"4 is an alkyl group or an aryl group, and R15 is an alkyl group; and is a heteroaryl group having five ring atoms, including 1, 2 or 3 heteroatoms selected from O, S, and N; or a pharmaceutically acceptable salt or solvate thereof, or a pharmaceutically acceptable prodrug thereof, said prodrug being different from a compound of the formula (I); with the proviso that if the compound of formula (I)" . . . . .