Preparation And Characterization Of PH/UV Responsive Polymeric Micelles For Tumor Extracellular Targeting

The main reason that it is technically difficult to design a nanosystem which can truly recognize tumor extracellular pH is that the extracellular pH（6.5⁷.2）of most tumors varies only slightly from those of the normal tissues and blood（pH 7.4）.As a result,we conceive that if a pH-sensitive system can be endowed with proton release function to enlarge the difference,maybe the enhanced tumor extracellular pH targeting can be achieved.Based on this idea,a pH sensitive/photoresponsive drug delivery system was constructed.The basic design is a drug-loaded micellar system which reaches tumor sites by EPR effect can release proton into its dispersion in respond to external light stimuli and trigger a rapid drug release.When the extracellular pH decreases to a certain value,the subsequently reaching micelles aggregate rapidly and again trigger a drug release.In this dissertation,the photoresponsiveness,pH sensitivity,photo-regulated aggregation behavior of the micelles and the release behavior of drug-loaded micelles were studied.The main results are as follows:Firstly,1-pyrenemethyl succinate（PMS）was prepared by the esterification of1-pyrenemethanol with succinic anhydride,which was grafted onto the carboxymethylchitosanbackbonetoobtainphotoresponsivepolymer（PMS-g-CMCS）.Their structures were determined by IR and ¹H NMR.PMS-g-CMCS could self-assemble into polymer micelles by ultrasonic treatment in deionized water.The minimum critical micelle concentration（CMC）of micelles determined by a spectrofluorophotometer using Nile red as a fluorescence probe was0.020 mg/mL.Transmission electron microscope（TEM）observation showed that the micelles had a core-shell structure with an average diameter of 210.2±10.7 nm.Secondly,the micelles showed good storage stability at pH 7.4 but aggregated at pH 6.0.DLS,TEM and fluorescence spectrophotometry studies showed that the micelles were disrupted in response to UV stimulus and accompanied by proton release.The photoresponsive mechanism of micelles may be attributed to the cleavage of the ester bond linked to the pyrenemethyl groups in the polymeric side chains,which was proved by UV-Vis and ¹H NMR.The proton release from micelles was correlated with the UV intensity,irradiation time and DS of PMS.The pH of micellar medium could be adjusted by changing the irradiation conditions.Further study of the photo-regulated aggregation behavior of the micelles showed that the micelles could accumulate in the irradiation for a certain period of time.Therefore we can regulate the aggregation behavior of the micelles by changing the irradiation conditions.Finally,in order to evaluate the potential of the PMS-g-CMCS micelles as a controlled release carrier,PTX was used as a model drug,which was encapsulated in PMS-g-CMCS micelles reaching encapsulation efficiency 23.2%.The drug-loaded micelles only showed an accumulative release rate of 3.35%within 12 h at pH 7.4.However,at pH 6.8（simulating tumor extracellular pH）with UV irradiation(365nm,500 mW cm^-2)for 4 min and subsequently by mixing with an aliquot sample without irradiation,a rapid drug release followed by a slower release was observed.These properties would be useful for selective accumulation of the micelles in tumor tissues by a synergistic effect with EPR and achieving a controlled-release of drug.