Histone deacetylases inhibitors (HDACIs) and compositions comprising the same are disclosed. Methods of treating diseases and conditions wherein inhibition of HDAC provides a benefit are also disclosed. In one aspect, the invention relates to a compound having the structure or a pharmaceutically acceptable salt, a solvate or hydrate, a prodrug, or a metabolite 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, or a salt thereof.
