Abstract: Pharmaceutically acceptable metal salts of compounds of formula (I): in which R1 is selected from the group consisting of (C1-C6)alkoxy(C1-C6)alkyl which may be substituted with (C1-C6)alkoxy, phenyl-(C1-C6)-alkyl and/or phenyloxy-(C1-C6)-alkyl group in which any phenyl group may be substituted with (C1-C6)alkyl, (C1-C6)alkoxy or halogen, and naphtyl-(C1-C6)-alkyl; R2 and R3 are each independently hydrogen or halogen; R4 is a biolabile ester forming group; and in which the salt is selected from the lithium salt and salts of bivalent metal ions such as magnesium, calcium and zinc.