Histone deacetylases (HDACs) inhibitors

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, Ris ethyl; Ris 2-phenylethyl; Ris hydrogen; Ris fluoro; Ris (2E)-3-N-hydroxyamino-3-oxo-propenyl; and Ris hydrogen, or a salt thereof. In another embodiment of the invention, Ris ethyl; Ris 2-phenylethyl; Ris hydrogen; Ris (2E)-3-N-hydroxyamino-3-oxo-propenyl; Ris chloro or fluoro; and Ris hydrogen, or a salt thereof.