Low‑dose 5‑fluorouracil (5‑FU), a widely used chemotherapeutic, has been reported to have immunomodulatory effects. This study aimed to evaluate the optimal dose of 5‑FU that produces antitumor and immunomodulatory effects. In a hepatoma 22 tumor‑bearing mouse model, 0, 10, 20 and 40 mg/kg 5‑FU (i.p.) was administered for 10 days. Tumor weight and volume were measured, thymus index (TI) and spleen index (SI) were calculated, and the number of white blood cells (WBCs) and lymphocytes (LYs) were counted following treatment. The percentages of CD3+, CD4+, CD8+ and natural killer (NK) cells were measured by flow cytometry. In addition, the body weights of the mice were measured and the average diet consumption was calculated. Administration of 5‑FU produced a potent antitumor effect in a dose‑dependent manner (P<0.01). At 20 and 40 mg/kg, a significant reduction of body weight and food consumption was observed. TI and SI decreased in the 20- and 40‑mg/kg groups (P<0.01) for 10 days. The number of WBCs significantly decreased in each group (P<0.01); however, the number of LYs only decreased in the 40‑mg/kg group (P<0.01). Percentages of CD3+ and CD4+ cells were increased in the 10- and 20‑mg/kg groups (P<0.01). Thus, 5‑FU at 10 mg/kg inhibits tumor growth while maintaining the immune function of the mice. 5‑FU may exert its antitumor effect at a low dose with low toxicity and stimulate the host immune system. Future clinical trials taking into account the immunostimulatory capacity of chemotherapeutic agents are desirable for certain patients.

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