The present invention relates to compounds formula (I): and to salts thereof, wherein R1-R4 have any of he values defined in the specification, and compositions and uses thereof. The compounds are useful as inhibitors of CDK8. Also included are pharmaceutically acceptable compositions comprising the compounds of the present invention and methods of using said compositions in the treatment of various disorders.本發明係關於式(I)化合物:及其鹽,其中R1至R4具有本說明書中所定義的任何值,以及其組合物及用途。該等化合物適用作CDK8抑制劑。亦包括包含本發明化合物之醫藥學上可接受之組合物及使用該組合物治療各種病症之方法。