The present invention provides compounds of formula (I) and pharmaceutically acceptable salts thereof, wherein R1 and R2 are as defined in the specification, processes for their preparation, pharmaceutical compositions containing them and their use in therapy.本發明提供式(I)的化合物及其之藥學上可接受的鹽,其中R1和R2係如說明書內所定義的,用於其等之製備的方法,包含其等之藥學組成物以及其等於療法中的用途。