Examples of compounds of formula (I) wherein A2 is a residue of a specific drug molecule are 1-hydroxy-3-(1-pyrrolidinyl)-propylidene diphosphonic acid, or cycloheptylamino-methylene diphosphonic acid, or 1-hydroxy-2-imidazo-(1,2-a)-pyridin-3-yl-ethylidene diphosphonic acid or 1-hydroxy-2-(3-pyridinyl)-ethylidene diphosphonic acid or (4-chlorophenyl)thio-methylene diphosphonic acid or 1-hydroxy-2-