Constructing doxorubicin-loaded polymeric micelles through amphiphilic graft polyphosphazenes containing ethyl tryptophan and PEG segments

By changing the molar ratio of hydrophilic and hydrophobic segments, a series of novel amphiphilic graft polyphosphazenes (PEG/EtTrp-PPPs) was synthesized via thermal ring-opening polymerization and a subsequent two-step substitution reaction of hydrophilic methoxyl polyethylene glycol (MPEG) and hydrophobic ethyl tryptophan (EtTrp). 1H-Nuclear magnetic resonance and Fourier transform infrared studies validated the expected synthesis of copolymers. The copolymer composition was also confirmed by UV–visible spectrophotometry. The molar ratio of the segment PEG to group EtTrp was 1.33:0.67, 1.01:0.99 and 0.78:1.22, respectively. Micellization behavior of PEG/EtTrp-PPPs in an aqueous phase was characterized by fluorescence technique, dynamic light scattering and transmission electron microscopy. The critical micelle concentration (CMC) of the graft copolymer in aqueous solution was 0.158, 0.033 and 0.020 g l−1, which decreased as the hydrophobic content in amphiphilic copolymers increased. Doxorubicin (DOX) was physically loaded into micelles prepared by an O/W emulsion method with a drug loading content increasing with DOX feeding. In vitro release of DOX from micelles can be accelerated in weak acidic solution. The results of cytotoxicity study using an MTT assay method with HeLa cell showed that amphiphilic graft polyphosphazenes were biocompatible while DOX-loaded micelles achieved comparable cytotoxicity with that of free DOX. In summary, these novel amphiphilic copolymers exhibited potential to be used as injectable drug carriers for tumor treatment.