– PDE1-IN-4 is a potent and selective inhibitor of PDE1 (phosphodiesterase-1), acting on PDE1C (IC50: 10 nM), PDE1A (IC50: 145 nM) and PDE1B (IC50: 354 nM). PDE1-IN-4 regulates cAMP (3′,5′-cyclic adenosine monophosphate) and cGMP (3′,5′-cyclic guanosine monophosphate) and exhibits antifibrotic effects. PDE1-IN-4 can inhibit TGF-β1-induced differentiation of human lung fibroblasts, and can be used to study idiopathic pulmonary fibrosis (IPF).