A compound useful for treating lymphoma, having the structure of formula (I): or a pharmaceutically acceptable salt thereof, wherein R1 is H or —C(═O)—O—R3, and R2 is H or —C(═O)—O—R4, provided R1 and R2 are not both H; and R3 and R4 are each independently selected from the group consisting of alkyl, alkenyl, alkynyl, aryl, arylalkyl, cycloalkyl, cycloheteroalkyl, and heteroalkyl. In an exemplary compound of formula (I), R1 is —C(═O)—O—CH2—CH3, and R2 is H.