PDE1-IN-8 (Compound 3f) is the inhibitor for PDE1 with an IC 50 of 11 nM. PDE1-IN-8 inhibits cAMP and cGMP signaling pathway, inhibits the differentiation and proliferation of cells into myofibroblast, and exhibits anti-fibrotic efficacy in Bleomycin -induced rat pulmonary fibrosis model.