Potential Applications Of Molecular Targeted Magnetic Resonance Imaging Contrast Agents In The Diagnosis Of Liver Fibrosis And Imaging Of Tumors

Magnetic resonance imaging（MRI）is a powerful non-invasive and non-invasive imaging technology,which has many advantages,such as high resolution,no ionizing radiation,multi-directional imaging,and so on,and has been widely used in clinical medical research.MRI contrast agents are important component of clinical MRI research.Molecular targeted contrast agents belong to molecular imaging probes,which are constructed by connecting small molecules with contrast agent molecules with strong affinity and high specificity,they are able to adhere to molecular targets with high specificity,providing extended local contrast enhancement for specific tissues or organs,it will be helpful for the detection,staging and treatment monitoring of some diseases which are difficult to be diagnosed by conventional MRI,and they have great research value and clinical application potential.Hepatic fibrosis is a necessary stage for the development of chronic liver diseases to liver cirrhosis and hepatocellular carcinoma.The non-invasive and accurate diagnosis and staging of hepatic fibrosis by MRI are of great value in clinical medical research.In this study,we designed and synthesized a series of MRI molecular probes based on the changes of cellular and molecular levels during the development of liver fibrosis,and evaluated their feasibility for early diagnosis of liver fibrosis,the feasibility of two of these probes for tumor detection was also evaluated.In the first chapter,we described the principle of MRI and MRI contrast agent,the research progress of targeted MRI contrast agents and the study of hepatic fibrosis,then,we explained the research ideas of this paper and briefly summarize the main content.In chapter 2,based on the fact that small cyclic peptide pPB can specifically bind to over-expressed PDGFR-βon the surface of activated hepatic stellate cells,two MRI molecular probes,pPB-（HP-DO3A）Gd and pPB-tris-3（HP-DO3A）Gd,were designed and synthesized.The relaxation rates of pPB-（HP-DO3A）Gd and pPB-tris-3（HP-DO3A）Gd were 6.91 m M^-1s^-1and 8.15 m M^-1s^-1,respectively,which were 62%and 91%higher than those of Prohance.Two probes had good complexation stability and biocompatibility,and both of them could specifically bind to hepatic stellate cells HSC and LX-2.In the mouse model of early and middle-stage liver fibrosis,two probes showed good imaging results compared with the non-targeted contrast agent Prohance,pPB-（HP-DO3A）Gd and pPB-tris-3（HP-DO3A）Gd significantly increased the image brightness and signal intensity of the liver in mice with hepatic fibrosis.In addition,at the same injection dose,pPB-tris-3（HP-DO3A）Gd was able to obtain a higher signal than pPB-（HP-DO3A）Gd.The distribution experiment also showed that two probes could specifically and rapidly reach the fibrotic liver.Although the two MRI molecular probes in chapter 2 obtained good results in MRI imaging of mice with liver fibrosis,we also wanted to find better targets to further enhance imaging.In Chapter 3,based on the fact that small molecule FAP inhibitors can specifically bind to FAP,which were expressed in large quantities in the process of liver fibrosis,we designed and synthesized two MRI molecular probes,FAPI-06-（HP-DO3A）Gd and FAPI-07-（HP-DO3A）Gd.The relaxation rates of FAPI-06-（HP-DO3A）Gd and FAPI-07-（HP-DO3A）Gd were 7.87 m M^-1s^-1 and 7.63m M^-1s^-1,respectively,it were up 84.7%and 79.1%compared to Prohance.The two probes had good complexation stability and biocompatibility.In the mouse model of early and middle-stage liver fibrosis,both of them showed good imaging results compared with the non-targeted contrast agent Prohance.The two probes were able to significantly increase image brightness and signal intensity of the liver in mice with hepatic fibrosis,and FAPI-07-（HP-DO3A）Gd was able to obtain higher signal.In addition,the distribution experiment showed that the two probes could specifically and rapidly reach the site of liver fibrosis and metabolized.Even though those probes in the first two chapters achieved good imaging in mice with liver fibrosis,due to the limited number of target cells,the imaging effects of these molecular probes were limited.In chapter 4,we focused on the most abundant parenchymal cells in the liver.Based on the fact that there was a large amount of ASGPR on the surface of hepatocytes and that it has a strong binding affinity for Gal NAc,we synthesized a MRI molecular probe Gal NAc-（HP-DO3A）Gd,using the Gal NAc with three antennaes as the target group.The relaxation rate of Gal NAc-（HP-DO3A）Gd was 7.88 m M^-1s^-1,which is 84.9%higher than Prohance.The probe had good complexation stability and biocompatibility.MRI experiments showed that the probe had a good targeting effect on hepatocytes,it was able to significantly increase image brightness and show higher signal enhancement at the liver site in both normal and fibrotic mice.Unfortunately,due to the limitation of experimental conditions and image quality,it is difficult to distinguish normal liver from fibrotic liver in terms of image quality.In addition,the distribution experiment also showed that the probe can specifically and quickly reach the liver and stay for a certain time.FAP was also present in the majority of tumors,in chapter 5,we used FAPI-06-（HP-DO3A）Gd and FAPI-07-（HP-DO3A）Gd for imaging of tumors.Cellular imaging demonstrated that two probes were specific for FAP-positive cells.In vivo MRI for tumors found that two probes could significantly enhance the image brightness of tumor formed by FAP-positive cells,the distribution experiment also proved that the two probes could be more enriched in the tumor formed by FAP-positive cells and stay for a certain period of time.In summary,based on the design principle of molecular targeted MRI contrast agents,a series of MRI molecular probes with targeting specificity were constructed in this dissertation,their properties were studied and evaluated at molecular level,cellular level and animal level.The results of this study provide a new idea and basis for early accurate diagnosis of hepatic fibrosis,liver-targeted imaging and tumor diagnosis at cellular and molecular levels.