Abstract:

Background and purpose: 5-fluorouracil (5-FU) is the backbone of drug for gastric cancer. Traditional high-dose 5-FU often leads to serious adverse reactions and drug resistance. The low-dose 5-FU metronomic chemotherapy can significantly reduce the toxicity of the drug without affecting the efficacy, but it is not clear which dose can achieve the best anti-tumor effect. The purpose of this study was to explore the optimal metronomic chemotherapy strategy for 5-FU and to explore the related molecular mechanisms. Methods: A nude mouse subcutaneous tumor model of SGC-7901 gastric cancer cell line was established. After tumor formation, mice were randomly divided into 4 groups: maximum tolerated dose (MTD) group, daily metronomic chemotherapy (MET-qd) group, every other day metronomic chemotherapy (MET-qod) group and twice-weekly metronomic chemotherapy (MET-biw) group. Duration of single course of treatment was 21 d, and mice received a total of 2 courses of treatment. During the treatment period, the general condition of nude mice was observed, the tumor mass was weighed and measured every week, and the tumor growth curve was drawn. Flow cytometry was used to detect circulating endothelial progenitors (CEP) in peripheral blood of nude mice; B cells, natural killer (NK) cells, tumor-associated macrophage (TAM) and myeloid-derived suppressor cell (MDSC) infiltrated in the tumor and spleen. Immunohistochemical staining was used to detect the expressions of CD11c and CD163 in the tumor. Enzyme-linked immunosorbent assay (ELISA) was used to detect the expressions of vascular endothelial growth factor (VEGF), platelet derived growth factor (PDGF), interleukin (IL)-10 and IL-12 in peripheral blood of nude mice. In addition, white blood cell counts and H-E stained sections of liver, lung, kidney, and heart were performed in order to monitor toxicity assessments. Results: The three metronomic chemotherapy strategies of 5-FU could inhibit the growth of xenograft tumor in nude mice similar to the MTD group, and the MET-qod group has the most obvious anti-tumor effect (P<0.05). Compared with the MTD group (45.3%±4.3%), the three metronomic chemotherapy strategies of 5-FU could significantly reduce the number of CEP in the peripheral blood of nude mice, among which the MET-qd group had the most significant decrease (14.8%±3.8%). VEGF in peripheral blood decreased most significantly in MET-qod group (P<0.001), but significantly increased in MET-biw group (P<0.001). The expression of PDGF was consistent with the trend of VEGF. Compared with the MTD group, all three 5-FU metronomic chemotherapy strategies resulted in an increase in the ratio of M1 to M2 TAM infiltrated in the spleen and xenograft tumor of nude mice, and the ratio increased most significantly in the MET-qod group (spleen: 1.78 ±0.21 vs 1.19±0.07; tumor: 0.57±0.11 vs 0.14±0.09; P<0.001). The expression of IL-10, which represented M2 TAM in peripheral blood, was the lowest in MET-qod group (P<0.001), while the expression of IL-12, which represented M1 TAM, was relatively high (P<0.001). Compared with the metronomic chemotherapy group, the body weight of nude mice in MTD group decreased slightly, and the absolute values of peripheral blood leukocytes and platelets decreased significantly (P<0.001), however there was no statistically significant difference between the three metronomic treatment groups. The MTD group showed obvious thickening and chronic inflammatory changes in the pulmonary interstitium of nude mice, which were not found in the metronomic chemotherapy groups. Conclusion: The metronomic administration of 5-FU MET-qod can achieve the best antitumor effect without increasing toxicity. In addition to anti-angiogenesis, it also exerts its anti-tumor effect by regulating the polarization of TAM.

Key words: 5-fluorouracil, Metronomic chemotherapy, Gastric cancer, Tumor-associated macrophage, Anti-angiogenesis