Indigo naturalis for inflammatory bowel disease: evidence from animal studies and molecular mechanisms

BACKGROUND: Indigo naturalis (IN) has been extensively used in prescriptions of traditional Chinese medicine to treat inflammatory bowel disease (IBD), particularly ulcerative colitis (UC). However, there is a lack of quantitative, evidence-based assessments from preclinical trials. AIMS: Quantitative statistical evidence regarding the efficacy of IN in animal models of IBD remains insufficient. This study performed a meta-analysis to evaluate the therapeutic effects of IN in experiments with IBD. METHODS: Relevant animal studies were identified from PubMed, Web of Science, Embase, China National Knowledge Infrastructure, SinoMed, and Wanfang databases. Two researchers independently conducted literature screening and risk-of-bias assessments using the CAMARADES 10-point quality checklist. Meta-analysis was performed using Review Manager 5.4, focusing on the histopathological index as the primary outcome measure. RESULTS: Of the 15 eligible studies included, over half had low risks of bias in more than five items. Compared to controls, the histopathological index significantly improved after IN treatment (n = 151/137; SMD = -2.69 [-3.36, -2.02]; p < 0.00001). Subgroup analysis showed that a high dose of IN (>600 mg/kg; 4 studies, n = 31/22; SMD = -3.55 [-5.72, -1.39]; p < 0.001) was most effective in reducing the histopathological index. The IN group showed a significantly lower final disease activity index (DAI) score (n = 121/89; WMD = -1.69 [-2.18, -1.20]; p < 0.00001), greater percentage body weight recovery (n = 77/63; WMD = 9.99 [6.50, 13.49]; p < 0.00001), and longer colon lengths (n = 65/51; WMD = 0.95 [0.67, 1.24]; p < 0.00001) compared to controls. Additionally, IN treatment reduced IL-1β, IL-6, IL-8, and TNF-α expression while increasing IL-10 levels. These findings suggest that IN ameliorates inflammation by balancing innate and adaptive immunity, modulating the AhR/CYP1A1 signaling pathway, and altering gut microbiota structure. CONCLUSION: IN demonstrated significant therapeutic efficacy in preclinical models of IBD, particularly at dosages exceeding 600 mg/kg. It protected colonic mucosal integrity and exerted beneficial effects through multiple molecular pathways.