Magnetic Resonance Imaging Probes Based On Iron Oxide Nanoparticles

Magnetic resonance imaging(MRI)is widely used as a non-invasive medical imaging technique with high spatial resolution,clear tissue anatomy and functional information.However,the sensitivity of MRI is relatively low,which greatly limits its medical imaging applications.Therefore,it’s urgent to develop MRI contrast agents with high sensitivity and high specificity for clinical and translational medicine applications.The MRI contrast agent,superparamagnetic iron oxide(SPIO)nanoparticles show high sensitivity,good biocompatibility and can be modified easily.Thus,this study designed and synthesized a series contrast agents based on SPIO with good biocompatibility and ultra-high sensitivity to explore the clinical problems.This study explored two types of applications with SPIO nanoparticle-based MRI probe.Firstly,we explored the feasibility of early tracing based on low molecular weight polyethyleneimine(PEI)amphiphilic polymer encapsulated SPIO nanoparticle with mouse model of systemic Candida infection,which may provide a valuable reference for the potential application.Besides,aggregation-inducing emission(AIE)molecule tetraphenylene(TPE)were enrolled to form the MR/fluorescence dual-mode probe,thereby integrating their advantages.Secondly,to meet the clinical needs,we developed MRI probes for soft tissue imaging.As there is no available MRI contract agent for lymph node imaging in clinic,we designed and synthesized a series of dextran encapsulated SPIO nanoparticles,and tested them on lymph node imaging of mouse,which may contribute to revealing the biology of lymph node.In addition,in order to obtain more accurate diagnostic information and to reduce the frequency of probe injection,we constructed a SPIO-Gadolinium(Gd)T1-T2 dual-mode probe,which realized the dual-mode multi-organ imaging with time sequence in cardiovascular and liver tissues of rats.As one of the non-viral gene carriers,low molecular weight PEI can label cell efficiently.Therefore,this thesis explored the Candida albicans tracing based on the PEI/SPIO contrast agent platform.Candida albicans infection has become a major public health problem,and invasive Candida infections shown high prevalence and mortality.Tracing the early invasion of Candida albicans by MRI may provide new insights for its treatment.In this thesis,an amphiphilic polymer Alkyl-PEI-LAC with alkyl chain as the hydrophobic core and lactobionic acid(LAC)-modified PEI as the hydrophilic shell was synthesized to load SPIOs,and then Candida albicans were labelled and traced in BALB/c mouse.The size distribution of Alkyl-PEI-LAC/SPIO nanoparticle was 65 nm with uniform spherical structure.Besides,the zeta potential of Alkyl-PEI-LAC/SPIO nanoparticle was negative.At 3.0 T MR scanner,the T2 relaxivity of Alkyl-PEI-LAC/SPIO nanoparticle was 499.8 Fe mM-1s-1,which is higher than the commercial contrast agent Feridex(120 mM-1s-1).The Alkyl-PEI-LAC/SPIO nanoparticle was labeled on the surface of Candida albicans,showing good labeling efficiency and in vitro MR imaging effect,while Alkyl-PEI-LAC/SPIO nanoparticle didn’t affect their virulence.The systemic Candida albicans infection models were built by injecting the Alkyl-PEI-LAC/SPIO-labeled Candida albicans to BALB/c mice through the tail vein.The pathological staining results showed that Candida albicans invaded the liver and kidneys.Compared with the T2-weighted signal of liver and kidney before infection,there was no significant change in post-infection.A small amount of Alkyl-PEI-LAC/SPIO nanoparticles were detected in liver,but didn’t coincide with the position of detected Candida albicans.The possible reasons are:1)Due to the limited pathological sections obtained,the detected Alkyl-PEI-LAC/SPIO nanoparticles and Candida albicans in the same position at one section may not be captured;2)After the Alkyl-PEI-LAC/SPIO labeled Candida albicans invaded the body,the probe fell off their surface because of the complexity the internal environment.Then the probe and Candida albicans arrived at the liver,respectively.In addition,T2-weighted imaging is affected by the kidney perfusion,the signal changes of the region of interest of kidney couldn’t be accurate detected.Although the mouse infection model of Alkyl-PEI-LAC/SPIO nanoparticles-labeled Candida albicans failed to fully realize the tracking of the Candida albicans in the early infection stage,the PEI/SPIO nanoparticles with excellent T2 relaxivity,high labeling efficiency and good biocompatibility reflects the potential in cell or fungal tracing.The previous studies of PEI/SPIO are mainly used for T2 MRI.Combining with other imaging technologies can achieve complementary advantages of imaging technologies,which may further expand the applications of PEI/SPIO.The fluorescence intensity of the AIE in the aggregation state is significantly increased,which improves the application of the fluorescent probes in vivo.Therefore,based on the Alkyl-PEI-LAC amphiphilic polymer studied in Candida albicans tracing,the study introduces the AIE molecule TPE as fluorescent probe.Then,through