Primulagenin A is a potent inverse agonist of the nuclear receptor RAR-related orphan receptor gamma (RORγ).

Throughout history, herbal medicines and natural products have played a crucial role as therapeutics for humans, yet their molecular mechanisms of action often remain elusive. Here, we investigate whether primulagenin A (PGA) from the traditionally used herbal substance Primula root acts via the nuclear receptor RORγ, a key regulator of pro-inflammatory Th17 cells, which are linked to autoimmune diseases like psoriasis. Full-length luciferase assays revealed a high potency (IC50 = 119 nmol/L) and efficacy (I max = 87%) of PGA as an inverse agonist of RORγ. To ensure sufficient supply, we established methods to isolate and synthesize PGA. Its binding to the human RORγ ligand binding domain was confirmed by nano differential scanning fluorimetry, and a structure-activity relationship was proposed by docking and site-directed mutagenesis. qPCR revealed PGA-mediated downregulation of RORγ target gene expression. Furthermore, PGA inhibited murine and human Th17 differentiation in a concentration-dependent manner and reduced the proportion of IL-17A-producing Th17 cells, as assessed by flow cytometry. In this work, we identify PGA as a new, potent, and efficacious inverse agonist of RORγ, with potential for modulating immune responses in inflammatory and autoimmune diseases.