Inhibitory Effects of Vandetanib on Catecholamine Synthesis in Rat Pheochromocytoma PC12 Cells

Gain-of-function gene alterations in rearranged during transfection (RET), a receptor tyrosine kinase, are observed in both sporadic and hereditary medullary thyroid cancers (MTCs) and pheochromocytomas and paragangliomas (PPGLs). Several tyrosine kinase inhibitors (TKIs) that target RET have been proven to be effective on MTCs and PCCs. Recently, TKIs, namely, sunitinib and selpercatinib, which were clinically used to target PPGLs, have been reported to decrease catecholamine levels without reducing tumor size. Our clinical case of metastatic medullary thyroid cancer, which is associated with RET mutations undergoing treatment with vandetanib, also suggests that vandetanib can decrease catecholamine levels. Therefore, we investigated the effect of vandetanib, a representative multi-targeted TKI for RET-related MTC, on cell proliferation and catecholamine synthesis in rat pheochromocytoma PC12 cells. Vandetanib reduced viable cells in a concentration-dependent manner. The dopamine and noradrenaline levels of the cell lysate were reduced in a concentration-dependent manner. They also decreased more prominently at lower concentrations of vandetanib compared to the inhibition of cell proliferation. The RNA knockdown study of Ret revealed that this inhibitory effect on catecholamine synthesis is mainly mediated by the suppression of RET signaling. Next, we focused on two signaling pathways downstream of RET, namely, ERK and AKT signaling. Treatment with vandetanib reduced both ERK and AKT phosphorylation in PC12 cells. Moreover, both an MEK inhibitor U0126 and a PI3K/AKT inhibitor LY294002 suppressed catecholamine synthesis without decreasing viable cells. This study in rat pheochromocytoma PC12 cells reveals the direct inhibitory effects of vandetanib on catecholamine synthesis via the suppression of RET-ERK and RET-AKT signaling.