Combination treatment with BRAFV600E inhibitor, vemurafenib, and BH3 mimetic, navitoclax, for BRAF mutant thyroid carcinoma.

Background Vemurafenib is a selective BRAF inhibitor (BRAFi) that has shown promising activity in BRAFV600E-positive papillary thyroid cancer (PTC). However, adverse events and resistance to a single-agent BRAFi often require discontinuation of the targeted therapy in BRAFV600E-positive PTC. Thus, we investigated the expression of anti-apoptotic B-cell lymphoma 2 (BCL-2) family members, which is frequently overexpressed in many human cancers to inhibit apoptosis, in PTC harboring the BRAFV600E mutation after BRAFi treatment and then evaluated the cytotoxic effects of a homology 3 domain (BH3)-mimetic in combination with a BRAFi. Methods K1 cells (BRAFV600E-positive human PTC) were treated with various concentrations of vemurafenib to investigate the effect of the BRAFi. In addition, we analyzed the protein expression profiles of phosphorylated ERK1/2 (p-ERK 1/2) and anti-apoptotic BCL-2 family after vemurafenib treatment and selected the target anti-apoptotic protein. Antitumor effects were measured by cell counting kit-8 assay, and effects on apoptosis were determined by TUNEL assay and western blot analysis. Results Vemurafenib, at a concentration of 10 µM, inhibited the growth of K1 cells by 49.4%. Western blot analysis following exposure to 10 μM vemurafenib revealed that p-ERK1/2 gradually decreased over 24 h, but the expression of B-cell lymphoma-extralarge (BCL-XL) and BCL-2 increased after 12 h of treatment. Based on this result, the K1 cells were treated with navitoclax (BCL-2 /BCL-XL inhibitor) for 24 h up to a concentration of 4 µM, which resulted in negligible effects on cell survival. However, a combination treatment of 0.5 µM navitoclax with 1 µM vemurafenib resulted in significantly enhanced cell growth inhibition and increased apoptosis. Conclusions The results of the present study showed that vemurafenib increased the expression of anti-apoptotic proteins of the BCL-2 family. Thus, the combination of vemurafenib with navitoclax may be effective in BRAFV600E-positive PTC treatment.