5-Fluorouracil (5-FU) has been a mainstay of chemotherapy for gastric cancer. Vandetanib is a tyrosine kinase inhibitor with inhibitory activity against vascular endothelial growth factor receptor and epidermal growth factor receptor (EGFR). We investigated the combination effect of vandetanib with 5-FU on gastric cancer cells.

Anticancer efficacy was assessed by 3-(4,5-dimethyl-2-tetrazolyl)-2,5-diphenyl-2H tetrazolium bromide assay of five gastric cancer cell lines, MKN1, MKN7, MKN45, MKN74, and TMK1. Signal expression was examined by western blot, and the cell-cycle distribution was assessed by flow cytometry. In vivo anticancer activity of vandetanib with/without 5-FU was tested in MKN74 cells on nude mice.

Vandetanib inhibited the growth of all cell lines. In MKN7 and MKN74 cells, the combination of 5-FU and vandetanib had synergistic effects, but effects were only additional against the other cell lines in vitro. Combination chemotherapy in vivo also significantly inhibited tumor growth compared to single use of each drug. Flow cytometry showed vandetanib increased the proportion in the G1 phase, and in MKN74, combination therapy increased the early S phase and caused bimodal peaks in the G1 phase. The level of expression of cyclin D1 was clearly strong in MKN7 and MKN74 in the natural state, and the expression of cyclin D1, E2 promoter binding factor 1 and thymidylate synthase (TYMS) was inhibited by vandetanib, but not in MKN1 cells. The synergistic effect disappeared in MKN7 and MKN74 cells in vitro when cyclin D1 was knocked-down by siRNA.

The synergistic effect of vandetanib with 5-FU is related to vandetanib-induced reduction of TYMS via down-regulation of cyclin D1. Hyperexpression of cyclin D1 might be a biomarker of the synergistic effect.