Targeting of fibroblast growth factor receptor pathway as a new anti-tumor strategy for medullary thyroid cancer

Medullary thyroid cancer (MTC) is a tumor highly resistant to chemo- and radiotherapy. Vandetanib and cabozantinib, two tyrosine kinase inhibitors (TKIs) that selectively inhibit RET, VEGFR, EGFR or cMET, have been recently approved for the treatment of locally advanced or metastatic MTC. However, with time the MTC becomes resistant to these drugs. Since fibroblast growth factor receptor (FGFR) signaling regulates proliferation and angiogenesis and its aberrant activation has been described in thyroid tumors, our study is aimed at evaluating if its pharmacologic manipulation may represent a new strategy for the treatment of advanced MTC. In this context, we evaluated the potential anti-neoplastic effect of FGFR signaling inhibition by incubating two human MTC cell lines (TT and MZ-CRC-1) with SU5402, an inhibitor of this pathway. SU5402 significantly inhibited in vitro cell growth in a dose-dependent manner, evaluated by MTT proliferation assay, after 6 days of treatment in MZ-CRC-1 (IC50 = 2.6 μM, maximal inhibition = -80.66%) and in TT (IC50 = 3.6 μM, maximal inhibition = -96.44%) cells. Moreover, this compound significantly decreased the population of TT and MZCRC-1 cells in S phase, with a concomitant decrease of TT cells in Go/G1 and G2/M phase. In addition, Annexin V/propidium iodide staining and flow-cytometric analysis showed a potent stimulation of late and early apoptosis in both MTC cell lines after SU5402 treatment compared to untreated control. The antitumor activity of SU5402 was also tested in vivo on Tg(fli1a:EGFP)y1 zebrafish embryos, comparing its antiangiogenic effect with that of Vandetanib. In zebrafish embryos the effects of both compounds were evaluated on: 1) the formation of vascular structures that originate by angiogenesis during embryonic development, 2) MTC-induced angiogenesis through the xenograft of TT cells. Interestingly, the treatment with SU5402 showed a more potent inhibition on angiogenesis compared to Vandetanib. In conclusion, this study allowed us to collect preliminary evidences about the validity of the FGFR signaling inhibition as a new pharmacological strategy for the treatment of MTC.