Compounds of formula (I) and salts thereof: wherein R1, R2, R3and a are as defined herein. Compounds of formula (I) and salts thereof have been found to inhibit the binding of the BET family of bromodomain containing proteins to, for example, acetylated lysine residues and thus may have use in therapy, for example in the treatment of autoimmune and inflammatory diseases, such as rheumatoid arthritis and cancers.本發明係關於式(I)化合物及其鹽, 其中R1、R2、R3及a係如本文中所定義。已發現式(I)化合物及其鹽抑制含有溴結構域之蛋白質之BET家族與例如乙醯化離胺酸殘基之結合且因此可用於療法中,例如用於治療自體免疫及發炎性疾病,例如類風濕性關節炎;及癌症。