kinase and Mammalian Target of Rapamycin in Differentiated and Anaplastic Thyroid Cancer

Context: Differentiated thyroid cancer and anaplastic thyroid cancer tumors frequently have activation of the ras/raf/MAPK kinase (MEK)/ERK and phosphatidylinositol 3-kinase (PI-3K)/AKT/mammalian target of rapamycin (mTOR) signaling pathways. Objective: The objective of the study was to investigate the efficacy of MEK and mTOR inhibitors in preclinical thyroid cancer treatment models with defined mutation status. Experimental Design: The MEK inhibitor AZD6244 (ARRY-142886) and mTOR inhibitor rapamycin were tested separately and in combination in 10 differentiated thyroid cancer and anaplastic thyroid cancer cell lines and in a xenograft model for evidence of pathway inhibition, growth inhibition, apoptosis, and long-range adaptation and resistance. Results: Seven of 10 tested lines had evidence of significant basal activity of the PI-3K/AKT/mTOR pathway, with elevated phosphorylated AKT and phosphorylated p70 S6 kinase. Activation of ras/RAF/MEK/ERK was equally common in this panel. All 10 lines exhibited better than 60% growth inhibition with combined MEK and mTOR inhibition, including lines with BRAF, Ret-PTC, ras, and PTEN mutations. Rapamycin or AZD6244 alone achieved this threshold in six and two lines, respectively. Dual-pathway inhibition in the Ret-PTC mutant cell line TPC1 caused an intense G1 arrest in cell culture and reversible cytostatic inhibition in a xenograft model. We did not observe significant feedback up-regulation of AKT activation in either acute or prolonged exposures. Conclusion: These preclinical results support the inclusion of thyroid cancer patients in early-phase clinical trials combining ras/RAF/MEK/ERK and PI-3K/AKT/mTOR pathway inhibition. (J Clin Endocrinol Metab 94: 4107-4112, 2009)