Work at Boehringer Ingelheim Pharma GmbH & Co. KG has led to the synthesis of pyrazolo[5,1-f][1,2,4]triazin-4-ones acting as NADPH oxidase 4 (NOX4) inhibitors. As such, they are described as potentially useful for the treatment of liver disease, portal hypertension, viral infection, cancer, interstitial lung diseases, retinopathy, inflammation and fibrosis.