Preparation and characterization of self-assembled polymer-metal complex micelle from cis-dichlorodiammineplatinum(II) and poly(ethylene glycol)-poly(α,β-aspartic acid) block copolymer in an aqueous medium

Block copolymer micelles (polymer-metal complex micelles) containing platinum(II) complexes in the core were prepared through the complexation of cis-dichlorodiammineplatinum(II) (cisplatin, CDDP) with poly(ethylene glycol)-poly(alpha,beta-aspartic acid) block copolymer (PEG-P(Asp)) in an aqueous medium. Dynamic light scattering measurements revealed that CDDP-complexed micelles had diameters of approximately 20 nm with considerably narrow distribution. The critical substitution molar ratio of CDDP to Asp residues in PEG-P(Asp) (CDDP/Asp) to form a stable micelle structure was determined to be 0.5. The CDDP-complexed micelle, which was stable in distilled water at room temperature, started to dissociate in approximately 10 h of the induction period accompanying the sustained release of platinum(II) complex from the micelle in physiological saline (0.15 M NaCl solution) at 37 degrees C. The release rate of CDDP was inversely correlated with the chain length of P(Asp) segments in the block copolymer. This property of sustained CDDP release with modulated micelle dissociation leads to the potential utility of the CDDP-complexed micelle as a tumor-directed carrier system.