New compounds represented by the formula or pharmaceutically acceptable salts or prodrugs thereof, wherein X1 and X2 are each independently selected from a CH group or a nitrogen atom; and R is an optionally substituted five or six membered nonaromatic carbocyclic ring or a nonaromatic or aromatic heterocyclic ring, whereby the ring is optionally condensed with a 6-membered, optionally substituted carbocyclic aromatic ring.