Background/Aims: Recent studies suggested a favorable effect of indigo naturalis (IN) in inducing remission for refractory ulcerative colitis (UC), however, the maintenance effect of IN for patients with UC remains unknown. Therefore, we conducted a prospective uncontrolled open-label study to analyze the efficacy and safety of IN for patients with UC. Methods: Patients with moderate to severe active UC (clinical activity index [CAI] ≥ 8) took 2 g/day of IN for 52 weeks. CAI at weeks 0, 4, 8, and 52 and Mayo endoscopic subscore (MES) and Geboes score (GS) at weeks 0, 4, and 52 were assessed. Clinical remission (CAI ≤ 4), mucosal healing (MES ≤ 1), and histological healing (GS ≤ 1) rates at each assessment were evaluated. Overall adverse events (AEs) during study period were also evaluated. The impact of IN on mucosal microbial composition was assessed using 16S ribosomal RNA gene sequences. Results: Thirty-three patients were enrolled. The rates of clinical remission at weeks 4, 8, and 52 were 67%, 76%, and 73%, respectively. The rates of mucosal healing at weeks 4 and 52 were 48% and 70%, respectively. AEs occurred in 17 patients (51.5%) during follow-up. Four patients (12.1%) showed severe AEs, among whom 3 manifested acute colitis. No significant alteration in the mucosal microbial composition was observed with IN treatment. Conclusions: One-year treatment of moderate to severe UC with IN was effective. IN might be a promising therapeutic option for maintaining remission in UC, although the relatively high rate of AEs should be considered.
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