Cytostatic activity of adenosine triphosphate-competitive kinase inhibitors in BRAF mutant thyroid carcinoma cells.

The V600E mutation accounts for the vast majority of thyroid carcinoma-associated BRAF mutations. The aim was to study the effects of the two BRAF V600E ATP-competitive kinase inhibitors, PLX4032 and PLX4720, in thyroid carcinoma cell lines. We examined the activity of PLX4032 and PLX4720 in thyroid carcinoma cell lines harboring BRAF V600E (8505C, BCPAP, SW1736, BHT101), NRAS Q61R (HTH7), KRAS G12R (CAL62), HRAS G13R (C643), or RET/PTC1 (TPC-1) oncogenes. Normal thyrocytes (PC Cl 3) were used as control. Both compounds inhibited the proliferation of BRAF mutant cell lines, but not normal thyrocytes, with a half maximal effective concentration (EC(50)) ranging from 78-113 nm for PLX4720 and from 29-97 nm for PLX4032. Doses equal to or higher than 500 nm were required to achieve a similar effect in BRAF wild-type cancer cells. Phosphorylation of ERK 1/2 and MAPK kinase (MEK) 1/2 decreased upon PLX4032 and PLX4720 treatment in BRAF mutant thyroid carcinoma cells but not in normal thyroid cells or in cell lines harboring mutations of RAS or RET/PTC1 rearrangements. PLX4032 and PLX4720 treatment induced a G(1) block and altered expression of genes involved in the control of G(1)-S cell-cycle transition. 8505C cell tumor xenografts were smaller in nude mice treated with PLX4032 than in control mice. This inhibition was associated with reduction of phospho-ERK and phospho-MEK levels. This study provides additional evidence of the promising nature of mutant BRAF as a molecular target for thyroid carcinoma cells.