Intratumoral (i.t.) butyrate impairs the antitumor response to IR. (A) Established tumors were collected on day 10 after tumor inoculation from vancomycin-treated and untreated C57BL/6 mice after 1 mo of vancomycin feeding. Butyrate concentration in tumor tissues (n = 9/group). (B) Established tumors were collected on day 10 after tumor inoculation from control and TSD-26–fed mice after 1 mo of feeding. Butyrate concentration in tumor tissues (control group, n = 13; TSD-26 group, n = 8). (C) C57BL/6 mice (n = 5/group) were injected s.c. with 106 MC38 cells. Established tumors were irradiated (20 Gy) on day 10 after tumor inoculation. Tumor-bearing mice were intratumorally injected with 2 μmol of NaBu on days 1, 4, and 7 after IR. (D) C57BL/6 mice (n = 5/group) were fed vancomycin for 1 mo and then injected s.c. with 106 MC38 cells. Established tumors were irradiated (15 Gy) on day 10 after tumor inoculation. Tumor-bearing mice were intratumorally injected with 2 μmol of NaBu on days 1, 4, and 7 after IR. Two-way ANOVA tests were used to analyze the tumor growth data. *, P < 0.05; **, P < 0.01; ****, P < 0.0001. One representative experiment (out of two experiments in A, B, and D or three experiments in C) is shown.