Histamine modulates immunity by binding to histamine receptor 2 (H2R). Cimetidine, an H2R antagonist that inhibits gastric acid secretion and treats gastrointestinal ulcers, interferes with histamine-mediated immunomodulation and may have anticancer activity. This study examined cimetidine's effect on the anticancer effect of anti-PD-L1 in colon cancer. The MTT assay, colony formation assay, and DNA histograms assessed cell viability, clonogenicity, and cell cycle distribution, respectively. Flow cytometry measured H2R and PD-L1 expression and estimated specific immune cell lineages. For the in vivo study, tumor cells were subcutaneously implanted into the right flank of BALB/c mice. Cimetidine had no significant effect on CT26 cell viability, clonogenicity, or cell cycle distribution. It also did not affect H2R and PD-L1 expression levels in CT26 cells. In vivo, anti-PD-1 and anti-PD-L1 suppressed CT26 tumor growth, whereas cimetidine showed mild antitumor activity. In the combined experiment, cimetidine significantly attenuated anti-PD-1 and anti-PD-L1' antitumor effects without major toxicity. In the tumor microenvironment, anti-PD-L1 increased CD3+ T, CD4+ T, and CD8+ T cells and M1 macrophages. Combined treatment with cimetidine reversed this. Cimetidine also reversed anti-PD-1 and anti-PD-L1's decrease in circulating and tumor-associated neutrophils. Cimetidine attenuated anti-PD-L1's antitumor effect and modulated the tumor microenvironment in colon cancer.
Keywords: cimetidine; immunomodulation; tumor microenvironment.