Combination BRAF^V600E Inhibition with the Multitargeting Tyrosine Kinase Inhibitor Axitinib Shows Additive Anticancer Activity in BRAF^V600E-Mutant Anaplastic Thyroid Cancer

Background: Anaplastic thyroid cancer (ATC) is uniformly lethal. BRAF(V600E) mutation is present in 45% of patients with ATC. Targeted therapy with combined BRAF and MEK inhibition in BRAF(V600E)-mutant ATC can be effective, but acquired resistance is common because this combination targets the same pathway. Drug matrix screening, in BRAF(V600E) ATC cells, of highly active compounds in combination with BRAF inhibition showed multitargeting tyrosine kinase inhibitors (MTKIs) had the highest synergistic/additive activity. Thus, we hypothesized that the combination of BRAF(V600E) inhibition and an MTKI is more effective than a single drug or combined BRAF and MEK inhibition in BRAF(V600E)-mutant ATC. We evaluated the effect of BRAF(V600E) inhibitors in combination with the MTKI axitinib and its mechanism(s) of action. Methods: We evaluated the effects of BRAF(V600E) inhibitors and axitinib alone and in combination in in vitro and in vivo models of BRAF(V600E)-mutant and wild-type ATC. Results: The combination of axitinib and BRAF(V600E) inhibitors (dabrafenib and PLX4720) showed an additive effect on inhibiting cell proliferation based on the Chou-Talalay algorithm in BRAF(V600E)-mutant ATC cell lines. This combination also significantly inhibited cell invasion and migration (p<0.001) compared with the control. Dabrafenib and PLX4720 arrested ATC cells in the G0/G1 phase. Axitinib arrested ATC cells in the G2/M phase by decreasing phosphorylation of aurora kinase B (Thr232) and histone H3 (Ser10) proteins and by upregulating the c-JUN signaling pathway. The combination of BRAF inhibition and axitinib significantly inhibited tumor growth and was associated with improved survival in an orthotopic ATC model. Conclusions: The novel combination of axitinib and BRAF(V600E) inhibition enhanced anticancer activity in in vitro and in vivo models of BRAF(V600E)-mutant ATC. This combination may have clinical utility in BRAF(V600E)-mutant ATC that is refractory to current standard therapy, namely combined BRAF and MEK inhibition.