at least one of R1 and R2 is a group that comprises the second hydrophobic drug, wherein if only one of R1 and R2 is the group that comprises the second hydrophobic drug, the other one of R1 and R2 is selected from the group consisting of hydrogen, a C1-10 alkyl group, a C6-20 aryl group, an ammonium group, an alkali metal, a polydentate ligand, a polydentate ligand precursor with protected oxygen