The invention provides diketopiperazines of formula I. The invention also provides pharmaceutical compositions comprising the diketopiperazines, or pharmaceutically-acceptable salts or prodrugs thereof, as the active ingredient. The invention further provides therapeutic treatments that utilize the diketopiperazines of formula I, including inhibition of a proliferative disease or condition, inhibition of angiogenesis, treatment of an angiogenic disease or condition, treatment of cancer and precancerous conditions, treatment of a fibrotic disorder, treatment of a viral infection, treatment of an Akt-mediated disease or condition, inhibition of the production, release or both of matrix metalloproteinase-9, and inhibition of Akt activation.