mTOR inhibitors sensitize thyroid cancer cells to cytotoxic effect of vemurafenib

Treatment options for advanced metastatic thyroid cancer patients are limited. Vemurafenib, a BRAFV600E inhibitor, has shown promise in clinical trials although cellular resistance occurs. Combination therapy that includes BRAFV600E inhibition and avoids resistance is a clinical need. We used an in vitro model to examine combination treatment with vemurafenib and mammalian target of rapamycin (mTOR) inhibitors, metformin and rapamycin. Cellular viability and apoptosis were analyzed in thyroid cell lines by trypan blue exclusion and TUNEL assays. Combination of vemurafenib and metformin decreased cell viability and increased apoptosis in both BCPAP papillary thyroid cancer cells and 8505c anaplastic thyroid cancer cells. This combination was also found to be active in vemurafenib-resistant BCPAP cells. Changes in expression of signaling molecules such as decreased mTOR expression in BCPAP and enhanced inhibition of phospho-MAPK in resistant BCPAP and 8505c were observed. The second combination of vemurafenib and rapamycin amplified cell death in BCPAP cells. We conclude that combination of BRAFV600E and mTOR inhibition forms the basis of a treatment regimen that should be further investigated in in vivo model systems. Metformin or rapamycin adjuvant treatment may provide clinical benefits with minimal side effects to BRAFV600E-positive advanced thyroid cancer patients treated with vemurafenib.