PEG-epirubicin conjugates with high drug loading

PEG is a polymer extensively studied in drug delivery and some of its conjugates with proteins are already on the market. Studies are also focusing on PEG as polymeric carrier for low molecular weight drugs, but limitations arise from the possibility to link the drug at the level of the only one or two hydroxyl residues of the polymer. The synthesis of dendrimeric structure at the level of these groups may be a convenient solution to increase the polymer drug payload. The present report deals with the preparation of dendrimers, based on amino adipic acid or beta-glutamic acid as branching molecule, built on a PEG diol of MW 10,000 Da. The polycyclic large drug epirubicin was chosen as a model to investigate the influence of the branching moiety structure in avoiding the drug steric hindrance during the coupling reaction. Several derivatives with increasing number of drug molecules linked to each PEG chain were synthesized and their physical, chemical and biological properties were studied. The study demonstrated that the use of proper amino bicarboxylic acid (amino adipic acid or β-glutamic acid), as branching moiety for the dendrimer synthesis, allows the linking of hindered molecule as epirubicin to multibranched PEG. It was found that the most drug loaded conjugates dissolve in water only following a pre-dissolution in DMSO. This solubility problem could be solved by adding a hydrophilic peptide linker between drug and polymer. The conjugates, synthesized in good yield and purity, showed higher stability than free epirubicin in buffers at different pHs and in plasma and a much prolonged residence time in blood. Dynamic light scattering investigation showed the high tendency of these products to aggregate forming stable micelles.