No abstract

Tapinarof (GSK2894512) is a naturally derived topical treatment with demonstrated efficacy for patients with psoriasis and atopic dermatitis, although the biologic target and mechanism of action had been unknown. We demonstrate that the anti-inflammatory properties of tapinarof are mediated through activation of the aryl hydrocarbon receptor (AhR). We show that tapinarof binds and activates AhR in multiple cell types, including cells of the target tissue-human skin. In addition, tapinarof moderates proinflammatory cytokine expression in stimulated peripheral blood CD4+ T cells and ex vivo human skin, and impacts barrier gene expression in primary human keratinocytes; both of these processes are likely to be downstream of AhR activation based on current evidence. That the anti-inflammatory properties of tapinarof derive from AhR agonism is conclusively demonstrated using the mouse model of imiquimod-induced psoriasiform skin lesions. Topical treatment of AhR-sufficient mice with tapinarof leads to compound-driven reductions in erythema, epidermal thickening, and tissue cytokine levels. In contrast, tapinarof has no impact on imiquimod-induced skin inflammation in AhR-deficient mice. In summary, these studies identify tapinarof as an AhR agonist and confirm that its efficacy is dependent on AhR.

Antagonizing the action of the human nuclear xenobiotic receptor pregnane X receptor (PXR) may have important clinical implications in preventing drug-drug interactions and improving therapeutic efficacy. We provide evidence that a naturally occurring phytoestrogen, coumestrol, is an antagonist of the nuclear receptor PXR (NR1I2). In transient transfection assays, coumestrol was able to suppress the agonist effects of SR12813 on human PXR activity. PXR activity was assessed and correlated with effects on the metabolism of the anesthetic tribromoethanol and on gene expression in primary human hepatocytes. We found that coumestrol was able to suppress the effects of PXR agonists on the expression of the known PXR target genes, CYP3A4 and CYP2B6, in primary human hepatocytes as well as inhibit metabolism of tribromoethanol in humanized PXR mice. Coumestrol at concentrations above 1.0 microm competed in scintillation proximity assays with a labeled PXR agonist for binding to the ligand-binding cavity. However, mammalian two-hybrid assays and transient transcription data using ligand-binding-cavity mutant forms of PXR show that coumestrol also antagonizes coregulator recruitment. This effect is likely by binding to a surface outside the ligand-binding pocket. Taken together, these data imply that there are antagonist binding site(s) for coumestrol on the surface of PXR. These studies provide the basis for development of novel small molecule inhibitors of PXR with the ultimate goal of clinical applications toward preventing drug-drug interactions.