Clinical efficacy and IL-17 targeting mechanism of Indigo naturalis as a topical agent in moderate psoriasis

Background: Indigo naturalis is a Traditional Chinese Medicine (TCM) ingredient long-recognized as a therapy for several inflammatory conditions, including psoriasis. However, its mechanism is unknown due to lack of knowledge about the responsible chemical entity. We took a different approach to this challenge by investigating the molecular profile of Indigo naturalis treatment and impacted pathways. Methods: A randomized, double-blind, placebo-controlled clinical study was conducted using Indigo naturalis as topical monotherapy to treat moderate plaque psoriasis in a Chinese cohort (n = 24). Patients were treated with Indigo naturalis ointment (n = 16) or matched placebo (n = 8) twice daily for 8 weeks, with 1 week of follow-up. Results: At week 8, significant improvements in Psoriasis Area and Severity Index (PASI) scores from baseline were observed in Indigo naturalis-treated patients (56.3% had 75% improvement [ PASI 75] response) compared with placebo (0. 0%). A gene expression signature of moderate psoriasis was established from baseline skin biopsies, which included the up-regulation of the interleukin (IL)-17 pathway as a key component; Indigo naturalis treatment resulted in most of these signature genes returning toward normal, including down-regulation of the IL-17 pathway. Using an in vitro keratinocyte assay, an IL-17-inhibitory effect was observed for tryptanthrin, a component of Indigo naturalis. Conclusions: This study demonstrated the clinical efficacy of Indigo naturalis in moderate psoriasis, and exemplified a novel experimental medicine approach to understand TCM targeting mechanisms.