self-assembles encapsulating SPIO to construct a MR/Fluorescent dual-mode probe with both high sensitivity and spatial-temporal resolution,named Alkyl-PEI-LAC-TPE/SPIO.The fluorescence intensity of Alkyl-PEI-LAC-TPE micelles of different alkyl chain branches(13%,27%and 40%)was different.Reducing the degree of grafting of the alkyl chain increased the tightness of the TPE in the hydrophobic core of Alkyl-PEI-LAC-TPE,and further limited the rotation of the TPE molecule,thereby prolonging the fluorescence lifetime and increasing the fluorescence intensity.At 1.5 T MR scanner,compared with the commercial contrast agent Feridex(120 mM-1s-1),higher T2 relaxivity were detected in Alkyl-PEI-LAC-TPE(13%)/SPIO,Alkyl-PEI-LAC-TPE(27%)/SPIO and Alkyl-PEI-LAC-TPE(40%)/SPIO nanoparticles with 285 Fe mM-1s-1,337 Fe mM-1s-1 and 352 Fe mM-1s-1,respectively.In addition,the Alkyl-PEI-LAC-TPE/SPIO nanoparticles can label HeLa cells effectively,showing good cell fluorescence imaging and MRI in vitro.In summary,Alkyl-PEI-LAC-TPE/SPIO nanoparticles,as magnetic resonance/optical probes based on SPIO and AIE,show great potential in medical imaging application.In addition to exploring the PEI/SPIO contrast agent platform,the second part of this thesis includes that dextran-encapsulated SPIO nanoparticles of different sizes were used in mouse lymph node imaging,which may serve as a reference for the lymph node staging in tumor metastasis.Finally,in order to obtain more accurate diagnostic information,a SPIO-based T1-T2 dual-mode probe was constructed,which realized the dual-mode multi-organ imaging in cardiovascular and liver tissues.Tumor lymph node staging is an important guidance for the treatment and prognosis of cancer patients.SPIO nanoparticles used for T2 modal MRI as a lymph node tracer can increase the detection rate of lymph node metastases,and provide a basis for tumor lymph node staging.In addition,the water-soluble polymer dextran with good biocompatibility is an ideal coating material.This study explored the dextran coating SPIO nanoparticles with different diameters(18,27,65 and 91 nm)in MR ly mphadenography.Dextran-encapsulated SPIO nanoparticles with good dispersibility were synthesized by co-precipitation method.Different sizes of dextran-encapsulated SPIO nanoparticles were injected through subcutaneous plantar.T2-weighted MRI was performed by 3.0 T MR scanner on mice popliteal lymph nodes and inguinal lymph nodes before and 2 h,24 h,and 1 week after injection,calculating the T2 relaxation time.The arriving time and T2 relaxation time of lymph node from dextran-encapsulated SPIO with different sizes were different.Compared with the large-size dextran-encapsulated SPIO,the smaller(18 nm and 27 nm)entered the lymph nodes through the afferent lymphatic vessels more quickly.Comparing with transparent lymph nodes before injection,the color of the popliteal lymph nodes’ outer periphery turned to brown after injection,which was consistent with the results of pathological staining,indicating dextran-encapsulated SPIO arrived at the lymph nodes.This work paved the way for the selection of appropriate dextran-encapsulated SPIO for staging diagnosis of lymph node metastases.As a T2 MRI contrast agent,the magnetic susceptibility artifacts and negative contrast effects of SPIO limit its clinical application.However,T1-T2 dual-mode contrast agent enhanced MRI can obtain more accurate information by obtaining T1-weighted images with high tissue resolution and T2-weighted images with high sensitivity to detect lesions.In this thesis,a three-step reaction of ligand exchange,diethylenetriaminepentaacetic acid(DTPA)grafting,and gadolinium(Gadolinium,Gd)chelation was performed to obtain T1-T2 dual-mode contrast agent.Polyethylene glycol(PEG)-based Gd-DTPA(T1 imaging component)was formed as a shell and SPIO(T2 imaging component)was used as a core,named SPIO@PEG-GdDTPA.In order to obtain a good T1-T2 imaging effect,SPIO@PEG-GdDTPA with different Gd/Fe molar ratios(0.94,1.28 and 1.67)were prepared.Both of them showed good gadolinium chelation stability.In addition,the SPIO@PEG-GdDTPA0.94 nanoparticles with high r1(8.38 mM-1s-1)and high r2(83.20 mM-1s-1)showed a relatively ideal r2/rl ratio(9.93).Besides,there was no obvious cytotoxicity within 200 μg(Gd+Fe)/mL.At 3.0 T MR scanner,SPIO@PEG-GdDTPA0.94 nanoparticles were injected into Sprague-Dawley(SD)rats via tail vein.Compared with signal before injection,the T1 MRI signal of the vascular was significantly enhanced within 2 hours after injection,while the T2 MRI signal of the liver tissue was also significantly enhanced within 24 hours after injection.Therefore,MRA and liver T1 and T2-weighted MR images can be sequentially obtained by once injection of SPIO@PEG-GdDTPA0.94,which shows potential in clinical application as a multi-organ dual-mode MRI contrast agent.In summary,this study designed and developed a series of the SPIO-based contrast agents with good biocompatibility and ultra-high sensitivity,widely used in Candida albicans tracing,cell imaging,lymphadenography,angiography and liver imaging,showing great potential in future clinical application.