Synthesis, characterization, and tumor selectivity of a polyphosphazene-platinum(II) conjugate

A new amphiphilic poly(organophosphazene) was synthesized by stepwise nucleophilic substitutions with a hydrophilic methoxy poly(ethylene glycol) with an average molecular weight of 350 (MPEG350) and a hydrophobic glycyl-l-glutamate as side groups, and then an antitumor (dach)platinum(II) (dach: trans-(±)-1,2-diaminocyclohexane) moiety was conjugated to the polymer using the dipeptide as a spacer. This polymeric platinum conjugate was found to be accumulated in the tumor tissue to a remarkably greater extent than in the normal tissue (tumor/tissue ratio >4), probably due to the excellent EPR effect and the long circulating properties of the polymer conjugate (t1/2β = 6.2 h and AUC = 4020 nmol h/ml) compared with carboplatin (t1/2β = 0.42 h and AUC = 120 nmol h/ml). The polymer conjugate also exhibited high in vitro cytotoxicity comparable to cisplatin against several human tumor cells tested.