Histone deacetylases inhibitors (HDACIs) and methods for treating cancer in a subject in need thereof are disclosed. The HDACIs comprise a compound of Formula X.or a pharmaceutically acceptable salt thereof. In one embodiment of the invention, R1 is ethyl； R2 is 2-phenylethyl； R3 is hydrogen； R4 is fluoro； R5 is (2E)-3-N-hydroxyamino-3-oxo-propenyl； and R6 is hydrogen, or a salt thereof. In another embodiment of the invention, R1 is ethyl: R2 is 2-phenylethyl； R3 is hydrogen； R4 is (2E)-3-N-hydroxyamino-3-oxo-propenyl； R5 is chloro or fluoro: and R6 is hydrogen.
