Fabrication Of Biodegradable Micelles Based On Amphiphilic Polymers As Drug Carriers

Chemotherapy plays an important role in the treatment of cancer because the chemotherapy drugs can effectively inhibit growth of various types of tumor cells. However, since anti-tumor drugs could not self-enrich to the tumor site due to lack of target specificity, leading to insufficient intratumor drug concentrations, which greatly reduces effect of treatment for tumor and increases its toxicity to normal tissues, a wide variety of Nano smart drug carriers based on stimuli-responsive polymers have aroused much attention. Not only Nano smart drug carriers were used to improve solubility of anti-tumor drugs in water, but also elongate the residence time in vivo. Meanwhile, anti-tumor drugs were delivered to tumor sites at high dose by passive tumor targeting through the high permeability and retention effect(EPR effect) or active tumor targeting. Then the drugs were released within the tumor cells by the disintegration of drug carriers when responded to a specific stimulation in tumor cells, so the purpose to improve therapeutic effect and reduce the side effects of drugs were achieved.Cathepsin B(CB), which is highly expressed in tumor cells, was choosed as a stimulus. The dipeptide segment valine-citrulline(Val-Cit, VC), which is stable in the plasma and could be specifically recognized and cut off by CB, was used as the connection segment. Two CB-sensitive amphiphilic polymers were synthesized by multi-step reaction.The size and morphology of micelles formed by amphiphilic polymers were characterized by dynamic light scattering(DLS) and transmission electron microscopy(TEM). Using pH 7.4 phosphate buffer solution(PBS) as a simulated plasma environment, the stability of polymeric micelles was studied. To evaluate the degradation behavior of the polymeric micelles, the change of their size with CB was observed by DLS. The in vitro release behavior of DOX-loaded micelles was studied under a simulated lysosomal environment. Finally, the cytotoxicity of polymeric micelles or DOX-loaded micelle was evaluated by MTT assay.The main findings of this paper are as follows:(1) Stearic-valine-citrulline-4,aminobenzylcarbonyl-poly(ethylene glycol) monomethyl ether, a biodegradable amphiphilic polymer was synthesized, using valine-citrulline as a coupling linker and 4,aminobenzylcarbonyl(PABC) as spacer and named as C18-VC-PABC-mPEG. The polymer C18-VC-PABC-mPEG was observed to self-assemble into spherical micelles in the aqueous environment with an average size of ca.179 nm and showed narrow size distribution, and then the micelles were used to encapsulate anti-tumor drugs doxorubicin(DOX). The average particle size of C18-VC-PABC-mPEG polymeric micelles in PBS was observed change little at 15 d, which indicated the micelles could realize long-term circulation in the plasma conditions. The decomposition behavior of C18-VC-PABC-mPEG micelles triggered by CB and drug release behavior of C18-VC-PABC-mPEG DOX-loaded micelles was investigated in a simulated condition of lysosomes. The average size of the C18-VC-PABC-mPEG micelles was increased from ca. 179 nm to ca. 280 nm at 7 d and the size continued to increase with the prolonging of culture time, which demonstrated the disintegration C18-VC-PABC-mPEG micelles. Meanwhile, the amount of released DOX and release rate of DOX-loaded C18-VC-PABC-mPEG micelles were higher than the micelles without CB. The in vitro cytotoxicity assay for the polymeric micelles, DOX-loaded micelles and free DOX against BxPC-3 cells was performed. The results indicated C18-VC-PABC-mPEG micelles showed almost no cytotoxicity and the cytotoxicity of the DOX-loaded micelles exhibited an essential decrease compared with the free DOX.(2) Another biodegradable amphiphilic polymer with similar and simple structure was synthesized. The polymer stearic-valine-citrulline-poly(ethylene glycol)(C18-VC-mPEG) tended to form spheroidal micelles in the aqueous media by the manner of self-assembly with an average size of ca.197 nm. In simulated plasma environment, the particle diameter of C18-VC-mPEG micelles did not change significantly in 7 days, implying good stability of the micelles. Similarly, the degradation of C18-VC-mPEG micelles was confirmed by the increased size in the presence of CB under a simulated lysosome environment. At the same time, the amount of released DOX and release rate of DOX-loaded C18-VC-mPEG micelles were higher than the micelles without CB. In addition, C18-VC-mPEG micelles showed almost no cytotoxicity to BxPC-3 cells and the DOX-loaded micelles exhibited lower cell cytotoxicity with respect to the free DOX. Hence, a couple of enzymatic degradation polymers have been synthesized and they were expected to be degraded within lysosomal followed by drug release, so they have the potential to use for intracellular drug delivery.