Compounds of the formula (Ib) in which R1 and R2 are identical or different and represent bromine, trifluoromethyl, ethyl, cyclopropyl and in particular represent methyl or chlorine, R3 represents a group of the formula ???NH???C(O)???CH2???C(O)???R10, in which R10 represents hydroxyl or the radical ???C(O)???R10 has the meanings of R10 indicated above for a group, which in the sense of a prodrug