Preparation Of Tumor Microenvironment Responsive Iron-based T₁-weighted MRI Contrast Agents And Their Application In Tumor Imaging

Due to its high soft-tissue resolution,multiplanarity and no radiation,magnetic resonance imaging（MRI）is preferred among the common imaging modalities,and widely applied for early diagnosis of cancer.To overcome the inherently low sensitivity and contrast of MRI,contrast agents（CAs）have been developed to amplify the contrast between tumor and normal tissues,which can generally be classified into T₁ and T₂ CAs,providing bright and darkening signals,respectively.Compared with T₂ CAs,T₁ CAs are more favored because the bright images provided are good for observation and identification.Among various T₁ CAs,gadolinium（Gd（III））-based CAs are the most representative and the only available ones in the clinic,but they face the problem of biosafety originated from the gadolinium ion.To reduce the potential biotoxicity related to Gd-based CAs,ferric ion（Fe（III））,endogenous metal ion indispensable to the human body,is considered as an alternative to Gd（III）for its high paramagnetic and better biocompatibility.However,lack of tumor specificity makes iron-based T₁CAs far from the need for highly accurate diagnosis.For the purpose of improving tumor specificity,two kinds of tumor-microenvironment responsive iron-based T₁CAs were constructed in this thesis,which could enhance the MRI signal intensity of tumor specifically,through increasing longitudinal relaxation rate（r₁）by reactive oxygen species（ROS）oxidation,or raising the accumulation in tumor tissue via switching surface charge from anion to cation with the stimuli of the decreasing p H of tumor microenvironment,respectively.Taken together,both of the CAs could promote the specificity of tumor MRI and ameliorate the diagnostic precision.The main contents and results could be summarized as follows:1.Due to the difference of electron spin-resonance spectroscopy,Fe（III）has higher longitudinal relaxivity than ferrous ion（Fe（II））.Based on the difference on relaxivity and the high level of ROS in tumor microenvironment,ROS responsive CA（GA-Fe（II）-PEG-FA）was designed and constructed for tumor MRI.Using polyvinyl pyrrolidone as a protective agent to control the nanoparticle growth,Fe（II）was chelated with gallic acid（GA）via the strong force between Fe（II）and catechol.During the process,polyethylene glycol（PEG）containing folic acid（FA）molecules and 3,4-dihydroxyphenylacetic acid（DOPAC）,was added simultaneously,to improve water solubility and tumor targeting.With the guidance of FA,GA-Fe（II）-PEG-FA could accumulate in tumor tissue specifically,and be oxidized to Fe（III）by highly-concentrated ROS.As a result,the intensity of T₁ contrast signal in tumor was enhanced more than that of other normal tissue.The longitudinal relaxation time（T₁）of GA-Fe（II）-PEG-FA decreased as the reacting time with hydrogen peroxide（H₂O₂）prolonged.After exposure to 10 m M H₂O₂ for 2 h,the r₁ of GA-Fe（II）-PEG-FA at 0.5T increased to 2.20 m M^-1s^-1,which is 5 times larger than the r₁(0.42 m M^-1s^-1)before oxidation.Simultaneously,GA-Fe（II）-PEG-FA exhibited good biocompatibility and significant targeting specificity to tumor cells and tumor tissue.Besides,in vivo MRI studies demonstrated that the enhanced T₁ contrast effect of tumor could be achieved after injecting the contrast agent for 4 h,indicating GA-Fe（II）-PEG-FA has the potential as an ideal tumor MRI contrast agent to increase MRI contrast and improve the diagnostic precision.2.A pH responsive T₁ contrast agent（Fe（III）-Mel-PEOz）was fabricated through employing melanin nanoparticle（Mel）as a carrier to load Fe（III）,with the modification of p H-sensitive poly（2-ethyl-2-oxazoline）（PEOz）.The surface charge could switch from anion to cation when Fe（III）-Mel-PEOz was exposed to weakly acidic environment,which leads to more tumor cellular uptake and accumulation,resulting from electrostatic interaction between cation and negatively charged tumor cell membrane.However,the change of p H has negligible effect on the r₁ of Fe（III）-Mel-PEOz,which is always maintained at around 1.0 m M^-1s^-1 at 0.5 T.Fe（III）-Mel-PEOz exhibited low cytotoxicity,and satisfactory tumor specificity to tumor cells and tumor tissue.Moreover,in vivo MRI study verified that the amount of accumulation in tumor would reach the maximum at the time point 3 h after intravenously injecting Fe（III）-Mel-PEOz,and the tumor region showed strongly positive contrast effect.In conclusion,Fe（III）-Mel-PEOz could be applied for tumor MRI as a biocompatible T₁ contrast agent,with the capacity of specifically enhancing tumor MR signal via the decreasing p H of tumor environment.