A series of compounds (including CCG-1423 and CCG-203971) discovered through an MRTF/SRF-dependent luciferase screen has shown remarkable efficacy in a variety of in vitro and in vivo models, including significant reduction of melanoma metastasis and bleomycin- induced fibrosis. Although these compounds are efficacious in these disease models, the molecular target is unknown. Here, we describe affinity isolation-based target identification efforts which yielded pirin, an iron-dependent cotranscription factor, as a target of this series of compounds. Using biophysical techniques including isothermal titration calorimetry and X-ray crystallography, we verify that pirin binds these compounds in vitro. We also show with genetic approaches that pirin modulates MRTF- dependent luciferase reporter activity. Finally, using both siRNA and a previously validated pirin inhibitor, we show a role for pirin in TGF-β- induced gene expression in primary dermal fibroblasts. A recently developed analog, CCG-257081, which co crystallizes with pirin, is also effective in the prevention of bleomycin-induced dermal fibrosis.

Identification of Pirin as a Molecular Target of the CCG-1423/CCG-203971 Series of Antifibrotic and Antimetastatic Compounds