Preparation And Bioimaging Study Of Novel Dendritic Magnetic Resonance Contrast Agents

Tumor imaging is very important for tumor diagnosis.Nowadays,magnetic resonance imaging?MRI?is widely used as a consequence of its advantages of non-radiation,noninvasion and high spatial resolution.It provides great help for tumor diagnosis.However,MRI imaging resolution of the tumor is not ideal,contrast agents were necessary for the clarity of their images.At present,most commercial contrast agents are small molecules with simple structure and good safety,but their relaxation rates are low and the removal rates are too fast to meet the requirements of tumor diagnosis and research.Therefore,the construction of a new type of contrast agent with high relaxation rate and long circulation has become an urgent need.Combining nanocarriers with MRI contrast agents is a very promising solution.Among all kinds of nanocarriers,dendrimer,with advantages of accurate chemical structure,uniform distribution and multifunctional groups,is very suitable as medical nano-carrier.However,the current common dendrimer is limited by the complexity of synthesis,poor biocompatibility and a high residual rate in vivo.In this paper,two novel dendrimer were constructed:a polyglycerol dendrimer and another dendrimer with internal hydroxyl residues at the side of branches,both with(3-cyclodextrin as core which simplified the synthesis.And zwitterionic groups and tumor targeting groups were introduced into the system to build tumor targeting dendritic contrast agents?DCA?.?-cyclodextrin-based polyglycerol dendrimer?G1?G4?,which was synthesized by alternating Willaims and Sharpless reactions,and charaterized by H NMR,GPC and MALDI-TOF-MS.The dendrimer has the advantages of compact structure,high biocompatibility and simple synthesis steps.And the dendrimer can easily reached 18.9kDa with 168 branches while the traditional one with only 5.1kDa and 48 brancheds.A variety of groups can be modified on the surface of the dendrimer to change the character of the dendrimer by click reactions.And their cytotoxicity,interaction with protein and organ distribution were measured.G3-NH₂ was highly toxic and with poor biocompatibility.G3-COOH was little toxic and hard to enter cells.G3-OH was excreted very quickly.G3-Cys was non-fouling under pH 7.4,but interactional with protein under acid environment.As a consequence,G3-Cys can make a better dilevery system than the others.On the basis of this,a dendritic contrast agent?DCA?with the structure of the zwitterionic surface was constructed,Gn-DOTA-Gd-Cys.The relaxation rate of it was 3.3 times to Magnevist.And the blood pool imaging in mice enhanced by Gn-DOTA-Gd-Cys was much better than that by Magnevist.Also,the the DCA with high generations had better circlulation time and imainge ehancement.And the residual of the DCA in vivo is very lower and without toxicity caused by Retention and metabolism than that of DCA based on PAMAM.We also have attached a polyethylene glycol with a targeting group to the polyglycerol dendrimer,and synthesized a tumor-targeted DCA.The Ri is about 6.9 times to Magnevist.It can enrich in the hypoxia region of the tumor and enhance the contrast between tumor and normal tissue.Finally,in order to avoid dendritic macromolecule surface introduction of contrast agent damage dendrimer zwitterionic properties.In this paper,cyclodextrin was used as the core to synthesize an internal hydroxylated dendrimer by "amine-epoxy" reaction and "thiol-allyl" reaction.The structures were characterized by ¹H NMR,GPC.At the same time,a new type of tumor-targeted dendritic macromolecule contrast agent was obtained by linking zwitterions and tumor-targeted benzenesulfonamide groups onto the surface of the dendrimer.Among them,zwitterionic groups can avoid non-specific protein adhesion and endocytosis,while the tumor targeting group on the surface can improve the tumor targeting performance of DCA.The contrast agent had a particle size of 6.2 nm and a relaxation rate of 11.3 mM^-1s^-1,which is about 2.7 times that of Magnevist.Cell experiments showed that the DCA had tumor targeting properties under hypoxia.And animal experiments showed that the tumor targeting DCA significantly enhanced tumor.Moreover,the DCA can quickly excrete through the kidneys without obvious systemic toxicity.All these characters make the DCA an ideal tumor-targeted contrast agent.