Tranilast Treatment Prevents Chronic Radiation-Induced Colitis in Rats by Inhibiting Mast Cell Infiltration

Introduction: Mast cells are the principal cells involved in acute and chronic colitis due to radiation, known as radiationinduced colitis (RIC). In this study, we investigated whether pretreatment with tranilast, a mast cell inhibitor, could alleviate chronic RIC. Methods: A total of 23 Sprague-Dawley rats were randomly divided into three groups: control group (n = 5), radiation group (RG, n = 9), and tranilast-pretreated radiation group (TG, n = 9). The rats in the RG and the TG were irradiated in the pelvic area (1.5 cm from the anus) with a single dose of 20 Gy under general anesthesia. Tranilast (100 mg/kg) was administered intraperitoneally to the rats of the TG for 10 days, starting from the day of pelvic radiation. Ten weeks after radiation, the rats were euthanized. Rectal tissue samples were histologically evaluated for the total inflammation score (TIS) and mast cell count. The expression of MUC2, MUC5AC, and matrix metalloproteinase-9 (MMP-9) was also assessed immunohistochemically. Results: Both the TIS and specific components of TIS such as epithelial atypia, vascular sclerosis, and colitis cystica profunda (CCP) were significantly higher in the RG than in the TG (p = 0.02, 0.038, 0.025, and 0.01, respectively). Thein number of infiltrating mast cells was significantly higher in the RG than in the TG (median [range]: 20 [3-54] versus 6 [3-25], respectively; p = 0.034). Quantitatively, the number of MMP-9-positive cells was significantly higher in the RG (23.67 ± 19.00) than in the TG (10.25 ± 8.45) (mean ± standard deviation; p < 0.05). TIS andMMP-9 exhibited a strong association (correlation coefficient r = 0.56, p < 0.05). Immunohistochemically, the mucin-lake of CCP showed no staining for MUC5AC but was stained positive for MUC2. Conclusion: Tranilast pretreatment of chronic RIC showed an anti-inflammatory effect associated with the reduction of mast cell infiltration and MMP-9 expression.