The present invention provides a compound of formula U, in which R1, R2, X and R3 are defined in the Summary of the Invention, or a pharmaceutically acceptable salt thereof; a method for manufacturing the compounds of the invention, and its therapeutic uses. The present invention further provides a combination of pharmacologically active agents and a pharmaceutical composition.本發明提供式I化合物,其中R1、R2、X及R3定義於[發明內容]中,或其醫藥上可接受之鹽;製造本發明化合物之方法,及其治療用途。本發明進一步提供醫藥活性劑之組合及醫藥組合物。