Provided herein are compounds of the formula (I) as well as pharmaceutically acceptable salts thereof, wherein the substituents are as those disclosed in the specification. These compounds, and the pharmaceutical compositions containing them, are useful for the treatment of cancer. Additionally, provided is a method for treating cancer, comprising the step of administering a therapeutically effective amount of a compound according to formula (I), or a pharmaceutically acceptable salt thereof, and a pharmaceutically acceptable carrier to a patient in need thereof.
